process loop in C

Question

I'm trying to write a code about a process that executes programs from $PATH using the execlp() command.(it doesn't need to be the execlp command but I've found it useful for this one) I've achieved my expected output, but I need to run more than one commands. More specifically I want the child process to run the exec command, then the parent process to print a text indicating that it's ready to accept another command. Then the child process will run the new exec command. My code is this:

int main ( int argc, char *argp[]) { 
pid_t progpid = fork(); // the fork command for the creation of the child process
int status = 0;
char com[256];

if (progpid < 0)  // the check in case of failure
   {
    printf("PROGRAM ABORTED!");
    return 0;
   }
do
   {
    if (progpid == 0)  // the child process
      {
       scanf( "%s", com);
         if (com == "exit")
          {
            exit(0);
          }
        else
            {
               execlp(com, com, NULL);
            }
      }
else //the parent process
    {
      wait(&status);
      printf("$");
    }
}while (com != "exit");
return 0;
}

The expected output is :

<program which I input from keyboard> ( for example  : ls )
<output of the program>
$<next program>
<output of the next program>
.
.
.
$exit

In short I want to keep running programs till I enter exit where it ends without doing anything else. However the output I get is this:

<program>
<output of program>
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

It keeps printing $ until I shut it down. I'm new to processes so please don't be too harsh about my code so far. Thank you in advance!

The logic of your loop is flawed. You only create *one* child process, at the beginning of the program. You need to create one child process *per command entered*. — Some programmer dude, Nov 01 '16 at 15:32

score 2 · Accepted Answer · edited May 23 '17 at 12:33

This

if (com == "exit")

should be

if (strcmp(com, "exit") == 0)

Similarly change the while condition as well.

In C, string comparisons are done using strcmp(). == in your case, simply compares the address of com and the address of the string literal "exit". (In expressions, an array gets converted into a pointer to its first element. Hence, "address" comparison. Also see: What is array decaying?).

Note that your execlp() call has an issue. NULL may be defined as 0, in which case execlp(), being a variadic function, may be able to recognize it as the last argument. I'd suggest to change it to:

execlp(com, com, (char*)0);

You'd also want to check if wait() failed or not by checking its return code.

Here's a simple example based on yours with improved error checking.

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

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

for(;;) {
    char com[1024];
    printf("$ ");
    fgets(com, sizeof com, stdin);
    com[strcspn(com, "\n")] = 0; /* Remove if there's a newline at the end */

    if (strcmp(com, "exit") == 0) {
       exit(0);
    }

    pid_t pid = fork();
    if (pid < 0) {
        perror("fork");
        exit(1);
    }

    if (pid == 0) { /* child process */
       execlp(com, com, (char*)0);
    }

    int status;
    int rc = wait(&status);
    /* You can inspect 'status' for further info. */
    if (rc == -1) {
        perror("wait");
        exit(1);
    }
}

return 0;
}

Note that if you want your to execute commands with arguments then you need to do argument processing.

wait did not fail. I have tested it without the do while loop and it works. — Dimitris Delis, Nov 01 '16 at 15:35
I am just saying it'd be a good practice to check the return code. If there's any problem with the execution `com` then you'd want to know about it. — P.P, Nov 01 '16 at 15:40

process loop in C

1 Answers1