avformatwriteheader

avformat_write_header is a function in FFmpeg’s libavformat that writes the header for an output format associated with a given AVFormatContext. It initializes the container file’s header based on the configured streams and metadata and is typically called after opening the output and configuring streams, but before writing any packet data.

Prototype and parameters:

int avformat_write_header(AVFormatContext *s, AVDictionary **options);

The first argument, s, is a pointer to an initialized AVFormatContext with an open IO context and configured streams. The second argument, options, is an optional dictionary of muxer-specific options that may influence how the header is written; pass NULL if no options are needed.

Return value:

0 on success, or a negative error code on failure. The error codes follow FFmpeg’s conventional negative values and can indicate IO errors, unsupported features, or invalid configuration.

Usage notes:

Call avformat_write_header after setting up the output context (including avio_open) and creating or configuring streams, and before writing packet data with av_write_frame or av_interleaved_write_frame. If the function fails, handle cleanup and free resources accordingly. The header may contain format-specific data such as file signatures, track metadata, and codec parameters serialized into the container’s header.

Remarks:

Some muxers may write necessary header information automatically during the first packet write, in which case the function could be a no-op. Always check the return value and use an options dictionary if you need to customize muxer behavior. This function is part of the typical FFmpeg muxing workflow that also includes avformat_write_trailer for finalization.