Question

Consider following source, reduced for simplicity

int main()
{
  int d[2];
  pipe(d);
  if(fork())
  {
    close(1);
    dup(d[1]);
    execlp("ls", "ls", NULL); 
  }
  else
  {
    close(0);
    dup(d[0]);
    execlp("cat", "cat", NULL); 
  }
}

So it creates a pipe and redirects the output from ls to cat.

It works perfectly fine, no problems. But change cat to more and bash breaks.

The symptoms are:

• you don’t see anything you type
• pressing “enter” shows up a new prompt, but not in a new line, but in the same one
• you can execute any command and see the output
• reset helps fixing things up.

So there is a problem with input from keyboard, it is there, but is not visible.

Why is that?

UPDATE:

• the output from ls | more is equivalent to the output of my program
• more process does not finish, it’s is orphaned by ls
• the only visible problem is with the state of the console after the parent process quits
• on some systems it does work like intended. E.g., on OpenSUSE I had no problems, on Kubuntu. I couldn’t find any information on what differences should I look for, more binaries are different on both systems

Answer 1

Because unlike cat, more is an interactive program that requires more than stdin, stdout and stderr — it requires a terminal, which your system call cannot provide. Try to run more in a pipe or from a script and see what happens.

Also note that bash is not involved here at any stage, the execlp function call replaces the current process by another one specified as an argument.