From bug-request at octave dot org Thu Dec 16 14:05:12 2004
Subject: signbit on linux/gcc 3.2+
From: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>
To: Orion Poplawski <orion at cora dot nwra dot com>
Cc: bug at octave dot org
Date: Thu, 16 Dec 2004 15:06:23 -0500

On 16-Dec-2004, Orion Poplawski <orion at cora dot nwra dot com> wrote:

| Octave uses the autoconf mechanism AC_CHECK_FUNCS() to look for signbit. 
|   This doesn't work for linux with gcc 3.2+ because signbit is a macro 
| in <math.c>, and is only exposed if _GNU_SOURCE, _ISOC99_SOURCE, 
| _ISOC9X_SOURCE, or __STDC_VERSION__ >= 199901L.  To make things more 
| complicated, in c++ if <cmath> is included, the macro is turned off and 
| it is replaced with the std::signbit() function.

I didn't know there was a std::signbit function.  If there was, then I
don't think we would need all the checks for signbit appearing in
various locations.

| In liboctave/lo-ieee.h, lo_ieee_signbit it declared as follows:
| 
| /* In the following definitions, only check x < 0 explicitly to avoid
|     a function call when it looks like signbit or copysign are actually
|     functions.  */
| 
| #if defined (signbit)
| # define lo_ieee_signbit(x) signbit (x)
| #elif defined (HAVE_SIGNBIT)
| # if defined (__MINGW32__)
| extern int signbit (double);
| # endif
| # define lo_ieee_signbit(x) (x < 0 || signbit (x))
| #elif defined (copysign)
| # define lo_ieee_signbit(x) (copysign (1.0, x) < 0)
| #elif defined (HAVE_COPYSIGN)
| # define lo_ieee_signbit(x) (x < 0 || copysign (1.0, x) < 0)
| #else
| # define lo_ieee_signbit(x) 0
| #endif
| 
| So, you end up with lo_ieee_signbit(x) defined as 0.  Which seems like a 
| pretty bad thing, and it is not advertised as such (no error is generated).
| 
| My proposed fix is as follows:
| 
| - Change configure.in to check for the signbit *declaration*:
| 
| --- configure.in.orig   2004-12-15 17:26:11.082647280 -0700
| +++ configure.in        2004-12-15 17:26:55.482848586 -0700
|  at @ -1152,7 +1152,8 @@
|       AC_DEFINE(HAVE_ISNAN, 1, [Define if you have isnan().])
|     ;;
|     *)
| -    AC_CHECK_FUNCS(finite isnan isinf copysign signbit)
| +    AC_CHECK_FUNCS(finite isnan isinf copysign)
| +    AC_CHECK_DECLS(signbit,,,[#include <math.h>])
|     ;;
|   esac
| --- liboctave/lo-ieee.h.orig    2003-09-05 10:55:42.000000000 -0600
| +++ liboctave/lo-ieee.h 2004-12-16 11:29:10.697873801 -0700
|  at @ -23,8 +23,14 @@
|   #if !defined (octave_liboctave_ieee_h)
|   #define octave_liboctave_ieee_h 1
| 
| +#include <math.h>
| +
|   #ifdef __cplusplus
| +/* Use standard c++ signbit */
| +#include <cmath>
| +using std::signbit;
| +

I would prefer to avoid including <math.h> in C++ code.

|  at @ -82,7 +88,7 @@
|   #elif defined (HAVE_COPYSIGN)
|   #define lo_ieee_signbit(x) (x < 0 || copysign (1.0, x) < 0)
|   #else
| -#define lo_ieee_signbit(x) 0
| +#error "No signbit definition?!"

It is probably bad to completely fail to compile if the system doesn't
have a signbit function/macro.

| Of course, all this probably breaks other platforms.

Yes, for example:

  sun-45:9> cat foo.cc
  #include <math.h>
  #include <cmath>
  using std::signbit;
  #include <iostream>
  int
  main (void)
  {
    std::cerr << signbit (-1.0) << std::endl;

    return 0;
  }
  sun-45:10> CC -V
  CC: Sun C++ 5.6 2004/07/15
  sun-45:11> CC foo.cc
  "foo.cc", line 3: Error: signbit is not a member of std.
  "foo.cc", line 8: Error: The function "signbit" must have a prototype.
  2 Error(s) detected.

so I don't think I can accept this patch as it is.

jwe



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