pidfd_open(2) System Calls Manual pidfd_open(2)
NAME
pidfd_open - obtain a file descriptor that refers to a process
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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).
DESCRIPTION
The pidfd_open() system call creates a file descriptor that refers to
the process whose PID is specified in pid. The file descriptor is re-
turned 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 at-
tempt to wait on the file descriptor using waitid(2) will imme-
diately return the error EAGAIN rather than blocking.
RETURN VALUE
On success, pidfd_open() returns a file descriptor (a nonnegative inte-
ger). On error, -1 is returned and errno is set to indicate the error.
ERRORS
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 getr-
limit(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.
VERSIONS
pidfd_open() first appeared in Linux 5.3.
STANDARDS
pidfd_open() is Linux specific.
NOTES
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, how-
ever, 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 process that it refers to terminates, these in-
terfaces 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 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 de-
scriptor.
• A PID file descriptor can be used as the argument of setns(2) in or-
der 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_mad-
vise(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).
EXAMPLES
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 <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
clone(2), kill(2), pidfd_getfd(2), pidfd_send_signal(2), poll(2),
process_madvise(2), select(2), setns(2), waitid(2), epoll(7)
Linux man-pages 6.03 2023-02-05 pidfd_open(2)
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