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     sigqueue — queue a signal to a process (REALTIME)


     Standard C Library (libc, -lc)


     #include <signal.h>

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


     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.


     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.


     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

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

     [ESRCH]            The process pid does not exist.


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


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


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