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. 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. double() is a C99 feature, so without any feature defines glibc's math.h does not declare it. Thus to avoid the warning and still used the optimized gcc builtin, you just need to define _GNU_SOURCE (or _ISOC99_SOURCE). Brian
