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53-666: Microsoft MACRO-80 (often shortened to M80 ) is a relocatable macro assembler for Intel 8080 and Zilog Z80 microcomputer systems. The complete MACRO-80 package includes the MACRO-80 Assembler , the LINK-80 Linking Loader , and the CREF-80 Cross Reference Facility . The LIB-80 Library Manager is included in CP/M versions only. The list price at the time was $ 200. A MACRO-80 source program consists of

106-473: 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,

159-529: 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 . DLLs provide

212-486: A DLL at compile-time. Delphi 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 AddNumbers . The DLL file 'Example.dll' may also have to be copied to

265-406: 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

318-412: 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. The 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

371-661: A file extension that describes the content of the file such as .ocx for ActiveX controls and .drv for a legacy (16-bit) device driver . A DLL that contains only resources can be called a resource DLL . Examples include the icon library , sometimes having extension .icl , and font library having extensions .fon and .fot . The file format of a DLL is the same as for an executable (a.k.a. EXE ), but different versions of Windows use different formats. 32-bit and 64-bit Windows versions use Portable Executable (PE), and 16-bit Windows versions use New Executable (NE). The main difference between DLL and EXE

424-669: 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

477-536: 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 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

530-603: A series of statements . Each statement must follow a predefined format. Source lines up to 132 characters in length are supported. M80 accepts source files almost identical to files for Intel -compatible assemblers. It also supports several switches in the command string. Some can be used to control the format of the source file. A switch can be set to allow support for Z80 mnemonics . MACRO-80 runs on Digital Research CP/M , Intel ISIS-II , Tandy TRSDOS , Tektronix TEKDOS , and Microsoft MSX-DOS . This article about software created, produced or developed by Microsoft

583-474: 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),

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636-449: 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

689-482: Is a stub . You can help Misplaced Pages by expanding it . Relocation (computing) Relocation is typically done by the linker at link time , but it can also be done at load time by a relocating loader , or at run time by the running program itself . Some architectures avoid relocation entirely by deferring address assignment to run time; as, for example, in stack machines with zero address arithmetic or in some segmented architectures where every compilation unit

742-406: 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

795-419: 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

848-451: 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

901-529: Is faster to link to the DLL directly. 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

954-450: Is loaded into a separate segment. Object files are segmented into various memory segment or section types. Example segment types include code segment (.text) , initialized data segment (.data) , uninitialized data segment (.bss) , or others as established by the programmer, such as common segments, or named static segments. The relocation table is a list of pointers created by the translator (a compiler or assembler ) and stored in

1007-455: 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

1060-401: Is not mandatory to provide relocation tables for EXE files, since they are the first image loaded into the virtual address space and thus will be loaded at their preferred base address. For both DLLs and for EXEs which opt into address space layout randomization (ASLR), an exploit mitigation technique introduced with Windows Vista , relocation tables once again become mandatory because of

1113-402: 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

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1166-415: 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. The first versions of Microsoft Windows ran programs together in

1219-411: 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

1272-500: 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

1325-453: 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

1378-460: 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

1431-428: 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,

1484-407: 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

1537-431: 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

1590-427: 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. 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

1643-705: The IAT). At run-time, the IAT is filled with appropriate addresses that point directly to a function in the separately loaded DLL. 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 ). Although its GNU Binutils toolchain can generate import libraries and link to them, it

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1696-644: 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

1749-463: The actual address of code/data depends on where the program is loaded in memory and this is not known until the program is loaded. Instead, segments are relative values in the DOS EXE file. These segments need to be corrected, when the executable has been loaded into memory. The EXE loader uses a relocation table to find the segments which need to be adjusted. With 32-bit Windows operating systems, it

1802-481: 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

1855-541: 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. 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

1908-402: 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

1961-510: 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. 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

2014-404: 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

2067-411: The current working directory from the DLL search path. The Python ctypes binding will use POSIX API on POSIX systems. The 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

2120-413: The current working directory is looked up before the system library directories), and thus to a malicious version of the library. See the reference for Microsoft's guidance on safe library loading: one should use SetDefaultDllDirectories in kernel32 to remove both the application directory and the current working directory from the DLL search path, or use SetDllDirectoryW in kernel32 to remove

2173-487: 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,

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2226-734: The frame buffer. When drawing to a printer, the API calls had to be transformed into requests to a printer. Although it could have been possible to provide hard-coded support for a limited set of devices (like the Color Graphics Adapter display, the HP LaserJet Printer Command Language ), Microsoft chose a different approach. GDI would work by loading different pieces of code, called " device drivers ", to work with different output devices. The same architectural concept that allowed GDI to load different device drivers also allowed

2279-462: The loader moves through the table. In some architectures a fixup that crosses certain boundaries (such as a segment boundary) or that is not aligned on a word boundary is illegal and flagged as an error by the linker. Far pointers ( 32-bit pointers with segment :offset, used to address 20-bit 640 KB memory space available to DOS programs ), which point to code or data within a DOS executable ( EXE ), do not have absolute segments, because

2332-649: The location where the .exe file would be generated by the following code: 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 (unless explicitly excluded the application directory goes before system library locations, and without HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\SafeDllSearchMode or HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\CWDIllegalInDLLSearch

2385-634: The object files and performs relocation by: The following example uses Donald Knuth 's MIX architecture and MIXAL assembly language. The principles are the same for any architecture, though the details will change. 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 , in any combination. A DLL file often has file extension .dll , but can have any file extension. Developers can choose to use

2438-437: The object or executable file. Each entry in the table, or "fixup", is a pointer to an absolute address in the object code that must be changed when the loader relocates the program so that it will refer to the correct location. Fixups are designed to support relocation of the program as a complete unit. In some cases, each fixup in the table is itself relative to a base address of zero, so the fixups themselves must be changed as

2491-557: The performance of the Pentium Pro microprocessor when launched, and ultimately limited the stability and scalability of the DOS-based versions of Windows. Although the DLL technology is core to the Windows architecture, it has drawbacks. DLL hell describes the bad behavior of an application when the wrong version of a DLL is consumed. Mitigation strategies include: The executable code of

2544-544: The possibility that the binary may be dynamically moved before being executed, even though they are still the first thing loaded in the virtual address space. When running native 64-bit binaries on Windows Vista and above, ASLR is mandatory , and thus relocation sections cannot be omitted by the compiler. The Executable and Linkable Format (ELF) executable format and shared library format used by most Unix-like systems allows several types of relocation to be defined. The linker reads segment information and relocation tables in

2597-509: 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

2650-487: 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

2703-427: 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

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2756-488: 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

2809-486: Was implemented in a DLL called GDI.EXE , the user interface in USER.EXE . These extra layers on top of DOS had to be shared across all running Windows programs, not just to enable Windows to work in a machine with less than a megabyte of RAM, but to enable the programs to co-operate with each other. The code in GDI needed to translate drawing commands to operations on specific devices. On the display, it had to manipulate pixels in

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