我应该在这段代码中使用wait和waitpid在哪里？

int main (   int argc , char *argv[] )
{

char *cmd_one_tokens[MAX];  /* Array of pointer to hold tokens of first command */
char *cmd_two_tokens[MAX];  /* Array of pointer to hold tokens of second command */

pid_t pid_one = -1 , /* variable to hold process id of first sub process */
      pid_two = -1 ; /* variable to hold process id of second sub process */
int status    = -1 ; /* variable to read the status of the child process */
int pipe_fd[2] = {-1,-1}; /* Array to hold descriptors returned by pipe sys call */
int ret_val    =-1 ; /* variable to hold return values by system calls */

/*Validate Number of command line arguments */
if(3 != argc)
{
    printf("Provide Appropriate Arguments  n <lab3> < "arg1" > < "arg2" > n");
    _exit(FAILURE);

}
/*Parse first command and get the tokens */
parse_command(argv[1],cmd_one_tokens);
/*Parse second command and get the tokens */
parse_command(argv[2],cmd_two_tokens);


/* Create pipe */
ret_val=pipe(pipe_fd);
/*Error check */
if(ERROR==ret_val)
{
    perror("Pipe creation error n");
    _exit(FAILURE);
}
/*Fork First Child */
pid_one = fork() ;
//Error check
if( 0 == pid_one ) /*child process block */
{

    /* redirect stdout to pipe's write end  for sub process one*/
    dup2(pipe_fd[1],1);

    /*close pipe read end */
    close(pipe_fd[0]);

    execvp(cmd_one_tokens[0],cmd_one_tokens);   

    /* if execvp returns then if must have failed */
    //Error check 

}
else /*main process block */
{

    /*Wait for first sub process to finish */
    waitpid ( pid_two , &status ,0); // <-------changes

    /*Fork second subprocess */
    pid_two = fork();
            //Error check
    if( 0 == pid_two ) /*second child process block */
    {
        /* redirect stdin  to pipe's read end  for sub process two */
        dup2(pipe_fd[0],0);

        /*close pipe write end */
        close(pipe_fd[1]);

        execvp(cmd_two_tokens[0] , cmd_two_tokens); 
        /* if execvp returns then if must have failed */
        //Error check 

    }
    else        /*main process block */
    {
        status=-1; /*reset status */

        /*Waiting for the second sub process to finish in No hang fashion */
             waitpid ( pid_two , &status ,0);  // <-------changes

    }
}
return(SUCCESS);        

}/*End of main */