flo@xxxxxxxxx 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.
