Re: Portable replacement function for `trunc'?

[Bruno Haible]

Hello Benoit,

> I was looking for a portable replacement of `trunc' for systems where  
> it is not be available.  I saw that gnulib had that (http:// 
> www.gnu.org/software/gnulib/manual/html_node/trunc.html)

Well, there is a misunderstanding here. That doc says that
1) gnulib has no module supporting this function,
2) all known portability problems are in the "not fixed by gnulib" category.

> invoked path/to/gnulib-tool --import trunc.  It did what it was meant  
> to do but there was still no trace of `trunc' in my source tree 

Now wonder...

But since trunc() is a particularly easy function to implement, I implemented
this now. Also updating the doc.

2007-10-03  Bruno Haible  <address@hidden>

        * modules/trunc: New file.
        * lib/trunc.c: New file.
        * m4/trunc.m4: New file.
        * lib/math.in.h (trunc): New declaration.
        * m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize GNULIB_TRUNC and
        HAVE_DECL_TRUNC.
        * modules/math (Makefile.am): Substitute also GNULIB_TRUNC and
        HAVE_DECL_TRUNC.
        * doc/functions/trunc.texi: Mention the 'trunc' module.

*** modules/trunc.orig  2003-09-23 19:59:22.000000000 +0200
--- modules/trunc       2007-10-04 02:22:04.000000000 +0200
***************
*** 0 ****
--- 1,29 ----
+ Description:
+ trunc() function: round towards zero.
+ 
+ Files:
+ lib/trunc.c
+ m4/trunc.m4
+ 
+ Depends-on:
+ math
+ float
+ 
+ configure.ac:
+ gl_FUNC_TRUNC
+ gl_MATH_MODULE_INDICATOR([trunc])
+ 
+ Makefile.am:
+ 
+ Include:
+ <math.h>
+ 
+ Link:
+ $(TRUNC_LIBM)
+ 
+ License:
+ LGPL
+ 
+ Maintainer:
+ Bruno Haible
+ 
*** lib/trunc.c.orig    2003-09-23 19:59:22.000000000 +0200
--- lib/trunc.c 2007-10-04 02:19:51.000000000 +0200
***************
*** 0 ****
--- 1,68 ----
+ /* Round towards zero.
+    Copyright (C) 2007 Free Software Foundation, Inc.
+ 
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2, or (at your option)
+    any later version.
+ 
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+ 
+    You should have received a copy of the GNU General Public License along
+    with this program; if not, write to the Free Software Foundation,
+    Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+ 
+ /* Written by Bruno Haible <address@hidden>, 2007.  */
+ 
+ #include <config.h>
+ 
+ /* Specification.  */
+ #include <math.h>
+ 
+ #include <float.h>
+ 
+ /* 2^(DBL_MANT_DIG-1).  */
+ static const double TWO_MANT_DIG =
+   /* Assume DBL_MANT_DIG <= 4 * 31.
+      Use the identity
+        n = floor(n/4) + floor((n+1)/4) + floor((n+2)/4) + floor((n+3)/4).  */
+   (double) (1U << ((DBL_MANT_DIG - 1) / 4))
+   * (double) (1U << ((DBL_MANT_DIG - 1 + 1) / 4))
+   * (double) (1U << ((DBL_MANT_DIG - 1 + 2) / 4))
+   * (double) (1U << ((DBL_MANT_DIG - 1 + 3) / 4));
+ 
+ double
+ trunc (double x)
+ {
+   /* The use of 'volatile' guarantees that excess precision bits are dropped
+      at each addition step and before the following comparison at the caller's
+      site.  It is necessary on x86 systems where double-floats are not IEEE
+      compliant by default, to avoid that the results become platform and 
compiler
+      option dependent.  'volatile' is a portable alternative to gcc's
+      -ffloat-store option.  */
+   volatile double y = x;
+   volatile double z = y;
+ 
+   if (z > 0)
+     {
+       /* Round to the next integer (nearest or up or down, doesn't matter).  
*/
+       z += TWO_MANT_DIG;
+       z -= TWO_MANT_DIG;
+       /* Enforce rounding down.  */
+       if (z > y)
+       z -= 1.0;
+     }
+   else if (z < 0)
+     {
+       /* Round to the next integer (nearest or up or down, doesn't matter).  
*/
+       z -= TWO_MANT_DIG;
+       z += TWO_MANT_DIG;
+       /* Enforce rounding up.  */
+       if (z < y)
+       z += 1.0;
+     }
+   return z;
+ }
*** m4/trunc.m4.orig    2003-09-23 19:59:22.000000000 +0200
--- m4/trunc.m4 2007-10-04 02:41:25.000000000 +0200
***************
*** 0 ****
--- 1,46 ----
+ # trunc.m4 serial 1
+ dnl Copyright (C) 2007 Free Software Foundation, Inc.
+ dnl This file is free software; the Free Software Foundation
+ dnl gives unlimited permission to copy and/or distribute it,
+ dnl with or without modifications, as long as this notice is preserved.
+ 
+ AC_DEFUN([gl_FUNC_TRUNC],
+ [
+   AC_REQUIRE([gl_MATH_H_DEFAULTS])
+   dnl Test whether trunc() is declared.
+   AC_CHECK_DECLS([trunc], , , [#include <math.h>])
+   if test "$ac_cv_have_decl_trunc" = yes; then
+     dnl Test whether trunc() can be used without libm.
+     TRUNC_LIBM=?
+     AC_TRY_LINK([
+        #ifndef __NO_MATH_INLINES
+        # define __NO_MATH_INLINES 1 /* for glibc */
+        #endif
+        #include <math.h>
+        double x;],
+       [x = trunc(x);],
+       [TRUNC_LIBM=])
+     if test "$TRUNC_LIBM" = "?"; then
+       save_LIBS="$LIBS"
+       LIBS="$LIBS -lm"
+       AC_TRY_LINK([
+          #ifndef __NO_MATH_INLINES
+          # define __NO_MATH_INLINES 1 /* for glibc */
+          #endif
+          #include <math.h>
+          double x;],
+         [x = trunc(x);],
+         [TRUNC_LIBM="-lm"])
+       LIBS="$save_LIBS"
+     fi
+     if test "$TRUNC_LIBM" = "?"; then
+       TRUNC_LIBM=
+     fi
+   else
+     HAVE_DECL_TRUNC=0
+     AC_LIBOBJ([trunc])
+     TRUNC_LIBM=
+   fi
+   AC_SUBST([HAVE_DECL_TRUNC])
+   AC_SUBST([TRUNC_LIBM])
+ ])
*** lib/math.in.h.orig  2007-10-04 03:07:40.000000000 +0200
--- lib/math.in.h       2007-10-04 02:00:57.000000000 +0200
***************
*** 211,216 ****
--- 211,229 ----
       tanl (x))
  #endif
  
+ #if @GNULIB_TRUNC@
+ # if address@hidden@
+ #  define trunc rpl_trunc
+ extern double trunc (double x);
+ # endif
+ #elif defined GNULIB_POSIXCHECK
+ # undef trunc
+ # define trunc(x) \
+     (GL_LINK_WARNING ("trunc is unportable - " \
+                       "use gnulib module trunc for portability"), \
+      trunc (x))
+ #endif
+ 
  
  #if @GNULIB_SIGNBIT@
  # if @REPLACE_SIGNBIT@
*** m4/math_h.m4.orig   2007-10-04 03:07:40.000000000 +0200
--- m4/math_h.m4        2007-10-04 01:58:05.000000000 +0200
***************
*** 1,4 ****
! # math_h.m4 serial 5
  dnl Copyright (C) 2007 Free Software Foundation, Inc.
  dnl This file is free software; the Free Software Foundation
  dnl gives unlimited permission to copy and/or distribute it,
--- 1,4 ----
! # math_h.m4 serial 6
  dnl Copyright (C) 2007 Free Software Foundation, Inc.
  dnl This file is free software; the Free Software Foundation
  dnl gives unlimited permission to copy and/or distribute it,
***************
*** 24,29 ****
--- 24,30 ----
    GNULIB_LDEXPL=0;  AC_SUBST([GNULIB_LDEXPL])
    GNULIB_MATHL=0;   AC_SUBST([GNULIB_MATHL])
    GNULIB_SIGNBIT=0; AC_SUBST([GNULIB_SIGNBIT])
+   GNULIB_TRUNC=0;   AC_SUBST([GNULIB_TRUNC])
    dnl Assume proper GNU behavior unless another module says otherwise.
    HAVE_DECL_ACOSL=1;  AC_SUBST([HAVE_DECL_ACOSL])
    HAVE_DECL_ASINL=1;  AC_SUBST([HAVE_DECL_ASINL])
***************
*** 38,43 ****
--- 39,45 ----
    HAVE_DECL_SINL=1;   AC_SUBST([HAVE_DECL_SINL])
    HAVE_DECL_SQRTL=1;  AC_SUBST([HAVE_DECL_SQRTL])
    HAVE_DECL_TANL=1;   AC_SUBST([HAVE_DECL_TANL])
+   HAVE_DECL_TRUNC=1;  AC_SUBST([HAVE_DECL_TRUNC])
    REPLACE_FREXP=0;    AC_SUBST([REPLACE_FREXP])
    REPLACE_FREXPL=0;   AC_SUBST([REPLACE_FREXPL])
    REPLACE_LDEXPL=0;   AC_SUBST([REPLACE_LDEXPL])
*** modules/math.orig   2007-10-04 03:07:40.000000000 +0200
--- modules/math        2007-10-04 02:02:34.000000000 +0200
***************
*** 27,32 ****
--- 27,33 ----
              -e 's|@''GNULIB_LDEXPL''@|$(GNULIB_LDEXPL)|g' \
              -e 's|@''GNULIB_MATHL''@|$(GNULIB_MATHL)|g' \
              -e 's|@''GNULIB_SIGNBIT''@|$(GNULIB_SIGNBIT)|g' \
+             -e 's|@''GNULIB_TRUNC''@|$(GNULIB_TRUNC)|g' \
              -e 's|@''HAVE_DECL_ACOSL''@|$(HAVE_DECL_ACOSL)|g' \
              -e 's|@''HAVE_DECL_ASINL''@|$(HAVE_DECL_ASINL)|g' \
              -e 's|@''HAVE_DECL_ATANL''@|$(HAVE_DECL_ATANL)|g' \
***************
*** 40,45 ****
--- 41,47 ----
              -e 's|@''HAVE_DECL_SINL''@|$(HAVE_DECL_SINL)|g' \
              -e 's|@''HAVE_DECL_SQRTL''@|$(HAVE_DECL_SQRTL)|g' \
              -e 's|@''HAVE_DECL_TANL''@|$(HAVE_DECL_TANL)|g' \
+             -e 's|@''HAVE_DECL_TRUNC''@|$(HAVE_DECL_TRUNC)|g' \
              -e 's|@''REPLACE_FREXP''@|$(REPLACE_FREXP)|g' \
              -e 's|@''REPLACE_FREXPL''@|$(REPLACE_FREXPL)|g' \
              -e 's|@''REPLACE_LDEXPL''@|$(REPLACE_LDEXPL)|g' \
*** doc/functions/trunc.texi.orig       2007-10-04 03:07:40.000000000 +0200
--- doc/functions/trunc.texi    2007-10-04 02:42:37.000000000 +0200
***************
*** 4,18 ****
  
  POSIX specification: @url{http://www.opengroup.org/susv3xsh/trunc.html}
  
! Gnulib module: ---
  
  Portability problems fixed by Gnulib:
  @itemize
  @end itemize
  
  Portability problems not fixed by Gnulib:
  @itemize
- @item
- This function is missing on some platforms:
- FreeBSD 5.2.1, NetBSD 3.0, OpenBSD 3.8, Solaris 9, Interix 3.5.
  @end itemize
--- 4,18 ----
  
  POSIX specification: @url{http://www.opengroup.org/susv3xsh/trunc.html}
  
! Gnulib module: trunc
  
  Portability problems fixed by Gnulib:
  @itemize
+ @item
+ This function is missing on some platforms:
+ FreeBSD 5.2.1, NetBSD 3.0, OpenBSD 3.8, Solaris 9, Interix 3.5.
  @end itemize
  
  Portability problems not fixed by Gnulib:
  @itemize
  @end itemize