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47-451: The original Unix object file format a.out is unable to adequately support shared libraries , foreign format identification, or explicit address linkage. As development of Unix-like systems continued both inside and outside AT&T , different solutions to these and other issues emerged. COFF was introduced in 1983, in AT&;T's UNIX System V for non- VAX 32-bit platforms such as

94-468: A Communication Pool (COMPOOL), roughly a library of header files. Another major contributor to the modern library concept came in the form of the subprogram innovation of FORTRAN . FORTRAN subprograms can be compiled independently of each other, but the compiler lacked a linker . So prior to the introduction of modules in Fortran-90, type checking between FORTRAN subprograms was impossible. By

141-559: A feature called smart linking whereby the linker is aware of or integrated with the compiler, such that the linker knows how external references are used, and code in a library that is never actually used , even though internally referenced, can be discarded from the compiled application. For example, a program that only uses integers for arithmetic, or does no arithmetic operations at all, can exclude floating-point library routines. This smart-linking feature can lead to smaller application file sizes and reduced memory usage. Some references in

188-404: A library, a programmer only needs to know high-level information such as what items it contains at and how to use the items – not all of the internal details of the library. Libraries can use other libraries resulting in a hierarchy of libraries in a program. A library of executable code has a well-defined interface by which the functionality is invoked. For example, in C , a library function

235-423: A process. The range of virtual addresses usually starts at a low address and can extend to the highest address allowed by the computer's instruction set architecture and supported by the operating system 's pointer size implementation, which can be 4 bytes for 32-bit or 8 bytes for 64-bit OS versions. This provides several benefits, one of which is security through process isolation assuming each process

282-436: A program are loaded from individual shared objects into memory at load time or runtime , rather than being copied by a linker when it creates a single monolithic executable file for the program. Shared libraries can be statically linked during compile-time, meaning that references to the library modules are resolved and the modules are allocated memory when the executable file is created. But often linking of shared libraries

329-589: A program or library module are stored in a relative or symbolic form which cannot be resolved until all code and libraries are assigned final static addresses. Relocation is the process of adjusting these references, and is done either by the linker or the loader . In general, relocation cannot be done to individual libraries themselves because the addresses in memory may vary depending on the program using them and other libraries they are combined with. Position-independent code avoids references to absolute addresses and therefore does not require relocation. When linking

376-407: A suffix of .a ( archive , static library) or of .so (shared object, dynamically linked library). Some systems might have multiple names for a dynamically linked library. These names typically share the same prefix and have different suffixes indicating the version number. Most of the names are names for symbolic links to the latest version. For example, on some systems libfoo.so.2 would be

423-440: Is created (static linking), or whenever the program is used at runtime (dynamic linking). The references being resolved may be addresses for jumps and other routine calls. They may be in the main program, or in one module depending upon another. They are resolved into fixed or relocatable addresses (from a common base) by allocating runtime memory for the memory segments of each module referenced. Some programming languages use

470-403: Is given a separate address space . When a new application on a 32-bit OS is executed, the process has a 4 GiB VAS: each one of the memory addresses (from 0 to 2 − 1) in that space can have a single byte as a value. Initially, none of them have values ('-' represents no value). Using or setting values in such a VAS would cause a memory exception . Then the application's executable file

517-400: Is invoked via C's normal function call capability. The linker generates code to call a function via the library mechanism if the function is available from a library instead of from the program itself. The functions of a library can be connected to the invoking program at different program lifecycle phases . If the code of the library is accessed during the build of the invoking program, then

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564-479: Is leveraged during software development to implement a computer program . Historically, a library consisted of subroutines (generally called functions today). The concept now includes other forms of executable code including classes and non-executable data including images and text . It can also refer to a collection of source code . For example, a program could use a library to indirectly make system calls instead of making those system calls directly in

611-538: Is mapped into the VAS. Addresses in the process VAS are mapped to bytes in the exe file. The OS manages the mapping: The v's are values from bytes in the mapped file . Then, required DLL files are mapped (this includes custom libraries as well as system ones such as kernel32.dll and user32.dll ): The process then starts executing bytes in the EXE file. However, the only way the process can use or set '-' values in its VAS

658-420: Is not usually known where in memory it will be loaded. The virtual address where the first byte of the file will be loaded is called image base address . The rest of the file is not necessarily loaded in a contiguous block, but in different sections . Relative virtual addresses (RVAs) are not to be confused with standard virtual addresses. A relative virtual address is the virtual address of an object from

705-430: Is performed during the creation of an executable or another object file, it is known as static linking or early binding . In this case, the linking is usually done by a linker , but may also be done by the compiler . A static library , also known as an archive , is one intended to be statically linked. Originally, only static libraries existed. Static linking must be performed when any modules are recompiled. All of

752-500: Is postponed until they are loaded. Although originally pioneered in the 1960s, dynamic linking did not reach the most commonly-used operating systems until the late 1980s. It was generally available in some form in most operating systems by the early 1990s. During this same period, object-oriented programming (OOP) was becoming a significant part of the programming landscape. OOP with runtime binding requires additional information that traditional libraries do not supply. In addition to

799-503: Is to ask the OS to map them to bytes from a file. A common way to use VAS memory in this way is to map it to the page file . The page file is a single file, but multiple distinct sets of contiguous bytes can be mapped into a VAS: And different parts of the page file can map into the VAS of different processes: On Microsoft Windows 32-bit, by default, only 2 GiB are made available to processes for their own use. The other 2 GiB are used by

846-631: The 3B20 . Improvements over the existing AT&T a.out format included arbitrary sections, explicit processor declarations, and explicit address linkage. However, the COFF design was both too limited and incompletely specified: there was a limit on the maximum number of sections, a limit on the length of section names, included source files, and the symbolic debugging information was incapable of supporting real world languages such as C , much less newer languages like C++ , or new processors. All real world implementations of COFF were necessarily violations of

893-533: The UNIX world, which uses different file extensions, when linking against .LIB file in Windows one must first know if it is a regular static library or an import library. In the latter case, a .DLL file must be present at runtime. Virtual address In computing , a virtual address space ( VAS ) or address space is the set of ranges of virtual addresses that an operating system makes available to

940-519: The COFF symbol table. Each symbol table entry includes a name, storage class, type, value and section number. Short names (8 characters or fewer) are stored directly in the symbol table; longer names are stored as an offset into the string table at the end of the COFF object. Storage classes describe the type entity the symbol represents, and may include external variables (C_EXT), automatic (stack) variables (C_AUTO), register variables (C_REG), functions (C_FCN), and many others. The symbol type describes

987-473: The date and time that the object file was created as a 32-bit binary integer, representing the number of seconds since the Unix epoch , 1 January 1970 00:00:00  UTC . Dates occurring after 19 January 2038 cannot be stored in this format, resulting in an instance of the year 2038 problem . Library (computing) In computer science , a library is a collection of resources that

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1034-556: The dependencies to external libraries in build configuration files (such as a Maven Pom in Java). Another library technique uses completely separate executables (often in some lightweight form) and calls them using a remote procedure call (RPC) over a network to another computer. This maximizes operating system re-use: the code needed to support the library is the same code being used to provide application support and security for every other program. Additionally, such systems do not require

1081-532: The engine would have a library of its own." In 1947 Goldstine and von Neumann speculated that it would be useful to create a "library" of subroutines for their work on the IAS machine , an early computer that was not yet operational at that time. They envisioned a physical library of magnetic wire recordings , with each wire storing reusable computer code. Inspired by von Neumann, Wilkes and his team constructed EDSAC . A filing cabinet of punched tape held

1128-475: The file once it is loaded into memory, minus the base address of the file image. If the file were to be mapped literally from disk to memory, the RVA would be the same as that of the offset into the file, but this is actually quite unusual. Note that the RVA term is only used with objects in the image file. Once loaded into memory, the image base address is added, and ordinary VAs are used. The COFF file header stores

1175-536: The filename for the second major interface revision of the dynamically linked library libfoo . The .la files sometimes found in the library directories are libtool archives, not usable by the system as such. The system inherits static library conventions from BSD , with the library stored in a .a file, and can use .so -style dynamically linked libraries (with the .dylib suffix instead). Most libraries in macOS, however, consist of "frameworks", placed inside special directories called " bundles " which wrap

1222-439: The instantiated objects residing only in memory (although potentially able to be made persistent in separate files). In others, like Smalltalk , the class libraries are merely the starting point for a system image that includes the entire state of the environment, classes and all instantiated objects. Today most class libraries are stored in a package repository (such as Maven Central for Java). Client code explicitly declare

1269-442: The interpretation of the symbol entity's value and includes values for all the C data types. When compiled with appropriate options, a COFF object file will contain line number information for each possible break point in the text section of the object file. Line number information takes two forms: in the first, for each possible break point in the code, the line number table entry records the address and its matching line number. In

1316-474: The library is called a static library . An alternative is to build the program executable to be separate from the library file. The library functions are connected after the executable is started, either at load-time or runtime . In this case, the library is called a dynamic library . Most compiled languages have a standard library , although programmers can also create their own custom libraries. Most modern software systems provide libraries that implement

1363-958: The library to exist on the same machine, but can forward the requests over the network. However, such an approach means that every library call requires a considerable amount of overhead. RPC calls are much more expensive than calling a shared library that has already been loaded on the same machine. This approach is commonly used in a distributed architecture that makes heavy use of such remote calls, notably client-server systems and application servers such as Enterprise JavaBeans . Code generation libraries are high-level APIs that can generate or transform byte code for Java . They are used by aspect-oriented programming , some data access frameworks, and for testing to generate dynamic proxy objects. They also are used to intercept field access. The system stores libfoo.a and libfoo.so files in directories such as /lib , /usr/lib or /usr/local/lib . The filenames always start with lib , and end with

1410-463: The library's required files and metadata. For example, a framework called MyFramework would be implemented in a bundle called MyFramework.framework , with MyFramework.framework/MyFramework being either the dynamically linked library file or being a symlink to the dynamically linked library file in MyFramework.framework/Versions/Current/MyFramework . Dynamic-link libraries usually have

1457-501: The majority of the system services. Such libraries have organized the services which a modern application requires. As such, most code used by modern applications is provided in these system libraries. The idea of a computer library dates back to the first computers created by Charles Babbage . An 1888 paper on his Analytical Engine suggested that computer operations could be punched on separate cards from numerical input. If these operation punch cards were saved for reuse then "by degrees

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1504-528: The mid 1960s, copy and macro libraries for assemblers were common. Starting with the popularity of the IBM System/360 , libraries containing other types of text elements, e.g., system parameters, also became common. In IBM's OS/360 and its successors this is called a partitioned data set . The first object-oriented programming language, Simula , developed in 1965, supported adding classes to libraries via its compiler. Libraries are important in

1551-488: The modules required by a program are sometimes statically linked and copied into the executable file. This process, and the resulting stand-alone file, is known as a static build of the program. A static build may not need any further relocation if virtual memory is used and no address space layout randomization is desired. A shared library or shared object is a file that is intended to be shared by executable files and further shared object files . Modules used by

1598-790: The most widespread use of the COFF format today is in Microsoft 's Portable Executable (PE) format. Developed for Windows NT , the PE format (sometimes written as PE/COFF) uses a COFF header for object files , and as a component of the PE header for executable files. COFF's main improvement over a.out was the introduction of multiple named sections in the object file. Different object files could have different numbers and types of sections. The COFF symbolic debugging information consists of symbolic ( string ) names for program functions and variables, and line number information, used for setting breakpoints and tracing execution. Symbolic names are stored in

1645-472: The names and entry points of the code located within, they also require a list of the objects they depend on. This is a side-effect of one of OOP's core concepts, inheritance, which means that parts of the complete definition of any method may be in different places. This is more than simply listing that one library requires the services of another: in a true OOP system, the libraries themselves may not be known at compile time , and vary from system to system. At

1692-657: The operating system artificially limits the user mode portion of the process's virtual address space to 2 GiB. This applies to both 32- and 64-bit executables. Processes running executables that were linked with the /LARGEADDRESSAWARE:YES option, which is the default for 64-bit Visual Studio 2010 and later, have access to more than 2 GiB of virtual address space: up to 4 GiB for 32-bit executables, up to 8 TiB for 64-bit executables in Windows through Windows 8, and up to 128 TiB for 64-bit executables in Windows 8.1 and later. Allocating memory via C 's malloc establishes

1739-435: The operating system. On later 32-bit editions of Microsoft Windows, it is possible to extend the user-mode virtual address space to 3 GiB while only 1 GiB is left for kernel-mode virtual address space by marking the programs as IMAGE_FILE_LARGE_ADDRESS_AWARE and enabling the /3GB switch in the boot.ini file. On Microsoft Windows 64-bit, in a process running an executable that was linked with /LARGEADDRESSAWARE:NO ,

1786-430: The program linking or binding process, which resolves references known as links or symbols to library modules. The linking process is usually automatically done by a linker or binder program that searches a set of libraries and other modules in a given order. Usually it is not considered an error if a link target can be found multiple times in a given set of libraries. Linking may be done when an executable file

1833-404: The program. A library can be used by multiple, independent consumers (programs and other libraries). This differs from resources defined in a program which can usually only be used by that program. When a consumer uses a library resource, it gains the value of the library without having to implement it itself. Libraries encourage code reuse in a modular fashion. When writing code that uses

1880-424: The rough OOP equivalent of older types of code libraries. They contain classes , which describe characteristics and define actions ( methods ) that involve objects. Class libraries are used to create instances , or objects with their characteristics set to specific values. In some OOP languages, like Java , the distinction is clear, with the classes often contained in library files (like Java's JAR file format ) and

1927-424: The same time many developers worked on the idea of multi-tier programs, in which a "display" running on a desktop computer would use the services of a mainframe or minicomputer for data storage or processing. For instance, a program on a GUI-based computer would send messages to a minicomputer to return small samples of a huge dataset for display. Remote procedure calls (RPC) already handled these tasks, but there

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1974-407: The second form, the entry identifies a symbol table entry representing the start of a function, enabling a breakpoint to be set using the function's name. Note that COFF was not capable of representing line numbers or debugging symbols for included source as with header files rendering the COFF debugging information virtually useless without incompatible extensions. When a COFF file is generated, it

2021-560: The standard as a result. This led to numerous COFF extensions. IBM used the XCOFF format in AIX ; DEC , SGI and others used ECOFF ; and numerous SysV ports and tool chains targeting embedded development each created their own, incompatible, variations. With the release of SVR4, AT&T replaced COFF with ELF . While extended versions of COFF continue to be used for some Unix and Unix-like platforms, primarily in embedded systems , perhaps

2068-528: The status of the "next big thing" in the programming world. There were a number of efforts to create systems that would run across platforms, and companies competed to try to get developers locked into their own system. Examples include IBM 's System Object Model (SOM/DSOM), Sun Microsystems ' Distributed Objects Everywhere (DOE), NeXT 's Portable Distributed Objects (PDO), Digital 's ObjectBroker , Microsoft's Component Object Model (COM/DCOM), and any number of CORBA -based systems. Class libraries are

2115-521: The subroutine library for this computer. Programs for EDSAC consisted of a main program and a sequence of subroutines copied from the subroutine library. In 1951 the team published the first textbook on programming, The Preparation of Programs for an Electronic Digital Computer , which detailed the creation and the purpose of the library. COBOL included "primitive capabilities for a library system" in 1959, but Jean Sammet described them as "inadequate library facilities" in retrospect. JOVIAL has

2162-475: The suffix *.DLL , although other file name extensions may identify specific-purpose dynamically linked libraries, e.g. *.OCX for OLE libraries. The interface revisions are either encoded in the file names, or abstracted away using COM-object interfaces. Depending on how they are compiled, *.LIB files can be either static libraries or representations of dynamically linkable libraries needed only during compilation, known as " import libraries ". Unlike in

2209-467: Was no standard RPC system. Soon the majority of the minicomputer and mainframe vendors instigated projects to combine the two, producing an OOP library format that could be used anywhere. Such systems were known as object libraries , or distributed objects , if they supported remote access (not all did). Microsoft's COM is an example of such a system for local use. DCOM, a modified version of COM, supports remote access. For some time object libraries held

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