pidfd_send_signal(2)

NAME
pidfd_send_signal - 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):

•   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 allows to modify the scope of the signal. By default, the scope of the signal will be inferred from the pidfd argument. For example, if pidfd refers to a specific thread —i.e., the pidfd was created through pidfd_open(2) using the PIDFD_THREAD flag or through clone3(2) using the CLONE_PIDFD flag together with the CLONE_THREAD flag— then passing pidfd to pidfd_send_signal(2) and leaving the flags argument as 0 will cause the signal to be sent to the specific thread referenced by the pidfd.

PIDFD_SIGNAL_THREAD (since Linux 6.9)
    Ensure that the signal is sent to the specific thread referenced by pidfd.

PIDFD_SIGNAL_THREAD_GROUP (since Linux 6.9)
    If pidfd refers to a thread-group leader, ensure that the signal is sent to the thread-group, even if pidfd was created to refer to a specific thread.

PIDFD_SIGNAL_PROCESS_GROUP (since Linux 6.9)
    If pidfd refers to a process-group leader, ensure that the signal is sent to the process-group, even if pidfd was created to refer to a specific thread or to a thread-group leader.

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 valid.

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).

STANDARDS
Linux.

HISTORY
Linux 5.1.

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)