ffmpeg中avio_alloc_context设置

/**
 * Allocate and initialize an AVIOContext for buffered I/O. It must be later
 * freed with av_free().
 *
 * @param buffer Memory block for input/output operations via AVIOContext.
 *        The buffer must be allocated with av_malloc() and friends.
 *        It may be freed and replaced with a new buffer by libavformat.
 *        AVIOContext.buffer holds the buffer currently in use,
 *        which must be later freed with av_free().
 * @param buffer_size The buffer size is very important for performance.
 *        For protocols with fixed blocksize it should be set to this blocksize.
 *        For others a typical size is a cache page, e.g. 4kb.
 * @param write_flag Set to 1 if the buffer should be writable, 0 otherwise.
 * @param opaque An opaque pointer to user-specific data.
 * @param read_packet  A function for refilling the buffer, may be NULL.
 * @param write_packet A function for writing the buffer contents, may be NULL.
 *        The function may not change the input buffers content.
 * @param seek A function for seeking to specified byte position, may be NULL.
 *
 * @return Allocated AVIOContext or NULL on failure.
 */
AVIOContext *avio_alloc_context(
                  unsigned char *buffer,
                  int buffer_size,
                  int write_flag,
                  void *opaque,
                  int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),
                  int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),
                  int64_t (*seek)(void *opaque, int64_t offset, int whence));

avio_alloc_context是ffmpeg中用来读写buffer的函数，根据读写需求分配函数参数。