Hamish wrote:
> when I try to compile a fresh check out of 6.4.3svn on an older
> system (glibc 2.3.6) I get this error from v.in.ogr:
>
> main.c: In function 'main':
> main.c:1208: error: implicit declaration of function 'log2'
> main.c:1208: warning: incompatible implicit declaration of built-in function
> 'log2'
>
>
> I thought it was because #include <math.h> was missing (for pow()
> and log10() too), but after adding that I still get the error.
>
> It turns out I need to add -std=c99 to the gcc line to get it
> to work, but the glibc man page for log2() has it conforming to
> SVr4, 4.3BSD, C89.
Huh? On my system, the log2 manual page says:
CONFORMING TO
C99, POSIX.1-2001. The variant returning double also conforms
to SVr4, 4.3BSD.
> maybe I could replace it with a log()/log() trick, but would
> pow() and log10() also be broken?
pow() and log10() are C89. As is exp2(), FWIW.
But if you're planning on replacing this:
new_snap = log2(xmax) - 52;
new_snap = pow(2, new_snap);
I'd suggest using:
int exp;
new_snap = frexp(xmax, &exp);
exp -= 52;
new_snap = ldexp(new_snap, exp);
frexp() and ldexp() are C89, don't introduce rounding errors, and
handle zero correctly (they're also likely to be more efficient, but
that's a trivial detail).
--
Glynn Clements <[email protected]>
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