Detailed Description
The QLibrary class provides a wrapper for handling shared libraries.
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();
}
See also Plugins.
Member Function Documentation
QLibrary::QLibrary ( const QString & filename )
Creates a QLibrary object for the shared library filename. The
library will be unloaded in the destructor.
Note that filename does not need to include the (platform specific)
file extension, so calling
QLibrary lib( "mylib" );
is equivalent to calling
QLibrary lib( "mylib.dll" );
on Windows, and
QLibrary lib( "libmylib.so" );
on Unix. Specifying the extension is not recommended, since
doing so introduces a platform dependency.
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().
QLibrary::~QLibrary () [virtual]
Deletes the QLibrary object.
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().
bool QLibrary::autoUnload () const
Returns TRUE if the library will be automatically unloaded when
this wrapper object is destructed; otherwise returns FALSE. The
default is TRUE.
See also setAutoUnload().
bool QLibrary::isLoaded () const
Returns TRUE if the library is loaded; otherwise returns FALSE.
See also unload().
QString QLibrary::library () const
Returns the filename of the shared library this QLibrary object
handles, including the platform specific file extension.
For example:
QLibrary lib( "mylib" );
QString str = lib.library();
will set str to "mylib.dll" on Windows, and "libmylib.so" on Linux.
bool QLibrary::load ()
Loads the library. 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.
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.
void * QLibrary::resolve ( const char * symb )
Returns the address of the exported symbol symb. The library is
loaded if necessary. The function returns 0 if the symbol could
not be resolved or the library could not be loaded.
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.
void * QLibrary::resolve ( const QString & filename, const char * symb ) [static]
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
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().
See also
void QLibrary::setAutoUnload ( bool enabled )
If enabled is TRUE (the default), the wrapper object is set to
automatically unload the library upon destruction. If enabled
is FALSE, the wrapper object is not unloaded unless you explicitly
call unload().
See also autoUnload().
bool QLibrary::unload () [virtual]
Unloads the library and returns TRUE if the library could be
unloaded; otherwise returns FALSE.
This function is called by the destructor if autoUnload() is
enabled.
See also resolve().
This file is part of the Qt toolkit.
Copyright © 1995-2005
Trolltech. All Rights Reserved.
