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pidfd_open(2) — Linux manual page

NAME \|LIBRARY \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|ERRORS \|STANDARDS \|HISTORY \|NOTES \|EXAMPLES \|SEE ALSO \|COLOPHON

pidfd_open(2)              System Calls Manualpidfd_open(2)

NAME top

       pidfd_open - obtain a file descriptor that refers to a process

LIBRARY top

       Standard C library (libc,-lc)

SYNOPSIS top

#include <sys/syscall.h>/* Definition ofSYS_*constants */#include <unistd.h>int syscall(SYS_pidfd_open, pid_tpid, unsigned intflags);Note: glibc provides no wrapper forpidfd_open(), necessitating       the use ofsyscall(2).

DESCRIPTION top

       Thepidfd_open() system call creates a file descriptor that refers       to the process whose PID is specified inpid.  The file descriptor       is returned as the function result; the close-on-exec flag is set       on the file descriptor.       Theflags argument either has the value 0, or contains the       following flag:PIDFD_NONBLOCK(since Linux 5.10)              Return a nonblocking file descriptor.  If the process              referred to by the file descriptor has not yet terminated,              then an attempt to wait on the file descriptor usingwaitid(2) will immediately return the errorEAGAINrather              than blocking.

RETURN VALUE top

       On success,pidfd_open() returns a file descriptor (a nonnegative       integer).  On error, -1 is returned anderrno is set to indicate       the error.

ERRORS top

EINVALflags is not valid.EINVALpid is not valid.EMFILEThe per-process limit on the number of open file              descriptors has been reached (see the description ofRLIMIT_NOFILEingetrlimit(2)).ENFILEThe system-wide limit on the total number of open files has              been reached.ENODEVThe anonymous inode filesystem is not available in this              kernel.ENOMEMInsufficient kernel memory was available.ESRCHThe process specified bypid does not exist.

STANDARDS top

       Linux.

HISTORY top

       Linux 5.3.

NOTES top

       The following code sequence can be used to obtain a file       descriptor for the child offork(2):           pid = fork();           if (pid > 0) {     /* If parent */               pidfd = pidfd_open(pid, 0);               ...           }       Even if the child has already terminated by the time of thepidfd_open() call, its PID will not have been recycled and the       returned file descriptor will refer to the resulting zombie       process.  Note, however, that this is guaranteed only if the       following conditions hold true:       •  the disposition ofSIGCHLDhas not been explicitly set toSIG_IGN(seesigaction(2));       •  theSA_NOCLDWAITflag was not specified while establishing a          handler forSIGCHLDor while setting the disposition of that          signal toSIG_DFL(seesigaction(2)); and       •  the zombie process was not reaped elsewhere in the program          (e.g., either by an asynchronously executed signal handler or          bywait(2) or similar in another thread).       If any of these conditions does not hold, then the child process       (along with a PID file descriptor that refers to it) should       instead be created usingclone(2) with theCLONE_PIDFDflag.Use cases for PID file descriptors       A PID file descriptor returned bypidfd_open() (or byclone(2)       with theCLONE_PIDflag) can be used for the following purposes:       •  Thepidfd_send_signal(2) system call can be used to send a          signal to the process referred to by a PID file descriptor.       •  A PID file descriptor can be monitored usingpoll(2),select(2), andepoll(7).  When the process that it refers to          terminates, these interfaces indicate the file descriptor as          readable.  Note, however, that in the current implementation,          nothing can be read from the file descriptor (read(2) on the          file descriptor fails with the errorEINVAL).       •  If the PID file descriptor refers to a child of the calling          process, then it can be waited on usingwaitid(2).       •  Thepidfd_getfd(2) system call can be used to obtain a          duplicate of a file descriptor of another process referred to          by a PID file descriptor.       •  A PID file descriptor can be used as the argument ofsetns(2)          in order to move into one or more of the same namespaces as the          process referred to by the file descriptor.       •  A PID file descriptor can be used as the argument ofprocess_madvise(2) in order to provide advice on the memory          usage patterns of the process referred to by the file          descriptor.       Thepidfd_open() system call is the preferred way of obtaining a       PID file descriptor for an already existing process.  The       alternative is to obtain a file descriptor by opening a/proc/pid       directory.  However, the latter technique is possible only if theproc(5) filesystem is mounted; furthermore, the file descriptor       obtained in this way isnot pollable and can't be waited on withwaitid(2).

EXAMPLES top

       The program below opens a PID file descriptor for the process       whose PID is specified as its command-line argument.  It then usespoll(2) to monitor the file descriptor for process exit, as       indicated by anEPOLLINevent.Program source       #define _GNU_SOURCE       #include <poll.h>       #include <stdio.h>       #include <stdlib.h>       #include <sys/syscall.h>       #include <sys/types.h>       #include <unistd.h>       static int       pidfd_open(pid_t pid, unsigned int flags)       {           return syscall(SYS_pidfd_open, pid, flags);       }       int       main(int argc, char *argv[])       {           int            pidfd, ready;           struct pollfd  pollfd;           if (argc != 2) {               fprintf(stderr, "Usage: %s <pid>\n", argv[0]);               exit(EXIT_SUCCESS);           }           pidfd = pidfd_open(atoi(argv[1]), 0);           if (pidfd == -1) {               perror("pidfd_open");               exit(EXIT_FAILURE);           }           pollfd.fd = pidfd;           pollfd.events = POLLIN;           ready = poll(&pollfd, 1, -1);           if (ready == -1) {               perror("poll");               exit(EXIT_FAILURE);           }           printf("Events (%#x): POLLIN is %sset\n", pollfd.revents,                  (pollfd.revents & POLLIN) ? "" : "not ");           close(pidfd);           exit(EXIT_SUCCESS);       }

COLOPHON top

       This page is part of theman-pages (Linux kernel and C library       user-space interface documentation) project.  Information about       the project can be found at        ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report       for this manual page, see       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.       This page was obtained from the tarball man-pages-6.15.tar.gz       fetched from       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on       2025-08-11.  If you discover any rendering problems in this HTML       version of the page, or you believe there is a better or more up-       to-date source for the page, or you have corrections or       improvements to the information in this COLOPHON (which isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-05-17pidfd_open(2)

Pages that refer to this page:pgrep(1), clone(2), fanotify_init(2), pidfd_getfd(2), pidfd_send_signal(2), process_madvise(2), seccomp_unotify(2), setns(2), syscalls(2), wait(2), id_t(3type), sd_bus_creds_get_pid(3), sd_bus_creds_new_from_pid(3), sd_event_add_child(3), sd_pid_get_owner_uid(3), org.freedesktop.systemd1(5), fanotify(7)