From: Andrew Haley <aph-gcc@littlepinkcloud.COM>
To: flo@redflo.de
Cc: gcc-help@gcc.gnu.org
Subject: Re: dealing with built-in functions
Date: Wed, 13 Jun 2007 12:27:00 -0000 [thread overview]
Message-ID: <18031.49175.192836.370113@zebedee.pink> (raw)
In-Reply-To: <466FBBA5.9060702@redflo.de>
flo@redflo.de writes:
> Brian Dessent schrieb:
> > Florian Gleixner wrote:
> >
> >> gcc r.c -lm
> >> r.c: In function 'main':
> >> r.c:9: warning: incompatible implicit declaration of built-in function
> >> 'round'
> >
> > I'm assuming that your C library is glibc, i.e. you're using Linux. It
> > is always a good idea to state what platform you are using, because gcc
> > supports many dozens of platforms so don't assume we know what you're
> > using.
> >
>
> Good guess. I will be more verbose next time.
>
> > The problem you are seeing is that glibc has a number of feature levels
> > that it supports. By default, it only exposes a fraction of available
> > features (C90) in its headers. You have to explicitly define the level
> > of feature support that you want:
> > <http://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html>.
> > Defining _GNU_SOURCE gets you everything and is the most commonly used,
> > e.g. by adding -D_GNU_SOURCE to CFLAGS. I think that if you use
> > autoconf, it takes care of this for you if it detects you're on a glibc
> > system, but I could be wrong.
>
> Autoconf did not help me here automatically. But maybe i have to change
> something in configure.ac?
> So what is the "right way"?
It depends on what you want. If you want to use Standard C (1999
version), use std=c99. If you want GNU extensions, use GNU_SOURCE.
Andrew.
next prev parent reply other threads:[~2007-06-13 10:00 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-06-12 22:41 Florian Gleixner
2007-06-13 9:36 ` Brian Dessent
2007-06-13 10:00 ` flo
2007-06-13 12:22 ` Brian Dessent
2007-06-13 12:27 ` Andrew Haley [this message]
2007-06-13 9:57 ` Andrew Haley
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