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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
PIDFD_SEND_SIGNAL(2) Linux Programmer's Manual PIDFD_SEND_SIGNAL(2)
pidfd_send_signal - send a signal to a process specified by a file
descriptor
#include <signal.h>
int pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
unsigned int flags);
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 cause of 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).
pidfd_send_signal() first appeared in Linux 5.1.
pidfd_send_signal() is Linux specific.
Currently, there is no glibc wrapper for this system call; call it
using syscall(2).
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 <limits.h>
#include <signal.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/syscall.h>
#ifndef __NR_pidfd_send_signal
#define __NR_pidfd_send_signal 424
#endif
static int
pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
unsigned int flags)
{
return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
}
int
main(int argc, char *argv[])
{
siginfo_t info;
char path[PATH_MAX];
int pidfd, sig;
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 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 2020-06-09 PIDFD_SEND_SIGNAL(2)
Pages that refer to this page: kill(2) , pidfd_open(2) , rt_sigaction(2) , rt_sigqueueinfo(2) , rt_tgsigqueueinfo(2) , sigaction(2) , syscalls(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) , signal(7)
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