Question

I have a task in Linux and I can’t get it work.

I have a program that receives a text file as parameter. It then creates a child process using fork() and sends to the child process, line by line the content of the text file received as parameter. The child process needs to count the lines and return to the parent process the number of lines received.

This is what I have until now, but somewhat the child process does not receive all the lines. For my test I used a text file with 9 lines. The parent sent 9 lines as strings but the child process received only 2 or 3 of them.

What am I doing wrong?

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

int main(int argc, char *argv[])
{  
  char string[80];
  char readbuffer[80];
  int pid, p[2];
  FILE *fp;
  int i=0;
  if(argc != 2)
  {
    printf("Syntax: %s [file_name]\n", argv[0]);
    return 0;    
  }
  fp = fopen(argv[1], "r");
  if(!fp) 
  {    
    printf("Error: File '%s' does not exist.\n", argv[1]);
    return 0;
  }
  if(pipe(p) == -1)
  {
    printf("Error: Creating pipe failed.\n");
    exit(0);
  } 
  // creates the child process
  if((pid=fork()) == -1)
  {
   printf("Error: Child process could not be created.\n");
    exit(0);
  }  

  /* Main process */
  if (pid) 
  { 
    // close the read
    close(p[0]);    
    while(fgets(string,sizeof(string),fp) != NULL)
    {                
       write(p[1], string, (strlen(string)+1));
       printf("%s\n",string);
    } 

    // close the write
    close(p[1]);
    wait(0);
  }

  // child process
  else 
  {   
    // close the write
    close(p[1]); 

    while(read(p[0],readbuffer, sizeof(readbuffer)) != 0) 
    {
      printf("Received string: %s\n", readbuffer);    
    }

    // close the read
    close(p[0]); 
  } 
  fclose(fp);       
}

Answer 1

A pipe is a unidirectional interprocess communication channel. You have to create 2 pipes, one to speak to the child process, the other to read data back.

Remember to close the unused side of the pipe on both processes.