Thread: Creating a Pipe between 2 processes

My task is to write C code that will create a pipe between two process, writer and reader.


The program will be called communicate and should be run as such: "communicate 10".


When this is done it should be doing the equivalent of "writer 10 | reader".


Writer just simply prints a message n number of times. Reader gets what writer prints and prints it out with no more than 50 characters per line.


My writer and reader programs are correct. Here they are:


writer.c

Code:

#include <stdio.h> /*typically in /usr/include*/
#include <stdlib.h>


int main (int argc, char* argv [ ])
{


   int count, i;
   if (argc != 2)
   {
      fprintf(stderr, "usage: %s  count\n", argv[0]);
      exit (-1); /*signal error (other than 0) to shell*/
   }
   else
      count = atoi (argv[1]);


   for ( i = 0; i < count; i++ )
      printf ("Hellohello");


   return 0;
}

reader.c

Code:

#include <stdio.h> /*typically in /usr/include*/
#include <stdlib.h>


int main (int argc, char* argv [ ])
{


   int count, c;
   if (argc != 1)
   {
      fprintf(stderr, "usage: %s", argv[0]);
      exit (-1); /*signal error (other than 0) to shell*/
   }
   else
   {
        count = 0;
        while ((c = getchar()) != EOF)
        {
                putchar(c);
                if(++count % 50  == 0)
                {
                         putchar((char)'\n');
                }
        }
        if(count % 50 != 0 )
        {
                putchar((char)'\n');
        }
   }


  return 0;
}

Now my problem must be with my communicate program. I think it is pretty well commented so it should be easy to follow what's going on:

Code:

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


#define READ_END 0
#define WRITE_END 1


int main(int argc, char* argv [ ])
{
  if (argc != 2)
    {
      fprintf(stderr, "usage: %s  count\n", argv[0]);
      exit (-1); /*signal error (other than 0) to shell*/
    }


  int fd[2];


  pid_t childpid;
  pid_t childpid2;


  /* create pipe */
  if (pipe(fd) == -1){
    fprintf(stderr,"Pipe failed");
    return 1;
  }


  childpid = fork();


  if (childpid == -1) {
    printf("Error in fork; program terminated\n");
    return -1;
  }


  if (childpid == 0) {// child #1
    close(1);// close standard output (fd = 1)
    dup(fd[WRITE_END]);// dup the writing end of pipe it it gets fd = 1


    // close both ends of the pipe
    close(fd[WRITE_END]);
    close(fd[READ_END]);


    execlp("writer", "writer", argv[1], NULL);// exec to writer code
  }
  else {// parent
    int status;
    wait(&status);


    childpid2 = fork();// fork again


    if (childpid2 == -1) {
      printf("Error in fork; program terminated\n");
      return -1;
    }


    if (childpid2 == 0) {// child #2
      close(0);// close standard input (fd = 0)
      dup(fd[READ_END]);// dup the reading end of the pipe it gets fd = 0


      // close both ends of the pipe
      close(fd[WRITE_END]);
      close(fd[READ_END]);


      execlp("reader", "reader", NULL);// exec to reader code
    }
    else {// parent
      int status;
      wait(&status);
    }


    //close both ends of the pipe
    close(fd[WRITE_END]);
    close(fd[READ_END]);
  }
  return 0;
}

When I run my communicate program, 2 things are going wrong:
1.) I have to hit like ctrl-d or ctrl-z to stop the program. Otherwise it just sits there after printing stuff. Not sure why this is happening, but that's not really a HUGE problem relatively speaking.
2.) It only works in multiples of 5. If I do communicate 5, it works great except for bullet point 1. If I do communicate 10, same story. However if I do anything not a multiple of 5 nothing happens. So communicate 1, nothing. communicate 7, nothing.


Could someone help me figure out what's wrong with my code? I'm a very novice C programmer, but not a novice programmer in general.


Thanks for the help!

You want to close your pipes in the parent before you wait() at the end; otherwise, the reader will continue to expect data from your unclosed pipe in the parent.

Also, just as a note, there's no need to cast '\n' to a char in reader.c: '\n' is an int, putchar() expects an int, so the char cast just creates two unnecessary conversions (int->char->int instead of just using the int). Even if the compiler's smart enough to not do the conversion, casts should be used sparingly, and only when necessary: they often mean that you're doing something behind the compiler's back, so to speak, and when readers of your code see a cast they might spend time wondering why it's there if it appears to be unnecessary.

Thanks guys. Got it working with your help.

Much appreciated.