pidfd_open
Section: System Calls (2)
Updated: 2023-02-05
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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 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)
will immediately return the error
EAGAIN
rather than blocking.
RETURN VALUE
On success,
pidfd_open()
returns a file descriptor (a nonnegative integer).
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
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.
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, 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 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 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.
- •
-
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).
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)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- STANDARDS
-
- NOTES
-
- Use cases for PID file descriptors
-
- EXAMPLES
-
- Program source
-
- SEE ALSO
-
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Time: 10:46:04 GMT, April 30, 2024