Benefits and drawbacks of making all functions in main.c static?

Question

I have heard that, when you have just 1 (main.c) file (or use a "unity build"), there are benefits to be had if you make all your functions static.

I am kind of confused why this (allegedly) isn't optimized by default, since it's not probable that you will include main.c into another file where you will use one of its functions.

I would like to know the benefits and dangers of doing this before implementing it.

Example:

main.c

static int my_func(void){ /*stuff*/ }

int main(void) {
    my_func();
    return 0;
}

Answer 1

You have various chunks of wisdom in the comments, assembled here into a Community Wiki answer.

Jonathan Leffler noted:

The primary benefit of static functions is that the compiler can (and will) aggressively inline them when it knows there is no other code that can call the function. I've had error messages from four levels of inlined function calls (three qualifying “inlined from” lines) on occasion. It's staggering what a compiler will do!

and:

FWIW: my rule of thumb is that every function should be static until it is known that it will be called from code in another file. When it is known that it will be used elsewhere, it should be declared in a header file that is included both where the function is defined and where it is used. (Similar rules apply to file scope variables — aka 'global variables'; they should be static until there's a proven need for them elsewhere, and then they should be declared in a header too.)

The main() function is always called from the startup code, so it is never static. Any function defined in the same file as an unconditionally compiled main() function cannot be reused by other programs. (Library code might contain a conditionally compiled test program for the library function(s) defined in the source file — most of my library code has #ifdef TEST / …test program… / #endif at the end.)

Eirc Postpischil generalized on that:

General rule: Anytime you can write code that says the use of something is limited, do it. Value will not be modified? Make it const . Name only needs to be used in a certain section? Declare it in the innermost enclosing scope. Name does not need to be linked externally? Make it static . Every limitation both shrinks the window for a bug to be created and may remove complications that interfere with optimization.