Signal Handling: signal.h

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Signal Handling: signal.h

A signal is a condition that can be reported during program execution. It is represented by a positive integer. The raise() function sends, or raises , a signal, and the signal() function sets the response to a particular signal.

The standard provides the macros listed in Table F.8 to represent possible signals; an implementation can add further values. They can be used as arguments to raise() and signal() .

Table  F.8. Signal macros.

Macro	Description
SIGABRT	Abnormal termination, such as initiated by a call to abort() .
SIGFPE	Erroneous arithmetic operation.
SIGILL	Invalid function image (such as illegal instruction) detected .
SIGINT	Interactive attention signal received (such as a DOS interrupt).
SIGSEGV	Invalid access to storage.
SIGTERM	Termination request sent to program.

The signal() function takes as its second argument a pointer to a void function that takes an int argument. It also returns a pointer of the same type. A function invoked in response to a signal is termed a signal handler . The standard defines three macros fitting this prototype:

 void (*funct)(int); 

Table F.9 lists these macros.

Table  F.9. Type void (*f)(int) macros.

Macro	Description
SIG_DFL	When used as an argument to signal() , along with a signal value, indicates that the default handling for that signal will occur.
SIG_ERR	Used as a return value for signal() if it cannot return its second argument.
SIG_IGN	When used as an argument to signal() , along with a signal value, indicates that the signal will be ignored.

If a signal sig is raised and func points to a function (see signal() prototype in Table F.10), first, under most circumstances, signal(sig, SIG_DFL) is called to reset signal handling to the default, and then (*func)(sig) is called. The signal-handling function pointed to by func can terminate by executing a return statement or by calling abort() , exit() , or longjmp() .

Table F.10 lists the signal functions.

Table  F.10. Signal functions.

Prototype	Description
void (*signal(int sig, void (*func)(int)))(int);	Causes the function pointed to by func to be executed if signal sig is raised. If possible, returns func ; otherwise , returns SIG_ERR .
int raise(int sig);	Sends the signal sig to the executing program; returns zero if successful, and non-zero otherwise.

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