#include <qlibrary.h>
An instance of a QLibrary object can handle a single shared library and provide access to the functionality in the library in a platform independent way. If the library is a component server, QLibrary provides access to the exported component and can directly query this component for interfaces.
QLibrary ensures that the shared library is loaded and stays in memory whilst it is in use. QLibrary can also unload the library on destruction and release unused resources.
A typical use of QLibrary is to resolve an exported symbol in a shared object, and to call the 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 went 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. When the QLibrary object is destroyed the library will be unloaded, making all references to memory allocated in the library invalid.
typedef void (*MyPrototype)();
MyPrototype myFunction;
QLibrary myLib( "mylib" );
myFunction = (MyPrototype) myLib.resolve( "mysymbol" );
if ( myFunction ) {
myFunction();
}
Note that filename does not need to include the (platform specific) file extension, so calling
is equivalent to calling
QLibrary lib( "mylib" );
on Windows, and
QLibrary lib( "mylib.dll" );
on Unix. Specifying the extension is not recommended, since doing so introduces a platform dependency.
QLibrary lib( "libmylib.so" );
If filename does not include a path, the library loader will look for the file in the platform specific search paths.
See also load(), unload(), and setAutoUnload().
The library will be unloaded if autoUnload() is TRUE (the default), otherwise it stays in memory until the application exits.
See also unload() and setAutoUnload().
For example:
will set str to "mylib.dll" on Windows, and "libmylib.so" on Linux.
QLibrary lib( "mylib" );
QString str = lib.library();
On Darwin and Mac OS X this function uses code from dlcompat, part of the OpenDarwin project.
Copyright (c) 2002 Jorge Acereda and Peter O'Gorman
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the" Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
typedef int (*avgProc)( int, int );
avgProc avg = (avgProc) 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 requires the extern "C" notation if the library is compiled with a C++ compiler. On Windows you also have to explicitly export the function from the DLL using the __declspec(dllexport) compiler directive.
extern "C" MY_EXPORT_MACRO 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
On Darwin and Mac OS X this function uses code from dlcompat, part of the OpenDarwin project.
Copyright (c) 2002 Jorge Acereda and Peter O'Gorman
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the" Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
Loads the library filename and returns the address of the exported symbol symb. Note that like the constructor, filename does not need to include the (platform specific) file extension. The library remains loaded until the process exits.
The function returns 0 if the symbol could not be resolved or the library could not be loaded.
This function is useful only if you want to resolve a single symbol, e.g. a function pointer from a specific library once:
typedef void (*FunctionType)();
static FunctionType *ptrFunction = 0;
static bool triedResolve = FALSE;
if ( !ptrFunction && !triedResolve )
ptrFunction = QLibrary::resolve( "mylib", "mysymb" );
if ( ptrFunction )
ptrFunction();
else
...
If you want to resolve multiple symbols, use a QLibrary object and call the non-static version of resolve().
This function is called by the destructor if autoUnload() is enabled.
See also resolve().