What is a "static" function?

The question was about plain c functions, not c++ static methods, as clarified in comments.

Ok, I understand what a static variable is, but what is a static function?

And why is it that if I declare a function, let's say void print_matrix , in let's say ac (WITHOUT ah ) and include "ac" - I get "print_matrix@@....) already defined in a.obj" , BUT if I declare it as static void print_matrix then it compiles?

UPDATE Just to clear things up - I know that including .c is bad, as many of you pointed out. I just do it to temporarily clear space in main.c until I have a better idea of how to group all those functions into proper .h and .c files. Just a temporary, quick solution.

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static functions are functions that are only visible to other functions in the same file (more precisely the same translation unit).

EDIT : For those who thought, that the author of the questions meant a 'class method': As the question is tagged C he means a plain old C function. For (C++/Java/...) class methods, static means that this method can be called on the class itself, no instance of that class necessary.

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There is a big difference between static functions in C and static member functions in C++. In C, a static function is not visible outside of its translation unit, which is the object file it is compiled into. In other words, making a function static limits its scope. You can think of a static function as being "private" to its *.c file (although that is not strictly correct).

In C++, "static" can also apply to member functions and data members of classes. A static data member is also called a "class variable", while a non-static data member is an "instance variable". This is Smalltalk terminology. This means that there is only one copy of a static data member shared by all objects of a class, while each object has its own copy of a non-static data member. So a static data member is essentially a global variable, that is a member of a class.

Non-static member functions can access all data members of the class: static and non-static. Static member functions can only operate on the static data members.

One way to think about this is that in C++ static data members and static member functions do not belong to any object, but to the entire class.

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There are two uses for the keyword static when it comes to functions in C++.

The first is to mark the function as having internal linkage so it cannot be referenced in other translation units. This usage is deprecated in C++. Unnamed namespaces are preferred for this usage.

// inside some .cpp file:

static void foo();    // old "C" way of having internal linkage

// C++ way:
namespace
{
   void this_function_has_internal_linkage()
   {
      // ...
   }
}

The second usage is in the context of a class. If a class has a static member function, that means the function is a member of the class (and has the usual access to other members), but it doesn't need to be invoked through a particular object. In other words, inside that function, there is no "this" pointer.

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