The traditional UNIX signal model is extended to threads in a fairly natural way. The key characteristics are that the signal disposition is process-wide, but the signal mask is per-thread. The process-wide disposition of signals is established using the traditional mechanisms signal(3C), sigaction(2) , and so on.
When a signal handler is marked SIG_DFL or SIG_IGN, the action on receipt of a signal is performed on the entire receiving process. These signals include exit, core dump, stop, continue, and ignore. The action on receipt of these signals is carried out on all threads in the process. Therefore, the issue of which thread picks the signal is nonexistent. The exit, core dump, stop, continue, and ignore signals have no handlers. See the signal.h(3HEAD) man page for basic information about signals.
Each thread has its own signal mask. The signal mask lets a thread block some signals while the thread uses memory or another state that is also used by a signal handler. All threads in a process share the set of signal handlers that are set up by sigaction(2) and its variants.
A thread in one process cannot send a signal to a specific thread in another process. A signal sent by kill(2), sigsend(2), or sigqueue(3RT) to a process is handled by any receptive threads in the process.
Signals are divided into the following categories: traps, exceptions, and interrupts. Traps and exceptions are synchronously generated signals. Interrupts are asynchronously generated signals.
As in traditional UNIX, if a signal is pending, additional occurrences of that signal normally have no additional effect. A pending signal is represented by a bit, not by a counter. However, signals that are posted through the sigqueue(3RT) interface allow multiple instances of the same signal to be queued to the process.
As is the case with single-threaded processes, when a thread receives a signal while blocked in a system call, the thread might return early. When a thread returns early, the thread either returns an EINTR error code, or, in the case of I/O calls, with fewer bytes transferred than requested.
Of particular importance to multithreaded programs is the effect of signals on pthread_cond_wait(3C). This call usually returns without error, a return value of zero, only in response to a pthread_cond_signal(3C) or a pthread_cond_broadcast(3C). However, if the waiting thread receives a traditional UNIX signal, pthread_cond_wait() returns with a return value of zero even though the wakeup was spurious.