signal implemention

/* signal.c: Don't compile this program. This is only the implementation of signal(3), *
 * but without existence of main(). So it can't be compiled unless a specific flag -c  *
 * is given to the compiler e.g. gcc -c. signal(3) complies with POSIX.1. But it is    *
 * only a wrapper function. signal(3) is actually implemented using sigaction(2).      */

#include <signal.h>

typedef void Sigfunc(int);

Sigfunc *signal(int signo, Sigfunc *func)
{
    struct sigaction act, oact;

    act.sa_handler = func;
    sigemptyset(&act.sa_mask); /* Empty signal set. */
    act.sa_flags = 0;
    /* the purpose of generating SIGALRM is normally to place a timeout *
     * on an I/O operation. If SIGALRM caught, system calls will never  *
     * restart regardless of SA_RESTART.                                */
    if (signo == SIGALRM) {
#ifdef SA_INTERRUPT
        act.sa_flags |= SA_INTERRUPT; /* SunOS 4.x */
#endif
    } else {
#ifdef SA_RESTART
        act.sa_flags |= SA_RESTART; /* SVR4 & BSD */
#endif
    }
    /* The 1st arg to sigaction(2) is signal number to be caught. *
     * The 2nd arg is ptr to the current disposition.             *
     * The 3rd arg is ptr to the old disposition.                 */
    if (sigaction(signo, &act, &oact) == -1)
        return SIG_ERR;
    return oact.sa_handler; /* the previous disposition of the signal */
}