Create FFmpeg源码分析：内存管理系统.md

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+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](https://img-blog.csdnimg.cn/ec570c9e0f8f49fdb21ec2819acee3d9.png?x-oss-process=image/watermark,type_d3F5LXplbmhlaQ,shadow_50,text_Q1NETiBA5b6Q56aP6K6wNDU2,size_20,color_FFFFFF,t_70,g_se,x_16)
+
+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：
+
+```c
+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：
+```
+
+```c
+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：
+
+```c
+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：
+
+```c
+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：
+
+```c
+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：
+
+```c
+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：
+
+```c
+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：
+```
+
+```c
+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：
+
+```c
+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：
+
+```c
+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：
+
+```c
+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:
+
+
+
+```c
+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：
+
+```c
+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：
+
+```c
+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：
+
+```c
+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：
+
+```c
+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：
+
+```c
+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：
+
+```c
+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：
+
+```c
+void av_freep(void *arg)
+{
+    void *val;
+memcpy(&val, arg, sizeof(val));
+memcpy(arg, &(void *){ NULL }, sizeof(val));
+av_free(val);
+}
+```