Re: Detecting gated functions w/ AC_CHECK_FUNCS()

[Bob Friesenhahn]

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="$CFLAGS"
> CFLAGS="CFLAGS -D_POSIX_C_SOURCE=200809L"
> AC_CHECK_FUNCS([open_memstream])
> CFLAGS="$save_CFLAGS"

Normally one should use CPPFLAGS rather than CFLAGS for options 
pertaining to the C pre-processor.

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
--
Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
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