Qt 5.6 | Qt Core | C++ Classes | QLibrary

Qt 5.6.2 Reference Documentation

# QLibrary Class

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

Header: | #include <QLibrary>
qmake: | QT += core
Inherits: | QObject

- List of all members, including inherited members

Note: All functions in this class are reentrant.

## Public Types

enum | LoadHint { ResolveAllSymbolsHint, ExportExternalSymbolsHint, LoadArchiveMemberHint, PreventUnloadHint, DeepBindHint }
flags | LoadHints

## Properties

- fileName : QString
- loadHints : LoadHints

- 1 property inherited from QObject

## Public Functions

| QLibrary(QObject *parent = Q_NULLPTR)
| QLibrary(const QString &fileName, QObject *parent = Q_NULLPTR)
| QLibrary(const QString &fileName, int verNum, QObject *parent = Q_NULLPTR)
| QLibrary(const QString &fileName, const QString &version, QObject *parent = Q_NULLPTR)
| ~QLibrary()
QString | errorString() const
QString | fileName() const
bool | isLoaded() const
bool | load()
LoadHints | loadHints() const
QFunctionPointer | resolve(const char *symbol)
void | setFileName(const QString &fileName)
void | setFileNameAndVersion(const QString &fileName, int versionNumber)
void | setFileNameAndVersion(const QString &fileName, const QString &version)
void | setLoadHints(LoadHints hints)
bool | unload()

- 31 public functions inherited from QObject

## Static Public Members

bool | isLibrary(const QString &fileName)
QFunctionPointer | resolve(const QString &fileName, const char *symbol)
QFunctionPointer | resolve(const QString &fileName, int verNum, const char *symbol)
QFunctionPointer | resolve(const QString &fileName, const QString &version, const char *symbol)

- 11 static public members inherited from QObject

### Additional Inherited Members

- 1 public slot inherited from QObject
- 2 signals inherited from QObject
- 9 protected functions inherited from QObject

## 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 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 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.

## Member Type Documentation

### enum QLibrary::LoadHint
flags QLibrary::LoadHints

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.
`QLibrary::PreventUnloadHint` | `0x08` | 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` | `0x10` | 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.

The LoadHints type is a typedef for QFlags<LoadHint>. It stores an OR combination of LoadHint values.

See also loadHints.

## 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 (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:

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

### loadHints : 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 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.

Setting PreventUnloadHint will only apply on Unix platforms.

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: 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:

LoadHints | loadHints() const
void | setLoadHints(LoadHints hints)

## Member Function Documentation

### QLibrary::QLibrary(QObject *parent = Q_NULLPTR)

Constructs a library with the given parent.

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

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.)

### QLibrary::QLibrary(const QString &fileName, int verNum, QObject *parent = Q_NULLPTR)

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.)

### QLibrary::QLibrary(const QString &fileName, const QString &version, QObject *parent = Q_NULLPTR)

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.)

### 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().

### 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.

This function was introduced in Qt 4.2.

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

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.

### 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().

### QFunctionPointer 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_OS_WIN
      #define MY_EXPORT __declspec(dllexport)
      #else
      #define MY_EXPORT
      #endif

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

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

See also resolve().

### `[static]` QFunctionPointer QLibrary::resolve(const QString &fileName, int verNum, const char *symbol)

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

See also resolve().

### `[static]` QFunctionPointer QLibrary::resolve(const QString &fileName, const QString &version, const char *symbol)

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

This function was introduced in Qt 4.4.

See also resolve().

### 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::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.

This function was introduced in Qt 4.4.

See also setFileName().

### 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 10.3 (Panther), dynamic libraries cannot be unloaded.

See also resolve() and load().