An ffmpeg and SDL Tutorial

Text version

Tutorial 07: Seeking

Code: tutorial07.c

Handling the seek command

Now we're going to add some seeking capabilities to our player, because it's really annoying when you can't rewind a movie. Plus, this will show you how easy the av_seek_frame function is to use.

We're going to make the left and right arrows go back and forth in the movie by a little and the up and down arrows a lot, where "a little" is 10 seconds, and "a lot" is 60 seconds. So we need to set up our main loop so it catches the keystrokes. However, when we do get a keystroke, we can't call av_seek_frame directly. We have to do that in our main decode loop, the decode_thread loop. So instead, we're going to add some values to the big struct that will contain the new position to seek to and some seeking flags:

  int             seek_req;
  int             seek_flags;
  int64_t         seek_pos;

Now we need to set up our main loop to catch the key presses:

  for(;;) {
    double incr, pos;

    switch(event.type) {
    case SDL_KEYDOWN:
      switch(event.key.keysym.sym) {
      case SDLK_LEFT:
	incr = -10.0;
	goto do_seek;
      case SDLK_RIGHT:
	incr = 10.0;
	goto do_seek;
      case SDLK_UP:
	incr = 60.0;
	goto do_seek;
      case SDLK_DOWN:
	incr = -60.0;
	goto do_seek;
	if(global_video_state) {
	  pos = get_master_clock(global_video_state);
	  pos += incr;
                      (int64_t)(pos * AV_TIME_BASE), incr);
To detect keypresses, we first look and see if we get an SDL_KEYDOWN event. Then we check and see which key got hit using event.key.keysym.sym. Once we know which way we want to seek, we calculate the new time by adding the increment to the value from our new get_master_clock function. Then we call a stream_seek function to set the seek_pos, etc., values. We convert our new time to avcodec's internal timestamp unit. Recall that timestamps in streams are measured in frames rather than seconds, with the formula seconds = frames * time_base (fps). avcodec defaults to a value of 1,000,000 fps (so a pos of 2 seconds will be timestamp of 2000000). We'll see why we need to convert this value later.

Here's our stream_seek function. Notice we set a flag if we are going backwards:

void stream_seek(VideoState *is, int64_t pos, int rel) {

  if(!is->seek_req) {
    is->seek_pos = pos;
    is->seek_flags = rel < 0 ? AVSEEK_FLAG_BACKWARD : 0;
    is->seek_req = 1;
Now let's go over to our decode_thread where we will actually perform our seek. You'll notice in the source files that we've marked an area "seek stuff goes here". Well, we're going to put it there now.

Seeking centers around the av_seek_frame function. This function takes a format context, a stream, a timestamp, and a set of flags as an argument. The function will seek to the timestamp you give it. The unit of the timestamp is the time_base of the stream you pass the function. However, you do not have to pass it a stream (indicated by passing a value of -1). If you do that, the time_base will be in avcodec's internal timestamp unit, or 1000000fps. This is why we multiplied our position by AV_TIME_BASE when we set seek_pos.

However, sometimes you can (rarely) run into problems with some files if you pass av_seek_frame -1 for a stream, so we're going to pick the first stream in our file and pass it to av_seek_frame. Don't forget we have to rescale our timestamp to be in the new unit too.

if(is->seek_req) {
  int stream_index= -1;
  int64_t seek_target = is->seek_pos;

  if     (is->videoStream >= 0) stream_index = is->videoStream;
  else if(is->audioStream >= 0) stream_index = is->audioStream;

    seek_target= av_rescale_q(seek_target, AV_TIME_BASE_Q,
  if(av_seek_frame(is->pFormatCtx, stream_index, 
                    seek_target, is->seek_flags) < 0) {
    fprintf(stderr, "%s: error while seeking\n",
  } else {
     /* handle packet queues... more later... */

av_rescale_q(a,b,c) is a function that will rescale a timestamp from one base to another. It basically computes a*b/c but this function is required because that calculation could overflow. AV_TIME_BASE_Q is the fractional version of AV_TIME_BASE. They're quite different: AV_TIME_BASE * time_in_seconds = avcodec_timestamp and AV_TIME_BASE_Q * avcodec_timestamp = time_in_seconds (but note that AV_TIME_BASE_Q is actually an AVRational object, so you have to use special q functions in avcodec to handle it).

Flushing our buffers

So we've set our seek correctly, but we aren't finished quite yet. Remember that we have a queue set up to accumulate packets. Now that we're in a different place, we have to flush that queue or the movie ain't gonna seek! Not only that, but avcodec has its own internal buffers that need to be flushed too by each thread.

To do this, we need to first write a function to clear our packet queue. Then, we need to have some way of instructing the audio and video thread that they need to flush avcodec's internal buffers. We can do this by putting a special packet on the queue after we flush it, and when they detect that special packet, they'll just flush their buffers.

Let's start with the flush function. It's really quite simple, so I'll just show you the code:

static void packet_queue_flush(PacketQueue *q) {
  AVPacketList *pkt, *pkt1;

  for(pkt = q->first_pkt; pkt != NULL; pkt = pkt1) {
    pkt1 = pkt->next;
  q->last_pkt = NULL;
  q->first_pkt = NULL;
  q->nb_packets = 0;
  q->size = 0;

Now that the queue is flushed, let's put on our "flush packet." But first we're going to want to define what that is and create it:

AVPacket flush_pkt;

main() {
  av_init_packet(&flush_pkt); = "FLUSH";
Now we put this packet on the queue:
  } else {
    if(is->audioStream >= 0) {
      packet_queue_put(&is->audioq, &flush_pkt);
    if(is->videoStream >= 0) {
      packet_queue_put(&is->videoq, &flush_pkt);
  is->seek_req = 0;
(This code snippet also continues the code snippet above for decode_thread.) We also need to change packet_queue_put so that we don't duplicate the special flush packet:
int packet_queue_put(PacketQueue *q, AVPacket *pkt) {

  AVPacketList *pkt1;
  if(pkt != &flush_pkt && av_dup_packet(pkt) < 0) {
    return -1;
And then in the audio thread and the video thread, we put this call to avcodec_flush_buffers immediately after packet_queue_get:
    if(packet_queue_get(&is->audioq, pkt, 1) < 0) {
      return -1;
    if(pkt->data == {
The above code snippet is exactly the same for the video thread, with "audio" being replaced by "video".

That's it! We're done! Go ahead and compile your player:

gcc -o tutorial07 tutorial07.c -lavutil -lavformat -lavcodec -lswscale -lz -lm \
`sdl-config --cflags --libs`
and enjoy your movie player made in less than 1000 lines of C!

Of course, there's a lot of things we glanced over that we could add.

>> What's Left?

This work is licensed under the Creative Commons Attribution-Share Alike 2.5 License. To view a copy of this license, visit or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA.

Code examples are based off of FFplay, Copyright (c) 2003 Fabrice Bellard, and a tutorial by Martin Bohme.