[flac-dev] vsnprintf_s and vsnprintf

[lvqcl]

Erik de Castro Lopo wrote:

>>   vsnprintf (MSVC, MinGW):
>>
>>   buf_size = 8;        ret = -1;        buffer = "01234567ijklmnopqrstuvwxyz"
>>   buf_size = 9;        ret = -1;        buffer = "012345678jklmnopqrstuvwxyz"
>>   buf_size = 10;       ret = 10;        buffer = "0123456789klmnopqrstuvwxyz"
>>   buf_size = 11;       ret = 10;        buffer = "0123456789"
>>   buf_size = 12;       ret = 10;        buffer = "0123456789"
>
> This is actually the only one we can use.

Why? We can use vsnprintf_s for MSVS, and vsnprintf for MinGW.

MSVS version of vsnprintf_s is located inside (statically linked) msvcp???.lib
or (dynamically linked) msvcp???.dll. They are part of MSVS runtime, and compatible
with WinXP. So it is safe to use it in FLAC.

OTOH MinGW uses vsnprintf_s from msvcrt.dll file that belongs to Windows.
WinXP version of msvcrt.dll doesn't have vsnprintf_s. So we have to use
vsnprintf when the compiler is MinGW.

> With a check on the the return value and a line or two of extra
> code, this function can made to behave resonably sensibly, even it
> its not possible to make it meet the requirements if the ISO C specs.
>
> I'm basically think of something like the (untested) diff below.
>
> @@ -62,8 +62,11 @@ flac_snprintf(char *str, size_t size, const char *fmt, ...)
>         va_start (va, fmt);
> #ifdef _MSC_VER
> -       rc = vsnprintf_s (str, size, _TRUNCATE, fmt, va);
> -       rc = (rc > 0) ? rc : (size == 0 ? 1024 : size * 2);
> +       rc = vsnprintf (str, size, fmt, va);
> +       if (rc < 0) {
> +               rc = size - 1;
> +               str [rc] = 0;
> +       }
>  #else
>         rc = vsnprintf (str, size, fmt, va);
>  #endif

>>   buf_size = 10;       ret = 10;        buffer = "0123456789klmnopqrstuvwxyz"

As you see, MSVC version of vsnprintf can return positive value
even when it didn't write '\0' at the end. So
	if (rc < 0) { ... str [...] = 0; }
is not enough.


What about this version --

int flac_snprintf(char *str, size_t size, const char *fmt, ...)
{
	va_list va;
	int rc;

	va_start (va, fmt);

#if defined _MSC_VER
	rc = vsnprintf_s (str, size, _TRUNCATE, fmt, va);
	if (rc < 0)
		rc = size - 1;
#elif defined __MINGW32__ && (!defined __USE_MINGW_ANSI_STDIO || __USE_MINGW_ANSI_STDIO == 0)
	rc = vsnprintf (str, size, fmt, va);
	if (rc < 0 || rc == size) {
		rc = size - 1;
		str [rc] = '\0'; /* assert(size > 0) */
	}
#else
	rc = vsnprintf (str, size, fmt, va);
#endif
	va_end (va);

	return rc;
}

Of course it is also possible to get rid of vsnprintf_s:

int flac_snprintf(char *str, size_t size, const char *fmt, ...)
{
	va_list va;
	int rc;

	va_start (va, fmt);

#if defined _MSC_VER || (defined __MINGW32__ && (!defined __USE_MINGW_ANSI_STDIO || __USE_MINGW_ANSI_STDIO == 0))
	rc = vsnprintf (str, size, fmt, va);
	if (rc < 0 || rc == size) {
		rc = size - 1;
		str [rc] = '\0'; /* assert(size > 0) */
	}
#else
	rc = vsnprintf (str, size, fmt, va);
#endif
	va_end (va);

	return rc;
}

Or forget about __USE_MINGW_ANSI_STDIO:

int flac_snprintf(char *str, size_t size, const char *fmt, ...)
{
	va_list va;
	int rc;

	va_start (va, fmt);

#if defined _MSC_VER || defined __MINGW32__
	rc = vsnprintf (str, size, fmt, va);
	if (rc < 0 || rc == size) {
		rc = size - 1;
		str [rc] = '\0'; /* assert(size > 0) */
	}
#else
	rc = vsnprintf (str, size, fmt, va);
#endif
	va_end (va);

	return rc;
}