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Every variable used in a C program needs to be made known by a
declaration. It can be used only after it has been declared.
It is an error to declare a variable name more than once in the same
scope; an exception is that extern declarations and tentative
definitions can coexist with another declaration of the same
variable.
Variables can be declared anywhere within a block or file. (Older versions of C required that all variable declarations within a block occur before any statements.)
Variables declared within a function or block are local to it. This means that the variable name is visible only until the end of that function or block, and the memory space is allocated only while control is within it.
Variables declared at the top level in a file are called file-scope. They are assigned fixed, distinct memory locations, so they retain their values for the whole execution of the program.