Define external variable declaration, Computer Engineering

Assignment Help:

Summarize the distinction between an external variable definition and an external variable declaration.

When we have ''declared'' a variable, we have meant that we have told the compiler about the variable; i.e. its type and its name, as well as allocated a memory cell for the variable (either locally or globally). This latter action of the compiler, allocation of storage, is more properly called the definition of the variable. The stricter definition of declaration is simply to describe information ''about'' the variable.  For automatic and register variables, there is no difference between definition and declaration. The process of declaring an automatic or a register variable defines the variable name and allocates appropriate memory. However, for external variables, these two operations may occur independently. This is important because memory for a variable must be allocated only once, to ensure that access to the variable always refers to the same cell.

If an external variable is to be used in a file other than the one in which it is defined, a mechanism is needed to ''connect'' such a use with the uniquely defined external variable cell allocated for it. This process of connecting the references of the same external variable in different files, is called resolving the references.

External  variables  may  be  defined  and  declared  with  a  declaration  statement outside any function, with no storage class specifier. Such a declaration allocates memory for the variable. A declaration statement may also be used to simply declare a variable name with the extern storage class specifier at the beginning of the declaration. Such a declaration specifies that the variable is defined elsewhere, i.e. memory for this variable is allocated in another file. Thus, access to an external variable in a file other than the one in which it is defined is possible if it is declared with the keyword extern; no new memory is allocated. Such a  declaration tells  the compiler  that  the  variable  is  defined  elsewhere,  and  the code  is compiled with the external variable left unresolved. The reference to the external variable is resolved during the linking process

 


Related Discussions:- Define external variable declaration

Write shorts notes on remote procedure call, Write shorts notes on Remote P...

Write shorts notes on Remote Procedure Call (RPC) The facility whihc was created to help the programmers write client server software is termed as Remote Procedure Call. Within

Disk operating system, Q. Disk operating system? The operating system (...

Q. Disk operating system? The operating system (OS) is the first program that should be loaded into the memory of your PC before you can use it for any application. You can st

Function of in network access layer in tcp/ip protocol stack, What is the f...

What is the function of in network access layer in TCP/IP protocol stack? Function of Network Access Layer: The network access layer is the lowest layer within Internet

Calculus, i want an assignment on application of integraton and my topc of ...

i want an assignment on application of integraton and my topc of assifment is area under the curve includin solved examples

How the temperature effecting the delays in a chip, How the temperature eff...

How the temperature effecting the delays in a chip The delays are directly proportional to the temperature. As the temperature enhances the delays are enhances and chip wil

Define syntax of barrier directive, Q. Define syntax of barrier directive? ...

Q. Define syntax of barrier directive? Barrier Directive  The syntax of barrier directive is  #pragma omp barrier When a thread attains barrier it waits till all threa

Define apple mobileme calendar, Define Apple MobileMe Calendar The Mobi...

Define Apple MobileMe Calendar The MobileMe Calendar is a web-based calendar that can be accessed from any computer linked to the Internet, Mac or Windows. What makes it more u

Propositional truth tables, Propositional truth tables: There {X\t} is...

Propositional truth tables: There {X\t} is a substitution that replaces all occurances of variable X with a term representing an object t as:  ∀X. A is true if and only

What are set up time and hold time constraints, What are set up time and ho...

What are set up time and hold time constraints? What do they mean? Which one is crucial for establishing maximum clock frequency of a circuit? Set up time is the amount of time

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd