Re: [PATCH v2 1/1] git-compat-util: add a test balloon for C99 support

[Phillip Wood <phillip.wood123@gmail.com>]

On 16/11/2021 12:19, Jeff King wrote:
> On Tue, Nov 16, 2021 at 02:12:41AM +0000, brian m. carlson wrote:
> 
>> The C99 standard was released in January 1999, now 22 years ago.  It
>> provides a variety of useful features, including variadic arguments for
>> macros, declarations after statements, variable length arrays, and a
>> wide variety of other useful features, many of which we already use.
> 
> I like the idea of being able to assume C99. And I know this list is
> just "here are some things we could do". But I'd like to express caution
> over variable length arrays. We've already had problems with alloca()
> causing stack exhaustion, and VLAs are basically the same thing. And the
> worst part is there's no way to recover; you just get a segfault.

I agree with this, also C11 and later make variable length array support 
an optional compiler feature which is another reason not to rely on them.

Best Wishes

Phillip

>> Let's add a test balloon to git-compat-util.h to see if anyone is using
>> an older compiler.  We'll add a comment telling people how to enable
>> this functionality on GCC and Clang, even though modern versions of both
>> will automatically do the right thing, and ask people still experiencing
>> a problem to report that to us on the list.
>>
>> Note that C89 compilers don't provide the __STDC_VERSION__ macro, so we
>> use a well-known hack of using "- 0".  On compilers with this macro, it
>> doesn't change the value, and on C89 compilers, the macro will be
>> replaced with nothing, and our value will be 0.
> 
> This part makes sense to me. The macro check will complain if any
> compiler isn't C99. But this hunk seems like it is going to cause
> headaches:
> 
>> diff --git a/Makefile b/Makefile
>> index 12be39ac49..893d533d22 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -1204,7 +1204,7 @@ endif
>>   # Set CFLAGS, LDFLAGS and other *FLAGS variables. These might be
>>   # tweaked by config.* below as well as the command-line, both of
>>   # which'll override these defaults.
>> -CFLAGS = -g -O2 -Wall
>> +CFLAGS = -g -O2 -Wall -std=gnu99
>>   LDFLAGS =
>>   CC_LD_DYNPATH = -Wl,-rpath,
>>   BASIC_CFLAGS = -I.
> 
> Do most compilers understand -std=gnu99? It seems like we're breaking
> the out-of-the-box build for everything that isn't gcc or clang.
> 
> I understand that older versions of gcc (prior to 5.1.0, from my
> digging) default to gnu89, and so they would be broken _without_ this.
> So it is a tradeoff one way or the other. But somehow this seems
> backwards to me. We should assume that modern compilers support C99 out
> of the box, and put the burden on older ones to trigger C99 support in
> whatever non-portable way they need.
> 
> I also checked clang, and it looks like it has defaulted to gnu11 since
> clang-7 (I'm not sure about clang-6; its documentation wasn't clear).
> 
> -Peff
>