.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH pidfd_send_signal 2 2025-10-29 "Linux man-pages (unreleased)"
.SH NAME
pidfd_send_signal \- send a signal to a process specified by a file descriptor
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.BR "#include <linux/signal.h>" "     /* Definition of " SIG* " constants */"
.BR "#include <signal.h>" "           /* Definition of " SI_* " constants */"
.BR "#include <sys/syscall.h>" "      /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.P
.BI "int syscall(SYS_pidfd_send_signal, int " pidfd ", int " sig ,
.BI "            siginfo_t *_Nullable " info ", unsigned int " flags );
.fi
.P
.IR Note :
glibc provides no wrapper for
.BR pidfd_send_signal (),
necessitating the use of
.BR syscall (2).
.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
.P
If the
.I info
argument points to a
.I siginfo_t
buffer, that buffer should be populated as described in
.BR rt_sigqueueinfo (2).
.P
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):
.P
.PD 0
.IP \[bu] 3
.I si_signo
is set to the signal number;
.IP \[bu]
.I si_errno
is set to 0;
.IP \[bu]
.I si_code
is set to
.BR SI_USER ;
.IP \[bu]
.I si_pid
is set to the caller's PID;
and
.IP \[bu]
.I si_uid
is set to the caller's real user ID.
.PD
.P
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.
.P
The
.I flags
argument allows to modify the scope of the signal.
By default,
the scope of the signal will be inferred from the
.I pidfd
argument.
For example,
if
.I pidfd
refers to a specific thread
\[em]i.e.,
the
.I pidfd
was created through
.BR pidfd_open (2)
using the
.B PIDFD_THREAD
flag
or through
.BR clone3 (2)
using the
.B CLONE_PIDFD
flag together with the
.B CLONE_THREAD
flag\[em]
then passing
.I pidfd
to
.BR pidfd_send_signal (2)
and leaving the
.I flags
argument as
.B 0
will cause the signal to be sent to the specific thread referenced by the
.IR pidfd .
.TP
.BR PIDFD_SIGNAL_THREAD " (since Linux 6.9)"
.\" commit e1fb1dc08e73466830612bcf2f9f72180965c9ba
Ensure that the signal is sent to the specific thread referenced by
.IR pidfd .
.TP
.BR PIDFD_SIGNAL_THREAD_GROUP " (since Linux 6.9)"
.\" commit e1fb1dc08e73466830612bcf2f9f72180965c9ba
If
.I pidfd
refers to a thread-group leader,
ensure that the signal is sent to the thread-group,
even if
.I pidfd
was created to refer to a specific thread.
.TP
.BR PIDFD_SIGNAL_PROCESS_GROUP " (since Linux 6.9)"
.\" commit e1fb1dc08e73466830612bcf2f9f72180965c9ba
If
.I pidfd
refers to a process-group leader,
ensure that the signal is sent to the process-group,
even if
.I pidfd
was created to refer to a specific thread or to a thread-group leader.
.SH RETURN VALUE
On success,
.BR pidfd_send_signal ()
returns 0.
On error, \-1 is returned and
.I errno
is set to indicate 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 valid.
.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
.I 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 STANDARDS
Linux.
.SH HISTORY
Linux 5.1.
.SH NOTES
.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 \[bu] 3
by opening a
.IR /proc/ pid
directory;
.IP \[bu]
using
.BR pidfd_open (2);
or
.IP \[bu]
via the PID file descriptor that is returned by a call to
.BR clone (2)
or
.BR clone3 (2)
that specifies the
.B CLONE_PIDFD
flag.
.P
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
.\" SRC BEGIN (pidfd_send_signal.c)
.EX
#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>\[rs]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 \[aq]siginfo_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
.\" SRC END
.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)