Static memory allocation

Static memory allocation refers to the process of allocating memory at compile-time before the associated program is executed, unlike dynamic memory allocation or automatic memory allocation where memory is allocated as required at run-time.^[1]

An application of this technique involves a program module (e.g. function or subroutine) declaring static data locally, such that these data are inaccessible in other modules unless references to it are passed as parameters or returned. A single copy of static data is retained and accessible through many calls to the function in which it is declared. Static memory allocation therefore has the advantage of modularising data within a program design in the situation where these data must be retained through the runtime of the program.

The use of static variables within a class in object oriented programming enables a single copy of such data to be shared between all the objects of that class.

Object constants known at compile-time, like string literals, are usually allocated statically. In object-oriented programming, the virtual method tables of classes are usually allocated statically. A statically defined value can also be global in its scope ensuring the sameimmutable value is used throughout a run for consistency.

References

   1.   Static memory allocation: The compiler allocates required memory space for a declared variable. By using the addressof operator, the reserved address is obtained and this address may be assigned to a pointer variable. Since most of the declared variables have static memory, this way of assigning pointer value to a pointer variable is known as static memory allocation. Memory is assigned during compilation time.

2. Dynamic memory allocation: It uses functions such as malloc( ) or calloc( ) to get memory dynamically. If these functions are used to get memory dynamically and the values returned by these functions are assigned to pointer variables, such assignments are known as dynamic memory allocation. Memory is assigned during run time.

What is static memory allocation and dynamic memory allocation?

Static Memory Allocation: Memory is allocated for the declared variable by the compiler. The address can be obtained by using ‘address of’ operator and can be assigned to a pointer. The memory is allocated during compile time. Since most of the declared variables have static memory, this kind of assigning the address of a variable to a pointer is known as static memory allocation.

Dynamic Memory Allocation: Allocation of memory at the time of execution (run time) is known as dynamic memory allocation. The functions calloc() and malloc() support allocating of dynamic memory. Dynamic allocation of memory space is done by using these functions when value is returned by functions and assigned to pointer variables.     

Rationale

The C programming language manages memory statically, automatically, or dynamically. Static-duration variables are allocated in main memory, usually along with the executable code of the program, and persist for the lifetime of the program; automatic-duration variables are allocated on the stack and come and go as functions are called and return. For static-duration and automatic-duration variables, the size of the allocation is required to be compile-time constant (before C99, which allows variable-length automatic arrays^[5]). If the required size is not known until run-time (for example, if data of arbitrary size is being read from the user or from a disk file), then using fixed-size data objects is inadequate.

The lifetime of allocated memory is also a concern. Neither static- nor automatic-duration memory is adequate for all situations. Automatic-allocated data cannot persist across multiple function calls, while static data persists for the life of the program whether it is needed or not. In many situations the programmer requires greater flexibility in managing the lifetime of allocated memory.

These limitations are avoided by using dynamic memory allocation in which memory is more explicitly (but more flexibly) managed, typically, by allocating it from the heap, an area of memory structured for this purpose. In C, the library function malloc is used to allocate a block of memory on the heap. The program accesses this block of memory via a pointer that malloc returns. When the memory is no longer needed, the pointer is passed to free which deallocates the memory so that it can be used for other purposes.

Some platforms provide library calls which allow run-time dynamic allocation from the C stack rather than the heap (e.g. Unixallction ends. The need for this is lessened by changes in the C99 standard, which added support for variable-leoca(),^[6] Microsoft Windows CRTL's malloca()^[7]). This memory is automatically freed when the calling funngth arrays of block scope having sizes determined at runtime.

Transmission medium

A transmission medium (plural transmission media) is a material substance (solid, liquid, gas, or plasma) that can propagate energywaves. For example, the transmission medium for sound received by the ears is usually air, but solids and liquids may also act as transmission media for

sound.

Transmission and reception of data is performed in four steps.

1.    The data is coded as binary numbers at the sender end

2.    A carrier signal is modulated as specified by the binary representation of the data

3.    At the receiving end, the incoming signal is demodulated into the respective binary numbers

4.    Decoding of the binary numbers is perform editcommunications channel

A transmission may be simplex, half-duplex, or full-duplex.

In simplex transmission, signals are transmitted in only one direction; one station is a transmitter and the other is the receiver. In the half-duplex operation, both stations may transmit, but only one at a time. In full duplex operation, both stations may transmit simultaneously. In the latter case, the medium is carrying signals in both directions at same time.

Transmission media is a pathway that carries the information from sender to receiver. We use different types of cables or waves to transmit data. Data is transmitted normally through electrical or electromagnetic signals.

An electrical signal is in the form of current. An electromagnetic signal is series of electromagnetic energy pulses at various frequencies. These signals can be transmitted through copper wires, optical fibers, atmosphere, water and vacuum Different Medias have different properties like bandwidth, delay, cost and ease of installation and maintenance. Transmission media is also calledCommunication channel.

Types of Transmission Media

Transmission media is broadly classified into two groups.

1. Wired or Guided Media or Bound Transmission Media

2. Wireless or Unguided Media or Unbound Transmission Media

Wired or Guided Media or Bound Transmission Media: Bound transmission media are the cables that are tangible or have physical existence and are limited by the physical geography. Popular bound transmission media in use are twisted pair cable, co-axial cable and fiber optical cable. Each of them has its own characteristics like transmission speed, effect of noise, physical appearance, cost etc.

Wireless or Unguided Media or Unbound Transmission Media: Unbound transmission media are the ways of transmitting data without using any cables. These media are not bounded by physical geography. This type of transmission is called Wireless communication. Nowadays wireless communication is becoming popular. Wireless LANs are being installed in office and college campuses. This transmission

A transmission medium can be classified as a:

·         Linear medium, if different waves at any particular point in the medium can be superposed;

·         Bounded medium, if it is finite in extent, otherwise unbounded medium;

·         Uniform medium or homogeneous medium, if its physical properties are unchanged at different points;

·         Isotropic medium, if its physical properties are the same in different directions.

Fiber Optics

Fiber optic cable uses electrical signals to transmit data. It uses light. In fiber optic cable light only moves in one direction for two way communication to take place a second connection must be made between the two devices. It is actually two stands of cable. Each stand is responsible for one

Twisted Pair Cable

The most popular network cabling is Twisted pair. It is light weight, easy to install, inexpensive and support many different types of network. It also supports the speed of 100 mps.Twisted pair cabling is made of pairs of solid or stranded copper twisted along each other. The twists are done to reduce vulnerably to EMI and cross talk. The number of pairs in the cable depends on the type. The copper core is usually 22-AWG or 24-AWG, as measured on the American wire gauge standard. There are two types of twisted pairs cabling

COAXIAL CABLE:

Coaxial cable is very common & widely used commutation media. For example TV wire is usually coaxial.

Coaxial cable gets its name because it contains two conductors that are parallel to each other. The center conductor in the cable is usually copper. The copper can be either a solid wire or stranded martial.

Outside this central Conductor is a non-conductive material. It is usually white, plastic material used to separate the inner Conductor form the outer Conductor. The other Conductor is a fine mesh made from Copper. It is used to help shield the cable form EMI.

Outside the copper mesh is the final protective cover. (as shown in Fig)