# 系统编程(系统函数)

eg:exit() _exit()

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
int main()
{
    printf("this is exit\n");
#if 0
//_exit 进程终止函数，不清理缓冲区的内容，进程直接结束
    printf("_exit test");
    _exit(0);
#else
//exit 进程终止函数，清理缓冲区的内容，然后进程结束
    printf("exit test");
    exit(0);
#endif
}

eg:wait()

//阻塞函数，在父进程不运行的情况下用于回收子进程(父进程不运行，子进程运行就是孤儿进程，必须有一父一子。)
//wait()返回值是子进程的进程id
#include<sys/wait.h>
#include<unistd.h>
#include<stdio.h>
int main()
{
    int pid=fork();
    if(pid>0){
        printf("this is main process start\n");
        wait(NULL);
        printf("this is main process over\n");
    }
    else if(pid==0){
        for(int i=0;i<5;i++){
        printf("this is child\n");
        sleep(1);
        }
    }
}

eg: WEXITSTATUS宏

#include<sys/wait.h>
#include<unistd.h>
#include<stdio.h>
int main()
{
    int pid=fork();
    if(pid>0){
        int ret;
        printf("this is main process start\n");
        int pid_p=wait(&ret);//等待子进程结束  返回值：子进程退出的进程号，&ret 获取子进程退出的进程id
        printf("ret:%d\n",ret);
        if(WIFEXITED(ret))    //判断子进程是否调用exit _exit函数 正常退出
            printf("wait status ret:%d\n",WEXITSTATUS(ret));//获取exit里的值
        printf("this is main process over,pid_p:\n",pid_p);
    }
    else if(pid==0){
        for(int i=0;i<5;i++){
        printf("this is child\n");
        sleep(1);
        }
        //abort();//程序异常退出
        exit(10);
    }
}

eg:waitpid()