It has been suggested that Software pointer be merged into this article or section. (Discuss) In computer science, a pointer is a programming language datatype whose value refers directly to ("points to") another value stored elsewhere in the computer memory using its address. Obtaining the value that a pointer refers to is called dereferencing the pointer. A pointer is a simple implementation of the general reference datatype (although it is quite different from the facility referred to as a reference in C++). Image File history File links Please see the file description page for further information. ...
It has been suggested that this article or section be merged into Pointer. ...
Computer science is the study of the theoretical foundations of information and computation and their implementation and application in computer systems. ...
Computer code (HTML with JavaScript) in a tool that uses syntax highlighting (colors) to help the developer see the purpose of each piece of code. ...
In computer science, a datatype or data type (often simply a type) is a name or label for a set of values and some operations which one can perform on that set of values. ...
The terms storage (U.K.) or memory (U.S.) refer to the parts of a digital computer that retain physical state (data) for some interval of time, possibly even after electrical power to the computer is turned off. ...
This article discusses a general notion of reference in computing. ...
In the C++ programming language, a reference is a simple reference datatype that is less powerful but safer than the pointer type inherited from C, which is a reference in the general sense but not in the sense used by C++. // Syntax and terminology The declaration of the form <Type...
Pointers are so commonly used as references that sometimes people use the word "pointer" to refer to references in general; however, more properly it only applies to data structures whose interface explicitly allows it to be manipulated as a memory address. If you are seeking general information on a small piece of data used to find an object, see reference (computer science). This article discusses a general notion of reference in computing. ...
Architectural roots 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, effectively transforming all of memory into a very large array. Then, if we have an address, the system provides an operation to retrieve the value stored in the memory unit at that address. Pointers are datatypes which hold addresses. See reference (computer science). In computer science, a memory address is a unique identifier for a memory location at which a CPU or other device can store a piece of data for later retrieval. ...
Look up Index in Wiktionary, the free dictionary Index can be defined as: an ordered list, plural indexes a number or variable, plural indices. ...
A byte is commonly used as a unit of storage measurement in computers, regardless of the type of data being stored. ...
In computing, word is a term for the natural unit of data used by a particular computer design. ...
In computer programming, an array, also known as a vector or list (for one-dimensional arrays) or a matrix (for two-dimensional arrays), is one of the simplest data structures. ...
In computer science, a datatype or data type (often simply a type) is a name or label for a set of values and some operations which one can perform on that set of values. ...
This article discusses a general notion of reference in computing. ...
In the usual case, a pointer is large enough to hold more different 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, called a segmentation 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. A segmentation fault (sometimes referred to as segfault for short) is a particular error condition that can occur during the operation of computer software. ...
Segmentation is one of the most common ways to achieve memory protection; another common one is paging. ...
In computer operating systems, paging memory allocation algorithms divide computer memory into small partitions, and allocates memory using a page as the smallest building block. ...
In order to provide 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. For more generic meanings of input/output port, see port (computing). ...
A Device Register is the view any device presents to a programmer. ...
Uses Pointers are directly supported without restrictions in C, C++, Pascal and 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 a program. The C Programming Language, Brian Kernighan and Dennis Ritchie, the original edition that served for many years as an informal specification of the language The C programming language is a standardized imperative computer programming language developed in the early 1970s by Dennis Ritchie for use on the Unix operating system. ...
C++ (generally pronounced see plus plus) is a general-purpose programming language. ...
Pascal is an imperative computer programming language, developed in 1970 by Niklaus Wirth as a language particularly suitable for structured programming. ...
Assembly language commonly called assembly or asm, is a human-readable notation for the machine language that a specific computer architecture uses. ...
This article discusses a general notion of reference in computing. ...
A binary tree, a simple type of branching linked data structure. ...
When dealing with arrays, the critical lookup operation typically involves a stage called address calculation which involves constructing 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. In computer programming, an array, also known as a vector or list (for one-dimensional arrays) or a matrix (for two-dimensional arrays), is one of the simplest data structures. ...
Pointers are used to pass parameters by reference. This is useful if we want a function's modifications to a parameter to be visible to the function's caller. This is also useful for returning multiple values from a function.
Examples of use Below is an example of the definition of a linked list in C; this is not possible in C without pointers. In computer science, a linked list is one of the fundamental data structures used in computer programming. ...
/* the empty linked list is represented by NULL or some other signal value */ #define EMPTY_LIST NULL struct link { void *data; /* the data of this link */ struct link *next; /* the next link; EMPTY_LIST if this is the last link */ }; Note that this pointer-recursive definition is essentially the same as the reference-recursive definition from the Haskell programming language: Haskell is a standardized pure functional programming language with non-strict semantics named after the logician Haskell Curry. ...
data Link a = Nil {- the empty list -} | Cons a (Link a) {- a cons cell of a value of type a and another link -} The definition with references, however, is type-checked and doesn't use potentially confusing signal values. For this reason, data structures in C are usually dealt with via wrapper functions, which are carefully checked for correctness.
Arrays in C are just pointers to consecutive areas of memory. Thus:
#include <stdio.h> int main() { int array[5] = { 2, 4, 3, 1, 5 }; printf("%pn", array); /* print the address of the array */ printf("%dn", array[0]); /* print the first item of the array, 2 */ printf("%dn", *array); /* print the first integer at the address * pointed to by array; this is the first * item, 2 */ printf("%dn", array[3]); /* print the fourth item of the array, 1 */ printf("%pn", array+3); /* print the third address past array */ printf("%dn", *(array+3)); /* print the value at the address just * printed this is the fourth item, 1 */ return 0; } This activity is called pointer arithmetic: direct arithmetic operations on pointers are used to index arrays. See below for more detail.
Lastly, pointers can be used to pass variables by reference, allowing their value to be changed. For example:
#include <stdio.h> void alter(int *n) { *n = 120; } int main() { int x = 24; int *address = &x; /* the '&' operator (read "reference") retrieves * the address of a variable */ printf("%dn", x); /* show x */ printf("%pn", address); /* show x's address */ alter(&x); /* pass x's address to alter, x is passed "by reference" */ printf("%dn", x); /* show x's new value */ printf("%p %pn", address, &x); /* notice that x's address is not altered */ return 0; }
Typed pointers and casting In many languages, pointers have the additional restriction that the object they point to has a specific type. For example, a pointer may be declared to point to an integer; the language will then attempt to prevent the programmer from pointing it to objects which are not integers, such as floating-point numbers, eliminating some errors. In computer science, a datatype or data type (often simply a type) is a name or label for a set of values and some operations which one can perform on that set of values. ...
The integers consist of the positive natural numbers (1, 2, 3, …), their negatives (−1, −2, −3, ...) and the number zero. ...
A floating-point number is a digital representation for a number in a certain subset of the rational numbers, and is often used to approximate an arbitrary real number on a computer. ...
However, few languages strictly enforce pointer types, because programmers often run into situations where they want to treat an object of one type as though it were of another type. For these cases, it is possible to typecast, or cast, the pointer. Some casts are always safe, while other casts are dangerous, possibly resulting in incorrect behavior. Although it's impossible in general to determine at compile-time which of these casts are safe, some languages store run-time type information which can be used to confirm that these dangerous casts are valid at runtime. Other languages merely accept a conservative approximation of safe casts, or none at all. This article should be merged with type_conversion. ...
This article or section should be merged with datatype or runtime. ...
Making pointers safer Because pointers allow a program to access objects that are not explicitly declared beforehand, they enable a variety of programming errors. However, the power they provide is so great that it can be difficult to do some programming tasks without them. To help deal with their problems, many languages have created objects that have some of the useful features of pointers, while avoiding some of their pitfalls. The word error has different meanings in different domains. ...
Anti-patterns, also referred to as pitfalls, are classes of commonly-reinvented bad solutions to problems. ...
One major problem with pointers is that, as long as they can be directly manipulated as a number, they can be made to point to unused addresses or to data which is being used for other purposes. Many languages, including most functional programming languages and recent imperative languages like Java, replace pointers with a more opaque type of reference, typically referred to as simply a reference, which can only be used to refer to objects and not manipulated as numbers, preventing this type of error. Array indexing is handled as a special case. Functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions. ...
Java is an object-oriented programming language developed by James Gosling and colleagues at Sun Microsystems in the early 1990s. ...
A pointer which does not have any address assigned to it is called a wild pointer. Any attempt to use such uninitialized pointers can cause unexpected behaviour, either because the initial value is not a valid address, or because using it may damage the runtime system and other unrelated parts of the program.
In systems with explicit memory allocation, it's possible to create a dangling pointer by deallocating the memory region it points into. This type of pointer is dangerous and subtle, because a deallocated memory region may contain the same data as it did before it was deallocated, but may be then reallocated and overwritten by unrelated code, unbeknownst to the earlier code. Languages with garbage collection prevent this type of error. In computer science, garbage collection (also known as GC) is a form of automatic memory management. ...
Some languages, like C++, support smart pointers, which use a simple form of reference counting to help track allocation of dynamic memory in addition to acting as a reference. In the absence of reference cycles, where an object refers to itself indirectly through a sequence of smart pointers, these eliminate the possibility of dangling pointers and memory leaks. A smart pointer is an abstract data type that simulates a pointer while providing additional features, such as automatic garbage collection or bounds checking. ...
In computer science, reference counting is a technique of storing the number of references, pointers, or handles to a resource such as an object or block of memory. ...
The null pointer A null pointer has a reserved value, often but not necessarily the value zero, indicating that it refers to no object. Null pointers are used routinely, particularly in C and C++, to represent exceptional conditions such as the lack of a successor to the last element of a linked list, while maintaining a consistent structure for the list nodes. This use of null pointers can be compared to the use of null values in relational databases and to the "Nothing" value in the "Maybe" monad. In C, each pointer type has its own null value, and sometimes they have different representations. In computer programming, null is a special value for a pointer (or other kind of reference) used to signify that the pointer intentionally does not have a target. ...
In computer science, a linked list is one of the fundamental data structures used in computer programming. ...
It has been suggested that this article or section be merged into Relational model. ...
This article contains information that has not been verified and thus might not be reliable. ...
Because it refers to nothing, an attempt to dereference a null pointer causes a run-time error that usually terminates the program immediately (in the case of C, often with a segmentation fault, since the address literally corresponding to the null pointer will likely not be allocated to the running program). In Java, access to a null reference triggers a Java.lang.NullPointerException, which can be caught (but a common practice is to attempt to ensure such exceptions never occur). In safe languages a possibly null pointer can be replaced with a tagged union which enforces explicit handling of the exceptional case; in fact, a possibly-null pointer can be seen as a tagged union with a computed tag. In computer science, a tagged union, also called a variant, variant record, or disjoint union, is a data structure used to hold a value that could take on several different, but fixed types. ...
A null pointer should not be confused with an uninitialized pointer: a null pointer is guaranteed to compare unequal to a pointer to any object or function, whereas an uninitialized pointer might have any value. Two separate null pointers are guaranteed to compare equal.
Wild pointers Wild pointers are pointers that have not been initialized (that is, set to point to a valid address) and may make a program crash or behave oddly . In the Pascal or C programming languages, pointers that are not specifically initialized may point to unpredictable addresses in memory. Pascal is an imperative computer programming language, developed in 1970 by Niklaus Wirth as a language particularly suitable for structured programming. ...
The C Programming Language, Brian Kernighan and Dennis Ritchie, the original edition that served for many years as an informal specification of the language The C programming language is a standardized imperative computer programming language developed in the early 1970s by Dennis Ritchie for use on the Unix operating system. ...
Examples The following example code shows a wild pointer: #include <stdio.h> #include <stdlib.h> int main(void) { char *p1 = (char *)malloc(sizeof(char)); // allocate memory and initialized pointer printf("p1 points to: %pn", p1); // points to some place in the heap printf("Value of *p1: %cn", *p1); // (undefined) value of some place in the heap char *p2; // wild (uninitialized) pointer printf("Address of p2: %pn", p2); // undefined value, may not be a valid address // if you are LUCKY, this will cause an addressing exception printf("Value of *p2: %cn", *p2); // random value at random address return 0; } The problems with invalid pointers include more than simply uninitialized values.
For instance, pointers can be used after the object or variable they point to no longer exists, or has gone out of scope, as in the example below. If such an invalid pointer is used, the program will probably not immediately crash, but the result will still probably be incorrect, and the failure will probably be hard to track down.
#include <stdio.h> #include <stdlib.h> int badIdea(int **p) // p is a pointer to a pointer to an int { int x = 1; // allocate an int on the stack **p = x; // assign value of x to int that pointer p points to *p = &x; // make the pointer that p points to point to x return x; // after return x is out of scope and undefined } int main(void) { int y = 0; int *p1 = &y; // initialized pointer to y int *p2 = NULL; // a good habit to form printf("Address of p1: %pn", p1); // prints address of y printf("Value of *p1: %dn", *p1); // prints value of y y = badIdea(&p1); // changes y and changes p1 // p1 now points to where x was // The place where x was will get clobbered, // for instance, on the next interrupt, or on // the next subroutine call, as below.... // some other code that also uses the stack p2 = (int *)malloc(5*sizeof(int)); // this probably will NOT crash, but value printed is unpredictable printf("Value of *p1: %pn", *p1); // prints value of where x was return 0; } A very common problem is using a pointer to the heap after that memory has been deallocated, as in this example. The invalid copies of the pointer are usually much harder to find than here. #include <stdio.h> #include <stdlib.h> int main(void) { int *p1 = (int *)malloc(sizeof(int)); // initialized pointer to heap int *p2 = p1; // make a copy *p1 = 0; // initialize the heap printf("Address of p2: %pn", p2); // points into the heap printf("Value of *p2: %dn", *p2); // should print zero free(p1); // deallocate the memory .... // other code, possibly using the heap // p2 still points to the original allocation, but who knows what is there printf("Value of *p2: %dn", *p2); // invalid use of p2 return 0; } A third way that pointers can be misused is to access outside the data structure they point to. Here is a simple example. #include <stdio.h> #include <stdlib.h> int main(void) { int y = 5; // create a variable int *p1 = &y; // initialized pointer to y printf("Address of p1: %pn", p1); // address of y printf("Value of *p1: %dn", *p1); // value of y p1 = p1 + y; // allowed pointer arithmetic printf("Value of *p1: %pn", *p1); // p1 no longer points to y return 0; } If a pointer is used to write beyond the end of a local buffer, the stack can be destroyed. In the case below, the problem will probably manifest when the main program returns. #include <stdio.h> #include <stdlib.h> // copy source to destination, no check on sizes void strcopy(char *d, char *s) { while (*d++ = *s++) // copy until '0' encountered ; } int main(void) { char y[3]; // create a local buffer char *p1 = (char *)malloc(10*sizeof(char)); // another buffer on heap p1[9] = '0'; // terminate the larger buffer strcopy(y, p1); // overflow the local buffer free(p1); return 0; // now bad stuff happens }
Support in various programming languages A number of languages support some type of pointer, although some are more restricted than others. If a pointer is significantly abstracted, such that it can no longer be manipulated as an address, the resulting data structure is no longer a pointer; see the more general reference article for more discussion of these. This article discusses a general notion of reference in computing. ...
Ada Ada is a strongly typed language where all pointers are typed and only safe type conversions are permitted. All pointers are by default initialized to null, and any attempt to access data through a null pointer causes an exception to be raised. Pointers in Ada are called access types. Ada 83 did not permit arithmetic on access types (although many compiler vendors provided for it as a non-standard feature), but Ada 95 supports "safe" arithmetic on access types via the package System.Storage_Elements. Ada is a structured, statically typed imperative computer programming language designed by a team led by Jean Ichbiah of CII Honeywell Bull during 1977–1983. ...
Exception handling is a programming language construct or computer hardware mechanism designed to handle runtime errors or other problems (exceptions) which occur during the execution of a computer program. ...
C/C++ In C and C++, pointers are variables that store addresses and can be null. Each pointer has a type it points to, but one can freely cast between pointer types. A special pointer type called the "void pointer" points to an object of unknown type and cannot be dereferenced. The address can be directly manipulated by casting a pointer to and from an integer. The C Programming Language, Brian Kernighan and Dennis Ritchie, the original edition that served for many years as an informal specification of the language The C programming language is a standardized imperative computer programming language developed in the early 1970s by Dennis Ritchie for use on the Unix operating system. ...
C++ (generally pronounced see plus plus) is a general-purpose programming language. ...
In the C Programming Language, a null pointer is a special pointer which is guaranteed to compare unequal to a pointer to any object or function. ...
C++ fully supports C pointers and C typecasting. It also supports a new group of typecasting operators to help catch some unintended dangerous casts at compile-time. The C++ standard library also provides auto ptr, a sort of smart pointer which can be used in some situations as a safe alternative to primitive C pointers. C++ also supports another form of reference, quite different from a pointer, called simply a reference or reference type. C++ (generally pronounced see plus plus) is a general-purpose programming language. ...
auto_ptr is a template class available in the C++ Standard Library (declared in <memory>) that provides some basic RAII features for C++ raw pointers. ...
A smart pointer is an abstract data type that simulates a pointer while providing additional features, such as automatic garbage collection or bounds checking. ...
In the C++ programming language, a reference is a simple reference datatype that is less powerful but safer than the pointer type inherited from C, which is a reference in the general sense but not in the sense used by C++. // Syntax and terminology The declaration of the form <Type...
Pointer arithmetic Pointer arithmetic is unrestricted; adding or subtracting from a pointer moves it by a multiple of the size of the datatype it points to. For example, adding 1 to a pointer to 4-byte integer values will increment the pointer by 4. This has the effect of incrementing the pointer to point at the next element in a contiguous array of integers -- which is often the intended result. Pointer arithmetic cannot be performed on void pointers. In computer science, a datatype or data type (often simply a type) is a name or label for a set of values and some operations which one can perform on that set of values. ...
Pointer arithmetic provides the programmer with a single way of dealing with different types: adding and subtracting the number of elements required instead of the actual offset in bytes. In particular, the C definition explicitly declares that the syntax a[n], which is the n-th element of the array pointed by a, is equivalent to *(a+n), which is the content of the element pointed by a+n.
While powerful, pointer arithmetic can be a source of computer bugs. It tends to confuse novice programmers, forcing them into different contexts: an expression can be an ordinary arithmetic one or a pointer arithmetic one, and sometimes it is easy to mistake one for the other. In response to this, many modern high level computer languages (for example Java) do not permit direct access to memory using addresses. Also, the safe C dialect Cyclone addresses many of the issues with pointers. See C programming language#Pointers for more criticism. A computer bug is an error, flaw, mistake, failure, or fault in a computer program that prevents it from working as intended, or produces an incorrect result. ...
A programmer or software developer is someone who programs computers, i. ...
Java is an object-oriented programming language developed by James Gosling and colleagues at Sun Microsystems in the early 1990s. ...
The Cyclone programming language is intended to be a safe dialect of the C programming language. ...
The C Programming Language, Brian Kernighan and Dennis Ritchie, the original edition that served for many years as an informal specification of the language The C programming language is a standardized imperative computer programming language developed in the early 1970s by Dennis Ritchie for use on the Unix operating system. ...
C# In the C# programming language, pointers are supported only under certain conditions: any block of code including pointers must be marked with the unsafe keyword. Such blocks usually require higher security permissions than pointerless code to be allowed to run. The syntax is essentially the same as in C++, and the address pointed can be either managed or unmanaged memory. However, pointers to managed memory (any pointer to a managed object) must be declared using the fixed keyword, which prevents the garbage collector from moving the pointed object as part of memory management while the pointer is in scope, thus keeping the pointer address valid. The title given to this article is incorrect due to technical limitations. ...
In computer science, garbage collection (also known as GC) is a form of automatic memory management. ...
The .NET framework includes many classes and methods in the System and System.Runtime.InteropServices namespaces (such as the Marshal class) which convert .NET types (for example, System.String) to and from many unmanaged types and pointers (for example, LPWSTR or void*) to allow communication with unmanaged code. The . ...
D The D programming language is a derivative of C and C++ which fully supports C pointers and C typecasting. But D also offers numerous constructs such as foreach loops, out function parameters, reference types, and advanced array handling which replace pointers for most routine programming tasks. D is an object-oriented, imperative systems programming language designed by Walter Bright of Digital Mars as a successor to C++. He has done this by adding some features and reducing the complexity of C++ syntax. ...
Modula-2 Pointers are implemented very much as in Pascal, as are VAR parameters in procedure calls. Modula 2 is even more strongly typed than Pascal, with fewer ways to escape the type system. Some of the variants of Modula 2 (such as Modula-3) include garbage collection. Modula-2 is a computer programming language invented by Niklaus Wirth at ETH around 1978, as a successor to Modula, another language by him that was never implemented. ...
Modula-3 is a programming language conceived as a successor to an upgraded version of Modula-2. ...
Oberon Much as with Modula-2, pointers are available. There are still fewer ways to evade the type system and so Oberon and its variants are still safer with respect to pointers than Modula-2 or its variants. As with Modula-3, garbage collection is a part of the language specification. Modula-2 is a computer programming language invented by Niklaus Wirth at ETH around 1978, as a successor to Modula, another language by him. ...
Oberon is a reflective programming language created in the late 1980s by Professor Niklaus Wirth (creator of the Pascal, Modula, and Modula-2 programming languages) and his associates at ETHZ in Switzerland. ...
Modula-3 is a programming language conceived as a successor to an upgraded version of Modula-2. ...
Pascal Pascal implements pointers in a straightforward, limited, and relatively safe way. It helps catch mistakes made by people who are new to programming, like dereferencing a pointer into the wrong datatype; however, a pointer can be cast from one pointer type to another. Pointer arithmetic is unrestricted; adding or subtracting from a pointer moves it by that number of bytes in either direction, but using the Inc or Dec standard procedures on it moves it by the size of the datatype it is declared to point to. Trying to dereference a null pointer, named nil in Pascal, or a pointer referencing unallocated memory, raises an exception in protected mode. Parameters may be passed using pointers (as VAR parameters) but are automatically handled by the runtime system. Pascal is an imperative computer programming language, developed in 1970 by Niklaus Wirth as a language particularly suitable for structured programming. ...
In computer science, a datatype or data type (often simply a type) is a name or label for a set of values and some operations which one can perform on that set of values. ...
In computer science, a datatype or data type (often simply a type) is a name or label for a set of values and some operations which one can perform on that set of values. ...
KK Null, a Japanese musician Null, a special value in computer programming. ...
Exception handling is a programming language construct or computer hardware mechanism designed to handle runtime errors or other problems (exceptions) which occur during the execution of a computer program. ...
Protected mode is an operational mode of x86-compatible CPUs of the 80286 series or later. ...
This article needs to be cleaned up to conform to a higher standard of quality. ...
See also In computer security and programming, a buffer overflow, or buffer overrun, is an anomalous condition where a process attempts to store more data in a buffer than there is memory allocated for it. ...
hazard pointer ...
In computer programming, an opaque pointer is a datatype that hides its internal implementation using a pointer. ...
In computer science, pointer swizzling is the conversion of references based on name or position to direct pointer references. ...
This article discusses a general notion of reference in computing. ...
Static analysis is the term applied to the analysis of computer software that is performed without actually executing programs built from that software (analysis performed on executing programs is known as dynamic analysis). ...
External links - Pointer Fun With Binky Introduction to pointers in a 3 minute educational video - Stanford Computer Science Education Library (this link has crashed some browsers -- use with caution)
- 0pointer.de A terse list of minimum length source codes that dereference a null pointer in several different programming languages
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