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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
PIDFD_OPEN(2) Linux Programmer's Manual PIDFD_OPEN(2)
pidfd_open - obtain a file descriptor that refers to a process
#include <sys/types.h>
int pidfd_open(pid_t pid, unsigned int flags);
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 is reserved for future use; currently, this
argument must be specified as 0.
On success, pidfd_open() returns a file descriptor (a nonnegative
integer). On error, -1 is returned and errno is set to indicate the
cause of the error.
EINVAL flags is not 0.
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.
pidfd_open() first appeared in Linux 5.3.
pidfd_open() is Linux specific.
Currently, there is no glibc wrapper for this system call; call it
using syscall(2).
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 condi‐
tions 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 han‐
dler 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 process that it refers to terminates, these
interfaces indicate the file descriptor as readable. Note, how‐
ever, that in the current implementation, nothing can be read from
the file descriptor (read(2) 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).
· 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.
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. How‐
ever, 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 <sys/types.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <poll.h>
#include <stdlib.h>
#include <stdio.h>
#ifndef __NR_pidfd_open
#define __NR_pidfd_open 434 /* System call # on most architectures */
#endif
static int
pidfd_open(pid_t pid, unsigned int flags)
{
return syscall(__NR_pidfd_open, pid, flags);
}
int
main(int argc, char *argv[])
{
struct pollfd pollfd;
int pidfd, ready;
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 (0x%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),
select(2), setns(2), waitid(2), epoll(7)
This page is part of release 5.08 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2020-08-13 PIDFD_OPEN(2)
Pages that refer to this page: clone2(2) , __clone2(2) , clone(2) , clone3(2) , pidfd_getfd(2) , pidfd_send_signal(2) , setns(2) , syscalls(2) , wait(2) , waitid(2) , waitpid(2) , sd_event_add_child(3) , sd_event_add_child_pidfd(3) , sd_event_child_handler_t(3) , sd_event_source_get_child_pid(3) , sd_event_source_get_child_pidfd(3) , sd_event_source_get_child_pidfd_own(3) , sd_event_source_get_child_process_own(3) , sd_event_source_send_child_signal(3) , sd_event_source_set_child_pidfd_own(3) , sd_event_source_set_child_process_own(3)
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