Interrupts, such as SIGINT and SIGIO, are asynchronous with any thread and result from some action outside the process. These interrupts might be signals sent explicitly by another process, or might represent external actions such as a user typing a Control-C.
An interrupt can be handled by any thread whose signal mask allows the interrupt. When more than one thread is able to receive the interrupt, only one thread is chosen.
When multiple occurrences of the same signal are sent to a process, then each occurrence can be handled by a separate thread. However, the available threads must not have the signal masked. When all threads have the signal masked, then the signal is marked pending and the first thread to unmask the signal handles the signal.