How to programmatically retrieve the GID of a running process

I don't know of any portable way to do this. I thought maybe ptrace(), but I can't see how from the manpage. Even if that works, "tracing" the other process in any way is probably unnecessarily invasive

For Linux, your suggestion to use fopen("/proc/PID/status", "r") is about as clean and direct as you're going to get. It seems to be what gdb does.