userland: add "hexdump" command

userland/Makefile

@@ -18,6 +18,7 @@ BIN_TARGET_NAMES := \
 	fib \
 	grep \
 	halt \
+	hexdump \
 	imgview \
 	init \
 	init-test \

userland/hexdump.c

@@ -0,0 +1,87 @@
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+static void dump_file(int fd, bool canonical) {
+    size_t offset = 0;
+    for (;;) {
+        uint8_t buf[16];
+        ssize_t nread;
+        size_t cursor = 0;
+        for (; cursor < sizeof(buf);) {
+            nread = read(fd, (char*)buf + cursor, sizeof(buf) - cursor);
+            if (nread < 0) {
+                perror("read");
+                return;
+            }
+            if (nread == 0)
+                break;
+            cursor += nread;
+        }
+        size_t len = cursor;
+        if (len == 0)
+            break;
+
+        printf("%08x", offset);
+        offset += 0x10;
+
+        if (canonical) {
+            printf(" ");
+            for (size_t i = 0; i < sizeof(buf); ++i) {
+                if (i < len)
+                    printf(" %02x", buf[i]);
+                else
+                    printf("   ");
+                if (i == 7)
+                    printf(" ");
+            }
+            printf("  |");
+            for (size_t i = 0; i < len; ++i)
+                putchar(isprint(buf[i]) ? buf[i] : '.');
+            printf("|\n");
+        } else {
+            for (size_t i = 0; i < len; i += 2) {
+                if (i + 1 < cursor)
+                    printf(" %04x", (buf[i + 1] << 8) | buf[i]);
+                else
+                    printf(" 00%02x", buf[i]);
+            }
+            puts("");
+        }
+
+        if (nread == 0)
+            break;
+    }
+}
+
+int main(int argc, char* argv[]) {
+    if (argc < 2) {
+        dprintf(STDERR_FILENO, "Usage: hexdump [-C] FILE...\n");
+        return EXIT_FAILURE;
+    }
+
+    bool canonical = false;
+    for (int i = 1; i < argc; ++i) {
+        if (!strcmp(argv[i], "-C")) {
+            canonical = true;
+            break;
+        }
+    }
+
+    for (int i = 1; i < argc; ++i) {
+        const char* filename = argv[i];
+        if (!strcmp(filename, "-C"))
+            continue;
+        int fd = open(filename, O_RDONLY);
+        if (fd < 0) {
+            perror("open");
+            continue;
+        }
+        dump_file(fd, canonical);
+        close(fd);
+    }
+
+    return EXIT_SUCCESS;
+}