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ffmpeg_develop_doc/FFmpeg源码分析:内存管理系统.md

8.6 KiB

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:

img

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;
}2av_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;
 
......
}7av_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);
}