swprintf() with narrow strings

Question

The following line of code gives garbage with Visual Studio 2010:

swprintf(buf, L"Value is %s", "abcd");

However, the same code works fine on Linux.

By trial, I could make it work by using %S instead of %s under Visual Studio.

swprintf(buf, L"Value is %S", "abcd");

I am wondering if this is a bug in Visual Studio 2010 or am I missing something. Regards.

Answer 1

This is a "bug," though the behavior is by design. The initial Visual C++ implementation of the wide string printf and scanf functions predated their standardization in C, and in some cases the behavior deviates from what is required by the C Standard Library specification.

In the C Standard Library specification, a %s or %c format specifier must always be paired with a char array or element, and the l length modifier must be used when a wchar_t array or element is provided.

In the Visual C++ implementation of these functions (documentation), the %s and %c format specifiers expect a corresponding argument of the "natural" width. For the narrow string printf and scanf functions, a char pointer or element is required and for the wide string functions a wchar_t pointer or element is required. To pass a string of the "other" width, the %S and %C format specifiers may be used. Alternatively, the h and l length modifiers may be used to explicitly specify that the string argument is a narrow or wide string.

Among other advantages, the Visual C++ implementation of these functions made it possible to easily migrate old code to use Unicode strings via the _TCHAR mappings in <tchar.h>. It is unfortunate that what was standardized was different from what had already been implemented in the Visual C++ implementation (I am not familiar with the history here; it may be that what was standardized matched another implementation.).

Answer 2

The Microsoft implementation of wide printf and scanf functions is not standard conforming, thus you get different results on Linux (standard-conforming) and Windows (broken).

The link Matt posted in his answer points to a MSDN page hinting at that misbehavior:

The C, S, and Z type characters, and the behavior of the c and s type characters when they are used with the printf and wprintf functions, are Microsoft extensions and are not ANSI compatible. Visual C++ does not support the F type character.

Emphasis mine. BTW: Here ANSI means all C standards, and by extension the C++ standards.

The specific issue is that MS redefined those conversion specifiers (c s) to expect UTF-16 when used with wide functions.