Input takes a video, any format/fps, converts to PPM and sends to a pipe with CFR:
[i] video.c: cmd= ffmpeg -loglevel quiet -i input.mp4 -pix_fmt gray -s 960x540 -vcodec rawvideo -vf vflip -f image2pipe -r 30 -vsync cfr -

Program processes frames and outputs to a pipe for encoding:
[i] mp4.c:   cmd= ffmpeg -loglevel quiet -re -vcodec ppm -f image2pipe -i pipe: -vcodec libx264 -pix_fmt yuv420p -r 30 -vsync cfr output.mp4