strtold: work around Cygwin bug

[Bruno Haible]

On Cygwin, I'm seeing a test failure:

FAIL: test-strtold

The strtold function of Cygwin does not set errno=ERANGE upon underflow, and
the gnulib substitute, so far, was working around other issues but still
calling strtold for the case of decimal numbers, thus inheriting this bug.

The fix is to avoid calling strtold altogether on this platform.


2019-12-12  Bruno Haible  <address@hidden>

        strtold: Work around Cygwin bug.
        * m4/strtold.m4 (gl_FUNC_STRTOLD): Add test for the underflow problem.
        If it is present, define STRTOLD_HAS_UNDERFLOW_BUG.
        * lib/strtod.c (HAVE_UNDERLYING_STRTOD): Set to 0 if
        STRTOLD_HAS_UNDERFLOW_BUG is defined.
        * doc/posix-functions/strtold.texi: Mention the Cygwin bug.

diff --git a/doc/posix-functions/strtold.texi b/doc/posix-functions/strtold.texi
index d8ed174..f4116c6 100644
--- a/doc/posix-functions/strtold.texi
+++ b/doc/posix-functions/strtold.texi
@@ -62,6 +62,11 @@ IRIX 6.5, mingw.
 In hexadecimal floats, this function allows whitespace between @samp{p}
 and the exponent on some platforms:
 HP-UX 11.31/ia64.
+
+@item
+This function fails to set @code{errno} upon underflow on some platforms:
+@c https://cygwin.com/ml/cygwin/2019-12/msg00072.html
+Cygwin 2.9.
 @end itemize
 
 Portability problems not fixed by Gnulib:
diff --git a/lib/strtod.c b/lib/strtod.c
index 69b1564..30abbae 100644
--- a/lib/strtod.c
+++ b/lib/strtod.c
@@ -45,6 +45,9 @@
    /* We cannot call strtold on HP-UX/hppa, because its return type is a 
struct,
       not a 'long double'.  */
 #  define HAVE_UNDERLYING_STRTOD 0
+# elif STRTOLD_HAS_UNDERFLOW_BUG
+   /* strtold would not set errno=ERANGE upon underflow.  */
+#  define HAVE_UNDERLYING_STRTOD 0
 # else
 #  define HAVE_UNDERLYING_STRTOD HAVE_STRTOLD
 # endif
diff --git a/m4/strtold.m4 b/m4/strtold.m4
index 15fea14..0c7cc3b 100644
--- a/m4/strtold.m4
+++ b/m4/strtold.m4
@@ -1,4 +1,4 @@
-# strtold.m4 serial 6
+# strtold.m4 serial 7
 dnl Copyright (C) 2002-2003, 2006-2019 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -13,7 +13,7 @@ AC_DEFUN([gl_FUNC_STRTOLD],
   if test $ac_cv_func_strtold != yes; then
     HAVE_STRTOLD=0
   else
-    AC_CACHE_CHECK([whether strtold obeys C99], [gl_cv_func_strtold_works],
+    AC_CACHE_CHECK([whether strtold obeys POSIX], [gl_cv_func_strtold_works],
       [AC_RUN_IFELSE([AC_LANG_PROGRAM([[
 #include <stdlib.h>
 #include <math.h>
@@ -35,7 +35,7 @@ numeric_equal (long double x, long double y)
     char *term;
     strtold (string, &term);
     if (term != string && *(term - 1) == 0)
-      result |= 2;
+      result |= 1;
   }
   {
     /* Older glibc and Cygwin mis-parse "-0x".  */
@@ -44,7 +44,7 @@ numeric_equal (long double x, long double y)
     long double value = strtold (string, &term);
     long double zero = 0.0L;
     if (1.0L / value != -1.0L / zero || term != (string + 2))
-      result |= 4;
+      result |= 2;
   }
   {
     /* IRIX 6.5, mingw do not parse hex floats.  */
@@ -52,7 +52,7 @@ numeric_equal (long double x, long double y)
     char *term;
     long double value = strtold (string, &term);
     if (value != 20.0L || term != (string + 6))
-      result |= 8;
+      result |= 4;
   }
   {
     /* IRIX 6.5 does not parse infinities.  HP-UX 11.31/ia64 parses inf,
@@ -63,7 +63,7 @@ numeric_equal (long double x, long double y)
     errno = 0;
     value = strtold (string, &term);
     if (value != HUGE_VAL || term != (string + 3) || errno)
-      result |= 16;
+      result |= 8;
   }
   {
     /* glibc-2.3.2, IRIX 6.5, mingw, Haiku misparse "nan()".  */
@@ -71,7 +71,7 @@ numeric_equal (long double x, long double y)
     char *term;
     long double value = strtold (string, &term);
     if (numeric_equal (value, value) || term != (string + 5))
-      result |= 32;
+      result |= 16;
   }
   {
     /* Mac OS X 10.5, IRIX 6.5 misparse "nan(".  */
@@ -79,12 +79,27 @@ numeric_equal (long double x, long double y)
     char *term;
     long double value = strtold (string, &term);
     if (numeric_equal (value, value) || term != (string + 3))
+      result |= 32;
+  }
+  {
+    /* In Cygwin 2.9, strtold does not set errno upon underflow.  */
+    const char *string = "1E-100000";
+    char *term;
+    long double value;
+    errno = 0;
+    value = strtold (string, &term);
+    if (term != (string + 9) || (value == 0.0L && errno != ERANGE))
       result |= 64;
   }
   return result;
 ]])],
         [gl_cv_func_strtold_works=yes],
-        [gl_cv_func_strtold_works=no],
+        [if expr $? '>=' 64 >/dev/null; then
+           gl_cv_func_strtold_works="no (underflow problem)"
+         else
+           gl_cv_func_strtold_works=no
+         fi
+        ],
         [dnl The last known bugs in glibc strtold(), as of this writing,
          dnl were fixed in version 2.8
          AC_EGREP_CPP([Lucky user],
@@ -101,6 +116,8 @@ numeric_equal (long double x, long double y)
            [case "$host_os" in
                        # Guess yes on musl systems.
               *-musl*) gl_cv_func_strtold_works="guessing yes" ;;
+                       # Guess 'no (underflow problem)' on Cygwin.
+              cygwin*) gl_cv_func_strtold_works="guessing no (underflow 
problem)" ;;
               *)       gl_cv_func_strtold_works="$gl_cross_guess_normal" ;;
             esac
            ])
@@ -110,6 +127,12 @@ numeric_equal (long double x, long double y)
       *yes) ;;
       *)
         REPLACE_STRTOLD=1
+        case "$gl_cv_func_strtold_works" in
+          *"no (underflow problem)")
+            AC_DEFINE([STRTOLD_HAS_UNDERFLOW_BUG], [1],
+              [Define to 1 if strtold does not set errno upon underflow.])
+            ;;
+        esac
         ;;
     esac
   fi