Variables with external linkage are often simply called external variables. They necessarily have static storage duration and file scope. External variables are defined outside, and hence external to, any function. For example, they could be declared above the
main() function or in a header file. You can use an external variable in any function that follows the external variable’s definition in the file. Thus, external variables are also termed global variables, in contrast to automatic variables, which are local variables.
On the one hand, an external variable has to be declared in each file that uses the variable. On the other hand, C++ has the “one definition rule” (also known as