static has only one meaning

static can be confusing with the wrong definition. Here's a simplified example:

The task is to write a counter function that takes no arguments and returns a different number each time, starting at 0 and adding 1 each time.

int current_counter = 0;
int counter(void) {
    return current_counter++;
}

Now imagine someone is using this library. They write a function called counter just for testing. But when they try to link their code, it fails, telling them that counter is being redefined. But no worry, We are C programmers and we have static at our disposal!

Many believe static to have multiple meanings in C.

• static functions are not exported globally
• static local variables are hidden globals
• static global variables are not exported globally What could have caused this mess of keyword overloading?

The answer lies in a shift of perspective. static means compilation unit local. A compilation unit in C is just the files given to the c compiler (gcc, clang, msvc, etc) and what they #include.

Using this definition we can say the following.

• static functions are owned by the compilation unit
• static global variables are owned by the compilation unit
• static local variables are owned by the compilation unit

Using this knowledge, we can rewrite the counter function to not leak counter into the API.

First, move current_counter into a static in counter

int counter(void) {
    static int current_counter = 0;
    return current_counter++;
} 

This will make current_counter not collide with anything else named current.

Next is to make counter a static function

static int counter(void) {
    static int current_counter = 0;
    return current_counter++;
}

This will make counter not visible to files not in the same compilation unit.

Now things are fixed! The counter function and current_counter variable cannot be used in files not in the same compilation unit.

I hope this helped someone.

Comments

[Paul J. Lucas]

static means compilation unit local.

The correct term is translation unit. Hence, in C, static actually means primarily:

• Is local to the translation unit (strictly speaking, has "internal linkage").

If it's a static variable (as opposed to a function), then it also means:

• Is automatically initialized to 0 (or equivalent) prior to the first statement in main().
• Persists for the entire duration of the program (hence, between function calls).

[matthew-salerno]

I really like this interpretation! Unfortunately I already had to learn this the hard way.
I think the additional tiered levels of local/global adds to the confusion, you ran into it yourself when saying that static global variables are not (exported) global.
C can be confusing to get into, but once you get used to it, it can be refreshingly simple. Hopefully this helps someone out!