c-使用提示符实现shell：子级在父waitpid之后打印

Question

I'm currently trying to implement a basic shell programm which wait for user to prompt a single command (just like "ls" or something else) and this following code is working, except that I get a weird output...

Code

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <errno.h>


char *username;
char hostname[1024];
char currentdir[1024];
char shell_input[256];
int error_code;

int main() {

    int pid, status;
    char *pos;
    username = getenv("USER");
    gethostname(hostname, sizeof(hostname));
    getcwd(currentdir, sizeof(currentdir));

    while(1){
        printf("%s@%s:%s$ ", username, hostname, currentdir);
        fgets(shell_input, sizeof(shell_input), stdin);

        // deleting the newline captured by fgets
        if ((pos=strchr(shell_input, '\n')) != NULL)
            *pos = '\0';

        if((pid = fork())==0){
            error_code= execlp(shell_input, shell_input, NULL);
            if(error_code!=0){
                printf(" %s \n", strerror(errno));
                exit(0);
            }
        }else{
            waitpid(pid, &status, -1);
        }
    }
}

As expected, when i run my program, i'v got this console prompt :

jeremy@jeremy-pc:/home/jeremy/Cours$ 

Then, if want to run the ls command in my program, the command works but i got this output :

jeremy@jeremy-pc:/home/jeremy/Cours$ ls
jeremy@jeremy-pc:/home/jeremy/Cours$ example    prototype.c      dir1    prototype

The problem here, is that the "prompt" string (jeremy@jeremy-pc etc...) is printed before the ls result which is supposed to be printed before the waitpid.

So my question is, what's wrong in my code as long as i don't get a result like this :

jeremy@jeremy-pc:/home/jeremy/Cours$ ls
example    prototype.c       dir1    prototype
jeremy@jeremy-pc:/home/jeremy/Cours$ 

Thank you in advance for your help and for your time, have a good day :)

Answer 1

You have a problem with the option argument of waitpid . By giving -1 (0xFFFFFFFF) as third argument, you probably enable all options including WNOHANG , which is described as return immediately if no child has exited. in manpage.

Disabling all flags would make waitpid wait for process to exit:

waitpid(pid, &status, 0);