I want to multiplex audio and video together using the ffmpeg encoding library with xvid264rgb codec. I can open the codec when I create the context specifying that codec, but not if I use NULL for the argument with allocate context. My understanding is I need to use NULL so I can open the audio codec as well.

In the example, the first open works with codec indicated in the allocate context. The second doesn’t.

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <libavcodec/avcodec.h>
#include <libavutil/opt.h>
#define WIDTH 400
#define HEIGHT 300

const AVCodec *videocodec;

bool init_video_encoder(AVCodecContext *c, int width, int height) {
    int ret;
    
    const AVRational tb_rat = {1, 30};
    const AVRational fr_rat = {30, 1};

    fprintf(stderr, "init video encodern");
    // video
    //fprintf(stderr, "speed= %d n", videocodec->speed);
    c->bit_rate = 400000;
    c->width = width;
    c->height = height;
    c->time_base = tb_rat;
    c->framerate = fr_rat;
    c->gop_size = 10;
    c->max_b_frames = 1;
    c->pix_fmt = AV_PIX_FMT_RGB24;
    if( videocodec->id == AV_CODEC_ID_H264 ) 
        av_opt_set(c->priv_data, "preset", "slow", 0);
        
    ret = avcodec_open2(c, videocodec, NULL);
    if( ret < 0 ) {
        fprintf(stderr, "failure to open codec: %sn", av_err2str(ret));
        return false;
    }
    //avcodec_free_context(&c);
    return true;
}

int main(int argc, char **argv) {

    AVCodecContext *context = NULL;

    videocodec = avcodec_find_encoder_by_name("libx264rgb");
    if( videocodec == NULL ) {
        fprintf(stderr, "video encoder 'libx264rgb' not found");
        exit;
    }

    context = avcodec_alloc_context3(videocodec);
    if( context == NULL ) {
        fprintf(stderr, "could not allocate context with codec");
        return false;
    }
    if( init_video_encoder( context, WIDTH, HEIGHT ) ) 
        fprintf(stderr, " success initializing video encoder case onen");
    else
        fprintf(stderr, " failure to initialize video encoder case onen");
    avcodec_close( context );

    context = avcodec_alloc_context3(NULL);
    if( context == NULL ) {
        fprintf(stderr, "could not allocate context without codec");
        return false;
    }
    if( init_video_encoder( context, WIDTH, HEIGHT ) ) 
        fprintf(stderr, " success initializing video encoder case twon");
    else
        fprintf(stderr, " failure to initialize video encoder case twon");
    avcodec_close( context );
}

Here is the error message:

broken ffmpeg default settings detected
use an encoding preset (e.g. -vpre medium)
preset usage: -vpre <speed> -vpre <profile>
speed presets are listed in x264 --help
profile is optional; x264 defaults to high
failure to open codec: Generic error in an external library
failure to initialize video encoder case two

Is this the right approach to use? How do I set the encoding preset? The code appears already to do that.