signalmaske

A signalmask, or signal mask, is a per-thread or per-process set of signals whose delivery is currently blocked in Unix-like operating systems. When a signal is masked, the kernel does not deliver it to the target thread immediately; instead the signal is marked as pending. If more signals arrive while masked, they may be queued for delivery depending on the signal type. When the mask is later changed to unmask a signal, the kernel delivers the signal, invoking any configured handler or performing the default action.

In POSIX systems the signal mask is typically manipulated with sigprocmask (or pthread_sigmask for threaded programs) and the type sigset_t. Helper operations include creating and modifying masks: sigemptyset, sigfillset, sigaddset, sigdelset, and sigismember. The constants SIG_BLOCK, SIG_UNBLOCK, and SIG_SETMASK specify how the mask should be updated. In modern systems, the mask is generally per-thread; the process-wide view applies to new threads unless overridden.

Key distinctions: the signal mask controls whether a signal is delivered, not what action occurs when it is delivered. Blocking a signal does not prevent its pending status from existing; it only delays the delivery. Standard signals are not queued beyond a single pending state, while real-time signals may be queued if multiple occurrences occur while masked.

Practical use typically involves protecting critical sections of code from asynchronous interruption by temporarily blocking certain signals and restoring the previous mask afterward. Mismanagement of the mask can lead to missed signals or deadlocks, so careful handling of saving and restoring masks is important.