Check if process exists given its pid

Issue a kill(2) system call with 0 as the signal. If the call succeeds, it means that a process with this pid exists.

If the call fails and errno is set to ESRCH, a process with such a pid does not exist.

Quoting the POSIX standard:

If sig is 0 (the null signal), error checking is performed but no signal is actually sent. The null signal can be used to check the validity of pid.

Note that you are not safe from race conditions: it is possible that the target process has exited and another process with the same pid has been started in the meantime. Or the process may exit very quickly after you check it, and you could do a decision based on outdated information.

Only if the given pid is of a child process (fork'ed from the current one), you can use waitpid(2) with the WNOHANG option, or try to catch SIGCHLD signals. These are safe from race conditions, but are only relevant to child processes.

Use procfs.

#include <sys/stat.h>[...]struct stat sts;if (stat("/proc/<pid>", &sts) == -1 && errno == ENOENT) {  // process doesn't exist}

Easily portable to

• Solaris
• IRIX
• Tru64 UNIX
• BSD
• Linux
• IBM AIX
• QNX
• Plan 9 from Bell Labs

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).