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Pointer a pointing to variable b. Note that b contains a number (hexadecimal 00000011 = decimal 17), where a contains the address of b in memory (hexadecimal 1008). In this case both the address and the data are contained in 32 bit words.

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. For high-level programming languages, pointers effectively take the place of general purpose registers in low level languages such as assembly language or machine code—but, in contrast, occupies part of the available memory. A pointer references a location in memory, and obtaining the value at the location a pointer refers to is known as dereferencing the pointer. A pointer is a simple, less abstracted implementation of the more abstracted reference data type (although it is not as directly usable as a C++ reference). Several languages support some type of pointer, although some are more restricted than others.

Pointers to data significantly improve performance for repetitive operations such as traversing strings, lookup tables, control tables and tree structures. In particular, it 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 what are called virtual method tables.

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 a memory address, as opposed to a magic cookie or capability where this is not possible.[citation needed] Because pointers allow both protected and unprotected access to memory addresses, there are risks associated with using them particularly in the latter case. For general information about references, see reference (computer science).

Contents

Formal description

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 a byte and 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, the aggregate can 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.

In the context of these definitions, a byte is the smallest primitive; each memory address specifies a different byte. The memory address of the first byte of a datum is considered the memory address (or base memory address) of the entire datum.

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 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 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 the pointer points.

Quotations

I do consider assignment statements and pointer variables to be among computer science's most valuable treasures.
 
Donald Knuth (Stanford Professor Emeritus of the Art of Computer Programming), "Structured Programming with go to Statements"[1]

Use in data structures

When setting up data structures like lists, queues and trees, it 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 less bits than a full address, but will usually require one additional arithmetic operation to resolve).

A two-byte offset, containing a 16-bit, unsigned integer, can be used to provide relative addressing for up to 64 kilobytes of a data structure. This can easily be extended to 128K,256K or 512K if the address pointed to is forced to be 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 the offset by a factor of 2,3 or 4, before its addition to the base address).

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 a usable sequential index and then to an absolute address without a lookup table.

Use in control tables

Control tables, that are used to control program flow, usually make extensive use of pointers. The pointers, usually embedded in a table entry, may, for instance, be used to hold the entry points to subroutines to be executed, based on certain conditions defined in the same table entry. The pointers can however be simply indexes to other separate, but associated, tables comprising an array of the actual addresses or the addresses themselves (depending upon the programming language constructs available). They can also be used to point (back) to earlier table entries (as in loop processing) or forward to skip some table entries (as in a switch or "early" exit from a loop). For this latter purpose, the "pointer" may simply be the table entry number itself and can be transformed into an actual address by simple arithmetic.

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 (usually utilizing the machines 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 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. 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.

Uses

Pointers are directly supported without restrictions in languages such as PL/I, 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. If the data elements in the array have lengths that are divisible by powers of two, this arithmetic is usually much more efficient. Padding is frequently used as a mechanism for ensuring this is the case, despite the increased memory requirement. 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 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).

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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 sometimes suggested to initialize the pointer to NULL (however initialising pointers unnecessarily can mask compiler analyses and hide bugs).

int *money = NULL;

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

Once a pointer has been declared, the next logical step is for it to point 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 means take the contents of money (which is 0x8130), "locate" that address in memory and set its value to 8. If a is later accessed again, its new value will be 8.

This example may be more clear if memory is 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 is 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 coding

 *money = 8;

the computer will take the contents of money (which is 0x8130), 'locate' 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.

C arrays

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).[2] Thus in C, arrays can be thought of as pointers to consecutive areas of memory (with no gaps),[2] 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 (per Plauger Standard C 1992) 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 */
*(1 + array) = 3;  /* Pointer addition is commutative */
2[array] = 4;      /* Subscript operator is commutative */

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, 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 (values are in hexadecimal, like the addresses):

0 1 2 3
1000 2 0 0 0
1004 4 0 0 0
1008 3 0 0 0
100C 1 0 0 0
1010 5 0 0 0

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 CPU 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. Considering the contents as a memory address (0x1000) , look up the value at that location (0x0002).
  • array[i] means the ith 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 ith 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.

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 does not 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.

Pass-by-address using pointers

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

/* a copy of the int n is changed */
void not_alter(int n) {
    n = 360;
}
 
/* the actual variable passed (by address) is changed */
void alter(int *n) {
    *n = 120;
}
 
void func(void) {
    int x = 24;
 
    /*pass x's address as the argument*/
    alter(&x);
 
    /* x now equal to 120 */
 
    not_alter(x);
 
    /* x still equal to 120 */
}

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

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.

A 2005 draft of the C standard requires that casting a pointer derived from one type to one of another type should maintain the alignment correctness for both types (6.3.2.3 Pointers, par. 7):[3]

char *external_buffer = "abcdef";
int *internal_data;
 
internal_data = (int *)external_buffer;  // UNDEFINED BEHAVIOUR if "the resulting pointer
                                         // is not correctly aligned"

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 1 bytes, then (money+1) will point to 0x2004 but (bags+1) will point to 0x2001. 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 is 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.

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. The result is often a segmentation fault or storage violation.

In systems with explicit memory allocation, it is 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. It is claimed that languages with garbage collection prevent this type of error (because deallocation is performed automatically) but the pointer itself is not removed by the garbage collector and it may point to irrelevant and unpredictable data if re-used at any time after it has been deallocated.

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.

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 (though the integer literal for zero is preferred in C++), 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 nullable types, null values in relational databases and to the Nothing value in an option type. Nullable references were invented by C.A.R. Hoare in 1965 as part of the Algol W language. Hoare later (2009) described his invention as a "billion-dollar mistake":[4][5]

I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.

Since a null-valued pointer does not refer to a meaningful object, an attempt to dereference a null pointer usually causes a run-time error. If this error is left unhandled, the program terminates immediately. In the case of C on a general computer, execution halts with a segmentation fault because the literal address of NULL is never allocated to a running program (with a C program in an embedded system, various things may occur). 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 pointer with a computed tag. Other languages, such as Objective-C, allow messages to be sent to a nil address (the value of a pointer that does not point to a valid object) without causing the program to be interrupted; the message will simply be ignored, and the return value (if any) is nil or 0, depending on the type[6].

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; furthermore the macro NULL is defined as a null pointer constant, that is value 0 (either as an integer type or converted to a pointer to void), so a null pointer will compare equal to NULL.

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 a null pointer 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.

Double indirection

In some languages a pointer can reference another pointer, requiring two dereference operations to get to the original value. While each level of indirection may add a performance cost, it is sometimes necessary in order to provide correct behavior for complex data structures. For example, in C it is typical to define a linked list in terms of an element that contains a pointer to the next element of the list:

struct element
{
    struct element * next;
    int              value;
};
 
struct element * head = NULL;

This implementation uses a pointer to the first element in the list as a surrogate for the entire list. If a new value is added to the beginning of the list, head has to be changed to point to the new element. Since C arguments are always passed by value, using double indirection allows the insertion to be implemented correctly, and has the desirable side-effect of eliminating special case code to deal with insertions at the front of the list:

// Given a sorted list at *head, insert the element item at the first
// location where all earlier elements have lesser or equal value.
void insert(struct element **head, struct element *item)
{
    struct element ** p;  // p points to a pointer to an element
 
    for (p = head; *p != NULL; p = &(*p)->next)
    {
        if (item->value <= (*p)->value)
            break;
    }
    item->next = *p;
    *p = item;
}
 
// Caller does this:
insert(&head, item);

In this case, if the value of item is less than that of head, the caller's head is properly updated to the address of the new item.

Wild pointers

Wild pointers are pointers that have not been initialized (that is, a wild pointer does not have any address assigned to it) 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, assuming malloc() has not returned NULL. */
    *p2 = 'b';      /* This invokes undefined behavior */
}

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.

Wild branch

Where a pointer is used as the address of the entry point to a program or start of a subroutine and is also either uninitialized or corrupted, if a call or jump is nevertheless made to this address, a "wild branch" is said to have occurred. The consequences are usually unpredictable and the error may present itself in several different ways depending upon whether or not the pointer is a "valid" address and whether or not there is (coincidentally) a valid instruction (/opcode) at that address. The detection of a wild branch can present one of the most difficult and frustrating debugging exercises since much of the evidence may already have been destroyed beforehand or by execution of one or more inappropriate instructions at the branch location. If available, an instruction set simulator can usually not only detect a wild branch before it takes effect, but also provide a complete or partial trace of its history.

Simulation using an array index

It is possible to simulate pointer behaviour using an index to an (normally one-dimensional) array.

Primarily for languages which do not support pointers explicitly but do support arrays, the array can be thought of and processed as if it were the entire memory range (within the scope of the particular array) and any index to it can be thought of as equivalent to a general purpose register in assembly language (that points to the individual bytes but whose actual value is relative to the start of the array, not its absolute address in memory). Assuming the array is, say, a contiguous 16 megabyte character data structure, individual bytes (or a string of contiguous bytes within the array) can be directly addressed and manipulated using the name of the array with a 31 bit unsigned integer as the simulated pointer (this is quite similar to the C arrays example shown above). Pointer arithmetic can be simulated by adding or subtracting from the index, with minimal additional overhead compared to genuine pointer arithmetic.

It is even theoretically possible, using the above technique, together with a suitable instruction set simulator to simulate any machine code or the intermediate (byte code) of any processor/language in another language that does not support pointers at all (for example Java / JavaScript). To achieve this, the binary code can initially be loaded into contiguous bytes of the array for the simulator to "read", interpret and action entirely within the memory contained of the same array. If necessary, to completely avoid buffer overflow problems, bounds checking can usually be actioned for the compiler (or if not, hand coded in the simulator).

Support in various programming languages

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.

BASIC

Originally, BASIC did not support pointers. Newer dialects of BASIC, such as FreeBASIC or BlitzMax, 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) )

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, although the behaviour is implementation-defined. A special pointer type called the “void pointer” allows pointing to any variable type, but is limited by the fact that it cannot be dereferenced directly. The address can be directly manipulated by casting a pointer to and from an integral type of sufficient size. While earlier standards did not have an integral type that was guaranteed to be large enough, C99 has added the uintptr_t type defined in <stdint.h>.

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, although gcc and other compilers will perform byte arithmetic on void* as a non-standard extension. 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], and one can write, e.g., a[3] or 3[a] equally well to access the fourth element of an array a.

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 implicitly converted 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 (before ANSI C).

int x = 4;
void* q = &x;
int* p = q;  /* void* implicity converted to int*: valid C, but not C++ */
int i = *p;
int j = *(int*)q; /* when dereferencing inline, there is no implicit conversion */

C++ does not allow the implicit conversion 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 implicit conversion 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.

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.

An exception to this is from using the IntPtr structure, which is a safe managed equivalent to int*, and does not require unsafe code. This type is often returned when using methods from the System.Runtime.InteropServices, for example:

// Get 16 bytes of memory from the process' unmanaged memory
IntPtr pointer = System.Runtime.InteropServices.Marshal.AllocHGlobal(16);
 
// Do something with the allocated memory
 
// Free the allocated memory
System.Runtime.InteropServices.Marshal.FreeHGlobal(pointer);

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.

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.

Eiffel

The Eiffel object-oriented language supports pointers in the form of references, which are typed and do not allow any form of pointer arithmetic. The ECMA standard for Eiffel includes an "attached type" mechanism that claims to guarantee void safety.

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.

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.

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.

Pascal

Pascal implements pointers in a straightforward, limited, and relatively safe way. It catches mistakes such as 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 with it moves the pointer 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.

Perl

The Perl programming language supports pointers in the form of references, which are typed and do not allow any form of pointer arithmetic. They are used to construct complex data structures[7].

See also

References

  1. ^ http://pplab.snu.ac.kr/courses/adv_pl05/papers/p261-knuth.pdf
  2. ^ a b Plauger, P J; Brodie, Jim. ANSI and ISO Standard C Programmer's Reference. Redmond, WA: Microsoft Press. pp. 108, 51. ISBN 1556153597. "An array type does not contain additional holes because all other types pack tightly when composed into arrays [at page 51]" 
  3. ^ WG14 N1124, C - Approved standards: ISO/IEC 9899 - Programming languages - C, 2005-05-06.
  4. ^ Tony Hoare (2009). "Null References: The Billion Dollar Mistake". QCon London. http://qconlondon.com/london-2009/presentation/Null+References:+The+Billion+Dollar+Mistake. 
  5. ^ Tony Hoare (2009-08-25). "Null References: The Billion Dollar Mistake". InfoQ.com. http://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare. 
  6. ^ http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/Articles/ocObjectsClasses.html#//apple_ref/doc/uid/TP30001163-CH11-SW7 - The Objective-C 2.0 Programming Language, section "Sending Messages to nil".
  7. ^ // Making References (Perl References and nested data structures)

External links


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