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

//http://msdn.microsoft.com/en-us/library/vstudio/db4fefz1.aspx

void main(int argc, char * argv[])
{
  int     dflag =1;
  char    x     = 2;
  size_t  bytes = 0;
  fpos_t  pos;
  fpos_t  eoff;
  long foo;
  char    buff;
  char    next;
  int xx [100];
  int     d =0;
  if (argc > 1) {
     x = atoi(argv[1]);
  }
	//открытие файла на запись
	FILE*fio=fopen("Test.txt","r+b");
	if(fio) {

     if (fseek(fio,0,SEEK_END)!=0){
       fprintf(stderr, "can't seek to end position");
   	   perror("end seek");
       goto error;
     }
     fgetpos(fio, &eoff);
     foo = eoff;
     fprintf(stderr, " length of file: %ld", foo);

     if (fseek(fio,0,SEEK_SET)!=0){
       fprintf(stderr, "can't seek to start position");
   	   perror("start seek");
       goto error;
     }
     fgetpos(fio, &pos);

     for (pos = 0; pos < eoff; pos++){
       fsetpos(fio, &pos);    //<---------------???
       fread(&buff, 1, 1, fio);
       next =  buff^x;
       fsetpos(fio, &pos);
       foo = pos;
       fprintf(stderr, "\n  position/new/old symbol: %d / '%c'/'%c'", foo, next, buff);
       fwrite(&next, 1, 1, fio);
     }
    foo = pos;
		fprintf(stderr, "\n end of file %d", foo );
    error:fclose(fio);
	}
	else {
	   perror("Test1.txt");
	}

	//показ результата на экран
}