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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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pidfd_open(2) System Calls Manual pidfd_open(2)
pidfd_open - obtain a file descriptor that refers to a task
Standard C library (libc, -lc)
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_pidfd_open, pid_t pid, unsigned int flags);
Note: glibc provides no wrapper for pidfd_open(), necessitating
the use of syscall(2).
The pidfd_open() system call creates a file descriptor that refers
to the task referenced by pid. The file descriptor is returned as
the function result; the close-on-exec flag is set on the file
descriptor.
The flags argument either has the value 0, or contains the
following flags:
PIDFD_NONBLOCK (since Linux 5.10)
Return a nonblocking file descriptor. If the task referred
to by the file descriptor has not yet terminated, then an
attempt to wait on the file descriptor using waitid(2) will
immediately return the error EAGAIN rather than blocking.
PIDFD_THREAD (since Linux 6.9)
Create a file descriptor that refers to a specific thread,
rather than a process (thread-group leader). If this flag
is not set, pid must refer to a process.
On success, pidfd_open() returns a file descriptor (a nonnegative
integer). On error, -1 is returned and errno is set to indicate
the error.
EINVAL flags is not valid.
EINVAL pid is not valid.
EMFILE The per-process limit on the number of open file
descriptors has been reached (see the description of
RLIMIT_NOFILE in getrlimit(2)).
ENFILE The system-wide limit on the total number of open files has
been reached.
ENODEV The anonymous inode filesystem is not available in this
kernel.
ENOMEM Insufficient kernel memory was available.
ESRCH The process specified by pid does not exist.
Linux.
Linux 5.3.
The following code sequence can be used to obtain a file
descriptor for the child of fork(2):
pid = fork();
if (pid > 0) { /* If parent */
pidfd = pidfd_open(pid, 0);
...
}
Even if the child has already terminated by the time of the
pidfd_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 of SIGCHLD has not been explicitly set to
SIG_IGN (see sigaction(2));
• the SA_NOCLDWAIT flag was not specified while establishing a
handler for SIGCHLD or while setting the disposition of that
signal to SIG_DFL (see sigaction(2)); and
• the zombie process was not reaped elsewhere in the program
(e.g., either by an asynchronously executed signal handler or
by wait(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 using clone(2) with the CLONE_PIDFD flag.
Use cases for PID file descriptors
A PID file descriptor returned by pidfd_open() (or by clone(2)
with the CLONE_PID flag) can be used for the following purposes:
• The pidfd_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 using poll(2),
select(2), and epoll(7).
When the task that it refers to terminates and becomes a
zombie, these interfaces indicate the file descriptor as
readable (EPOLLIN). When the task is reaped, these interfaces
produce a hangup event (EPOLLHUP).
Note, however, that in the current implementation, nothing can
be read from the file descriptor (read(2) on the file
descriptor fails with the error EINVAL). The polling behavior
depends on whether PIDFD_THREAD flag was used when obtaining
the file descriptor:
• With PIDFD_THREAD, the file descriptor becomes readable when
the task exits and becomes a zombie, even if the thread-
group is not empty.
• Without PIDFD_THREAD, the file descriptor becomes readable
only when the last thread in the thread group exits.
• If the PID file descriptor refers to a child of the calling
process, then it can be waited on using waitid(2).
• The pidfd_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 of setns(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 of
process_madvise(2) in order to provide advice on the memory
usage patterns of the process referred to by the file
descriptor.
The pidfd_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 the
proc(5) filesystem is mounted; furthermore, the file descriptor
obtained in this way is not pollable and can't be waited on with
waitid(2).
The program below opens a PID file descriptor for the process
whose PID is specified as its command-line argument. It then uses
poll(2) to monitor the file descriptor for process exit, as
indicated by an EPOLLIN event.
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);
}
clone(2), kill(2), pidfd_getfd(2), pidfd_send_signal(2), poll(2),
process_madvise(2), select(2), setns(2), waitid(2), epoll(7)
This page is part of the man-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-6.16.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2026-01-16. 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 is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.16 2025-10-29 pidfd_open(2)
Pages that refer to this page: getino(1), 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)
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HTML rendering created 2026-01-16 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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