.\" Copyright (c) 2019 by Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .TH PIDFD_SEND_SIGNAL 2 2020-06-09 "Linux" "Linux Programmer's Manual" .SH NAME pidfd_send_signal \- send a signal to a process specified by a file descriptor .SH SYNOPSIS .nf .B "#include " .PP .BI "int pidfd_send_signal(int " pidfd ", int " sig ", siginfo_t *" info , .BI " unsigned int " flags ); .fi .SH DESCRIPTION The .BR pidfd_send_signal () system call sends the signal .I sig to the target process referred to by .IR pidfd , a PID file descriptor that refers to a process. .\" See the very detailed commit message for kernel commit .\" 3eb39f47934f9d5a3027fe00d906a45fe3a15fad .PP If the .I info argument points to a .I siginfo_t buffer, that buffer should be populated as described in .BR rt_sigqueueinfo (2). .PP If the .I info argument is a NULL pointer, this is equivalent to specifying a pointer to a .I siginfo_t buffer whose fields match the values that are implicitly supplied when a signal is sent using .BR kill (2): .PP .PD 0 .IP * 3 .I si_signo is set to the signal number; .IP * .I si_errno is set to 0; .IP * .I si_code is set to .BR SI_USER ; .IP * .I si_pid is set to the caller's PID; and .IP * .I si_uid is set to the caller's real user ID. .PD .PP The calling process must either be in the same PID namespace as the process referred to by .IR pidfd , or be in an ancestor of that namespace. .PP The .I flags argument is reserved for future use; currently, this argument must be specified as 0. .SH RETURN VALUE On success, .BR pidfd_send_signal () returns 0. On error, \-1 is returned and .I errno is set to indicate the cause of the error. .SH ERRORS .TP .B EBADF .I pidfd is not a valid PID file descriptor. .TP .B EINVAL .I sig is not a valid signal. .TP .B EINVAL The calling process is not in a PID namespace from which it can send a signal to the target process. .TP .B EINVAL .I flags is not 0. .TP .B EPERM The calling process does not have permission to send the signal to the target process. .TP .B EPERM .I pidfd doesn't refer to the calling process, and .IR info.si_code is invalid (see .BR rt_sigqueueinfo (2)). .TP .B ESRCH The target process does not exist (i.e., it has terminated and been waited on). .SH VERSIONS .BR pidfd_send_signal () first appeared in Linux 5.1. .SH CONFORMING TO .BR pidfd_send_signal () is Linux specific. .SH NOTES Currently, there is no glibc wrapper for this system call; call it using .BR syscall (2). .\" .SS PID file descriptors The .I 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: .IP * 3 by opening a .IR /proc/[pid] directory; .IP * using .BR pidfd_open (2); or .IP * via the PID file descriptor that is returned by a call to .BR clone (2) or .BR clone3 (2) that specifies the .BR CLONE_PIDFD flag. .PP The .BR pidfd_send_signal () system call allows the avoidance of race conditions that occur when using traditional interfaces (such as .BR 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, .BR pidfd_send_signal () fails with the error .BR ESRCH . .SH EXAMPLES .EX #define _GNU_SOURCE #include #include #include #include #include #include #include #include #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 \en", 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 \(aqsiginfo_t\(aq 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); } .EE .SH SEE ALSO .BR clone (2), .BR kill (2), .BR pidfd_open (2), .BR rt_sigqueueinfo (2), .BR sigaction (2), .BR pid_namespaces (7), .BR signal (7) .SH COLOPHON This page is part of release 5.10 of the Linux .I 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/.