pidfd_send_signal(2) — Linux manual page


pidfd_send_signal(2)       System Calls Manual      pidfd_send_signal(2)

NAME         top

       pidfd_send_signal - send a signal to a process specified by a
       file descriptor

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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         top

       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

       •  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

       The flags argument is reserved for future use; currently, this
       argument must be specified as 0.

RETURN VALUE         top

       On success, pidfd_send_signal() returns 0.  On error, -1 is
       returned and errno is set to indicate the error.

ERRORS         top

       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         top


HISTORY         top

       Linux 5.1.

NOTES         top

   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         top

       #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);

       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]);

           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) {

           /* 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) {


SEE ALSO         top

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-06-15           pidfd_send_signal(2)

Pages that refer to this page: kill(2)pidfd_open(2)rt_sigqueueinfo(2)sigaction(2)syscalls(2)sd_event_add_child(3)signal(7)system_data_types(7)