pidfd_send_signal - Man Page

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.

#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)

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).