进程控制 Linux C fork() execl() exit() wait()

进程控制实验：

在linux下面使用c语言利用系统调用fork(), execl(), exit(), wait()
fork()用来复制进程
int fork() turns a single process into 2 identical processes, known as the parent and the child. On success, fork() returns 0 to the child process and returns the process ID of the child process to the parent process. On failure, fork() returns -1 to the parent process, sets errno to indicate the error, and no child process is created.
execl()调用外部命令
execl stands for execute and leave which means that a process will get executed and then terminated by execl.
exit()
int exit(int status) - terminates the process which calls this function and returns the exit status value. Both UNIX and C (forked) programs can read the status value.
wait()等待子进程执行结束
int wait (int *status_location) - will force a parent process to wait for a child process to stop or terminate. wait() return the pid of the child or -1 for an error. The exit status of the child is returned to status_location.

#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>

int main() {
        printf("main");
        pid_t pid;
        pid = fork();
        // son process create here, exact copy from parent process
        // son process will execute the main funciton after parent process
        if(pid<0) {
                printf("return");
                return 0;
        }
        if(pid==0) {
                printf("I am the child, my pid is %d\n", getpid());
        }else {
                printf("I am the parent, my pid is %d, my child pid is%d\n", getpid(), pid);
        }
//      sleep(6);
}

#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>

void main() {
//	execl("/bin/echo", "echo", "I am in execl", (char*)0);
	pid_t pid;
	int status = -10;
	pid=fork();
	if(pid==0) {
		printf("i am in son proc\n");
		printf("pid = %d\n", getpid());
		int t = wait(&status);
		printf("t = %d\n", t);
		printf("status = %d\n", status);
	//子进程下面没有子进程，wait返回-1，status也未变化
		sleep(3);
	}else {	
	//wait(&status) 返回子进程的pid， status为执行exit(5)的一个值
	// 当然不为5， WEXITSTATUS(status)返回值为5
	//	sleep(3);
	// wait()会等待子进程执行结束，然后回过来的执行父进程
		printf("i am in father proc\n");
       		int t = wait(&status);
		printf("t = %d\n", t);
		printf("status = %d\n", status);
		printf("WEXITSTATUS(status)=%d\n",WEXITSTATUS(status));
	}
	exit(5);
}