CODEWITHSUNDEEP
clinuxexec
user3146118
user3146118

Reputation: 11

why my program execute scan and print twice?

I write this program which must execute mkdir and rmdir, the parent must send argument to child and child execute it. 

void main()
{
  int r; 
  int i;
  int fd[2];
  pipe(fd);
  char arg[100];
  printf("Please Choose one of the following commands: \n");
  printf("1 MKDIR \n");
  printf("2 RMDIR \n");

  while(1)
  {
    printf("You choose: \n");
    scanf("%d ", &r);

    pid_t pid;
    pid = fork();

    printf("Please enter arguments: ");
    scanf("%s ", arg);
    write(fd[1],arg,sizeof(arg));

    for(i=0;i<4;i++)
    {
      if(pid==0)
      {
        switch(r)
        {
          case 1: 
          read(fd[0],arg,sizeof(arg));
          execl("/bin/mkdir", "MKDIR", arg, NULL);

          case 2: 
          read(fd[0],arg,sizeof(arg));
          execl("/bin/rmdir", "RMDIR", arg, NULL);
        }
      }
      break;
    }
  }
}

The problem is that when I execute it it, I I need to scan r two times, then it repeat please enter the argument twice, and it execute the second argument of r.

Upvotes: 0

Views: 583

Answers (2)

Some programmer dude
Some programmer dude

Reputation: 409136

Remember that the child process starts executing the code directly after the fork call, that means that both the parent and child processes will call scanf. You need to check if you are in the child or parent process directly after the fork call.

You also need to have a break statement between the cases in the switch statement, or else when you do the first case the it will continue and to the second as well.

There is also the problem that the child processes never exit, but continue the endless loop. Once the child process is done it should exit.

You are also creating lots of "defunct" processes, in that you never wait for your child processes. You need to call the wait function (or one of its related functions like waitpid) to "reap" the child. Otherwise the process will still be existing even even if it's in a state where it's paused.

Upvotes: 1

Wojtek Surowka
Wojtek Surowka

Reputation: 20993

Add break instruction after each case in switch.

Upvotes: 1

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