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An exception to
the non-declaration of global variables occurs where a program has several
source files. 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. That declaration must be
proceeded by the word extern. The external variable is really a special type
of global variable. 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.
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