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dlmalloc.cc
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#include "dlmalloc.h"
/*------------------------------ internal #includes ---------------------- */
#ifdef _MSC_VER
#pragma warning( disable : 4146 ) /* no "unsigned" warnings */
#endif /* _MSC_VER */
#if !NO_MALLOC_STATS
#include <stdio.h> /* for printing in malloc_stats */
#endif /* NO_MALLOC_STATS */
#ifndef LACKS_ERRNO_H
#include <errno.h> /* for MALLOC_FAILURE_ACTION */
#endif /* LACKS_ERRNO_H */
#ifdef DEBUG
#if ABORT_ON_ASSERT_FAILURE
#undef assert
#define assert(x) if(!(x)) ABORT
#else /* ABORT_ON_ASSERT_FAILURE */
#include <assert.h>
#endif /* ABORT_ON_ASSERT_FAILURE */
#else /* DEBUG */
#ifndef assert
#define assert(x)
#endif
#define DEBUG 0
#endif /* DEBUG */
#if !defined(WIN32) && !defined(LACKS_TIME_H)
#include <time.h> /* for magic initialization */
#endif /* WIN32 */
#ifndef LACKS_STDLIB_H
#include <stdlib.h> /* for abort() */
#endif /* LACKS_STDLIB_H */
#ifndef LACKS_STRING_H
#include <string.h> /* for memset etc */
#endif /* LACKS_STRING_H */
#if USE_BUILTIN_FFS
#ifndef LACKS_STRINGS_H
#include <strings.h> /* for ffs */
#endif /* LACKS_STRINGS_H */
#endif /* USE_BUILTIN_FFS */
#if HAVE_MMAP
#ifndef LACKS_SYS_MMAN_H
/* On some versions of linux, mremap decl in mman.h needs __USE_GNU set */
#if (defined(linux) && !defined(__USE_GNU))
#define __USE_GNU 1
#include <sys/mman.h> /* for mmap */
#undef __USE_GNU
#else
#include <sys/mman.h> /* for mmap */
#endif /* linux */
#endif /* LACKS_SYS_MMAN_H */
#ifndef LACKS_FCNTL_H
#include <fcntl.h>
#endif /* LACKS_FCNTL_H */
#endif /* HAVE_MMAP */
#ifndef LACKS_UNISTD_H
#include <unistd.h> /* for sbrk, sysconf */
#else /* LACKS_UNISTD_H */
#if !defined(__FreeBSD__) && !defined(__OpenBSD__) && !defined(__NetBSD__)
extern void* sbrk(ptrdiff_t);
#endif /* FreeBSD etc */
#endif /* LACKS_UNISTD_H */
/* Declarations for locking */
#if USE_LOCKS
#ifndef WIN32
#if defined (__SVR4) && defined (__sun) /* solaris */
#include <thread.h>
#elif !defined(LACKS_SCHED_H)
#include <sched.h>
#endif /* solaris or LACKS_SCHED_H */
#if (defined(USE_RECURSIVE_LOCKS) && USE_RECURSIVE_LOCKS != 0) || !USE_SPIN_LOCKS
#include <pthread.h>
#endif /* USE_RECURSIVE_LOCKS ... */
#elif defined(_MSC_VER)
#ifndef _M_AMD64
/* These are already defined on AMD64 builds */
extern "C" {
LONG __cdecl _InterlockedCompareExchange(LONG volatile *Dest, LONG Exchange, LONG Comp);
LONG __cdecl _InterlockedExchange(LONG volatile *Target, LONG Value);
}
#endif /* _M_AMD64 */
#pragma intrinsic (_InterlockedCompareExchange)
#pragma intrinsic (_InterlockedExchange)
#define interlockedcompareexchange _InterlockedCompareExchange
#define interlockedexchange _InterlockedExchange
#elif defined(WIN32) && defined(__GNUC__)
#define interlockedcompareexchange(a, b, c) __sync_val_compare_and_swap(a, c, b)
#define interlockedexchange __sync_lock_test_and_set
#endif /* Win32 */
#else /* USE_LOCKS */
#endif /* USE_LOCKS */
#ifndef LOCK_AT_FORK
#define LOCK_AT_FORK 0
#endif
/* Declarations for bit scanning on win32 */
#if defined(_MSC_VER) && _MSC_VER>=1300
#ifndef BitScanForward /* Try to avoid pulling in WinNT.h */
extern "C" {
unsigned char _BitScanForward(unsigned long *index, unsigned long mask);
unsigned char _BitScanReverse(unsigned long *index, unsigned long mask);
}
#define BitScanForward _BitScanForward
#define BitScanReverse _BitScanReverse
#pragma intrinsic(_BitScanForward)
#pragma intrinsic(_BitScanReverse)
#endif /* BitScanForward */
#endif /* defined(_MSC_VER) && _MSC_VER>=1300 */
#ifndef WIN32
#ifndef malloc_getpagesize
# ifdef _SC_PAGESIZE /* some SVR4 systems omit an underscore */
# ifndef _SC_PAGE_SIZE
# define _SC_PAGE_SIZE _SC_PAGESIZE
# endif
# endif
# ifdef _SC_PAGE_SIZE
# define malloc_getpagesize sysconf(_SC_PAGE_SIZE)
# else
# if defined(BSD) || defined(DGUX) || defined(HAVE_GETPAGESIZE)
extern size_t getpagesize();
# define malloc_getpagesize getpagesize()
# else
# ifdef WIN32 /* use supplied emulation of getpagesize */
# define malloc_getpagesize getpagesize()
# else
# ifndef LACKS_SYS_PARAM_H
# include <sys/param.h>
# endif
# ifdef EXEC_PAGESIZE
# define malloc_getpagesize EXEC_PAGESIZE
# else
# ifdef NBPG
# ifndef CLSIZE
# define malloc_getpagesize NBPG
# else
# define malloc_getpagesize (NBPG * CLSIZE)
# endif
# else
# ifdef NBPC
# define malloc_getpagesize NBPC
# else
# ifdef PAGESIZE
# define malloc_getpagesize PAGESIZE
# else /* just guess */
# define malloc_getpagesize ((size_t)4096U)
# endif
# endif
# endif
# endif
# endif
# endif
# endif
#endif
#endif
/* ------------------- size_t and alignment properties -------------------- */
/* The byte and bit size of a size_t */
#define SIZE_T_BITSIZE (sizeof(size_t) << 3)
/* Some constants coerced to size_t */
/* Annoying but necessary to avoid errors on some platforms */
#define HALF_MAX_SIZE_T (MAX_SIZE_T / 2U)
// The bit mask value corresponding to MALLOC_ALIGNMENT
#define CHUNK_ALIGN_MASK (MALLOC_ALIGNMENT - 1)
// True if address a has acceptable alignment
bool is_aligned(void *p) { return ((size_t)p & CHUNK_ALIGN_MASK) == 0; }
// the number of bytes to offset an address to align it
size_t align_offset(void *p)
{
return (((size_t)p & CHUNK_ALIGN_MASK) == 0) ? 0 :
((MALLOC_ALIGNMENT - ((size_t)p & CHUNK_ALIGN_MASK)) & CHUNK_ALIGN_MASK);
}
/* -------------------------- MMAP preliminaries ------------------------- */
/*
If HAVE_MORECORE or HAVE_MMAP are false, we just define calls and
checks to fail so compiler optimizer can delete code rather than
using so many "#if"s.
*/
/* MORECORE and MMAP must return MFAIL on failure */
#define MFAIL ((void*)(MAX_SIZE_T))
#define CMFAIL ((char*)(MFAIL)) /* defined for convenience */
#if HAVE_MMAP
#ifndef WIN32
#define MUNMAP_DEFAULT(a, s) munmap((a), (s))
#define MMAP_PROT (PROT_READ | PROT_WRITE)
#if !defined(MAP_ANONYMOUS) && defined(MAP_ANON)
#define MAP_ANONYMOUS MAP_ANON
#endif /* MAP_ANON */
#ifdef MAP_ANONYMOUS
#define MMAP_FLAGS (MAP_PRIVATE | MAP_ANONYMOUS)
#define MMAP_DEFAULT(s) mmap(0, (s), MMAP_PROT, MMAP_FLAGS, -1, 0)
#else /* MAP_ANONYMOUS */
/*
Nearly all versions of mmap support MAP_ANONYMOUS, so the following
is unlikely to be needed, but is supplied just in case.
*/
#define MMAP_FLAGS (MAP_PRIVATE)
static int dev_zero_fd = -1; /* Cached file descriptor for /dev/zero. */
void MMAP_DEFAULT(size_t s) {
if (dev_zero_fd < 0)
dev_zero_fd = open("/dev/zero", O_RDWR);
mmap(0, s, MMAP_PROT, MMAP_FLAGS, dev_zero_fd, 0);
}
#endif /* MAP_ANONYMOUS */
#define DIRECT_MMAP_DEFAULT(s) MMAP_DEFAULT(s)
#else /* WIN32 */
/* Win32 MMAP via VirtualAlloc */
static FORCEINLINE void* win32mmap(size_t size) {
void* ptr = VirtualAlloc(0, size, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
return (ptr != 0)? ptr: MFAIL;
}
/* For direct MMAP, use MEM_TOP_DOWN to minimize interference */
static FORCEINLINE void* win32direct_mmap(size_t size) {
void* ptr = VirtualAlloc(0, size, MEM_RESERVE | MEM_COMMIT | MEM_TOP_DOWN,
PAGE_READWRITE);
return (ptr != 0)? ptr: MFAIL;
}
/* This function supports releasing coalesed segments */
static FORCEINLINE int win32munmap(void* ptr, size_t size) {
MEMORY_BASIC_INFORMATION minfo;
char* cptr = (char*)ptr;
while (size) {
if (VirtualQuery(cptr, &minfo, sizeof(minfo)) == 0)
return -1;
if (minfo.BaseAddress != cptr || minfo.AllocationBase != cptr ||
minfo.State != MEM_COMMIT || minfo.RegionSize > size)
return -1;
if (VirtualFree(cptr, 0, MEM_RELEASE) == 0)
return -1;
cptr += minfo.RegionSize;
size -= minfo.RegionSize;
}
return 0;
}
#define MMAP_DEFAULT(s) win32mmap(s)
#define MUNMAP_DEFAULT(a, s) win32munmap((a), (s))
#define DIRECT_MMAP_DEFAULT(s) win32direct_mmap(s)
#endif /* WIN32 */
#endif /* HAVE_MMAP */
#if HAVE_MREMAP
#ifndef WIN32
#define MREMAP_DEFAULT(addr, osz, nsz, mv) mremap((addr), (osz), (nsz), (mv))
#endif /* WIN32 */
#endif /* HAVE_MREMAP */
/**
* Define CALL_MORECORE
*/
#if HAVE_MORECORE
#ifdef MORECORE
#define CALL_MORECORE(S) MORECORE(S)
#else /* MORECORE */
#define CALL_MORECORE(S) MORECORE_DEFAULT(S)
#endif /* MORECORE */
#else /* HAVE_MORECORE */
#define CALL_MORECORE(S) MFAIL
#endif /* HAVE_MORECORE */
/**
* Define CALL_MMAP/CALL_MUNMAP/CALL_DIRECT_MMAP
*/
#if HAVE_MMAP
#define USE_MMAP_BIT 1
#ifdef MMAP
#define CALL_MMAP(s) MMAP(s)
#else /* MMAP */
#define CALL_MMAP(s) MMAP_DEFAULT(s)
#endif /* MMAP */
#ifdef MUNMAP
#define CALL_MUNMAP(a, s) MUNMAP((a), (s))
#else /* MUNMAP */
#define CALL_MUNMAP(a, s) MUNMAP_DEFAULT((a), (s))
#endif /* MUNMAP */
#ifdef DIRECT_MMAP
#define CALL_DIRECT_MMAP(s) DIRECT_MMAP(s)
#else /* DIRECT_MMAP */
#define CALL_DIRECT_MMAP(s) DIRECT_MMAP_DEFAULT(s)
#endif /* DIRECT_MMAP */
#else /* HAVE_MMAP */
#define USE_MMAP_BIT 0
#define MMAP(s) MFAIL
#define MUNMAP(a, s) (-1)
#define DIRECT_MMAP(s) MFAIL
#define CALL_DIRECT_MMAP(s) DIRECT_MMAP(s)
#define CALL_MMAP(s) MMAP(s)
#define CALL_MUNMAP(a, s) MUNMAP((a), (s))
#endif /* HAVE_MMAP */
/**
* Define CALL_MREMAP
*/
#if HAVE_MMAP && HAVE_MREMAP
#ifdef MREMAP
#define CALL_MREMAP(addr, osz, nsz, mv) MREMAP((addr), (osz), (nsz), (mv))
#else /* MREMAP */
#define CALL_MREMAP(addr, osz, nsz, mv) MREMAP_DEFAULT((addr), (osz), (nsz), (mv))
#endif /* MREMAP */
#else /* HAVE_MMAP && HAVE_MREMAP */
#define CALL_MREMAP(addr, osz, nsz, mv) MFAIL
#endif /* HAVE_MMAP && HAVE_MREMAP */
/* mstate bit set if continguous morecore disabled or failed */
#define USE_NONCONTIGUOUS_BIT (4U)
/* segment bit set in create_mspace_with_base */
#define EXTERN_BIT (8U)
/* --------------------------- Lock preliminaries ------------------------ */
/*
When locks are defined, there is one global lock, plus
one per-mspace lock.
The global lock_ensures that mparams.magic and other unique
mparams values are initialized only once. It also protects
sequences of calls to MORECORE. In many cases sys_alloc requires
two calls, that should not be interleaved with calls by other
threads. This does not protect against direct calls to MORECORE
by other threads not using this lock, so there is still code to
cope the best we can on interference.
Per-mspace locks surround calls to malloc, free, etc.
By default, locks are simple non-reentrant mutexes.
Because lock-protected regions generally have bounded times, it is
OK to use the supplied simple spinlocks. Spinlocks are likely to
improve performance for lightly contended applications, but worsen
performance under heavy contention.
If USE_LOCKS is > 1, the definitions of lock routines here are
bypassed, in which case you will need to define the type MLOCK_T,
and at least INITIAL_LOCK, DESTROY_LOCK, ACQUIRE_LOCK, RELEASE_LOCK
and TRY_LOCK. You must also declare a
static MLOCK_T malloc_global_mutex = { initialization values };.
*/
#if !USE_LOCKS
#define USE_LOCK_BIT (0U)
#define INITIAL_LOCK(l) (0)
#define DESTROY_LOCK(l) (0)
#define ACQUIRE_MALLOC_GLOBAL_LOCK()
#define RELEASE_MALLOC_GLOBAL_LOCK()
#else
#if USE_LOCKS > 1
/* ----------------------- User-defined locks ------------------------ */
/* Define your own lock implementation here */
/* #define INITIAL_LOCK(lk) ... */
/* #define DESTROY_LOCK(lk) ... */
/* #define ACQUIRE_LOCK(lk) ... */
/* #define RELEASE_LOCK(lk) ... */
/* #define TRY_LOCK(lk) ... */
/* static MLOCK_T malloc_global_mutex = ... */
#elif USE_SPIN_LOCKS
/* First, define CAS_LOCK and CLEAR_LOCK on ints */
/* Note CAS_LOCK defined to return 0 on success */
#if defined(__GNUC__)&& (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 1))
#define CAS_LOCK(sl) __sync_lock_test_and_set(sl, 1)
#define CLEAR_LOCK(sl) __sync_lock_release(sl)
#elif (defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__)))
/* Custom spin locks for older gcc on x86 */
static FORCEINLINE int x86_cas_lock(int *sl) {
int ret;
int val = 1;
int cmp = 0;
__asm__ __volatile__ ("lock; cmpxchgl %1, %2"
: "=a" (ret)
: "r" (val), "m" (*(sl)), "0"(cmp)
: "memory", "cc");
return ret;
}
static FORCEINLINE void x86_clear_lock(int* sl) {
assert(*sl != 0);
int prev = 0;
int ret;
__asm__ __volatile__ ("lock; xchgl %0, %1"
: "=r" (ret)
: "m" (*(sl)), "0"(prev)
: "memory");
}
#define CAS_LOCK(sl) x86_cas_lock(sl)
#define CLEAR_LOCK(sl) x86_clear_lock(sl)
#else /* Win32 MSC */
#define CAS_LOCK(sl) interlockedexchange(sl, 1)
#define CLEAR_LOCK(sl) interlockedexchange (sl, 0)
#endif /* ... gcc spins locks ... */
/* How to yield for a spin lock */
#define SPINS_PER_YIELD 63
#if defined(_MSC_VER)
#define SLEEP_EX_DURATION 50 /* delay for yield/sleep */
#define SPIN_LOCK_YIELD SleepEx(SLEEP_EX_DURATION, FALSE)
#elif defined (__SVR4) && defined (__sun) /* solaris */
#define SPIN_LOCK_YIELD thr_yield();
#elif !defined(LACKS_SCHED_H)
#define SPIN_LOCK_YIELD sched_yield();
#else
#define SPIN_LOCK_YIELD
#endif /* ... yield ... */
#if !defined(USE_RECURSIVE_LOCKS) || USE_RECURSIVE_LOCKS == 0
/* Plain spin locks use single word (embedded in malloc_states) */
static int spin_acquire_lock(unsigned *sl) {
int spins = 0;
while (*(volatile unsigned *)sl != 0 || CAS_LOCK(sl)) {
if ((++spins & SPINS_PER_YIELD) == 0) {
SPIN_LOCK_YIELD;
}
}
return 0;
}
#define MLOCK_T unsigned
#define TRY_LOCK(sl) !CAS_LOCK(sl)
#define RELEASE_LOCK(sl) CLEAR_LOCK(sl)
#define ACQUIRE_LOCK(sl) (CAS_LOCK(sl)? spin_acquire_lock(sl) : 0)
#define INITIAL_LOCK(sl) (*sl = 0)
#define DESTROY_LOCK(sl) (0)
static MLOCK_T malloc_global_mutex = 0;
#else /* USE_RECURSIVE_LOCKS */
/* types for lock owners */
#ifdef WIN32
#define THREAD_ID_T DWORD
#define CURRENT_THREAD GetCurrentThreadId()
#define EQ_OWNER(X,Y) ((X) == (Y))
#else
/*
Note: the following assume that pthread_t is a type that can be
initialized to (casted) zero. If this is not the case, you will need to
somehow redefine these or not use spin locks.
*/
#define THREAD_ID_T pthread_t
#define CURRENT_THREAD pthread_self()
#define EQ_OWNER(X,Y) pthread_equal(X, Y)
#endif
struct malloc_recursive_lock {
int sl;
unsigned int c;
THREAD_ID_T threadid;
};
#define MLOCK_T struct malloc_recursive_lock
static MLOCK_T malloc_global_mutex = { 0, 0, (THREAD_ID_T)0};
static FORCEINLINE void recursive_release_lock(MLOCK_T *lk) {
assert(lk->sl != 0);
if (--lk->c == 0) {
CLEAR_LOCK(&lk->sl);
}
}
static FORCEINLINE int recursive_acquire_lock(MLOCK_T *lk) {
THREAD_ID_T mythreadid = CURRENT_THREAD;
int spins = 0;
for (;;) {
if (*((volatile int *)(&lk->sl)) == 0) {
if (!CAS_LOCK(&lk->sl)) {
lk->threadid = mythreadid;
lk->c = 1;
return 0;
}
}
else if (EQ_OWNER(lk->threadid, mythreadid)) {
++lk->c;
return 0;
}
if ((++spins & SPINS_PER_YIELD) == 0) {
SPIN_LOCK_YIELD;
}
}
}
static FORCEINLINE int recursive_try_lock(MLOCK_T *lk) {
THREAD_ID_T mythreadid = CURRENT_THREAD;
if (*((volatile int *)(&lk->sl)) == 0) {
if (!CAS_LOCK(&lk->sl)) {
lk->threadid = mythreadid;
lk->c = 1;
return 1;
}
}
else if (EQ_OWNER(lk->threadid, mythreadid)) {
++lk->c;
return 1;
}
return 0;
}
#define RELEASE_LOCK(lk) recursive_release_lock(lk)
#define TRY_LOCK(lk) recursive_try_lock(lk)
#define ACQUIRE_LOCK(lk) recursive_acquire_lock(lk)
#define INITIAL_LOCK(lk) ((lk)->threadid = (THREAD_ID_T)0, (lk)->sl = 0, (lk)->c = 0)
#define DESTROY_LOCK(lk) (0)
#endif /* USE_RECURSIVE_LOCKS */
#elif defined(WIN32) /* Win32 critical sections */
#define MLOCK_T CRITICAL_SECTION
#define ACQUIRE_LOCK(lk) (EnterCriticalSection(lk), 0)
#define RELEASE_LOCK(lk) LeaveCriticalSection(lk)
#define TRY_LOCK(lk) TryEnterCriticalSection(lk)
#define INITIAL_LOCK(lk) (!InitializeCriticalSectionAndSpinCount((lk), 0x80000000 | 4000))
#define DESTROY_LOCK(lk) (DeleteCriticalSection(lk), 0)
#define NEED_GLOBAL_LOCK_INIT
static MLOCK_T malloc_global_mutex;
static volatile LONG malloc_global_mutex_status;
/* Use spin loop to initialize global lock */
static void init_malloc_global_mutex() {
for (;;) {
long stat = malloc_global_mutex_status;
if (stat > 0)
return;
/* transition to < 0 while initializing, then to > 0) */
if (stat == 0 &&
interlockedcompareexchange(&malloc_global_mutex_status, (LONG)-1, (LONG)0) == 0) {
InitializeCriticalSection(&malloc_global_mutex);
interlockedexchange(&malloc_global_mutex_status, (LONG)1);
return;
}
SleepEx(0, FALSE);
}
}
#else /* pthreads-based locks */
#define MLOCK_T pthread_mutex_t
#define ACQUIRE_LOCK(lk) pthread_mutex_lock(lk)
#define RELEASE_LOCK(lk) pthread_mutex_unlock(lk)
#define TRY_LOCK(lk) (!pthread_mutex_trylock(lk))
#define INITIAL_LOCK(lk) pthread_init_lock(lk)
#define DESTROY_LOCK(lk) pthread_mutex_destroy(lk)
#if defined(USE_RECURSIVE_LOCKS) && USE_RECURSIVE_LOCKS != 0 && defined(linux) && !defined(PTHREAD_MUTEX_RECURSIVE)
/* Cope with old-style linux recursive lock initialization by adding */
/* skipped internal declaration from pthread.h */
extern int pthread_mutexattr_setkind_np __P ((pthread_mutexattr_t *__attr,
int __kind));
#define PTHREAD_MUTEX_RECURSIVE PTHREAD_MUTEX_RECURSIVE_NP
#define pthread_mutexattr_settype(x,y) pthread_mutexattr_setkind_np(x,y)
#endif /* USE_RECURSIVE_LOCKS ... */
static MLOCK_T malloc_global_mutex = PTHREAD_MUTEX_INITIALIZER;
static int pthread_init_lock (MLOCK_T *lk) {
pthread_mutexattr_t attr;
if (pthread_mutexattr_init(&attr)) return 1;
#if defined(USE_RECURSIVE_LOCKS) && USE_RECURSIVE_LOCKS != 0
if (pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE)) return 1;
#endif
if (pthread_mutex_init(lk, &attr)) return 1;
if (pthread_mutexattr_destroy(&attr)) return 1;
return 0;
}
#endif /* ... lock types ... */
/* Common code for all lock types */
#define USE_LOCK_BIT (2U)
#ifndef ACQUIRE_MALLOC_GLOBAL_LOCK
#define ACQUIRE_MALLOC_GLOBAL_LOCK() ACQUIRE_LOCK(&malloc_global_mutex);
#endif
#ifndef RELEASE_MALLOC_GLOBAL_LOCK
#define RELEASE_MALLOC_GLOBAL_LOCK() RELEASE_LOCK(&malloc_global_mutex);
#endif
#endif /* USE_LOCKS */
/* ----------------------- Chunk representations ------------------------ */
/*
(The following includes lightly edited explanations by Colin Plumb.)
The malloc_chunk declaration below is misleading (but accurate and
necessary). It declares a "view" into memory allowing access to
necessary fields at known offsets from a given base.
Chunks of memory are maintained using a `boundary tag' method as
originally described by Knuth. (See the paper by Paul Wilson
ftp://ftp.cs.utexas.edu/pub/garbage/allocsrv.ps for a survey of such
techniques.) Sizes of free chunks are stored both in the front of
each chunk and at the end. This makes consolidating fragmented
chunks into bigger chunks fast. The head fields also hold bits
representing whether chunks are free or in use.
Here are some pictures to make it clearer. They are "exploded" to
show that the state of a chunk can be thought of as extending from
the high 31 bits of the head field of its header through the
prev_foot and PINUSE_BIT bit of the following chunk header.
A chunk that's in use looks like:
chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size of previous chunk (if P = 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |P|
| Size of this chunk 1| +-+
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- -+
| :
+- size - sizeof(size_t) available payload bytes -+
: |
chunk-> +- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|
| Size of next chunk (may or may not be in use) | +-+
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
And if it's free, it looks like this:
chunk-> +- -+
| User payload (must be in use, or we would have merged!) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |P|
| Size of this chunk 0| +-+
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prev pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+- size - sizeof(struct chunk) unused bytes -+
: |
chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size of this chunk |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|
| Size of next chunk (must be in use, or we would have merged)| +-+
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+- User payload -+
: |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|
+-+
Note that since we always merge adjacent free chunks, the chunks
adjacent to a free chunk must be in use.
Given a pointer to a chunk (which can be derived trivially from the
payload pointer) we can, in O(1) time, find out whether the adjacent
chunks are free, and if so, unlink them from the lists that they
are on and merge them with the current chunk.
Chunks always begin on even word boundaries, so the mem portion
(which is returned to the user) is also on an even word boundary, and
thus at least double-word aligned.
The P (PINUSE_BIT) bit, stored in the unused low-order bit of the
chunk size (which is always a multiple of two words), is an in-use
bit for the *previous* chunk. If that bit is *clear*, then the
word before the current chunk size contains the previous chunk
size, and can be used to find the front of the previous chunk.
The very first chunk allocated always has this bit set, preventing
access to non-existent (or non-owned) memory. If pinuse is set for
any given chunk, then you CANNOT determine the size of the
previous chunk, and might even get a memory addressing fault when
trying to do so.
The C (CINUSE_BIT) bit, stored in the unused second-lowest bit of
the chunk size redundantly records whether the current chunk is
inuse (unless the chunk is mmapped). This redundancy enables usage
checks within free and realloc, and reduces indirection when freeing
and consolidating chunks.
Each freshly allocated chunk must have both cinuse and pinuse set.
That is, each allocated chunk borders either a previously allocated
and still in-use chunk, or the base of its memory arena. This is
ensured by making all allocations from the `lowest' part of any
found chunk. Further, no free chunk physically borders another one,
so each free chunk is known to be preceded and followed by either
inuse chunks or the ends of memory.
Note that the `foot' of the current chunk is actually represented
as the prev_foot of the NEXT chunk. This makes it easier to
deal with alignments etc but can be very confusing when trying
to extend or adapt this code.
The exceptions to all this are
1. The special chunk `top' is the top-most available chunk (i.e.,
the one bordering the end of available memory). It is treated
specially. Top is never included in any bin, is used only if
no other chunk is available, and is released back to the
system if it is very large (see M_TRIM_THRESHOLD). In effect,
the top chunk is treated as larger (and thus less well
fitting) than any other available chunk. The top chunk
doesn't update its trailing size field since there is no next
contiguous chunk that would have to index off it. However,
space is still allocated for it (TOP_FOOT_SIZE) to enable
separation or merging when space is extended.
3. Chunks allocated via mmap, have both cinuse and pinuse bits
cleared in their head fields. Because they are allocated
one-by-one, each must carry its own prev_foot field, which is
also used to hold the offset this chunk has within its mmapped
region, which is needed to preserve alignment. Each mmapped
chunk is trailed by the first two fields of a fake next-chunk
for sake of usage checks.
*/
#define PINUSE_BIT 1
#define CINUSE_BIT 2
#define FLAG4_BIT 4
#define INUSE_BITS (PINUSE_BIT | CINUSE_BIT)
#define FLAG_BITS (PINUSE_BIT | CINUSE_BIT | FLAG4_BIT)
/* ------------------- Chunks sizes and alignments ----------------------- */
#define MCHUNK_SIZE (sizeof(mchunk))
#if FOOTERS
#define CHUNK_OVERHEAD (2 * sizeof(size_t))
#else // FOOTERS
#define CHUNK_OVERHEAD (sizeof(size_t))
#endif // FOOTERS
/* MMapped chunks need a second word of overhead ... */
#define MMAP_CHUNK_OVERHEAD (2 * sizeof(size_t))
/* ... and additional padding for fake next-chunk at foot */
#define MMAP_FOOT_PAD (4 * sizeof(size_t))
/* The smallest size we can malloc is an aligned minimal chunk */
#define MIN_CHUNK_SIZE ((MCHUNK_SIZE + CHUNK_ALIGN_MASK) & ~CHUNK_ALIGN_MASK)
// ===============================================================================
struct malloc_chunk_header {
void set_size_and_pinuse_of_free_chunk(size_t s) {
_head = s | PINUSE_BIT;
set_foot(s);
}
void set_foot(size_t s) {
((malloc_chunk_header *)((char*)this + s))->_prev_foot = s;
}
// extraction of fields from head words
bool cinuse() const { return !!(_head & CINUSE_BIT); }
bool pinuse() const { return !!(_head & PINUSE_BIT); }
bool flag4inuse() const { return !!(_head & FLAG4_BIT); }
bool is_inuse() const { return (_head & INUSE_BITS) != PINUSE_BIT; }
bool is_mmapped() const { return (_head & INUSE_BITS) == 0; }
size_t chunksize() const { return _head & ~(FLAG_BITS); }
void clear_pinuse() { _head &= ~PINUSE_BIT; }
void set_flag4() { _head |= FLAG4_BIT; }
void clear_flag4() { _head &= ~FLAG4_BIT; }
// Treat space at ptr +/- offset as a chunk
malloc_chunk_header * chunk_plus_offset(size_t s) {
return (malloc_chunk_header *)((char*)this + s);
}
malloc_chunk_header * chunk_minus_offset(size_t s) {
return (malloc_chunk_header *)((char*)this - s);
}
// Ptr to next or previous physical malloc_chunk.
malloc_chunk_header * next_chunk() {
return (malloc_chunk_header *)((char*)this + (_head & ~FLAG_BITS));
}
malloc_chunk_header * prev_chunk() {
return (malloc_chunk_header *)((char*)this - (_prev_foot));
}
// extract next chunk's pinuse bit
size_t next_pinuse() { return next_chunk()->_head & PINUSE_BIT; }
size_t _prev_foot; // Size of previous chunk (if free).
size_t _head; // Size and inuse bits.
};
// ===============================================================================
struct malloc_chunk : public malloc_chunk_header {
// Set size, pinuse bit, foot, and clear next pinuse
void set_free_with_pinuse(size_t s, malloc_chunk* n)
{
n->clear_pinuse();
set_size_and_pinuse_of_free_chunk(s);
}
// Get the internal overhead associated with chunk p
size_t overhead_for() { return is_mmapped() ? MMAP_CHUNK_OVERHEAD : CHUNK_OVERHEAD; }
// Return true if malloced space is not necessarily cleared
bool calloc_must_clear()
{
#if MMAP_CLEARS
return !is_mmapped();
#else
return true;
#endif
}
struct malloc_chunk* _fd; // double links -- used only if free.
struct malloc_chunk* _bk;
};
typedef malloc_chunk mchunk;
typedef malloc_chunk* mchunkptr;
typedef malloc_chunk_header *hchunkptr;
typedef malloc_chunk* sbinptr; // The type of bins of chunks
typedef unsigned int bindex_t; // Described below
typedef unsigned int binmap_t; // Described below
typedef unsigned int flag_t; // The type of various bit flag sets
// conversion from malloc headers to user pointers, and back
static FORCEINLINE void *chunk2mem(const void *p) { return (void *)((char *)p + 2 * sizeof(size_t)); }
static FORCEINLINE mchunkptr mem2chunk(const void *mem) { return (mchunkptr)((char *)mem - 2 * sizeof(size_t)); }
// chunk associated with aligned address A
static FORCEINLINE mchunkptr align_as_chunk(char *A) { return (mchunkptr)(A + align_offset(chunk2mem(A))); }
// Bounds on request (not chunk) sizes.
#define MAX_REQUEST ((-MIN_CHUNK_SIZE) << 2)
#define MIN_REQUEST (MIN_CHUNK_SIZE - CHUNK_OVERHEAD - 1)
// pad request bytes into a usable size
static FORCEINLINE size_t pad_request(size_t req)
{
return (req + CHUNK_OVERHEAD + CHUNK_ALIGN_MASK) & ~CHUNK_ALIGN_MASK;
}
// pad request, checking for minimum (but not maximum)
static FORCEINLINE size_t request2size(size_t req)
{
return req < MIN_REQUEST ? MIN_CHUNK_SIZE : pad_request(req);
}
/* ------------------ Operations on head and foot fields ----------------- */
/*
The head field of a chunk is or'ed with PINUSE_BIT when previous
adjacent chunk in use, and or'ed with CINUSE_BIT if this chunk is in
use, unless mmapped, in which case both bits are cleared.
FLAG4_BIT is not used by this malloc, but might be useful in extensions.
*/
// Head value for fenceposts
#define FENCEPOST_HEAD (INUSE_BITS | sizeof(size_t))
/* ---------------------- Overlaid data structures ----------------------- */
/*
When chunks are not in use, they are treated as nodes of either
lists or trees.
"Small" chunks are stored in circular doubly-linked lists, and look
like this:
chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size of previous chunk |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
`head:' | Size of chunk, in bytes |P|
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forward pointer to next chunk in list |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Back pointer to previous chunk in list |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused space (may be 0 bytes long) .
. .
. |
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
`foot:' | Size of chunk, in bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Larger chunks are kept in a form of bitwise digital trees (aka
tries) keyed on chunksizes. Because malloc_tree_chunks are only for
free chunks greater than 256 bytes, their size doesn't impose any
constraints on user chunk sizes. Each node looks like:
chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size of previous chunk |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
`head:' | Size of chunk, in bytes |P|
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forward pointer to next chunk of same size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Back pointer to previous chunk of same size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pointer to left child (child[0]) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pointer to right child (child[1]) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pointer to parent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| bin index of this chunk |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused space .
. |
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
`foot:' | Size of chunk, in bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each tree holding treenodes is a tree of unique chunk sizes. Chunks
of the same size are arranged in a circularly-linked list, with only
the oldest chunk (the next to be used, in our FIFO ordering)
actually in the tree. (Tree members are distinguished by a non-null
parent pointer.) If a chunk with the same size an an existing node
is inserted, it is linked off the existing node using pointers that
work in the same way as fd/bk pointers of small chunks.
Each tree contains a power of 2 sized range of chunk sizes (the
smallest is 0x100 <= x < 0x180), which is is divided in half at each
tree level, with the chunks in the smaller half of the range (0x100
<= x < 0x140 for the top nose) in the left subtree and the larger
half (0x140 <= x < 0x180) in the right subtree. This is, of course,
done by inspecting individual bits.
Using these rules, each node's left subtree contains all smaller
sizes than its right subtree. However, the node at the root of each
subtree has no particular ordering relationship to either. (The
dividing line between the subtree sizes is based on trie relation.)
If we remove the last chunk of a given size from the interior of the
tree, we need to replace it with a leaf node. The tree ordering
rules permit a node to be replaced by any leaf below it.
The smallest chunk in a tree (a common operation in a best-fit
allocator) can be found by walking a path to the leftmost leaf in
the tree. Unlike a usual binary tree, where we follow left child
pointers until we reach a null, here we follow the right child
pointer any time the left one is null, until we reach a leaf with
both child pointers null. The smallest chunk in the tree will be
somewhere along that path.
The worst case number of steps to add, find, or remove a node is
bounded by the number of bits differentiating chunks within
bins. Under current bin calculations, this ranges from 6 up to 21
(for 32 bit sizes) or up to 53 (for 64 bit sizes). The typical case
is of course much better.
*/
// ===============================================================================
struct malloc_tree_chunk : public malloc_chunk_header {
malloc_tree_chunk *leftmost_child() {
return _child[0] ? _child[0] : _child[1];
}
malloc_tree_chunk* _fd;
malloc_tree_chunk* _bk;
malloc_tree_chunk* _child[2];
malloc_tree_chunk* _parent;
bindex_t _index;
};
typedef malloc_tree_chunk tchunk;
typedef malloc_tree_chunk* tchunkptr;
typedef malloc_tree_chunk* tbinptr; // The type of bins of trees
/* ----------------------------- Segments -------------------------------- */
/*
Each malloc space may include non-contiguous segments, held in a