dispatch: split DISPATCH_EXPORT and prepare for static linking
#872
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This extracts a new `DISPATCH_EXTERN` macro from the existing definition of `DISPATCH_EXPORT`. This follows the standard style of many of Darwin's frameworks. Extracting and re-using this macro allows us to make the definition of `DISPATCH_EXPORT` more concise.
When building with CMake, `dispatch_EXPORT` is automatically defined when building a shared library. We should additionally rely on `dispatch_STATIC` to indicate that we are building a static library. When building statically, we should ensure that the ABI of dispatch does not participate in dynamic linking as otherwise any consuming library would vend the dispatch ABI. Particularly for Linux, this would be problematic as the symbol resolution is ill-defined and can end up with multiple incompatible definitions.
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etcwilde
reviewed
May 14, 2025
Co-authored-by: Evan Wilde <[email protected]>
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This change accomplishes two items:
DISPATCH_EXTERNmacro to decorate withexternorextern "C"based on the compilation mode (C vs C++).dispatch_STATIC) that allows building with libdispatch for static linking. The default remains dynamic linking.The motivation for
DISPATCH_EXTERNis primarily being able to concisely defineDISPATCH_EXPORT. This follows the naming scheme that most other Apple frameworks use (e.g. UIKit, Foundation, etc).The change to introduce support for static linking is motivated by Windows. With the gradual roll out of static linking of the runtime, it would be convenient to be able to statically link dispatch into a fully sealed Swift program. Doing so requires building libdispatch without the
__declspec(dllexport)and__declspec(dllimport)attributes on Windows. Similarly, it would be unfortunate to have dispatch's ABI be subsumed by a client library and have it participate in dynamic linking.The user is responsible for defining
dispatch_STATICwhen using a static copy of libdispatch. This allows the default to remain dynamic linking (which is the preferred style on Darwin and Windows).