What is the difference between “File scope” and “program scope”

Question

A variable declared globally is said to having program scope
A variable declared globally with static keyword is said to have file scope.

For example:

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Answer 1

In C99, there's nothing called "program scope". In your example variable x has a file scope which terminates at the end of translation unit. Variables y and z which are declared static also have the file scope but with internal linkage.

C99 (6.2.2/3) If the declaration of a file scope identifier for an object or a function contains the storage class specifier static, the identifier has internal linkage

Also, the variable x has an external linkage which means the name x can is accessible to other translation units or throughout the program.

C99 (6.2.2/5) If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.