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# QLibrary Class Reference
[QtCore module]

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

Inherits QObject.

### Types

- enum LoadHint { ResolveAllSymbolsHint, ExportExternalSymbolsHint, LoadArchiveMemberHint }
- class LoadHints

### Methods

- __init__ (self, QObject parent = None)
- __init__ (self, QString fileName, QObject parent = None)
- __init__ (self, QString fileName, int verNum, QObject parent = None)
- QString errorString (self)
- QString fileName (self)
- bool isLoaded (self)
- bool load (self)
- LoadHints loadHints (self)
- sip.voidptr resolve (self, str symbol)
- setFileName (self, QString fileName)
- setFileNameAndVersion (self, QString fileName, int verNum)
- setLoadHints (self, LoadHints hints)
- bool unload (self)

### Static Methods

- bool isLibrary (QString fileName)
- sip.voidptr resolve (QString fileName, str symbol)
- sip.voidptr resolve (QString fileName, int verNum, str symbol)

## 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 cannot 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 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.

## Type Documentation

### 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.

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

See also loadHints.

## Method Documentation

### QLibrary.__init__ (self, QObject parent = None)

The parent argument, if not None, causes self to be owned by Qt instead of PyQt.

Constructs a library with the given parent.

### QLibrary.__init__ (self, QString fileName, QObject parent = None)

The parent argument, if not None, causes self to be owned by Qt instead of PyQt.

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 fileName.)

### QLibrary.__init__ (self, QString fileName, int verNum, QObject parent = None)

The parent argument, if not None, causes self to be owned by Qt instead of PyQt.

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 fileName.)

### QString QLibrary.errorString (self)

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.

### QString QLibrary.fileName (self)

### bool QLibrary.isLibrary (QString fileName)

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

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

Trailing versioning numbers on Unix are ignored.

### bool QLibrary.isLoaded (self)

Returns true if the library is loaded; otherwise returns false.

See also load().

### bool QLibrary.load (self)

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 (self)

### sip.voidptr QLibrary.resolve (self, str 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_WS_WIN
     #define MY_EXPORT __declspec(dllexport)
     #else
     #define MY_EXPORT
     #endif

### sip.voidptr QLibrary.resolve (QString fileName, str symbol)

This is an overloaded member function, provided for convenience.

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().

### sip.voidptr QLibrary.resolve (QString fileName, int verNum, str symbol)

This is an overloaded member function, provided for convenience.

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().

### QLibrary.setFileName (self, QString fileName)

### QLibrary.setFileNameAndVersion (self, QString fileName, int verNum)

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

See also setFileName().

### QLibrary.setLoadHints (self, LoadHints hints)

### bool QLibrary.unload (self)

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().