Where a global variable is declared in one file, but used by functions from another, then the variable is called an external variable in these functions, and must be declared as such. The declaration must be preceeded by the word extern. The declaration is required so the compiler can find the type of the variable without having to search through several source files for the declaration.
Global and external variables can be of any legal type. They can be initialised, but the initialisation takes place when the program starts up, before entry to the main function.
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