Check if process exists given its pid

Question

Given the pid of a Linux process, I want to check, from a C program, if the process is still running.

Answer 1

kill(pid, 0) is the typical approach, as @blagovest-buyukliev said. But if the process you are checking might be owned by a different user, and you don't want to take the extra steps to check whether errno == ESRCH, it turns out that

(getpgid(pid) >= 0)

is an effective one-step method for determining if any process has the given PID (since you are allowed to inspect the process group ID even for processes that don't belong to you).