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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.
stdarg.h is a header in the C standard library of the C programming language that allows functions to accept an indefinite number of arguments. [1] It provides facilities for stepping through a list of function arguments of unknown number and type. C++ provides this functionality in the header cstdarg.
Such extern declarations are often placed in a shared header file, since it is common practice for all .c files in a project to include at least one .h file: the standard header file errno.h is an example, making the errno variable accessible to all modules in a project.
The details differ between C (where only objects and functions - but not types - have linkage) and C++ and between this simplified overview. Linkage between languages must be done with some care, as different languages adorn their external symbols differently. A common idiom uses extern "C" to link C++ and C code.
Likewise, for C code to call a C++ function bar(), the C++ code for bar() must be declared with extern "C". A common practice for header files to maintain both C and C++ compatibility is to make its declaration be extern "C" for the scope of the header: [ 21 ]
Some things, like types, templates, and extern inline functions, can be defined in more than one translation unit. For a given entity, each definition must have the same sequence of tokens. Non-extern objects and functions in different translation units are different entities, even if their names and types are the same.
Templates are a feature of the C++ programming language that allows functions and classes to operate with generic types.This allows a function or class declaration to reference via a generic variable another different class (built-in or newly declared data type) without creating full declaration for each of these different classes.