Dynamic-link library
A dynamic-link library (DLL) is a shared library in the Microsoft Windows or OS/2 operating system. A DLL can contain executable code (functions), data, and resources. A DLL file often has file extension A DLL that contains only resources can be called a resource DLL. Examples include an icon library, with common extension The file format of a DLL is the same as for an executable (a.k.a. EXE). The main difference between a DLL file and an EXE file is that a DLL cannot be run directly since the operating system requires an entry point to start execution. Windows provides a utility program (RUNDLL.EXE/RUNDLL32.EXE) to execute a function exposed by a DLL. Since they have the same format, an EXE can be used as a DLL. Consuming code can load an EXE via the same mechanism as loading a DLL. BackgroundThe first versions of Microsoft Windows ran programs together in a single address space. Every program was meant to co-operate by yielding the CPU to other programs so that the graphical user interface (GUI) could multitask and be maximally responsive. All operating-system level operations were provided by the underlying operating system: MS-DOS. All higher-level services were provided by Windows Libraries "Dynamic Link Library". The Drawing API, Graphics Device Interface (GDI), was implemented in a DLL called The same architectural concept that allowed GDI to load different device drivers also allowed the Windows shell to load different Windows programs, and for these programs to invoke API calls from the shared USER and GDI libraries. That concept was "dynamic linking". In a conventional non-shared static library, sections of code are simply added to the calling program when its executable is built at the "linking" phase; if two programs call the same routine, the routine is included in both the programs during the linking stage of the two. With dynamic linking, shared code is placed into a single, separate file. The programs that call this file are connected to it at run time, with the operating system (or, in the case of early versions of Windows, the OS-extension), performing the binding. For those early versions of Windows (1.0 to 3.11), the DLLs were the foundation for the entire GUI. As such, display drivers were merely DLLs with a .DRV extension that provided custom implementations of the same drawing API through a unified device driver interface (DDI), and the Drawing (GDI) and GUI (USER) APIs were merely the function calls exported by the GDI and USER, system DLLs with .EXE extension. This notion of building up the operating system from a collection of dynamically loaded libraries is a core concept of Windows that persists as of 2015[update]. DLLs provide the standard benefits of shared libraries, such as modularity. Modularity allows changes to be made to code and data in a single self-contained DLL shared by several applications without any change to the applications themselves. Another benefit of modularity is the use of generic interfaces for plug-ins. A single interface may be developed which allows old as well as new modules to be integrated seamlessly at run-time into pre-existing applications, without any modification to the application itself. This concept of dynamic extensibility is taken to the extreme with the Component Object Model, the underpinnings of ActiveX. In Windows 1.x, 2.x and 3.x, all Windows applications shared the same address space as well as the same memory. A DLL was only loaded once into this address space; from then on, all programs using the library accessed it. The library's data was shared across all the programs. This could be used as an indirect form of inter-process communication, or it could accidentally corrupt the different programs. With the introduction of 32-bit libraries in Windows 95, every process ran in its own address space. While the DLL code may be shared, the data is private except where shared data is explicitly requested by the library. That said, large swathes of Windows 95, Windows 98 and Windows Me were built from 16-bit libraries, which limited the performance of the Pentium Pro microprocessor when launched, and ultimately limited the stability and scalability of the DOS-based versions of Windows. LimitationsAlthough the DLL technology is core to the Windows architecture, it has drawbacks. DLL HellDLL hell describes the bad behavior of an application when the wrong version of a DLL is consumed.[2] Mitigation strategies include:
Shared memory spaceThe executable code of a DLL runs in the memory space of the calling process and with the same access permissions, which means there is little overhead in their use, but also that there is no protection for the calling program if the DLL has any sort of bug. FeaturesUpgradabilityThe DLL technology allows for an application to be modified without requiring consuming components to be re-compiled or re-linked. A DLL can be replaced so that the next time the application runs it uses the new DLL version. To work correctly, the DLL changes must maintain backward compatibility. Even the operating system can be upgraded since it is exposed to the applications via DLLs. System DLLs can be replaced so that the next time the applications run, they use the new system DLLs. Memory managementIn Windows API, DLL files are organized into sections. Each section has its own set of attributes, such as being writable or read-only, executable (for code) or non-executable (for data), and so on. The code in a DLL is usually shared among all the processes that use the DLL; that is, they occupy a single place in physical memory, and do not take up space in the page file. Windows does not use position-independent code for its DLLs; instead, the code undergoes relocation as it is loaded, fixing addresses for all its entry points at locations which are free in the memory space of the first process to load the DLL. In older versions of Windows, in which all running processes occupied a single common address space, a single copy of the DLL's code would always be sufficient for all the processes. However, in newer versions of Windows which use separate address spaces for each program, it is only possible to use the same relocated copy of the DLL in multiple programs if each program has the same virtual addresses free to accommodate the DLL's code. If some programs (or their combination of already-loaded DLLs) do not have those addresses free, then an additional physical copy of the DLL's code will need to be created, using a different set of relocated entry points. If the physical memory occupied by a code section is to be reclaimed, its contents are discarded, and later reloaded directly from the DLL file as necessary. In contrast to code sections, the data sections of a DLL are usually private; that is, each process using the DLL has its own copy of all the DLL's data. Optionally, data sections can be made shared, allowing inter-process communication via this shared memory area. However, because user restrictions do not apply to the use of shared DLL memory, this creates a security hole; namely, one process can corrupt the shared data, which will likely cause all other sharing processes to behave undesirably. For example, a process running under a guest account can in this way corrupt another process running under a privileged account. This is an important reason to avoid the use of shared sections in DLLs. If a DLL is compressed by certain executable packers (e.g. UPX), all of its code sections are marked as read and write, and will be unshared. Read-and-write code sections, much like private data sections, are private to each process. Thus DLLs with shared data sections should not be compressed if they are intended to be used simultaneously by multiple programs, since each program instance would have to carry its own copy of the DLL, resulting in increased memory consumption. Import librariesLike static libraries, import libraries for DLLs are noted by the Linking to dynamic libraries is usually handled by linking to an import library when building or linking to create an executable file. The created executable then contains an import address table (IAT) by which all DLL function calls are referenced (each referenced DLL function contains its own entry in the IAT). At run-time, the IAT is filled with appropriate addresses that point directly to a function in the separately loaded DLL.[3] In Cygwin/MSYS and MinGW, import libraries are conventionally given the suffix Symbol resolution and bindingEach function exported by a DLL is identified by a numeric ordinal and optionally a name. Likewise, functions can be imported from a DLL either by ordinal or by name. The ordinal represents the position of the function's address pointer in the DLL Export Address table. It is common for internal functions to be exported by ordinal only. For most Windows API functions only the names are preserved across different Windows releases; the ordinals are subject to change. Thus, one cannot reliably import Windows API functions by their ordinals. Importing functions by ordinal provides only slightly better performance than importing them by name: export tables of DLLs are ordered by name, so a binary search can be used to find a function. The index of the found name is then used to look up the ordinal in the Export Ordinal table. In 16-bit Windows, the name table was not sorted, so the name lookup overhead was much more noticeable. It is also possible to bind an executable to a specific version of a DLL, that is, to resolve the addresses of imported functions at compile-time. For bound imports, the linker saves the timestamp and checksum of the DLL to which the import is bound. At run-time, Windows checks to see if the same version of library is being used, and if so, Windows bypasses processing the imports. Otherwise, if the library is different from the one which was bound to, Windows processes the imports in a normal way. Bound executables load somewhat faster if they are run in the same environment that they were compiled for, and exactly the same time if they are run in a different environment, so there is no drawback for binding the imports. For example, all the standard Windows applications are bound to the system DLLs of their respective Windows release. A good opportunity to bind an application's imports to its target environment is during the application's installation. This keeps the libraries "bound" until the next OS update. It does, however, change the checksum of the executable, so it is not something that can be done with signed programs, or programs that are managed by a configuration management tool that uses checksums (such as MD5 checksums) to manage file versions. As more recent Windows versions have moved away from having fixed addresses for every loaded library (for security reasons), the opportunity and value of binding an executable is decreasing. Explicit run-time linkingDLL files may be explicitly loaded at run-time, a process referred to simply as run-time dynamic linking by Microsoft, by using the The procedure for explicit run-time linking is the same in any language that supports pointers to functions, since it depends on the Windows API rather than language constructs. Delayed loadingNormally, an application that is linked against a DLL’s import library will fail to start if the DLL cannot be found, because Windows will not run the application unless it can find all of the DLLs that the application may need. However an application may be linked against an import library to allow delayed loading of the dynamic library.[6]
In this case, the operating system will not try to find or load the DLL when the application starts; instead, a stub is included in the application by the linker which will try to find and load the DLL through The delayed loading mechanism also provides notification hooks, allowing the application to perform additional processing or error handling when the DLL is loaded and/or any DLL function is called. Compiler and language considerationsDelphiIn a source file, the keyword Delphi does not need Microsoft Visual BasicIn Visual Basic (VB), only run-time linking is supported; but in addition to using When importing DLL functions through declarations, VB will generate a run-time error if the When creating DLLs in VB, the IDE will only allow creation of ActiveX DLLs, however methods have been created[7] to allow the user to explicitly tell the linker to include a .DEF file which defines the ordinal position and name of each exported function. This allows the user to create a standard Windows DLL using Visual Basic (Version 6 or lower) which can be referenced through a "Declare" statement. C and C++Microsoft Visual C++ (MSVC) provides several extensions to standard C++ which allow functions to be specified as imported or exported directly in the C++ code; these have been adopted by other Windows C and C++ compilers, including Windows versions of GCC. These extensions use the attribute Besides specifying imported or exported functions using DLL compilation will produce both Programming examplesUsing DLL importsThe following examples show how to use language-specific bindings to import symbols for linking against a DLL at compile-time. Delphi {$APPTYPE CONSOLE}
program Example;
// import function that adds two numbers
function AddNumbers(a, b : Double): Double; StdCall; external 'Example.dll';
// main program
var
R: Double;
begin
R := AddNumbers(1, 2);
Writeln('The result was: ', R);
end.
C 'Example.lib' file must be included (assuming that Example.dll is generated) in the project before static linking. The file 'Example.lib' is automatically generated by the compiler when compiling the DLL. Not executing the above statement would cause linking error as the linker would not know where to find the definition of #include <windows.h>
#include <stdio.h>
// Import function that adds two numbers
extern "C" __declspec(dllimport) double AddNumbers(double a, double b);
int main(int argc, char *argv[])
{
double result = AddNumbers(1, 2);
printf("The result was: %f\n", result);
return 0;
}
Using explicit run-time linkingThe following examples show how to use the run-time loading and linking facilities using language-specific Windows API bindings. Note that all of the four samples are vulnerable to DLL preloading attacks, since example.dll can be resolved to a place unintended by the author (unless explicitly excluded the application directory goes before system library locations, and without Microsoft Visual BasicOption Explicit
Declare Function AddNumbers Lib "Example.dll" _
(ByVal a As Double, ByVal b As Double) As Double
Sub Main()
Dim Result As Double
Result = AddNumbers(1, 2)
Debug.Print "The result was: " & Result
End Sub
Delphiprogram Example;
{$APPTYPE CONSOLE}
uses Windows;
var
AddNumbers:function (a, b: integer): Double; StdCall;
LibHandle:HMODULE;
begin
LibHandle := LoadLibrary('example.dll');
if LibHandle <> 0 then
AddNumbers := GetProcAddress(LibHandle, 'AddNumbers');
if Assigned(AddNumbers) then
Writeln( '1 + 2 = ', AddNumbers( 1, 2 ) );
Readln;
end.
C#include <windows.h>
#include <stdio.h>
// DLL function signature
typedef double (*importFunction)(double, double);
int main(int argc, char **argv)
{
importFunction addNumbers;
double result;
HINSTANCE hinstLib;
// Load DLL file
hinstLib = LoadLibrary(TEXT("Example.dll"));
if (hinstLib == NULL) {
printf("ERROR: unable to load DLL\n");
return 1;
}
// Get function pointer
addNumbers = (importFunction) GetProcAddress(hinstLib, "AddNumbers");
if (addNumbers == NULL) {
printf("ERROR: unable to find DLL function\n");
FreeLibrary(hinstLib);
return 1;
}
// Call function.
result = addNumbers(1, 3);
// Unload DLL file
FreeLibrary(hinstLib);
// Display result
printf("The result was: %f\n", result);
return 0;
}
PythonThe Python ctypes binding will use POSIX API on POSIX systems. import ctypes
my_dll = ctypes.cdll.LoadLibrary("Example.dll")
# The following "restype" method specification is needed to make
# Python understand what type is returned by the function.
my_dll.AddNumbers.restype = ctypes.c_double
p = my_dll.AddNumbers(ctypes.c_double(1.0), ctypes.c_double(2.0))
print("The result was:", p)
Component Object ModelThe Component Object Model (COM) defines a binary standard to host the implementation of objects in DLL and EXE files. It provides mechanisms to locate and version those files as well as a language-independent and machine-readable description of the interface. Hosting COM objects in a DLL is more lightweight and allows them to share resources with the client process. This allows COM objects to implement powerful back-ends to simple GUI front ends such as Visual Basic and ASP. They can also be programmed from scripting languages.[12] DLL hijackingDue to a vulnerability commonly known as DLL hijacking, DLL spoofing, DLL preloading or binary planting, many programs will load and execute a malicious DLL contained in the same folder as a data file opened by these programs.[13][14][15][16] The vulnerability was discovered by Georgi Guninski in 2000.[17] In August 2010 it gained worldwide publicity after ACROS Security rediscovered it and many hundreds of programs were found vulnerable.[18] Programs that are run from unsafe locations, i.e. user-writable folders like the Downloads or the Temp directory, are almost always susceptible to this vulnerability.[19][20][21][22][23][24][25] See also
References
External links
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