Pointers In C Ppt Download

[Niccoletta Boyer]

They are important in C, because they allow us to manipulate the data in the computer's memory. This can reduce the code and improve the performance. If you are familiar with data structures like lists, trees and graphs, you should know that pointers are especially useful for implementing those. And sometimes you even have to use pointers, for example when working with files.

pointers in c ppt download

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Using pointers significantly improves performance for repetitive operations, like traversing iterable data structures (e.g. 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 virtual method tables.

A pointer is a simple, more concrete implementation of the more abstract reference data type. Several languages, especially low-level languages, support some type of pointer, although some have more restrictions on their use than others. While "pointer" has been used to refer to references in general, it more properly applies to data structures whose interface explicitly allows the pointer to be manipulated (arithmetically via .mw-parser-output .vanchor>:target.vanchor-textbackground-color:#b1d2ffpointer arithmetic) as a memory address, as opposed to a magic cookie or capability which does not allow such.[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. Primitive pointers are often stored in a format similar to an integer; however, attempting to dereference or "look up" such a pointer whose value is not a valid memory address could cause a program to crash (or contain invalid data). To alleviate this potential problem, as a matter of type safety, pointers are considered a separate type parameterized by the type of data they point to, even if the underlying representation is an integer. Other measures may also be taken (such as validation and bounds checking), to verify that the pointer variable contains a value that is both a valid memory address and within the numerical range that the processor is capable of addressing.

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

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.

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.

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.

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).

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).[8] Thus in C, arrays can be thought of as pointers to consecutive areas of memory (with no gaps),[8] 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:

This allocates a block of five integers and names the block array, which acts as a pointer to the block. Another common use of pointers is to point to dynamically allocated memory from malloc which returns a consecutive block of memory of no less than the requested size that can be used as an array.

While most operators on arrays and pointers are equivalent, the result of the sizeof operator differs. In this example, sizeof(array) will evaluate to 5*sizeof(int) (the size of the array), while sizeof(ptr) will evaluate to sizeof(int*), the size of the pointer itself.

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:

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.

In many languages, pointers have the additional restriction that the object they point to has a specific type. For example, a pointer may be declared to point to an integer; the language will then attempt to prevent the programmer from pointing it to objects which are not integers, such as floating-point numbers, eliminating some errors.

In 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 byte, then money + 1 will point to 0x2004, but bags + 1 would 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.

In C and C++, even if two pointers compare as equal that doesn't mean they are equivalent. In these languages and LLVM, the rule is interpreted to mean that "just because two pointers point to the same address, does not mean they are equal in the sense that they can be used interchangeably", the difference between the pointers referred to as their provenance.[10] Casting to an integer type such as uintptr_t is implementation-defined and the comparison it provides does not provide any more insight as to whether the two pointers are interchangeable. In addition, further conversion to bytes and arithmetic will throw off optimizers trying to keep track the use of pointers, a problem still being elucidated in academic research.[11]

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.

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