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 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 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).

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)