The QLibrary class loads shared libraries at runtime. More...

Inheritance diagram for QLibrary:

Public Typedefs
using	LoadHints = QFlags< LoadHint >

Public Types
enum	LoadHint

Public Methods
	QLibrary (const QString &fileName, const QString &version, QObject *parent=nullptr)

	QLibrary (const QString &fileName, int verNum, QObject *parent=nullptr)

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

	QLibrary (QObject *parent=nullptr)

	~QLibrary ()

QString	errorString () const

QString	fileName () const

bool	isLoaded () const

bool	load ()

LoadHints	loadHints () const

void *	resolve (const QString &symbol)

void	setFileName (const QString &fileName)

void	setFileNameAndVersion (const QString &fileName, const QString &version)

void	setFileNameAndVersion (const QString &fileName, int versionNumber)

void	setLoadHints (LoadHints hints)

bool	unload ()

Public Methods inherited from QObject
	QObject (QObject *parent=nullptr)

	~QObject ()

bool	blockSignals (bool block)

const QList< QObject * > &	children () const

bool	connect (const QObject *sender, const QString &signalMethod, const QString &location, const QString &slotMethod, Qt::ConnectionType type=Qt::AutoConnection)

bool	connect (const QObject *sender, const QString &signalMethod, const QString &slotMethod, Qt::ConnectionType type=Qt::AutoConnection)

bool	disconnect (const QObject *receiver, const QString &slotMethod=QString ()) const

bool	disconnect (const QString &signalMethod, const QString &location, const QObject *receiver=nullptr, const QString &slotMethod=QString ()) const

bool	disconnect (const QString &signalMethod=QString (), const QObject *receiver=nullptr, const QString &slotMethod=QString ()) const

void	dumpObjectInfo ()

void	dumpObjectTree ()

QList< QString >	dynamicPropertyNames () const

virtual bool	event (QEvent *event)

virtual bool	eventFilter (QObject watched, QEvent event)

template<typename T >
T	findChild (const QString &childName=QString ()) const

template<class T >
QList< T >	findChildren (const QRegularExpression &regExp, Qt::FindChildOptions options=Qt::FindChildrenRecursively) const

template<class T >
QList< T >	findChildren (const QString &childName=QString (), Qt::FindChildOptions options=Qt::FindChildrenRecursively) const

bool	inherits (const QString &className) const

void	installEventFilter (QObject *filterObj)

bool	isWidgetType () const

bool	isWindowType () const

void	killTimer (int id)

const QMetaObject *	metaObject () const

void	moveToThread (QThread *targetThread)

QString	objectName () const

QObject *	parent () const

template<class T = QVariant>
T	property (const QString &name) const

void	removeEventFilter (QObject *obj)

void	setObjectName (const QString &name)

void	setParent (QObject *parent)

bool	setProperty (const QString &name, const QVariant &value)

bool	signalsBlocked () const

int	startTimer (int interval, Qt::TimerType timerType=Qt::CoarseTimer)

QThread *	thread () const

Static Public Methods
static bool	isLibrary (const QString &fileName)

static void *	resolve (const QString &fileName, const QString &symbol)

static void *	resolve (const QString &fileName, const QString &version, const QString &symbol)

static void *	resolve (const QString &fileName, int verNum, const QString &symbol)

Static Public Methods inherited from QObject
static bool	connect (const QObject sender, const QMetaMethod &signalMethod, const QObject receiver, const QMetaMethod &slotMethod, Qt::ConnectionType type=Qt::AutoConnection)

static bool	connect (const QObject sender, const QString &signalMethod, const QObject receiver, const QString &slotMethod, Qt::ConnectionType type=Qt::AutoConnection, const QString &location=QString ())

static bool	connect (const QObject sender, const QString &signalMethod, const QString &location, const QObject receiver, const QString &slotMethod, Qt::ConnectionType type=Qt::AutoConnection)

template<class Sender , class SignalClass , class... SignalArgs, class Receiver , class SlotClass , class... SlotArgs, class SlotReturn >
static bool	connect (const Sender sender, void (SignalClass::signalMethod)(SignalArgs...), const Receiver receiver, SlotReturn (SlotClass::slotMethod)(SlotArgs...), Qt::ConnectionType type=Qt::AutoConnection)

template<class Sender , class SignalClass , class... SignalArgs, class Receiver , class T >
static bool	connect (const Sender sender, void (SignalClass::signalMethod)(SignalArgs...), const Receiver *receiver, T slotLambda, Qt::ConnectionType type=Qt::AutoConnection)

static bool	disconnect (const QObject sender, const QMetaMethod &signalMethod, const QObject receiver, const QMetaMethod &slotMethod)

static bool	disconnect (const QObject sender, const QString &signalMethod, const QObject receiver, const QString &slotMethod)

static bool	disconnect (const QObject sender, const QString &signalMethod, const QString &location, const QObject receiver, const QString &slotMethod)

static bool	disconnect (const QObject sender, std::nullptr_t, const QObject receiver, std::nullptr_t)

template<class Sender , class SignalClass , class... SignalArgs, class Receiver , class SlotClass , class... SlotArgs, class SlotReturn >
static bool	disconnect (const Sender sender, void (SignalClass::signalMethod)(SignalArgs...), const Receiver receiver, SlotReturn (SlotClass::slotMethod)(SlotArgs...))

template<class Sender , class SignalClass , class... SignalArgs, class Receiver >
static bool	disconnect (const Sender sender, void (SignalClass::signalMethod)(SignalArgs...), const Receiver *receiver, std::nullptr_t slotMethod=nullptr)

template<class Sender , class SignalClass , class... SignalArgs, class Receiver , class T >
static bool	disconnect (const Sender sender, void (SignalClass::signalMethod)(SignalArgs...), const Receiver *receiver, T slotMethod)

static QMetaObject &	staticMetaObject ()

static QString	tr (const char text, const char comment=nullptr, std::optional< int > numArg=std::optional< int >())

Properties
	fileName

	loadHints

Properties inherited from QObject
	objectName

Additional Inherited Members
Public Signals inherited from QObject
void	destroyed (QObject *obj=nullptr)

void	objectNameChanged (const QString &objectName)

Public Slots inherited from QObject
void	deleteLater ()

Protected Methods inherited from QObject
virtual void	childEvent (QChildEvent *event)

virtual void	connectNotify (const QMetaMethod &signalMethod) const

virtual void	customEvent (QEvent *event)

virtual void	disconnectNotify (const QMetaMethod &signalMethod) const

bool	isSignalConnected (const QMetaMethod &signalMethod) const

int	receivers (const QString &signal) const

QObject *	sender () const

int	senderSignalIndex () const

virtual void	timerEvent (QTimerEvent *event)

Related Functions inherited from QObject
T	qobject_cast (QObject *object)

	QObjectList

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 can not 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 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 will not 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. Refer to 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

Member Typedef Documentation

QLibrary::LoadHints

Typedef for QFlags<LoadHint>. Refer to QLibrary::LoadHint for documentation.

Member Enumeration Documentation

enum QLibrary::LoadHint

This enum describes the possible hints that can be used to change the way libraries are handled when they are loaded. These values indicate how symbols are resolved when libraries are loaded, and are specified using the setLoadHints() function.

Constant	Value	Description
`QLibrary::ResolveAllSymbolsHint`	`0x01`	Causes all symbols in a library to be resolved when it is loaded, not simply when resolve() is called.
`QLibrary::ExportExternalSymbolsHint`	`0x02`	Exports unresolved and external symbols in the library so that they can be resolved in other dynamically-loaded libraries loaded later.
`QLibrary::LoadArchiveMemberHint`	`0x04`	Allows the file name of the library to specify a particular object file within an archive file. If this hint is given, the filename of the library consists of a path, which is a reference to an archive file, followed by a reference to the archive member.

See also: loadHints

Constructor & Destructor Documentation

QLibrary::QLibrary ( QObject * parent = nullptr )

explicit

Constructs a library with the given parent.

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

explicit

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 also: fileName

QLibrary::QLibrary	(	const QString &	fileName,
		int	verNum,
		QObject *	parent = `nullptr`
	)

explicit

Constructs a library object with the given parent that will load the library specified by fileName and major version number verNum. Currently, the version number is ignored on Windows.

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 also: fileName

QLibrary::QLibrary	(	const QString &	fileName,
		const QString &	version,
		QObject *	parent = `nullptr`
	)

explicit

Constructs a library object with the given parent that will load the library specified by fileName and full version number version. Currently, the version number is ignored on Windows.

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 also: fileName

QLibrary::~QLibrary ( )

Destroys the QLibrary object.

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

See also: isLoaded(), unload()

Method Documentation

QString QLibrary::errorString ( ) const

Returns a text string with the description of the last error that occurred. Currently, errorString will only be set if load(), unload() or resolve() for some reason fails.

QString QLibrary::fileName ( ) const

Returns the value of the property fileName.

bool QLibrary::isLibrary ( const QString & fileName )

static

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

Platform	Valid suffixes
Windows	`.dll`, `.DLL`
Unix/Linux	`.so`
AIX	`.a`
HP-UX	`.sl`, `.so` (HP-UXi)
Mac OS X	`.dylib`, `.bundle`, `.so`

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.

See also: unload()

LoadHints QLibrary::loadHints ( ) const

Returns the value of the loadHints property.

void * QLibrary::resolve	(	const QString &	fileName,
		const QString &	symbol
	)

static

Loads the library fileName and returns the address of the exported symbol. The fileName should not include the platform specific file suffix. The library remains loaded until the application exits.

Returns a nullptr if the symbol could not be resolved or if the library could not be loaded.

See also: resolve(), fileName

void * QLibrary::resolve	(	const QString &	fileName,
		const QString &	version,
		const QString &	symbol
	)

static

Loads the library fileName with full version number version and returns the address of the exported symbol. The fileName should not include the platform specific file suffix. The library remains loaded until the application exits. The version is ignored on Windows.

Returns a nullptr if the symbol could not be resolved or if the library could not be loaded.

See also: resolve(), fileName

void * QLibrary::resolve	(	const QString &	fileName,
		int	verNum,
		const QString &	symbol
	)

static

Loads the library fileName with major version number verNum and returns the address of the exported symbol. The fileName should not include the platform specific file suffix. The library remains loaded until the application exits. The value for verNum is ignored on Windows.

Returns a nullptr if the symbol could not be resolved or if the library could not be loaded.

See also: resolve(), fileName

void * QLibrary::resolve ( const QString & symbol )

Returns the address of the exported symbol. The library is loaded if necessary. Returns a nullptr if the symbol could not be resolved or if the library could not be loaded.

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.

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

with MY_EXPORT defined as

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

void QLibrary::setFileName ( const QString & fileName )

Sets the value of the property fileName to fileName.

void QLibrary::setFileNameAndVersion	(	const QString &	fileName,
		const QString &	version
	)

Sets the fileName property and full version number to fileName and version respectively. The version parameter is ignored on Windows.

See also: setFileName()

void QLibrary::setFileNameAndVersion	(	const QString &	fileName,
		int	versionNumber
	)

Sets the fileName property and major version number to fileName and versionNumber respectively. The versionNumber is ignored on Windows.

See also: setFileName()

void QLibrary::setLoadHints ( LoadHints hints )

Sets the value of the property loadHints to hints.

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 should not 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().

See also: resolve(), load()

Property Documentation

QLibrary::fileName

This property holds the file name of the library. It is recommended to omit the file's suffix in the file name since QLibrary will automatically look for the file with the appropriate suffix.

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, including the full path to the library if one was given in the constructor or passed to setFileName().

For example, after successfully loading the "GL" library on Unix platforms, fileName() will return "libGL.so". If the file name was originally passed as "/usr/lib/libGL", fileName() will return "/usr/lib/libGL.so".

See also: isLibrary()

Properties	Class Methods
read	fileName
write	setFileName

QLibrary::loadHints

This property holds give the load() function some hints on how it should behave.

You can give some hints on how the symbols are resolved. Usually, the symbols are not resolved at load time, but resolved lazily, (that is, when resolve() is called). If you set the loadHint to ResolveAllSymbolsHint, then all symbols will be resolved at load time if the platform supports it.

Setting ExportExternalSymbolsHint will make the external symbols in the library available for resolution in subsequent loaded libraries.

If LoadArchiveMemberHint is set, the file name is composed of two components: A path which is a reference to an archive file followed by the second component which is the reference to the archive member. For instance, the fileName libGL.a(shr_64.o) will refer to the library shr_64.o in the archive file named libGL.a. This is only supported on the AIX platform.

The interpretation of the load hints is platform dependent, and if you use it you are probably making some assumptions on which platform you are compiling for, so use them only if you understand the consequences of them.

By default, none of these flags are set, so libraries will be loaded with lazy symbol resolution, and will not export external symbols for resolution in other dynamically-loaded libraries.

Properties	Class Methods
read	loadHints
write	setLoadHints

Public Typedefs

Public Types

Public Methods

Static Public Methods

Properties

Additional Inherited Members

Detailed Description

Member Typedef Documentation

Member Enumeration Documentation

Constructor & Destructor Documentation

Method Documentation

Property Documentation