Re: report a bug of gcc implementation of std::vsnprintf()

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Mar 30, 2023 at 08:29:45AM +0000, Qiang Ren via Gcc-bugs wrote:
> I found a bug in the result of std::vsnprintf(), here is the test app that can reproduce it.
> I tested with g++ 11.2 and 12.2 and both have the bug,  and this issue does not happen with visual c++.

gcc-bugs is not the right way to report GCC bugs, though in this case
the only bug is in your program.
You can't use the same va_list multiple times as you pass it to
std::vsnprintf, either you need to use va_copy before the first call
and use the copy in one of the two calls, or you need to va-end after
the first vsnprintf call and va_start again before the second call.
> 
> #include <stdio.h>
> #include <string>
> #include <iostream>
> #include <cstdarg>
> 
> int Sprintf(std::string& s, const char* pszFormat, ...) {
>   va_list argptr;
>   va_start(argptr, pszFormat);
> 
>   auto n = std::vsnprintf(nullptr, 0, pszFormat, argptr);
>   char* buf = (char*)malloc(n + 1);
>   n = std::vsnprintf(buf, n + 1, pszFormat, argptr);
>   s = buf;
>   free(buf);
> 
>   va_end(argptr);
>   return n;
> }

	Jakub