sleep function error （"Advanced Programming in the UNIX Environment" Third Edition No.374）

#include <unistd.h>
#include <fcntl.h>
#include <time.h>
#include "apue.h"

static void sig_alrm(int signo)
{
    /* nothing to do, just returning wakes up sigsuspend */
}

static void sig_usr1(int signo)
{

}

//static void sig_alrm_test(int signo)
//{
//    err_msg("left an alrm");
//}

unsigned int sleep(unsigned int seconds)
{
    struct sigaction newact, oldact;
    sigset_t newmask, oldmask, suspmask;
    unsigned int unslept;

    timespec ts;            // ++
    ts.tv_sec = 25;         // ++
    ts.tv_nsec = 0;         // ++

    /* 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(seconds);
    suspmask = oldmask;

    /* make sure SIGALRM isn't block */
    sigdelset(&suspmask, SIGALRM);

    /* wait for any signal to be caught */
    sigsuspend(&suspmask);

    nanosleep(&ts, NULL);                   // ++

    /* some signal has been caught, SIGALRM is now blocked */
    unslept = alarm(0);

    /* reset previous action */
    sigaction(SIGALRM, &oldact, NULL);

    /* reset signal mask, which unblocks SIGALRM */
    sigprocmask(SIG_SETMASK, &oldmask, NULL);
    return unslept;
}

int main(int argc, char *argv[])
{
    struct sigaction useract;
    struct sigaction alrmact;

    useract.sa_handler = sig_usr1;
    sigemptyset(&useract.sa_mask);
    useract.sa_flags = 0;
    if (sigaction(SIGUSR1, &useract, NULL) < 0)
        err_sys("SIG SUR1 error");

//    alrmact.sa_handler = sig_alrm_test;
//    sigemptyset(&alrmact.sa_mask);
//    alrmact.sa_flags = 0;
//    if (sigaction(SIGALRM, &alrmact, NULL) < 0)
//        err_sys("SIG ALRM error");

    sleep(20);

    return 0;
}