Brad King wrote:
> On 5/5/2012 4:27 PM, Stephen Kelly wrote:
>>> Ok. Then factoring out how it is set is the first step. Looking at the
>>> output of 'git grep -i " \?pic\b"' and 'git grep -wi +Z', there are many
>>> different ways of telling the compiler that we want this property. Most
>>> of them are ancient unix which I can't test. Should I change those at
>>> all?
>>
>> This issue is the subject of a TODO comment in my wip branch on this
>> topic.
>
> They all set the CMAKE_SHARED_LIBRARY_${lang}_FLAGS one way or
> another. AFAIK CMake platform files use that variable exclusively
> for -fPIC or equivalent flag.
It appears so to me too, yes.
> It is possible that projects set
> the value to add their own flags though. I also know there are
> projects that *read* the variable to get the flag so it can be
> used in a static library (working around the lack of the feature
> you're adding).
>
> I think the most compatible way to do this is:
>
> (1) Define a new platform variable named specifically to hold this
> flag, like CMAKE_${lang}_PIC_FLAG or something.
I've done something similar already.
> (2) Document the property as using that flag *and* suppressing
> use of CMAKE_SHARED_LIBRARY_${lang}_FLAGS altogether.
Hmm, I don't think I understand why this is necessary.
> (3) Leave the property undefined even for shared libs and let
> the project set the property to get new behavior.
That's not compatible with people thinking they'll get PIC shared libraries
by default already though. I don't see why changing that is necessary.
>> https://gitorious.org/~steveire/cmake/steveires-cmake/commits/position-
independent-targets
>
> Good start. I'd like to amend my suggested property name to be
> "POSITION_INDEPENDENT_CODE".
Done.
> How should this interact with OBJECT library support? If I compile
> a set of objects meant for use in an executable shouldn't they use
> -fPIE instead of -fPIC? CMake cannot know where $<TARGET_OBJECTS:...>
> will be used when it compiles an object library.
My understanding is that it is ok to link objects compiled with -fPIC (ie,
not -fPIE) into an executable, but you don't get all the position
independence benefit by using that instead of -fPIE. So it is probably ok to
use -fPIC for OBJECT_LIBRARY compilations.
Thanks,
Steve.
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