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An external variable can be accessed by all the functions in all the modules of a program. It is a global variable.For a function to be able to use the variable, a declaration or the definition of the external variable must lie before the function definition in the source code.
The C-language version was preceded by Kernighan's own 1972 A Tutorial Introduction to the Language B, [4] where the first known version of the program is found in an example used to illustrate external variables:
Variables declared with file scope are visible between their declaration and the end of the compilation unit (.c file) (unless shadowed by a like-named object in a nearer scope, such as a local variable); and they implicitly have external linkage and are thus visible to not only the .c file or compilation unit containing their declarations but ...
A basic example is in the argv argument to the main function in C (and C++), which is given in the prototype as char **argv—this is because the variable argv itself is a pointer to an array of strings (an array of arrays), so *argv is a pointer to the 0th string (by convention the name of the program), and **argv is the 0th character of the ...
A snippet of C code which prints "Hello, World!". The syntax of the C programming language is the set of rules governing writing of software in C. It is designed to allow for programs that are extremely terse, have a close relationship with the resulting object code, and yet provide relatively high-level data abstraction.
A typical example is the static variables in C and C++. A Stack-dynamic variable is known as local variable, which is bound when the declaration statement is executed, and it is deallocated when the procedure returns. The main examples are local variables in C subprograms and Java methods.
In programming languages, particularly the compiled ones like C, C++, and D, linkage describes how names can or can not refer to the same entity throughout the whole program or one single translation unit. The static keyword is used in C to restrict the visibility of a function or variable to its translation unit. This is also valid in C++.
In C and C++, volatile is a type qualifier, like const, and is a part of a type (e.g. the type of a variable or field). The behavior of the volatile keyword in C and C++ is sometimes given in terms of suppressing optimizations of an optimizing compiler: 1- don't remove existing volatile reads and writes, 2- don't add new volatile reads and writes, and 3- don't reorder volatile reads and writes.