[ILUG] round and drand48

[Pádraig Brady]

David Malone wrote:
> I want to use both the c99 function round() and the drand48() function
> in a single C program. To get the round() function on a recent
> version of Ubuntu I need to give gcc the "--std=c99" flag, otherwise
> I get the warnings:
> 
> 	warning: incompatible implicit declaration of built-in function â

[snip]

> Slightly less ugly is that I can mess with glibc's
> internal macros before including stdlib.h:
> 
> 	#define __USE_XOPEN
> 	#include <stdlib.h>
> 
> This works, but I can't say I'm that happy with it. Does anyone
> know what the blessed way of doing this on recent Linux/glibc/gcc
> boxes is?

You should not use the internal flags as you noted.
Instead #define _XOPEN_SOURCE in addition to the -std=c99 flag
For details on these macros see: info libc "feature test macros"

An alternative is just to use the -std=gnu99 flag

Note they following can be handy to determine directly
which features are set by the various options:

$ echo "#include <features.h>" | cpp -dN | grep "#define __USE_"
#define __USE_ANSI
#define __USE_POSIX
#define __USE_POSIX2
#define __USE_POSIX199309
#define __USE_POSIX199506
#define __USE_XOPEN2K
#define __USE_MISC
#define __USE_BSD
#define __USE_SVID
#define __USE_FORTIFY_LEVEL

$ echo "#include <features.h>" | cpp -dN -std=c99 | grep "#define __USE_"
#define __USE_ANSI
#define __USE_ISOC99
#define __USE_FORTIFY_LEVEL