QLibrary Class Reference
^{[QtCore module]}

Detailed Description

The QLibrary class loads shared libraries at runtime.

An instance of a QLibrary object operates on a single shared object file (which we call a "library", but is also known as a "DLL"). A QLibrary provides access to the functionality in the library in a platform independent way. You can either pass a file name in the constructor, or set it explicitly with setFileName(). When loading the library, QLibrary searches in all the system-specific library locations (e.g. LD_LIBRARY_PATH on Unix), unless the file name has an absolute path. If the file cannot be found, QLibrary tries the name with different platform-specific file suffixes, like ".so" on Unix, ".dylib" on the Mac, or ".dll" on Windows. This makes it possible to specify shared libraries that are only identified by their basename (i.e. without their suffix), so the same code will work on different operating systems.

The most important functions are load() to dynamically load the library file, isLoaded() to check whether loading was successful, and resolve() to resolve a symbol in the library. The resolve() function implicitly tries to load the library if it has not been loaded yet. Multiple instances of QLibrary can be used to access the same physical library. Once loaded, libraries remain in memory until the application terminates. You can attempt to unload a library using unload(), but if other instances of QLibrary are using the same library, the call will fail, and unloading will only happen when every instance has called unload().

A typical use of QLibrary is to resolve an exported symbol in a library, and to call the C function that this symbol represents. This is called "explicit linking" in contrast to "implicit linking", which is done by the link step in the build process when linking an executable against a library.

The following code snippet loads a library, resolves the symbol "mysymbol", and calls the function if everything succeeded. If something goes wrong, e.g. the library file does not exist or the symbol is not defined, the function pointer will be 0 and won't be called.

    QLibrary myLib("mylib");
    typedef void (*MyPrototype)();
    MyPrototype myFunction = (MyPrototype) myLib.resolve("mysymbol");
    if (myFunction)
        myFunction();

The symbol must be exported as a C function from the library for resolve() to work. This means that the function must be wrapped in an extern "C" block if the library is compiled with a C++ compiler. On Windows, this also requires the use of a dllexport macro; see resolve() for the details of how this is done. For convenience, there is a static resolve() function which you can use if you just want to call a function in a library without explicitly loading the library first:

    typedef void (*MyPrototype)();
    MyPrototype myFunction =
            (MyPrototype) QLibrary::resolve("mylib", "mysymbol");
    if (myFunction)
        myFunction();

See also QPluginLoader.

Property Documentation

fileName : QString

This property holds the file name of the library.

We recommend omitting the file's suffix in the file name, since QLibrary will automatically look for the file with the appropriate suffix (see isLibrary()).

When loading the library, QLibrary searches in all system-specific library locations (e.g. LD_LIBRARY_PATH on Unix), unless the file name has an absolute path. After loading the library successfully, fileName() returns the fully qualified file name of the library. For example, after successfully loading the "GL" library on unix, fileName() will return "libGL.so".

Access functions:

• QString fileName () const
• void setFileName ( const QString & fileName )

Member Function Documentation

QLibrary::QLibrary ( QObject * parent = 0 )

Constructs a library with the given parent.

QLibrary::QLibrary ( const QString & fileName, QObject * parent = 0 )

Constructs a library object with the given parent that will load the library specified by fileName.

We recommend omitting the file's suffix in fileName, since QLibrary will automatically look for the file with the appropriate suffix in accordance with the platform, e.g. ".so" on Unix, ".dylib" on Mac OS X, and ".dll" on Windows. (See fileName.)

QLibrary::~QLibrary ()

Destroys the QLibrary object.

Unless unload() was called explicitly, the library stays in memory until the application terminates.

See also isLoaded() and unload().

bool QLibrary::isLibrary ( const QString & fileName )   [static]

Returns true if fileName has a valid suffix for a loadable library; otherwise returns false.

Trailing versioning numbers on Unix are ignored.

bool QLibrary::isLoaded () const

Returns true if the library is loaded; otherwise returns false.

See also load().

bool QLibrary::load ()

Loads the library and returns true if the library was loaded successfully; otherwise returns false. Since resolve() always calls this function before resolving any symbols it is not necessary to call it explicitly. In some situations you might want the library loaded in advance, in which case you would use this function.

On Darwin and Mac OS X this function uses code from dlcompat, part of the OpenDarwin project.

    Copyright (c) 2002 Jorge Acereda and Peter O'Gorman.

    Permission is hereby granted, free of charge, to any person obtaining
    a copy of this software and associated documentation files (the
    "Software"), to deal in the Software without restriction, including
    without limitation the rights to use, copy, modify, merge, publish,
    distribute, sublicense, and/or sell copies of the Software, and to
    permit persons to whom the Software is furnished to do so, subject to
    the following conditions:

    The above copyright notice and this permission notice shall be
    included in all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
    EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
    MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
    NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
    LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
    OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
    WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

See also unload().

void * QLibrary::resolve ( const char * symbol )

Returns the address of the exported symbol symbol. The library is loaded if necessary. The function returns 0 if the symbol could not be resolved or if the library could not be loaded.

Example:

    typedef int (*AvgFunction)(int, int);

    AvgFunction avg = (AvgFunction) library->resolve("avg");
    if (avg)
        return avg(5, 8);
    else
        return -1;

The symbol must be exported as a C function from the library. This means that the function must be wrapped in an extern "C" if the library is compiled with a C++ compiler. On Windows you must also explicitly export the function from the DLL using the __declspec(dllexport) compiler directive, for example:

    extern "C" MY_EXPORT int avg(int a, int b)
    {
        return (a + b) / 2;
    }

with MY_EXPORT defined as

    #ifdef Q_WS_WIN
    #define MY_EXPORT __declspec(dllexport)
    #else
    #define MY_EXPORT
    #endif

On Darwin and Mac OS X this function uses code from dlcompat, part of the OpenDarwin project.

    Copyright (c) 2002 Jorge Acereda and Peter O'Gorman.

    Permission is hereby granted, free of charge, to any person obtaining
    a copy of this software and associated documentation files (the
    "Software"), to deal in the Software without restriction, including
    without limitation the rights to use, copy, modify, merge, publish,
    distribute, sublicense, and/or sell copies of the Software, and to
    permit persons to whom the Software is furnished to do so, subject to
    the following conditions:

    The above copyright notice and this permission notice shall be
    included in all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
    EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
    MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
    NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
    LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
    OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
    WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

void * QLibrary::resolve ( const QString & fileName, const char * symbol )   [static]

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Loads the library fileName and returns the address of the exported symbol symbol. Note that fileName should not include the platform-specific file suffix; (see fileName). The library remains loaded until the application exits.

The function returns 0 if the symbol could not be resolved or if the library could not be loaded.

See also resolve().

bool QLibrary::unload ()

Unloads the library and returns true if the library could be unloaded; otherwise returns false.

This happens automatically on application termination, so you shouldn't normally need to call this function.

If other instances of QLibrary are using the same library, the call will fail, and unloading will only happen when every instance has called unload().

Note that on Mac OS X, dynamic libraries cannot be unloaded.

See also resolve() and load().