FFmpeg有专门的内存管理系统，包括：内存分配、内存拷贝、内存释放。其中内存分配包含分配内存与对齐、内存分配与清零、分配指定大小的内存块、重新分配内存块、快速分配内存、分配指定最大值的内存、分配数组内存、快速分配数组内存、重新分配数组内存。
FFmpeg的内存管理位于libavutil/mem.c，相关函数如下图所示：
一、内存分配 1、av_malloc av_malloc()内存分配，并且内存对齐，方便系统快速访问内存。代码如下：
void *av_malloc(size_t size)
{
void *ptr = NULL;
if (size > max_alloc_size)
return NULL;
#if HAVE_POSIX_MEMALIGN
if (size)
if (posix_memalign(&ptr, ALIGN, size))
ptr = NULL;
#elif HAVE_ALIGNED_MALLOC
ptr = _aligned_malloc(size, ALIGN);
#elif HAVE_MEMALIGN
#ifndef __DJGPP__
ptr = memalign(ALIGN, size);
#else
ptr = memalign(size, ALIGN);
#endif
/* Why 64?
* Indeed, we should align it:
* on 4 for 386
* on 16 for 486
* on 32 for 586, PPro - K6-III
* on 64 for K7 (maybe for P3 too).
* Because L1 and L2 caches are aligned on those values.
* But I don't want to code such logic here!
*/
/* Why 32?
* For AVX ASM. SSE / NEON needs only 16.
* Why not larger? Because I did not see a difference in benchmarks ...
*/
/* benchmarks with P3
* memalign(64) + 1 3071, 3051, 3032
* memalign(64) + 2 3051, 3032, 3041
* memalign(64) + 4 2911, 2896, 2915
* memalign(64) + 8 2545, 2554, 2550
* memalign(64) + 16 2543, 2572, 2563
* memalign(64) + 32 2546, 2545, 2571
* memalign(64) + 64 2570, 2533, 2558
*
* BTW, malloc seems to do 8-byte alignment by default here.
*/
#else
ptr = malloc(size);
#endif
if(!ptr && !size) {
size = 1;
ptr= av_malloc(1);
}
#if CONFIG_MEMORY_POISONING
if (ptr)
memset(ptr, FF_MEMORY_POISON, size);
#endif
return ptr;
}2、av_mallocz
av_mallocz()是在av_malloc()基础上，调用memset()进行内存清零：void *av_mallocz(size_t size)
{
void *ptr = av_malloc(size);
if (ptr)
memset(ptr, 0, size);
return ptr;
}3、av_malloc_array av_malloc_array()先计算数组所需要内存块大小，然后用av_malloc()分配数组内存：
void *av_malloc_array(size_t nmemb, size_t size)
{
size_t result;
if (av_size_mult(nmemb, size, &result) < 0)
return NULL;
return av_malloc(result);
}4、av_mallocz_array av_mallocz_array()先计算数组所需要内存块大小，然后用av_mallocz()分配数组内存：
void *av_mallocz_array(size_t nmemb, size_t size)
{
size_t result;
if (av_size_mult(nmemb, size, &result) < 0)
return NULL;
return av_mallocz(result);
}5、av_calloc av_calloc()操作与av_mallocz_array()，先计算内存大小再用av_mallocz()分配内存：
void *av_calloc(size_t nmemb, size_t size)
{
size_t result;
if (av_size_mult(nmemb, size, &result) < 0)
return NULL;
return av_mallocz(result);
}6、av_max_alloc av_max_alloc()主要是指定分配内存的最大值：
static size_t max_alloc_size= INT_MAX;
void av_max_alloc(size_t max)
{
max_alloc_size = max;
}在av_malloc()用于判断size是否超出最大值：
void *av_malloc(size_t size)
{
void *ptr = NULL;
if (size > max_alloc_size)
return NULL;
......
}7、av_realloc
av_realloc()是对系统的realloc函数进行封装，重新分配内存块：void *av_realloc(void *ptr, size_t size)
{
if (size > max_alloc_size)
return NULL;
#if HAVE_ALIGNED_MALLOC
return _aligned_realloc(ptr, size + !size, ALIGN);
#else
return realloc(ptr, size + !size);
#endif
}8、av_realloc_array av_realloc_array()先计算内存块大小，然后用av_realloc()重新分配数组内存：
void *av_realloc_array(void *ptr, size_t nmemb, size_t size)
{
size_t result;
if (av_size_mult(nmemb, size, &result) < 0)
return NULL;
return av_realloc(ptr, result);
}9、av_fast_realloc av_fast_realloc()快速重新分配内存，如果原始内存块足够大直接复用：
void *av_fast_realloc(void *ptr, unsigned int *size, size_t min_size)
{
if (min_size <= *size)
return ptr;
if (min_size > max_alloc_size) {
*size = 0;
return NULL;
}
min_size = FFMIN(max_alloc_size, FFMAX(min_size + min_size / 16 + 32, min_size));
ptr = av_realloc(ptr, min_size);
/* we could set this to the unmodified min_size but this is safer
* if the user lost the ptr and uses NULL now
*/
if (!ptr)
min_size = 0;
*size = min_size;
return ptr;
}10、av_fast_malloc av_fast_malloc()快速分配内存：
void av_fast_malloc(void *ptr, unsigned int *size, size_t min_size)
{
ff_fast_malloc(ptr, size, min_size, 0);
}其中ff_fast_malloc()代码位于libavutil/mem_internal.h:
static inline int ff_fast_malloc(void *ptr, unsigned int *size, size_t min_size, int zero_realloc)
{
void *val;
memcpy(&val, ptr, sizeof(val));
if (min_size <= *size) {
av_assert0(val || !min_size);
return 0;
}
min_size = FFMAX(min_size + min_size / 16 + 32, min_size);
av_freep(ptr);
val = zero_realloc ? av_mallocz(min_size) : av_malloc(min_size);
memcpy(ptr, &val, sizeof(val));
if (!val)
min_size = 0;
*size = min_size;
return 1;
}11、av_fast_mallocz av_fast_mallocz()快速分配内存，并且内存清零：
void av_fast_mallocz(void *ptr, unsigned int *size, size_t min_size)
{
ff_fast_malloc(ptr, size, min_size, 1);
}二、内存拷贝 1、av_strdup av_strdup()用于重新分配内存与拷贝字符串：
char *av_strdup(const char *s)
{
char *ptr = NULL;
if (s) {
size_t len = strlen(s) + 1;
ptr = av_realloc(NULL, len);
if (ptr)
memcpy(ptr, s, len);
}
return ptr;
}2、av_strndup av_strndup()用于分配指定大小内存与拷贝字符串，先用memchr()获取有效字符串长度，然后使用av_realloc()重新分配内存，再用memcpy()拷贝字符串：
char *av_strndup(const char *s, size_t len)
{
char *ret = NULL, *end;
if (!s)
return NULL;
end = memchr(s, 0, len);
if (end)
len = end - s;
ret = av_realloc(NULL, len + 1);
if (!ret)
return NULL;
memcpy(ret, s, len);
ret[len] = 0;
return ret;
}3、av_memdup av_memdup()用于内存分配与内存拷贝，先用av_malloc()分配内存，再用memcpy()拷贝内存：
void *av_memdup(const void *p, size_t size)
{
void *ptr = NULL;
if (p) {
ptr = av_malloc(size);
if (ptr)
memcpy(ptr, p, size);
}
return ptr;
}4、av_memcpy_backptr av_memcpy_backptr()用于内存拷贝，与系统提供的memcpy()类似，并且考虑16位、24位、32位内存对齐：
void av_memcpy_backptr(uint8_t *dst, int back, int cnt)
{
const uint8_t *src = &dst[-back];
if (!back)
return;
if (back == 1) {
memset(dst, *src, cnt);
} else if (back == 2) {
fill16(dst, cnt);
} else if (back == 3) {
fill24(dst, cnt);
} else if (back == 4) {
fill32(dst, cnt);
} else {
if (cnt >= 16) {
int blocklen = back;
while (cnt > blocklen) {
memcpy(dst, src, blocklen);
dst += blocklen;
cnt -= blocklen;
blocklen <<= 1;
}
memcpy(dst, src, cnt);
return;
}
if (cnt >= 8) {
AV_COPY32U(dst, src);
AV_COPY32U(dst + 4, src + 4);
src += 8;
dst += 8;
cnt -= 8;
}
if (cnt >= 4) {
AV_COPY32U(dst, src);
src += 4;
dst += 4;
cnt -= 4;
}
if (cnt >= 2) {
AV_COPY16U(dst, src);
src += 2;
dst += 2;
cnt -= 2;
}
if (cnt)
*dst = *src;
}} 三、内存释放 1、av_free av_free()用于释放内存块，主要是调用系统free()进行释放。如果宏定义了对齐分配，那么要对齐释放：
void av_free(void *ptr)
{
#if HAVE_ALIGNED_MALLOC
_aligned_free(ptr);
#else
free(ptr);
#endif
}2、av_freep av_freep()用于释放内存指针，先备份内存指针，然后把指针地址清空，再释放内存：
void av_freep(void *arg)
{
void *val;
memcpy(&val, arg, sizeof(val));
memcpy(arg, &(void *){ NULL }, sizeof(val));
av_free(val);
}