SIGQUEUE(2)

NAME
sigqueue - queue a signal to a process (REALTIME)

LIBRARY
libc

SYNOPSIS
#include <signal.h>
int
sigqueue(pid_t pid, int signo, const union sigval value);

DESCRIPTION
The sigqueue() system call causes the signal specified by signo to be sent with the value specified by value to the process specified by pid. If signo is zero (the null signal), error checking is performed but no signal is actually sent. The null signal can be used to check the validity of PID.

The conditions required for a process to have permission to queue a signal to another process are the same as for the kill(2) system call. The sigqueue() system call queues a signal to a single process specified by the pid argument.

The sigqueue() system call returns immediately. If the resources were available to queue the signal, the signal will be queued and sent to the receiving process.

If the value of pid causes signo to be generated for the sending process, and if signo is not blocked for the calling thread and if no other thread has signo unblocked or is waiting in a sigwait() system call for signo, either signo or at least the pending, unblocked signal will be delivered to the calling thread before sigqueue() returns. Should any multiple pending signals in the range SIGRTMIN to SIGRTMAX be selected for delivery, it is the lowest numbered one. The selection order between realtime and non-realtime signals, or between multiple pending non-realtime signals, is unspecified.

RETURN VALUES

ERRORS
The sigqueue() system call will fail if:

[EAGAIN]
    No resources are available to queue the signal. The process has already queued {SIGQUEUE_MAX} signals that are still pending at the receiver(s), or a system-wide resource limit has been exceeded.

[EINVAL]
    The value of the signo argument is an invalid or unsupported signal number.

[EPERM]
    The process does not have the appropriate privilege to send the signal to the receiving process.

[ESRCH]
    The process pid does not exist.

SEE ALSO
kill(2), sigaction(2), sigpending(2), sigsuspend(2), sigtimedwait(2), sigwait(2), sigwaitinfo(2), pause(3), pthread_sigmask(3), siginfo(3)

STANDARDS
The sigqueue() system call conforms to p1003.1-2004.

HISTORY
Support for POSIX realtime signal queue first appeared in FreeBSD 7.0.

CAVEATS
When using sigqueue to send signals to a process which might have a different ABI (for instance, one is 32-bit and the other 64-bit), the sival_int member of value can be delivered reliably, but the sival_ptr may be truncated in endian dependent ways and must not be relied on. Further, many pointer integrity schemes disallow sending pointers to other processes, and this technique should not be used in programs intended to be portable. In particular, CheriABI programs may only send pointers to themselves.