If you register & log-in, you can use HTML and you'd be able to present your code in a readable layout:

void WriteString(char* str) 
{ 
   while(str){ 
      WR_DATA(*str); 
      str++; 
   }; 
}

Your while loop is testing the value of the pointer - you need to check the value that it is pointing to; ie, *str

BTW: the style issue of placing the braces { } is hotly debated but, whatever your views, it's usually best to stick to just one style! ;-)