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46-732: [REDACTED] Look up relocation in Wiktionary, the free dictionary. Relocation may refer to: Relocation (computing) Relocation of professional sports teams Major League Baseball relocations of 1950s–1960s Relocation of professional sports teams in Australia and New Zealand Relocation of professional sports teams in China Relocation of professional sports teams in Europe Relocation of professional sports teams in

92-405: A byte and a word are primitives). A data aggregate (or just aggregate ) is a group of primitives that are logically contiguous in memory and that are viewed collectively as one datum (for instance, an aggregate could be 3 logically contiguous bytes, the values of which represent the 3 coordinates of a point in space). When an aggregate is entirely composed of the same type of primitive,

138-530: A DOS executable ( EXE ), do not have absolute segments, because 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

184-412: A is later accessed again, its new value will be 8. This example may be clearer if memory is examined directly. Assume that a is located at address 0x8130 in memory and ptr at 0x8134; also assume this is a 32-bit machine such that an int is 32-bits wide. The following is what would be in memory after the following code snippet is executed: (The NULL pointer shown here is 0x00000000.) By assigning

230-418: A pointer is an object in many programming languages that stores a memory address . This can be that of another value located in computer memory , or in some cases, that of memory-mapped computer hardware . A pointer references a location in memory, and obtaining the value stored at that location is known as dereferencing the pointer. As an analogy, a page number in a book's index could be considered

276-724: A consistent interface, some architectures provide memory-mapped I/O , which allows some addresses to refer to units of memory while others refer to device registers of other devices in the computer. There are analogous concepts such as file offsets, array indices, and remote object references that serve some of the same purposes as addresses for other types of objects. Pointers are directly supported without restrictions in languages such as PL/I , C , C++ , Pascal , FreeBASIC , and implicitly in most assembly languages . They are primarily used for constructing references , which in turn are fundamental to constructing nearly all data structures , as well as in passing data between different parts of

322-491: A form of manual memory segmentation , and share many of its advantages and disadvantages. A two-byte offset, containing a 16-bit, unsigned integer, can be used to provide relative addressing for up to 64 KiB (2 bytes) of a data structure. This can easily be extended to 128, 256 or 512 KiB if the address pointed to is forced to be aligned on a half-word, word or double-word boundary (but, requiring an additional "shift left" bitwise operation —by 1, 2 or 3 bits—in order to adjust

368-565: A function. Pointers can also be used to allocate and deallocate dynamic variables and arrays in memory. Since a variable will often become redundant after it has served its purpose, it is a waste of memory to keep it, and therefore it is good practice to deallocate it (using the original pointer reference) when it is no longer needed. Failure to do so may result in a memory leak (where available free memory gradually, or in severe cases rapidly, diminishes because of an accumulation of numerous redundant memory blocks). The basic syntax to define

414-400: A level of indirection: A pointer's value determines which memory address (that is, which datum) is to be used in a calculation. Because indirection is a fundamental aspect of algorithms, pointers are often expressed as a fundamental data type in programming languages ; in statically (or strongly ) typed programming languages, the type of a pointer determines the type of the datum to which

460-401: A memory address, as opposed to a magic cookie or capability which does not allow such. Because pointers allow both protected and unprotected access to memory addresses , there are risks associated with using them, particularly in the latter case. Primitive pointers are often stored in a format similar to an integer ; however, attempting to dereference or "look up" such a pointer whose value

506-443: A non-canonical pointer is dereferenced, the processor raises a general protection fault . On the other hand, some systems have more units of memory than there are addresses. In this case, a more complex scheme such as memory segmentation or paging is employed to use different parts of the memory at different times. The last incarnations of the x86 architecture support up to 36 bits of physical memory addresses, which were mapped to

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552-404: A pointer as an arrow that points to a specific spot in a computer's memory, allowing you to interact with the data stored at that location. A memory pointer (or just pointer ) is a primitive, the value of which is intended to be used as a memory address; it is said that a pointer points to a memory address . It is also said that a pointer points to a datum [in memory] when the pointer's value

598-475: A pointer be explicitly initialized to the null pointer value, which is traditionally specified in C with the standardized macro NULL : Dereferencing a null pointer in C produces undefined behavior , which could be catastrophic. However, most implementations simply halt execution of the program in question, usually with a segmentation fault . However, initializing pointers unnecessarily could hinder program analysis, thereby hiding bugs. In any case, once

644-443: A pointer has been declared, the next logical step is for it to point at something: This assigns the value of the address of a to ptr . For example, if a is stored at memory location of 0x8130 then the value of ptr will be 0x8130 after the assignment. To dereference the pointer, an asterisk is used again: This means take the contents of ptr (which is 0x8130), "locate" that address in memory and set its value to 8. If

690-415: A pointer is: This declares ptr as the identifier of an object of the following type: This is usually stated more succinctly as " ptr is a pointer to int ." Because the C language does not specify an implicit initialization for objects of automatic storage duration, care should often be taken to ensure that the address to which ptr points is valid; this is why it is sometimes suggested that

736-515: A pointer to the corresponding page; dereferencing such a pointer would be done by flipping to the page with the given page number and reading the text found on that page. The actual format and content of a pointer variable is dependent on the underlying computer architecture . Using pointers significantly improves performance for repetitive operations, like traversing iterable data structures (e.g. strings , lookup tables , control tables , linked lists , and tree structures). In particular, it

782-420: A pointer to the desired data element in the array. In other data structures, such as linked lists , pointers are used as references to explicitly tie one piece of the structure to another. Pointers are used to pass parameters by reference. This is useful if the programmer wants a function's modifications to a parameter to be visible to the function's caller. This is also useful for returning multiple values from

828-538: A program. In functional programming languages that rely heavily on lists, data references are managed abstractly by using primitive constructs like cons and the corresponding elements car and cdr , which can be thought of as specialised pointers to the first and second components of a cons-cell. This gives rise to some of the idiomatic "flavour" of functional programming. By structuring data in such cons-lists , these languages facilitate recursive means for building and processing data—for example, by recursively accessing

874-516: A usable sequential index and then to an absolute address without a lookup table . In C, array indexing is formally defined in terms of pointer arithmetic; that is, the language specification requires that array[i] be equivalent to *(array + i) . Thus in C, arrays can be thought of as pointers to consecutive areas of memory (with no gaps), and the syntax for accessing arrays is identical for that which can be used to dereference pointers. For example, an array array can be declared and used in

920-506: A value that is both a valid memory address and within the numerical range that the processor is capable of addressing. In 1955, Soviet Ukrainian computer scientist Kateryna Yushchenko created the Address programming language that made possible indirect addressing and addresses of the highest rank – analogous to pointers. This language was widely used on the Soviet Union computers. However, it

966-468: A very large array . The system would then also provide an operation to retrieve the value stored in the memory unit at a given address (usually utilizing the machine's general-purpose registers ). In the usual case, a pointer is large enough to hold more addresses than there are units of memory in the system. This introduces the possibility that a program may attempt to access an address which corresponds to no unit of memory, either because not enough memory

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1012-461: Is a simple, more concrete implementation of the more abstract reference data type . Several languages, especially low-level languages , support some type of pointer, although some have more restrictions on their use than others. While "pointer" has been used to refer to references in general, it more properly applies to data structures whose interface explicitly allows the pointer to be manipulated (arithmetically via pointer arithmetic ) as

1058-487: Is installed (i.e. beyond the range of available memory) or the architecture does not support such addresses. The first case may, in certain platforms such as the Intel x86 architecture, be called a segmentation fault (segfault). The second case is possible in the current implementation of AMD64 , where pointers are 64 bit long and addresses only extend to 48 bits. Pointers must conform to certain rules (canonical addresses), so if

1104-511: Is necessary to have pointers to help manage how the structure is implemented and controlled. Typical examples of pointers are start pointers, end pointers, and stack pointers. These pointers can either be absolute (the actual physical address or a virtual address in virtual memory ) or relative (an offset from an absolute start address ("base") that typically uses fewer bits than a full address, but will usually require one additional arithmetic operation to resolve). Relative addresses are

1150-422: Is not a valid memory address could cause a program to crash (or contain invalid data). To alleviate this potential problem, as a matter of type safety , pointers are considered a separate type parameterized by the type of data they point to, even if the underlying representation is an integer. Other measures may also be taken (such as validation and bounds checking ), to verify that the pointer variable contains

1196-401: 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

1242-458: Is often much cheaper in time and space to copy and dereference pointers than it is to copy and access the data to which the pointers point. Pointers are also used to hold the addresses of entry points for called subroutines in procedural programming and for run-time linking to dynamic link libraries (DLLs) . In object-oriented programming , pointers to functions are used for binding methods , often using virtual method tables . A pointer

1288-416: Is the datum's memory address. More generally, a pointer is a kind of reference , and it is said that a pointer references a datum stored somewhere in memory ; to obtain that datum is to dereference the pointer . The feature that separates pointers from other kinds of reference is that a pointer's value is meant to be interpreted as a memory address, which is a rather low-level concept. References serve as

1334-649: 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 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

1380-442: The sizeof operator differs. In this example, sizeof(array) will evaluate to 5*sizeof(int) (the size of the array), while sizeof(ptr) will evaluate to sizeof(int*) , the size of the pointer itself. Default values of an array can be declared like: If array is located in memory starting at address 0x1000 on a 32-bit little-endian machine then memory will contain the following (values are in hexadecimal , like

1426-532: The 32-bit linear address space through the PAE paging mechanism. Thus, only 1/16 of the possible total memory may be accessed at a time. Another example in the same computer family was the 16-bit protected mode of the 80286 processor, which, though supporting only 16 MB of physical memory, could access up to 1 GB of virtual memory, but the combination of 16-bit address and segment registers made accessing more than 64 KB in one data structure cumbersome. In order to provide

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1472-526: The United States and Canada Relocation (personal) , the process of vacating a fixed residence for a different one Population transfer Rental relocation Structure relocation Car relocation See also [ edit ] Relocation service Myka Relocate Moving (disambiguation) Relocation center → Internment Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with

1518-486: The address of a to ptr : yields the following memory values: Then by dereferencing ptr by coding: the computer will take the contents of ptr (which is 0x8130), 'locate' that address, and assign 8 to that location yielding the following memory: Clearly, accessing a will yield the value of 8 because the previous instruction modified the contents of a by way of the pointer ptr . When setting up data structures like lists , queues and trees, it

1564-403: The aggregate may be called an array ; in a sense, a multi-byte word primitive is an array of bytes, and some programs use words in this way. A pointer is a programming concept used in computer science to reference or point to a memory location that stores a value or an object. It is essentially a variable that stores the memory address of another variable or data structure rather than storing

1610-665: The concepts appeared in the June 1967 issue of CACM entitled: PL/I List Processing. According to the Oxford English Dictionary , the word pointer first appeared in print as a stack pointer in a technical memorandum by the System Development Corporation . In computer science , a pointer is a kind of reference . A data primitive (or just primitive ) is any datum that can be read from or written to computer memory using one memory access (for instance, both

1656-474: The data itself. Pointers are commonly used in programming languages that support direct memory manipulation, such as C and C++. They allow programmers to work with memory directly, enabling efficient memory management and more complex data structures. By using pointers, you can access and modify data located in memory, pass data efficiently between functions, and create dynamic data structures like linked lists, trees, and graphs. In simpler terms, you can think of

1702-406: The following manner: This allocates a block of five integers and names the block array , which acts as a pointer to the block. Another common use of pointers is to point to dynamically allocated memory from malloc which returns a consecutive block of memory of no less than the requested size that can be used as an array. While most operators on arrays and pointers are equivalent, the result of

1748-440: The head and tail elements of lists of lists; e.g. "taking the car of the cdr of the cdr". By contrast, memory management based on pointer dereferencing in some approximation of an array of memory addresses facilitates treating variables as slots into which data can be assigned imperatively . When dealing with arrays, the critical lookup operation typically involves a stage called address calculation which involves constructing

1794-477: 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. Pointer (computer programming) I do consider assignment statements and pointer variables to be among computer science's "most valuable treasures." Donald Knuth , Structured Programming, with go to Statements In computer science ,

1840-509: The offset by a factor of 2, 4 or 8, before its addition to the base address). Generally, though, such schemes are a lot of trouble, and for convenience to the programmer absolute addresses (and underlying that, a flat address space ) is preferred. A one byte offset, such as the hexadecimal ASCII value of a character (e.g. X'29') can be used to point to an alternative integer value (or index) in an array (e.g., X'01'). In this way, characters can be very efficiently translated from ' raw data ' to

1886-437: The pointer points. Pointers are a very thin abstraction on top of the addressing capabilities provided by most modern architectures . In the simplest scheme, an address , or a numeric index , is assigned to each unit of memory in the system, where the unit is typically either a byte or a word – depending on whether the architecture is byte-addressable or word-addressable – effectively transforming all of memory into

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1932-544: 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

1978-556: 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 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

2024-460: 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 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

2070-472: The title Relocation . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=Relocation&oldid=1225599371 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages Relocation (computing) Relocation

2116-528: Was unknown outside the Soviet Union and usually Harold Lawson is credited with the invention, in 1964, of the pointer. In 2000, Lawson was presented the Computer Pioneer Award by the IEEE "[f]or inventing the pointer variable and introducing this concept into PL/I, thus providing for the first time, the capability to flexibly treat linked lists in a general-purpose high-level language". His seminal paper on

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