1、　　了解系统调用fork()、execl()、exit()、getpid()和waitpid()的功能和实现过程

2、　　编写一段程序实现以下功能：

a)　　使用系统调用fork()创建两个子进程

b)　　父进程重复显示字符串”parent:”，并使用函数getpid()显示自己的进程ID。

c)　　两个子进程分别重复显示字符串”child:”，并使用函数getpid()显示自己的进程ID

3、　　编写一段程序实现以下功能：

a)　　使用系统调用fork()创建一个子进程

b)　　子进程显示自己的进程ID和字符串\": The child is calling an exec.\\n\"，然后通过execl()调用系统命令ps显示当前运行的进程情况，从而更换自己的执行代码，最后调用exit()结束。

c)　　父进程显示自己的进程ID和字符串” \": The parent is waiting for child to exit.\\n \"，然后调用waitpid()等待子进程结束，并在子进程结束后显示”The parent exit.\\n

-1, the understanding of the system call fork (), execl (), exit (), getpid (), and waitpid () function and process 2, write a procedure to achieve the following functions : a) The use of system calls fork () to create two child process b) the parent process repeated string "parent : ", and the use of function getpid () show their ID process. C) two separate child process repeated string "child :" and the use of function getpid () show their ID 3 process, to prepare a process to achieve the following functions : a) The use of system calls fork () to create a child process b) children show their ID process, and the string " : The child is calling an exec. \\ n " Then execl () call system commands ps shows currently running process, thereby replacing the execution