Using pipe and fork to form a ring

[PetitePi]

I've found some sample code online about how to create a ring with pipes and fork(). I'm to pass messages from one process to another through the pipe.

A process reads in a number from its parent process, but apparently, fscanf() doesn't work as expected. (in main, after creation of pipe in the while loop) The output is not the same.

Any idea what goes wrong? Thanks!

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

#define MAX         512

void make_trivial_ring(){
  int  fd[2];     
  pipe (fd);
  dup2(fd[0], STDIN_FILENO);
  dup2(fd[1], STDOUT_FILENO);
  close(fd[0]);
  close(fd[1]); 
}


void add_new_node(int *pid){
  int   fd[2];
  pipe (fd);
  *pid = fork();
  if (*pid > 0)  dup2(fd[1], STDOUT_FILENO);
  else           dup2(fd[0], STDIN_FILENO);
  close(fd[0]); 
  close(fd[1]); 
}


int main(int argc,  char *argv[ ])
{
  int  k, i, childpid;
  int maxpid, maxnode, maxnum;

 
  make_trivial_ring();
  for (i = 1; i < atoi(argv[1]); i++){
    add_new_node(&childpid);
    if (childpid) break;   
  }

fprintf(stderr, "123\n");
fscanf(stderr, "%d", &maxpid);
	fprintf(stderr, "%d\n", maxpid);
  
   //sleep(5);
   exit(0); 
}

Output:

123
1307813
123
1307813
123
1307813

[anduril462]

You can't fscanf an stderr. fscanf requires an input stream.

[PetitePi]

Is there any advice on what should I use instead of fscanf to read message from another process ? Thanks

[anduril462]

You could use read() to read from a socket file descriptor (an integer, not the FILE * like fscanf takes) into some buffer. Then you can use sscanf to parse the buffer in the same fashion as fscanf.

[Salem]

Can you draw a picture of what you're trying to do?

Like which processes are reading/writing to.

> if (*pid > 0) dup2(fd[1], STDOUT_FILENO);
The parent has only one stdout.
So each time you dup2(), the previous pipe end is lost.