Detect platforms where memset(0) doesn't set doubles to 0.0.

This is allowed by the C statndard, which permits you to represent
doubles any way you like, but in practice we have some code that
assumes that memset() clears doubles in structs.  Noticed as part of
7802 review; see 8081 for more info.

changes/double-0-check

+  o Build improvements (bizarre platform detection):
+    - Try to detect it if we are ever building on a platform where
+      memset(...,0,...) does not set the value of a double to 0.0.  Such
+      platforms are permitted by the C standard, though in practice
+      they're pretty rare (since IEEE 754 is nigh-ubiquitous). We don't
+      currently support them, but it's better to detect them and fail
+      than to perform erroneously.
+

configure.ac

@@ -1033,6 +1033,30 @@ if test "$tor_cv_null_is_zero" != no; then
             [Define to 1 iff memset(0) sets pointers to NULL])
 fi
 
+AC_CACHE_CHECK([whether memset(0) sets doubles to 0.0], tor_cv_dbl0_is_zero,
+[AC_RUN_IFELSE([AC_LANG_SOURCE(
+[[#include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
+#ifdef HAVE_STDDEF_H
+#include <stddef.h>
+#endif
+int main () { double d1,d2; d1=0; memset(&d2,0,sizeof(d2));
+return memcmp(&d1,&d2,sizeof(d1))?1:0; }]])],
+       [tor_cv_dbl0_is_zero=yes],
+       [tor_cv_dbl0_is_zero=no],
+       [tor_cv_dbl0_is_zero=cross])])
+
+if test "$tor_cv_dbl0_is_zero" = cross ; then
+  # Cross-compiling; let's hope that the target isn't raving mad.
+  AC_MSG_NOTICE([Cross-compiling: we'll assume that 0.0 can be represented as a sequence of 0-valued bytes.])
+fi
+
+if test "$tor_cv_dbl0_is_zero" != no; then
+  AC_DEFINE([DOUBLE_0_REP_IS_ZERO_BYTES], 1,
+            [Define to 1 iff memset(0) sets doubles to 0.0])
+fi
+
 # And what happens when we malloc zero?
 AC_CACHE_CHECK([whether we can malloc(0) safely.], tor_cv_malloc_zero_works,
 [AC_RUN_IFELSE([AC_LANG_SOURCE(

src/common/compat.h

@@ -74,6 +74,11 @@
 #error "It seems your platform does not represent NULL as zero. We can't cope."
 #endif
 
+#ifndef DOUBLE_0_REP_IS_ZERO_BYTES
+#error "It seems your platform does not represent 0.0 as zeros. We can't cope."
+#endif
+
+
 #if 'a'!=97 || 'z'!=122 || 'A'!=65 || ' '!=32
 #error "It seems that you encode characters in something other than ASCII."
 #endif

src/win32/orconfig.h

@@ -151,6 +151,9 @@
 /* Define to 1 iff NULL is represented by a 0 in memory. */
 #define NULL_REP_IS_ZERO_BYTES 1
 
+/* Define to 1 iff memset(0) sets doubles to 0.0 */
+#define DOUBLE_0_REP_IS_ZERO_BYTES 1
+
 /* Name of package */
 #define PACKAGE "tor"