avioalloccontext

avio_alloc_context, often referred to as avio_alloc_context in FFmpeg, is a API function that creates an AVIOContext configured to use a user-supplied I/O buffer together with callback functions for reading, writing, and seeking. It enables custom input/output sources to be integrated with FFmpeg’s demuxing and muxing pipeline, such as in-memory data, network streams, or device interfaces.

The function signature is: AVIOContext *avio_alloc_context(unsigned char *buffer, int buffer_size, int write_flag, void *opaque, int (*read_packet)(void *opaque, unsigned char *buf, int buf_size), int (*write_packet)(void *opaque, unsigned char *buf, int buf_size), int64_t (*seek)(void *opaque, int64_t offset, int whence)). The parameters are: buffer and buffer_size specify the memory region used as the I/O buffer; write_flag indicates whether the context is for writing (non-zero) or reading (zero); opaque is a user-defined context passed to the callbacks; read_packet fills a portion of the buffer with data; write_packet writes data from the buffer to the underlying medium; seek repositions the read/write pointer and takes a whence value (such as SEEK_SET, SEEK_CUR, SEEK_END).

The returned AVIOContext is typically attached to a format context, for example by assigning it to ctx->pb, allowing FFmpeg to read from or write to the custom source. If seeking is unsupported, the seek callback may be NULL.

Memory management is handled by avio_context_free, which frees the AVIOContext and its associated buffer when finished. This function enables flexible, custom I/O while leveraging FFmpeg’s standard demuxing and muxing facilities.