On 21/01/20 21:36 -0300, Alexandre Oliva wrote:
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?
Under what conditions does this fail? Only for -std=gnu++98 and not
later standards, I assume?
Because since C++11 state_type() does perform zero-initialization of
the whole object (including padding) even if it has a default
constructor.
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?),
I don't think so, that wouldn't work. I think pos02 could just be
removed from the test.
