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.

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

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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.