Find parent pid given a pid

A few days ago I needed to get the parent process id (pid) of a given pid on Linux. There are shell commands for that, .e.g, ps -o ppid= p the-pid, but I needed to get the pid programmatically from C.

For some reason the getppid function can only return the parent pid of the current process, not any process. I searched the web after an alternative solution but I found nothing.

So I took a peek at the source for the ps command, which revealed that there is a function called get_proc_stats defined in proc/readproc.h that (among other things) returns the parent pid of a given pid. You need to do install libproc-dev to get this function. You can then do:

#include <proc/readproc.h>
void printppid(pid_t pid) {
proc_t process_info;
get_proc_stats(pid, &process_info);
printf("Parent of pid=%d is pid=%d\n", pid, process_info.ppid);
}

and compile it with gcc the-file.c -lproc. I hope you find this useful.