#include <stdio.h>

void main()
{ FILE *f; 
  char *str1="abc\r\n";
  char *str2="123\r\n";
  char s;
  int res;
  int pos;
// part one
  f=fopen("test.txt","wb");
  fwrite(str1,5,1,f);
  fclose(f);

// part two
  f=fopen("test.txt","rb+");
  
// move cursor to end line
  while(1)
  { res=fread(&s,1,1,f);
    if (res==0)
     break;
    if(s=='\n')
    { break;
    }
  }
  pos=ftell(f);

// if uncomment this - will OK
//  fseek(f,0,SEEK_END);
//  pos=ftell(f);
  
  res=fwrite(str2,1,5,f);
  
  if (res==0)
  { printf("What's a problem?\n");
  }

  fclose(f); 
}