pidfd_send_signal - send a signal to a process specified by a file descriptor
Standard C library (libc, -lc)
#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).
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 allows to modify the scope of the signal. By default the scope of the signal will be infered from the pidfd argument.
For example, if pidfd refers to a specific thread, i.e., the pidfd was created through pidfd_open(2) passing the PIDFD_THREAD flag or through clone3(2) using the CLONE_PIDFD flag together with the CLONE_THREAD flag then passing pidfd to pidfd_send_signal(2) and leaving the flags argument as 0 will cause the signal to be sent to the specific thread referenced by the pidfd.
The flags mask of pidfd_send_signal(2) system call allows the caller to override the default signal scope indicated by pidfd.
PIDFD_SIGNAL_THREAD (since Linux v6.9) Ensure that the signal is sent to the specific thread referenced by pidfd.
PIDFD_SIGNAL_THREAD_GROUP (since Linux v6.9) If pidfd refers to a thread-group leader ensure that the signal is sent to the thread-group even if pidfd was created to refer to a specific thread.
PIDFD_SIGNAL_PROCESS_GROUP (since Linux v6.9) If pidfd refers to a process-group leader ensure that the signal is sent to the process-group even if pidfd was created to refer to a specific thread or to a thread-group leader.
On success, pidfd_send_signal() returns 0. On error, -1 is returned and errno is set to indicate the error.
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 value is not valid.
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).
Linux.
Linux 5.1.
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.
#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);
}
clone(2), kill(2), pidfd_open(2), rt_sigqueueinfo(2), sigaction(2), pid_namespaces(7), signal(7)