QLibrary Class Reference

The QLibrary class provides a wrapper for handling shared libraries. More...

All the functions in this class are reentrant when Qt is built with thread support.

#include <qlibrary.h>

List of all member functions.

Public Members

QLibrary ( const QString & filename )
virtual ~QLibrary ()
void * resolve ( const char * symb )
bool load ()
virtual bool unload ()
bool isLoaded () const
bool autoUnload () const
void setAutoUnload ( bool enabled )
QString library () const

Static Public Members

void * resolve ( const QString & filename, const char * symb )

Detailed Description

The QLibrary class provides a wrapper for handling shared libraries.

An instance of a QLibrary object can handle a single shared library and provide access to the functionality in the library in a platform independent way. If the library is a component server, QLibrary provides access to the exported component and can directly query this component for interfaces.

QLibrary ensures that the shared library is loaded and stays in memory whilst it is in use. QLibrary can also unload the library on destruction and release unused resources.

A typical use of QLibrary is to resolve an exported symbol in a shared object, and to call the 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 went 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. When the QLibrary object is destroyed the library will be unloaded, making all references to memory allocated in the library invalid.

    typedef void (*MyPrototype)();
    MyPrototype myFunction;

    QLibrary myLib( "mylib" );
    myFunction = (MyPrototype) myLib.resolve( "mysymbol" );
    if ( myFunction ) {
        myFunction();
    }

Member Function Documentation

QLibrary::QLibrary ( const QString & filename )

Creates a QLibrary object for the shared library filename. The library will be unloaded in the destructor.

Note that filename does not need to include the (platform specific) file extension, so calling

    QLibrary lib( "mylib" );

is equivalent to calling

    QLibrary lib( "mylib.dll" );

on Windows, and

    QLibrary lib( "libmylib.so" );

on Unix. Specifying the extension is not recommended, since doing so introduces a platform dependency.

If filename does not include a path, the library loader will look for the file in the platform specific search paths.

See also load(), unload(), and setAutoUnload().

QLibrary::~QLibrary () `[virtual]`

Deletes the QLibrary object.

The library will be unloaded if autoUnload() is TRUE (the default), otherwise it stays in memory until the application exits.

bool QLibrary::autoUnload () const

Returns TRUE if the library will be automatically unloaded when this wrapper object is destructed; otherwise returns FALSE. The default is TRUE.

bool QLibrary::isLoaded () const

Returns TRUE if the library is loaded; otherwise returns FALSE.

QString QLibrary::library () const

Returns the filename of the shared library this QLibrary object handles, including the platform specific file extension.

For example:

    QLibrary lib( "mylib" );
    QString str = lib.library();

will set str to "mylib.dll" on Windows, and "libmylib.so" on Linux.

bool QLibrary::load ()

Loads the library. 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.

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 char * symb )

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

    typedef int (*avgProc)( int, int );

    avgProc avg = (avgProc) 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 requires the extern "C" notation if the library is compiled with a C++ compiler. On Windows you also have to explicitly export the function from the DLL using the __declspec(dllexport) compiler directive.

    extern "C" MY_EXPORT_MACRO 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.

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

void * QLibrary::resolve ( const QString & filename, const char * symb ) `[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 symb. Note that like the constructor, filename does not need to include the (platform specific) file extension. The library remains loaded until the process exits.

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

This function is useful only if you want to resolve a single symbol, e.g. a function pointer from a specific library once:

    typedef void (*FunctionType)();
    static FunctionType *ptrFunction = 0;
    static bool triedResolve = FALSE;
    if ( !ptrFunction && !triedResolve )
        ptrFunction = QLibrary::resolve( "mylib", "mysymb" );

    if ( ptrFunction )
        ptrFunction();
    else
        ...

If you want to resolve multiple symbols, use a QLibrary object and call the non-static version of resolve().

void QLibrary::setAutoUnload ( bool enabled )

If enabled is TRUE (the default), the wrapper object is set to automatically unload the library upon destruction. If enabled is FALSE, the wrapper object is not unloaded unless you explicitly call unload().

bool QLibrary::unload () `[virtual]`

Unloads the library and returns TRUE if the library could be unloaded; otherwise returns FALSE.

This function is called by the destructor if autoUnload() is enabled.