/* File: ftdemo.cpp 					 */
/* Demonstrate file tables. Open a file, fork, open it again.    */
/* 	Show when system file table entries are shared, and when */
/*	they aren't						 */
/* Test some std I/O stuff					 */

#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <iostream>

using namespace std;

main()
{int inward,inout;  // file descriptors
 char ch;
 // Fill a file with numbers, say 1000 digits
 inout=open("abc.txt",O_RDWR|O_CREAT|O_TRUNC);
 for (int k=0;k<1000;k++) {
	char z=(char)(k%10+48);
	write(inout,&z,sizeof(char)); 
	if (k%80==79) {
		ch='\n';
		write(inout,&ch,sizeof(char));
	}
 }
 lseek(inout,0,SEEK_SET);  // back to start
 pid_t IsParent;
 IsParent=fork();
 if (IsParent) {
  read(inout,&ch,sizeof(char)); 
  cout << "Parent reads " << ch << " from shared stream\n";
  wait(NULL);
  cout << "wait() Returns!\n"; 
 }
 else {
  sleep(1);			// let parent go first
  read(inout,&ch,sizeof(char)); 
  cout << "Child reads " << ch << " from shared stream and closes it\n";
  close(inout);
  cout << "Original Data file:\n";
  system("cat abc.txt");
  cout << endl;
  inward=open("abc.txt",O_RDONLY);
  cout << "Child opens new stream on same file and reads 10 characters:\n";
  for (int j=0;j<10;j++) {
	read(inward,&ch,sizeof(char));
	cout << ch;
  }	
  cout << endl;
  inout=open("abc.txt",O_RDWR);
  cout << "Child writes the upper case alphabet to start of file\n";
  for (int jj=0;jj<26;jj++) {
	char p=(char)(jj+65);
	write(inout,&p,sizeof(char)); 
  }	
  cout << "Child writes the lowcase alphabet to 50th spot of file, using cerr\n";
  lseek(inout,50,SEEK_SET);
  dup2(inout,2);
  for (int jj=0;jj<26;jj++) {
	cerr << (char)(jj+97);
  }	
  close(inout);
  cout << "Data file:\n";
  system("cat abc.txt > /dev/tty ");  // what is /dev/tty??
  cout << endl;
  cout <<  "Now, read next 10 chars via new stream (not formerly shared one)\n";
  for (int n=0;n<10;n++) {
	read(inward,&ch,sizeof(char));
	cout << ch;
  }	
  cout << endl;
 }
}