QLibrary Class Reference

\inmodule QtCore \reentrant More...

#include <qlibrary.h>

Inheritance diagram for QLibrary:

Collaboration diagram for QLibrary:

Public Types
enum	LoadHint {   ResolveAllSymbolsHint = 0x01 , ExportExternalSymbolsHint = 0x02 , LoadArchiveMemberHint = 0x04 , PreventUnloadHint = 0x08 ,   DeepBindHint = 0x10 }
	This enum describes the possible hints that can be used to change the way libraries are handled when they are loaded. More...

Public Member Functions
	QLibrary (QObject *parent=nullptr)
	Constructs a library with the given parent.
	QLibrary (const QString &fileName, QObject *parent=nullptr)
	Constructs a library object with the given parent that will load the library specified by fileName.
	QLibrary (const QString &fileName, int verNum, QObject *parent=nullptr)
	Constructs a library object with the given parent that will load the library specified by fileName and major version number verNum.
	QLibrary (const QString &fileName, const QString &version, QObject *parent=nullptr)
	Constructs a library object with the given parent that will load the library specified by fileName and full version number version.
	~QLibrary ()
	Destroys the QLibrary object.
QFunctionPointer	resolve (const char *symbol)
	Returns the address of the exported symbol symbol.
bool	load ()
	Loads the library and returns true if the library was loaded successfully; otherwise returns false.
bool	unload ()
	Unloads the library and returns true if the library could be unloaded; otherwise returns false.
bool	isLoaded () const
	Returns true if load() succeeded; otherwise returns false.
void	setFileName (const QString &fileName)
QString	fileName () const
void	setFileNameAndVersion (const QString &fileName, int verNum)
	Sets the fileName property and major version number to fileName and versionNumber respectively.
void	setFileNameAndVersion (const QString &fileName, const QString &version)
QString	errorString () const
void	setLoadHints (LoadHints hints)
LoadHints	loadHints () const
Public Member Functions inherited from QObject
Q_INVOKABLE	QObject (QObject *parent=nullptr)
	Constructs an object with parent object parent.
virtual	~QObject ()
	Destroys the object, deleting all its child objects.
virtual bool	event (QEvent *event)
	This virtual function receives events to an object and should return true if the event e was recognized and processed.
virtual bool	eventFilter (QObject *watched, QEvent *event)
	Filters events if this object has been installed as an event filter for the watched object.
QString	objectName () const
Q_WEAK_OVERLOAD void	setObjectName (const QString &name)
	Sets the object's name to name.
void	setObjectName (QAnyStringView name)
	This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
QBindable< QString >	bindableObjectName ()
bool	isWidgetType () const
	Returns true if the object is a widget; otherwise returns false.
bool	isWindowType () const
	Returns true if the object is a window; otherwise returns false.
bool	isQuickItemType () const
	Returns true if the object is a QQuickItem; otherwise returns false.
bool	signalsBlocked () const noexcept
	Returns true if signals are blocked; otherwise returns false.
bool	blockSignals (bool b) noexcept
	If block is true, signals emitted by this object are blocked (i.e., emitting a signal will not invoke anything connected to it).
QThread *	thread () const
	Returns the thread in which the object lives.
bool	moveToThread (QThread *thread QT6_DECL_NEW_OVERLOAD_TAIL)
	Changes the thread affinity for this object and its children and returns true on success.
int	startTimer (int interval, Qt::TimerType timerType=Qt::CoarseTimer)
	This is an overloaded function that will start a timer of type timerType and a timeout of interval milliseconds.
int	startTimer (std::chrono::nanoseconds time, Qt::TimerType timerType=Qt::CoarseTimer)
void	killTimer (int id)
	Kills the timer with timer identifier, id.
void	killTimer (Qt::TimerId id)
template<typename T >
T	findChild (QAnyStringView aName, Qt::FindChildOptions options=Qt::FindChildrenRecursively) const
	Returns the child of this object that can be cast into type T and that is called name, or \nullptr if there is no such object.
template<typename T >
QList< T >	findChildren (QAnyStringView aName, Qt::FindChildOptions options=Qt::FindChildrenRecursively) const
	Returns all children of this object with the given name that can be cast to type T, or an empty list if there are no such objects.
template<typename T >
T	findChild (Qt::FindChildOptions options=Qt::FindChildrenRecursively) const
	This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
template<typename T >
QList< T >	findChildren (Qt::FindChildOptions options=Qt::FindChildrenRecursively) const
	This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
const QObjectList &	children () const
	Returns a list of child objects.
void	setParent (QObject *parent)
	Makes the object a child of parent.
void	installEventFilter (QObject *filterObj)
	Installs an event filter filterObj on this object.
void	removeEventFilter (QObject *obj)
	Removes an event filter object obj from this object.
QMetaObject::Connection	connect (const QObject *sender, const char *signal, const char *member, Qt::ConnectionType type=Qt::AutoConnection) const
bool	disconnect (const char *signal=nullptr, const QObject *receiver=nullptr, const char *member=nullptr) const
bool	disconnect (const QObject *receiver, const char *member=nullptr) const
void	dumpObjectTree () const
	Dumps a tree of children to the debug output.
void	dumpObjectInfo () const
	Dumps information about signal connections, etc.
bool	setProperty (const char *name, const QVariant &value)
	Sets the value of the object's name property to value.
bool	setProperty (const char *name, QVariant &&value)
QVariant	property (const char *name) const
	Returns the value of the object's name property.
QList< QByteArray >	dynamicPropertyNames () const
QBindingStorage *	bindingStorage ()
const QBindingStorage *	bindingStorage () const
QObject *	parent () const
	Returns a pointer to the parent object.
bool	inherits (const char *classname) const
	Returns true if this object is an instance of a class that inherits className or a QObject subclass that inherits className; otherwise returns false.

Static Public Member Functions
static QFunctionPointer	resolve (const QString &fileName, const char *symbol)
	This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.Loads the library fileName and returns the address of the exported symbol symbol.
static QFunctionPointer	resolve (const QString &fileName, int verNum, const char *symbol)
	This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.Loads the library fileName with major version number verNum and returns the address of the exported symbol symbol.
static QFunctionPointer	resolve (const QString &fileName, const QString &version, const char *symbol)
	This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
static bool	isLibrary (const QString &fileName)
	Returns true if fileName has a valid suffix for a loadable library; otherwise returns false.
Static Public Member Functions inherited from QObject
static QMetaObject::Connection	connect (const QObject *sender, const char *signal, const QObject *receiver, const char *member, Qt::ConnectionType=Qt::AutoConnection)
	\threadsafe
static QMetaObject::Connection	connect (const QObject *sender, const QMetaMethod &signal, const QObject *receiver, const QMetaMethod &method, Qt::ConnectionType type=Qt::AutoConnection)
template<typename Func1 , typename Func2 >
static QMetaObject::Connection	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object *sender, Func1 signal, const typename QtPrivate::ContextTypeForFunctor< Func2 >::ContextType *context, Func2 &&slot, Qt::ConnectionType type=Qt::AutoConnection)
template<typename Func1 , typename Func2 >
static QMetaObject::Connection	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object *sender, Func1 signal, Func2 &&slot)
static bool	disconnect (const QObject *sender, const char *signal, const QObject *receiver, const char *member)
	\threadsafe
static bool	disconnect (const QObject *sender, const QMetaMethod &signal, const QObject *receiver, const QMetaMethod &member)
static bool	disconnect (const QMetaObject::Connection &)
	Disconnect a connection.
template<typename Func1 , typename Func2 >
static bool	disconnect (const typename QtPrivate::FunctionPointer< Func1 >::Object *sender, Func1 signal, const typename QtPrivate::FunctionPointer< Func2 >::Object *receiver, Func2 slot)
template<typename Func1 >
static bool	disconnect (const typename QtPrivate::FunctionPointer< Func1 >::Object *sender, Func1 signal, const QObject *receiver, void **zero)

Properties
QString	fileName
	the file name of the library
LoadHints	loadHints
	Give the load() function some hints on how it should behave.
Properties inherited from QObject
QString	objectName
	the name of this object

Additional Inherited Members
Public Slots inherited from QObject
void	deleteLater ()
	\threadsafe
Signals inherited from QObject
void	destroyed (QObject *=nullptr)
	This signal is emitted immediately before the object obj is destroyed, after any instances of QPointer have been notified, and cannot be blocked.
void	objectNameChanged (const QString &objectName, QPrivateSignal)
	This signal is emitted after the object's name has been changed.
Protected Member Functions inherited from QObject
QObject *	sender () const
	Returns a pointer to the object that sent the signal, if called in a slot activated by a signal; otherwise it returns \nullptr.
int	senderSignalIndex () const
int	receivers (const char *signal) const
	Returns the number of receivers connected to the signal.
bool	isSignalConnected (const QMetaMethod &signal) const
virtual void	timerEvent (QTimerEvent *event)
	This event handler can be reimplemented in a subclass to receive timer events for the object.
virtual void	childEvent (QChildEvent *event)
	This event handler can be reimplemented in a subclass to receive child events.
virtual void	customEvent (QEvent *event)
	This event handler can be reimplemented in a subclass to receive custom events.
virtual void	connectNotify (const QMetaMethod &signal)
virtual void	disconnectNotify (const QMetaMethod &signal)
	QObject (QObjectPrivate &dd, QObject *parent=nullptr)
Protected Attributes inherited from QObject
QScopedPointer< QObjectData >	d_ptr
Related Symbols inherited from QObject
template< class T > T	qobject_cast (const QObject *object)
	Returns the given object cast to type T if the object is of type T (or of a subclass); otherwise returns \nullptr.
template< typename T > T	qFindChildqFindChildren (const QObject *obj, const QString &name)()
template< typename T > QList< T >	qFindChildrenqFindChildren (const QObject *obj, const QString &name)()
	QObjectList
	\macro Q_CLASSINFO(Name, Value)

Detailed Description

\inmodule QtCore \reentrant

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 name is an absolute path then an attempt is made to load this path first. If the file cannot be found, QLibrary tries the name with different platform-specific file prefixes, like "lib" on Unix and Mac, and suffixes, like ".so" on Unix, ".dylib" on the Mac, or ".dll" on Windows.

If the file path is not absolute then QLibrary modifies the search order to try the system-specific prefixes and suffixes first, followed by the file path specified.

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 yet still minimise the number of attempts to find the library.

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 \nullptr 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

Definition at line 16 of file qlibrary.h.

Member Enumeration Documentation

LoadHint

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.

\value ResolveAllSymbolsHint Causes all symbols in a library to be resolved when it is loaded, not simply when resolve() is called. \value ExportExternalSymbolsHint Exports unresolved and external symbols in the library so that they can be resolved in other dynamically-loaded libraries loaded later. \value LoadArchiveMemberHint 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. \value PreventUnloadHint Prevents the library from being unloaded from the address space if close() is called. The library's static variables are not reinitialized if open() is called at a later time. \value DeepBindHint Instructs the linker to prefer definitions in the loaded library over exported definitions in the loading application when resolving external symbols in the loaded library. This option is only supported on Linux.

See also

loadHints

Enumerator
ResolveAllSymbolsHint
ExportExternalSymbolsHint
LoadArchiveMemberHint
PreventUnloadHint
DeepBindHint

Definition at line 22 of file qlibrary.h.

Constructor & Destructor Documentation

QLibrary() [1/4]

QLibrary::QLibrary ( QObject * parent = nullptr )

explicit

Constructs a library with the given parent.

Definition at line 872 of file qlibrary.cpp.

QLibrary() [2/4]

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 \macos and iOS, and ".dll" on Windows. (See \l{fileName}.)

Definition at line 886 of file qlibrary.cpp.

QLibrary() [3/4]

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 \macos and iOS, and ".dll" on Windows. (See \l{fileName}.)

Definition at line 901 of file qlibrary.cpp.

QLibrary() [4/4]

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 \macos and iOS, and ".dll" on Windows. (See \l{fileName}.)

Definition at line 916 of file qlibrary.cpp.

~QLibrary()

QLibrary::~QLibrary

(

)

Destroys the QLibrary object.

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

See also

isLoaded(), unload()

Definition at line 930 of file qlibrary.cpp.

Member Function Documentation

errorString()

QString QLibrary::errorString

(

)

const

Since

4.2

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.

Definition at line 1098 of file qlibrary.cpp.

fileName()

QString QLibrary::fileName

(

)

const

Definition at line 962 of file qlibrary.cpp.

isLibrary()

bool QLibrary::isLibrary ( const QString & fileName )

static

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

\table \header

• Platform
• Valid suffixes \row
• Windows
• .dll, .DLL \row
• Unix/Linux
• .so \row
• AIX
• .a \row
• HP-UX
• .sl, .so (HP-UXi) \row
• \macos and iOS
• .dylib, .bundle, .so \endtable

Trailing versioning numbers on Unix are ignored.

Definition at line 616 of file qlibrary.cpp.

isLoaded()

bool QLibrary::isLoaded

(

)

const

Returns true if load() succeeded; otherwise returns false.

Note

Prior to Qt 6.6, this function would return true even without a call to load() if another QLibrary object on the same library had caused it to be loaded.

See also

load()

Definition at line 863 of file qlibrary.cpp.

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()

Definition at line 815 of file qlibrary.cpp.

loadHints()

QLibrary::LoadHints QLibrary::loadHints

(

)

const

Definition at line 1156 of file qlibrary.cpp.

resolve() [1/4]

QFunctionPointer QLibrary::resolve

(

const char *

symbol

)

Returns the address of the exported symbol symbol.

The library is loaded if necessary. The function returns \nullptr 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_OS_WIN
#define MY_EXPORT __declspec(dllexport)
#else
#define MY_EXPORT
#endif

Definition at line 1025 of file qlibrary.cpp.

resolve() [2/4]

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

static

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.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 \l{fileName}). The library remains loaded until the application exits.

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

See also

resolve()

Definition at line 1045 of file qlibrary.cpp.

resolve() [3/4]

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

static

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Since

4.4

Loads the library fileName with full version number version and returns the address of the exported symbol symbol. Note that fileName should not include the platform-specific file suffix; (see \l{fileName}). The library remains loaded until the application exits. version is ignored on Windows.

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

See also

resolve()

Definition at line 1086 of file qlibrary.cpp.

resolve() [4/4]

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

static

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.Loads the library fileName with major version number verNum and returns the address of the exported symbol symbol.

Note that fileName should not include the platform-specific file suffix; (see \l{fileName}). The library remains loaded until the application exits. verNum is ignored on Windows.

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

See also

resolve()

Definition at line 1065 of file qlibrary.cpp.

setFileName()

void QLibrary::setFileName

(

const QString &

fileName

)

Definition at line 957 of file qlibrary.cpp.

setFileNameAndVersion() [1/2]

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

Since

4.4

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

See also

setFileName()

Definition at line 994 of file qlibrary.cpp.

setFileNameAndVersion() [2/2]

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()

Definition at line 980 of file qlibrary.cpp.

setLoadHints()

void QLibrary::setLoadHints

(

LoadHints

hints

)

Definition at line 1147 of file qlibrary.cpp.

unload()

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 \macos, dynamic libraries cannot be unloaded. QLibrary::unload() will return true, but the library will remain loaded into the process.

See also

resolve(), load()

Definition at line 845 of file qlibrary.cpp.

Property Documentation

fileName

QString QLibrary::fileName

readwrite

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 (for example, 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".

Definition at line 19 of file qlibrary.h.

loadHints

LoadHints QLibrary::loadHints

readwrite

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 loadHints 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.

Note

Hints can only be cleared when this object is not associated with a file. Hints can only be added once the file name is set (hints will be or'ed with the old hints).

Setting this property after the library has been loaded has no effect and loadHints() will not reflect those changes.

This property is shared among all QLibrary instances that refer to the same library.

Definition at line 20 of file qlibrary.h.

---

The documentation for this class was generated from the following files:

• qtbase/src/corelib/plugin/qlibrary.h (05fc3aef53348fb58be6308076e000825b704e58)
• qtbase/src/corelib/plugin/qlibrary.cpp (1e1c68017338c89265d6664a27f4137fc8960473)