Types of signals

Signals fall into two categories that correspond to the events by which they are generated.

Type of event	Type of signal
Exception	Synchronous signal
External event	Asynchronous signal

Exceptions and synchronous signals

Synchronous signals are generated by exceptions, or errors, which are caused by invalid operations in a program. Examples of exceptions include attempts to access invalid memory addresses and attempts to divide by zero.

Examples of synchronous signals include SIGILL, SIGFPE, SIGBUS, SIGSEGV, SIGSYS, and SIGPIPE.

External events and asynchronous signals

Asynchronous signals are generated by events outside the control of the process that receives them, and arrive at unpredictable times. Asynchronous signals are delivered to the process without regard to the instruction that is executing.

Typically, the asynchronous signals are SIGHUP, SIGINT, SIGQUIT, SIGALRM, SIGTERM, SIGUSR1, SIGUSR2, SIGCHLD, SIGPWR, SIGVTALRM, SIGPROF, SIGIO, SIGWINCH, SIGTSTP, SIGCONT, SIGTTIN, SIGTTOU, and SIGURG.

The Sybase libraries treat the asynchronous signal SIGTRAP as a synchronous signal. See “SIGTRAP signal” for more information.