ISO C90 compilation error

ISO C90 compilation error

[Paz Offer]

[-- Attachment #1: Type: text/plain, Size: 608 bytes --]

Hi,

I am trying to build my code with QEMU and getting compilation error according to the ISO C90 standard:

      const size_t buf_size = 31;
      char buffer[buf_size + 1];

      error: ISO C90 forbids array ‘buffer’ whose size can’t be evaluated [-Werror=vla]

I noticed that the code builds with '-std=gnu11', which is newer then C90, so this is not clear to me why I get this error.
Where is the correct place to specify the language version for this?

I am building QEMU using the instructions on QEMU site (configure, make).

Thanks,
Paz


[-- Attachment #2: Type: text/html, Size: 4088 bytes --]

Re: ISO C90 compilation error

[Thomas Huth]

On 29/02/2024 08.03, Paz Offer wrote:
> Hi,
> 
> I am trying to build my code with QEMU and getting compilation error 
> according to the /ISO C90 /standard:
> 
>       const size_t buf_size = 31;
>       char buffer[buf_size + 1];
> 
>       error: ISO C90 forbids array ‘buffer’ whose size can’t be evaluated 
> [-Werror=vla]
> 
> I noticed that the code builds with '-std=gnu11', which is newer then C90, 
> so this is not clear to me why I get this error.
> Where is the correct place to specify the language version for this?

The "ISO C90" part of the error message is quite misleading here. It's 
rather that we explicitly enabled -Werror=vla in our codebase recently:

  https://gitlab.com/qemu-project/qemu/-/commit/64c1a5443528ac09d8cd50f365d6

See the commit description there for the rationale.

  Thomas

Re: ISO C90 compilation error

[Daniel P. Berrangé]

On Thu, Feb 29, 2024 at 07:03:35AM +0000, Paz Offer wrote:
> Hi,
> 
> I am trying to build my code with QEMU and getting compilation error according to the ISO C90 standard:
> 
>       const size_t buf_size = 31;
>       char buffer[buf_size + 1];
> 
>       error: ISO C90 forbids array ‘buffer’ whose size can’t be evaluated [-Werror=vla]
> 
> I noticed that the code builds with '-std=gnu11', which is newer then
> C90, so this is not clear to me why I get this error.
> Where is the correct place to specify the language version for this?

QEMU has set compiler flags to explicitly /forbid/ use of variable
sized arrays on the stack, as it is a known dangerous language
feature. You must refactor your changes to avoid this by using either
a statically sized array, or allocating on the heap.

With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|

Re: ISO C90 compilation error

[Philippe Mathieu-Daudé]

On 29/2/24 08:59, Daniel P. Berrangé wrote:
> On Thu, Feb 29, 2024 at 07:03:35AM +0000, Paz Offer wrote:
>> Hi,
>>
>> I am trying to build my code with QEMU and getting compilation error according to the ISO C90 standard:
>>
>>       const size_t buf_size = 31;
>>       char buffer[buf_size + 1];
>>
>>       error: ISO C90 forbids array ‘buffer’ whose size can’t be evaluated [-Werror=vla]
>>
>> I noticed that the code builds with '-std=gnu11', which is newer then
>> C90, so this is not clear to me why I get this error.
>> Where is the correct place to specify the language version for this?
> 
> QEMU has set compiler flags to explicitly /forbid/ use of variable
> sized arrays on the stack, as it is a known dangerous language
> feature. You must refactor your changes to avoid this by using either
> a statically sized array, or allocating on the heap.

If you array has a fixed size, you could use a definition, so the
proprocessor can evaluate the buffer size:

   #define BUF_SIZE 31
   char buffer[BUF_SIZE + 1];

Regards,

Phil.