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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 de-
       scriptor

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 specify-
       ing 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  us-
       ing 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 condi-
       tions that occur when using traditional interfaces (such as kill(2)) to
       signal a process.  The problem is that the traditional interfaces spec-
       ify 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 orig-
       inally  intended target process has terminated and its PID has been re-
       cycled 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);
       }

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 5.05 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                             2019-09-19              PIDFD_SEND_SIGNAL(2)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON