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.

1 files changed, 24 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index f047ab902..6c4a0792f 100644
--- a/configure.ac
+++ b/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(