Dynamic-link library

In 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.

Like static libraries, import libraries for DLLs are noted by the .lib file extension. For example, kernel32.dll, the primary dynamic library for Windows's base functions such as file creation and memory management, is linked via kernel32.lib. The usual way to tell an import library from a proper static library is by size: the import library is much smaller as it only contains symbols referring to the actual DLL, to be processed at link-time. Both nevertheless are Unix ar format files.

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 .dll.a, combining both the Windows DLL suffix and the Unix ar suffix. The file format is similar, but the symbols used to mark the imports are different (_head_foo_dll vs __IMPORT_DESCRIPTOR_foo).[4] Although its GNU Binutils toolchain can generate import libraries and link to them, it is faster to link to the DLL directly.[5] An experimental tool in MinGW called genlib can be used to generate import libs with MSVC-style symbols.

Each 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.

DLL files may be explicitly loaded at run-time, a process referred to simply as run-time dynamic linking by Microsoft, by using the LoadLibrary (or LoadLibraryEx) API function. The GetProcAddress API function is used to look up exported symbols by name, and FreeLibrary – to unload the DLL. These functions are analogous to dlopen, dlsym, and dlclose in the POSIX standard API.

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.

Normally, 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 LoadLibrary and GetProcAddress when one of its functions is called. If the DLL cannot be found or loaded, or the called function does not exist, the application will generate an exception, which may be caught and handled appropriately. If the application does not handle the exception, it will be caught by the operating system, which will terminate the program with an error message.

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.

In a source file, the keyword library is used instead of program. At the end of the file, the functions to be exported are listed in exports clause.

Delphi does not need LIB files to import functions from DLLs; to link to a DLL, the external keyword is used in the function declaration to signal the DLL name, followed by name to name the symbol (if different) or index to identify the index.

In Visual Basic (VB), only run-time linking is supported; but in addition to using LoadLibrary and GetProcAddress API functions, declarations of imported functions are allowed.

When importing DLL functions through declarations, VB will generate a run-time error if the DLL file cannot be found. The developer can catch the error and handle it appropriately.

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.

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 __declspec before a function declaration. Note that when C functions are accessed from C++, they must also be declared as extern "C" in C++ code, to inform the compiler that the C linkage should be used.[8]

Besides specifying imported or exported functions using __declspec attributes, they may be listed in IMPORT or EXPORTS section of the DEF file used by the project. The DEF file is processed by the linker, rather than the compiler, and thus it is not specific to C++.

DLL compilation will produce both DLL and LIB files. The LIB file (import library) is used to link against a DLL at compile-time; it is not necessary for run-time linking. Unless the DLL is a Component Object Model (COM) server, the DLL file must be placed in one of the directories listed in the PATH environment variable, in the default system directory, or in the same directory as the program using it. COM server DLLs are registered using regsvr32.exe, which places the DLL's location and its globally unique ID (GUID) in the registry. Programs can then use the DLL by looking up its GUID in the registry to find its location or create an instance of the COM object indirectly using its class identifier and interface identifier.

The 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 (Add Existing Item option for 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 AddNumbers. The DLL Example.dll may also have to be copied to the location where the .exe file would be generated by the following code.

#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;
}

The 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 (the current working directory goes before system library locations), and thus to a malicious version of the library. See the reference for Microsoft's guidance on safe library loading: one should use SetDllDirectoryW in kernel32 to remove the current-directory lookup before any libraries are loaded.[9]

The 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)