Re: Is it safe to use ax_gcc_builtin to detect __builtin_offsetof?

Jeffrey Walton wrote:
> On Sun, Feb 25, 2024 at 2:09 AM Jeffrey Walton <[email protected]> wrote:
>   
>> The page <https://www.gnu.org/software/autoconf-archive/ax_gcc_builtin.html>
>> does not list __builtin_offsetof in the list of documented builtins.
>> But the page says "Unsupported built-ins will be tested with an empty
>> parameter set and the result of the check might be wrong or
>> meaningless so use with care."
>>
>> Is it safe to use ax_gcc_builtin to detect __builtin_offsetof?
>>
>> Sorry to ask. Google is not returning useful results.
>>     
>
> Cancel. We can't use AX_GCC_BUILTIN in production. Using
> AX_GCC_BUILTIN produces broken configure's:
>
> checking for getpagesize... yes
> checking for working mmap... yes
> checking for gettimeofday... yes
> ./configure: line 15023: syntax error near unexpected token `__builtin_offsetof'
> ./configure: line 15023: `AX_GCC_BUILTIN(__builtin_offsetof)'
>   

Between your error and Peter Hull's response, I suspect that 
AX_GCC_BUILTIN is extremely sensitive to underquoted arguments.  Try 
`AX_GCC_BUILTIN([__builtin_offsetof])' just to see if it solves this 
problem, but I would recommend also following Zack Weinberg's advice to 
#include <stddef.h> and use the offsetof that is defined in C89 instead 
of directly depending on the GCC builtin.


-- Jacob