[clang] [-Wunsafe-buffer-usage] Warning Libc functions (PR #101583)

[Ziqing Luo via cfe-commits]

ziqingluo-90 wrote:

> Btw a question about the new warning: So with -Wunsafe-buffer-usage-in-libc-call clang now warns on the following?
> 
> ```
> #include <stdio.h>
> 
> void foo(void) {
>   char q[10];
>   snprintf(q, 10, "%s", "hello");
> }
> ```
> 
> It says
> 
> ```
> foo.c:5:3: warning: function 'snprintf' is unsafe [-Wunsafe-buffer-usage-in-libc-call]
>     5 |   snprintf(q, 10, "%s", "hello");
>       |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> foo.c:5:12: note:  buffer pointer and size may not match
>     5 |   snprintf(q, 10, "%s", "hello");
>       |            ^
> 1 warning generated.
> ```
> 
> Is that as expected? If so, how should snprintf be used to avoid the warning?

Yes, this is expected.  According to the C++ Safe Buffers programming model, buffer pointers should be changed to `std::span`.   Then  `snprintf(span.data(), span.size(), ...)` is considered safe and will not be warned.  We may also allow the use of the form `snprintf(span.first(10).data(), 10, ...)` later.

https://github.com/llvm/llvm-project/pull/101583