Reading a file located in memory with libavformat

Question

I\'m currently trying to read small video files sent from a server

In order to read a file using libavformat, you are supposed to call

av_open_input_         


        

Answer 1

Tomaka17's excellent answer gave me a good start toward solving an analogous problem using Qt QIODevice rather than std::istream. I found I needed to blend aspects of Tomaka17's solution, with aspects of the related experience at http://cdry.wordpress.com/2009/09/09/using-custom-io-callbacks-with-ffmpeg/

My custom Read function looks like this:

int readFunction(void* opaque, uint8_t* buf, int buf_size)
{
    QIODevice* stream = (QIODevice*)opaque;
    int numBytes = stream->read((char*)buf, buf_size);
    return numBytes;
}

...but I also needed to create a custom Seek function:

int64_t seekFunction(void* opaque, int64_t offset, int whence)
{
    if (whence == AVSEEK_SIZE)
        return -1; // I don't know "size of my handle in bytes"
    QIODevice* stream = (QIODevice*)opaque;
    if (stream->isSequential())
        return -1; // cannot seek a sequential stream
    if (! stream->seek(offset) )
        return -1;
    return stream->pos();
}

...and I tied it together like this:

...
const int ioBufferSize = 32768;
unsigned char * ioBuffer = (unsigned char *)av_malloc(ioBufferSize + FF_INPUT_BUFFER_PADDING_SIZE); // can get av_free()ed by libav
AVIOContext * avioContext = avio_alloc_context(ioBuffer, ioBufferSize, 0, (void*)(&fileStream), &readFunction, NULL, &seekFunction);
AVFormatContext * container = avformat_alloc_context();
container->pb = avioContext;
avformat_open_input(&container, "dummyFileName", NULL, NULL);
...

Note I have not yet worked out the memory management issues.