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FFmpeg内存管理相关的源码分析：
FFmpeg中内存分配和释放相关的源码：av_malloc函数、av_mallocz函数、av_free函数和av_freep函数分析
FFmpeg源码：av_realloc、av_reallocp、size_mult、av_realloc_f函数分析
FFmpeg引用计数数据缓冲区相关的结构体：AVBuffer、AVBufferRef简介
FFmpeg源码：buffer_create、av_buffer_create、av_buffer_default_free、av_buffer_alloc、av_buffer_allocz函数分析
FFmpeg源码：av_buffer_ref、av_buffer_unref函数分析
FFmpeg源码：av_buffer_is_writable、av_buffer_realloc函数分析
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一、av_buffer_is_writable函数
（一）av_buffer_is_writable函数的声明
av_buffer_is_writable函数声明在FFmpeg源码（本文演示用的FFmpeg源码版本为7.0.1）的头文件libavutil/buffer.h中：
/**
* @return 1 if the caller may write to the data referred to by buf (which is
* true if and only if buf is the only reference to the underlying AVBuffer).
* Return 0 otherwise.
* A positive answer is valid until av_buffer_ref() is called on buf.
*/
int av_buffer_is_writable(const AVBufferRef *buf);该函数作用是：判断是否可以写入buf引用的数据，如果调用者可以写入(当且仅当buf是对底层AVBuffer的唯一引用时），返回1，否则返回0。
（二）av_buffer_is_writable函数的定义
av_buffer_is_writable函数定义在FFmpeg源码的源文件libavutil/buffer.c中：
/**
* Always treat the buffer as read-only, even when it has only one
* reference.
*/
#define AV_BUFFER_FLAG_READONLY (1 << 0)
int av_buffer_is_writable(const AVBufferRef *buf)
{
if (buf->buffer->flags & AV_BUFFER_FLAG_READONLY)
return 0;
return atomic_load(&buf->buffer->refcount) == 1;
}该函数内部，首先通过buf->buffer->flags判断是否始终将缓冲区视为只读的，如果是，返回0：
if (buf->buffer->flags & AV_BUFFER_FLAG_READONLY)
return 0;然后通过atomic_load函数读取&buf->buffer->refcount的值。如果该值为1，表示引用buf缓冲区的现有AVBufferRef实例的数目为1，即引用计数为1，此时av_buffer_is_writable函数返回1，表示调用者可以写入；如果引用计数不为1，av_buffer_is_writable函数返回0，表示不可写入：
return atomic_load(&buf->buffer->refcount) == 1;二、av_buffer_realloc函数
（一）av_buffer_realloc函数的声明
av_buffer_realloc函数声明在头文件libavutil/buffer.h中：
/**
* Reallocate a given buffer.
*
* @param buf a buffer reference to reallocate. On success, buf will be
* unreferenced and a new reference with the required size will be
* written in its place. On failure buf will be left untouched. *buf
* may be NULL, then a new buffer is allocated.
* @param size required new buffer size.
* @return 0 on success, a negative AVERROR on failure.
*
* @note the buffer is actually reallocated with av_realloc() only if it was
* initially allocated through av_buffer_realloc(NULL) and there is only one
* reference to it (i.e. the one passed to this function). In all other cases
* a new buffer is allocated and the data is copied.
*/
int av_buffer_realloc(AVBufferRef **buf, size_t size);该函数作用是：给(*pbuf)指向的AVBufferRef结构体、(*pbuf)->buffer分配内存，给(*pbuf)->data重新分配大小为size个字节内存。
返回值：返回0表示成功，返回AVERROR(ENOMEM)表示失败。
（二）av_buffer_realloc函数的定义
av_buffer_realloc函数定义在源文件libavutil/buffer.c中：
int av_buffer_realloc(AVBufferRef **pbuf, size_t size)
{
AVBufferRef *buf = *pbuf;
uint8_t *tmp;
int ret;
if (!buf) {
/* allocate a new buffer with av_realloc(), so it will be reallocatable
* later */
uint8_t *data = av_realloc(NULL, size);
if (!data)
return AVERROR(ENOMEM);
buf = av_buffer_create(data, size, av_buffer_default_free, NULL, 0);
if (!buf) {
av_freep(&data);
return AVERROR(ENOMEM);
}
buf->buffer->flags_internal |= BUFFER_FLAG_REALLOCATABLE;
*pbuf = buf;
return 0;
} else if (buf->size == size)
return 0;
if (!(buf->buffer->flags_internal & BUFFER_FLAG_REALLOCATABLE) ||
!av_buffer_is_writable(buf) || buf->data != buf->buffer->data) {
/* cannot realloc, allocate a new reallocable buffer and copy data */
AVBufferRef *new = NULL;
ret = av_buffer_realloc(&new, size);
if (ret < 0)
return ret;
memcpy(new->data, buf->data, FFMIN(size, buf->size));
buffer_replace(pbuf, &new);
return 0;
}
tmp = av_realloc(buf->buffer->data, size);
if (!tmp)
return AVERROR(ENOMEM);
buf->buffer->data = buf->data = tmp;
buf->buffer->size = buf->size = size;
return 0;
}