/*
 * A simple program to illustrate the work of a pair of pipes
 * Textbook Figure 3.26 Page 144
 */
#include <stdio.h>
#include <unistd.h>
#include <string.h>

#define BUFFER_SIZE 32
#define READ_END    0     // read 0 and write 1 can't be chosen randomly
#define WRITE_END   1

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

  char sent_msg[BUFFER_SIZE] = "Greetings from child";
  char read_msg[BUFFER_SIZE];
  pid_t pid;
  int fd[2];
  int amt_read;   // number of bytes read
  char c;

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

  /* create a child process */
  pid = fork();
  if (pid < 0) { // error occured
    fprintf(stderr," fork failed \n");
    return -2;
  }
  
  if (pid > 0) { // parent process

    /* close the pipe end not in use */
    close(fd[READ_END]);

    char * c_msg = "===Hello from parent==";

    /* write to the pipe */
    write(fd[WRITE_END], c_msg, strlen(c_msg));

    printf("In parent process, type a char to continue ...\n");
    scanf("%c", &c);
    printf("In parent process, just wrote the message %s\n", sent_msg);
    printf("type a char to continue ...\n");
    scanf("%c", &c);

    /* close the write end of the pipe */
    close(fd[WRITE_END]);

  } else { // child process
    printf("In child process, waiting for message ...\n");

    /* write to the pipe */
    write(fd[WRITE_END], sent_msg, strlen(sent_msg));

    /* read the messages from both the child and the parent itself */
    amt_read = read(fd[READ_END], read_msg, BUFFER_SIZE);
    while (amt_read > 0) {
      read_msg[amt_read] = 0;  // terminate the string
      printf("bytes read = %d\n", amt_read);
      printf("read -->%s<--\n", read_msg);
      amt_read = read(fd[READ_END], read_msg, BUFFER_SIZE);
    }

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

  return 0;
}