Processing Signals

The sender can use sigqueue(3RT) to send a signal together with a small amount of information to a target process.

To queue subsequent occurrences of a pending signal, the target process must have the SA_SIGINFO bit set for the specified signal. See the sigaction(2) man page.

The target process normally receive signals asynchronously. To receive signals synchronously, block the signal and call either sigwaitinfo(3RT) or sigtimedwait(3RT). See the sigprocmask(2) man page. This procedure causes the signal to be received synchronously. The value sent by the caller of sigqueue(3RT) is stored in the si_value member of the siginfo_t argument. Leaving the signal unblocked causes the signal to be delivered to the signal handler specified by sigaction(2), with the value appearing in the si_value of the siginfo_t argument to the handler.

A specified number of signals with associated values can be sent by a process and remain undelivered. Storage for {SIGQUEUE_MAX} signals is allocated at the first call to sigqueue(3RT). Thereafter, a call to sigqueue(3RT) either successfully enqueues at the target process or fails within a bounded amount of time.