signal---specify handler subroutine for a signal

#include <signal.h>
void ( * signal(int sig, void(*func)(int)) )(int);

void ( * _signal_r(void *reent, 
    int sig, void(*func)(int)) )(int);

int raise (int sig);

int _raise_r (void *reent, int sig);

signal allows you to request changed treatment for a particular signal sig. You can use one of the predefined macros SIG_DFL (select system default handling) or SIG_IGN (ignore this signal) as the value of func; otherwise, func is a function pointer that identifies a subroutine in your program as the handler for this signal.

Some of the execution environment for signal handlers is unpredictable; notably, the only library function required to work correctly from within a signal handler is signal itself, and only when used to redefine the handler for the current signal value.

Static storage is likewise unreliable for signal handlers, with one exception: if you declare a static storage location as `volatile sig_atomic_t', then you may use that location in a signal handler to store signal values.

If your signal handler terminates using return (or implicit return), your program's execution continues at the point where it was when the signal was raised (whether by your program itself, or by an external event). Signal handlers can also use functions such as exit and abort to avoid returning.

raise sends the signal sig to the executing program. It returns zero if successful, non-zero if unsuccessful.

The alternate functions _signal_r, _raise_r are the reentrant versions. The extra argument reent is a pointer to a reentrancy structure.

Returns
If your request for a signal handler cannot be honored, the result is SIG_ERR; a specific error number is also recorded in errno.

Otherwise, the result is the previous handler (a function pointer or one of the predefined macros).

Portability
ANSI C requires raise, signal.

No supporting OS subroutines are required to link with signal, but it will not have any useful effects, except for software generated signals, without an operating system that can actually raise exceptions.