Like the Windows functions, the C run-time library offers one set of functions to manipulate ANSI characters and strings and another set of functions to manipulate Unicode characters and strings. However, unlike Windows, the ANSI functions do the work; they do not translate the strings to Unicode and then call the Unicode version of the functions internally. And, of course, the Unicode versions do the work themselves too; they do not internally call the ANSI versions.

An example of a C run-time function that returns the length of an ANSI string is strlen, and an example of an equivalent C run-time function that returns the length of a Unicode string is wcslen.

Both of these functions are prototyped in String.h. To write source code that can be compiled for either ANSI or Unicode, you must also include TChar.h, which defines the following macro:

   #ifdef _UNICODE
#define _tcslen     wcslen
#else
#define _tcslen     strlen
#endif