This is the mail archive of the cygwin-developers mailing list for the Cygwin project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: src/winsup/cygwin ChangeLog Makefile.in autolo ...


On 22/04/2011 3:49 PM, Yaakov (Cygwin/X) wrote:
CVSROOT:	/cvs/src
Module name:	src
Changes by:	corinna@...	2011-04-19 10:02:07

	* registry.cc (reg_key): Rewrite reg_key class to use native NT registry
	functions.  Use WCHAR string parameters throughout.  Use PCWSTR rather
	than const WCHAR.  Drop multibyte char functionality.  Drop unused
	methods.
This change introduced a compiler warning with GCC 4.5:

cc1plus: warnings being treated as errors
registry.cc: In member function âint reg_key::get_int(const WCHAR*,
int)â:
registry.cc:145:32: error: dereferencing type-punned pointer will break
strict-aliasing rules
For the benefit of folks reading the archives later:

*(DWORD*) is asking for trouble, and technically, even the cast from alloca's void* breaks C++ strict aliasing rules.

The common suggestion is to compile with -fno-strict-aliasing, but gcc allows to turn off strict aliasing on a case-by-case basis by converting casts into members of a union:

- PKEY_VALUE_PARTIAL_INFORMATION vbuf = (PKEY_VALUE_PARTIAL_INFORMATION)
- alloca (size);
+ union { void* v; PKEY_VALUE_PARTIAL_INFORMATION vbuf; DWORD *dst; } u =
+ {alloca (size)};
...
- status = NtQueryValueKey (key, &uname, KeyValuePartialInformation, vbuf,
+ status = NtQueryValueKey (key, &uname, KeyValuePartialInformation, u.vbuf,
...
- DWORD dst = *(DWORD *) vbuf->Data;
- return (int) dst;
+ return (int) *u.dst;


In my experience, the union also allows the compiler to generate better code in some cases.

Ryan



Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]