fork系统调用

/*
 * simple fork usage
 */
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

int main()
{
	pid_t child;
	if ((child = fork()) == -1)
	{
		perror("fork");
		exit(EXIT_FAILURE);
	}
	else if(child == 0)
	{
		puts("in child");
		printf ("\tchild pid = %d\n", getpid());
		printf ("\tchild ppid = %d\n", getppid());
		exit(EXIT_SUCCESS);
	}
	else
	{
		puts("in parent");
		printf ("\tparent pid = %d\n", getpid());
		printf ("\tparent ppid = %d\n", getppid());
	}
	return (EXIT_SUCCESS);
}

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第一次运行结果如下：

in parent

in child
parent pid = 4836
parent ppid = 4835
child pid = 4837

child ppid = 4836

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第二次运行结果变这样了：

in parent
parent pid = 4855
in child
parent ppid = 4854
child pid = 4856
child ppid = 4855

使用fork时， 夫进程和子进程的运行顺序是随机的，也就是说他的执行是异步的。