Signals

Signals is the mechanism by which processes are made aware of events occurring in the system. An important feature of signals is that they are asynchronous, which means that a process may receive a signal at any time during its execution, and must be prepared to respond to the signal at that time. It is even possible for a process to receive a signal while it is executing a system call. Generally, if a system call is interrupted by a signal, then it will return its error value and set errno to the value EINTR. It is then the responsibility of the signaled process to re-try the interrupted call.

Each of the signal types, of which there are currently thirty odd, is given a symbolic name accessed via the <signal.h> header file. The most common signal values and their symbolic names are:

value	symbol	purpose
1	SIGHUP	hangup on control terminal
2	SIGINT	interrupt from keyboard (Ctrl-c)
3	SIGQUIT	quit from keyboard (Ctrl-\)
8	SIGFPE	floating point exception (e.g. divide by 0)
9	SIGKILL	terminate receiving process
14	SIGALRM	end of alarm() system call timeout
15	SIGTERM	default kill command signal
17	SIGCHLD	child process stopped or terminated
19	SIGSTOP	stop execution from keyboard Ctrl-z

Each signal type has associated with it an action which the kernel will perform on behalf of a process when the process is sent that signal. For most of the signals, the default action is to terminate the receiving process, though, usually, a process can request some alternative action of the system. The various alternatives are:

Ignore the signal. With this option set, the process will be unaware that a signal has occurred. Two of the signals cannot be ignored: SIGKILL, which will terminate the process, and SIGSTOP, which will suspend its execution as part of the job control mechanism.
Restore the signal's default action.
Execute a pre-arranged signal handling function. A process can register a special signal handling function which will be called like an interrupt service routine when the signal is received by the process. When the handler returns, control is passed back to the main code and normal process execution continues.

Next: signal System Call