- class QLibrary#
The
QLibrary
class loads shared libraries at runtime. More…Synopsis#
Properties#
fileNameᅟ
- The file name of the libraryloadHintsᅟ
- Give the load() function some hints on how it should behave
Methods#
def
__init__()
def
errorString()
def
fileName()
def
isLoaded()
def
load()
def
loadHints()
def
resolve()
def
setFileName()
def
setLoadHints()
def
unload()
Static functions#
def
isLibrary()
def
resolve()
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”). AQLibrary
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 withsetFileName()
. 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, andresolve()
to resolve a symbol in the library. Theresolve()
function implicitly tries to load the library if it has not been loaded yet. Multiple instances ofQLibrary
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 usingunload()
, but if other instances ofQLibrary
are using the same library, the call will fail, and unloading will only happen when every instance has calledunload()
.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 anextern "C"
block if the library is compiled with a C++ compiler. On Windows, this also requires the use of adllexport
macro; seeresolve()
for the details of how this is done. For convenience, there is a staticresolve()
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
- 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 thesetLoadHints()
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
- 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 (seeisLibrary()
).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 toResolveAllSymbolsHint
, 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, thefileName
libGL.a(shr_64.o)
will refer to the libraryshr_64.o
in the archive file namedlibGL.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:
Constructs a library object with the given
parent
that will load the library specified byfileName
.We recommend omitting the file’s suffix in
fileName
, sinceQLibrary
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. (SeefileName
.)- __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 byfileName
and full version numberversion
. Currently, the version number is ignored on Windows.We recommend omitting the file’s suffix in
fileName
, sinceQLibrary
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. (SeefileName
.)- __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 byfileName
and major version numberverNum
. Currently, the version number is ignored on Windows.We recommend omitting the file’s suffix in
fileName
, sinceQLibrary
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. (SeefileName
.)- __init__([parent=None])
- Parameters:
parent –
QObject
Constructs a library with the given
parent
.- 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()
orresolve()
for some reason fails.- fileName()#
- Return type:
str
See also
Getter of property
fileNameᅟ
.- static isLibrary(fileName)#
- Parameters:
fileName – str
- Return type:
bool
Returns
true
iffileName
has a valid suffix for a loadable library; otherwise returnsfalse
.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
ifload()
succeeded; otherwise returnsfalse
.Note
Prior to Qt 6.6, this function would return
true
even without a call toload()
if anotherQLibrary
object on the same library had caused it to be loaded.See also
- load()#
- Return type:
bool
Loads the library and returns
true
if the library was loaded successfully; otherwise returnsfalse
. Sinceresolve()
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
Getter of property
loadHintsᅟ
.- 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 numberversion
and returns the address of the exported symbolsymbol
. Note thatfileName
should not include the platform-specific file suffix; (seefileName
). 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
- 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 symbolsymbol
. Note thatfileName
should not include the platform-specific file suffix; (seefileName
). 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
- 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 numberverNum
and returns the address of the exported symbolsymbol
. Note thatfileName
should not include the platform-specific file suffix; (seefileName
). 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(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 returnsNone
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
- setFileName(fileName)#
- Parameters:
fileName – str
See also
Setter of property
fileNameᅟ
.- setFileNameAndVersion(fileName, version)#
- Parameters:
fileName – str
version – str
Sets the
fileName
property and full version number tofileName
andversion
respectively. Theversion
parameter is ignored on Windows.See also
- setFileNameAndVersion(fileName, verNum)
- Parameters:
fileName – str
verNum – int
Sets the
fileName
property and major version number tofileName
andversionNumber
respectively. TheversionNumber
is ignored on Windows.See also
Setter of property
loadHintsᅟ
.- unload()#
- Return type:
bool
Unloads the library and returns
true
if the library could be unloaded; otherwise returnsfalse
.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.