--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,16 @@
+2010-03-07  Bruno Haible  <bruno@clisp.org>
+
+	logb: Provide missing declaration for Cygwin.
+	* lib/math.in.h (logb): New declaration.
+	* m4/logb.m4: New file.
+	* modules/logb (Files): Add m4/logb.m4.
+	(Depends-on): Add math.
+	(configure.ac): Invoke gl_FUNC_LOGB, gl_MATH_MODULE_INDICATOR.
+	* m4/math_h.m4 (gl_MATH_H): Check also for logb declaration.
+	(gl_MATH_H_DEFAULTS): Initialize GNULIB_LOGB, HAVE_DECL_LOGB.
+	* modules/math (Makefile.am): Substitute GNULIB_LOGB, HAVE_DECL_LOGB.
+	* doc/posix-functions/logb.texi: Mention the Cygwin bug.
+
 2010-03-07  Bruno Haible  <bruno@clisp.org>
 
 	Fix test-cond link error.

--- a/doc/posix-functions/logb.texi
+++ b/doc/posix-functions/logb.texi
@@ -12,4 +12,7 @@
 
 Portability problems not fixed by Gnulib:
 @itemize
+@item
+This function is missing a declaration on some platforms:
+Cygwin 1.5.x.
 @end itemize

--- a/lib/math.in.h
+++ b/lib/math.in.h
@@ -97,6 +97,7 @@
 # define HUGE_VAL (1.0 / 0.0)
 #endif
 
+
 /* Write x as
      x = mantissa * 2^exp
    where
@@ -117,6 +118,19 @@
 #endif
 
 
+#if @GNULIB_LOGB@
+# if !@HAVE_DECL_LOGB@
+extern double logb (double x);
+# endif
+#elif defined GNULIB_POSIXCHECK
+# undef logb
+# if HAVE_RAW_DECL_LOGB
+_GL_WARN_ON_USE (logb, "logb is unportable - "
+                 "use gnulib module logb for portability");
+# endif
+#endif
+
+
 #if @GNULIB_ACOSL@
 # if !@HAVE_ACOSL@ || !@HAVE_DECL_ACOSL@
 extern long double acosl (long double x);

new file mode 100644
--- /dev/null
+++ b/m4/logb.m4
@@ -0,0 +1,44 @@
+# logb.m4 serial 1
+dnl Copyright (C) 2010 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_LOGB],
+[
+  AC_REQUIRE([gl_MATH_H_DEFAULTS])
+  dnl Test whether logb() is declared.
+  AC_CHECK_DECLS([logb], , , [#include <math.h>])
+  if test "$ac_cv_have_decl_logb" != yes; then
+    HAVE_DECL_LOGB=0
+  fi
+  dnl Test whether logb() can be used without libm.
+  LOGB_LIBM=?
+  AC_TRY_LINK([
+     #ifndef __NO_MATH_INLINES
+     # define __NO_MATH_INLINES 1 /* for glibc */
+     #endif
+     #include <math.h>
+     extern double logb (double x);
+     double x;],
+    [x = logb(x);],
+    [LOGB_LIBM=])
+  if test "$LOGB_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>
+       extern double logb (double x);
+       double x;],
+      [x = logb(x);],
+      [LOGB_LIBM="-lm"])
+    LIBS="$save_LIBS"
+  fi
+  if test "$LOGB_LIBM" = "?"; then
+    LOGB_LIBM=
+  fi
+  AC_SUBST([LOGB_LIBM])
+])

--- a/m4/math_h.m4
+++ b/m4/math_h.m4
@@ -1,4 +1,4 @@
-# math_h.m4 serial 17
+# math_h.m4 serial 18
 dnl Copyright (C) 2007-2010 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -40,7 +40,7 @@
   dnl corresponding gnulib module is not in use.
   gl_WARN_ON_USE_PREPARE([[#include <math.h>
     ]], [acosl asinl atanl ceilf ceill cosl expl floorf floorl frexpl
-    ldexpl logl round roundf roundl sinl sqrtl tanl trunc truncf truncl])
+    ldexpl logb logl round roundf roundl sinl sqrtl tanl trunc truncf truncl])
 ])
 
 AC_DEFUN([gl_MATH_MODULE_INDICATOR],
@@ -70,6 +70,7 @@
   GNULIB_ISNAND=0;   AC_SUBST([GNULIB_ISNAND])
   GNULIB_ISNANL=0;   AC_SUBST([GNULIB_ISNANL])
   GNULIB_LDEXPL=0;   AC_SUBST([GNULIB_LDEXPL])
+  GNULIB_LOGB=0;     AC_SUBST([GNULIB_LOGB])
   GNULIB_LOGL=0;     AC_SUBST([GNULIB_LOGL])
   GNULIB_ROUND=0;    AC_SUBST([GNULIB_ROUND])
   GNULIB_ROUNDF=0;   AC_SUBST([GNULIB_ROUNDF])
@@ -101,6 +102,7 @@
   HAVE_DECL_EXPL=1;            AC_SUBST([HAVE_DECL_EXPL])
   HAVE_DECL_FREXPL=1;          AC_SUBST([HAVE_DECL_FREXPL])
   HAVE_DECL_LDEXPL=1;          AC_SUBST([HAVE_DECL_LDEXPL])
+  HAVE_DECL_LOGB=1;            AC_SUBST([HAVE_DECL_LOGB])
   HAVE_DECL_LOGL=1;            AC_SUBST([HAVE_DECL_LOGL])
   HAVE_DECL_SINL=1;            AC_SUBST([HAVE_DECL_SINL])
   HAVE_DECL_SQRTL=1;           AC_SUBST([HAVE_DECL_SQRTL])

--- a/modules/logb
+++ b/modules/logb
@@ -2,12 +2,15 @@
 logb() function: get exponent.
 
 Files:
+m4/logb.m4
 m4/mathfunc.m4
 
 Depends-on:
+math
 
 configure.ac:
-gl_MATHFUNC([logb], [double], [(double)])
+gl_FUNC_LOGB
+gl_MATH_MODULE_INDICATOR([logb])
 
 Makefile.am:
 

--- a/modules/math
+++ b/modules/math
@@ -42,6 +42,7 @@
 	      -e 's|@''GNULIB_ISNAND''@|$(GNULIB_ISNAND)|g' \
 	      -e 's|@''GNULIB_ISNANL''@|$(GNULIB_ISNANL)|g' \
 	      -e 's|@''GNULIB_LDEXPL''@|$(GNULIB_LDEXPL)|g' \
+	      -e 's|@''GNULIB_LOGB''@|$(GNULIB_LOGB)|g' \
 	      -e 's|@''GNULIB_LOGL''@|$(GNULIB_LOGL)|g' \
 	      -e 's|@''GNULIB_ROUND''@|$(GNULIB_ROUND)|g' \
 	      -e 's|@''GNULIB_ROUNDF''@|$(GNULIB_ROUNDF)|g' \
@@ -72,6 +73,7 @@
 	      -e 's|@''HAVE_DECL_EXPL''@|$(HAVE_DECL_EXPL)|g' \
 	      -e 's|@''HAVE_DECL_FREXPL''@|$(HAVE_DECL_FREXPL)|g' \
 	      -e 's|@''HAVE_DECL_LDEXPL''@|$(HAVE_DECL_LDEXPL)|g' \
+	      -e 's|@''HAVE_DECL_LOGB''@|$(HAVE_DECL_LOGB)|g' \
 	      -e 's|@''HAVE_DECL_LOGL''@|$(HAVE_DECL_LOGL)|g' \
 	      -e 's|@''HAVE_DECL_SINL''@|$(HAVE_DECL_SINL)|g' \
 	      -e 's|@''HAVE_DECL_SQRTL''@|$(HAVE_DECL_SQRTL)|g' \