/*************************************************************************> File Name: fork.c> Author: ma6174> Mail: ma6174@163.com > Created Time: Thu 18 Jan 2018 09:54:34 PM PST************************************************************************/#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
#include<sys/wait.h>
#include<errno.h>int main()
{pid_t pid;if((pid = fork()) < 0)perror("fork error");else if(pid == 0){if((pid = fork()) < 0)perror("fork error");else if(pid > 0)exit(0);sleep(2);printf("second child , parent pid = %d\n",(int)getppid());exit(0);}if(waitpid(pid,NULL,0) != pid)perror("waitpid error\n");exit(0);
} 

最后来看一下我们的结果是什么:

while(getppid() != 1)
sleep(1);

这种形式的循环称为轮询，他的问题是浪费了cpu时间，因为调用者每隔一秒都被唤醒，然后进行条件测试.

#include <signal.h>int sigsuspend(const sigset_t *mask);

/*************************************************************************> File Name: mysleep2.c> Author: ma6174> Mail: ma6174@163.com > Created Time: Thu 18 Jan 2018 11:29:18 PM PST************************************************************************/#include<stdio.h>
#include<signal.h>
#include<unistd.h>void sig_alrm(int sig)
{//DO SOMETHING
}unsigned int mysleep(unsigned int nsecs)
{struct sigaction new,old;sigset_t newmask,oldmask,suspendmask;unsigned int unslept = 0;new.sa_handler = sig_alrm;sigemptyset(&new.sa_mask);new.sa_flags = 0;sigaction(SIGALRM,&new,&old);sigemptyset(&newmask);sigaddset(&newmask,SIGALRM);sigprocmask(SIG_BLOCK,&newmask,&oldmask);alarm(nsecs);suspendmask = oldmask;sigdelset(&suspendmask,SIGALRM);sigsuspend(&suspendmask);unslept = alarm(0);sigaction(SIGALRM,&old,NULL);sigprocmask(SIG_BLOCK,&oldmask,NULL);return unslept;
}int main()
{while(1){mysleep(1);printf("1 seconds passed\n");}
}