pidfd_send_signal - Man Page

send a signal to a process specified by a file descriptor

Library

Standard C library (libc, -lc)

Synopsis

#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).

Description

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):

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.

Return Value

On success, pidfd_send_signal() returns 0. On error, -1 is returned and errno is set to indicate the error.

Errors

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).

Versions

pidfd_send_signal() first appeared in Linux 5.1.

Standards

pidfd_send_signal() is Linux specific.

Notes

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.

Examples

#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);
}

See Also

clone(2), kill(2), pidfd_open(2), rt_sigqueueinfo(2), sigaction(2), pid_namespaces(7), signal(7)

Referenced By

kill(2), notcurses_fds(3), pidfd_open(2), rt_sigqueueinfo(2), sd_event_add_child(3), sigaction(2), signal(7), syscalls(2), system_data_types(7).

2022-12-03 Linux man-pages 6.02