Signals

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

The target process must have the SA_SIGINFO bit set for the specified signal (see sigaction(2)), for subsequent occurrences of a pending signal to be queued also.

The target process can receive signals either synchronously or asynchronously. Blocking a signal (see sigprocmask(2)) and calling either sigwaitinfo(3RT) or sigtimedwait(3RT), causes the signal to be received synchronously, with the value sent by the caller of sigqueue(3RT) 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.

Only a fixed 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.