[CMake] Proper way to export a library

Thu Oct 31 16:25:28 EDT 2013

On Thu, Oct 31, 2013 at 1:21 PM, Hendrik Sattler
<post at hendrik-sattler.de> wrote:
>
>
> Giordano Khouri <kgiordano at nikon.net> schrieb:
>>I am a firm believer that static libraries should have hidden
>>visibility. This is based on my experience on the Mac and may be
>>different in Linux.
>
> This is about Windows, not Linux or Mac. Linking libraries works a bit different there.

While that's true, I will concur that hidden is a good thing; there
have been several times I have accidentally defined a function with
the same name as some library included by a library by a library by a
library that ended up with a stack overflow or other bad behavior;
everyone should know how to target every platform :)  Microsoft
believed in hidden by default for shared libraries.

>
>>A shared library (dylib, framework) can export a symbol that came in
>>from a static library as a private extern. If the symbol is extern
>>(visibility=default) in the static library, the shared library cannot
>>hide it unless the shared library is linked using an explicit list of
>>the symbols to export. A list of exports maybe isn't so bad for a small
>>to intermediate C library, but for any kind of C++ library, it's a huge
>>pain. The proper use of visibility avoids all that pain.
>>
>>The static library symbol visibility makes no difference when the
>>static library is linked into an executable.
>>
>>> -----Original Message-----
>>> From: cmake-bounces at cmake.org [mailto:cmake-bounces at cmake.org] On
>>> Behalf Of J Decker
>>> Sent: Thursday, October 31, 2013 3:27 PM
>>> To: Hendrik Sattler
>>> Cc: Matthew Woehlke; cmake at cmake.org
>>> Subject: Re: [CMake] Proper way to export a library
>>>
>>> something like.... (but probably mostly in a common header)
>>>
>>> if( IS_THIS_PROJECT )
>>>   if( UNIX or STATIC )
>>>       set( EXPORT  )  # nothing, everything is exported by default;
>>gcc/unix
>>>   else( UNIX or STATIC )
>>>       set( EXPORT __declspec(dllexport) )
>>>   endif( UNIX or STATIC )
>>> else( IS_THIS_PROJECT )
>>>   # if something else includes this....
>>>   if( UNIX or STATIC )
>>>       set( EXPORT extern )
>>>   else( UNIX or STATIC )
>>>       set( EXPORT __declspec(dllimport) )
>>>   endif( UNIX or STATIC )
>>> endif( IS_THIS_PROJECT )
>>>
>>> On Thu, Oct 31, 2013 at 11:46 AM, Hendrik Sattler
>><post at hendrik-sattler.de>
>>> wrote:
>>> >
>>> >
>>> > Matthew Woehlke <matthew.woehlke at kitware.com> schrieb:
>>> >>On 2013-10-31 05:26, Cyrille Faucheux wrote:
>>> >>> Can you tell me a bit more about "implicit compile flags [...]
>>for
>>> >>imported
>>> >>> tagets"?
>>> >>
>>> >>See documentation on target_compile_definitions.
>>> >>
>>> >>> On the library I'm currently working on, with Visual Studio, I
>>have
>>> >>to
>>> >>> declare some compile definition in order to get the proper
>>> >>> "__declspec(dllimport)" or "__declspec(dllexport)" defined (or
>>not
>>> >>defined
>>> >>> when compiling/using a static version of this library).
>>> >>
>>> >>This sounds like you're doing it wrong.
>>> >
>>> > Not really as you can only cover the export import with this case
>>but not
>>> static vs. dynamic linking to that library with the same header file.
>>> > That it's usually solved with a define when linking statically
>>> >
>>> >>When building with CMake, the preferred way is to unconditionally
>>> >>define an ABI export symbol via a header which exists for that
>>purpose
>>> >>(e.g.
>>> >>config.h, my_package_exports.h, etc.). The value of the definition
>>> >>however is conditional on a symbol that is only defined when
>>building
>>> >>the library, to choose between import and export as appropriate.
>>> >>
>>> >>Even better, CMake will define <target>_EXPORTS for you when
>>building
>>> >>a
>>> >>
>>> >>library, so you shouldn't need to manually specify any definitions
>>at
>>> >>all.
>>> >>
>>> >>IOW you libraries headers would somewhere contain:
>>> >>
>>> >>#if defined(_WIN32)
>>> >>#  define MYPROJECT_ABI_EXPORT __declspec(dllexport) #  define
>>> >>MYPROJECT_ABI_IMPORT __declspec(dllimport) #elif __GNUC__ >= 4 #
>>> >>define MYPROJECT_ABI_EXPORT __attribute__ ((visibility("default")))
>>#
>>> >>define MYPROJECT_ABI_IMPORT __attribute__ ((visibility("default")))
>>> >>#else #  define MYPROJECT_ABI_EXPORT #  define
>>> MYPROJECT_ABI_IMPORT
>>> >>#endif
>>> >>
>>> >>...and:
>>> >>
>>> >>#ifdef mylibrary_EXPORTS
>>> >>#  define MYLIBRARY_EXPORT MYPROJECT_ABI_EXPORT #else #  define
>>> >>MYLIBRARY_EXPORT MYPROJECT_ABI_IMPORT #endif
>>> >>
>>> >>
>>> >>That said...
>>> >>
>>> >>> Does "implicit compile flags" means that those compile definition
>>> >>could be
>>> >>> automatically declared by the imported target?
>>> >>
>>> >>...this was my understanding of how
>>target_compile_definitions(PUBLIC)
>>> >>is supposed to work. (Note: I haven't actually used this feature
>>myself
>>> >>
>>> >>yet.)
>>> >
>>> >
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>>
>>
>>Khouri  Giordano
>>Software Technology Researcher
>>
>>Nikon Inc.
>>1300 Walt Whitman Road
>>Melville NY 11747-3064
>>
>>Office: 631-547-4335    Fax: 631-547-0361
>>
>>kgiordano at nikon.net     www.nikonusa.com
>>
>>
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