SIGQUEUE

Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
Page Index

NAME

sigqueue - queue a signal and data to a process

SYNOPSIS

#include <signal.h>

int sigqueue(pid_t pid, int sig, const union sigval value);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

sigqueue(): _POSIX_C_SOURCE >= 199309L

DESCRIPTION

sigqueue() sends the signal specified in sig to the process whose PID is given in pid. The permissions required to send a signal are the same as for kill(2). As with kill(2), the null signal (0) can be used to check if a process with a given PID exists.

The 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:

union sigval {
int sival_int;
void *sival_ptr; };

If the receiving process has installed a handler for this signal using the SA_SIGINFO flag to sigaction(2), then it can obtain this data via the si_value field of the siginfo_t structure passed as the second argument to the handler. Furthermore, the si_code field of that structure will be set to SI_QUEUE.

RETURN VALUE

On success, sigqueue() returns 0, indicating that the signal was successfully queued to the receiving process. Otherwise, -1 is returned and errno is set to indicate the error.

ERRORS

EAGAIN: The limit of signals which may be queued has been reached. (See signal(7) for further information.)
EINVAL: sig was invalid.
EPERM: The process does not have permission to send the signal to the receiving process. For the required permissions, see kill(2).
ESRCH: No process has a PID matching pid.

VERSIONS

sigqueue() and the underlying rt_sigqueueinfo() system call first appeared in Linux 2.2.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
sigqueue()	Thread safety	MT-Safe

CONFORMING TO

POSIX.1-2001, POSIX.1-2008.

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 sigwait(3)), then at least some signal must be delivered to this thread before this function returns.

C library/kernel differences

On Linux, sigqueue() is implemented using the rt_sigqueueinfo(2) system call. The system call differs in its third argument, which is the siginfo_t structure that will be supplied to the receiving process's signal handler or returned by the receiving process's sigtimedwait(2) call. Inside the glibc sigqueue() wrapper, this argument, uinfo, is initialized as follows:

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() */

COLOPHON

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