Re: doc isinf isnan function portability

Re: doc isinf isnan function portability

[Paul Eggert]

Kevin Ryde <user42@zip.com.au> writes:

>         * doc/autoconf.texi (Function Portability): Add notes on isinf and
>         isnan as macros versus functions.
>
> This arose in guile, where Andreas Vögele found AC_CHECK_FUNCS for
> isinf and isnan insufficient on HP-UX.  We ended up with an
> AC_LINK_IFELSE that included <math.h>, which is what I've tried to
> alude to in the words below.

Thanks for mentionin the problem.  I installed the following instead:
it sidesteps the portability mess that guile got its feet stuck into,
and it produces more accurate results for "long double".  Hope it
helps with guile...

2004-08-12  Paul Eggert  <eggert@cs.ucla.edu>

	* doc/autoconf.texi (Function Portability): Document isinf and
	and isnan.  From a suggestion by Kevin Ryde.

--- doc/autoconf.texi	3 Aug 2004 22:06:10 -0000	1.826
+++ doc/autoconf.texi	12 Aug 2004 21:51:48 -0000	1.827
@@ -3670,6 +3670,58 @@ tradition of it returning @code{int}.
 The ISO C99 standard says a call @code{free(NULL)} does nothing, but
 some old systems don't support this (eg.@: NextStep).
 
+@item @code{isinf}
+@itemx @code{isnan}
+@c @fuindex isinf
+@c @fuindex isnan
+@prindex @code{isinf}
+@prindex @code{isnan}
+The ISO C99 standard specifies that @code{isinf} and @code{isnan} are
+macros.  On some systems just macros are available (e.g., HP-UX), on
+some systems both macros and functions (e.g., glibc 2.3.2), and on some
+systems only functions (e.g., IRIX 6 and Solaris 9).  In some cases
+these functions are declared in nonstandard headers like
+@code{<sunmath.h>} and defined in non-default libraries like
+@option{-lm} or @option{-lsunmath}.
+
+The C99 @code{isinf} and @code{isnan} macros work correctly with
+@code{long double} arguments, but pre-C99 systems that use functions
+typically assume @code{double} arguments.  On such a system,
+@code{isinf} incorrectly returns true for a finite @code{long double}
+argument that is outside the range of @code{double}.
+
+To work around this porting mess, you can use code like the following.
+
+@example
+#include <math.h>
+
+#ifndef isnan
+# define isnan(x) \
+    (sizeof (x) == sizeof (long double) ? isnan_ld (x) \
+     : sizeof (x) == sizeof (double) ? isnan_d (x) \
+     : isnan_f (x))
+static inline int isnan_f  (float       x) @{ return x != x; @}
+static inline int isnan_d  (double      x) @{ return x != x; @}
+static inline int isnan_ld (long double x) @{ return x != x; @}
+#endif
+
+#ifndef isinf
+# define isinf(x) \
+    (sizeof (x) == sizeof (long double) ? isinf_ld (x) \
+     : sizeof (x) == sizeof (double) ? isinf_d (x) \
+     : isinf_f (x))
+static inline int isinf_f  (float       x) @{ return isnan (x - x); @}
+static inline int isinf_d  (double      x) @{ return isnan (x - x); @}
+static inline int isinf_ld (long double x) @{ return isnan (x - x); @}
+#endif
+@end example
+
+Use @code{AC_C_INLINE} (@pxref{C Compiler}) so that this code works on
+compilers that lack the @code{inline} keyword.  Some optimizing
+compilers mishandle these definitions, but systems with that bug
+typically have missing or broken @code{isnan} functions anyway, so it's
+probably not worth worrying about.
+
 @item @code{malloc}
 @c @fuindex malloc
 @prindex @code{malloc}


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