Two functions, one object
int visited_entries = 0; void mark_visited(void) { ++visited_entries; } int main(void) { mark_visited(); return visited_entries != 1; } The declaration of visited_entries is outside either function, at file scope. Both functions name the same object: mark_visited changes it, and main reads the changed value. section 2.2 section 10.2.2 section 9.2
A global variable is a file-scope object used as shared state. Its name is not confined to a function’s block; where it can be used depends on its scope and linkage. Function parameters, by contrast, receive values when a function is called. If two functions need to manipulate the same object rather than merely exchange values, making its declaration visible to both is one possibility. section 2.2 section 10.2.2
TAKEAWAY: Avoid a global variable when the object can instead be supplied through a function’s interface. Shared global state fixes which object a function operates on, while modifications may be scattered across the program. A function that modifies a global object—or reads changeable state other than through its arguments—is not pure: its behavior is not determined solely by its parameter values and return value. section 10.2.2
When a file-scope variable is necessary, name the state it represents. Names such as visited_entries or an initialization flag describe changing state, unlike a global constant, whose name identifies an artifact with a particular meaning. section 9.2
