Command Section
SIGQUEUE(2)               FreeBSD System Calls Manual              SIGQUEUE(2)

     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

     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.

     kill(2), sigaction(2), sigpending(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

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

     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.

FreeBSD 11.1-RELEASE-p4           May 5, 2017          FreeBSD 11.1-RELEASE-p4
Command Section