dealing with built-in functions

[Andrew Haley]

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.