Pointer (computing)

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In computer science, a pointer is a programming language data type whose value refers directly to (or "points to") another value stored elsewhere in the computer memory using its address. Obtaining the value to which a pointer refers is called dereferencing the pointer. A pointer is a simple implementation of the general reference data type (although it is quite different from the facility referred to as a reference in C++). Pointers to data improve performance for repetitive operations such as traversing string and tree structures, and pointers to functions are used for binding methods in Object-oriented programming and run-time linking to Dynamic Link Libraries (DLLs).

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 as a memory address. Because pointers allow largely unprotected access to memory addresses, there are risks associated with using them. For general information about references, see reference (computer science).

[edit] Pointers in data structures

When setting up data structures like lists, queues and trees, it is necessary to have pointers to help manage the way in which the structure is implemented and controlled. Typical examples of pointers would be start pointers, end pointers, or stack pointers.

[edit] 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.

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 is installed or the architecture does not support such addresses. The first case may, in certain platforms 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. There, pointers must conform to certain rules (canonical addresses), so if a noncanonical 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 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 MiB of physical memory, could access up to 1 GiB of virtual memory, but the combination of 16-bit address and segment registers made accessing more than 64 KiB in one data structure cumbersome. Some restrictions of ANSI pointer arithmetic may have been due to the segmented memory models of this processor family.

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.

[edit] Uses

Pointers are directly supported without restrictions in languages such as 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.

In functional programming languages that rely heavily on lists, pointers and references are managed abstractly by the language using internal constructs like cons.

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.

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.

[edit] C pointers

The basic syntax to define a pointer is

int *money;

This declares money as a pointer to an integer. Since the contents of memory are not guaranteed to be of any specific value in C, care must be taken to ensure that the address that money points to is valid. This is why it is suggested to initialize the pointer to NULL

int *money = NULL;

If a NULL pointer is dereferenced then a runtime error will occur and execution will stop likely with a segmentation fault.

Once a pointer has been declared then, perhaps, the next logical step is to point it at something

int a = 5;
int *money = NULL;
 
money = &a;

This assigns the value of money to be the address of a. For example, if a is stored at memory location of 0x8130 then the value of money will be 0x8130 after the assignment. To dereference the pointer, an asterisk is used again

*money = 8;

This says to take the contents of money (which is 0x8130), go to that address in memory and set its value to 8. If a were then accessed then its value will be 8.

This example may be more clear if memory were examined directly. Assume that a is located at address 0x8130 in memory and money 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 were executed

int a = 5;
int *money = NULL;

Address	Contents
0x8130	0x00000005
0x8134	0x00000000

(The NULL pointer shown here is 0x00000000.) By assigning the address of a to money

money = &a;

yields the following memory values

Address	Contents
0x8130	0x00000005
0x8134	0x00008130

Then by dereferencing money by doing

*money = 8;

the computer will take the contents of money (which is 0x8130), go to that address, and assign 8 to that location yielding the following memory.

Address	Contents
0x8130	0x00000008
0x8134	0x00008130

Clearly, accessing a will yield the value of 8 because the previous instruction modified the contents of a by way of the pointer money.

[edit] C arrays

Taking C pointers to the next step is the array.

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, 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 the following manner:

int array[5];      /* Declares 5 contiguous integers */
int *ptr = array;  /* Arrays can be used as pointers */
ptr[0] = 1;        /* Pointers can be indexed with array syntax */
*(array + 1) = 2;  /* Arrays can be dereferenced with pointer syntax */

This allocates a block of five integers and declares array as a pointer to this 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, it is important to note that the sizeof operator will differ. 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:

int array[5] = {2,4,3,1,5};

If you assume that array is located in memory starting at address 0x1000 on a 32-bit little-endian machine then memory will contain the following:

	0	1	2	3
1000	02	00	00	00
1004	04	00	00	00
1008	03	00	00	00
100C	01	00	00	00
1010	05	00	00	00

Represented here are five integers: 2, 4, 3, 1, and 5. These five integers occupy 32 bits (4 bytes) each with the least-significant byte stored first (this is a little-endian architecture) and are stored consecutively starting at address 0x1000.

The syntax for C with pointers is:

• array means 0x1000
• array+1 means 0x1004 (note that the "+1" really means to add one times the size of an int (4 bytes) not literally "plus one")
• *array means to dereference the contents of array which means to consider the contents as a memory address (0x1000) and to go look up the value at that memory location (0x1000)
• array[i] means the i^th index of array which is translated into *(array + i)

The last example is how to access the contents of array. Breaking it down:

• array + i is the memory location of the i^th element of array
• *(array + i) takes that memory address and dereferences it to access the value.

E.g. array[3] is synonymous with *(array+3), meaning *(0x1000 + 3*sizeof(int)), which says "dereference the value stored at 0x100C", in this case 0x0001.

[edit] C linked list

Below is an example of the definition of a linked list in C.

/* the empty linked list is
 * represented by NULL or some
 * other signal value */
#define EMPTY_LIST NULL
 
struct link {
    /* the data of this link */
    void *data;
    /* the next link; EMPTY_LIST if this is the last link */
    struct link *next;
};

Note that this pointer-recursive definition is essentially the same as the reference-recursive definition from the Haskell programming language:

data Link a = Nil
            | Cons a (Link a)

Nil is the empty list, and Cons a (Link a) is a cons cell of type a with another link also of type a.

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.

[edit] Pass by reference

Pointers can be used to pass variables by reference, allowing their value to be changed. For example:

void not_alter(int n) {
    n = 360;
}
 
void alter(int *n) {
    *n = 120;
}
 
void func(void) {
    int x = 24;
 
    not_alter(x);
 
    /* x still equal to 24 */
 
    alter(&x);
 
    /* x now equal to 120 */
}

[edit] Memory-mapped hardware

On some computing architectures, pointers can be used to directly manipulate memory or memory-mapped devices.

Assigning addresses to pointers is an invaluable tool when programming microcontrollers. Below is a simple example declaring a pointer of type int and initialising it to a hexadecimal address in this example the constant 0x7FFF:

int *hardware_address = (int *)0x7FFF;

In the mid 80s, using the BIOS to access the video capabilities of PCs was slow. Applications that were display-intensive typically used to access CGA video memory directly by casting the hexadecimal constant 0xB8000000 to a pointer to an array of 80 unsigned 16-bit int values. Each value consisted of an ASCII code in the low byte, and a colour in the high byte. Thus, to put the letter 'A' at row 5, column 2 in bright white on blue, one would write code like the following:

#define VID ((unsigned (*)[80])0xB8000000)
 
void foo() {
    VID[4][1] = 0x1F00 | 'A';
}

[edit] 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.

For example, in C

int *money;
char *bags;

money would be an integer pointer and bags would be a char pointer. The following would yield a compiler warning of "assignment from incompatible pointer type" under GCC

bags = money;

because money and bags were declared with different types. To suppress the compiler warning, it must be made explicit that you do indeed wish to make the assignment by typecasting it

bags = (char *)money;

which says to cast the integer pointer of money to a char pointer and assign to bags.

In languages that allow pointer arithmetic, arithmetic on pointers takes into account the size of the type. For example, adding an integer number to a pointer produces another pointer that points to an address that is higher by that number times the size of the type. This allows us to easily compute the address of elements of an array of a given type, as was shown in the C arrays example above. When a pointer of one type is cast to another type of a different size, the programmer should expect that pointer arithmetic will be calculated differently. In C, for example, if the money array starts at 0x2000 and sizeof(int) is 4 bytes whereas sizeof(char) is 2 bytes, then (money+1) will point to 0x2004 but (bags+1) will point to 0x2002. Other risks of casting include loss of data when "wide" data is written to "narrow" locations (e.g. bags[0]=65537;), unexpected results when bit-shifting values, and comparison problems, especially with signed vs unsigned values.

Although it's impossible in general to determine at compile-time which 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.

[edit] 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.

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.

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, unknown to the earlier code. Languages with garbage collection prevent this type of error.

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. Delphi strings support reference counting natively.

[edit] 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++ where the compile-time constant NULL is used, to represent 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.

Because it does not refer to a meaningful object, an attempt to dereference a null pointer usually causes a run-time error that, if unhandled, terminates the program immediately. In the case of C, execution halts with a segmentation fault because the literal address of NULL is never allocated to a running program. In Java, access to a null reference triggers a NullPointerException, which can be caught by error handling code, but the preferred practice is to ensure that 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 C and C++ programming, two null pointers are guaranteed to compare equal; ANSI C guarantees that any NULL pointer will be equal to 0 in a comparison with an integer type.

A null pointer should not be confused with an uninitialized pointer: a null pointer is guaranteed to compare unequal to any valid pointer, whereas depending on the language and implementation an uninitialized pointer might have either an indeterminate (random or meaningless) value or might be initialised to an initial constant (possibly but not necessarily NULL).

In most C programming environments malloc returns a NULL pointer if it is unable to allocate the memory region requested, which notifies the caller that there is insufficient memory available. However, some implementations of malloc allow malloc(0) with the return of a NULL pointer and instead indicate failure by both returning NULL and setting errno to an appropriate value.

Computer systems based on a tagged architecture are able to distinguish in hardware between a NULL dereference and a legitimate attempt to access a word or structure at address zero.

In some programming language environments (at least one proprietary Lisp implementation, for example) the value used as the null pointer (called nil in Lisp) may actually be a pointer to a block of internal data useful to the implementation (but not explicitly reachable from user programs), thus allowing the same register to be used as a useful constant and a quick way of accessing implementation internals. This is known as the nil vector.

[edit] Double indirection

In C, it is possible to have a pointer point at another pointer. Although a higher number of pointer dereferences will add a performance penalty, this can make manipulating certain data structures particularly neat and elegant. For instance, consider this code to insert an item into a simple linked list:

struct element {
        struct element *next;
        int value;
};
 
struct element *head = NULL;
 
void insert(struct element *item) {
        struct element **p;
        for(p = &head; *p != NULL; p = &(*p)->next) {
                if(item->value <= (*p)->value) {
                        break;
                }
        }
        item->next = *p;
        *p = item;
}

[edit] 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.

The following example code shows a wild pointer:

int func(void)
{
    char *p1 = malloc(sizeof(char)); /* (undefined) value of some place on the heap */
    char *p2;       /* wild (uninitialized) pointer */
    *p1 = 'a';      /* This is OK */
    *p2 = 'b';      /* ERROR: the result is undefined, and may lead to program crash */
}

Here, p2 may point to anywhere in memory, so performing the assignment *p2 = 'b' will corrupt an unknown area of memory that may contain sensitive data.

Note that in C and derived languages static variables without an initializer is initialized to zero on the program's start. Thus, the example above will dereference a NULL pointer which will lead to a segmentation fault.

[edit] 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.

[edit] 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.

[edit] BASIC

BASIC does not support pointers. Some dialects of BASIC, including FreeBASIC, have exhaustive pointer implementations, however.

In FreeBASIC, maths on ANY pointers (equivalent to C's void*) are treated as though the ANY pointer was a byte width. ANY pointers cannot be dereferenced, as in C. Also, casting between ANY and any other type's pointers will not generate any warnings.

dim as integer f = 257
dim as any ptr g = @f
dim as integer ptr i = g
assert(*i = 257)
assert( (g + 4) = (@f + 1) )

[edit] C and 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 unspecified type and cannot be dereferenced. The address can be directly manipulated by casting a pointer to and from an integral type of sufficient size (not defined in the language itself, but possibly in standard headers).

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.

Pointer arithmetic, that is, the ability to modify a pointer's target address with arithmetic operations (as well as magnitude comparisons), is restricted by the language standard to remain within the bounds of a single array object (or just after it), though many non-segmented architectures will allow for more lenient arithmetic. 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 because the void type has no size, and thus the pointed address can not be added to. For working 'directly' with bytes they usually cast pointers to BYTE*, or unsigned char* if BYTE isn't defined in the standard library used.

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. (though the char pointer, char being defined as always having a size of one byte, allows the element offset of pointer arithmetic to in practice be equal to a byte offset) In particular, the C definition explicitly declares that the syntax a[n], which is the n-th element of the array a, is equivalent to *(a+n), which is the content of the element pointed by a+n. This implies that n[a] is equivalent to 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 for more criticism.

The void pointer, or void*, is supported in ANSI C and C++ as a generic pointer type. A pointer to void can store an address to any data type, and, in C, is automatically cast to any other pointer type on assignment, but it must be explicitly cast if dereferenced inline. K&R C used char* for the “type-agnostic pointer” purpose.

int x = 4;
void* q = &x;
int* p = q;  /* void* automatically cast to int*: valid C, but not C++ */
int i = *p;
int j = *((int*)q); /* when dereferencing inline, there is no automatic casting */

C++ does not allow the automatic casting of void* to other pointer types, not even in assignments. This was a design decision to avoid careless and even unintended casts, though most compilers only output warnings, not errors, when encountering other ill casts.

int x = 4;
void* q = &x;
// int* p = q; // This fails in C++: there is no autocast from void*
int* a = (int*)q; // C-style cast
int* b = static_cast<int*>(q); // C++ cast

In C++, there is no void& (reference to void) to complement void* (pointer to void), because references behave like aliases to the variables they point to, and there can never be a variable whose type is void.

[edit] 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 .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.

[edit] D

The D programming language is a derivative of C and C++ which fully supports C pointers and C typecasting. However 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.

[edit] Fortran

Fortran-90 introduced a strongly-typed pointer capability. Fortran pointers contain more than just a simple memory address. They also encapsulate the lower and upper bounds of array dimensions, strides (for example, to support arbitrary array sections), and other metadata. An association operator, => is used to associate a POINTER to a variable which has a TARGET attribute. The Fortran-90 ALLOCATE statement may also be used to associate a pointer to a block of memory. For example, the following code might be used to define and create a linked list structure:

type real_list_t
  real :: sample_data(100)
  type (real_list_t), pointer :: next => null ()
end type
 
type (real_list_t), target :: my_real_list
type (real_list_t), pointer :: real_list_temp
 
real_list_temp => my_real_list
do
  read (1,iostat=ioerr) real_list_temp%sample_data
  if (ioerr /= 0) exit
  allocate (real_list_temp%next)
  real_list_temp => real_list_temp%next
end do

Fortran-2003 adds support for procedure pointers. Also, as part of the C Interoperability feature, Fortran-2003 supports intrinsic functions for converting C-style pointers into Fortran pointers and back.

[edit] 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.

[edit] 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.

[edit] 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 static compilation system.

[edit] See also

• Buffer overflow
• Hazard pointer
• Opaque pointer
• Pointer swizzling
• Reference (computer science)
• Static code analysis
• Bounded pointer

[edit] External links

• Pointers and Memory Introduction to pointers - Stanford Computer Science Education Library
• 0pointer.de A terse list of minimum length source codes that dereference a null pointer in several different programming languages
• A tutorial in C Pointers and Arrays by Ted Jensen
• Pointers | Resourceful Idiot Brief Overview of Pointers and Why they are important