[LLVMdev] Fwd: [windows) how to use weak references with llvm 3.4 and windows?

[Reid Kleckner]

COFF doesn't support the same kind of concept of 'weak' that ELF does.
 This is the issue that users brought up with mingw:
https://sourceware.org/bugzilla/show_bug.cgi?id=9687

Instead, COFF supports weak externals, which you can use like:

$ cat t.c
int foo() __attribute__((weak));
int main() {
  if (foo)
    return foo();
  return 13;
}

$ cat t2.c
int foo() {
  return 42;
}

$ clang t.c t2.c -o t && ./t ; echo $?
42

So, we got the definition of foo from t2.c.  If t2.c hadn't been linked,
foo would be null.


On Mon, Feb 10, 2014 at 6:28 AM, Carl <name.is.carl at gmail.com> wrote:

>
> Hello,
>
> I'm generating C code (and the resulting obj files) using llvm 3.4 for
> both unix and windows.
> And I use the dreaded weak references, that, for windows, are not too
> widely supported.
>
> When I link my application on linux, I have no issue.
> But when I'm doing the same on windows using mingw I got a duplicate
> symbol error :
>
> ..\robovm-0.0.8\lib\robovm-rt.jar\dalvik\system\BlockGuard$BlockGuardPolicyException.class.o:(.text$dalvik_system_BlockGuard$24BlockGuardPolicyException_getPolicy__I_lookup[_dalvik_system_BlockGuard$24BlockGuardPolicyException_getPolicy__I_lookup]+0x0):
> multiple definition of
> `dalvik_system_BlockGuard$24BlockGuardPolicyException_getPolicy__I_lookup'
> ..\Temp\robovm3774596063679264132.tmp\linker.o:(.text+0x340): first
> defined here
> ..\robovm-0.0.8\lib\robovm-rt.jar\dalvik\system\BlockGuard$BlockGuardPolicyException.class.o:(.text$dalvik_system_BlockGuard$24BlockGuardPolicyException_getPolicyViolation__I_lookup[_dalvik_system_BlockGuard$24BlockGuardPolicyException_getPolicyViolation__I_lookup]+0x0):
> multiple definition of
> `dalvik_system_BlockGuard$24BlockGuardPolicyException_getPolicyViolation__I_lookup'
> ..\Temp\robovm3774596063679264132.tmp\linker.o:(.text+0x350): first
> defined here
> ..\robovm-0.0.8\lib\robovm-rt.jar\dalvik\system\BlockGuard$BlockGuardPolicyException.class.o:(.text$dalvik_system_BlockGuard$24BlockGuardPolicyException_getMessage__Ljava_lang_String$3B_lookup[_dalvik_system_BlockGuard$24BlockGuardPolicyException_getMessage__Ljava_lang_String$3B_lookup]+0x0):
> multiple definition of
> `dalvik_system_BlockGuard$24BlockGuardPolicyException_getMessage__Ljava_lang_String$3B_lookup'
>
> Because those symbols are declared as weak.
>
> What is the status about the support of weak symbols on windows?
> Are they supposed to work? Are they supposed to *never *work ?
>
> Do you know any way to fix this (or work around it). I use weak symbols
> everywhere in my code generation, so I would prefer a fix that doesn't
> involve to rewrite my code.
>
> I'm considering rewriting the obj files using objcopy.
> Has this any chance to work ?
> Here are the o.
>
> Cheers,
> Carl.
>
>
> _______________________________________________
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>
>
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