函数 av_demuxer_iterate 

pub unsafe extern "C" fn av_demuxer_iterate(
    opaque: *mut *mut c_void,
) -> *const AVInputFormat

Iterate over all registered demuxers.

@param opaque a pointer where libavformat will store the iteration state. Must point to NULL to start the iteration.

@return the next registered demuxer or NULL when the iteration is finished