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In computing, a segmentation fault (often shortened to segfault) or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restricted area of memory (a memory access violation). On standard x86 computers, this is a form of general protection fault. The OS kernel will, in response, usually perform some corrective action, generally passing the fault on to the offending process by sending the process a signal. Processes can in some cases install a custom signal handler, allowing them to recover on their own, but otherwise the OS default signal handler is used, generally causing abnormal termination of the process (a program crash), and sometimes a core dump.
Segmentation faults are a common class of error in programs written in languages like C that provide low-level memory access. They arise primarily due to errors in use of pointers for virtual memory addressing, particularly illegal access. Another type of memory access error is a bus error, which also has various causes, but is today much rarer; these occur primarily due to incorrect physical memory addressing, or due to misaligned memory access – these are memory references that the hardware cannot address, rather than references that a process is not allowed to address.
Newer programming languages may employ mechanisms designed to avoid segmentation faults and improve memory safety. For example, the Rust programming language which appeared in 2010 employs an 'Ownership' based model to ensure memory safety.
A segmentation fault occurs when a program attempts to access a memory location that it is not allowed to access, or attempts to access a memory location in a way that is not allowed (for example, attempting to write to a read-only location, or to overwrite part of the operating system).
The term "segmentation" has various uses in computing; in the context of "segmentation fault", a term used since the 1950s, it refers to the address space of a program. With memory protection, only the program's own address space is readable, and of this, only the stack and the read-write portion of the data segment of a program are writable, while read-only data and the code segment are not writable. Thus attempting to read outside of the program's address space, or writing to a read-only segment of the address space, results in a segmentation fault, hence the name.
On systems using hardware memory segmentation to provide virtual memory, a segmentation fault occurs when the hardware detects an attempt to refer to a non-existent segment, or to refer to a location outside the bounds of a segment, or to refer to a location in a fashion not allowed by the permissions granted for that segment. On systems using only paging, an invalid page fault generally leads to a segmentation fault, and segmentation faults and page faults are both faults raised by the virtual memory management system. Segmentation faults can also occur independently of page faults: illegal access to a valid page is a segmentation fault, but not an invalid page fault, and segmentation faults can occur in the middle of a page (hence no page fault), for example in a buffer overflow that stays within a page but illegally overwrites memory.
At the hardware level, the fault is initially raised by the memory management unit (MMU) on illegal access (if the referenced memory exists), as part of its memory protection feature, or an invalid page fault (if the referenced memory does not exist). If the problem is not an invalid logical address but instead an invalid physical address, a bus error is raised instead, though these are not always distinguished.
At the operating system level, this fault is caught and a signal is passed on to the offending process, activating the process's handler for that signal. Different operating systems have different signal names to indicate that a segmentation fault has occurred. On Unix-like operating systems, a signal called SIGSEGV (abbreviated from segmentation violation) is sent to the offending process. On Microsoft Windows, the offending process receives a STATUS_ACCESS_VIOLATION exception.
The conditions under which segmentation violations occur and how they manifest themselves are specific to hardware and the operating system: different hardware raises different faults for given conditions, and different operating systems convert these to different signals that are passed on to processes. The proximate cause is a memory access violation, while the underlying cause is generally a software bug of some sort. Determining the root cause – debugging the bug – can be simple in some cases, where the program will consistently cause a segmentation fault (e.g., dereferencing a null pointer), while in other cases the bug can be difficult to reproduce and depend on memory allocation on each run (e.g., dereferencing a dangling pointer).
The following are some typical causes of a segmentation fault:
- Attempting to access a nonexistent memory address (outside process's address space)
- Attempting to access memory the program does not have rights to (such as kernel structures in process context)
- Attempting to write read-only memory (such as code segment)
These in turn are often caused by programming errors that result in invalid memory access:
- Dereferencing a null pointer, which usually points to an address that's not part of the process's address space
- Dereferencing or assigning to an uninitialized pointer (wild pointer, which points to a random memory address)
- Dereferencing or assigning to a freed pointer (dangling pointer, which points to memory that has been freed/deallocated/deleted)
- A buffer overflow
- A stack overflow
- Attempting to execute a program that does not compile correctly. (Some compilers will output an executable file despite the presence of compile-time errors.)
In C code, segmentation faults most often occur because of errors in pointer use, particularly in C dynamic memory allocation. Dereferencing a null pointer will always result in a segmentation fault, but wild pointers and dangling pointers point to memory that may or may not exist, and may or may not be readable or writable, and thus can result in transient bugs. For example:
Now, dereferencing any of these variables could cause a segmentation fault: dereferencing the null pointer generally will cause a segfault, while reading from the wild pointer may instead result in random data but no segfault, and reading from the dangling pointer may result in valid data for a while, and then random data as it is overwritten.
The default action for a segmentation fault or bus error is abnormal termination of the process that triggered it. A core file may be generated to aid debugging, and other platform-dependent actions may also be performed. For example, Linux systems using the grsecurity patch may log SIGSEGV signals in order to monitor for possible intrusion attempts using buffer overflows.
Writing to read-only memory
Writing to read-only memory raises a segmentation fault. At the level of code errors, this occurs when the program writes to part of its own code segment or the read-only portion of the data segment, as these are loaded by the OS into read-only memory.
Here is an example of ANSI C code that will generally cause a segmentation fault on platforms with memory protection. It attempts to modify a string literal, which is undefined behavior according to the ANSI C standard. Most compilers will not catch this at compile time, and instead compile this to executable code that will crash:
When the program containing this code is compiled, the string "hello world" is placed in the rodata section of the program executable file: the read-only section of the data segment. When loaded, the operating system places it with other strings and constant data in a read-only segment of memory. When executed, a variable, s, is set to point to the string's location, and an attempt is made to write an H character through the variable into the memory, causing a segmentation fault. Compiling such a program with a compiler that does not check for the assignment of read-only locations at compile time, and running it on a Unix-like operating system produces the following runtime error:
Backtrace of the core file from GDB:
This code can be corrected by using an array instead of a character pointer, as this allocates memory on stack and initializes it to the value of the string literal:
Even though string literals should not be modified (this has undefined behavior in the C standard), in C they are of type, so there is no implicit conversion in the original code (which points a at that array), while in C++ they are of type, and thus there is an implicit conversion, so compilers will generally catch this particular error.
Null pointer dereference
Because a very common program error is a null pointerdereference (a read or write through a null pointer, used in C to mean "pointer to no object" and as an error indicator), most operating systems map the null pointer's address such that accessing it causes a segmentation fault.
This sample code creates a null pointer, and then tries to access its value (read the value). Doing so causes a segmentation fault at runtime on many operating systems.
Dereferencing a null pointer and then assigning to it (writing a value to a non-existent target) also usually causes a segmentation fault:
The following code includes a null pointer dereference, but when compiled will often not result in a segmentation fault, as the value is unused and thus the dereference will often be optimized away by dead code elimination:
Main article: Buffer overflow
Main article: Stack overflow
Another example is recursion without a base case:
which causes the stack to overflow which results in a segmentation fault. Infinite recursion may not necessarily result in a stack overflow depending on the language, optimizations performed by the compiler and the exact structure of a code. In this case, the behavior of unreachable code (the return statement) is undefined, so the compiler can eliminate it and use a tail call optimization that might result in no stack usage. Other optimizations could include translating the recursion into iteration, which given the structure of the example function would result in the program running forever, while probably not overflowing its stack.
In computer science, a pointer is a programming language object, whose value refers to (or "points to") another value stored elsewhere in the computer memory using its memory address. A pointer references a location in memory, and obtaining the value stored at that location is known as dereferencing the pointer. As an analogy, a page number in a book's index could be considered a pointer to the corresponding page; dereferencing such a pointer would be done by flipping to the page with the given page number and reading the text found on the indexed page.
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 bindingmethods, often using what are called virtual method tables.
A pointer is a simple, more concrete implementation of the more abstract referencedata type. Several languages support some type of pointer, although some have more restrictions on their use than others. While "pointer" has been used to refer to references in general, it more properly applies to data structures whose interface explicitly allows the pointer to be manipulated (arithmetically via pointer arithmetic) as a memory address, as opposed to a magic cookie or capability where this is not possible. Because pointers allow both protected and unprotected access to memory addresses, there are risks associated with using them particularly in the latter case. Primitive pointers are often stored in a format similar to an integer; however, attempting to dereference or "look up" a pointer whose value was never a valid memory address would cause a program to crash. To alleviate this potential problem, as a matter of type safety, pointers are considered a separate type parameterized by the type of data they point to, even if the underlying representation is an integer. Other measures may also be taken (such as validation & bounds checking), to verify the contents of the pointer variable contain a value that is both a valid memory address and within the numerical range that the processor is capable of addressing.
Harold Lawson is credited with the 1964 invention of the pointer. In 2000, Lawson was presented the Computer Pioneer Award by the IEEE “[f]or inventing the pointer variable and introducing this concept into PL/I, thus providing for the first time, the capability to flexibly treat linked lists in a general-purpose high level language”. According to the Oxford English Dictionary, the wordpointer first appeared in print as a stack pointer in a technical memorandum by the System Development Corporation.
In computer science, a pointer is a kind of reference.
A data primitive (or just primitive) is any datum that can be read from or written to computer memory using one memory access (for instance, both a byte and a word are primitives).
A data aggregate (or just aggregate) is a group of primitives that are logically contiguous in memory and that are viewed collectively as one datum (for instance, an aggregate could be 3 logically contiguous bytes, the values of which represent the 3 coordinates of a point in space). When an aggregate is entirely composed of the same type of primitive, the aggregate may be called an array; in a sense, a multi-byte word primitive is an array of bytes, and some programs use words in this way.
In the context of these definitions, a byte is the smallest primitive; each memory address specifies a different byte. The memory address of the initial 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.
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 fewer bits than a full address, but will usually require one additional arithmetic operation to resolve).
Relative addresses are a form of manual memory segmentation, and share many of its advantages and disadvantages. A two-byte offset, containing a 16-bit, unsigned integer, can be used to provide relative addressing for up to 64 kilobytes of a data structure. This can easily be extended to 128K, 256K or 512K if the address pointed to is forced to be aligned on a half-word, word or double-word boundary (but, requiring an additional "shift left" bitwise operation—by 1, 2 or 3 bits—in order to adjust the offset by a factor of 2, 4 or 8, before its addition to the base address). Generally, though, such schemes are a lot of trouble, and for convenience to the programmer absolute addresses (and underlying that, a flat address space) is preferred.
A one byte offset, such as the hexadecimal ASCII value of a character (e.g. X'29') can be used to point to an alternative integer value (or index) in an array (e.g. X'01'). In this way, characters can be very efficiently translated from 'raw data' to 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 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.
Pointers are a very thin abstraction on top of the addressing capabilities provided by most modern architectures. In the simplest scheme, an address, or a numeric index, is assigned to each unit of memory in the system, where the unit is typically either a byte or a word – depending on whether the architecture is byte-addressable or word-addressable – effectively transforming all of memory into a very large array. The system would then also provide an operation to retrieve the value stored in the memory unit at a given address (usually utilizing the machine's general purpose registers).
In the usual case, a pointer is large enough to hold more addresses than there are units of memory in the system. This introduces the possibility that a program may attempt to access an address which corresponds to no unit of memory, either because not enough memory is installed (i.e. beyond the range of available memory) or the architecture does not support such addresses. The first case may, in certain platforms such as the Intel x86 architecture, be called a segmentation fault (segfault). The second case is possible in the current implementation of AMD64, where pointers are 64 bit long and addresses only extend to 48 bits. Pointers must conform to certain rules (canonical addresses), so if a non-canonical pointer is dereferenced, the processor raises a general protection fault.
On the other hand, some systems have more units of memory than there are addresses. In this case, a more complex scheme such as memory segmentation or paging is employed to use different parts of the memory at different times. The last incarnations of the x86 architecture support up to 36 bits of physical memory addresses, which were mapped to the 32-bit linear address space through the PAE paging mechanism. Thus, only 1/16 of the possible total memory may be accessed at a time. Another example in the same computer family was the 16-bit protected mode of the 80286 processor, which, though supporting only 16 MB of physical memory, could access up to 1 GB of virtual memory, but the combination of 16-bit address and segment registers made accessing more than 64 KB in one data structure cumbersome.
In order to provide a consistent interface, some architectures provide memory-mapped I/O, which allows some addresses to refer to units of memory while others refer to device registers of other devices in the computer. There are analogous concepts such as file offsets, array indices, and remote object references that serve some of the same purposes as addresses for other types of objects.
Pointers are directly supported without restrictions in languages such as PL/I, C, C++, Pascal, FreeBASIC, and implicitly in most assembly languages. They are primarily used for constructing references, which in turn are fundamental to constructing nearly all data structures, as well as in passing data between different parts of 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 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).
The basic syntax to define a pointer is:
This declares as the identifier of an object of the following type:
- pointer that points to an object of type
This is usually stated more succinctly as " is a pointer to ."
Because the C language does not specify an implicit initialization for objects of automatic storage duration, care should often be taken to ensure that the address to which points is valid; this is why it is sometimes suggested that a pointer be explicitly initialized to the null pointer value, which is traditionally specified in C with the standardized macro :
Dereferencing a null pointer in C produces undefined behavior, which could be catastrophic. However, most implementations simply halt execution of the program in question, usually with a segmentation fault.
However, initializing pointers unnecessarily could hinder program analysis, thereby hiding bugs.
In any case, once a pointer has been declared, the next logical step is for it to point at something:
This assigns the value of the address of to . For example, if is stored at memory location of 0x8130 then the value of will be 0x8130 after the assignment. To dereference the pointer, an asterisk is used again:
This means take the contents of (which is 0x8130), "locate" that address in memory and set its value to 8. If is later accessed again, its new value will be 8.
This example may be clearer if memory is examined directly. Assume that is located at address 0x8130 in memory and 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:
Address Contents 0x8130 0x00000005 0x8134 0x00000000
(The NULL pointer shown here is 0x00000000.) By assigning the address of to :
yields the following memory values:
Address Contents 0x8130 0x00000005 0x8134 0x00008130
Then by dereferencing by coding:
the computer will take the contents of (which is 0x8130), 'locate' that address, and assign 8 to that location yielding the following memory:
Address Contents 0x8130 0x00000008 0x8134 0x00008130
Clearly, accessing will yield the value of 8 because the previous instruction modified the contents of by way of the pointer .
In C, array indexing is formally defined in terms of pointer arithmetic; that is, the language specification requires that be equivalent to . Thus in C, arrays can be thought of as pointers to consecutive areas of memory (with no gaps), and the syntax for accessing arrays is identical for that which can be used to dereference pointers. For example, an array can be declared and used in the following manner:
This allocates a block of five integers and names the block , which acts as a pointer to the block. Another common use of pointers is to point to dynamically allocated memory from malloc which returns a consecutive block of memory of no less than the requested size that can be used as an array.
While most operators on arrays and pointers are equivalent, the result of the operator differs. In this example, will evaluate to (the size of the array), while will evaluate to , the size of the pointer itself.
Default values of an array can be declared like:
If 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:
- means 0x1000;
- means 0x1004: the "+ 1" means to add the size of 1 , which is 4 bytes;
- means to dereference the contents of . Considering the contents as a memory address (0x1000), look up the value at that location (0x0002);
- means element number , 0-based, of which is translated into .
The last example is how to access the contents of . Breaking it down:
- is the memory location of the (i)th element of , starting at i=0;
- takes that memory address and dereferences it to access the value.
C linked list
Below is an example definition of a linked list in C.
This pointer-recursive definition is essentially the same as the reference-recursive definition from the Haskell programming language:
is the empty list, and is a cons cell of type with another link also of type .
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, consider the following C code:
Dynamic memory allocation
In some programs, the required memory depends on what the user may enter. In such cases the programmer needs to allocate memory dynamically. This is done by allocating memory at the heap rather than on the stack, where variables usually are stored. (Variables can also be stored in the CPU registers, but that's another matter) Dynamic memory allocation can only be made through pointers, and names (like with common variables) can't be given.
Pointers are used to store and manage the addresses of dynamically allocated blocks of memory. Such blocks are used to store data objects or arrays of objects. Most structured and object-oriented languages provide an area of memory, called the heap or free store, from which objects are dynamically allocated.
The example C code below illustrates how structure objects are dynamically allocated and referenced. The standard C library provides the function for allocating memory blocks from the heap. It takes the size of an object to allocate as a parameter and returns a pointer to a newly allocated block of memory suitable for storing the object, or it returns a null pointer if the allocation failed.
The code below illustrates how memory objects are dynamically deallocated, i.e., returned to the heap or free store. The standard C library provides the function for deallocating a previously allocated memory block and returning it back to the heap.
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:
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 0xB8000 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:
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
would be an integer pointer and would be a char pointer. The following would yield a compiler warning of "assignment from incompatible pointer type" under GCC
because and 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
which says to cast the integer pointer of to a char pointer and assign to .
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 (188.8.131.52 Pointers, par. 7):
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 array starts at 0x2000 and is 4 bytes whereas is 1 byte, then will point to 0x2004, but would point to 0x2001. Other risks of casting include loss of data when "wide" data is written to "narrow" locations (e.g. ), 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
As a pointer allows a program to attempt to access an object that may not be defined, pointers can be the origin of a variety of programming errors. However, the usefulness of pointers is so great that it can be difficult to perform programming tasks without them. Consequently, many languages have created constructs designed to provide some of the useful features of pointers without some of their pitfalls, also sometimes referred to as pointer hazards. In this context, pointers that directly address memory (as used in this article) are referred to as raw pointers, by contrast with smart pointers or other variants.
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 behavior, either because the initial value is not a valid address, or because using it may damage other parts of the program. The result is often a segmentation fault, storage violation or wild branch (if used as a function pointer or branch address).
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. Languages with garbage collection prevent this type of error because deallocation is performed automatically when there are no more references in scope.
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.
The Rust programming language introduces a borrow checker, pointer lifetimes, and an optimisation based around optional types for null pointers to eliminate pointer bugs, without resorting to a garbage collector.
Main article: Null pointer
A null pointer has a value reserved for indicating that the pointer does not refer to a valid object. Null pointers are routinely used to represent conditions such as the end of a list of unknown length or the failure to perform some action; this use of null pointers can be compared to nullable types and to the Nothing value in an option type.
An autorelative pointer is a pointer whose value is interpreted as an offset from the address of the pointer itself; thus, if a data structure has an autorelative pointer member that points to some portion of the data structure itself, then the data structure may be relocated in memory without having to update the value of the auto relative pointer.
The cited patent also uses the term self-relative pointer to mean the same thing. However, the meaning of that term has been used in other ways:
- to mean an offset from the address of a structure rather than from the address of the pointer itself;
- to mean a pointer containing its own address, which can be useful for reconstructing in any arbitrary region of memory a collection of data structures that point to each other.
A based pointer is a pointer whose value is an offset from the value of another pointer. This can be used to store and load blocks of data, assigning the address of the beginning of the block to the base pointer.
In some languages, a pointer can reference another pointer, requiring multiple 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:
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, 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:
In this case, if the value of is less than that of , the caller's is properly updated to the address of the new item.
A basic example is in the argv argument to the main function in C (and C++), which is given in the prototype as —this is because the variable itself is a pointer to an array of strings (an array of arrays), so is a pointer to the 0th string (by convention the name of the program), and is the 0th character of the 0th string.
In some languages, a pointer can reference executable code, i.e., it can point to a function, method, or procedure. A function pointer will store the address of a function to be invoked. While this facility can be used to call functions dynamically, it is often a favorite technique of virus and other malicious software writers.
Main article: Dangling pointer
A dangling pointer is a pointer that does not point to a valid object and consequently 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 dangling pointer:
Here, may point to anywhere in memory, so performing the assignment can corrupt an unknown area of memory or trigger a segmentation fault.
In doubly linked lists or tree structures, a back pointer held on an element 'points back' to the item referring to the current element. These are useful for navigation and manipulation, at the expense of greater memory use.
Pointer declaration syntax overview
These pointer declarations cover most variants of pointer declarations. Of course it is possible to have triple pointers, but the main principles behind a triple pointer already exist in a double pointer.
The () and  have a higher priority than *. 
Where a pointer is used as the address of the entry point to a program or start of a function which doesn't return anything 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 behavior 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.
Support in various programming languages
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 , and any attempt to access data through a 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 .
Several old versions of BASIC for the Windows platform had support for STRPTR() to return the address of a string, and for VARPTR() to return the address of a variable. Visual Basic 5 also had support for OBJPTR() to return the address of an object interface, and for an ADDRESSOF operator to return the address of a function. The types of all of these are integers, but their values are equivalent to those held by pointer types.
Newer dialects of BASIC, such as FreeBASIC or BlitzMax, have exhaustive pointer implementations, however. In FreeBASIC, arithmetic on pointers (equivalent to C's ) are treated as though the pointer was a byte width. pointers cannot be dereferenced, as in C. Also, casting between and any other type's pointers will not generate any warnings.
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 (but not between a function pointer and an object pointer). A special pointer type called the “void pointer” allows pointing to any (non-function) object, but is limited by the fact that it cannot be dereferenced directly (it shall be cast). The address itself can often be directly manipulated by casting a pointer to and from an integral type of sufficient size, though the results are implementation-defined and may indeed cause undefined behavior; while earlier C standards did not have an integral type that was guaranteed to be large enough, C99 specifies the typedef name defined in , but an implementation need not provide it.
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. Since C++11, the C++ standard library also provides smart pointers (, and ) which can be used in some situations as a safer 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), and will otherwise invoke undefined behavior. Adding or subtracting from a pointer moves it by a multiple of the size of its datatype. For example, adding 1 to a pointer to 4-byte integer values will increment the pointer's pointed-to byte-address 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 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 as a non-standard extension, treating it as if it were .
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. (Pointer arithmetic with pointers uses byte offsets, because is 1 by definition.) In particular, the C definition explicitly declares that the syntax , which is the -th element of the array , is equivalent to , which is the content of the element pointed by . This implies that is equivalent to , and one can write, e.g., or equally well to access the fourth element of an array .
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 discussion.
The pointer, or , is supported in ANSI C and C++ as a generic pointer type. A pointer to can store the address of any object (not function), and, in C, is implicitly converted to any other object pointer type on assignment, but it must be explicitly cast if dereferenced. K&R C used for the “type-agnostic pointer” purpose (before ANSI C).
C++ does not allow the implicit conversion of to other pointer types, 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 casts.
In C++, there is no (reference to void) to complement (pointer to void), because references behave like aliases to the variables they point to, and there can never be a variable whose type is .