tolerate padding in mbstate_t

[Alexandre Oliva]

Padding in mbstate_t objects may get the memcmp to fail.
Attempt to avoid the failure with zero initialization.


Regstrapped on x86_64-linux-gnu, and also tested on a platform that used
to fail because of padding in std::mbstate_t.  Ok to install?


for  libstdc++-v3/ChangeLog

	* testsuite/27_io/fpos/mbstate_t/1.cc: Zero-init mbstate_t.
---
 testsuite/27_io/fpos/mbstate_t/1.cc |   28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git libstdc++-v3/testsuite/27_io/fpos/mbstate_t/1.cc libstdc++-v3/testsuite/27_io/fpos/mbstate_t/1.cc
index f92d68f..559bd8d 100644
--- libstdc++-v3/testsuite/27_io/fpos/mbstate_t/1.cc
+++ libstdc++-v3/testsuite/27_io/fpos/mbstate_t/1.cc
@@ -28,8 +28,24 @@
 void test01()
 {
   typedef std::mbstate_t state_type;
-  state_type state01 = state_type();
-  state_type state02 = state_type();
+  // Use zero-initialization of the underlying memory so that padding
+  // bytes, if any, stand a better chance of comparing the same.
+  // Zero-initialized memory is guaranteed to be a valid initial
+  // state.  This doesn't quite guarantee that any padding bits won't
+  // be overwritten when copying from other instances that haven't
+  // been fully initialized: this data type is compatible with C, so
+  // it is likely plain old data, but it could have a default ctor
+  // that initializes only the relevant fields, whereas copy-ctor and
+  // operator= could be implemented as a full-object memcpy, including
+  // padding bits, rather than fieldwise copying.  However, since
+  // we're comparing two values copied from the same state_type
+  // instance (or was this meant to take one of them from pos02 rather
+  // than both from pos01?), if padding bits are copied, we'll get the
+  // same for both of them, and if they aren't, we'll keep the values
+  // we initialized them with, so this should be good.
+  state_type state[2];
+  std::memset(state, 0, sizeof (state));
+
 
   std::streampos pos01(0);
   std::streampos pos02(0);
@@ -39,13 +55,13 @@ void test01()
   // state_type state();
 
   // XXX Need to have better sanity checking for the mbstate_t type,
-  // or whatever the insantiating type for class fpos happens to be
+  // or whatever the instantiating type for class fpos happens to be
   // for streampos, as things like equality operators and assignment
   // operators, increment and deincrement operators need to be in
   // place.
-  pos01.state(state02);
-  state01 = pos01.state();
-  VERIFY( std::memcmp(&state01, &state02, sizeof(state_type)) == 0 );
+  pos01.state(state[1]);
+  state[0] = pos01.state();
+  VERIFY( std::memcmp(&state[0], &state[1], sizeof(state_type)) == 0 );
 }
 
 int main() 

-- 
Alexandre Oliva, freedom fighter   he/him   https://FSFLA.org/blogs/lxo
Free Software Evangelist           Stallman was right, but he's left :(
GNU Toolchain Engineer    FSMatrix: It was he who freed the first of us
FSF & FSFLA board member                The Savior shall return (true);