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A variable declaration at the top level in a file (not inside a function definition) declares a file-scope variable. Loading a program allocates the storage for all the file-scope variables in it, and initializes them too.
Each file-scope variable is either static (limited to one
compilation module) or global (shared with all compilation
modules in the program). To make the variable static, write the
static at the start of the declaration. Omitting
static makes the variable global.
The initial value for a file-scope variable can’t depend on the contents of storage, and can’t call any functions.
int foo = 5; /* Valid. */ int bar = foo; /* Invalid! */ int bar = sin (1.0); /* Invalid! */
But it can use the address of another file-scope variable:
int foo; int *bar = &foo; /* Valid. */ int arr; int *bar3 = &arr; /* Valid. */ int *bar4 = arr + 4; /* Valid. */
It is valid for a module to have multiple declarations for a file-scope variable, as long as they are all global or all static, but at most one declaration can specify an initial value for it.