Question

A (very long) while ago I regularly used the following code – then on MSVC 6 – to determine the memory needed to format a string for a function with variadic arguments:

void LogPrint(const char *pszFormat, ...)
{
    int          nBytes;
    char        *pszBuffer;
    va_list      args;

    va_start(args, pszFormat);
    nBytes = vsnprintf(0, 0, pszFormat, va);
    va_end(args);

    // error checking omitted for brevity
    pszBuffer = new char[nBytes + 1];

    va_start(args, pszFormat);
    vsnprintf(pszBuffer, nBytes, pszFormat, va);
    va_end();

    // ...
}

The obvious error you’re getting in a more recent version of MSVC (I’m using 2010 now) is:

warning C4996: ‘vsnprintf’: This function or variable may be unsafe. Consider using vsnprintf_s instead. To disable deprecation use _CRT_SECURE_NO_WARNINGS. See online help for details.

I’m a big fan of the “treat warnings as errors” option for any C(++)-compiler, and obviously my build fails. It feels like cheating to me to simply employ #pragma warning (disable:4996) and get on with it.

The suggested “safer” alternative vsnprintf_s(), however is doomed to return -1 when input conditions of its “unsafe” predecessor occur.

TL/DR: Is there a way to implement the expected behavior of vsnprintf() to return the memory needed to fulfil its task using the new, safer variants of it?

---

EDIT: simply defining _CRT_SECURE_NO_WARNINGS won’t cut it; there’s a lot of strcpy() flying around, too. The new variant of which isn’t broken, so I’d like to still see these.

Answer 1

The function you want to look at is _vscprintf, which “returns the number of characters that would be generated if the string pointed to by the list of arguments was printed or sent to a file or buffer using the specified formatting codes”. There’s a widechar variant (_vscwprintf) as well.