/* gcc -Wall -std=c99 open_pipe_command.c && ./a.out */

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

#define HELLO "hello world!" // 12 

int open_pipe_command(int * fd, char * cmd, char **argv)
{
  int pr[2]; pipe(pr); 
  int pw[2]; pipe(pw); 
  /* user reads in fd[0]=pr[0] & writes in fd[1]=pw[1] */
  fd[0] = pr[0]; fd[1] = pw[1];   
  int pid = fork();
  if(pid == 0) { /* child */        
    close(pw[1]); close(pr[0]); /* useless */
    /* cmd writes in pr[1]=1 & reads in pw[0]=0 */
    dup2(pw[0],0); dup2(pr[1],1); 
    close(pw[0]); close(pr[1]); 
    execvp(cmd, argv); 
    perror("Error execvp");
    exit(EXIT_FAILURE);
  }
  close(pw[0]); close(pr[1]); /* useless */
  return pid; /* success > 0*/
}

int main(int argc, char * argv[])
{
  int fd[2];
  char* cmd[] = {"tr", "a-z", "A-Z", NULL};
  int pid = open_pipe_command(fd, *cmd, cmd);
  if(pid <= 0) return EXIT_FAILURE;
  write(fd[1], HELLO, strlen(HELLO)+1);
  close(fd[1]);
  char msg[128];
  int r = read(fd[0], msg, 128);  
  printf("msg[%d]: %s -> %s\n", r, HELLO, msg);
  close(fd[0]);
  waitpid(pid, NULL, 0);
  return EXIT_SUCCESS;
}