pidfd_open(2)

Synopsis

#include <sys/syscall.h>
#include <unistd.h>

int syscall(SYS_pidfd_open, pid_t pid, unsigned int flags);

Rod's take

A PID is a number the kernel will hand to someone else the moment your child dies, and kill(2) has trusted that number since Edition 6. pidfd_open gives you a reference instead. Poll it, signal it, waitid(P_PIDFD) on it, and you're talking to the process you meant, not whatever inherited the slot. Fine print: open it after fork() and it's only race-free if nobody else reaped the child and SIGCHLD isn't ignored; otherwise use clone3 with CLONE_PIDFD. PIDs for logging, pidfds for control. Fifty years late, but correct.

Flags and arguments that matter

Errors you will actually hit

Example

int pfd = syscall(SYS_pidfd_open, pid, 0);
if (pfd < 0)
    err(1, "pidfd_open");           /* ESRCH: already gone */

struct pollfd p = { .fd = pfd, .events = POLLIN };
poll(&p, 1, -1);                     /* readable once it exits */

siginfo_t info = {0};
waitid(P_PIDFD, pfd, &info, WEXITED); /* only if it's your child */
close(pfd);

Trimmed from the full walkthrough in pidfd: Unix finally has a handle on your processes (only took 50 years).

BSD equivalent

FreeBSD: process descriptors created at fork time with pdfork(2), plus pdgetpid(2) and EVFILT_PROCDESC in kqueue(2) (FreeBSD 9.0). OpenBSD: none.