[ILUG] round and drand48

[David Malone]

> 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"

Ah - that works. It also works with -D_XOPEN_SOURCE on the command
line (-D__USE_XOPEN doesn't 'cos I think it gets redefined by gcc
or glibc internally later).

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

Does that mean "the c99 language, plus any libraries and headers I
have installed"? I guess this also allows GNU extensions to the C
language?

> 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

Is it documented anywhere what functions/macros depend on what
_*_SOURCE, or do you just have to read the header files to find
out? For example, I've just discovered that I also need -D_BSD_SOURCE
in order to get random() and srandom(), but this isn't obvious (to
me) from the glibc man pages.

	David.