Question

I’m trying to do a simple fork -> execute another program -> say “hello” to that child process -> read back something -> print what received.

The program used as child just waits for any line of input and prints something to the stdout like “hello there!”

This is my “host” program (that is not working):

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>

#define IN 0
#define OUT 1
#define CHILD 0

main ()
{
  pid_t pid;
  int pipefd[2];
  FILE* output;
  char buf[256];

  pipe(pipefd);

  pid = fork();    
  if (pid == CHILD)
  {
    printf("child\n");
    dup2(pipefd[IN], IN);
    dup2(pipefd[OUT], OUT);
    execl("./test", "test", (char*) NULL);
  }
  else
  {
    sleep(1);
    printf("parent\n");
    write(pipefd[IN], "hello!", 10); // write message to the process    
    read(pipefd[OUT], buf, sizeof(buf));
    printf("received: %s\n", buf);
  }
}

I get this:

child
[.. waits 1 second ..]
parent
received: 

What am I missing? Thanks!

EDIT (test.c):

By request, this is the child program:

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

int getln(char line[])
{
  int nch = 0;
  int c;

  while((c = getchar()) != EOF)
    {
      if(c == '\n') break;
    line[nch] = c;
    nch++;
    }

 if(c == EOF && nch == 0) return EOF;
 return nch;
}

main()
{
  char line[20];

  getln(line);
  printf("hello there!", line);
  fflush(stdout);  
  return 0;
}

Answer 1

You’re always suppose to read from file-descriptor 0, and write to file-descriptor 1 with pipes … you have this relationship reversed in the parent process. For what you’re wanting to-do, you may end up needing two pipes for two-way communication between the parent and child that avoids situations where the parent ends up reading the contents it wrote to the pipe since process scheduling is non-deterministic (i.e., the child is not guaranteed to read what the parent wrote to the pipe if the parent is also reading from the same pipe since the parent could just end up writing and then reading with no interleaving of the child process to read what the parent wrote).

Change your code to the following:

main ()
{
  pid_t pid;
  int pipe_to_child[2];
  int pipe_from_child[2];
  FILE* output;
  char buf[256];

  pipe(pipe_to_child);
  pipe(pipe_from_child);

  pid = fork();    
  if (pid == CHILD)
  {
    printf("child\n");

    //child process not using these ends of the pipe, so close them
    close(pipe_to_child[1]);
    close(pipe_from_child[0]);

    dup2(pipe_to_child[0], fileno(stdin));
    dup2(pipe_from_child[1], fileno(stdout));

    execl("./test", "test", (char*) NULL);
  }
  else
  {
    sleep(1);
    printf("parent\n");
    write(pipe_to_child[1], "hello!\n", 10); // write message to the process    
    read(pipe_from_child[0], buf, sizeof(buf));
    printf("received: %s\n", buf);
  }
}