FILE is opaque -- you're not supposed to mess with what's inside, because it might be different everywhere you go. It's like a system call, you aren't supposed to ask the kernel what's inside its file tables. Technically you can get the fd with fileno() but since any actions on fp after you mess with its raw file descriptor may be unpredictable, I'd suggest making a pipe(), forking, and redirecting through the pipe instead. That's how popen works anyhow.