pidfd_open(2) - Linux manual page (original) (raw)
pidfdopen(2) System Calls Manual pidfdopen(2)
NAME top
pidfd_open - obtain a file descriptor that refers to a processLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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)](../man2/syscall.2.html).DESCRIPTION top
The **pidfd_open**() system call creates a file descriptor that refers
to the process whose PID is specified in _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 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 using
[waitid(2)](../man2/waitid.2.html) will immediately return the error **EAGAIN** rather
than blocking.RETURN VALUE top
On success, **pidfd_open**() returns a file descriptor (a nonnegative
integer). On error, -1 is returned and _[errno](../man3/errno.3.html)_ is set to indicate
the error.ERRORS top
**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)](../man2/getrlimit.2.html)).
**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.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 of [fork(2)](../man2/fork.2.html):
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)](../man2/sigaction.2.html));
• 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)](../man2/sigaction.2.html)); and
• the zombie process was not reaped elsewhere in the program
(e.g., either by an asynchronously executed signal handler or
by [wait(2)](../man2/wait.2.html) 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)](../man2/clone.2.html) 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)](../man2/pidfd%5Fsend%5Fsignal.2.html) 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)](../man2/poll.2.html),
[select(2)](../man2/select.2.html), and [epoll(7)](../man7/epoll.7.html). 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)](../man2/read.2.html) on the
file descriptor fails with the error **EINVAL**).
• If the PID file descriptor refers to a child of the calling
process, then it can be waited on using [waitid(2)](../man2/waitid.2.html).
• The [pidfd_getfd(2)](../man2/pidfd%5Fgetfd.2.html) 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)](../man2/setns.2.html)
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)](../man2/process%5Fmadvise.2.html) 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)](../man5/proc.5.html) filesystem is mounted; furthermore, the file descriptor
obtained in this way is _not_ pollable and can't be waited on with
[waitid(2)](../man2/waitid.2.html).EXAMPLES top
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)](../man2/poll.2.html) 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);
}SEE ALSO top
[clone(2)](../man2/clone.2.html), [kill(2)](../man2/kill.2.html), [pidfd_getfd(2)](../man2/pidfd%5Fgetfd.2.html), [pidfd_send_signal(2)](../man2/pidfd%5Fsend%5Fsignal.2.html), [poll(2)](../man2/poll.2.html),
[process_madvise(2)](../man2/process%5Fmadvise.2.html), [select(2)](../man2/select.2.html), [setns(2)](../man2/setns.2.html), [waitid(2)](../man2/waitid.2.html), [epoll(7)](../man7/epoll.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 pidfdopen(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)