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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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pidfd_send_signal(2) System Calls Manual pidfd_send_signal(2)
pidfd_send_signal - send a signal to a process specified by a file
descriptor
Standard C library (libc, -lc)
#include <linux/signal.h> /* Definition of SIG* constants */
#include <signal.h> /* Definition of SI_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_pidfd_send_signal, int pidfd, int sig,
siginfo_t *_Nullable info, unsigned int flags);
Note: glibc provides no wrapper for pidfd_send_signal(),
necessitating the use of syscall(2).
The pidfd_send_signal() system call sends the signal sig to the
target process referred to by pidfd, a PID file descriptor that
refers to a process.
If the info argument points to a siginfo_t buffer, that buffer
should be populated as described in rt_sigqueueinfo(2).
If the info argument is a null pointer, this is equivalent to
specifying a pointer to a siginfo_t buffer whose fields match the
values that are implicitly supplied when a signal is sent using
kill(2):
• si_signo is set to the signal number;
• si_errno is set to 0;
• si_code is set to SI_USER;
• si_pid is set to the caller's PID; and
• si_uid is set to the caller's real user ID.
The calling process must either be in the same PID namespace as
the process referred to by pidfd, or be in an ancestor of that
namespace.
The flags argument is reserved for future use; currently, this
argument must be specified as 0.
On success, pidfd_send_signal() returns 0. On error, -1 is
returned and errno is set to indicate the error.
EBADF pidfd is not a valid PID file descriptor.
EINVAL sig is not a valid signal.
EINVAL The calling process is not in a PID namespace from which it
can send a signal to the target process.
EINVAL flags is not 0.
EPERM The calling process does not have permission to send the
signal to the target process.
EPERM pidfd doesn't refer to the calling process, and
info.si_code is invalid (see rt_sigqueueinfo(2)).
ESRCH The target process does not exist (i.e., it has terminated
and been waited on).
Linux.
Linux 5.1.
PID file descriptors
The pidfd argument is a PID file descriptor, a file descriptor
that refers to process. Such a file descriptor can be obtained
in any of the following ways:
• by opening a /proc/pid directory;
• using pidfd_open(2); or
• via the PID file descriptor that is returned by a call to
clone(2) or clone3(2) that specifies the CLONE_PIDFD flag.
The pidfd_send_signal() system call allows the avoidance of race
conditions that occur when using traditional interfaces (such as
kill(2)) to signal a process. The problem is that the traditional
interfaces specify the target process via a process ID (PID), with
the result that the sender may accidentally send a signal to the
wrong process if the originally intended target process has
terminated and its PID has been recycled for another process. By
contrast, a PID file descriptor is a stable reference to a
specific process; if that process terminates, pidfd_send_signal()
fails with the error ESRCH.
#define _GNU_SOURCE
#include <fcntl.h>
#include <limits.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>
static int
pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
unsigned int flags)
{
return syscall(SYS_pidfd_send_signal, pidfd, sig, info, flags);
}
int
main(int argc, char *argv[])
{
int pidfd, sig;
char path[PATH_MAX];
siginfo_t info;
if (argc != 3) {
fprintf(stderr, "Usage: %s <pid> <signal>\n", argv[0]);
exit(EXIT_FAILURE);
}
sig = atoi(argv[2]);
/* Obtain a PID file descriptor by opening the /proc/PID directory
of the target process. */
snprintf(path, sizeof(path), "/proc/%s", argv[1]);
pidfd = open(path, O_RDONLY);
if (pidfd == -1) {
perror("open");
exit(EXIT_FAILURE);
}
/* Populate a 'siginfo_t' structure for use with
pidfd_send_signal(). */
memset(&info, 0, sizeof(info));
info.si_code = SI_QUEUE;
info.si_signo = sig;
info.si_errno = 0;
info.si_uid = getuid();
info.si_pid = getpid();
info.si_value.sival_int = 1234;
/* Send the signal. */
if (pidfd_send_signal(pidfd, sig, &info, 0) == -1) {
perror("pidfd_send_signal");
exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);
}
clone(2), kill(2), pidfd_open(2), rt_sigqueueinfo(2),
sigaction(2), pid_namespaces(7), signal(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-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. 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
[email protected]
Linux man-pages 6.15 2025-05-17 pidfd_send_signal(2)
Pages that refer to this page: pgrep(1), kill(2), pidfd_open(2), rt_sigqueueinfo(2), sigaction(2), syscalls(2), sd_event_add_child(3), signal(7), system_data_types(7)
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HTML rendering created 2025-09-06 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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