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sharedlibapi

This C header allows its users to flexibly create shared library attributes for headers. The library works with any mainstream C/C++ compiler that implements the standard preprocessor for any platform.

Sometimes multiple libraries must be built, which depend on one another. The library interfaces are defined in dedicated header files, where the symbols are marked as exported/imported. These headers are used by both the library's code and the library's users. When compiling these headers within the context of the library, the annotations imply the exporting of resources, but when the headers are compiled within the context of the user, the annotations imply importing. A macro is commonly introduced to control whether we're exporting or importing. For example:

#if defined(COMPILING_SHARED_LIB)
#   define EXPORTFUNC __attribute__ ((visibility ("default")))
#else
#   define EXPORTFUNC
#endif

This problem with this approach is that each library requires the creation of many macros in order to create a single library that exports its own interface, and imports interfaces from other libraries. This can become cumbersome to manage correctly. Some build systems, like CMake offer automation features to support this use case, but then a rigid external tool dependency is introduced. This library provides a standard C library that solves this problem.

Usage example

In this example, we will implement the IcecreamFarm and IcecreamTruck libraries. Create the following files in the following directory structure:

IcecreamExample
├───IcecreamFarm
│   ├───include
│   │       icecreamfarm.h
│   └───src
│           icecreamfarm.c
│
├───IcecreamTruck
│   ├───include
│   │       icecreamtruck.h
│   └───src
│           icecreamtruck.c
│
└───sharedlibapi
    │   LICENSE
    │   README.md
    └───include
            isempty.h
            sharedlibapi.h

Where icecreamfarm.h specifies the function GetIcecream:

#include "../../sharedlibapi/include/sharedlibapi.h"

LIBAPI(ICECREAMFARM) void GetIcecream();

And icecreamtruck.h specifies the function LoadIcecream:

#include "../../sharedlibapi/include/sharedlibapi.h"

LIBAPI(ICECREAMTRUCK) void LoadIcecream();

The LoadIcecream function's implementation uses the GetIcecream function from IcecreamFarm.

When building icecreamfarm.c, add the -DSHARED_LIB_ID_ICECREAMFARM= compiler command-line argument. When building icecreamtruck.c, add the -DSHARED_LIB_ID_ICECREAMTRUCK= compiler command-line argument, and of course import the IcecreamFarm library. (On the Microsoft Visual C and Clang.cl compilers, replace the - with a /.) This results in two libraries:

  • libicecreamfarm.so (or similar), which exports the GetIcecream function.
  • libicecreamtruck.so that uses the GetIcecream function, and exports the LoadIcecream function.

Additional notes

When using a GNU-compatible compiler, don't forget to add the -fvisibility=hidden command-line option to hide all non-exported symbols.