Example (C Programming Language)
File 1:
int GlobalVariable; // implicit definition void SomeFunction(void); // function prototype (declaration) int main { GlobalVariable = 1; SomeFunction; return 0; }File 2:
extern int GlobalVariable; // explicit declaration void SomeFunction(void) { // function header (definition) ++GlobalVariable; }In this example, the variable GlobalVariable is defined in File 1. In order to utilize the same variable in File 2, it must be declared. Regardless of the number of files, a global variable is only defined once, however, it must be declared in any file outside of the one containing the definition.
If the program is in several source files, and a variable is defined in file1 and used in file2 and file3, then extern declarations are needed in file2 and file3 to connect the occurrences of the variable. The usual practice is to collect extern declarations of variables and functions in a separate file, historically called a header, that is included by #include at the front of each source file. The suffix .h is conventional for header names.
—The C Programming LanguageRead more about this topic: External Variable
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