Creating Shared Libraries

Using Symbols from Shared Libraries

Symbols - functions, variables or classes - contained in shared libraries intended to be used by clients, such as applications or other libraries, must be marked in a special way. These symbols are called public symbols that are exported or made publicly visible.

The remaining symbols should not be visible from the outside. On most platforms, compilers will hide them by default. On some platforms, a special compiler option is required to hide these symbols.

When compiling a shared library, it must be marked for export. To use the shared library from a client, some platforms may require a special import declaration as well.

Depending on your target platform, Qt provides special macros that contain the necessary definitions:

• Q_DECL_EXPORT must be added to the declarations of symbols used when compiling a shared library.
• Q_DECL_IMPORT must be added to the declarations of symbols used when compiling a client that uses the shared library.

Now, we need to ensure that the right macro is invoked -- whether we compile a shared library itself, or just the client using the shared library. Typically, this can be solved by adding a special header.

Let us assume we want to create a shared library called mysharedlib. A special header for this library, mysharedlib_global.h, looks like this:

 #include <QtCore/QtGlobal>

 #if defined(MYSHAREDLIB_LIBRARY)
 #  define MYSHAREDLIB_EXPORT Q_DECL_EXPORT
 #else
 #  define MYSHAREDLIB_EXPORT Q_DECL_IMPORT
 #endif

In the .pro file of the shared library, we add:

 DEFINES += MYSHAREDLIB_LIBRARY

In each header of the library, we specify the following:

 #include "mysharedlib_global.h"

 MYSHAREDLIB_EXPORT void foo();
 class MYSHAREDLIB_EXPORT MyClass...

This ensures that the right macro is seen by both library and clients. We also use this technique in Qt's sources.

 #include <footronics/device.h>

 class MyDevice {
 private:
     FOOTRONICS_DEVICE_HANDLE handle;
 };

 #include "ui_widget.h"

 class MyWidget : public QWidget {
 private:
     Ui::MyWidget m_ui;
 };