NAME

pidfd_send_signal - send a signal to a process specified by a file descriptor

SYNOPSIS

#include <signal.h>

int pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
                      unsigned int flags);

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):

*: 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.

RETURN VALUE

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

CONFORMING TO

pidfd_send_signal() is Linux specific.

NOTES

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.

EXAMPLE

#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);
}