sigqueue man page on FreeBSD

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SIGQUEUE(2)		    BSD System Calls Manual		   SIGQUEUE(2)

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

LIBRARY
     Standard C Library (libc, -lc)

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 valid‐
     ity of PID.

     The conditions required for a process to have permission to queue a sig‐
     nal 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 deliv‐
     ery, it is the lowest numbered one.  The selection order between realtime
     and non-realtime signals, or between multiple pending non-realtime sig‐
     nals, is unspecified.

RETURN VALUES
     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

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.

     [EEPERM]		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
     sigaction(2), sigpending(2), sigqueue(2), sigsuspend(2), sigtimedwait(2),
     sigwait(2), sigwaitinfo(2), pause(3), pthread_sigmask(3), siginfo(3)

STANDARDS
     The sigqueue() system call conforms to IEEE Std 1003.1-2004 (“POSIX.1”)

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

BSD			       November 11, 2005			   BSD
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