diff --git a/icu4c/source/common/ucase.h b/icu4c/source/common/ucase.h
index e03b311870f8..af7387360880 100644
--- a/icu4c/source/common/ucase.h
+++ b/icu4c/source/common/ucase.h
@@ -357,7 +357,7 @@ enum {
 /* definitions for 16-bit case properties word ------------------------------ */
 
 U_CFUNC const UTrie2 * U_EXPORT2
-ucase_getTrie();
+ucase_getTrie(void);
 
 /* 2-bit constants for types of cased characters */
 #define UCASE_TYPE_MASK     3
diff --git a/icu4c/source/common/unicode/ulocbuilder.h b/icu4c/source/common/unicode/ulocbuilder.h
index f2e98612f0a0..9b147292f49e 100644
--- a/icu4c/source/common/unicode/ulocbuilder.h
+++ b/icu4c/source/common/unicode/ulocbuilder.h
@@ -74,7 +74,7 @@ typedef struct ULocaleBuilder ULocaleBuilder;
  * @draft ICU 74
  */
 U_CAPI ULocaleBuilder* U_EXPORT2
-ulocbld_open();
+ulocbld_open(void);
 
 /**
  * Close the builder and destroy it's internal states.
diff --git a/icu4c/source/samples/cal/cal.c b/icu4c/source/samples/cal/cal.c
index 95fe48ff837b..2725d44aa807 100644
--- a/icu4c/source/samples/cal/cal.c
+++ b/icu4c/source/samples/cal/cal.c
@@ -201,7 +201,7 @@ main(int argc,
 
 /* Usage information */
 static void
-usage()
+usage(void)
 {  
     puts("Usage: icucal [OPTIONS] [[MONTH] YEAR]");
     puts("");
@@ -219,7 +219,7 @@ usage()
 
 /* Version information */
 static void
-version()
+version(void)
 {
     printf("icucal version %s (ICU version %s), created by Stephen F. Booth.\n", 
         CAL_VERSION, U_ICU_VERSION); 
diff --git a/icu4c/source/samples/date/date.c b/icu4c/source/samples/date/date.c
index a8abb369b765..065ec71bf848 100644
--- a/icu4c/source/samples/date/date.c
+++ b/icu4c/source/samples/date/date.c
@@ -186,7 +186,7 @@ main(int argc,
 
 /* Usage information */
 static void
-usage()
+usage(void)
 {  
   puts("Usage: icudate [OPTIONS]");
   puts("Options:");
@@ -207,7 +207,7 @@ usage()
 
 /* Version information */
 static void
-version()
+version(void)
 {
   UErrorCode status = U_ZERO_ERROR;
   const char *tzVer;
diff --git a/icu4c/source/samples/layout/gnomeglue.h b/icu4c/source/samples/layout/gnomeglue.h
index 13e434e1acea..29b29dac6fe9 100644
--- a/icu4c/source/samples/layout/gnomeglue.h
+++ b/icu4c/source/samples/layout/gnomeglue.h
@@ -25,7 +25,7 @@ typedef void fm_fontMap;
 
 U_CDECL_BEGIN
 
-gs_guiSupport *gs_gnomeGuiSupportOpen();
+gs_guiSupport *gs_gnomeGuiSupportOpen(void);
 void gs_gnomeGuiSupportClose(gs_guiSupport *guiSupport);
 
 rs_surface *rs_gnomeRenderingSurfaceOpen(GtkWidget *theWidget);
diff --git a/icu4c/source/test/cintltst/callcoll.c b/icu4c/source/test/cintltst/callcoll.c
index ff9023db0379..855ce5d8b6a6 100644
--- a/icu4c/source/test/cintltst/callcoll.c
+++ b/icu4c/source/test/cintltst/callcoll.c
@@ -739,7 +739,7 @@ void genericRulesStarter(const char *rules, const char * const s[], uint32_t siz
   genericRulesStarterWithResult(rules, s, size, UCOL_LESS);
 }
 
-static void TestTertiary()
+static void TestTertiary(void)
 {
     int32_t len,i;
     UCollator *myCollation;
@@ -764,7 +764,7 @@ static void TestTertiary()
     myCollation = 0;
 }
 
-static void TestPrimary( )
+static void TestPrimary(void)
 {
     int32_t len,i;
     UCollator *myCollation;
@@ -790,7 +790,7 @@ static void TestPrimary( )
     myCollation = 0;
 }
 
-static void TestSecondary()
+static void TestSecondary(void)
 {
     int32_t i;
     int32_t len;
@@ -815,7 +815,7 @@ static void TestSecondary()
     myCollation = 0;
 }
 
-static void TestIdentical()
+static void TestIdentical(void)
 {
     int32_t i;
     int32_t len;
@@ -839,7 +839,7 @@ static void TestIdentical()
     myCollation = 0;
 }
 
-static void TestExtra()
+static void TestExtra(void)
 {
     int32_t i, j;
     int32_t len;
@@ -1173,7 +1173,7 @@ static void TestSurrogates(void)
  *
  */
 static void 
-TestInvalidRules(){
+TestInvalidRules(void){
 #define MAX_ERROR_STATES 2
 
     static const char* rulesArr[MAX_ERROR_STATES] = {
@@ -1219,7 +1219,7 @@ TestInvalidRules(){
 }
 
 static void
-TestJitterbug1098(){
+TestJitterbug1098(void){
     UChar rule[1000];
     UCollator* c1 = NULL;
     UErrorCode status = U_ZERO_ERROR;
diff --git a/icu4c/source/test/cintltst/capitst.c b/icu4c/source/test/cintltst/capitst.c
index 5d76ea0548a2..4797096310b7 100644
--- a/icu4c/source/test/cintltst/capitst.c
+++ b/icu4c/source/test/cintltst/capitst.c
@@ -271,7 +271,7 @@ void TestGetDefaultRules(){
 /* Collator Properties
  ucol_open, ucol_strcoll,  getStrength/setStrength
  getDecomposition/setDecomposition, getDisplayName*/
-void TestProperty()
+void TestProperty(void)
 {
     UCollator *col, *ruled;
     const UChar *rules;
@@ -449,7 +449,7 @@ void TestProperty()
 }
 
 /* Test RuleBasedCollator and getRules*/
-void TestRuleBasedColl()
+void TestRuleBasedColl(void)
 {
     UCollator *col1, *col2, *col3, *col4;
     UCollationElements *iter1, *iter2;
@@ -562,7 +562,7 @@ void TestRuleBasedColl()
     ucol_close(col1);
 }
 
-void TestCompare()
+void TestCompare(void)
 {
     UErrorCode status = U_ZERO_ERROR;
     UCollator *col;
@@ -612,7 +612,7 @@ void TestCompare()
 ---------------------------------------------
  tests decomposition setting
 */
-void TestDecomposition() {
+void TestDecomposition(void) {
     UErrorCode status = U_ZERO_ERROR;
     UCollator *en_US, *el_GR, *vi_VN;
     en_US = ucol_open("en_US", &status);
@@ -651,7 +651,7 @@ void TestDecomposition() {
 
 #define CLONETEST_COLLATOR_COUNT 4
 
-void TestSafeClone() {
+void TestSafeClone(void) {
     UChar test1[6];
     UChar test2[6];
     static const UChar umlautUStr[] = {0x00DC, 0};
@@ -808,7 +808,7 @@ void TestSafeClone() {
     }
 }
 
-void TestClone() {
+void TestClone(void) {
     UChar test1[6];
     UChar test2[6];
     static const UChar umlautUStr[] = {0x00DC, 0};
@@ -926,7 +926,7 @@ void TestClone() {
     }
 }
 
-void TestCloneBinary(){
+void TestCloneBinary(void){
     UErrorCode err = U_ZERO_ERROR;
     UCollator * col = ucol_open("en_US", &err);
     UCollator * c;
@@ -1046,7 +1046,7 @@ static void TestBengaliSortKey(void)
     TestOpenVsOpenRules ensures that collators from ucol_open and ucol_openRules
     will generate identical sort keys
 */
-void TestOpenVsOpenRules(){
+void TestOpenVsOpenRules(void){
 
     /* create an array of all the locales */
     int32_t numLocales = uloc_countAvailable();
@@ -1189,7 +1189,7 @@ void TestOpenVsOpenRules(){
 ----------------------------------------------------------------------------
  ctor -- Tests the getSortKey
 */
-void TestSortKey()
+void TestSortKey(void)
 {
     uint8_t *sortk1 = NULL, *sortk2 = NULL, *sortk3 = NULL, *sortkEmpty = NULL;
     int32_t sortklen, osortklen;
@@ -1334,7 +1334,7 @@ void TestSortKey()
     free(sortk2);
 
 }
-void TestHashCode()
+void TestHashCode(void)
 {
     uint8_t *sortk1, *sortk2, *sortk3;
     int32_t sortk1len, sortk2len, sortk3len;
@@ -1388,7 +1388,7 @@ void TestHashCode()
  * Tests the UCollatorElements API.
  *
  */
-void TestElemIter()
+void TestElemIter(void)
 {
     int32_t offset;
     int32_t order1, order2, order3;
@@ -1518,7 +1518,7 @@ doAssert( (ucol_tertiaryOrder(order1) != ucol_tertiaryOrder(order3)), "The terti
     log_verbose("testing CollationElementIterator ends...\n");
 }
 
-void TestGetLocale() {
+void TestGetLocale(void) {
   UErrorCode status = U_ZERO_ERROR;
   const char *rules = "&a<x<y<z";
   UChar rlz[256] = {0};
@@ -1619,7 +1619,7 @@ void TestGetLocale() {
 }
 
 
-void TestGetAll()
+void TestGetAll(void)
 {
     int32_t i, count;
     count=ucol_countAvailable();
@@ -1646,7 +1646,7 @@ static int compare_teststruct(const void *string1, const void *string2) {
     return(strcmp((const char *)((struct teststruct *)string1)->key, (const char *)((struct teststruct *)string2)->key));
 }
 
-void TestBounds() {
+void TestBounds(void) {
     UErrorCode status = U_ZERO_ERROR;
 
     UCollator *coll = ucol_open("sh", &status);
@@ -1861,7 +1861,7 @@ void TestSortKeyBufferOverrun(void) {
     ucol_close(coll);
 }
 
-static void TestAttribute()
+static void TestAttribute(void)
 {
     UErrorCode error = U_ZERO_ERROR;
     UCollator *coll = ucol_open(NULL, &error);
@@ -1964,7 +1964,7 @@ static void TestAttribute()
     ucol_close(coll);
 }
 
-void TestGetTailoredSet() {
+void TestGetTailoredSet(void) {
   struct {
     const char *rules;
     const char *tests[20];
diff --git a/icu4c/source/test/cintltst/cbiapts.c b/icu4c/source/test/cintltst/cbiapts.c
index 7fc43289a944..147bdc968458 100644
--- a/icu4c/source/test/cintltst/cbiapts.c
+++ b/icu4c/source/test/cintltst/cbiapts.c
@@ -156,7 +156,7 @@ static void freeToUCharStrings(void **hook) {
 
 
 #if !UCONFIG_NO_FILE_IO
-static void TestBreakIteratorCAPI()
+static void TestBreakIteratorCAPI(void)
 {
     UErrorCode status = U_ZERO_ERROR;
     UBreakIterator *word, *sentence, *line, *character, *b, *bogus;
@@ -633,7 +633,7 @@ static UBreakIterator * testOpenRules(char *rules) {
  *  TestBreakIteratorRules - Verify that a break iterator can be created from
  *                           a set of source rules.
  */
-static void TestBreakIteratorRules() {
+static void TestBreakIteratorRules(void) {
     /*  Rules will keep together any run of letters not including 'a', OR
      *             keep together 'abc', but only when followed by 'def', OTHERWISE
      *             just return one char at a time.
@@ -720,7 +720,7 @@ static void TestBreakIteratorRules() {
     ubrk_close(bi);
 }
 
-static void TestBreakIteratorRuleError() {
+static void TestBreakIteratorRuleError(void) {
 /*
  *  TestBreakIteratorRuleError -   Try to create a BI from rules with syntax errors,
  *                                 check that the error is reported correctly.
@@ -754,7 +754,7 @@ static void TestBreakIteratorRuleError() {
 /*
 *   TestsBreakIteratorStatusVals()   Test the ubrk_getRuleStatusVec() function
 */
-static void TestBreakIteratorStatusVec() {
+static void TestBreakIteratorStatusVec(void) {
     #define RULE_STRING_LENGTH 200
     UChar          rules[RULE_STRING_LENGTH];
 
diff --git a/icu4c/source/test/cintltst/ccaltst.c b/icu4c/source/test/cintltst/ccaltst.c
index 0744ba14abfa..33d5c39bf2dd 100644
--- a/icu4c/source/test/cintltst/ccaltst.c
+++ b/icu4c/source/test/cintltst/ccaltst.c
@@ -112,7 +112,7 @@ static const UCalGetTypeTest ucalGetTypeTests[] = {
     { NULL, 0, NULL } /* terminator */
 };    
     
-static void TestCalendar()
+static void TestCalendar(void)
 {
     UCalendar *caldef = 0, *caldef2 = 0, *calfr = 0, *calit = 0, *calfrclone = 0;
     UEnumeration* uenum = NULL;
@@ -527,7 +527,7 @@ static void TestCalendar()
 /*------------------------------------------------------*/
 /*Testing the getMillis, setMillis, setDate and setDateTime functions extensively*/
 
-static void TestGetSetDateAPI()
+static void TestGetSetDateAPI(void)
 {
     UCalendar *caldef = 0, *caldef2 = 0, *caldef3 = 0;
     UChar tzID[4];
@@ -795,7 +795,7 @@ static void TestGetSetDateAPI()
 /**
  * Confirm the functioning of the calendar field related functions.
  */
-static void TestFieldGetSet()
+static void TestFieldGetSet(void)
 {
     UCalendar *cal = 0;
     UChar tzID[4];
@@ -955,7 +955,7 @@ static const TransitionItem transitionItems[] = {
 /**
  * Execute adding and rolling in Calendar extensively,
  */
-static void TestAddRollExtensive()
+static void TestAddRollExtensive(void)
 {
     const TransitionItem * itemPtr;
     UCalendar *cal = 0;
@@ -1149,7 +1149,7 @@ static void TestAddRollExtensive()
 
 /*------------------------------------------------------ */
 /*Testing the Limits for various Fields of Calendar*/
-static void TestGetLimits()
+static void TestGetLimits(void)
 {
     UCalendar *cal = 0;
     int32_t min, max, gr_min, le_max, ac_min, ac_max, val;
@@ -1245,7 +1245,7 @@ static void TestGetLimits()
  * Test that the days of the week progress properly when add is called repeatedly
  * for increments of 24 days.
  */
-static void TestDOWProgression()
+static void TestDOWProgression(void)
 {
     int32_t initialDOW, DOW, newDOW, expectedDOW;
     UCalendar *cal = 0;
@@ -1311,7 +1311,7 @@ static void TestDOWProgression()
 /**
  * Confirm that the offset between local time and GMT behaves as expected.
  */
-static void TestGMTvsLocal()
+static void TestGMTvsLocal(void)
 {
     log_verbose("\nTesting the offset between the GMT and local time\n");
     testZones(1999, 1, 1, 12, 0, 0);
@@ -1551,7 +1551,7 @@ static void verify2(const char* msg, UCalendar* c, UDateFormat* dat, int32_t yea
         
 }
 
-void TestGregorianChange() {
+void TestGregorianChange(void) {
     static const UChar utc[] = { 0x45, 0x74, 0x63, 0x2f, 0x47, 0x4d, 0x54, 0 }; /* "Etc/GMT" */
     const int32_t dayMillis = 86400 * INT64_C(1000);    /* 1 day = 86400 seconds */
     UCalendar *cal;
@@ -1615,7 +1615,7 @@ void TestGregorianChange() {
     ucal_close(cal);
 }
 
-static void TestGetKeywordValuesForLocale() {
+static void TestGetKeywordValuesForLocale(void) {
 #define PREFERRED_SIZE 26
 #define MAX_NUMBER_OF_KEYWORDS 5
     const char *PREFERRED[PREFERRED_SIZE][MAX_NUMBER_OF_KEYWORDS+1] = {
@@ -1816,7 +1816,7 @@ static const UChar logDateFormat[] = { 0x0045,0x0045,0x0045,0x0020,0x004D,0x004D
                                        0x0073,0x0073,0x002E,0x0053,0x0053,0x0053,0 }; /* "EEE MMM dd yyyy G HH:mm:ss.SSS" */
 enum { kFormattedDateMax = 2*UPRV_LENGTHOF(logDateFormat) };
 
-static void TestWeekend() {
+static void TestWeekend(void) {
     const TestWeekendDatesList * testDatesPtr = testDates;
     const TestDaysOfWeekList *   testDaysPtr = testDays;
     int32_t count, subCount;
@@ -1956,7 +1956,7 @@ static const TFDItem tfdItems[] = {
     { NULL,        NULL,           0.0,               0.0,              false,    0,   0,     0,      0,        0,          0 }  /* terminator */
 };
 
-void TestFieldDifference() {
+void TestFieldDifference(void) {
     const TFDItem * tfdItemPtr;
     for (tfdItemPtr = tfdItems; tfdItemPtr->timezone != NULL; tfdItemPtr++) {
         UErrorCode status = U_ZERO_ERROR;
@@ -2031,7 +2031,7 @@ void TestFieldDifference() {
     }
 }
 
-void TestAmbiguousWallTime() {
+void TestAmbiguousWallTime(void) {
     UErrorCode status = U_ZERO_ERROR;
     UChar tzID[32];
     UCalendar* ucal;
@@ -2162,7 +2162,7 @@ static const EraTestItem eraTestItems[] = {
 
 static const UChar zoneGMT[] = { 0x47,0x4D,0x54,0 };
 
-void TestAddRollEra0AndEraBounds() {
+void TestAddRollEra0AndEraBounds(void) {
     const EraTestItem * eraTestItemPtr;
     for (eraTestItemPtr = eraTestItems; eraTestItemPtr->locale != NULL; eraTestItemPtr++) {
         UErrorCode status = U_ZERO_ERROR;
@@ -2385,7 +2385,7 @@ static const TZTransitionItem tzTransitionItems[] = {
     { NULL,                 NULL,             0,         0, 0, false, false } /* terminator */
 };
 
-void TestGetTZTransition() {
+void TestGetTZTransition(void) {
     UErrorCode status = U_ZERO_ERROR;
     UCalendar * ucal = ucal_open(zoneGMT, -1, "en", UCAL_GREGORIAN, &status);
     if ( U_SUCCESS(status) ) {
@@ -2455,7 +2455,7 @@ static const UChar sBogusWithVariantCharacters[] = /* Bogus with Variant charact
     {0x48,0xE8,0x2113,0x2113,0xF4,0x20,0x57,0xF4,0x159,0x2113,0x3B4,0x00};
 #endif
 
-void TestGetWindowsTimeZoneID() {
+void TestGetWindowsTimeZoneID(void) {
     UErrorCode status;
     UChar winID[64];
     int32_t len;
@@ -2489,7 +2489,7 @@ void TestGetWindowsTimeZoneID() {
     }
 }
 
-void TestGetTimeZoneIDByWindowsID() {
+void TestGetTimeZoneIDByWindowsID(void) {
     UErrorCode status;
     UChar tzID[64];
     int32_t len;
@@ -2543,7 +2543,7 @@ void TestGetTimeZoneIDByWindowsID() {
 
 // The following currently assumes that Reiwa is the last known/valid era.
 // Filed ICU-20551 to generalize this when we have more time...
-void TestJpnCalAddSetNextEra() {
+void TestJpnCalAddSetNextEra(void) {
     UErrorCode status = U_ZERO_ERROR;
     UCalendar *jCal = ucal_open(NULL, 0, "ja_JP@calendar=japanese", UCAL_DEFAULT, &status);
     if ( U_FAILURE(status) ) {
@@ -2588,7 +2588,7 @@ void TestJpnCalAddSetNextEra() {
     }
 }
 
-void TestUcalOpenBufferRead() {
+void TestUcalOpenBufferRead(void) {
     // ICU-21004: The issue shows under valgrind or as an Address Sanitizer failure.
     UErrorCode status = U_ZERO_ERROR;
     // string length: 157 + 1 + 100 = 258
@@ -2602,7 +2602,7 @@ void TestUcalOpenBufferRead() {
  * Testing ucal_getTimeZoneOffsetFromLocal
  */
 void
-TestGetTimeZoneOffsetFromLocal() {
+TestGetTimeZoneOffsetFromLocal(void) {
     static const UChar utc[] = u"Etc/GMT";
 
     const int32_t HOUR = 60*60*1000;
@@ -2814,7 +2814,7 @@ TestGetTimeZoneOffsetFromLocal() {
 }
 
 void
-TestFWWithISO8601() {
+TestFWWithISO8601(void) {
     /* UCAL_SUNDAY is 1, UCAL_MONDAY is 2, ..., UCAL_SATURDAY is 7 */
     const char* LOCALES[] = {
         "",
@@ -2843,7 +2843,7 @@ TestFWWithISO8601() {
 }
 
 void
-TestGetIanaTimeZoneID() {
+TestGetIanaTimeZoneID(void) {
     const UChar* UNKNOWN = u"Etc/Unknown";
     typedef struct {
         const UChar* id;
diff --git a/icu4c/source/test/cintltst/ccapitst.c b/icu4c/source/test/cintltst/ccapitst.c
index 48ad77c92f2b..c8bbea539176 100644
--- a/icu4c/source/test/cintltst/ccapitst.c
+++ b/icu4c/source/test/cintltst/ccapitst.c
@@ -241,7 +241,7 @@ static void ListNames(void) {
 }
 
 
-static void TestConvert() 
+static void TestConvert(void)
 {
 #if !UCONFIG_NO_LEGACY_CONVERSION
     char                myptr[4];
@@ -1153,7 +1153,7 @@ static void TestFlushCache(void) {
  * aliases map back to itself.  Check some hard-coded UTF-8 and
  * ISO_2022 aliases to make sure they work.
  */
-static void TestAlias() {
+static void TestAlias(void) {
     int32_t i, ncnv;
     UErrorCode status = U_ZERO_ERROR;
 
@@ -1341,7 +1341,7 @@ static void TestDuplicateAlias(void) {
 
 /* Test safe clone callback */
 
-static uint32_t    TSCC_nextSerial()
+static uint32_t    TSCC_nextSerial(void)
 {
     static uint32_t n = 1;
     
@@ -1492,7 +1492,7 @@ static void TSCC_print_log(TSCCContext *q, const char *name)
     }
 }
 
-static void TestConvertSafeCloneCallback()
+static void TestConvertSafeCloneCallback(void)
 {
     UErrorCode err = U_ZERO_ERROR;
     TSCCContext from1, to1;
@@ -1671,7 +1671,7 @@ containsAnyOtherByte(uint8_t *p, int32_t length, uint8_t b) {
     return false;
 }
 
-static void TestConvertSafeClone()
+static void TestConvertSafeClone(void)
 {
     /* one 'regular' & all the 'private stateful' converters */
     static const char *const names[] = {
@@ -1909,7 +1909,7 @@ static void TestConvertSafeClone()
 }
 
 
-static void TestConvertClone()
+static void TestConvertClone(void)
 {
     /* one 'regular' & all the 'private stateful' converters */
     static const char *const names[] = {
@@ -2040,7 +2040,7 @@ static void TestConvertClone()
     }
 }
 
-static void TestCCSID() {
+static void TestCCSID(void) {
 #if !UCONFIG_NO_LEGACY_CONVERSION
     UConverter *cnv;
     UErrorCode errorCode;
@@ -2104,7 +2104,7 @@ TestJ932(void)
  *
  * Bug report and test code provided by Edward J. Batutis.
  */
-static void bug1()
+static void bug1(void)
 {
 #if !UCONFIG_NO_LEGACY_CONVERSION
    char char_in[CHUNK_SIZE+32];
@@ -2150,7 +2150,7 @@ static void bug1()
 }
 
 /* bug2: pre-flighting loop bug: simple overflow causes bug */
-static void bug2()
+static void bug2(void)
 {
     /* US-ASCII "1234567890" */
     static const char source[]={ 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39 };
@@ -2224,7 +2224,7 @@ static void bug2()
  * bug3: when the characters expand going from source to target codepage
  *       you get bug3 in addition to bug2
  */
-static void bug3()
+static void bug3(void)
 {
 #if !UCONFIG_NO_LEGACY_CONVERSION && !UCONFIG_ONLY_HTML_CONVERSION
     char char_in[CHUNK_SIZE*4];
@@ -2394,7 +2394,7 @@ convertExMultiStreaming(UConverter *srcCnv, UConverter *targetCnv,
                        7, testName, expectCode);
 }
 
-static void TestConvertEx() {
+static void TestConvertEx(void) {
 #if !UCONFIG_NO_LEGACY_CONVERSION
     static const uint8_t
     utf8[]={
@@ -2788,7 +2788,7 @@ static void testFromBadUTF8(UConverter *utf8Cnv, UConverter *cnv, const char *co
 }
 
 /* Test illegal UTF-8 input. */
-static void TestConvertExFromUTF8() {
+static void TestConvertExFromUTF8(void) {
     static const char *const converterNames[]={
 #if !UCONFIG_NO_LEGACY_CONVERSION
         "windows-1252",
@@ -2831,7 +2831,7 @@ static void TestConvertExFromUTF8() {
     ucnv_close(utf8Cnv);
 }
 
-static void TestConvertExFromUTF8_C5F0() {
+static void TestConvertExFromUTF8_C5F0(void) {
     static const char *const converterNames[]={
 #if !UCONFIG_NO_LEGACY_CONVERSION
         "windows-1251",
@@ -2917,7 +2917,7 @@ static void TestConvertExFromUTF8_C5F0() {
 }
 
 static void
-TestConvertAlgorithmic() {
+TestConvertAlgorithmic(void) {
 #if !UCONFIG_NO_LEGACY_CONVERSION
     static const uint8_t
     utf8[]={
@@ -3340,7 +3340,7 @@ testSwap(const char *name, UBool swap) {
 }
 
 static void
-TestEBCDICSwapLFNL() {
+TestEBCDICSwapLFNL(void) {
     static const struct {
         const char *name;
         UBool swap;
@@ -3365,7 +3365,7 @@ TestEBCDICSwapLFNL() {
 }
 #endif
 
-static void TestFromUCountPending(){
+static void TestFromUCountPending(void){
 #if !UCONFIG_NO_LEGACY_CONVERSION
     UErrorCode status = U_ZERO_ERROR;
 /*       const UChar expectedUnicode[] = { 0x20ac, 0x0005, 0x0006, 0x000b, 0xdbc4, 0xde34, 0xd84d, 0xdc56, 0xfffd}; */
@@ -3469,7 +3469,7 @@ static void TestFromUCountPending(){
 }
 
 static void
-TestToUCountPending(){
+TestToUCountPending(void){
 #if !UCONFIG_NO_LEGACY_CONVERSION
     UErrorCode status = U_ZERO_ERROR;
     static const struct {
@@ -3667,7 +3667,7 @@ compareNames(const char **names) {
 }
 
 static void
-TestCompareNames() {
+TestCompareNames(void) {
     static const char *equalUTF8[]={ "=", "UTF-8", "utf_8", "u*T@f08", "Utf 8", NULL };
     static const char *equalIBM[]={ "=", "ibm-37", "IBM037", "i-B-m  00037", "ibm-0037", "IBM00037", NULL };
     static const char *lessMac[]={ "<", "macos-0_1-10.2", "macos-1-10.0.2", "macos-1-10.2", NULL };
@@ -3680,7 +3680,7 @@ TestCompareNames() {
 }
 
 static void
-TestSubstString() {
+TestSubstString(void) {
     static const UChar surrogate[1]={ 0xd900 };
     char buffer[16];
 
@@ -3771,7 +3771,7 @@ TestSubstString() {
 }
 
 static void
-InvalidArguments() {
+InvalidArguments(void) {
     UConverter *cnv;
     UErrorCode errorCode;
     char charBuffer[2] = {1, 1};
@@ -3824,7 +3824,7 @@ InvalidArguments() {
     ucnv_close(cnv);
 }
 
-static void TestGetName() {
+static void TestGetName(void) {
     static const char *const names[] = {
         "Unicode",                  "UTF-16",
         "UnicodeBigUnmarked",       "UTF-16BE",
@@ -3848,7 +3848,7 @@ static void TestGetName() {
     }
 }
 
-static void TestUTFBOM() {
+static void TestUTFBOM(void) {
     static const UChar a16[] = { 0x61 };
     static const char *const names[] = {
         "UTF-16",
diff --git a/icu4c/source/test/cintltst/ccurrtst.c b/icu4c/source/test/cintltst/ccurrtst.c
index f2111f4e2280..f54af9cf9881 100644
--- a/icu4c/source/test/cintltst/ccurrtst.c
+++ b/icu4c/source/test/cintltst/ccurrtst.c
@@ -37,7 +37,7 @@ void addCurrencyCollTest(TestNode** root)
 }
 
 
-void currTest()
+void currTest(void)
 {
     /* All the currency symbols, in UCA order*/
     static const UChar currency[][2] =
diff --git a/icu4c/source/test/cintltst/cdateintervalformattest.c b/icu4c/source/test/cintltst/cdateintervalformattest.c
index 4c535830aa3c..298dcae4247f 100644
--- a/icu4c/source/test/cintltst/cdateintervalformattest.c
+++ b/icu4c/source/test/cintltst/cdateintervalformattest.c
@@ -94,7 +94,7 @@ enum {
     kFormatBufLen = 128
 };
 
-static void TestDateIntervalFormat()
+static void TestDateIntervalFormat(void)
 {
     const DateIntervalFormatTestItem * testItemPtr;
     UErrorCode status = U_ZERO_ERROR;
@@ -317,7 +317,7 @@ static const LocaleAndSkeletonItem locSkelItems[] = {
 
 enum { kSizeUBuf = 96, kSizeBBuf = 192 };
 
-static void TestFPos_SkelWithSeconds()
+static void TestFPos_SkelWithSeconds(void)
 {
 	const LocaleAndSkeletonItem * locSkelItemPtr;
 	for (locSkelItemPtr = locSkelItems; locSkelItemPtr->locale != NULL; locSkelItemPtr++) {
@@ -359,7 +359,7 @@ static void TestFPos_SkelWithSeconds()
     }
 }
 
-static void TestFormatToResult() {
+static void TestFormatToResult(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UDateIntervalFormat* fmt = udtitvfmt_open("de", u"dMMMMyHHmm", -1, zoneGMT, -1, &ec);
     UFormattedDateInterval* fdi = udtitvfmt_openResult(&ec);
@@ -420,7 +420,7 @@ static void TestFormatToResult() {
     udtitvfmt_closeResult(fdi);
 }
 
-static void TestFormatCalendarToResult() {
+static void TestFormatCalendarToResult(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UCalendar* ucal1 = ucal_open(zoneGMT, -1, "de", UCAL_DEFAULT, &ec);
     ucal_setMillis(ucal1, Date201009270800, &ec);
diff --git a/icu4c/source/test/cintltst/cdattst.c b/icu4c/source/test/cintltst/cdattst.c
index c390f1e4ad6c..c1880f0975a4 100644
--- a/icu4c/source/test/cintltst/cdattst.c
+++ b/icu4c/source/test/cintltst/cdattst.c
@@ -78,7 +78,7 @@ void addDateForTest(TestNode** root)
     TESTCASE(TestHourCycle);
 }
 /* Testing the DateFormat API */
-static void TestDateFormat()
+static void TestDateFormat(void)
 {
     UDateFormat *def, *fr, *it, *de, *def1, *fr_pat;
     UDateFormat *any;
@@ -438,7 +438,7 @@ enum { kDateOrTimeOutMax = 96, kDateAndTimeOutMax = 192 };
 static const UDate minutesTolerance = 2 * 60.0 * 1000.0;
 static const UDate daysTolerance = 2 * 24.0 * 60.0 * 60.0 * 1000.0;
 
-static void TestRelativeDateFormat()
+static void TestRelativeDateFormat(void)
 {
     UDate today = 0.0;
     const UDateFormatStyle * stylePtr;
@@ -588,7 +588,7 @@ static void TestRelativeDateFormat()
 }
 
 /*Testing udat_getSymbols() and udat_setSymbols() and udat_countSymbols()*/
-static void TestSymbols()
+static void TestSymbols(void)
 {
     UDateFormat *def, *fr, *zhChiCal, *esMX;
     UErrorCode status = U_ZERO_ERROR;
@@ -862,7 +862,7 @@ free(pattern);
 /**
  * Test DateFormat(Calendar) API
  */
-static void TestDateFormatCalendar() {
+static void TestDateFormatCalendar(void) {
     UDateFormat *date=0, *time=0, *full=0;
     UCalendar *cal=0;
     UChar buf[256];
@@ -989,7 +989,7 @@ static void TestDateFormatCalendar() {
 /**
  * Test parsing two digit year against "YY" vs. "YYYY" patterns
  */
-static void TestCalendarDateParse() {
+static void TestCalendarDateParse(void) {
 
     int32_t result;
     UErrorCode ec = U_ZERO_ERROR;
@@ -1069,7 +1069,7 @@ static void TestCalendarDateParse() {
 
 
 /*INTERNAL FUNCTIONS USED*/
-static int getCurrentYear() {
+static int getCurrentYear(void) {
     static int currentYear = 0;
     if (currentYear == 0) {
         UErrorCode status = U_ZERO_ERROR;
@@ -1302,7 +1302,7 @@ static UBool _aux2ExtremeDates(UDateFormat* fmt, UDate small, UDate large,
  *  0.75*10^30, etc.  A logarithmic search will find 10^15, then 10^7.5
  *  and 10^22.5, etc.
  */
-static void TestExtremeDates() {
+static void TestExtremeDates(void) {
     UDateFormat *fmt;
     UErrorCode ec;
     UChar buf[256];
diff --git a/icu4c/source/test/cintltst/cdetst.c b/icu4c/source/test/cintltst/cdetst.c
index 697633c96069..778fa297e66f 100644
--- a/icu4c/source/test/cintltst/cdetst.c
+++ b/icu4c/source/test/cintltst/cdetst.c
@@ -99,7 +99,7 @@ void addGermanCollTest(TestNode** root)
 
 }
 
-static void TestTertiary( )
+static void TestTertiary(void)
 {
     
     int32_t i;
@@ -119,7 +119,7 @@ static void TestTertiary( )
     ucol_close(myCollation);
 }
 
-static void TestPrimary()
+static void TestPrimary(void)
 {
     
     int32_t i;
diff --git a/icu4c/source/test/cintltst/cdtdptst.c b/icu4c/source/test/cintltst/cdtdptst.c
index 5bc0c152a9c5..174c469558a5 100644
--- a/icu4c/source/test/cintltst/cdtdptst.c
+++ b/icu4c/source/test/cintltst/cdtdptst.c
@@ -50,7 +50,7 @@ void addDtFrDepTest(TestNode** root)
 /**
  * Test the parsing of 2-digit years.
  */
-void TestTwoDigitYearDSTParse()
+void TestTwoDigitYearDSTParse(void)
 {
     UDateFormat *fullFmt, *fmt;
     UErrorCode status = U_ZERO_ERROR;
@@ -114,7 +114,7 @@ void TestTwoDigitYearDSTParse()
  * correctly.  In some instances, this means not being parsed at all, and
  * returning an appropriate error.
  */
-void TestPartialParse994()
+void TestPartialParse994(void)
 {
     int32_t pos;
     UDateFormat *f;
@@ -197,7 +197,7 @@ void tryPat994(UDateFormat* format, const char* pattern, const char* s, UDate ex
  * Verify the behavior of patterns in which digits for different fields run together
  * without intervening separators.
  */
-void TestRunTogetherPattern985()
+void TestRunTogetherPattern985(void)
 {
     int32_t pos;
     UChar *pattern=NULL, *now=NULL, *then=NULL;
@@ -231,7 +231,7 @@ void TestRunTogetherPattern985()
  * Verify the handling of Czech June and July, which have the unique attribute that
  * one is a proper prefix substring of the other.
  */
-void TestCzechMonths459()
+void TestCzechMonths459(void)
 {
     int32_t lneed, pos;
     UChar *pattern=NULL, *tzID=NULL;
@@ -305,7 +305,7 @@ void TestCzechMonths459()
 /**
  * Test the handling of single quotes in patterns.
  */
-void TestQuotePattern161()
+void TestQuotePattern161(void)
 {
     UDateFormat *format = NULL;
     UCalendar *cal = NULL;
diff --git a/icu4c/source/test/cintltst/cdtrgtst.c b/icu4c/source/test/cintltst/cdtrgtst.c
index 5ee21e294abb..54bdc7228fd1 100644
--- a/icu4c/source/test/cintltst/cdtrgtst.c
+++ b/icu4c/source/test/cintltst/cdtrgtst.c
@@ -52,7 +52,7 @@ void addDateForRgrTest(TestNode** root)
 /**
  * @bug 4029195
  */
-void Test4029195() 
+void Test4029195(void)
 {
     int32_t resultlength, resultlengthneeded;
     UChar  *fmdt, *todayS, *rt;
@@ -128,7 +128,7 @@ void Test4029195()
  * @bug 4056591
  * Verify the function of the [s|g]et2DigitYearStart() API.
  */
-void Test4056591() 
+void Test4056591(void)
 {
     int i;
     UCalendar *cal;
@@ -210,7 +210,7 @@ void Test4056591()
  * SimpleDateFormat does not properly parse date strings without delimiters
  * @bug 4059917
  */
-void Test4059917() 
+void Test4059917(void)
 {
     UDateFormat* def;
     UChar *myDate;
@@ -311,7 +311,7 @@ void aux917( UDateFormat *fmt, UChar* str)
 /**
  * @bug 4060212
  */
-void Test4060212() 
+void Test4060212(void)
 {
     int32_t pos;
     UCalendar *cal;
@@ -359,7 +359,7 @@ void Test4060212()
 /**
  * @bug 4061287
  */
-void Test4061287() 
+void Test4061287(void)
 {
     UBool ok;
     int32_t pos;
@@ -411,7 +411,7 @@ void Test4061287()
 /**
  * @bug 4073003
  */
-void Test4073003() 
+void Test4073003(void)
 {
     int32_t pos,i;
     UDate d,dd;
@@ -480,7 +480,7 @@ void Test4073003()
 /**
  * @bug 4162071
  **/
-void Test4162071() 
+void Test4162071(void)
 {
     int32_t pos;
     UDate x;
diff --git a/icu4c/source/test/cintltst/cestst.c b/icu4c/source/test/cintltst/cestst.c
index 228fe811ba67..38c432c2dd88 100644
--- a/icu4c/source/test/cintltst/cestst.c
+++ b/icu4c/source/test/cintltst/cestst.c
@@ -84,7 +84,7 @@ void addSpanishCollTest(TestNode** root)
 }
 
 
-static void TestTertiary( )
+static void TestTertiary(void)
 {
     
     int32_t i;
@@ -103,7 +103,7 @@ static void TestTertiary( )
     ucol_close(myCollation);
 }
 
-static void TestPrimary()
+static void TestPrimary(void)
 {
     
     int32_t i;
diff --git a/icu4c/source/test/cintltst/cfintst.c b/icu4c/source/test/cintltst/cfintst.c
index d81d90c6396b..8b8b35c62a1d 100644
--- a/icu4c/source/test/cintltst/cfintst.c
+++ b/icu4c/source/test/cintltst/cfintst.c
@@ -75,7 +75,7 @@ void addFinnishCollTest(TestNode** root)
 }
 
 
-static void TestTertiary( )
+static void TestTertiary(void)
 {
     
     int32_t i;
@@ -93,7 +93,7 @@ static void TestTertiary( )
     ucol_close(myCollation);
 }
 
-static void TestPrimary()
+static void TestPrimary(void)
 {
     
     int32_t i;
diff --git a/icu4c/source/test/cintltst/cfrtst.c b/icu4c/source/test/cintltst/cfrtst.c
index de356466e11b..a4e358620dbe 100644
--- a/icu4c/source/test/cintltst/cfrtst.c
+++ b/icu4c/source/test/cintltst/cfrtst.c
@@ -143,7 +143,7 @@ void addFrenchCollTest(TestNode** root)
 }
 
 
-static void TestTertiary( )
+static void TestTertiary(void)
 {
 
     int32_t i;
@@ -168,7 +168,7 @@ static void TestTertiary( )
     ucol_close(myCollation);
 }
 
-static void TestSecondary()
+static void TestSecondary(void)
 {
     int32_t i,j, testAcuteSize;
     UCollationResult expected=UCOL_EQUAL;
@@ -199,7 +199,7 @@ static void TestSecondary()
     ucol_close(myCollation);
 }
 
-static void TestExtra()
+static void TestExtra(void)
 {
     int32_t i, j;
     UErrorCode status = U_ZERO_ERROR;
@@ -220,7 +220,7 @@ static void TestExtra()
     ucol_close(myCollation);
 }
 
-static void TestGetSortKey() {
+static void TestGetSortKey(void) {
     /* This is meant to test a buffer reallocation crash while using
     French secondary sorting with a large buffer.
     The fact that Japanese characters are used is irrelevant. */
diff --git a/icu4c/source/test/cintltst/cg7coll.c b/icu4c/source/test/cintltst/cg7coll.c
index e857afbdd0a0..1bc827ede7fe 100644
--- a/icu4c/source/test/cintltst/cg7coll.c
+++ b/icu4c/source/test/cintltst/cg7coll.c
@@ -135,7 +135,7 @@ void addRuleBasedCollTest(TestNode** root)
     
 }
 
-static void TestG7Locales()
+static void TestG7Locales(void)
 {
     UCollator *myCollation;
     UErrorCode status = U_ZERO_ERROR;
@@ -195,7 +195,7 @@ static void TestG7Locales()
     }
 }
 
-static void TestDemo1()
+static void TestDemo1(void)
 {
     UCollator *myCollation;
     int32_t j, n;
@@ -226,7 +226,7 @@ static void TestDemo1()
     ucol_close(myCollation); 
 }
 
-static void TestDemo2()
+static void TestDemo2(void)
 {
     UCollator *myCollation;
     int32_t j, n;
@@ -255,7 +255,7 @@ static void TestDemo2()
     ucol_close(myCollation); 
 }
 
-static void TestDemo3()
+static void TestDemo3(void)
 {
     UCollator *myCollation;
     int32_t j, n;
@@ -285,7 +285,7 @@ static void TestDemo3()
     ucol_close(myCollation); 
 }
 
-static void TestDemo4()
+static void TestDemo4(void)
 {
     UCollator *myCollation;
     int32_t j, n;
diff --git a/icu4c/source/test/cintltst/chashtst.c b/icu4c/source/test/cintltst/chashtst.c
index 2ea34939b28d..a4b774d93a1c 100644
--- a/icu4c/source/test/cintltst/chashtst.c
+++ b/icu4c/source/test/cintltst/chashtst.c
@@ -155,7 +155,7 @@ static void TestBasic(void) {
     uhash_close(hash);
 }
 
-static void TestAllowZero() {
+static void TestAllowZero(void) {
     UErrorCode status = U_ZERO_ERROR;
     UHashtable *hash = uhash_open(hashChars, isEqualChars, NULL,  &status);
     if (U_FAILURE(status)) {
diff --git a/icu4c/source/test/cintltst/cintltst.c b/icu4c/source/test/cintltst/cintltst.c
index 7afdb60af5fb..4c6d5fc6dec7 100644
--- a/icu4c/source/test/cintltst/cintltst.c
+++ b/icu4c/source/test/cintltst/cintltst.c
@@ -295,7 +295,7 @@ static void ctest_appendToDataDirectory(const char *toAppend)
 */
 
 /* returns the path to icu/source/data */
-const char *  ctest_dataSrcDir()
+const char *  ctest_dataSrcDir(void)
 {
     static const char *dataSrcDir = NULL;
 
@@ -362,7 +362,7 @@ const char *  ctest_dataSrcDir()
 }
 
 /* returns the path to icu/source/data/out */
-const char *ctest_dataOutDir()
+const char *ctest_dataOutDir(void)
 {
     static const char *dataOutDir = NULL;
 
@@ -439,7 +439,7 @@ const char *ctest_dataOutDir()
  *                       picked up via a static (build time) reference, but the
  *                       tests dynamically load some data.
  */
-void ctest_setICU_DATA() {
+void ctest_setICU_DATA(void) {
 
     /* No location for the data dir was identifiable.
      *   Add other fallbacks for the test data location here if the need arises
@@ -454,7 +454,7 @@ void ctest_setICU_DATA() {
  *    The ICU data directory must be preserved across these operations.
  *    Here is a helper function to assist with that.
  */
-static char *safeGetICUDataDirectory() {
+static char *safeGetICUDataDirectory(void) {
     const char *dataDir = u_getDataDirectory();  /* Returned string vanashes with u_cleanup */
     char *retStr = NULL;
     if (dataDir != NULL) {
@@ -464,7 +464,7 @@ static char *safeGetICUDataDirectory() {
     return retStr;
 }
 
-UBool ctest_resetICU() {
+UBool ctest_resetICU(void) {
     UErrorCode   status = U_ZERO_ERROR;
     char         *dataDir = safeGetICUDataDirectory();
 
@@ -675,7 +675,7 @@ void *ctst_malloc(size_t size) {
 }
 
 #ifdef CTST_LEAK_CHECK
-static void ctst_freeAll() {
+static void ctst_freeAll(void) {
     int i;
     if(ctst_free == false) { /* only free up to the allocated mark */
         for(i=0; i<ctst_allocated; i++) {
diff --git a/icu4c/source/test/cintltst/citertst.c b/icu4c/source/test/cintltst/citertst.c
index 4c50f32e3bc8..0a44d450a17f 100644
--- a/icu4c/source/test/cintltst/citertst.c
+++ b/icu4c/source/test/cintltst/citertst.c
@@ -65,7 +65,7 @@ void addCollIterTest(TestNode** root)
 
 static const char * LOCALES[] = {"en_AU", "en_BE", "en_CA"};
 
-static void TestBug672() {
+static void TestBug672(void) {
     UErrorCode  status = U_ZERO_ERROR;
     UChar       pattern[20];
     UChar       text[50];
@@ -138,7 +138,7 @@ static void TestBug672() {
 
 /*  Running this test with normalization enabled showed up a bug in the incremental
     normalization code. */
-static void TestBug672Normalize() {
+static void TestBug672Normalize(void) {
     UErrorCode  status = U_ZERO_ERROR;
     UChar       pattern[20];
     UChar       text[50];
@@ -217,7 +217,7 @@ static void TestBug672Normalize() {
  * Test for CollationElementIterator previous and next for the whole set of
  * unicode characters.
  */
-static void TestUnicodeChar()
+static void TestUnicodeChar(void)
 {
     UChar source[0x100];
     UCollator *en_us;
@@ -283,7 +283,7 @@ static void TestUnicodeChar()
  * Test for CollationElementIterator previous and next for the whole set of
  * unicode characters with normalization on.
  */
-static void TestNormalizedUnicodeChar()
+static void TestNormalizedUnicodeChar(void)
 {
     UChar source[0x100];
     UCollator *th_th;
@@ -347,7 +347,7 @@ static void TestNormalizedUnicodeChar()
 /**
 * Test the incremental normalization
 */
-static void TestNormalization()
+static void TestNormalization(void)
 {
           UErrorCode          status = U_ZERO_ERROR;
     const char               *str    =
@@ -418,7 +418,7 @@ static void TestNormalization()
  * @bug 4108758 - Make sure it works with contracting characters
  *
  */
-static void TestPrevious()
+static void TestPrevious(void)
 {
     UCollator *coll=NULL;
     UChar rule[50];
@@ -558,7 +558,7 @@ static void TestPrevious()
 /**
  * Test for getOffset() and setOffset()
  */
-static void TestOffset()
+static void TestOffset(void)
 {
     UErrorCode status= U_ZERO_ERROR;
     UCollator *en_us=NULL;
@@ -703,7 +703,7 @@ static void TestOffset()
 /**
  * Test for setText()
  */
-static void TestSetText()
+static void TestSetText(void)
 {
     int32_t c,i;
     UErrorCode status = U_ZERO_ERROR;
@@ -776,7 +776,7 @@ static void TestSetText()
 /** @bug 4108762
  * Test for getMaxExpansion()
  */
-static void TestMaxExpansion()
+static void TestMaxExpansion(void)
 {
     UErrorCode          status = U_ZERO_ERROR;
     UCollator          *coll   ;/*= ucol_open("en_US", &status);*/
@@ -934,7 +934,7 @@ static void assertEqual(UCollationElements *i1, UCollationElements *i2)
 /**
  * Testing iterators with extremely small buffers
  */
-static void TestSmallBuffer()
+static void TestSmallBuffer(void)
 {
     UErrorCode          status = U_ZERO_ERROR;
     UCollator          *coll;
@@ -1007,7 +1007,7 @@ static void TestSmallBuffer()
 /**
 * Testing the discontiguous contractions
 */
-static void TestDiscontiguos() {
+static void TestDiscontiguos(void) {
     const char               *rulestr    =
                             "&z < AB < X\\u0300 < ABC < X\\u0300\\u0315";
           UChar               rule[50];
diff --git a/icu4c/source/test/cintltst/cjaptst.c b/icu4c/source/test/cintltst/cjaptst.c
index c16ae8f06ca1..4f1e110cbb8e 100644
--- a/icu4c/source/test/cintltst/cjaptst.c
+++ b/icu4c/source/test/cintltst/cjaptst.c
@@ -115,7 +115,7 @@ void addKannaCollTest(TestNode** root)
     addTest(root, &TestChooonKigoo, "tscoll/cjacoll/TestChooonKigoo");
 }
 
-static void TestTertiary( )
+static void TestTertiary(void)
 {
     int32_t i;
     UErrorCode status = U_ZERO_ERROR;
@@ -135,7 +135,7 @@ static void TestTertiary( )
 }
 
 /* Testing base letters */
-static void TestBase()
+static void TestBase(void)
 {
     int32_t i;
     UErrorCode status = U_ZERO_ERROR;
diff --git a/icu4c/source/test/cintltst/cloctst.c b/icu4c/source/test/cintltst/cloctst.c
index d4015df96fd4..1f0a84b866c9 100644
--- a/icu4c/source/test/cintltst/cloctst.c
+++ b/icu4c/source/test/cintltst/cloctst.c
@@ -64,7 +64,7 @@ static void TestExcessivelyLongIDs(void);
 static void TestUldnNameVariants(void);
 #endif
 
-void PrintDataTable();
+void PrintDataTable(void);
 
 /*---------------------------------------------------
   table of valid data
@@ -295,7 +295,7 @@ void addLocaleTest(TestNode** root)
 
 
 /* testing uloc(), uloc_getName(), uloc_getLanguage(), uloc_getVariant(), uloc_getCountry() */
-static void TestBasicGetters() {
+static void TestBasicGetters(void) {
     int32_t i;
     int32_t cap;
     UErrorCode status = U_ZERO_ERROR;
@@ -370,7 +370,7 @@ static void TestBasicGetters() {
     }
 }
 
-static void TestNullDefault() {
+static void TestNullDefault(void) {
     UErrorCode status = U_ZERO_ERROR;
     char original[ULOC_FULLNAME_CAPACITY];
 
@@ -434,7 +434,7 @@ static void TestNullDefault() {
 
 #define PREFIXBUFSIZ 128
 
-static void TestPrefixes() {
+static void TestPrefixes(void) {
     int row = 0;
     int n;
     const char *loc, *expected;
@@ -570,7 +570,7 @@ static void TestPrefixes() {
 
 
 /* testing uloc_getISO3Language(), uloc_getISO3Country(),  */
-static void TestSimpleResourceInfo() {
+static void TestSimpleResourceInfo(void) {
     int32_t i;
     char* testLocale = 0;
     UChar* expected = 0;
@@ -696,7 +696,7 @@ static int32_t UCharsToEscapedAscii(const UChar* utext, int32_t len, char* resul
  * The lookup of display names must not fall back through the default
  * locale because that yields useless results.
  */
-static void TestDisplayNames()
+static void TestDisplayNames(void)
 {
     UChar buffer[100];
     UErrorCode errorCode=U_ZERO_ERROR;
@@ -846,7 +846,7 @@ static void TestDisplayNames()
  * ICU-21160 test the pre-flighting call to uloc_getDisplayScript returns the actual length needed
  * for the result buffer.
  */
-static void TestGetDisplayScriptPreFlighting21160()
+static void TestGetDisplayScriptPreFlighting21160(void)
 {
     const char* locale = "und-Latn";
     const char* inlocale = "de";
@@ -864,7 +864,7 @@ static void TestGetDisplayScriptPreFlighting21160()
 }
 
 /* test for uloc_getAvailable()  and uloc_countAvailable()*/
-static void TestGetAvailableLocales()
+static void TestGetAvailableLocales(void)
 {
 
     const char *locList;
@@ -889,7 +889,7 @@ static void TestGetAvailableLocales()
     }
 }
 
-static void TestGetAvailableLocalesByType() {
+static void TestGetAvailableLocalesByType(void) {
     UErrorCode status = U_ZERO_ERROR;
 
     UEnumeration* uenum = uloc_openAvailableByType(ULOC_AVAILABLE_DEFAULT, &status);
@@ -961,7 +961,7 @@ static void TestGetAvailableLocalesByType() {
 }
 
 /* test for u_getDataDirectory, u_setDataDirectory, uloc_getISO3Language */
-static void TestDataDirectory()
+static void TestDataDirectory(void)
 {
 
     char            oldDirectory[512];
@@ -1197,7 +1197,7 @@ static const DisplayNameBracketsItem displayNameBracketsItems[] = {
 
 enum { kDisplayNameBracketsMax = 128 };
 
-static void TestDisplayNameBrackets()
+static void TestDisplayNameBrackets(void)
 {
     const DisplayNameBracketsItem * itemPtr = displayNameBracketsItems;
     for (; itemPtr->displayLocale != NULL; itemPtr++) {
@@ -1252,7 +1252,7 @@ static void TestDisplayNameBrackets()
  * TestIllegalArgumentWhenNoDataWithNoSubstitute
  */
 
-static void TestIllegalArgumentWhenNoDataWithNoSubstitute()
+static void TestIllegalArgumentWhenNoDataWithNoSubstitute(void)
 {
 #if !UCONFIG_NO_FORMATTING
     UErrorCode status = U_ZERO_ERROR;
@@ -1320,7 +1320,7 @@ static void TestIllegalArgumentWhenNoDataWithNoSubstitute()
 
 #if !UCONFIG_NO_FILE_IO && !UCONFIG_NO_LEGACY_CONVERSION
 /* test for uloc_getISOLanguages, uloc_getISOCountries */
-static void TestISOFunctions()
+static void TestISOFunctions(void)
 {
     const char* const* str=uloc_getISOLanguages();
     const char* const* str1=uloc_getISOCountries();
@@ -1452,7 +1452,7 @@ static void TestISOFunctions()
 }
 #endif
 
-static void setUpDataTable()
+static void setUpDataTable(void)
 {
     int32_t i,j;
     dataTable = (UChar***)(calloc(sizeof(UChar**),LOCALE_INFO_SIZE));
@@ -1465,7 +1465,7 @@ static void setUpDataTable()
     }
 }
 
-static void cleanUpDataTable()
+static void cleanUpDataTable(void)
 {
     int32_t i,j;
     if(dataTable != NULL) {
@@ -1483,7 +1483,7 @@ static void cleanUpDataTable()
 /**
  * @bug 4011756 4011380
  */
-static void TestISO3Fallback()
+static void TestISO3Fallback(void)
 {
     const char* test="xx_YY";
 
@@ -1505,7 +1505,7 @@ static void TestISO3Fallback()
 /**
  * @bug 4118587
  */
-static void TestSimpleDisplayNames()
+static void TestSimpleDisplayNames(void)
 {
   /*
      This test is different from TestDisplayNames because TestDisplayNames checks
@@ -1550,7 +1550,7 @@ static void TestSimpleDisplayNames()
 /**
  * @bug 4118595
  */
-static void TestUninstalledISO3Names()
+static void TestUninstalledISO3Names(void)
 {
   /* This test checks to make sure getISO3Language and getISO3Country work right
      even for locales that are not installed (and some installed ones). */
@@ -1583,7 +1583,7 @@ static void TestUninstalledISO3Names()
 }
 
 
-static void TestVariantParsing()
+static void TestVariantParsing(void)
 {
     static const char* en_US_custom="en_US_De Anza_Cupertino_California_United States_Earth";
     static const char* dispName="English (United States, DE ANZA_CUPERTINO_CALIFORNIA_UNITED STATES_EARTH)";
@@ -3250,7 +3250,7 @@ static UBool isLocaleAvailable(UResourceBundle* resIndex, const char* loc){
     return true;
 }
 
-static void TestCalendar() {
+static void TestCalendar(void) {
 #if !UCONFIG_NO_FORMATTING
     int i;
     UErrorCode status = U_ZERO_ERROR;
@@ -3286,7 +3286,7 @@ static void TestCalendar() {
 #endif
 }
 
-static void TestDateFormat() {
+static void TestDateFormat(void) {
 #if !UCONFIG_NO_FORMATTING
     int i;
     UErrorCode status = U_ZERO_ERROR;
@@ -3329,7 +3329,7 @@ static void TestDateFormat() {
 #endif
 }
 
-static void TestCollation() {
+static void TestCollation(void) {
 #if !UCONFIG_NO_COLLATION
     int i;
     UErrorCode status = U_ZERO_ERROR;
@@ -3401,7 +3401,7 @@ static const char* ULayoutTypeToString(ULayoutType type)
     return "Unknown enum value for ULayoutType!";
 }
 
-static void  TestOrientation()
+static void  TestOrientation(void)
 {
     static const OrientationStruct toTest [] = {
         { "ar", ULOC_LAYOUT_RTL, ULOC_LAYOUT_TTB },
@@ -3454,7 +3454,7 @@ static void  TestOrientation()
     }
 }
 
-static void  TestULocale() {
+static void  TestULocale(void) {
     int i;
     UErrorCode status = U_ZERO_ERROR;
     UResourceBundle *resIndex = ures_open(NULL,"res_index", &status);
@@ -3494,7 +3494,7 @@ static void  TestULocale() {
 
 }
 
-static void TestUResourceBundle() {
+static void TestUResourceBundle(void) {
     const char* us1;
     const char* us2;
 
@@ -3542,7 +3542,7 @@ static void TestUResourceBundle() {
     ures_close(resIndex);
 }
 
-static void TestDisplayName() {
+static void TestDisplayName(void) {
     
     UChar oldCountry[256] = {'\0'};
     UChar newCountry[256] = {'\0'};
@@ -3584,7 +3584,7 @@ static void TestDisplayName() {
     }
 }
 
-static void TestGetLocaleForLCID() {
+static void TestGetLocaleForLCID(void) {
     int32_t i, length, lengthPre;
     const char* testLocale = 0;
     UErrorCode status = U_ZERO_ERROR;
@@ -6030,7 +6030,7 @@ static int32_t getBufferSize(const errorData* data, int32_t actualSize)
     }
 }
 
-static void TestLikelySubtags()
+static void TestLikelySubtags(void)
 {
     char buffer[ULOC_FULLNAME_CAPACITY + ULOC_KEYWORD_AND_VALUES_CAPACITY + 1];
     int32_t i = 0;
@@ -6929,7 +6929,7 @@ static void TestUnicodeDefines(void) {
   TEST_UNICODE_DEFINE(ULOC_KEYWORD_ITEM_SEPARATOR, ULOC_KEYWORD_ITEM_SEPARATOR_UNICODE);
 }
 
-static void TestIsRightToLeft() {
+static void TestIsRightToLeft(void) {
     // API test only. More test cases in intltest/LocaleTest.
     if(uloc_isRightToLeft("root") || !uloc_isRightToLeft("EN-HEBR")) {
         log_err("uloc_isRightToLeft() failed");
@@ -6955,7 +6955,7 @@ static const BadLocaleItem badLocaleItems[] = {
 
 enum { kUBufDispNameMax = 128, kBBufDispNameMax = 256 };
 
-static void TestBadLocaleIDs() {
+static void TestBadLocaleIDs(void) {
     const BadLocaleItem* itemPtr;
     for (itemPtr = badLocaleItems; itemPtr->badLocaleID != NULL; itemPtr++) {
         UChar ubufExpect[kUBufDispNameMax], ubufGet[kUBufDispNameMax];
@@ -6979,7 +6979,7 @@ static void TestBadLocaleIDs() {
 
 // Test case for ICU-20370.
 // The issue shows as an Address Sanitizer failure.
-static void TestBug20370() {
+static void TestBug20370(void) {
     const char *localeID = "x-privatebutreallylongtagfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobarfoobar";
     uint32_t lcid = uloc_getLCID(localeID);
     if (lcid != 0) {
@@ -6989,7 +6989,7 @@ static void TestBug20370() {
 
 // Test case for ICU-20149
 // Handle the duplicate U extension attribute
-static void TestBug20149() {
+static void TestBug20149(void) {
     const char *localeID = "zh-u-foo-foo-co-pinyin";
     char locale[256];
     UErrorCode status = U_ZERO_ERROR;
@@ -7144,7 +7144,7 @@ static const UldnLocAndOpts uldnLocAndOpts[] = {
 
 enum { kUNameBuf = 128, kBNameBuf = 256 };
 
-static void TestUldnNameVariants() {
+static void TestUldnNameVariants(void) {
     const UldnLocAndOpts * uloPtr;
     for (uloPtr = uldnLocAndOpts; uloPtr->displayLocale != NULL; uloPtr++) {
         UErrorCode status = U_ZERO_ERROR;
@@ -7228,7 +7228,7 @@ static void TestUldnNameVariants() {
 }
 #endif
 
-static void TestUsingDefaultWarning() {
+static void TestUsingDefaultWarning(void) {
     UChar buff[256];
     char errorOutputBuff[256];
     UErrorCode status = U_ZERO_ERROR;
@@ -7301,7 +7301,7 @@ static void TestUsingDefaultWarning() {
 // Test case for ICU-20575
 // This test checks if the environment variable LANG is set, 
 // and if so ensures that both C and C.UTF-8 cause ICU's default locale to be en_US_POSIX.
-static void TestCDefaultLocale() {
+static void TestCDefaultLocale(void) {
     const char *defaultLocale = uloc_getDefault();
     char *env_var = getenv("LANG");
     if (env_var == NULL) {
@@ -7318,7 +7318,7 @@ static void TestCDefaultLocale() {
 }
 
 // Test case for ICU-21449
-static void TestBug21449InfiniteLoop() {
+static void TestBug21449InfiniteLoop(void) {
     UErrorCode status = U_ZERO_ERROR;
     const char* invalidLocaleId = RES_PATH_SEPARATOR_S;
 
diff --git a/icu4c/source/test/cintltst/cnormtst.c b/icu4c/source/test/cintltst/cnormtst.c
index 24946e2eb628..6f933ac3d28d 100644
--- a/icu4c/source/test/cintltst/cnormtst.c
+++ b/icu4c/source/test/cintltst/cnormtst.c
@@ -212,23 +212,23 @@ static void TestNormCases(UNormalizationMode mode,
     }
 }
 
-void TestDecomp() {
+void TestDecomp(void) {
     TestNormCases(UNORM_NFD, canonTests, UPRV_LENGTHOF(canonTests));
 }
 
-void TestCompatDecomp() {
+void TestCompatDecomp(void) {
     TestNormCases(UNORM_NFKD, compatTests, UPRV_LENGTHOF(compatTests));
 }
 
-void TestCanonDecompCompose() {
+void TestCanonDecompCompose(void) {
     TestNormCases(UNORM_NFC, canonTests, UPRV_LENGTHOF(canonTests));
 }
 
-void TestCompatDecompCompose() {
+void TestCompatDecompCompose(void) {
     TestNormCases(UNORM_NFKC, compatTests, UPRV_LENGTHOF(compatTests));
 }
 
-void TestFCD() {
+void TestFCD(void) {
     TestNormCases(UNORM_FCD, fcdTests, UPRV_LENGTHOF(fcdTests));
 }
 
@@ -285,7 +285,7 @@ static void TestNull_check(UChar *src, int32_t srcLen,
     log_verbose("unorm_normalize(%s) with 0x0000: OK\n", name);
 }
 
-void TestNull() 
+void TestNull(void)
 {
 
     UChar   source_comp[] = { 0x0061, 0x0000, 0x0044, 0x0307 };
@@ -322,7 +322,7 @@ void TestNull()
 
 }
 
-static void TestQuickCheckResultNO() 
+static void TestQuickCheckResultNO(void)
 {
   const UChar CPNFD[] = {0x00C5, 0x0407, 0x1E00, 0x1F57, 0x220C, 
                          0x30AE, 0xAC00, 0xD7A3, 0xFB36, 0xFB4E};
@@ -369,7 +369,7 @@ static void TestQuickCheckResultNO()
 }
 
  
-static void TestQuickCheckResultYES() 
+static void TestQuickCheckResultYES(void)
 {
   const UChar CPNFD[] = {0x00C6, 0x017F, 0x0F74, 0x1000, 0x1E9A, 
                          0x2261, 0x3075, 0x4000, 0x5000, 0xF000};
@@ -441,7 +441,7 @@ static void TestQuickCheckResultYES()
   }
 }
 
-static void TestQuickCheckResultMAYBE() 
+static void TestQuickCheckResultMAYBE(void)
 {
   const UChar CPNFC[] = {0x0306, 0x0654, 0x0BBE, 0x102E, 0x1161, 
                          0x116A, 0x1173, 0x1175, 0x3099, 0x309A};
@@ -472,7 +472,7 @@ static void TestQuickCheckResultMAYBE()
   }
 }
 
-static void TestQuickCheckStringResult() 
+static void TestQuickCheckStringResult(void)
 {
   int count;
   UChar *d = NULL;
@@ -528,7 +528,7 @@ static void TestQuickCheckStringResult()
   }  
 }
 
-void TestQuickCheck() 
+void TestQuickCheck(void)
 {
   TestQuickCheckResultNO();
   TestQuickCheckResultYES();
@@ -605,7 +605,7 @@ static void TestIsNormalized(void) {
     }
 }
 
-void TestCheckFCD() 
+void TestCheckFCD(void)
 {
   UErrorCode status = U_ZERO_ERROR;
   static const UChar FAST_[] = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 
@@ -711,7 +711,7 @@ void TestCheckFCD()
 }
 
 static void
-TestAPI() {
+TestAPI(void) {
     static const UChar in[]={ 0x68, 0xe4 };
     UChar out[20]={ 0xffff, 0xffff, 0xffff, 0xffff };
     UErrorCode errorCode;
@@ -767,7 +767,7 @@ enum {
 };
 
 static void
-TestNormCoverage() {
+TestNormCoverage(void) {
     UChar input[1000], expect[1000], output[1000];
     UErrorCode errorCode;
     int32_t i, length, inLength, expectLength, hangulPrefixLength, preflightLength;
@@ -1148,7 +1148,7 @@ _testIter(const UChar *src, int32_t srcLength,
 }
 
 static void
-TestNextPrevious() {
+TestNextPrevious(void) {
     static const UChar
     src[]={ /* input string */
         0xa0, 0xe4, 0x63, 0x302, 0x327, 0xac00, 0x3133
@@ -1363,7 +1363,7 @@ TestFCNFKCClosure(void) {
 }
 
 static void
-TestQuickCheckPerCP() {
+TestQuickCheckPerCP(void) {
     UErrorCode errorCode;
     UChar32 c, lead, trail;
     UChar s[U16_MAX_LENGTH], nfd[16];
@@ -1487,7 +1487,7 @@ TestComposition(void) {
 }
 
 static void
-TestGetDecomposition() {
+TestGetDecomposition(void) {
     UChar decomp[32];
     int32_t length;
 
@@ -1530,7 +1530,7 @@ TestGetDecomposition() {
 }
 
 static void
-TestGetRawDecomposition() {
+TestGetRawDecomposition(void) {
     UChar decomp[32];
     int32_t length;
 
@@ -1595,7 +1595,7 @@ TestGetRawDecomposition() {
 }
 
 static void
-TestAppendRestoreMiddle() {
+TestAppendRestoreMiddle(void) {
     UChar a[20]={ 0x61, 0x62, 0x63, 0x41, 0x327, 0 };  /* last chars are 'A' and 'cedilla' NFC */
     static const UChar b[]={ 0x30A, 0x64, 0x65, 0x66, 0 };  /* first char is 'ring above' NFC */
     /* NFC: C5 is 'A with ring above' */
@@ -1632,7 +1632,7 @@ TestAppendRestoreMiddle() {
 }
 
 static void
-TestGetEasyToUseInstance() {
+TestGetEasyToUseInstance(void) {
     static const UChar in[]={
         0xA0,  /* -> <noBreak> 0020 */
         0xC7, 0x301  /* = 1E08 = 0043 0327 0301 */
@@ -1716,7 +1716,7 @@ TestGetEasyToUseInstance() {
 }
 
 static void
-TestAPICoverage() {
+TestAPICoverage(void) {
     UErrorCode errorCode = U_ZERO_ERROR;
     const UNormalizer2 *n2 = unorm2_getNFDInstance(&errorCode);
     if (U_FAILURE(errorCode)) {
diff --git a/icu4c/source/test/cintltst/cnumtst.c b/icu4c/source/test/cintltst/cnumtst.c
index 17bfbde3c40f..63bb8c2dc7e4 100644
--- a/icu4c/source/test/cintltst/cnumtst.c
+++ b/icu4c/source/test/cintltst/cnumtst.c
@@ -131,7 +131,7 @@ void addNumForTest(TestNode** root)
 
 /* test Parse int 64 */
 
-static void TestInt64Parse()
+static void TestInt64Parse(void)
 {
 
     UErrorCode st = U_ZERO_ERROR;
@@ -175,7 +175,7 @@ static void TestInt64Parse()
 }
 
 /* test Number Format API */
-static void TestNumberFormat()
+static void TestNumberFormat(void)
 {
     UChar *result=NULL;
     UChar temp1[512];
@@ -1097,7 +1097,7 @@ static const ParseCurrencyItem parseCurrencyItems[] = {
     { NULL,    NULL,       NULL,        NULL,           0,             0, 0.0, 0,             0, 0.0, NULL  }
 };
 
-static void TestParseCurrency()
+static void TestParseCurrency(void)
 {
     const ParseCurrencyItem * itemPtr;
     for (itemPtr = parseCurrencyItems; itemPtr->locale != NULL; ++itemPtr) {
@@ -1208,7 +1208,7 @@ static const SpelloutParseTest spelloutParseTests[] = {
     { NULL,    NULL, NULL,        0,   0,  0, 0 } /* terminator */
 };
 
-static void TestSpelloutNumberParse()
+static void TestSpelloutNumberParse(void)
 {
     const SpelloutParseTest * testPtr;
     for (testPtr = spelloutParseTests; testPtr->testname != NULL; ++testPtr) {
@@ -1231,7 +1231,7 @@ static void TestSpelloutNumberParse()
     }
 }
 
-static void TestSignificantDigits()
+static void TestSignificantDigits(void)
 {
     UChar temp[128];
     int32_t resultlengthneeded;
@@ -1273,7 +1273,7 @@ static void TestSignificantDigits()
     unum_close(fmt);
 }
 
-static void TestSigDigRounding()
+static void TestSigDigRounding(void)
 {
     UErrorCode status = U_ZERO_ERROR;
     UChar expected[128];
@@ -1310,7 +1310,7 @@ static void TestSigDigRounding()
     unum_close(fmt);
 }
 
-static void TestNumberFormatPadding()
+static void TestNumberFormatPadding(void)
 {
     UChar *result=NULL;
     UChar temp1[512];
@@ -1433,7 +1433,7 @@ withinErr(double a, double b, double err) {
     return uprv_fabs(a - b) < uprv_fabs(a * err);
 }
 
-static void TestInt64Format() {
+static void TestInt64Format(void) {
     UChar temp1[512];
     UChar result[512];
     UNumberFormat *fmt;
@@ -1706,7 +1706,7 @@ static void test_fmt(UNumberFormat* fmt, UBool isDecimal) {
     }
 }
 
-static void TestNonExistentCurrency() {
+static void TestNonExistentCurrency(void) {
     UNumberFormat *format;
     UErrorCode status = U_ZERO_ERROR;
     UChar currencySymbol[8];
@@ -1730,7 +1730,7 @@ static void TestNonExistentCurrency() {
     unum_close(format);
 }
 
-static void TestRBNFFormat() {
+static void TestRBNFFormat(void) {
     UErrorCode status;
     UParseError perr;
     UChar pat[1024];
@@ -1840,7 +1840,7 @@ static void TestRBNFFormat() {
     }
 }
 
-static void TestRBNFRounding() {
+static void TestRBNFRounding(void) {
     UChar fmtbuf[FORMAT_BUF_CAPACITY];
     UChar expectedBuf[FORMAT_BUF_CAPACITY];
     int32_t len;
@@ -3035,7 +3035,7 @@ static void TestVariousStylesAndAttributes(void) {
 static const UChar currpat[]  = { 0xA4,0x23,0x2C,0x23,0x23,0x30,0x2E,0x30,0x30,0};
 static const UChar parsetxt[] = { 0x78,0x30,0x79,0x24,0 }; /* x0y$ */
 
-static void TestParseCurrPatternWithDecStyle() {
+static void TestParseCurrPatternWithDecStyle(void) {
     UErrorCode status = U_ZERO_ERROR;
     UNumberFormat *unumfmt = unum_open(UNUM_DECIMAL, NULL, 0, "en_US", NULL, &status);
     if (U_FAILURE(status)) {
@@ -3162,7 +3162,7 @@ static void TestFormatForFields(void) {
     }
 }
 
-static void Test12052_NullPointer() {
+static void Test12052_NullPointer(void) {
     UErrorCode status = U_ZERO_ERROR;
     static const UChar input[] = u"199a";
     UChar currency[200] = {0};
diff --git a/icu4c/source/test/cintltst/cpluralrulestest.c b/icu4c/source/test/cintltst/cpluralrulestest.c
index e2b4e3629784..6ce6cd1c42a8 100644
--- a/icu4c/source/test/cintltst/cpluralrulestest.c
+++ b/icu4c/source/test/cintltst/cpluralrulestest.c
@@ -76,7 +76,7 @@ enum {
     kKeywordBufLen = 32
 };
 
-static void TestPluralRules()
+static void TestPluralRules(void)
 {
     const PluralRulesTestItem * testItemPtr;
     log_verbose("\nTesting uplrules_open() and uplrules_select() with various parameters\n");
@@ -133,7 +133,7 @@ static void TestPluralRules()
     }
 }
 
-static void TestOrdinalRules() {
+static void TestOrdinalRules(void) {
     U_STRING_DECL(two, "two", 3);
     UChar keyword[8];
     int32_t length;
@@ -195,7 +195,7 @@ static const KeywordsForLang getKeywordsItems[] = {
     { NULL, { NULL } }
 };
 
-static void TestGetKeywords() {
+static void TestGetKeywords(void) {
     /*
      * We don't know the order in which the enumeration will return keywords,
      * so we have an array with known keywords in a fixed order and then
@@ -260,7 +260,7 @@ static void TestGetKeywords() {
     }
 }
 
-static void TestFormatted() {
+static void TestFormatted(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UNumberFormatter* unumf = NULL;
     UFormattedNumber* uresult = NULL;
@@ -300,7 +300,7 @@ static void TestFormatted() {
     unumf_closeResult(uresult);
 }
 
-static void TestSelectRange() {
+static void TestSelectRange(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UNumberRangeFormatter* unumrf = NULL;
     UFormattedNumberRange* uresult = NULL;
diff --git a/icu4c/source/test/cintltst/crelativedateformattest.c b/icu4c/source/test/cintltst/crelativedateformattest.c
index 55643b92ff66..bb75f1814789 100644
--- a/icu4c/source/test/cintltst/crelativedateformattest.c
+++ b/icu4c/source/test/cintltst/crelativedateformattest.c
@@ -348,7 +348,7 @@ static const RelDateTimeFormatTestItem fmtTestItems[] = {
 
 enum { kUBufMax = 64, kBBufMax = 256 };
 
-static void TestRelDateFmt()
+static void TestRelDateFmt(void)
 {
     const RelDateTimeFormatTestItem *itemPtr;
     log_verbose("\nTesting ureldatefmt_open(), ureldatefmt_format(), ureldatefmt_formatNumeric() with various parameters\n");
@@ -429,7 +429,7 @@ static void TestRelDateFmt()
     }
 }
 
-static void TestNumericField()
+static void TestNumericField(void)
 {
     const RelDateTimeFormatTestItem *itemPtr;
     log_verbose("\nTesting ureldatefmt_open(), ureldatefmt_formatForFields(), ureldatefmt_formatNumericForFields() with various parameters\n");
@@ -591,7 +591,7 @@ static const CombineDateTimeTestItem combTestItems[] = {
     { NULL,  (UDateRelativeDateTimeFormatterStyle)0, (UDisplayContext)0, NULL, NULL, NULL } /* terminator */
 };
 
-static void TestCombineDateTime()
+static void TestCombineDateTime(void)
 {
     const CombineDateTimeTestItem *itemPtr;
     log_verbose("\nTesting ureldatefmt_combineDateAndTime() with various parameters\n");
@@ -645,7 +645,7 @@ static void TestCombineDateTime()
     }
 }
 
-static void TestFields() {
+static void TestFields(void) {
     UErrorCode ec = U_ZERO_ERROR;
     URelativeDateTimeFormatter* fmt = ureldatefmt_open(
         "en-us",
diff --git a/icu4c/source/test/cintltst/crestst.c b/icu4c/source/test/cintltst/crestst.c
index 2ccd2fc98eb6..7d0476462e66 100644
--- a/icu4c/source/test/cintltst/crestst.c
+++ b/icu4c/source/test/cintltst/crestst.c
@@ -150,7 +150,7 @@ void TestAliasConflict(void) {
 }
 
 
-void TestResourceBundles()
+void TestResourceBundles(void)
 {
     // The test expectation only works if the default locale is not one of the
     // locale bundle in the testdata which have those info. Therefore, we skip
@@ -180,7 +180,7 @@ void TestResourceBundles()
     log_verbose("Passed:=  %d   Failed=   %d \n", pass, fail);
 }
 
-void TestConstruction1()
+void TestConstruction1(void)
 {
     UResourceBundle *test1 = 0, *test2 = 0;
     const UChar *result1, *result2;
@@ -431,12 +431,12 @@ UBool testTag(const char* frag,
     return (UBool)(passNum == pass);
 }
 
-void record_pass()
+void record_pass(void)
 {
   ++pass;
 }
 
-void record_fail()
+void record_fail(void)
 {
   ++fail;
 }
@@ -446,7 +446,7 @@ void record_fail()
  * are set correctly
  */
 
-static void TestFallback()
+static void TestFallback(void)
 {
     UErrorCode status = U_ZERO_ERROR;
     UResourceBundle *fr_FR = NULL;
diff --git a/icu4c/source/test/cintltst/creststn.c b/icu4c/source/test/cintltst/creststn.c
index cb251ea3cdac..4438bd88e238 100644
--- a/icu4c/source/test/cintltst/creststn.c
+++ b/icu4c/source/test/cintltst/creststn.c
@@ -51,7 +51,7 @@ static int32_t fail;
  */
 
 static uint32_t
-randul()
+randul(void)
 {
     uint32_t l=0;
     int32_t i;
@@ -72,7 +72,7 @@ randul()
  * Return a random double x where 0.0 <= x < 1.0.
  */
 static double
-randd()
+randd(void)
 {
     return ((double)randul()) / UINT32_MAX;
 }
@@ -403,7 +403,7 @@ static void TestAliasConflict(void) {
     }
 }
 
-static void TestDecodedBundle(){
+static void TestDecodedBundle(void){
 
     UErrorCode error = U_ZERO_ERROR;
 
@@ -479,7 +479,7 @@ static void TestDecodedBundle(){
     ures_close(resB);
 }
 
-static void TestNewTypes() {
+static void TestNewTypes(void) {
     UResourceBundle* theBundle = NULL;
     char action[256];
     const char* testdatapath;
@@ -781,7 +781,7 @@ static void TestNewTypes() {
 
 }
 
-static void TestEmptyTypes() {
+static void TestEmptyTypes(void) {
     UResourceBundle* theBundle = NULL;
     char action[256];
     const char* testdatapath;
@@ -926,7 +926,7 @@ static void TestEmptyTypes() {
     ures_close(theBundle);
 }
 
-static void TestEmptyBundle(){
+static void TestEmptyBundle(void){
     UErrorCode status = U_ZERO_ERROR;
     const char* testdatapath=NULL;
     UResourceBundle *resb=0, *dResB=0;
@@ -950,7 +950,7 @@ static void TestEmptyBundle(){
     ures_close(resb);
 }
 
-static void TestBinaryCollationData(){
+static void TestBinaryCollationData(void){
 #if !UCONFIG_NO_COLLATION
     UErrorCode status=U_ZERO_ERROR;
     const char*      locale="te";
@@ -1008,7 +1008,7 @@ static void TestBinaryCollationData(){
 #endif
 }
 
-static void TestAPI() {
+static void TestAPI(void) {
     UErrorCode status=U_ZERO_ERROR;
     int32_t len=0;
     const char* key=NULL;
@@ -1170,7 +1170,7 @@ static void TestAPI() {
     free(utestdatapath);
 }
 
-static void TestErrorConditions(){
+static void TestErrorConditions(void){
     UErrorCode status=U_ZERO_ERROR;
     const char *key=NULL;
     const UChar *value=NULL;
@@ -1405,7 +1405,7 @@ static void TestErrorConditions(){
 
 }
 
-static void TestGetVersion(){
+static void TestGetVersion(void){
     UVersionInfo minVersionArray = {0x01, 0x00, 0x00, 0x00};
     UVersionInfo maxVersionArray = {0x50, 0xff, 0xcf, 0xcf};
     UVersionInfo versionArray;
@@ -1443,7 +1443,7 @@ static void TestGetVersion(){
 }
 
 
-static void TestGetVersionColl(){
+static void TestGetVersionColl(void){
 #if !UCONFIG_NO_COLLATION
     UVersionInfo minVersionArray = {0x00, 0x00, 0x00, 0x00};
     UVersionInfo maxVersionArray = {0x50, 0x80, 0xcf, 0xcf};
@@ -1507,7 +1507,7 @@ static void TestGetVersionColl(){
 #endif  /* !UCONFIG_NO_COLLATION */
 }
 
-static void TestResourceBundles()
+static void TestResourceBundles(void)
 {
     // The test expectation only works if the default locale is not one of the
     // locale bundle in the testdata which have those info. Therefore, we skip
@@ -1539,7 +1539,7 @@ static void TestResourceBundles()
 }
 
 
-static void TestConstruction1()
+static void TestConstruction1(void)
 {
     // The test expectation only works if the default locale is not one of the
     // locale bundle in the testdata which have those info. Therefore, we skip
@@ -2054,17 +2054,17 @@ static UBool testTag(const char* frag,
     return (UBool)(failNum == fail);
 }
 
-static void record_pass()
+static void record_pass(void)
 {
     ++pass;
 }
 
-static void record_fail()
+static void record_fail(void)
 {
     ++fail;
 }
 
-static void TestPreventFallback() {
+static void TestPreventFallback(void) {
     UResourceBundle* theBundle = NULL;
     const char* testdatapath;
     UErrorCode status = U_ZERO_ERROR;
@@ -2124,7 +2124,7 @@ static void TestPreventFallback() {
  * are set correctly
  */
 
-static void TestFallback()
+static void TestFallback(void)
 {
     UErrorCode status = U_ZERO_ERROR;
     UResourceBundle *fr_FR = NULL;
@@ -3039,7 +3039,7 @@ tres_getString(const UResourceBundle *resB,
  * UTF-16 ures_getStringXYZ() that are called internally.
  */
 static void
-TestGetUTF8String() {
+TestGetUTF8String(void) {
     UResourceBundle *res;
     const char *testdatapath;
     char buffer8[16];
diff --git a/icu4c/source/test/cintltst/cstrtest.c b/icu4c/source/test/cintltst/cstrtest.c
index d8006fd1c8e7..d6c762f9fd7f 100644
--- a/icu4c/source/test/cintltst/cstrtest.c
+++ b/icu4c/source/test/cintltst/cstrtest.c
@@ -170,7 +170,7 @@ static void TestAPI(void)
 
 /* test invariant-character handling */
 static void
-TestInvariant() {
+TestInvariant(void) {
     /* all invariant graphic chars and some control codes (not \n!) */
     const char invariantChars[]=
         "\t\r \"%&'()*+,-./"
@@ -306,7 +306,7 @@ static int32_t getSign(int32_t n) {
 }
 
 static void
-TestCompareInvEbcdicAsAscii() {
+TestCompareInvEbcdicAsAscii(void) {
     static const char *const invStrings[][2]={
         /* invariant-character strings in ascending ASCII order */
         /* EBCDIC       native */
@@ -359,7 +359,7 @@ static const UChar *asciiInvChars =
     u"0123456789 \"%&'()*+,-./:;<=>?_";
 
 static void
-TestLocaleAtSign() {
+TestLocaleAtSign(void) {
     static const char *invLocale = "de-Latn_DE@PHONEBOOK";
     for (int32_t i = 0;; ++i) {
         char ic = invLocale[i];
@@ -374,7 +374,7 @@ TestLocaleAtSign() {
 
 // The at sign is not an invariant character.
 static void
-TestNoInvariantAtSign() {
+TestNoInvariantAtSign(void) {
     for (int32_t i = 0;; ++i) {
         char ic = nativeInvChars[i];
         UBool actual = uprv_isAtSign(ic);
@@ -386,7 +386,7 @@ TestNoInvariantAtSign() {
 }
 
 static void
-TestInvCharToAscii() {
+TestInvCharToAscii(void) {
     for (int32_t i = 0;; ++i) {
         char ic = nativeInvChars[i];
         uint8_t ac = (uint8_t)asciiInvChars[i];
diff --git a/icu4c/source/test/cintltst/cturtst.c b/icu4c/source/test/cintltst/cturtst.c
index 31eebc49481f..b3b4ba322167 100644
--- a/icu4c/source/test/cintltst/cturtst.c
+++ b/icu4c/source/test/cintltst/cturtst.c
@@ -90,7 +90,7 @@ void addTurkishCollTest(TestNode** root)
 
 }
 
-static void TestTertiary( )
+static void TestTertiary(void)
 {
     
     int32_t i;
@@ -110,7 +110,7 @@ static void TestTertiary( )
     ucol_close(myCollation);
 }
 
-static void TestPrimary()
+static void TestPrimary(void)
 {
     
     int32_t i;
diff --git a/icu4c/source/test/cintltst/cucdapi.c b/icu4c/source/test/cintltst/cucdapi.c
index 4f9986017664..9b7c6ebf3c71 100644
--- a/icu4c/source/test/cintltst/cucdapi.c
+++ b/icu4c/source/test/cintltst/cucdapi.c
@@ -46,7 +46,7 @@ static void assertEqualScripts(const char *msg,
     }
 }
 
-void TestUScriptCodeAPI(){
+void TestUScriptCodeAPI(void){
     int i =0;
     int numErrors =0;
     {
@@ -518,7 +518,7 @@ void TestUScriptCodeAPI(){
     }
 }
 
-void TestHasScript() {
+void TestHasScript(void) {
     if(!(
         !uscript_hasScript(0x063f, USCRIPT_COMMON) &&
         uscript_hasScript(0x063f, USCRIPT_ARABIC) &&  /* main Script value */
@@ -578,7 +578,7 @@ static UBool scriptsContain(UScriptCode scripts[], int32_t length, UScriptCode s
     return contain;
 }
 
-void TestGetScriptExtensions() {
+void TestGetScriptExtensions(void) {
     UScriptCode scripts[20];
     int32_t length;
     UErrorCode errorCode;
@@ -666,7 +666,7 @@ void TestGetScriptExtensions() {
     }
 }
 
-void TestScriptMetadataAPI() {
+void TestScriptMetadataAPI(void) {
     /* API & code coverage. More testing in intltest/ucdtest.cpp. */
     UErrorCode errorCode=U_ZERO_ERROR;
     UChar sample[8];
@@ -729,7 +729,7 @@ void TestScriptMetadataAPI() {
     }
 }
 
-void TestBinaryValues() {
+void TestBinaryValues(void) {
     /*
      * Unicode 5.1 explicitly defines binary property value aliases.
      * Verify that they are all recognized.
diff --git a/icu4c/source/test/cintltst/cucdtst.c b/icu4c/source/test/cintltst/cucdtst.c
index e50c6924c727..1173caab20e5 100644
--- a/icu4c/source/test/cintltst/cucdtst.c
+++ b/icu4c/source/test/cintltst/cucdtst.c
@@ -208,7 +208,7 @@ void addUnicodeTest(TestNode** root)
 /*==================================================== */
 /* test u_toupper() and u_tolower()                    */
 /*==================================================== */
-static void TestUpperLower()
+static void TestUpperLower(void)
 {
     const UChar upper[] = {0x41, 0x42, 0x00b2, 0x01c4, 0x01c6, 0x01c9, 0x01c8, 0x01c9, 0x000c, 0x0000};
     const UChar lower[] = {0x61, 0x62, 0x00b2, 0x01c6, 0x01c6, 0x01c9, 0x01c9, 0x01c9, 0x000c, 0x0000};
@@ -440,7 +440,7 @@ compareUSets(const USet *a, const USet *b,
 }
 
 /* test isLetter(u_isapha()) and isDigit(u_isdigit()) */
-static void TestLetterNumber()
+static void TestLetterNumber(void)
 {
     UChar i = 0x0000;
 
@@ -542,7 +542,7 @@ static void testSampleCharProps(UBool propFn(UChar32), const char *propName,
 }
 
 /* Tests for isDefined(u_isdefined)(, isBaseForm(u_isbase()), isSpaceChar(u_isspace()), isWhiteSpace(), u_CharDigitValue() */
-static void TestMisc()
+static void TestMisc(void)
 {
     static const UChar32 sampleSpaces[] = {0x0020, 0x00a0, 0x2000, 0x2001, 0x2005};
     static const UChar32 sampleNonSpaces[] = {0x61, 0x62, 0x63, 0x64, 0x74};
@@ -822,7 +822,7 @@ static const struct {
 };
 
 static void
-TestPOSIX() {
+TestPOSIX(void) {
     uint32_t mask;
     int32_t cl, i;
     UBool expect;
@@ -841,7 +841,7 @@ TestPOSIX() {
 }
 
 /* Tests for isControl(u_iscntrl()) and isPrintable(u_isprint()) */
-static void TestControlPrint()
+static void TestControlPrint(void)
 {
     const UChar32 sampleControl[] = {0x1b, 0x97, 0x82, 0x2028, 0x2029, 0x200c, 0x202b};
     const UChar32 sampleNonControl[] = {0x61, 0x0031, 0x00e2};
@@ -889,7 +889,7 @@ static void TestControlPrint()
 }
 
 /* u_isJavaIDStart, u_isJavaIDPart, u_isIDStart(), u_isIDPart(), u_isIDIgnorable()*/
-static void TestIdentifier()
+static void TestIdentifier(void)
 {
     const UChar32 sampleJavaIDStart[] = {0x0071, 0x00e4, 0x005f};
     const UChar32 sampleNonJavaIDStart[] = {0x0020, 0x2030, 0x0082};
@@ -1313,7 +1313,7 @@ enumDefaultsRange(const void *context, UChar32 start, UChar32 limit, UCharCatego
 }
 
 /* tests for several properties */
-static void TestUnicodeData()
+static void TestUnicodeData(void)
 {
     UVersionInfo expectVersionArray;
     UVersionInfo versionArray;
@@ -1401,7 +1401,7 @@ static void TestUnicodeData()
     u_enumCharTypes(enumDefaultsRange, NULL);
 }
 
-static void TestCodeUnit(){
+static void TestCodeUnit(void){
     const UChar codeunit[]={0x0000,0xe065,0x20ac,0xd7ff,0xd800,0xd841,0xd905,0xdbff,0xdc00,0xdc02,0xddee,0xdfff,0};
 
     int32_t i;
@@ -1448,7 +1448,7 @@ static void TestCodeUnit(){
     }
 }
 
-static void TestCodePoint(){
+static void TestCodePoint(void){
     const UChar32 codePoint[]={
         /*surrogate, notvalid(codepoint), not a UnicodeChar, not Error */
         0xd800,
@@ -1586,7 +1586,7 @@ static void TestCodePoint(){
     }
 }
 
-static void TestCharLength()
+static void TestCharLength(void)
 {
     const int32_t codepoint[]={
         1, 0x0061,
@@ -1770,7 +1770,7 @@ u_charToUChar(char c) {
 }
 
 static void
-TestCharNames() {
+TestCharNames(void) {
     static char name[80];
     UErrorCode errorCode=U_ZERO_ERROR;
     struct enumExtCharNamesContext extContext;
@@ -2010,7 +2010,7 @@ TestCharNames() {
 }
 
 static void
-TestUCharFromNameUnderflow() {
+TestUCharFromNameUnderflow(void) {
     // Ticket #10889: Underflow crash when there is no dash.
     const char *name="<NO BREAK SPACE>";
     UErrorCode errorCode=U_ZERO_ERROR;
@@ -2074,7 +2074,7 @@ TestUCharFromNameUnderflow() {
 /* test u_isMirrored() and u_charMirror() ----------------------------------- */
 
 static void
-TestMirroring() {
+TestMirroring(void) {
     USet *set;
     UErrorCode errorCode;
 
@@ -2186,7 +2186,7 @@ CheckScriptRuns(UScriptRun *scriptRun, int32_t *runStarts, const RunTestData *te
 }
 
 static void
-TestUScriptRunAPI()
+TestUScriptRunAPI(void)
 {
     static const RunTestData testData1[] = {
         {"\\u0020\\u0946\\u0939\\u093F\\u0928\\u094D\\u0926\\u0940\\u0020", USCRIPT_DEVANAGARI},
@@ -2350,7 +2350,7 @@ TestUScriptRunAPI()
 
 /* test additional, non-core properties */
 static void
-TestAdditionalProperties() {
+TestAdditionalProperties(void) {
     /* test data for u_charAge() */
     static const struct {
         UChar32 c;
@@ -3210,7 +3210,7 @@ TestPropertyValues(void) {
 
 /* various tests for consistency of UCD data and API behavior */
 static void
-TestConsistency() {
+TestConsistency(void) {
     char buffer[300];
     USet *set1, *set2, *set3, *set4;
     UErrorCode errorCode;
@@ -3540,7 +3540,7 @@ caseFoldingLineFn(void *context,
 }
 
 static void
-TestCaseFolding() {
+TestCaseFolding(void) {
     CaseFoldingData data={ NULL, 0, 0, {0}, 0, 0 };
     char *fields[3][2];
     UErrorCode errorCode;
@@ -3580,7 +3580,7 @@ TestCaseFolding() {
     uset_close(data.notSeen);
 }
 
-static void TestBinaryCharacterPropertiesAPI() {
+static void TestBinaryCharacterPropertiesAPI(void) {
     // API test only. See intltest/ucdtest.cpp for functional test.
     UErrorCode errorCode = U_ZERO_ERROR;
     const USet *set = u_getBinaryPropertySet(-1, &errorCode);
@@ -3599,7 +3599,7 @@ static void TestBinaryCharacterPropertiesAPI() {
     }
 }
 
-static void TestIntCharacterPropertiesAPI() {
+static void TestIntCharacterPropertiesAPI(void) {
     // API test only. See intltest/ucdtest.cpp for functional test.
     UErrorCode errorCode = U_ZERO_ERROR;
     const UCPMap *map = u_getIntPropertyMap(UCHAR_INT_START - 1, &errorCode);
diff --git a/icu4c/source/test/cintltst/custrtrn.c b/icu4c/source/test/cintltst/custrtrn.c
index d794d79baa3b..7987dacc9ff6 100644
--- a/icu4c/source/test/cintltst/custrtrn.c
+++ b/icu4c/source/test/cintltst/custrtrn.c
@@ -208,7 +208,7 @@ static void Test_strToUTF32(void){
 }
 
 /* test unpaired surrogates */
-static void Test_strToUTF32_surrogates() {
+static void Test_strToUTF32_surrogates(void) {
     UErrorCode err = U_ZERO_ERROR;
     UChar32 u32Target[400];
     int32_t len16, u32DestLen;
@@ -388,7 +388,7 @@ static void Test_strFromUTF32(void){
 }
 
 /* test surrogate code points */
-static void Test_strFromUTF32_surrogates() {
+static void Test_strFromUTF32_surrogates(void) {
     UErrorCode err = U_ZERO_ERROR;
     UChar uTarget[400];
     int32_t len32, uDestLen;
@@ -1381,7 +1381,7 @@ static void Test_UChar_WCHART_API(void){
 #endif
 } 
 
-static void Test_widestrs()
+static void Test_widestrs(void)
 {
 #if (defined(U_WCHAR_IS_UTF16) || defined(U_WCHAR_IS_UTF32)) || (!UCONFIG_NO_CONVERSION && !UCONFIG_NO_LEGACY_CONVERSION)
         wchar_t ws[100];
@@ -1425,7 +1425,7 @@ static void Test_widestrs()
 }
 
 static void
-Test_WCHART_LongString(){
+Test_WCHART_LongString(void){
 #if (defined(U_WCHAR_IS_UTF16) || defined(U_WCHAR_IS_UTF32)) || (!UCONFIG_NO_CONVERSION && !UCONFIG_NO_LEGACY_CONVERSION)
     UErrorCode status = U_ZERO_ERROR;
     const char* testdatapath=loadTestData(&status);
@@ -1513,7 +1513,7 @@ Test_WCHART_LongString(){
 #endif
 }
 
-static void Test_strToJavaModifiedUTF8() {
+static void Test_strToJavaModifiedUTF8(void) {
     static const UChar src[]={
         0x61, 0x62, 0x63, 0xe1, 0xe2, 0xe3,
         0xe01, 0xe02, 0xe03, 0xe001, 0xe002, 0xe003,
@@ -1689,7 +1689,7 @@ static void Test_strToJavaModifiedUTF8() {
     }
 }
 
-static void Test_strFromJavaModifiedUTF8() {
+static void Test_strFromJavaModifiedUTF8(void) {
     static const uint8_t src[]={
         0x61, 0x62, 0x63, 0xc3, 0xa1, 0xc3, 0xa2, 0xc3, 0xa3,
         0xe0, 0xb8, 0x81, 0xe0, 0xb8, 0x82, 0xe0, 0xb8, 0x83,
@@ -1992,7 +1992,7 @@ static void Test_strFromJavaModifiedUTF8() {
 }
 
 /* test that string transformation functions permit NULL source pointer when source length==0 */
-static void TestNullEmptySource() {
+static void TestNullEmptySource(void) {
     char dest8[4]={ 3, 3, 3, 3 };
     UChar dest16[4]={ 3, 3, 3, 3 };
     UChar32 dest32[4]={ 3, 3, 3, 3 };
diff --git a/icu4c/source/test/cintltst/custrtst.c b/icu4c/source/test/cintltst/custrtst.c
index f276b94cc2d0..f59dfa29b726 100644
--- a/icu4c/source/test/cintltst/custrtst.c
+++ b/icu4c/source/test/cintltst/custrtst.c
@@ -72,7 +72,7 @@ static const char* raw[3][4] = {
     {   "English_United States", "French_France", "Croatian_Croatia", "English_United States"}
 };
 
-static void setUpDataTable()
+static void setUpDataTable(void)
 {
     int32_t i,j;
     if(dataTable == NULL) {
@@ -88,7 +88,7 @@ static void setUpDataTable()
     }
 }
 
-static void cleanUpDataTable()
+static void cleanUpDataTable(void)
 {
     int32_t i,j;
     if(dataTable != NULL) {
@@ -104,7 +104,7 @@ static void cleanUpDataTable()
 }
 
 /*Tests  for u_strcat(),u_strcmp(), u_strlen(), u_strcpy(),u_strncat(),u_strncmp(),u_strncpy, u_uastrcpy(),u_austrcpy(), u_uastrncpy(); */
-static void TestStringFunctions()
+static void TestStringFunctions(void)
 {
     int32_t i,j,k;
     UChar temp[512];
@@ -480,7 +480,7 @@ static void TestStringFunctions()
     cleanUpDataTable();
 }
 
-static void TestStringSearching()
+static void TestStringSearching(void)
 {
     const UChar testString[] = {0x0061, 0x0062, 0x0063, 0x0064, 0x0064, 0x0061, 0};
     const UChar testSurrogateString[] = {0xdbff, 0x0061, 0x0062, 0xdbff, 0xdfff, 0x0063, 0x0064, 0x0064, 0xdbff, 0xdfff, 0xdb00, 0xdf00, 0x0061, 0};
@@ -691,7 +691,7 @@ static void TestStringSearching()
  * There are some new functions, too - just test them all.
  */
 static void
-TestSurrogateSearching() {
+TestSurrogateSearching(void) {
     static const UChar s[]={
         /* 0       1       2     3       4     5       6     7       8       9    10 11 */
         0x61, 0xd801, 0xdc02, 0x61, 0xdc02, 0x61, 0xd801, 0x61, 0xd801, 0xdc02, 0x61, 0
@@ -979,7 +979,7 @@ TestSurrogateSearching() {
     }
 }
 
-static void TestStringCopy()
+static void TestStringCopy(void)
 {
     UChar temp[40];
     UChar *result=0;
@@ -1098,7 +1098,7 @@ static void TestStringCopy()
 /* test u_unescape() and u_unescapeAt() ------------------------------------- */
 
 static void
-TestUnescape() {
+TestUnescape(void) {
     static UChar buffer[200];
     
     static const char* input =
@@ -1130,7 +1130,7 @@ TestUnescape() {
 }
 
 static void
-TestUnescapeRepeatedSurrogateLead20725() {
+TestUnescapeRepeatedSurrogateLead20725(void) {
     const int32_t repeat = 20000;
     const int32_t srclen = repeat * 6 + 1;
     char *src = (char*)malloc(srclen);
@@ -1197,7 +1197,7 @@ _testStrHasMoreChar32Than(const UChar *s, int32_t i, int32_t length, int32_t num
 }
 
 static void
-TestCountChar32() {
+TestCountChar32(void) {
     static const UChar string[]={
         0x61, 0x62, 0xd800, 0xdc00,
         0xd801, 0xdc01, 0x63, 0xd802,
@@ -1498,7 +1498,7 @@ testIteratorState(UCharIterator *iter1, UCharIterator *iter2, const char *n, int
 }
 
 static void
-TestUCharIterator() {
+TestUCharIterator(void) {
     static const UChar text[]={
         0x61, 0x62, 0x63, 0xd801, 0xdffd, 0x78, 0x79, 0x7a, 0
     };
diff --git a/icu4c/source/test/cintltst/encoll.c b/icu4c/source/test/cintltst/encoll.c
index e54f898f0e9d..4bdadd553c11 100644
--- a/icu4c/source/test/cintltst/encoll.c
+++ b/icu4c/source/test/cintltst/encoll.c
@@ -262,7 +262,7 @@ void addEnglishCollTest(TestNode** root)
 
 }
 
-static void TestTertiary( )
+static void TestTertiary(void)
 {
     int32_t testMoreSize;
     UCollationResult expected=UCOL_EQUAL;
@@ -305,7 +305,7 @@ static void TestTertiary( )
     ucol_close(myCollation);
 }
 
-static void TestPrimary()
+static void TestPrimary(void)
 {
     
     int32_t i;
@@ -324,7 +324,7 @@ static void TestPrimary()
     ucol_close(myCollation);
 }
 
-static void TestSecondary()
+static void TestSecondary(void)
 {
     UCollationResult expected=UCOL_EQUAL;
     int32_t i,j, testAcuteSize;
diff --git a/icu4c/source/test/cintltst/eurocreg.c b/icu4c/source/test/cintltst/eurocreg.c
index 9aac51b099a8..618ee98ce37d 100644
--- a/icu4c/source/test/cintltst/eurocreg.c
+++ b/icu4c/source/test/cintltst/eurocreg.c
@@ -121,7 +121,7 @@ static const char convertersToCheck[][15] = {
 
 UBool isEuroAware(UConverter*);
 
-void TestEuroRegression()
+void TestEuroRegression(void)
 {
     int32_t i=0;
 
diff --git a/icu4c/source/test/cintltst/hpmufn.c b/icu4c/source/test/cintltst/hpmufn.c
index 558120de3d2f..f27e57939959 100644
--- a/icu4c/source/test/cintltst/hpmufn.c
+++ b/icu4c/source/test/cintltst/hpmufn.c
@@ -62,7 +62,7 @@ static int32_t gMutexFailures = 0;
  *    The ICU data directory must be preserved across these operations.
  *    Here is a helper function to assist with that.
  */
-static char *safeGetICUDataDirectory() {
+static char *safeGetICUDataDirectory(void) {
     const char *dataDir = u_getDataDirectory();  /* Returned string vanashes with u_cleanup */
     char *retStr = NULL;
     if (dataDir != NULL) {
@@ -122,7 +122,7 @@ static void * U_CALLCONV myMemRealloc(const void *context, void *mem, size_t siz
 }
 
 
-static void TestHeapFunctions() {
+static void TestHeapFunctions(void) {
     UErrorCode       status = U_ZERO_ERROR;
     UResourceBundle *rb     = NULL;
     char            *icuDataDir;
diff --git a/icu4c/source/test/cintltst/idnatest.c b/icu4c/source/test/cintltst/idnatest.c
index bd979f0d4c52..293e92074dae 100644
--- a/icu4c/source/test/cintltst/idnatest.c
+++ b/icu4c/source/test/cintltst/idnatest.c
@@ -427,7 +427,7 @@ static const char * const domainNames[] = {
 };
 
 static void
-TestToASCII(){
+TestToASCII(void){
 
     int32_t i;
     UChar buf[MAX_DEST_SIZE];
@@ -441,7 +441,7 @@ TestToASCII(){
 }
 
 static void
-TestToUnicode(){
+TestToUnicode(void){
 
     int32_t i;
     UChar buf[MAX_DEST_SIZE];
@@ -455,7 +455,7 @@ TestToUnicode(){
 
 
 static void
-TestIDNToUnicode(){
+TestIDNToUnicode(void){
     int32_t i;
     UChar buf[MAX_DEST_SIZE];
     UChar expected[MAX_DEST_SIZE];
@@ -484,7 +484,7 @@ TestIDNToUnicode(){
 }
 
 static void
-TestIDNToASCII(){
+TestIDNToASCII(void){
     int32_t i;
     UChar buf[MAX_DEST_SIZE];
     UChar expected[MAX_DEST_SIZE];
@@ -564,7 +564,7 @@ testCompareWithSrc(const UChar* s1, int32_t s1Len,
 
 
 static void
-TestCompare(){
+TestCompare(void){
     int32_t i;
 
     const char* testName ="uidna_compare";
@@ -641,7 +641,7 @@ TestCompare(){
     }
 }
 
-static void TestJB4490(){
+static void TestJB4490(void){
     static const UChar data[][50]= {
         {0x00F5,0x00dE,0x00dF,0x00dD, 0x0000},
         {0xFB00,0xFB01}
@@ -673,7 +673,7 @@ static void TestJB4490(){
     }
 }
 
-static void TestJB4475(){
+static void TestJB4475(void){
     
     static const UChar input[][10] = {
         {0x0054,0x0045,0x0053,0x0054,0x0000},/* TEST */
@@ -700,7 +700,7 @@ static void TestJB4475(){
     }
 }
 
-static void TestLength(){
+static void TestLength(void){
     {
         static const char* cl = "my_very_very_very_very_very_very_very_very_very_very_very_very_very_long_and_incredibly_uncreative_domain_label";
         UChar ul[128] = {'\0'};
@@ -823,7 +823,7 @@ static void TestLength(){
         }
     }    
 }
-static void TestJB5273(){
+static void TestJB5273(void){
     static const char INVALID_DOMAIN_NAME[] = "xn--m\\u00FCller.de";
     UChar invalid_idn[25] = {'\0'};
     int32_t len = u_unescape(INVALID_DOMAIN_NAME, invalid_idn, (int32_t)strlen(INVALID_DOMAIN_NAME));
@@ -857,7 +857,7 @@ static void TestJB5273(){
  * Test the new (ICU 4.6/2010) C API that was added for UTS #46.
  * Just an API test: Functionality is tested via C++ intltest.
  */
-static void TestUTS46() {
+static void TestUTS46(void) {
     static const UChar fA_sharps16[] = { 0x66, 0x41, 0xdf, 0 };
     static const char fA_sharps8[] = { 0x66, 0x41, (char)0xc3, (char)0x9f, 0 };
     static const UChar fa_sharps16[] = { 0x66, 0x61, 0xdf, 0 };
diff --git a/icu4c/source/test/cintltst/nccbtst.c b/icu4c/source/test/cintltst/nccbtst.c
index 0f5817f70d01..825e918192fc 100644
--- a/icu4c/source/test/cintltst/nccbtst.c
+++ b/icu4c/source/test/cintltst/nccbtst.c
@@ -104,7 +104,7 @@ void addTestConvertErrorCallBack(TestNode** root)
     addTest(root, &TestCallBackFailure,  "tsconv/nccbtst/TestCallBackFailure");
 }
 
-static void TestSkipCallBack()
+static void TestSkipCallBack(void)
 {
     TestSkip(NEW_MAX_BUFFER, NEW_MAX_BUFFER);
     TestSkip(1,NEW_MAX_BUFFER);
@@ -112,7 +112,7 @@ static void TestSkipCallBack()
     TestSkip(NEW_MAX_BUFFER, 1);
 }
 
-static void TestStopCallBack()
+static void TestStopCallBack(void)
 {
     TestStop(NEW_MAX_BUFFER, NEW_MAX_BUFFER);
     TestStop(1,NEW_MAX_BUFFER);
@@ -120,7 +120,7 @@ static void TestStopCallBack()
     TestStop(NEW_MAX_BUFFER, 1);
 }
 
-static void TestSubCallBack()
+static void TestSubCallBack(void)
 {
     TestSub(NEW_MAX_BUFFER, NEW_MAX_BUFFER);
     TestSub(1,NEW_MAX_BUFFER);
@@ -135,7 +135,7 @@ static void TestSubCallBack()
 #endif
 }
 
-static void TestSubWithValueCallBack()
+static void TestSubWithValueCallBack(void)
 {
     TestSubWithValue(NEW_MAX_BUFFER, NEW_MAX_BUFFER);
     TestSubWithValue(1,NEW_MAX_BUFFER);
@@ -144,7 +144,7 @@ static void TestSubWithValueCallBack()
 }
 
 #if !UCONFIG_NO_LEGACY_CONVERSION
-static void TestLegalAndOtherCallBack()
+static void TestLegalAndOtherCallBack(void)
 {
     TestLegalAndOthers(NEW_MAX_BUFFER, NEW_MAX_BUFFER);
     TestLegalAndOthers(1,NEW_MAX_BUFFER);
@@ -152,7 +152,7 @@ static void TestLegalAndOtherCallBack()
     TestLegalAndOthers(NEW_MAX_BUFFER, 1);
 }
 
-static void TestSingleByteCallBack()
+static void TestSingleByteCallBack(void)
 {
     TestSingleByte(NEW_MAX_BUFFER, NEW_MAX_BUFFER);
     TestSingleByte(1,NEW_MAX_BUFFER);
diff --git a/icu4c/source/test/cintltst/ncnvtst.c b/icu4c/source/test/cintltst/ncnvtst.c
index 0f84b86fe2b1..77b45db53274 100644
--- a/icu4c/source/test/cintltst/ncnvtst.c
+++ b/icu4c/source/test/cintltst/ncnvtst.c
@@ -140,7 +140,7 @@ void addExtraTests(TestNode** root)
 }
 
 /*test surrogate behaviour*/
-static void TestSurrogateBehaviour(){
+static void TestSurrogateBehaviour(void){
     log_verbose("Testing for SBCS and LATIN_1\n");
     {
         UChar sampleText[] = {0x0031, 0xd801, 0xdc01, 0x0032};
@@ -334,7 +334,7 @@ static void TestSurrogateBehaviour(){
 }
 
 /*test various error behaviours*/
-static void TestErrorBehaviour(){
+static void TestErrorBehaviour(void){
     log_verbose("Testing for SBCS and LATIN_1\n");
     {
         static const UChar    sampleText[] =   { 0x0031, 0xd801};
@@ -633,7 +633,7 @@ static void TestErrorBehaviour(){
 
 #if !UCONFIG_NO_LEGACY_CONVERSION
 /*test different convertToUnicode error behaviours*/
-static void TestToUnicodeErrorBehaviour()
+static void TestToUnicodeErrorBehaviour(void)
 {
     log_verbose("Testing error conditions for DBCS\n");
     {
@@ -665,7 +665,7 @@ static void TestToUnicodeErrorBehaviour()
     }
 }
 
-static void TestGetNextErrorBehaviour(){
+static void TestGetNextErrorBehaviour(void){
    /*Test for unassigned character*/
 #define INPUT_SIZE 1
     static const char input1[INPUT_SIZE]={ 0x70 };
@@ -689,7 +689,7 @@ static void TestGetNextErrorBehaviour(){
 #define MAX_UTF8_LEN 4
 
 /*Regression test for utf8 converter*/
-static void TestRegressionUTF8(){
+static void TestRegressionUTF8(void){
     UChar32 currCh = 0;
     int32_t offset8;
     int32_t offset16;
@@ -759,7 +759,7 @@ static void TestRegressionUTF8(){
 
 #define MAX_UTF32_LEN 1
 
-static void TestRegressionUTF32(){
+static void TestRegressionUTF32(void){
 #if !UCONFIG_ONLY_HTML_CONVERSION
     UChar32 currCh = 0;
     int32_t offset32;
@@ -896,7 +896,7 @@ static void TestRegressionUTF32(){
 }
 
 /*Walk through the available converters*/
-static void TestAvailableConverters(){
+static void TestAvailableConverters(void){
     UErrorCode status=U_ZERO_ERROR;
     UConverter *conv=NULL;
     int32_t i=0;
@@ -913,7 +913,7 @@ static void TestAvailableConverters(){
 
 }
 
-static void TestFlushInternalBuffer(){
+static void TestFlushInternalBuffer(void){
     TestWithBufferSize(MAX_LENGTH, 1);
     TestWithBufferSize(1, 1);
     TestWithBufferSize(1, MAX_LENGTH);
@@ -1834,7 +1834,7 @@ doTestTruncated(const char *cnvName, const uint8_t *bytes, int32_t length) {
 }
 
 static void
-TestTruncated() {
+TestTruncated(void) {
     static const struct {
         const char *cnvName;
         uint8_t bytes[8]; /* partial input bytes resulting in no output */
@@ -1884,7 +1884,7 @@ typedef struct NameRange {
 } NameRange;
 
 static void
-TestUnicodeSet() {
+TestUnicodeSet(void) {
     UErrorCode errorCode;
     UConverter *cnv;
     USet *set;
diff --git a/icu4c/source/test/cintltst/nucnvtst.c b/icu4c/source/test/cintltst/nucnvtst.c
index 9bf835bcbcce..6c97419f4cd8 100644
--- a/icu4c/source/test/cintltst/nucnvtst.c
+++ b/icu4c/source/test/cintltst/nucnvtst.c
@@ -1257,7 +1257,7 @@ static void TestNewConvertWithBufferSizes(int32_t outsize, int32_t insize )
     }
 }
 
-static void TestCoverageMBCS(){
+static void TestCoverageMBCS(void){
 #if 0
     UErrorCode status = U_ZERO_ERROR;
     const char *directory = loadTestData(&status);
@@ -1371,7 +1371,7 @@ static void TestConverterType(const char *convName, UConverterType convType) {
     ucnv_close(myConverter);
 }
 
-static void TestConverterTypesAndStarters()
+static void TestConverterTypesAndStarters(void)
 {
 #if !UCONFIG_NO_LEGACY_CONVERSION
     UConverter* myConverter;
@@ -1514,7 +1514,7 @@ TestAmbiguousConverter(UConverter *cnv) {
     }
 }
 
-static void TestAmbiguous()
+static void TestAmbiguous(void)
 {
     UErrorCode status = U_ZERO_ERROR;
     UConverter *ascii_cnv = 0, *sjis_cnv = 0, *cnv;
@@ -1598,7 +1598,7 @@ static void TestAmbiguous()
 }
 
 static void
-TestSignatureDetection(){
+TestSignatureDetection(void){
     /* with null terminated strings */
     {
         static const char* data[] = {
@@ -1811,7 +1811,7 @@ TestSignatureDetection(){
     }
 }
 
-static void TestUTF7() {
+static void TestUTF7(void) {
     /* test input */
     static const uint8_t in[]={
         /* H - +Jjo- - ! +- +2AHcAQ */
@@ -1854,7 +1854,7 @@ static void TestUTF7() {
     ucnv_close(cnv);
 }
 
-static void TestIMAP() {
+static void TestIMAP(void) {
     /* test input */
     static const uint8_t in[]={
         /* H - &Jjo- - ! &- &2AHcAQ- \ */
@@ -1897,7 +1897,7 @@ static void TestIMAP() {
     ucnv_close(cnv);
 }
 
-static void TestUTF8() {
+static void TestUTF8(void) {
     /* test input */
     static const uint8_t in[]={
         0x61,
@@ -1962,7 +1962,7 @@ static void TestUTF8() {
     ucnv_close(cnv);
 }
 
-static void TestCESU8() {
+static void TestCESU8(void) {
     /* test input */
     static const uint8_t in[]={
         0x61,
@@ -2033,7 +2033,7 @@ static void TestCESU8() {
     ucnv_close(cnv);
 }
 
-static void TestUTF16() {
+static void TestUTF16(void) {
     /* test input */
     static const uint8_t in1[]={
         0xfe, 0xff, 0x4e, 0x00, 0xfe, 0xff
@@ -2091,7 +2091,7 @@ static void TestUTF16() {
     ucnv_close(cnv);
 }
 
-static void TestUTF16BE() {
+static void TestUTF16BE(void) {
     /* test input */
     static const uint8_t in[]={
         0x00, 0x61,
@@ -2147,7 +2147,7 @@ static void TestUTF16BE() {
 }
 
 static void
-TestUTF16LE() {
+TestUTF16LE(void) {
     /* test input */
     static const uint8_t in[]={
         0x61, 0x00,
@@ -2201,7 +2201,7 @@ TestUTF16LE() {
     ucnv_close(cnv);
 }
 
-static void TestUTF32() {
+static void TestUTF32(void) {
     /* test input */
     static const uint8_t in1[]={
         0x00, 0x00, 0xfe, 0xff,   0x00, 0x10, 0x0f, 0x00,   0x00, 0x00, 0xfe, 0xff
@@ -2260,7 +2260,7 @@ static void TestUTF32() {
 }
 
 static void
-TestUTF32BE() {
+TestUTF32BE(void) {
     /* test input */
     static const uint8_t in[]={
         0x00, 0x00, 0x00, 0x61,
@@ -2331,7 +2331,7 @@ TestUTF32BE() {
 }
 
 static void
-TestUTF32LE() {
+TestUTF32LE(void) {
     /* test input */
     static const uint8_t in[]={
         0x61, 0x00, 0x00, 0x00,
@@ -2402,7 +2402,7 @@ TestUTF32LE() {
 }
 
 static void
-TestLATIN1() {
+TestLATIN1(void) {
     /* test input */
     static const uint8_t in[]={
        0x61,
@@ -2530,7 +2530,7 @@ TestLATIN1() {
 }
 
 static void
-TestSBCS() {
+TestSBCS(void) {
     /* test input */
     static const uint8_t in[]={ 0x61, 0xc0, 0x80, 0xe0, 0xf0, 0xf4};
     /* expected test results */
@@ -2565,7 +2565,7 @@ TestSBCS() {
 }
 
 static void
-TestDBCS() {
+TestDBCS(void) {
     /* test input */
     static const uint8_t in[]={
         0x44, 0x6a,
@@ -2612,7 +2612,7 @@ TestDBCS() {
 }
 
 static void
-TestMBCS() {
+TestMBCS(void) {
     /* test input */
     static const uint8_t in[]={
         0x01,
@@ -2665,7 +2665,7 @@ TestMBCS() {
 
 #if !UCONFIG_NO_LEGACY_CONVERSION && !UCONFIG_NO_FILE_IO
 static void
-TestICCRunout() {
+TestICCRunout(void) {
 /*    { "ibm-1363", :bin{ a2aea2 }, "\u00a1\u001a", :intvector{ 0, 2 }, :int{1}, :int{0}, "\", "?", :bin{""} } */
 
     const char *cnvName = "ibm-1363";
@@ -3072,7 +3072,7 @@ static int TestJitterbug930(const char* enc){
 }
 
 static void
-TestHZ() {
+TestHZ(void) {
     /* test input */
     static const uint16_t in[]={
             0x3000, 0x3001, 0x3002, 0x00B7, 0x02C9, 0x02C7, 0x00A8, 0x3003, 0x3005, 0x2014,
@@ -3155,7 +3155,7 @@ TestHZ() {
 }
 
 static void
-TestISCII(){
+TestISCII(void){
         /* test input */
     static const uint16_t in[]={
         /* test full range of Devanagari */
@@ -3298,7 +3298,7 @@ TestISCII(){
 }
 
 static void
-TestISO_2022_JP() {
+TestISO_2022_JP(void) {
     /* test input */
     static const uint16_t in[]={
         0x0041,/*0x00E9,*/0x3000, 0x3001, 0x3002, 0x0020, 0x000D, 0x000A,
@@ -3611,7 +3611,7 @@ TestRoundTrippingAllUTF(void){
 }
 
 static void
-TestSCSU() {
+TestSCSU(void) {
 
     static const uint16_t germanUTF16[]={
         0x00d6, 0x006c, 0x0020, 0x0066, 0x006c, 0x0069, 0x0065, 0x00df, 0x0074
@@ -3790,7 +3790,7 @@ TestSCSU() {
 }
 
 #if !UCONFIG_NO_LEGACY_CONVERSION
-static void TestJitterbug2346(){
+static void TestJitterbug2346(void){
     char source[] = { 0x1b,0x24,0x42,0x3d,0x45,0x1b,0x28,0x4a,0x0d,0x0a,
                       0x1b,0x24,0x42,0x3d,0x45,0x1b,0x28,0x4a,0x0d,0x0a};
     uint16_t expected[] = {0x91CD,0x000D,0x000A,0x91CD,0x000D,0x000A};    
@@ -3839,7 +3839,7 @@ static void TestJitterbug2346(){
 }
 
 static void
-TestISO_2022_JP_1() {
+TestISO_2022_JP_1(void) {
     /* test input */
     static const uint16_t in[]={
         0x3000, 0x3001, 0x3002, 0x0020, 0xFF0E, 0x30FB, 0xFF1A, 0xFF1B, 0x000D, 0x000A,
@@ -3922,7 +3922,7 @@ TestISO_2022_JP_1() {
 }
 
 static void
-TestISO_2022_JP_2() {
+TestISO_2022_JP_2(void) {
     /* test input */
     static const uint16_t in[]={
         0x00A8, 0x3003, 0x3005, 0x2015, 0xFF5E, 0x2016, 0x2026, 0x2018, 0x000D, 0x000A,
@@ -4016,7 +4016,7 @@ TestISO_2022_JP_2() {
 }
 
 static void
-TestISO_2022_KR() {
+TestISO_2022_KR(void) {
     /* test input */
     static const uint16_t in[]={
                     0x9F4B,0x9F4E,0x9F52,0x9F5F,0x9F61,0x9F67,0x9F6A,0x000A,0x000D
@@ -4098,7 +4098,7 @@ TestISO_2022_KR() {
 }
 
 static void
-TestISO_2022_KR_1() {
+TestISO_2022_KR_1(void) {
     /* test input */
     static const uint16_t in[]={
                     0x9F4B,0x9F4E,0x9F52,0x9F5F,0x9F61,0x9F67,0x9F6A,0x000A,0x000D
@@ -4180,7 +4180,7 @@ TestISO_2022_KR_1() {
     free(offsets);
 }
 
-static void TestJitterbug2411(){
+static void TestJitterbug2411(void){
     static const char* source = "\x1b\x24\x29\x43\x6b\x6b\x6e\x6e\x6a\x68\x70\x6f\x69\x75\x79\x71\x77\x65\x68\x67\x0A"
                          "\x1b\x24\x29\x43\x6a\x61\x73\x64\x66\x6a\x61\x73\x64\x66\x68\x6f\x69\x75\x79\x1b\x24\x29\x43";
     UConverter* kr=NULL, *kr1=NULL;
@@ -4218,7 +4218,7 @@ static void TestJitterbug2411(){
 }
 
 static void
-TestJIS(){
+TestJIS(void){
     /* From Unicode moved to testdata/conversion.txt */
     /*To Unicode*/
     {
@@ -4489,7 +4489,7 @@ TestISO_2022_CN_EXT() {
 #endif
 
 static void
-TestISO_2022_CN() {
+TestISO_2022_CN(void) {
     /* test input */
     static const uint16_t in[]={
          /* jitterbug 951 */
@@ -4661,7 +4661,7 @@ static void TestJitterbug6175(void) {
 }
 
 static void
-TestEBCDIC_STATEFUL() {
+TestEBCDIC_STATEFUL(void) {
     /* test input */
     static const uint8_t in[]={
         0x61,
@@ -4728,7 +4728,7 @@ TestEBCDIC_STATEFUL() {
 }
 
 static void
-TestGB18030() {
+TestGB18030(void) {
     /* test input */
     static const uint8_t in[]={
         0x24,
@@ -4784,7 +4784,7 @@ TestGB18030() {
 }
 
 static void
-TestLMBCS() {
+TestLMBCS(void) {
     /* LMBCS-1 string */
     static const uint8_t pszLMBCS[]={
         0x61,
@@ -5345,7 +5345,7 @@ TestLMBCS() {
 }
 
 
-static void TestJitterbug255()
+static void TestJitterbug255(void)
 {
     static const uint8_t testBytes[] = { 0x95, 0xcf, 0x8a, 0xb7, 0x0d, 0x0a, 0x00 };
     const char *testBuffer = (const char *)testBytes;
@@ -5371,7 +5371,7 @@ static void TestJitterbug255()
     ucnv_close(cnv);
 }
 
-static void TestEBCDICUS4XML()
+static void TestEBCDICUS4XML(void)
 {
     UChar unicodes_x[] = {0x0000, 0x0000, 0x0000, 0x0000};
     static const UChar toUnicodeMaps_x[] = {0x000A, 0x000A, 0x000D, 0x0000};
@@ -5411,7 +5411,7 @@ static void TestEBCDICUS4XML()
 
 #if !UCONFIG_NO_COLLATION
 
-static void TestJitterbug981(){
+static void TestJitterbug981(void){
     const UChar* rules;
     int32_t rules_length, target_cap, bytes_needed, buff_size;
     UErrorCode status = U_ZERO_ERROR;
@@ -5466,7 +5466,7 @@ static void TestJitterbug981(){
 #endif
 
 #if !UCONFIG_NO_LEGACY_CONVERSION
-static void TestJitterbug1293(){
+static void TestJitterbug1293(void){
     static const UChar src[] = {0x30DE, 0x30A4, 0x5E83, 0x544A, 0x30BF, 0x30A4, 0x30D7,0x000};
     char target[256];
     UErrorCode status = U_ZERO_ERROR;
@@ -5495,7 +5495,7 @@ static void TestJitterbug1293(){
 }
 #endif
 
-static void TestJB5275_1(){
+static void TestJB5275_1(void){
 
     static const char* data = "\x3B\xB3\x0A" /* Easy characters */
                                 "\xC0\xE9\xBF\xE9\xE8\xD8\x0A" /* Gurmukhi test */
@@ -5543,7 +5543,7 @@ static void TestJB5275_1(){
     ucnv_close(conv);
 }
 
-static void TestJB5275(){
+static void TestJB5275(void){
     static const char* data = 
     /* "\xEF\x42\xEF\x41\xA4\xD5\xE5\xB3\xEA\x0A"  unsupported sequence \xEF\x41 */
     /* "\xEF\x42\xEF\x41\xD4\xDA\xB3\xE8\xEA\x0A"  unsupported sequence \xEF\x41  */
@@ -5592,7 +5592,7 @@ static void TestJB5275(){
 }
 
 static void
-TestIsFixedWidth() {
+TestIsFixedWidth(void) {
     UErrorCode status = U_ZERO_ERROR;
     UConverter *cnv = NULL;
     int32_t i;
diff --git a/icu4c/source/test/cintltst/reapits.c b/icu4c/source/test/cintltst/reapits.c
index 3d657968e9fa..502d36aadd3d 100644
--- a/icu4c/source/test/cintltst/reapits.c
+++ b/icu4c/source/test/cintltst/reapits.c
@@ -2274,7 +2274,7 @@ static UBool U_CALLCONV MatchCallback(const void *context, int32_t steps) {
     return false;
 }
 
-static void TestBug10815() {
+static void TestBug10815(void) {
   /* Bug 10815:   uregex_findNext() does not set U_REGEX_STOPPED_BY_CALLER 
    *              when the callback function specified by uregex_setMatchCallback() returns false
    */
diff --git a/icu4c/source/test/cintltst/spooftest.c b/icu4c/source/test/cintltst/spooftest.c
index 64562bcbc476..9f59051e4a83 100644
--- a/icu4c/source/test/cintltst/spooftest.c
+++ b/icu4c/source/test/cintltst/spooftest.c
@@ -122,7 +122,7 @@ const char goodLatinUTF8[]    = {0x75, 0x77, 0};
 
 // Test open from source rules.
 // Run this in isolation to verify initialization.
-static void TestOpenFromSource() {
+static void TestOpenFromSource(void) {
     // No TEST_SETUP because that calls uspoof_open().
     UErrorCode status = U_ZERO_ERROR;
     const char *dataSrcDir;
diff --git a/icu4c/source/test/cintltst/spreptst.c b/icu4c/source/test/cintltst/spreptst.c
index 73e4fe3dc466..be374263ebcb 100644
--- a/icu4c/source/test/cintltst/spreptst.c
+++ b/icu4c/source/test/cintltst/spreptst.c
@@ -597,7 +597,7 @@ Test_nfs4_cs_prep(void){
 
 
 
-static void TestBEAMWarning(){
+static void TestBEAMWarning(void){
     UErrorCode status = U_ZERO_ERROR;
     UParseError parseError;
     UStringPrepProfile* profile = NULL;
diff --git a/icu4c/source/test/cintltst/stdnmtst.c b/icu4c/source/test/cintltst/stdnmtst.c
index 818478107abe..c59ee26d3f3b 100644
--- a/icu4c/source/test/cintltst/stdnmtst.c
+++ b/icu4c/source/test/cintltst/stdnmtst.c
@@ -55,7 +55,7 @@ static int dotestname(const char *name, const char *standard, const char *expect
     return res;
 }
 
-static void TestStandardName()
+static void TestStandardName(void)
 {
     int res = 1;
 
@@ -139,7 +139,7 @@ static int dotestconv(const char *name, const char *standard, const char *expect
     return res;
 }
 
-static void TestCanonicalName()
+static void TestCanonicalName(void)
 {
     /* Test for some expected results. */
 
@@ -267,7 +267,7 @@ static UBool doTestUCharNames(const char *name, const char *standard, const char
     return 1;
 }
 
-static void TestStandardNames()
+static void TestStandardNames(void)
 {
     static const char *asciiIANA[] = {
         "ANSI_X3.4-1968",
diff --git a/icu4c/source/test/cintltst/tracetst.c b/icu4c/source/test/cintltst/tracetst.c
index 9e43b7d003cb..e456d68d1e4f 100644
--- a/icu4c/source/test/cintltst/tracetst.c
+++ b/icu4c/source/test/cintltst/tracetst.c
@@ -209,7 +209,7 @@ static void pseudo_ucnv_close(UConverter * cnv)
 /*
  *   TestTraceAPI
  */
-static void TestTraceAPI() {
+static void TestTraceAPI(void) {
 
 
     UTraceEntry   *originalTEntryFunc;
diff --git a/icu4c/source/test/cintltst/ucnvseltst.c b/icu4c/source/test/cintltst/ucnvseltst.c
index ed45ccd5088a..b4f48ae05667 100644
--- a/icu4c/source/test/cintltst/ucnvseltst.c
+++ b/icu4c/source/test/cintltst/ucnvseltst.c
@@ -46,7 +46,7 @@ static const char **gAvailableNames = NULL;
 static int32_t gCountAvailable = 0;
 
 static UBool
-getAvailableNames() {
+getAvailableNames(void) {
   int32_t i;
   if (gAvailableNames != NULL) {
     return true;
@@ -69,7 +69,7 @@ getAvailableNames() {
 }
 
 static void
-releaseAvailableNames() {
+releaseAvailableNames(void) {
   uprv_free((void *)gAvailableNames);
   gAvailableNames = NULL;
   gCountAvailable = 0;
@@ -377,7 +377,7 @@ serializeAndUnserialize(UConverterSelector *sel, char **buffer, UErrorCode *stat
   return sel;
 }
 
-static void TestSelector()
+static void TestSelector(void)
 {
   TestText text;
   USet* excluded_sets[3] = { NULL };
@@ -507,7 +507,7 @@ static void TestSelector()
 }
 
 /* Improve code coverage of UPropsVectors */
-static void TestUPropsVector() {
+static void TestUPropsVector(void) {
     UErrorCode errorCode = U_ILLEGAL_ARGUMENT_ERROR;
     UPropsVectors *pv = upvec_open(100, &errorCode);
     if (pv != NULL) {
diff --git a/icu4c/source/test/cintltst/udatatst.c b/icu4c/source/test/cintltst/udatatst.c
index dc3e8463e58f..0a10ddd0b5f6 100644
--- a/icu4c/source/test/cintltst/udatatst.c
+++ b/icu4c/source/test/cintltst/udatatst.c
@@ -117,7 +117,7 @@ static void lots_of_mallocs()
 #endif
 
 #if !UCONFIG_NO_FILE_IO && !UCONFIG_NO_LEGACY_CONVERSION
-static void TestUDataOpen(){
+static void TestUDataOpen(void){
     UDataMemory *result;
     UErrorCode status=U_ZERO_ERROR;
     const char* memMap[][2]={
@@ -442,7 +442,7 @@ static const ICU_COMMON_Data_Header gEmptyHeader = {
 };
 
 
-static void TestUDataSetAppData(){
+static void TestUDataSetAppData(void){
 /*    UDataMemory      *dataItem;*/
 
     UErrorCode        status=U_ZERO_ERROR;
@@ -497,7 +497,7 @@ static void TestUDataSetAppData(){
         dataItem = udata_open("appData1", "res", "te_IN", &status); **/
 }
 
-static char *safeGetICUDataDirectory() {
+static char *safeGetICUDataDirectory(void) {
     const char *dataDir = u_getDataDirectory();  /* Returned string vanashes with u_cleanup */
     char *retStr = NULL;
     if (dataDir != NULL) {
@@ -507,7 +507,7 @@ static char *safeGetICUDataDirectory() {
     return retStr;
 }
     
-static void TestUDataFileAccess(){
+static void TestUDataFileAccess(void){
     UErrorCode status;
     char            *icuDataDir;
     icuDataDir = safeGetICUDataDirectory();   /* save icu data dir, so we can put it back
@@ -644,7 +644,7 @@ isAcceptable3(void *context,
 }
 
 #if !UCONFIG_NO_FILE_IO && !UCONFIG_NO_LEGACY_CONVERSION
-static void TestUDataOpenChoiceDemo1() {
+static void TestUDataOpenChoiceDemo1(void) {
     UDataMemory *result;
     UErrorCode status=U_ZERO_ERROR;
 
@@ -744,7 +744,7 @@ isAcceptable(void *context,
 
 /* This test checks to see if the isAcceptable function is being called correctly. */
 
-static void TestUDataOpenChoiceDemo2() {
+static void TestUDataOpenChoiceDemo2(void) {
     UDataMemory *result;
     UErrorCode status=U_ZERO_ERROR;
     int i;
@@ -791,7 +791,7 @@ static void TestUDataOpenChoiceDemo2() {
     }
 }
 
-static void TestUDataGetInfo() {
+static void TestUDataGetInfo(void) {
 
     UDataMemory *result;
     /* UDataInfo cf. udata.h */
@@ -868,7 +868,7 @@ static void TestUDataGetInfo() {
     udata_close(result);
 }
 
-static void TestUDataGetMemory() {
+static void TestUDataGetMemory(void) {
 
     UDataMemory *result;
     const int32_t *table=NULL;
@@ -916,7 +916,7 @@ static void TestUDataGetMemory() {
 
 }
 
-static void TestErrorConditions(){
+static void TestErrorConditions(void){
 
     UDataMemory *result=NULL;
     UErrorCode status=U_ZERO_ERROR;
@@ -1033,7 +1033,7 @@ static void TestErrorConditions(){
 }
 
 /* Test whether apps and ICU can each have their own root.res */
-static void TestAppData()
+static void TestAppData(void)
 {
     UResourceBundle *icu, *app;
     UResourceBundle *tmp = NULL;
@@ -1121,7 +1121,7 @@ static void TestAppData()
 }
 #endif
 
-static void TestICUDataName()
+static void TestICUDataName(void)
 {
     UVersionInfo icuVersion;
     char expectDataName[20];
@@ -1631,7 +1631,7 @@ printErrorToString(void *context, const char *fmt, va_list args) {
 
 #if !UCONFIG_NO_FILE_IO && !UCONFIG_NO_LEGACY_CONVERSION
 static void
-TestSwapData() {
+TestSwapData(void) {
     char name[100];
     UDataSwapper *ds;
     UDataMemory *pData;
@@ -1747,7 +1747,7 @@ TestSwapData() {
 }
 #endif
 
-static void PointerTableOfContents() {
+static void PointerTableOfContents(void) {
     UDataMemory      *dataItem;
     UErrorCode        status=U_ZERO_ERROR;
        
diff --git a/icu4c/source/test/cintltst/udatpg_test.c b/icu4c/source/test/cintltst/udatpg_test.c
index a28ab2b9d793..7f01d5d0b946 100644
--- a/icu4c/source/test/cintltst/udatpg_test.c
+++ b/icu4c/source/test/cintltst/udatpg_test.c
@@ -87,7 +87,7 @@ static const UChar sampleFormatted[] = {0x31, 0x30, 0x20, 0x6A, 0x75, 0x69, 0x6C
 static const UChar skeleton[]= {0x4d, 0x4d, 0x4d, 0x64, 0};  /* MMMd */
 static const UChar timeZoneGMT[] = { 0x0047, 0x004d, 0x0054, 0x0000 };  /* "GMT" */
 
-static void TestOpenClose() {
+static void TestOpenClose(void) {
     UErrorCode errorCode=U_ZERO_ERROR;
     UDateTimePatternGenerator *dtpg, *dtpg2;
     const UChar *s;
@@ -143,7 +143,7 @@ static const AppendItemNameData appendItemNameData[] = { /* for Finnish */
     { UDATPG_FIELD_COUNT,   {0}        }  /* terminator */
 };
 
-static void TestUsage() {
+static void TestUsage(void) {
     UErrorCode errorCode=U_ZERO_ERROR;
     UDateTimePatternGenerator *dtpg;
     const AppendItemNameData * appItemNameDataPtr;
@@ -254,7 +254,7 @@ static void TestUsage() {
     udatpg_close(dtpg);
 }
 
-static void TestBuilder() {
+static void TestBuilder(void) {
     UErrorCode errorCode=U_ZERO_ERROR;
     UDateTimePatternGenerator *dtpg;
     UDateTimePatternConflict conflict;
@@ -409,7 +409,7 @@ static const UChar patn_hpmm_a[]  = u"h.mm\u202Fa";
 static const UChar patn_Hpmm[]    = u"H.mm";
 static const UChar patn_hhpmm_a[] = u"hh.mm\u202Fa";
 
-static void TestOptions() {
+static void TestOptions(void) {
     const DTPtnGenOptionsData testData[] = {
         /*loc   skel       options                       expectedPattern */
         { "en", skel_Hmm,  UDATPG_MATCH_NO_OPTIONS,        patn_HHcmm   },
@@ -461,7 +461,7 @@ typedef struct FieldDisplayNameData {
 } FieldDisplayNameData;
 enum { kFieldDisplayNameMax = 32, kFieldDisplayNameBytesMax  = 64};
 
-static void TestGetFieldDisplayNames() {
+static void TestGetFieldDisplayNames(void) {
     const FieldDisplayNameData testData[] = {
         /*loc      field                              width               expectedName */
         { "de",    UDATPG_QUARTER_FIELD,              UDATPG_WIDE,        "Quartal" },
@@ -528,7 +528,7 @@ typedef struct HourCycleData {
     UDateFormatHourCycle   expected;
 } HourCycleData;
 
-static void TestGetDefaultHourCycle() {
+static void TestGetDefaultHourCycle(void) {
     const HourCycleData testData[] = {
         /*loc      expected */
         { "ar_EG",    UDAT_HOUR_CYCLE_12 },
@@ -566,7 +566,7 @@ static void TestGetDefaultHourCycle() {
 }
 
 // Ensure that calling udatpg_getDefaultHourCycle on an empty instance doesn't call UPRV_UNREACHABLE_EXIT/abort.
-static void TestGetDefaultHourCycleOnEmptyInstance() {
+static void TestGetDefaultHourCycleOnEmptyInstance(void) {
     UErrorCode status = U_ZERO_ERROR;
     UDateTimePatternGenerator * dtpgen = udatpg_openEmpty(&status);
 
diff --git a/icu4c/source/test/cintltst/uformattedvaluetst.c b/icu4c/source/test/cintltst/uformattedvaluetst.c
index 392d76e3404a..edc745a3a888 100644
--- a/icu4c/source/test/cintltst/uformattedvaluetst.c
+++ b/icu4c/source/test/cintltst/uformattedvaluetst.c
@@ -37,7 +37,7 @@ void addUFormattedValueTest(TestNode** root) {
 }
 
 
-static void TestBasic() {
+static void TestBasic(void) {
     UErrorCode status = U_ZERO_ERROR;
     UConstrainedFieldPosition* ucfpos = ucfpos_open(&status);
     assertSuccess("opening ucfpos", &status);
@@ -56,7 +56,7 @@ static void TestBasic() {
     ucfpos_close(ucfpos);
 }
 
-void TestSetters() {
+void TestSetters(void) {
     UErrorCode status = U_ZERO_ERROR;
     UConstrainedFieldPosition* ucfpos = ucfpos_open(&status);
     assertSuccess("opening ucfpos", &status);
diff --git a/icu4c/source/test/cintltst/ulistfmttest.c b/icu4c/source/test/cintltst/ulistfmttest.c
index b0a1f6bc3496..0696456acdcd 100644
--- a/icu4c/source/test/cintltst/ulistfmttest.c
+++ b/icu4c/source/test/cintltst/ulistfmttest.c
@@ -69,7 +69,7 @@ enum {
   kBBufMax = 256
 };
 
-static void TestUListFmt() {
+static void TestUListFmt(void) {
     const ListFmtTestEntry * lftep;
     for (lftep = listFmtTestEntries; lftep->locale != NULL ; lftep++ ) {
         UErrorCode status = U_ZERO_ERROR;
@@ -135,7 +135,7 @@ static void TestUListFmt() {
     }
 }
 
-static void TestUListFmtToValue() {
+static void TestUListFmtToValue(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UListFormatter* fmt = ulistfmt_open("en", &ec);
     UFormattedList* fl = ulistfmt_openResult(&ec);
@@ -216,7 +216,7 @@ static void TestUListFmtToValue() {
     ulistfmt_closeResult(fl);
 }
 
-static void TestUListOpenStyled() {
+static void TestUListOpenStyled(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UListFormatter* fmt = ulistfmt_openForType("en", ULISTFMT_TYPE_OR, ULISTFMT_WIDTH_SHORT, &ec);
     UFormattedList* fl = ulistfmt_openResult(&ec);
@@ -256,7 +256,7 @@ static void TestUListOpenStyled() {
 
 #include <stdio.h>
 
-static void TestUList21871_A() {
+static void TestUList21871_A(void) {
     UErrorCode status = U_ZERO_ERROR;
     UListFormatter *fmt = ulistfmt_openForType("en", ULISTFMT_TYPE_AND, ULISTFMT_WIDTH_WIDE, &status);
     assertSuccess("ulistfmt_openForType", &status);
@@ -304,7 +304,7 @@ static void TestUList21871_A() {
     ulistfmt_close(fmt);
 }
 
-static void TestUList21871_B() {
+static void TestUList21871_B(void) {
     UErrorCode status = U_ZERO_ERROR;
     UListFormatter *fmt = ulistfmt_openForType("en", ULISTFMT_TYPE_AND, ULISTFMT_WIDTH_WIDE, &status);
     assertSuccess("ulistfmt_openForType", &status);
diff --git a/icu4c/source/test/cintltst/ulocaletst.c b/icu4c/source/test/cintltst/ulocaletst.c
index 6d3fae84c72d..a4d58e370e70 100644
--- a/icu4c/source/test/cintltst/ulocaletst.c
+++ b/icu4c/source/test/cintltst/ulocaletst.c
@@ -35,7 +35,7 @@ static const char* const rawData[][6] = {
     { "x-klingon", "Latn", "ZX", "", "x-klingon_Latn_ZX.utf32be@special", "x-klingon_Latn_ZX.utf32be@special"},
 };
 
-static void TestBasicGetters() {
+static void TestBasicGetters(void) {
     for (int32_t i = 0; i < UPRV_LENGTHOF(rawData); i++)  {
         UErrorCode status = U_ZERO_ERROR;
         ULocale* l = ulocale_openForLocaleID(rawData[i][NAME], -1, &status);
@@ -64,7 +64,7 @@ static void VerifyMatch(const char* localeID, const char* tag) {
     }
 }
 
-static void TestForLanguageTag() {
+static void TestForLanguageTag(void) {
     VerifyMatch("en_US", "en-US");
     VerifyMatch("en_GB_OXENDICT", "en-GB-oed");
     VerifyMatch("af@calendar=coptic;t=ar-i0-handwrit;x=foo", "af-t-ar-i0-handwrit-u-ca-coptic-x-foo");
@@ -88,7 +88,7 @@ static void TestForLanguageTag() {
     VerifyMatch("__1994_BISKE_ROZAJ@x=private", "und-1994-biske-rozaj-x-private");
 }
 
-static void TestGetKeywords() {
+static void TestGetKeywords(void) {
     UErrorCode status = U_ZERO_ERROR;
     ULocale* l = ulocale_openForLocaleID("de@calendar=buddhist;collation=phonebook", -1, &status);
     if (assertSuccess(WHERE "ulocale_openForLocaleID()", &status)) {
@@ -117,7 +117,7 @@ static void TestGetKeywords() {
     }
 }
 
-static void TestGetKeywordsEmpty() {
+static void TestGetKeywordsEmpty(void) {
     UErrorCode status = U_ZERO_ERROR;
     ULocale* l = ulocale_openForLocaleID("de", -1, &status);
     if (assertSuccess(WHERE "ulocale_openForLocaleID()", &status)) {
@@ -130,7 +130,7 @@ static void TestGetKeywordsEmpty() {
     }
 }
 
-static void TestGetKeywordsWithPrivateUse() {
+static void TestGetKeywordsWithPrivateUse(void) {
     UErrorCode status = U_ZERO_ERROR;
     ULocale* l = ulocale_openForLanguageTag("en-US-u-ca-gregory-x-foo", -1, &status);
     if (assertSuccess(WHERE "ulocale_openForLanguageTag()", &status)) {
@@ -157,7 +157,7 @@ static void TestGetKeywordsWithPrivateUse() {
     }
 }
 
-static void TestGetUnicodeKeywords() {
+static void TestGetUnicodeKeywords(void) {
     UErrorCode status = U_ZERO_ERROR;
     ULocale* l = ulocale_openForLocaleID("de@calendar=buddhist;collation=phonebook", -1, &status);
     if (assertSuccess(WHERE "ulocale_openForLocaleID()", &status)) {
@@ -186,7 +186,7 @@ static void TestGetUnicodeKeywords() {
     }
 }
 
-static void TestGetUnicodeKeywordsEmpty() {
+static void TestGetUnicodeKeywordsEmpty(void) {
     UErrorCode status = U_ZERO_ERROR;
     ULocale* l = ulocale_openForLocaleID("de", -1, &status);
     if (assertSuccess(WHERE "ulocale_openForLocaleID()", &status)) {
@@ -199,7 +199,7 @@ static void TestGetUnicodeKeywordsEmpty() {
     }
 }
 
-static void TestGetUnicodeKeywordsWithPrivateUse() {
+static void TestGetUnicodeKeywordsWithPrivateUse(void) {
     UErrorCode status = U_ZERO_ERROR;
     ULocale* l = ulocale_openForLanguageTag("en-US-u-ca-gregory-x-foo", -1, &status);
     if (assertSuccess(WHERE "ulocale_openForLanguageTag()", &status)) {
@@ -220,7 +220,7 @@ static void TestGetUnicodeKeywordsWithPrivateUse() {
     }
 }
 
-static void TestGetKeywordValue() {
+static void TestGetKeywordValue(void) {
     UErrorCode status = U_ZERO_ERROR;
     ULocale* l = ulocale_openForLanguageTag("fa-u-nu-thai", -1, &status);
     if (assertSuccess(WHERE "ulocale_openForLanguageTag()", &status)) {
@@ -242,7 +242,7 @@ static void TestGetKeywordValue() {
     }
 }
 
-static void TestGetUnicodeKeywordValue() {
+static void TestGetUnicodeKeywordValue(void) {
     UErrorCode status = U_ZERO_ERROR;
     ULocale* l = ulocale_openForLanguageTag("fa-u-nu-thai", -1, &status);
     if (assertSuccess(WHERE "ulocale_openForLanguageTag()", &status)) {
diff --git a/icu4c/source/test/cintltst/ulocbuildertst.c b/icu4c/source/test/cintltst/ulocbuildertst.c
index af811277b4bb..d128d074b6e0 100644
--- a/icu4c/source/test/cintltst/ulocbuildertst.c
+++ b/icu4c/source/test/cintltst/ulocbuildertst.c
@@ -40,7 +40,7 @@ static void Verify(ULocaleBuilder* bld, const char* expected, const char* msg) {
     }
 }
 
-static void TestLocaleBuilder() {
+static void TestLocaleBuilder(void) {
 
     // The following test data are copy from
     // icu4j/main/core/src/test/java/com/ibm/icu/dev/test/util/LocaleBuilderTest.java
@@ -274,7 +274,7 @@ static void TestLocaleBuilder() {
     ulocbld_close(bld);
 }
 
-static void TestLocaleBuilderBasic() {
+static void TestLocaleBuilderBasic(void) {
     ULocaleBuilder* bld = ulocbld_open();
     ulocbld_setLanguage(bld, "zh", -1);
     Verify(bld, "zh", "ulocbld_setLanguage('zh') got Error: %s\n");
@@ -331,7 +331,7 @@ static void TestLocaleBuilderBasic() {
     ulocbld_close(bld);
 }
 
-static void TestLocaleBuilderBasicWithExtensionsOnDefaultLocale() {
+static void TestLocaleBuilderBasicWithExtensionsOnDefaultLocale(void) {
     // Change the default locale to one with extension tags.
     UErrorCode status = U_ZERO_ERROR;
     char originalDefault[ULOC_FULLNAME_CAPACITY];
@@ -351,7 +351,7 @@ static void TestLocaleBuilderBasicWithExtensionsOnDefaultLocale() {
     }
 }
 
-static void TestSetLanguageWellFormed() {
+static void TestSetLanguageWellFormed(void) {
     // http://www.unicode.org/reports/tr35/tr35.html#unicode_language_subtag
     // unicode_language_subtag = alpha{2,3} | alpha{5,8};
     // ICUTC decided also support alpha{4}
@@ -408,7 +408,7 @@ static void TestSetLanguageWellFormed() {
     }
 }
 
-static void TestSetLanguageIllFormed() {
+static void TestSetLanguageIllFormed(void) {
     static const char* illFormed[] = {
         "a",
         "z",
@@ -477,7 +477,7 @@ static void TestSetLanguageIllFormed() {
     }
 }
 
-static void TestSetScriptWellFormed() {
+static void TestSetScriptWellFormed(void) {
     // http://www.unicode.org/reports/tr35/tr35.html#unicode_script_subtag
     // unicode_script_subtag = alpha{4} ;
     static const char* wellFormedScripts[] = {
@@ -508,7 +508,7 @@ static void TestSetScriptWellFormed() {
         ulocbld_close(bld);
     }
 }
-static void TestSetScriptIllFormed() {
+static void TestSetScriptIllFormed(void) {
     static const char* illFormed[] = {
         "a",
         "z",
@@ -580,7 +580,7 @@ static void TestSetScriptIllFormed() {
     }
 }
 
-static void TestSetRegionWellFormed() {
+static void TestSetRegionWellFormed(void) {
     // http://www.unicode.org/reports/tr35/tr35.html#unicode_region_subtag
     // unicode_region_subtag = (alpha{2} | digit{3})
     static const char* wellFormedRegions[] = {
@@ -613,7 +613,7 @@ static void TestSetRegionWellFormed() {
     }
 }
 
-static void TestSetRegionIllFormed() {
+static void TestSetRegionIllFormed(void) {
     static const char* illFormed[] = {
         "a",
         "z",
@@ -685,7 +685,7 @@ static void TestSetRegionIllFormed() {
     }
 }
 
-static void TestSetVariantWellFormed() {
+static void TestSetVariantWellFormed(void) {
     // http://www.unicode.org/reports/tr35/tr35.html#unicode_variant_subtag
     // (sep unicode_variant_subtag)*
     // unicode_variant_subtag = (alphanum{5,8} | digit alphanum{3}) ;
@@ -763,7 +763,7 @@ static void TestSetVariantWellFormed() {
     }
 }
 
-static void TestSetVariantIllFormed() {
+static void TestSetVariantIllFormed(void) {
     static const char* illFormed[] = {
         "a",
         "z",
@@ -871,7 +871,7 @@ static void TestSetVariantIllFormed() {
     }
 }
 
-static void TestSetUnicodeLocaleKeywordWellFormed() {
+static void TestSetUnicodeLocaleKeywordWellFormed(void) {
     // http://www.unicode.org/reports/tr35/tr35.html#unicode_locale_extensions
     // keyword = key (sep type)? ;
     // key = alphanum alpha ;
@@ -901,7 +901,7 @@ static void TestSetUnicodeLocaleKeywordWellFormed() {
     }
 }
 
-static void TestSetUnicodeLocaleKeywordIllFormedKey() {
+static void TestSetUnicodeLocaleKeywordIllFormedKey(void) {
     static const char* illFormed[] = {
         "34",
         "ab-cde",
@@ -925,7 +925,7 @@ static void TestSetUnicodeLocaleKeywordIllFormedKey() {
     }
 }
 
-static void TestSetUnicodeLocaleKeywordIllFormedValue() {
+static void TestSetUnicodeLocaleKeywordIllFormedValue(void) {
     static const char* illFormed[] = {
         "34",
         "ab-",
@@ -952,7 +952,7 @@ static void TestSetUnicodeLocaleKeywordIllFormedValue() {
     ulocbld_close(bld);
 }
 
-static void TestAddRemoveUnicodeLocaleAttribute() {
+static void TestAddRemoveUnicodeLocaleAttribute(void) {
     ULocaleBuilder* bld = ulocbld_open();
     ulocbld_setLanguage(bld, "fr", -1);
     ulocbld_addUnicodeLocaleAttribute(bld, "abc", -1);
@@ -1050,7 +1050,7 @@ static void TestAddRemoveUnicodeLocaleAttribute() {
     ulocbld_close(bld);
 }
 
-static void TestAddRemoveUnicodeLocaleAttributeWellFormed() {
+static void TestAddRemoveUnicodeLocaleAttributeWellFormed(void) {
     // http://www.unicode.org/reports/tr35/tr35.html#unicode_locale_extensions
     // attribute = alphanum{3,8} ;
     static const char* wellFormedAttributes[] = {
@@ -1126,7 +1126,7 @@ static void TestAddRemoveUnicodeLocaleAttributeWellFormed() {
     ulocbld_close(bld);
 }
 
-static void TestAddUnicodeLocaleAttributeIllFormed() {
+static void TestAddUnicodeLocaleAttributeIllFormed(void) {
     static const char* illFormed[] = {
         "aa",
         "34",
@@ -1154,7 +1154,7 @@ static void TestAddUnicodeLocaleAttributeIllFormed() {
     }
 }
 
-static void TestSetExtensionU() {
+static void TestSetExtensionU(void) {
     ULocaleBuilder* bld = ulocbld_open();
     ulocbld_setLanguage(bld, "zhABC", 2);
     Verify(bld, "zh",
@@ -1195,7 +1195,7 @@ static void TestSetExtensionU() {
     ulocbld_close(bld);
 }
 
-static void TestSetExtensionValidateUWellFormed() {
+static void TestSetExtensionValidateUWellFormed(void) {
     static const char* wellFormedExtensions[] = {
         // keyword
         //   keyword = key (sep type)? ;
@@ -1244,7 +1244,7 @@ static void TestSetExtensionValidateUWellFormed() {
     }
 }
 
-static void TestSetExtensionValidateUIllFormed() {
+static void TestSetExtensionValidateUIllFormed(void) {
     static const char* illFormed[] = {
         // bad key
         "-",
@@ -1287,7 +1287,7 @@ static void TestSetExtensionValidateUIllFormed() {
     }
 }
 
-static void TestSetExtensionT() {
+static void TestSetExtensionT(void) {
     ULocaleBuilder* bld = ulocbld_open();
     ulocbld_setLanguage(bld, "fr", 2);
     Verify(bld, "fr",
@@ -1324,7 +1324,7 @@ static void TestSetExtensionT() {
     ulocbld_close(bld);
 }
 
-static void TestSetExtensionValidateTWellFormed() {
+static void TestSetExtensionValidateTWellFormed(void) {
     // ((sep tlang (sep tfield)*) | (sep tfield)+)
     static const char* wellFormedExtensions[] = {
         // tlang
@@ -1387,7 +1387,7 @@ static void TestSetExtensionValidateTWellFormed() {
     ulocbld_close(bld);
 }
 
-static void TestSetExtensionValidateTIllFormed() {
+static void TestSetExtensionValidateTIllFormed(void) {
     static const char* illFormed[] = {
         "a",
         "a-",
@@ -1437,7 +1437,7 @@ static void TestSetExtensionValidateTIllFormed() {
     ulocbld_close(bld);
 }
 
-static void TestSetExtensionPU() {
+static void TestSetExtensionPU(void) {
     ULocaleBuilder* bld = ulocbld_open();
     ulocbld_setLanguage(bld, "ar123", 2);
     Verify(bld, "ar",
@@ -1465,7 +1465,7 @@ static void TestSetExtensionPU() {
     ulocbld_close(bld);
 }
 
-static void TestSetExtensionValidatePUWellFormed() {
+static void TestSetExtensionValidatePUWellFormed(void) {
     // ((sep tlang (sep tfield)*) | (sep tfield)+)
     static const char* wellFormedExtensions[] = {
         "a",  // Short subtag
@@ -1499,7 +1499,7 @@ static void TestSetExtensionValidatePUWellFormed() {
     ulocbld_close(bld);
 }
 
-static void TestSetExtensionValidatePUIllFormed() {
+static void TestSetExtensionValidatePUIllFormed(void) {
     static const char* illFormed[] = {
         "123456789",  // Too long
         "abcdefghi",  // Too long
@@ -1522,7 +1522,7 @@ static void TestSetExtensionValidatePUIllFormed() {
     ulocbld_close(bld);
 }
 
-static void TestSetExtensionOthers() {
+static void TestSetExtensionOthers(void) {
     ULocaleBuilder* bld = ulocbld_open();
     ulocbld_setLanguage(bld, "fr", -1);
     Verify(bld, "fr",
@@ -1562,7 +1562,7 @@ static void TestSetExtensionOthers() {
     ulocbld_close(bld);
 }
 
-static void TestSetExtensionValidateOthersWellFormed() {
+static void TestSetExtensionValidateOthersWellFormed(void) {
     static const char* wellFormedExtensions[] = {
         "ab",
         "abc",
@@ -1616,7 +1616,7 @@ static void TestSetExtensionValidateOthersWellFormed() {
     ulocbld_close(bld);
 }
 
-static void TestSetExtensionValidateOthersIllFormed() {
+static void TestSetExtensionValidateOthersIllFormed(void) {
     static const char* illFormed[] = {
         "0",  // Too short
         "a",  // Too short
@@ -1645,7 +1645,7 @@ static void TestSetExtensionValidateOthersIllFormed() {
     ulocbld_close(bld);
 }
 
-static void TestSetLocale() {
+static void TestSetLocale(void) {
     ULocaleBuilder* bld1 = ulocbld_open();
     ULocaleBuilder* bld2 = ulocbld_open();
     UErrorCode status = U_ZERO_ERROR;
@@ -1679,7 +1679,7 @@ static void TestSetLocale() {
     ulocbld_close(bld2);
 }
 
-static void TestBuildULocale() {
+static void TestBuildULocale(void) {
     ULocaleBuilder* bld1 = ulocbld_open();
     UErrorCode status = U_ZERO_ERROR;
 
@@ -1735,7 +1735,7 @@ static void TestBuildULocale() {
 }
 
 
-static void TestPosixCases() {
+static void TestPosixCases(void) {
     UErrorCode status = U_ZERO_ERROR;
     ULocaleBuilder* bld = ulocbld_open();
     ulocbld_setLanguage(bld, "en", -1);
diff --git a/icu4c/source/test/cintltst/unumberformattertst.c b/icu4c/source/test/cintltst/unumberformattertst.c
index 93f039f81624..94c2c7e2e76e 100644
--- a/icu4c/source/test/cintltst/unumberformattertst.c
+++ b/icu4c/source/test/cintltst/unumberformattertst.c
@@ -63,7 +63,7 @@ void addUNumberFormatterTest(TestNode** root) {
 
 #define CAPACITY 30
 
-static void TestSkeletonFormatToString() {
+static void TestSkeletonFormatToString(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UChar buffer[CAPACITY];
     UFormattedNumber* result = NULL;
@@ -104,7 +104,7 @@ static void TestSkeletonFormatToString() {
 }
 
 
-static void TestSkeletonFormatToFields() {
+static void TestSkeletonFormatToFields(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UFieldPositionIterator* ufpositer = NULL;
 
@@ -188,7 +188,7 @@ static void TestSkeletonFormatToFields() {
 }
 
 
-static void TestExampleCode() {
+static void TestExampleCode(void) {
     // This is the example code given in unumberformatter.h.
 
     // Setup:
@@ -219,7 +219,7 @@ static void TestExampleCode() {
 }
 
 
-static void TestSimpleNumberFormatterExample() {
+static void TestSimpleNumberFormatterExample(void) {
     // This is the example in usimplenumberformatter.h
     UErrorCode ec = U_ZERO_ERROR;
     USimpleNumberFormatter* uformatter = usnumf_openForLocale("bn", &ec);
@@ -239,7 +239,7 @@ static void TestSimpleNumberFormatterExample() {
 }
 
 
-static void TestSimpleNumberFormatterFull() {
+static void TestSimpleNumberFormatterFull(void) {
     UErrorCode ec = U_ZERO_ERROR;
     USimpleNumberFormatter* uformatter = usnumf_openForLocaleAndGroupingStrategy("de-CH", UNUM_GROUPING_ON_ALIGNED, &ec);
     UFormattedNumber* uresult = unumf_openResult(&ec);
@@ -273,7 +273,7 @@ static void TestSimpleNumberFormatterFull() {
 }
 
 
-static void TestFormattedValue() {
+static void TestFormattedValue(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UNumberFormatter* uformatter = unumf_openForSkeletonAndLocale(
             u".00 compact-short", -1, "en", &ec);
@@ -306,7 +306,7 @@ static void TestFormattedValue() {
 }
 
 
-static void TestSkeletonParseError() {
+static void TestSkeletonParseError(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UNumberFormatter* uformatter;
     UParseError perror;
@@ -331,7 +331,7 @@ static void TestSkeletonParseError() {
 }
 
 
-static void TestToDecimalNumber() {
+static void TestToDecimalNumber(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UNumberFormatter* uformatter = unumf_openForSkeletonAndLocale(
         u"currency/USD",
@@ -359,7 +359,7 @@ static void TestToDecimalNumber() {
 }
 
 
-static void TestPerUnitInArabic() {
+static void TestPerUnitInArabic(void) {
     const char* simpleMeasureUnits[] = {
         "area-acre",
         "digital-bit",
@@ -445,7 +445,7 @@ static void TestPerUnitInArabic() {
 }
 
 
-static void Test21674_State() {
+static void Test21674_State(void) {
     UErrorCode status = U_ZERO_ERROR;
     UNumberFormatter* nf = NULL;
     UFormattedNumber* result = NULL;
diff --git a/icu4c/source/test/cintltst/unumberrangeformattertst.c b/icu4c/source/test/cintltst/unumberrangeformattertst.c
index ad1375c98381..6861538f4b5a 100644
--- a/icu4c/source/test/cintltst/unumberrangeformattertst.c
+++ b/icu4c/source/test/cintltst/unumberrangeformattertst.c
@@ -43,7 +43,7 @@ void addUNumberRangeFormatterTest(TestNode** root) {
 #define CAPACITY 30
 
 
-static void TestExampleCode() {
+static void TestExampleCode(void) {
     // This is the example code given in unumberrangeformatter.h.
 
     // Setup:
@@ -77,7 +77,7 @@ static void TestExampleCode() {
 }
 
 
-static void TestFormattedValue() {
+static void TestFormattedValue(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UNumberRangeFormatter* uformatter = unumrf_openForSkeletonWithCollapseAndIdentityFallback(
         u"K",
@@ -123,7 +123,7 @@ static void TestFormattedValue() {
 }
 
 
-static void TestSkeletonParseError() {
+static void TestSkeletonParseError(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UNumberRangeFormatter* uformatter;
     UParseError perror;
@@ -160,7 +160,7 @@ static void TestSkeletonParseError() {
 }
 
 
-static void TestGetDecimalNumbers() {
+static void TestGetDecimalNumbers(void) {
     UErrorCode ec = U_ZERO_ERROR;
     UNumberRangeFormatter* uformatter = unumrf_openForSkeletonWithCollapseAndIdentityFallback(
         u"currency/USD",
diff --git a/icu4c/source/test/cintltst/uregiontest.c b/icu4c/source/test/cintltst/uregiontest.c
index edb7aa39067c..0dd896817c0a 100644
--- a/icu4c/source/test/cintltst/uregiontest.c
+++ b/icu4c/source/test/cintltst/uregiontest.c
@@ -362,7 +362,7 @@ static KnownRegion knownRegions[] = {
     };
 
 
-static void TestKnownRegions() {
+static void TestKnownRegions(void) {
     const KnownRegion * rd;
     for (rd = knownRegions; rd->code != NULL ; rd++ ) {
         UErrorCode status = U_ZERO_ERROR;
@@ -389,7 +389,7 @@ static void TestKnownRegions() {
     }
 }
 
-static void TestGetContainedRegions() {
+static void TestGetContainedRegions(void) {
     const KnownRegion * rd;
     for (rd = knownRegions; rd->code != NULL ; rd++ ) {
         UErrorCode status = U_ZERO_ERROR;
@@ -418,7 +418,7 @@ static void TestGetContainedRegions() {
     }
 }
 
-static void TestGroupingChildren() {
+static void TestGroupingChildren(void) {
     const char* testGroupings[] = {
         "003", "021,013,029",
         "419", "013,029,005",
@@ -470,7 +470,7 @@ static void TestGroupingChildren() {
     }
 }
 
-static void TestGetContainedRegionsWithType() {
+static void TestGetContainedRegionsWithType(void) {
     const KnownRegion * rd;
     for (rd = knownRegions; rd->code != NULL ; rd++ ) {
         UErrorCode status = U_ZERO_ERROR;
@@ -499,7 +499,7 @@ static void TestGetContainedRegionsWithType() {
     }
 }
 
-static void TestGetContainingRegion() {        
+static void TestGetContainingRegion(void) {
     const KnownRegion * rd;
     for (rd = knownRegions; rd->code != NULL ; rd++ ) {
         UErrorCode status = U_ZERO_ERROR;
@@ -523,7 +523,7 @@ static void TestGetContainingRegion() {
     }
 }
 
-static void TestGetContainingRegionWithType() {        
+static void TestGetContainingRegionWithType(void) {
     const KnownRegion * rd;
     for (rd = knownRegions; rd->code != NULL ; rd++ ) {
         UErrorCode status = U_ZERO_ERROR;
@@ -563,7 +563,7 @@ static const char ** expectPrefRegionsTestData[] = {
     NULL
 };
 
-static void TestGetPreferredValues() {
+static void TestGetPreferredValues(void) {
     const char *** testDataPtr = expectPrefRegionsTestData;
     const char ** regionListPtr;
     while ( (regionListPtr = *testDataPtr++) != NULL ) {
@@ -600,7 +600,7 @@ static void TestGetPreferredValues() {
     }
 }
 
-static void TestContains() {
+static void TestContains(void) {
     const KnownRegion * rd;
     for (rd = knownRegions; rd->code != NULL ; rd++ ) {
         UErrorCode status = U_ZERO_ERROR;
diff --git a/icu4c/source/test/cintltst/usettest.c b/icu4c/source/test/cintltst/usettest.c
index c07e4073f9db..73f26a4fee71 100644
--- a/icu4c/source/test/cintltst/usettest.c
+++ b/icu4c/source/test/cintltst/usettest.c
@@ -54,7 +54,7 @@ addUSetTest(TestNode** root) {
  * Tests
  *------------------------------------------------------------------*/
 
-static void Testj2269() {
+static void Testj2269(void) {
   UErrorCode status = U_ZERO_ERROR;
   UChar a[4] = { 0x61, 0x62, 0x63, 0 };
   USet *s = uset_open(1, 0);
@@ -82,7 +82,7 @@ static const int32_t STR_ab_LEN = UPRV_LENGTHOF(STR_ab) - 1;
 /**
  * Basic API test for uset.x
  */
-static void TestAPI() {
+static void TestAPI(void) {
     USet* set;
     USet* set2;
     UErrorCode ec;
@@ -540,7 +540,7 @@ static void expectItems(const USet* set,
 }
 
 static void
-TestSerialized() {
+TestSerialized(void) {
     uint16_t buffer[1000];
     USerializedSet sset;
     USet *set;
@@ -583,7 +583,7 @@ TestSerialized() {
  * JB#3795.
  */
 static void
-TestNonInvariantPattern() {
+TestNonInvariantPattern(void) {
     UErrorCode ec = U_ZERO_ERROR;
     /* The critical part of this test is that the following pattern
        must contain a non-invariant character. */
@@ -608,14 +608,14 @@ static void TestBadPattern(void) {
     }
 }
 
-static USet *openIDSet() {
+static USet *openIDSet(void) {
     UErrorCode errorCode = U_ZERO_ERROR;
     U_STRING_DECL(pattern, "[:ID_Continue:]", 15);
     U_STRING_INIT(pattern, "[:ID_Continue:]", 15);
     return uset_openPattern(pattern, 15, &errorCode);
 }
 
-static void TestFreezable() {
+static void TestFreezable(void) {
     USet *idSet;
     USet *frozen;
     USet *thawed;
@@ -666,7 +666,7 @@ static void TestFreezable() {
     uset_close(thawed);
 }
 
-static void TestSpan() {
+static void TestSpan(void) {
     static const UChar s16[2]={ 0xe01, 0x3000 };
     static const char* s8="\xE0\xB8\x81\xE3\x80\x80";
 
diff --git a/icu4c/source/test/cintltst/utf16tst.c b/icu4c/source/test/cintltst/utf16tst.c
index 2394cbcef771..d6253f9c7ddc 100644
--- a/icu4c/source/test/cintltst/utf16tst.c
+++ b/icu4c/source/test/cintltst/utf16tst.c
@@ -67,7 +67,7 @@ addUTF16Test(TestNode** root)
     addTest(root, &TestSurrogate,               "utf16tst/TestSurrogate");
 }
 
-static void TestCodeUnitValues()
+static void TestCodeUnitValues(void)
 {
     static uint16_t codeunit[]={0x0000,0xe065,0x20ac,0xd7ff,0xd800,0xd841,0xd905,0xdbff,0xdc00,0xdc02,0xddee,0xdfff,0};
     
@@ -105,7 +105,7 @@ static void TestCodeUnitValues()
     }
 }
 
-static void TestCharLength()
+static void TestCharLength(void)
 {
     static uint32_t codepoint[]={
         1, 0x0061,
@@ -144,7 +144,7 @@ static void TestCharLength()
     }
 }
 
-static void TestGetChar()
+static void TestGetChar(void)
 {
     static UChar input[]={
     /*  code unit,*/
@@ -223,7 +223,7 @@ static void TestGetChar()
     }
 }
 
-static void TestNextPrevChar(){
+static void TestNextPrevChar(void){
 
     static UChar input[]={0x0061, 0xd800, 0xdc00, 0xdbff, 0xdfff, 0x0062, 0xd841, 0xd7ff, 0xd841, 0xdc41, 0xdc00, 0x0000};
     static UChar32 result[]={
@@ -398,7 +398,7 @@ static void TestNextPrevChar(){
 }
 
 /* keep this in sync with utf8tst.c's TestNulTerminated() */
-static void TestNulTerminated() {
+static void TestNulTerminated(void) {
     static const UChar input[]={
         /*  0 */  0x61,
         /*  1 */  0xd801, 0xdc01,
@@ -478,7 +478,7 @@ static void TestNulTerminated() {
     } while(c!=0);
 }
 
-static void TestFwdBack(){ 
+static void TestFwdBack(void){
     static UChar input[]={0x0061, 0xd800, 0xdc00, 0xdbff, 0xdfff, 0x0062, 0xd841, 0xd7ff, 0xd841, 0xdc41, 0xdc00, 0x0000};
     static uint16_t fwd_unsafe[] ={1, 3, 5, 6,  8, 10, 11, 12};
     static uint16_t fwd_safe[]   ={1, 3, 5, 6, 7, 8, 10, 11, 12};
@@ -643,7 +643,7 @@ static void TestFwdBack(){
     }
 }
 
-static void TestSetChar(){
+static void TestSetChar(void){
     static UChar input[]={0x0061, 0xd800, 0xdc00, 0xdbff, 0xdfff, 0x0062, 0xd841, 0xd7ff, 0xd841, 0xdc41, 0xdc00, 0x0000};
     static uint16_t start_unsafe[]={0, 1, 1, 3, 3, 5, 6, 7, 8, 8, 9, 11};
     static uint16_t start_safe[]  ={0, 1, 1, 3, 3, 5, 6, 7, 8, 8, 10, 11};
@@ -703,7 +703,7 @@ static void TestSetChar(){
     }
 }
 
-static void TestAppendChar(){
+static void TestAppendChar(void){
 #if !U_HIDE_OBSOLETE_UTF_OLD_H
     static UChar s[5]={0x0061, 0x0062, 0x0063, 0x0064, 0x0000};
     static uint32_t test[]={
@@ -803,7 +803,7 @@ static void TestAppendChar(){
 #endif
 }
 
-static void TestAppend() {
+static void TestAppend(void) {
     static const UChar32 codePoints[]={
         0x61, 0xdf, 0x901, 0x3040,
         0xac00, 0xd800, 0xdbff, 0xdcde,
@@ -864,7 +864,7 @@ static void TestAppend() {
     }
 }
 
-static void TestSurrogate(){
+static void TestSurrogate(void){
     static UChar32 s[] = {0x10000, 0x10ffff, 0x50000, 0x100000, 0x1abcd};
     int i = 0;
     while (i < 5) {
diff --git a/icu4c/source/test/cintltst/utf8tst.c b/icu4c/source/test/cintltst/utf8tst.c
index d0dd95c7b6b1..a5f3379fe028 100644
--- a/icu4c/source/test/cintltst/utf8tst.c
+++ b/icu4c/source/test/cintltst/utf8tst.c
@@ -123,7 +123,7 @@ addUTF8Test(TestNode** root)
     addTest(root, &TestSurrogates,              "utf8tst/TestSurrogates");
 }
 
-static void TestCodeUnitValues()
+static void TestCodeUnitValues(void)
 {
     static const uint8_t codeunit[]={0x00, 0x65, 0x7e, 0x7f, 0xc2, 0xc4, 0xf0, 0xf4, 0x80, 0x81, 0xbc, 0xbe,};
 
@@ -162,7 +162,7 @@ static void TestCodeUnitValues()
     }
 }
 
-static void TestCharLength()
+static void TestCharLength(void)
 {
     static const uint32_t codepoint[]={
         1, 0x0061,
@@ -205,7 +205,7 @@ static void TestCharLength()
     }
 }
 
-static void TestGetChar()
+static void TestGetChar(void)
 {
     static const uint8_t input[]={
     /*  code unit,*/
@@ -291,7 +291,7 @@ static void TestGetChar()
     }
 }
 
-static void TestNextPrevChar() {
+static void TestNextPrevChar(void) {
     static const uint8_t input[]={
         0x61,
         0xf0, 0x90, 0x90, 0x81,
@@ -450,7 +450,7 @@ static void TestNextPrevChar() {
 }
 
 /* keep this in sync with utf16tst.c's TestNulTerminated() */
-static void TestNulTerminated() {
+static void TestNulTerminated(void) {
     static const uint8_t input[]={
         /*  0 */  0x61,
         /*  1 */  0xf0, 0x90, 0x90, 0x81,
@@ -544,7 +544,7 @@ static void TestNulTerminated() {
     } while(c!=0);
 }
 
-static void TestNextPrevNonCharacters() {
+static void TestNextPrevNonCharacters(void) {
     /* test non-characters */
     static const uint8_t nonChars[]={
         0xef, 0xb7, 0x90,       /* U+fdd0 */
@@ -587,7 +587,7 @@ static void TestNextPrevNonCharacters() {
 #endif
 }
 
-static void TestNextPrevCharUnsafe() {
+static void TestNextPrevCharUnsafe(void) {
     /*
      * Use a (mostly) well-formed UTF-8 string and test at code point boundaries.
      * The behavior of _UNSAFE macros for ill-formed strings is undefined.
@@ -657,7 +657,7 @@ static void TestNextPrevCharUnsafe() {
     }
 }
 
-static void TestFwdBack() {
+static void TestFwdBack(void) {
     static const uint8_t input[]={
         0x61,
         0xF0, 0x90, 0x90, 0x81,
@@ -769,7 +769,7 @@ static void TestFwdBack() {
 #pragma optimize( "", off )
 #endif
 
-static void TestFwdBackUnsafe() {
+static void TestFwdBackUnsafe(void) {
     /*
      * Use a (mostly) well-formed UTF-8 string and test at code point boundaries.
      * The behavior of _UNSAFE macros for ill-formed strings is undefined.
@@ -859,7 +859,7 @@ static void TestFwdBackUnsafe() {
 #pragma optimize( "", on )
 #endif
 
-static void TestSetChar() {
+static void TestSetChar(void) {
     static const uint8_t input[]
         = {0x61, 0xe4, 0xba, 0x8c, 0x7f, 0xfe, 0x62, 0xc5, 0x7f, 0x61, 0x80, 0x80, 0xe0, 0x00 };
     static const int16_t start_safe[]
@@ -901,7 +901,7 @@ static void TestSetChar() {
     }
 }
 
-static void TestSetCharUnsafe() {
+static void TestSetCharUnsafe(void) {
     static const uint8_t input[]
         = {0x61, 0xe4, 0xba, 0x8c, 0x7f, 0x2e, 0x62, 0xc5, 0x7f, 0x61, 0x80, 0x80, 0xe0, 0x80, 0x80, 0x00 };
     static const int16_t start_unsafe[]
@@ -946,7 +946,7 @@ static void TestSetCharUnsafe() {
     }
 }
 
-static void TestTruncateIfIncomplete() {
+static void TestTruncateIfIncomplete(void) {
     // Difference from U8_SET_CP_START():
     // U8_TRUNCATE_IF_INCOMPLETE() does not look at s[length].
     // Therefore, if the last byte is a lead byte, then this macro truncates
@@ -1004,7 +1004,7 @@ static void TestTruncateIfIncomplete() {
     }
 }
 
-static void TestAppendChar(){
+static void TestAppendChar(void){
 #if !U_HIDE_OBSOLETE_UTF_OLD_H
     static const uint8_t s[11]={0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6a, 0x00};
     static const uint32_t test[]={
@@ -1173,7 +1173,7 @@ static void TestAppendChar(){
 #endif
 }
 
-static void TestAppend() {
+static void TestAppend(void) {
     static const UChar32 codePoints[]={
         0x61, 0xdf, 0x901, 0x3040,
         0xac00, 0xd800, 0xdbff, 0xdcde,
@@ -1235,7 +1235,7 @@ static void TestAppend() {
 }
 
 static void
-TestSurrogates() {
+TestSurrogates(void) {
     static const uint8_t b[]={
         0xc3, 0x9f,             /*  00DF */
         0xed, 0x9f, 0xbf,       /*  D7FF */
diff --git a/icu4c/source/test/cintltst/utmstest.c b/icu4c/source/test/cintltst/utmstest.c
index ba921070f403..c797ce892636 100644
--- a/icu4c/source/test/cintltst/utmstest.c
+++ b/icu4c/source/test/cintltst/utmstest.c
@@ -443,7 +443,7 @@ static const DotNetDateTimeTicks dotNetDateTimeTicks[]={
  * any date that we test, that is, before 0001 AD.
  */
 static void
-TestDotNet() {
+TestDotNet(void) {
     static const UChar utc[] = { 0x45, 0x74, 0x63, 0x2f, 0x47, 0x4d, 0x54, 0 }; /* "Etc/GMT" */
     const int32_t dayMillis = 86400 * INT64_C(1000);    /* 1 day = 86400 seconds */
     const int64_t dayTicks = 86400 * INT64_C(10000000);
diff --git a/icu4c/source/test/cintltst/utransts.c b/icu4c/source/test/cintltst/utransts.c
index 118625f31a39..322473b3b6e0 100644
--- a/icu4c/source/test/cintltst/utransts.c
+++ b/icu4c/source/test/cintltst/utransts.c
@@ -142,7 +142,7 @@ static void InitXReplaceableCallbacks(UReplaceableCallbacks* callbacks) {
  * Tests
  *------------------------------------------------------------------*/
 
-static void TestAPI() {
+static void TestAPI(void) {
     enum { BUF_CAP = 128 };
     char buf[BUF_CAP], buf2[BUF_CAP];
     UErrorCode status = U_ZERO_ERROR;
@@ -184,7 +184,7 @@ static void TestAPI() {
     }
 }
 
-static void TestUnicodeIDs() {
+static void TestUnicodeIDs(void) {
     UEnumeration *uenum;
     UTransliterator *utrans;
     const UChar *id, *id2;
@@ -249,7 +249,7 @@ static void TestUnicodeIDs() {
     uenum_close(uenum);
 }
 
-static void TestOpenInverse(){
+static void TestOpenInverse(void){
     UErrorCode status=U_ZERO_ERROR;
     UTransliterator* t1=NULL;
     UTransliterator* inverse1=NULL;
@@ -297,7 +297,7 @@ static void TestOpenInverse(){
    }
 }
 
-static void TestClone(){
+static void TestClone(void){
     UErrorCode status=U_ZERO_ERROR;
     UTransliterator* t1=NULL;
     UTransliterator* t2=NULL;
@@ -344,7 +344,7 @@ static void TestClone(){
 
 }
 
-static void TestRegisterUnregister(){
+static void TestRegisterUnregister(void){
     UErrorCode status=U_ZERO_ERROR;
     UTransliterator* t1=NULL;
     UTransliterator* rules=NULL, *rules2;
@@ -439,7 +439,7 @@ static void TestRegisterUnregister(){
     utrans_close(inverse1);
 }
 
-static void TestSimpleRules() {
+static void TestSimpleRules(void) {
     /* Test rules */
     /* Example: rules 1. ab>x|y
      *                2. yc>z
@@ -492,7 +492,7 @@ static void TestSimpleRules() {
                  "abc ababc aba", "xy abxy z"); 
 }
 
-static void TestFilter() {
+static void TestFilter(void) {
     UErrorCode status = U_ZERO_ERROR;
     UChar filt[128];
     UChar buf[128];
@@ -558,7 +558,7 @@ static void TestFilter() {
  * Test the UReplaceableCallback extractBetween support.  We use a
  * transliterator known to rely on this call.
  */
-static void TestExtractBetween() {
+static void TestExtractBetween(void) {
 
     UTransliterator *trans;
     UErrorCode status = U_ZERO_ERROR;
@@ -587,7 +587,7 @@ static const UChar transSimpleID[] = { 0x79,0x6F,0x2D,0x79,0x6F,0x5F,0x42,0x4A,0
 static const char* transSimpleCName = "yo-yo_BJ";
 
 enum { kUBufMax = 512 };
-static void TestGetRulesAndSourceSet() {
+static void TestGetRulesAndSourceSet(void) {
     UErrorCode status = U_ZERO_ERROR;
     UTransliterator *utrans = utrans_openU(transSimpleID, -1, UTRANS_FORWARD, NULL, 0, NULL, &status);
     if ( U_SUCCESS(status) ) {
@@ -672,7 +672,7 @@ static const TransIDSourceTarg dataVarCompItems[] = {
 };
 
 enum { kBBufMax = 1024 };
-static void TestDataVariantsCompounds() {
+static void TestDataVariantsCompounds(void) {
     const TransIDSourceTarg* itemsPtr;
     for (itemsPtr = dataVarCompItems; itemsPtr->transID != NULL; itemsPtr++) {
         UErrorCode status = U_ZERO_ERROR;
diff --git a/icu4c/source/tools/ctestfw/ctest.c b/icu4c/source/tools/ctestfw/ctest.c
index f7bbb3ff023a..3ae9694a8308 100644
--- a/icu4c/source/tools/ctestfw/ctest.c
+++ b/icu4c/source/tools/ctestfw/ctest.c
@@ -651,27 +651,27 @@ static void go_offline_with_marker(const char *mrk) {
   }
 }
 
-static void go_offline() {
+static void go_offline(void) {
 	go_offline_with_marker(NULL);
 }
 
-static void go_offline_err() {
+static void go_offline_err(void) {
 	go_offline();
 }
 
-static void first_line_verbose() {
+static void first_line_verbose(void) {
     go_offline_with_marker("v");
 }
 
-static void first_line_err() {
+static void first_line_err(void) {
     go_offline_with_marker("!");
 }
 
-static void first_line_info() {
+static void first_line_info(void) {
     go_offline_with_marker("\"");
 }
 
-static void first_line_test() {
+static void first_line_test(void) {
 	fputs(" ", stdout);
 }
 
diff --git a/icu4c/source/tools/genrb/errmsg.c b/icu4c/source/tools/genrb/errmsg.c
index a99d797ec5ce..d0d9af357034 100644
--- a/icu4c/source/tools/genrb/errmsg.c
+++ b/icu4c/source/tools/genrb/errmsg.c
@@ -42,19 +42,19 @@ U_CFUNC void setShowWarning(UBool val)
     gShowWarning = val;
 }
 
-U_CFUNC UBool getShowWarning(){
+U_CFUNC UBool getShowWarning(void){
     return gShowWarning;
 }
 
 static UBool gStrict =false;
-U_CFUNC UBool isStrict(){
+U_CFUNC UBool isStrict(void){
     return gStrict;
 }
 U_CFUNC void setStrict(UBool val){
     gStrict = val;
 }
 static UBool gVerbose =false;
-U_CFUNC UBool isVerbose(){
+U_CFUNC UBool isVerbose(void){
     return gVerbose;
 }
 U_CFUNC void setVerbose(UBool val){
diff --git a/icu4c/source/tools/genrb/read.c b/icu4c/source/tools/genrb/read.c
index 0d4a318a8984..0041bedc0210 100644
--- a/icu4c/source/tools/genrb/read.c
+++ b/icu4c/source/tools/genrb/read.c
@@ -53,7 +53,7 @@ static void    seekUntilEndOfComment (UCHARBUF *buf, struct UString *token, UErr
 static UBool   isWhitespace          (UChar32 c);
 static UBool   isNewline             (UChar32 c);
 
-U_CFUNC void resetLineNumber() {
+U_CFUNC void resetLineNumber(void) {
     lineCount = 1;
 }
 
diff --git a/icu4c/source/tools/gensprep/store.c b/icu4c/source/tools/gensprep/store.c
index c3712febb4ca..f0f4d7844be8 100644
--- a/icu4c/source/tools/gensprep/store.c
+++ b/icu4c/source/tools/gensprep/store.c
@@ -199,7 +199,7 @@ static UNewTrie *sprepTrie;
 
 
 extern void
-init() {
+init(void) {
 
     sprepTrie = (UNewTrie *)uprv_calloc(1, sizeof(UNewTrie));
 
diff --git a/icu4c/source/tools/toolutil/toolutil.h b/icu4c/source/tools/toolutil/toolutil.h
index b32a0b876283..98b2155551e4 100644
--- a/icu4c/source/tools/toolutil/toolutil.h
+++ b/icu4c/source/tools/toolutil/toolutil.h
@@ -99,7 +99,7 @@ findDirname(const char *path, char *buffer, int32_t bufLen, UErrorCode* status);
  * Return the current year in the Gregorian calendar. Used for copyright generation.
  */
 U_CAPI int32_t U_EXPORT2
-getCurrentYear();
+getCurrentYear(void);
 
 /*
  * Creates a directory with pathname.