[ros-dev] __attribute__((weak)) - in mingw

[Danny Smith]

Boaz Harrosh wrote:
>> Danny Smith wrote:
>>
>>> Boaz Harrosh wrote:
>>>
>>>
>>>> [Q] I'm (well ATL is) using  __attribute__((weak)) (translated from
>>>> __declspec( selectany) ) for instantiation of members and variables in
>>>> headers. I had no problem with it On GCC in Linux (gcc 3.2.2). On MinGW
>>>>
>>>>
>>>
>>> IMAGE_COMDAT_SELECT_ANY is not quite same as PECOFF version of "weak" , but
>>> AFAICT is equivalent to the GCC section flag ".linkonce discard".  I don't
>>> think there is a way for the user to specify that for data using an
>>> attribute, but it could be done with asm statements.
>>>
>>> .weak directive is partially supported in current binutils CVS.
>>> __attribute__((weak)) is not supported by GCC-3.4.x but will be in the next
>>> major GCC release (4.0.0).
>>>

      Sorry,  that was unclear.  I should have qualified with "on windows
targets"

>>> The semantics of weak for PECOFF differ from that on Linux.See the PECOFF60
>>> specs (Microsoft Portable Executable and Common Object File Format
>>> Specification)  section on weak externals
>>>
>>> Danny
>>>
>>>
>> (binutils at sources dot redhat dot com please also cc me as I'm not on
>> the list)
>>
>> Attached is a proof (See fooInt.h) that gcc (gcc version 3.2 (mingw
>> special 20020817-1)) has support for weak symbols. Just not with the
>> regular syntax.
>> But when templates are used duplicate symbols are merged by the linker.
>>
>> What would be the assembler magic to cram into the __WEAK__ definition
>> that would make this project link?
>>

I am currently testing a patch to GCC to add an __attribute__ ((selectany)) that
would work the way it is discribed in MS docs and  in the spec of
IMAGE_COMDAT_SELECT_ANY  in PECOFF doc.  Basically it puts the symbol into its
own section with ".linkonce discard"  charcteristics.

__declspec (selectany) int foo = 1;

becomes:

.globl _foo
 .section .data$foo,"w"
 .linkonce discard
 .align 4
_foo:
 .long 1



 So far the only problem I've seen is that, although it works for global objects
with non-trivial constructors

eg:


struct X {
  static int s;
  int m;
  X(int _i) : m(_i) {
    m++;
  }
};

__declspec(selectany) int X::s = -1;  // OK
__declspec(selectany) X x(1); // This is OK but app will be bloated with
                              // duplicate initialization code.
.
I haven't figured out how to get rid of the duplicated inititialization code

I will submit the patch for comment  once  GCC trunk branches.


Danny

>> Free Life
>> Boaz
>>
>
>