The sigqueue() function causes the signal specified by signo to be sent with the value specified by value to the process specified by pid. If signo is 0 (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) function.

The sigqueue() function returns immediately. If SA_SIGINFO is set for signo and if the resources were available to queue the signal, the signal is queued and sent to the receiving process. If SA_SIGINFO is not set for signo, then signo is sent at least once to the receiving process; it is unspecified whether value will be sent to the receiving process as a result of this call.

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(2) function for signo, either signo or at least the pending, unblocked signal will be delivered to the calling thread before the sigqueue() function returns. Should any of multiple pending signals in the range SIGRTMIN to SIGRTMAX be selected for delivery, it will be 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 specified signal will have been queued, and the sigqueue() function returns 0. Otherwise, the function returns -1 and sets errno to indicate the error.

The sigqueue() function 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 signo is an invalid or unsupported signal number.

ENOSYS

The sigqueue() function is not supported by the system.

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 attributes(5) for descriptions of the following attributes:

kill(2), sigwaitinfo(3RT), attributes(5), siginfo(3HEAD), signal(3HEAD)