pidfd_send_signal

PIDFD_SEND_SIGNAL(2)        Linux Programmer's Manual       PIDFD_SEND_SIGNAL(2)



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.

EXAMPLES
       #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);
       }

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

COLOPHON
       This page is part of release 5.10 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)