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Re: Detecting gated functions w/ AC_CHECK_FUNCS()

From: Philip Prindeville
Subject: Re: Detecting gated functions w/ AC_CHECK_FUNCS()
Date: Tue, 3 May 2022 13:55:35 -0600

> On May 2, 2022, at 2:02 PM, Bob Friesenhahn <> 
> wrote:
> On Mon, 2 May 2022, Philip Prindeville wrote:
>> Hi,
>> I was wondering how to do discovery of functions like open_memstream() which 
>> is only exposed by <features.h> when compilation used -D_GNU_SOURCE or 
>> -D_POSIX_C_SOURCE=200809L.
>> It might be the case that I compile everything with a certain flag, or I 
>> might just compile one-off modules such as:
>> time.o: _ASTCFLAGS+=-D_XOPEN_SOURCE=700
>> What's best practices for handling situations like this?
>> How do I bracket a particular AC_CHECK_FUNCS() invocation with defines that 
>> might be critical?
>> I tried using:
>> save_CFLAGS="$CFLAGS"
>> AC_CHECK_FUNCS([open_memstream])
>> CFLAGS="$save_CFLAGS"
> Normally one should use CPPFLAGS rather than CFLAGS for options pertaining to 
> the C pre-processor.

Noted... and the comment about the missing '$' in the other message...

Turns out that I don't need this...  AC_USE_SYSTEM_EXTENSIONS is causing it to 
be exposed.

> When library function availability is checked, it is normally done using the 
> linker.
> When in doubt, check config.log, which will show the test program and steps 
> which were used while performing the test.
> It is not wise to mix C/OS standard influencing pre-processor options when 
> compiling since they may require a different ABI, which could be provided by 
> alternate libraries.  It might not be possible to support more than one ABI 
> at a time.
> Bob

Hmm... good point.


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