man3/sigqueue.3

.\" Copyright (c) 2002 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" added note on self-signaling, aeb, 2002-06-07
.\" added note on CAP_KILL, mtk, 2004-06-16
.\"
.TH SIGQUEUE 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
sigqueue \- queue a signal and data to a process
.SH SYNOPSIS
.nf
.B #include <signal.h>
.PP
.BI "int sigqueue(pid_t " pid ", int " sig ", const union sigval " value );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR sigqueue ():
.nf
    _POSIX_C_SOURCE >= 199309L
.fi
.SH DESCRIPTION
.BR sigqueue ()
sends the signal specified in
.I sig
to the process whose PID is given in
.IR pid .
The permissions required to send a signal are the same as for
.BR kill (2).
As with
.BR kill (2),
the null signal (0) can be used to check if a process with a given
PID exists.
.PP
The
.I value
argument is used to specify an accompanying item of data (either an integer
or a pointer value) to be sent with the signal, and has the following type:
.PP
.in +4n
.EX
union sigval {
    int   sival_int;
    void *sival_ptr;
};
.EE
.in
.PP
If the receiving process has installed a handler for this signal using the
.B SA_SIGINFO
flag to
.BR sigaction (2),
then it can obtain this data via the
.I si_value
field of the
.I siginfo_t
structure passed as the second argument to the handler.
Furthermore, the
.I si_code
field of that structure will be set to
.BR SI_QUEUE .
.SH RETURN VALUE
On success,
.BR sigqueue ()
returns 0, indicating that the signal was successfully
queued to the receiving process.
Otherwise, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EAGAIN
The limit of signals which may be queued has been reached.
(See
.BR signal (7)
for further information.)
.TP
.B EINVAL
.I sig
was invalid.
.TP
.B EPERM
The process does not have permission to send the signal
to the receiving process.
For the required permissions, see
.BR kill (2).
.TP
.B ESRCH
No process has a PID matching
.IR pid .
.SH VERSIONS
.BR sigqueue ()
and the underlying
.BR rt_sigqueueinfo ()
system call first appeared in Linux 2.2.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR sigqueue ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
.SH NOTES
If this function results in the sending of a signal to the process
that invoked it, and that signal was not blocked by the calling thread,
and no other threads were willing to handle this signal (either by
having it unblocked, or by waiting for it using
.BR sigwait (3)),
then at least some signal must be delivered to this thread before this
function returns.
.SS C library/kernel differences
On Linux,
.BR sigqueue ()
is implemented using the
.BR rt_sigqueueinfo (2)
system call.
The system call differs in its third argument, which is the
.I siginfo_t
structure that will be supplied to the receiving process's
signal handler or returned by the receiving process's
.BR sigtimedwait (2)
call.
Inside the glibc
.BR sigqueue ()
wrapper, this argument,
.IR uinfo ,
is initialized as follows:
.PP
.in +4n
.EX
uinfo.si_signo = sig;      /* Argument supplied to sigqueue() */
uinfo.si_code = SI_QUEUE;
uinfo.si_pid = getpid();   /* Process ID of sender */
uinfo.si_uid = getuid();   /* Real UID of sender */
uinfo.si_value = val;      /* Argument supplied to sigqueue() */
.EE
.in
.SH SEE ALSO
.BR kill (2),
.BR rt_sigqueueinfo (2),
.BR sigaction (2),
.BR signal (2),
.BR pthread_sigqueue (3),
.BR sigwait (3),
.BR signal (7)