30. Visual C++ and wprintf() function

The fragment is taken from Energy Checker SDK. The code contains an error that PVS-Studio analyzer diagnoses in the following way: V576 Incorrect format. Consider checking the second actual argument of the 'wprintf' function. The pointer to string of wchar_t type symbols is expected.

int main(void) {
  ...
  char *p = NULL;
  ...
  wprintf(
    _T("Using power link directory: %s\n"), 
    p
  );
  ...
}

Explanation

Note: The first error is in the usage of _T for specifying a string in wide-character format. To use L prefix will be the correct variant here. However this mistake is not a crucial one and is not of a big interest to us. The code simply won't be compiled if we don't use a wide-character format and _T will expand into nothing.

If you want a wprintf() function to print a char* type string, you should use "%S" in the format string.

Many Linux programmers don't see where the pitfall is. The thing is that Microsoft quite strangely implemented such functions as wsprintf. If we work in Visual C++ with the wsprintf function, then we should use "%s" to print wide-character strings, at the same time to print char* strings we need "%S". So it's just a weird case. Those who develop cross platform applications quite often fall into this trap.

Correct code

The code I give here as a way to correct the issue is really not the most graceful one, but I still want to show the main point of corrections to make. 

char *p = NULL;
...
.#ifdef defined(_WIN32)
wprintf(L"Using power link directory: %S\n"), p);
.#else
wprintf(L"Using power link directory: %s\n"), p);
.#endif

Recommendation

I don't have any particular recommendation here. I just wanted to warn you about some surprises you may get if you use functions such as wprintf().

Starting from Visual Studio 2015 there was a solution suggested for writing a portable code. For compatibility with ISO C (C99), you should point out to the preprocessor a _CRT_STDIO_ISO_WIDE_SPECIFIERS macro.

In this case the code:

const wchar_t *p = L"abcdef";
const char *x = "xyz";
wprintf(L"%S %s", p, x);

is correct.

The analyzer knows about _CRT_STDIO_ISO_WIDE_SPECIFIERS and takes it into account when doing the analysis.

By the way, if you turn on the compatibility mode with ISO C (the _CRT_STDIO_ISO_WIDE_SPECIFIERS macro is declared), you can get the old behavior, using the specifier of "%Ts" format.

In general the story about the wide - character symbols is quite intricate, and goes beyond the frames of one short article. To investigate the topic more thoroughly, I recommend doing some reading on the topic:

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