class QLibrary

The QLibrary class loads shared libraries at runtime. More…

Inheritance diagram of PySide6.QtCore.QLibrary

Synopsis

Properties

  • fileNameᅟ - The file name of the library

  • loadHintsᅟ - Give the load() function some hints on how it should behave

Methods

Static functions

Note

This documentation may contain snippets that were automatically translated from C++ to Python. We always welcome contributions to the snippet translation. If you see an issue with the translation, you can also let us know by creating a ticket on https:/bugreports.qt.io/projects/PYSIDE

Detailed Description

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

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 None and won’t be called.

myLib = QLibrary("mylib")
void = typedef(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:

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

See also

QPluginLoader

class LoadHint

(inherits enum.Flag) 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

Description

QLibrary.ResolveAllSymbolsHint

Causes all symbols in a library to be resolved when it is loaded, not simply when resolve() is called.

QLibrary.ExportExternalSymbolsHint

Exports unresolved and external symbols in the library so that they can be resolved in other dynamically-loaded libraries loaded later.

QLibrary.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.

QLibrary.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.

QLibrary.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

class LoadStatusTag

Note

Properties can be used directly when from __feature__ import true_property is used or via accessor functions otherwise.

property fileNameᅟ: str

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 (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”.

Access functions:
property loadHintsᅟ: Combination of QLibrary.LoadHint

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 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).

Note

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

Note

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

Access functions:
__init__([parent=None])
Parameters:

parent – QObject

Constructs a library with the given parent.

__init__(fileName[, parent=None])
Parameters:

  • fileName – str

  • parent – QObject

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 fileName .)

__init__(fileName, version[, parent=None])
Parameters:

  • fileName – str

  • version – str

  • parent – QObject

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 fileName .)

__init__(fileName, verNum[, parent=None])
Parameters:

  • fileName – str

  • verNum – int

  • parent – QObject

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 fileName .)

errorString()
Return type:

str

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.

fileName()
Return type:

str

See also

setFileName()

Getter of property fileNameᅟ .

static isLibrary(fileName)
Parameters:

fileName – str

Return type:

bool

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)

macOS and iOS

.dylib, .bundle, .so

Trailing versioning numbers on Unix are ignored.

isLoaded()
Return type:

bool

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()

load()
Return type:

bool

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()
Return type:

Combination of LoadHint

See also

setLoadHints()

Getter of property loadHintsᅟ .

resolve(symbol)
Parameters:

symbol – str

Return type:

QFunctionPointer

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

Returns the address of the exported symbol symbol. The library is loaded if necessary. The function returns None if the symbol could not be resolved or if the library could not be loaded.

Example:

typedef int (AvgFunction)(int, int)
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
static resolve(fileName, symbol)
Parameters:

  • fileName – str

  • symbol – str

Return type:

QFunctionPointer

This is an overloaded 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 None if the symbol could not be resolved or if the library could not be loaded.

See also

resolve()

static resolve(fileName, version, symbol)
Parameters:

  • fileName – str

  • version – str

  • symbol – str

Return type:

QFunctionPointer

This is an overloaded function.

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 fileName ). The library remains loaded until the application exits. version is ignored on Windows.

The function returns None if the symbol could not be resolved or if the library could not be loaded.

See also

resolve()

static resolve(fileName, verNum, symbol)
Parameters:

  • fileName – str

  • verNum – int

  • symbol – str

Return type:

QFunctionPointer

This is an overloaded function.

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 fileName ). The library remains loaded until the application exits. verNum is ignored on Windows.

The function returns None if the symbol could not be resolved or if the library could not be loaded.

See also

resolve()

setFileName(fileName)
Parameters:

fileName – str

See also

fileName()

Setter of property fileNameᅟ .

setFileNameAndVersion(fileName, version)
Parameters:

  • fileName – str

  • version – str

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

See also

setFileName()

setFileNameAndVersion(fileName, verNum)
Parameters:

  • fileName – str

  • verNum – int

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

See also

setFileName()

setLoadHints(hints)
Parameters:

hints – Combination of LoadHint

See also

loadHints()

Setter of property loadHintsᅟ .

unload()
Return type:

bool

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()