C++ static local function vs global function

What is the utility of having static functions in a file ?

How are they different from having global functions in a file ?

static int Square(int i)
{
   return i * i;
}

int Square(int i)
{
   return i * i;
}

What is the utility of having static functions in a file?

You can use these functions to provide shared implementation logic to other functions within the same file. Various helper functions specific to a file are good candidates to be declared file-static.

How are they different from having global functions in a file?

They are invisible to the linker, allowing other compilation units to define functions with the same signature. Using namespaces alleviates this problem to a large degree, but file-static functions predate namespaces, because they are a feature inherited from the C programming language.

A static function simply means that the linker cannot export the function (i.e. make it visible to other translation units). It makes the function "private" to the current translation unit. It is the same as wrapping the function in an anonymous namespace.

namespace {

    int Square(int i)
    {
       return i * i;
    } 

}

Generally, using an anonymous namespace is the preferred C++ way of achieving this, instead of using the static keyword.

Static functions are visible on the file where they were defined only. You can't refer to them outside of that particular file.