2012/6/12 Xiaofan Chen <xiaof...@gmail.com>:
> On Mon, Jun 11, 2012 at 9:44 PM, Pete Batard <p...@akeo.ie> wrote:
>> Tarball archive at
>> https://sourceforge.net/projects/libusbx/files/releases/1.0.12/source/
>>
>> If possible please test the archive and report issues or potential
>> enhancements.
>>
>
> As already mentioned before, there are a few warnings
> under NetBSD but as per Pete we are going to wait for
> some BSD people to chime in to help resolve the warnings.
> I think that is fair.
>
> os/openbsd_usb.c: In function '_sync_control_transfer':
> os/openbsd_usb.c:638:2: warning: dereferencing type-punned pointer
> will break strict-aliasing rules
> os/openbsd_usb.c:639:2: warning: dereferencing type-punned pointer
> will break strict-aliasing rules
> os/openbsd_usb.c:640:2: warning: dereferencing type-punned pointer
> will break strict-aliasing rules
I have similar warnings under Mac OS X when using -fstrict-aliasing in CFLAGS.
$ make
make all-recursive
Making all in libusb
CC libusb_1_0_la-core.lo
CC libusb_1_0_la-descriptor.lo
CC libusb_1_0_la-io.lo
CC libusb_1_0_la-sync.lo
CC libusb_1_0_la-darwin_usb.lo
CC libusb_1_0_la-threads_posix.lo
os/darwin_usb.c: In function 'usb_setup_device_iterator':
os/darwin_usb.c:176: warning: type-punning to incomplete type might
break strict-aliasing rules
os/darwin_usb.c:180: warning: type-punning to incomplete type might
break strict-aliasing rules
os/darwin_usb.c: In function 'usb_get_next_device':
os/darwin_usb.c:226: warning: type-punning to incomplete type might
break strict-aliasing rules
os/darwin_usb.c:235: warning: type-punning to incomplete type might
break strict-aliasing rules
os/darwin_usb.c: In function 'darwin_devices_detached':
os/darwin_usb.c:304: warning: type-punning to incomplete type might
break strict-aliasing rules
os/darwin_usb.c: In function 'darwin_kernel_driver_active':
os/darwin_usb.c:1287: warning: type-punning to incomplete type might
break strict-aliasing rules
CCLD libusb-1.0.la
Making all in doc
make[2]: Nothing to be done for `all'.
According to [1] it is a bug in (Apple) GCC.
-fstrict-aliasing
Allows the compiler to assume the strictest aliasing rules
applicable to the language being compiled. For C (and C++), this
activates optimizations based on the type of expressions. In
particular, an object of one type is assumed never to reside at the
same address as an object of a different type, unless the types are
almost the same. For example, an "unsigned int" can alias an
"int", but not a "void*" or a "double". A character type may alias
any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one
most recently written to (called "type-punning") is common. Even
with -fstrict-aliasing, type-punning is allowed, provided the
memory is accessed through the union type. So, the code above will
work as expected. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Every language that wishes to perform language-specific alias
analysis should define a function that computes, given an "tree"
node, an alias set for the node. Nodes in different alias sets are
not allowed to alias. For an example, see the C front-end function
"c_get_alias_set".
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
Bye
[1] http://lists.apple.com/archives/xcode-users/2008/Jul/msg00745.html
--
Dr. Ludovic Rousseau
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