Hello Christian and all, Below, I have the rendered version of the current draft of the pidfd_send_signal(2) manual page that I have written. The page source can be found in a Git branch at: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/log/?h=draft_pidfd I would be pleased to receive corrections and notes on any details that should be added. (For example, are there error cases that I have missed?) Would you be able to review please? Thanks, Michael NAME pidfd_send_signal - send a signal to a process specified by a file descriptor SYNOPSIS 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 success, -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. 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 termi‐ nated and its PID has been recycled for another process. By con‐ trast, a PID file descriptor is a stable reference to a specific process; if that process terminates, then the file descriptor ceases to be valid and the caller of pidfd_send_signal() is informed of this fact via an ESRCH error. 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), sigac‐ tion(2), pid_namespaces(7), signal(7)
