c10-21-the implemention of sleep function

#include "apue.h"
static void
sig_alrm(int signo)
{
    /* nothing to do ,just returning wakes up to sigsuspend() */
}
unsigned int
sleep(unsigned int nsecs)
{
    struct sigaction newact,oldact;
    sigset_t newmask,oldmask,susmask;
    unsigned int unslept;

    /*
     * set our handler,save previous information
     */
    newact.sa_handler = sig_alrm;
    sigemptyset(&newact.sa_mask);
    newact.sa_flags = 0;
    sigaction(SIGALRM,&newact,&oldact);

    /*
     * block sigalrm and save current signal mask
     */
    sigemptyset(&newmask);
    sigaddset(&newmask,SIGALRM);
        sigprocmask(SIG_BLOCK,&newmask,&oldmask);

    alarm(nsecs);
    susmask = oldmask; /* make sure SIGALRM not block */
    sigdelset(&susmask,SIGALRM);
    sigsuspend(&susmask); /* wait for any signal ro be caught */

    /*
     * some signal has been caught,sigalrm is now block
     */
    unslept = alarm(0);
    sigaction(SIGALRM,&oldact,NULL); /* reset previous action */
    /*
     * reset signal mask,which unblock sigalrm
     */
    return(unslept);
}