Re: How to get autoconf to respect CC="gcc -std=c89"?

Evgeny Grin <[email protected]> Wed, 11 Oct 2023 17:47:58 +0300

Newsgroups	gmane.comp.sysutils.autoconf.general
Message-ID	<[email protected]>

On 11.10.2023 17:10, Zack Weinberg wrote:
> On Wed, Oct 11, 2023, at 9:34 AM, Evgeny Grin wrote:
>> It works, actually. ./configure CC=gcc CFLAGS=-std=c89
>>
>> C99 check would fail, compiler would be accepted as C89-compatible.
> 
> This only works if both configure.ac and all the Makefiles have been
> written "correctly".  I put "correctly" in quotes because, yes, it *is*
> how you should write them, and it *is* what you get if you aren't trying
> to do anything complicated -- but if you *do* need to do something
> complicated, it is very easy to break this property by accident and
> without noticing.  And I don't think everything you need to do to avoid
> breaking it is documented, so we can't even claim that we warned people.

It can be broken if:
* CFLAGS are filtered by 'configure' / Makefiles
* CFLAGS value is not passed to Makefiles / not unsed in Makefiles
* Some '-std=xxx' flag is added *after* user CFLAGS in configure / Makefiles

The first case is an intentional break of autoconf design.
The second option is very badly designed configuration, not supported at 
all.
The last one is incorrect as well and intentional prevention of 
user-specified C standard.

In other cases it will work.
Configure will not add -std= overriding flags and CFLAGS are used in 
Makefiles.

However, I agree that current autoconf + automake design is confusing.
At first user's CFLAGS are passed to 'configure'.
The configure *must* pass the same CFLAGS to automake, so if 'make' is 
executed with some CFLAGS=... override, such override must replace only 
user-supplied (for configure) CFLAGS and not automatically added 
compiler flags. At the same time, user CFLAGS should be added *after* 
automatic/configure-detected flags to be able to override them.

Knowing that each and every configure check (for C compiler) uses 
CFLAGS, it is very tempting just to modify CFLAGS inside the configure.

I'm not sure whether there are any suggested way of 
using/passing/modifying flags, which is documented for configure. In my 
projects I use special variable to hold user-supplied CFLAGS, in other 
variable I accumulate configure-detected additional flags. Before every 
check I combine flags like CFLAGS="conf_CFLAGS user_CFLAGS". Finally the 
value of "user_CFLAGS" is passed to automake files as "CFLAGS", value of 
"conf_CFLAGS" is used as part of AM_CFLAGS.
Not sure whether it is the best way and why it is not fully documented, 
but this is another story.

-- 
Evgeny

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