First make sure FFmpeg command could work in your command line. If you don't set up a FFmpeg yet, you could down load an executable file from here, and set the path of ffmpeg to you environment variable PATH. After that, open the command line and input ffmpeg to ensure that the system can recognize the command correctly.
gcc main.c -fPIC -O3 -lm -DNDEBUG -o test./test cactus.mp4 out.mp4 -crf 29