Question

referring to the old homework question : /* implementing "/usr/bin/ps -ef | /usr/bin/more" */
using pipes.

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

int main()
{
  int fds[2];
  int child[2];
  char *argv[3];
  pipe(fds);
  if (fork()== 0) {
    close(fds[1]);
    close(STDIN_FILENO); dup(fds[0]); /* redirect standard input to fds[1] */
    argv[0] = "/bin/more";
    argv[1] = NULL;           /* check how the argv array is set */
    execv(argv[0], argv);// here how execv reads from stdin ??
    exit(0);


  }
  if (fork() == 0) {
    close(fds[0]);
    close(STDOUT_FILENO); dup(fds[1]);  /* redirect standard output to fds[0] */
    argv[0] = "/bin/ps";
    argv[1] = "-e"; argv[2] = NULL;
    execv(argv[0], argv);
    exit(0);

  }

  close(fds[1]);
  wait(&child[0]);
  wait(&child[0]);  
} 

After redirecting the fd to standard output, how does execv reads from it. Is it inbuilt in execv that it reads from standard input before executing the command? I am unable to get this concept.

Answer 1

Your question is based on a false premise — execv doesn’t read from anywhere, nor does it need to. It is more that reads from the stdin it inherits across the call to execv. The reason more reads from stdin is because it’s a filter and, like most filters, it defaults to reading from stdin if another input source isn’t specified on the command line. (Otherwise, /usr/bin/ps -ef | /usr/bin/more wouldn’t work.)