#include <iostream>

#include <sys/types.h>  // read
#include <sys/stat.h>   //read
#include <fcntl.h>      //read
#include <unistd.h>     // lseek

#include <errno.h>      // errno, 
#include <string.h>     // strerror

const char * FILE_NAME = "seekData";
const int POSITIONS = 10;

using namespace std;

// for clarity I will assume no errors on read/write.

void MyError(int errmsg, string message) {
       cerr << message << endl;
       cerr << strerror(errmsg) << endl;
       return;
}

int main() {
    int fd;
    char data;
    int i;
    int location;
    int offset;
    int pos;
    int realPos;
    bool done = false;

    // open the file, read write
    // make it if it does not exist
    // and wipe it out to boot.
    fd  = open(FILE_NAME, O_RDWR|O_CREAT|O_TRUNC, S_IRWXU | S_IRGRP | S_IROTH );
    if (-1 == fd) {
       string message = FILE_NAME;
       message = "Could not open" + message;
       MyError(errno, message);
    }

    data = 'X';
    // first fill the file with X
    for(i=0;i<POSITIONS; i++) {
        write(fd, &data, 1);
    }

    // move the pointer to the beginning of the file.
    realPos = lseek(fd, 0, SEEK_SET);
    if (-1 == realPos) {
       MyError(errno, "lseek failed");
       done = true;
    } 
    pos = realPos;

    // let the user seek a bit.
    while (not done) {
       cout << "Enter a location (0 - " << POSITIONS -1 
            << ") or -1 to exit => ";
       cin >> location;

       if (-1 == location) {
          done = true;
       } else if (location >= POSITIONS or location < 0) {
          cout << location << "is not a valid position" << endl;
       } else {
          // data to write.
          cout << "Enter a character to write at position " << location;
	  cout << "=> ";
	  cin >> data;

          // calculate the offset and give some information
	  offset = location - pos;
	  cout << "\tCurrently at " << pos << endl;
	  cout << "\tGoing to " << location << endl;
	  cout << "\tThe offset is " << offset << endl;

          // seek the location
	  realPos == lseek(fd, offset, SEEK_CUR);

	  if (-1 == realPos) {
	     // argh, we failed.
             MyError(errno, "lseek failed");
	     done = true;
	  } else {
	      // the sweet smell of success
	      cout << "\tLseek reports that we have moved to " 
	           << realPos << endl;
	      cout << "\tWriting a " << data << " at location " 
	           << location << endl;
	      cout << endl;
	      write(fd, &data, 1);
	      pos = location + 1;
	  }
       }
    }

    close(fd);

    return 0;
}