What is a “static” function?

问题:

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 a.c (WITHOUT a.h) and include \"a.c\" - 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.

回答1:

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.

回答2:

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.

回答3:

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.

回答4:

Minimal runnable multi-file scope example

Here I illustrate how static affects the scope of function definitions across multiple files.

a.c

#include <stdio.h>

/* Undefined behavior: already defined in main.
 * Binutils 2.24 gives an error and refuses to link.
 * https://stackoverflow.com/questions/27667277/why-does-borland-compile-with-multiple-definitions-of-same-object-in-different-c
 */
/*void f() { puts(\"a f\"); }*/

/* OK: only declared, not defined. Will use the one in main. */
void f(void);

/* OK: only visible to this file. */
static void sf() { puts(\"a sf\"); }

void a() {
    f();
    sf();
}

main.c

#include <stdio.h>

void a(void);        

void f() { puts(\"main f\"); }

static void sf() { puts(\"main sf\"); }

void m() {
    f();
    sf();
}

int main() {
    m();
    a();
    return 0;
}

Compile

gcc -c a.c -o a.o
gcc -c main.c -o main.o
gcc -o main main.o a.o

Output

main f
main sf
main f
a sf

Interpretation

• there are two separate functions sf, one for each file
• there is a single shared function f

As usual, the smaller the scope, the better, so always declare functions static if you can.

In C programming, files are often used to represent \"classes\", and static functions represent \"private\" methods of the class.

A common C pattern is to pass a this struct around as the first \"method\" argument, which is basically what C++ does under the hood.

What standards say about it

C99 N1256 draft 6.7.1 \"Storage-class specifiers\" says that static is a \"storage-class specifier\".

6.2.2/3 \"Linkages of identifiers\" says static implies internal linkage:

If the declaration of a file scope identifier for an object or a function contains the storage-class specifier static, the identifier has internal linkage.

and 6.2.2/2 says that internal linkage behaves like in our example:

In the set of translation units and libraries that constitutes an entire program, each declaration of a particular identifier with external linkage denotes the same object or function. Within one translation unit, each declaration of an identifier with internal linkage denotes the same object or function.

where \"translation unit\" is a source file after preprocessing.

How GCC implements it for ELF (Linux)?

With the STB_LOCAL binding.

If we compile:

int f() { return 0; }
static int sf() { return 0; }

and disassemble the symbol table with:

readelf -s main.o

the output contains:

Num:    Value          Size Type    Bind   Vis      Ndx Name
  5: 000000000000000b    11 FUNC    LOCAL  DEFAULT    1 sf
  9: 0000000000000000    11 FUNC    GLOBAL DEFAULT    1 f

so the binding is the only significant difference between them. Value is just their offset into the .bss section, so we expect it to differ.

STB_LOCAL is documented on the ELF spec at http://www.sco.com/developers/gabi/2003-12-17/ch4.symtab.html:

STB_LOCAL Local symbols are not visible outside the object file containing their definition. Local symbols of the same name may exist in multiple files without interfering with each other

which makes it a perfect choice to represent static.

Functions without static are STB_GLOBAL, and the spec says:

When the link editor combines several relocatable object files, it does not allow multiple definitions of STB_GLOBAL symbols with the same name.

which is coherent with the link errors on multiple non static definitions.

If we crank up the optimization with -O3, the sf symbol is removed entirely from the symbol table: it cannot be used from outside anyways. TODO why keep static functions on the symbol table at all when there is no optimization? Can they be used for anything?

See also

• Same for variables: https://stackoverflow.com/a/14339047/895245
• extern is the opposite of static, and functions are already extern by default: How do I use extern to share variables between source files?

Try it yourself

Example on GitHub for you to play with.

回答5:

The following is about plain C functions - in a C++ class the modifier \'static\' has another meaning.

If you have just one file, this modifier makes absolutely no difference. The difference comes in bigger projects with multiple files:

In C, every \"module\" (a combination of sample.c and sample.h) is compiled independently and afterwards every of those compiled object files (sample.o) are linked together to an executable file by the linker.

Let\'s say you have several files that you include in your main file and two of them have a function that is only used internally for convenience called add(int a, b) - the compiler would easily create object files for those two modules, but the linker will throw an error, because it finds two functions with the same name and it does not know which one it should use (even if there\'s nothing to link, because they aren\'t used somewhere else but in it\'s own file).

This is why you make this function, which is only used internal, a static function. In this case the compiler does not create the typical \"you can link this thing\"-flag for the linker, so that the linker does not see this function and will not generate an error.

回答6:

First: It\'s generally a bad idea to include a .cpp file in another file - it leads to problems like this :-) The normal way is to create separate compilation units, and add a header file for the included file.

Secondly:

C++ has some confusing terminology here - I didn\'t know about it until pointed out in comments.

a) static functions - inherited from C, and what you are talking about here. Outside any class. A static function means that it isn\'t visible outside the current compilation unit - so in your case a.obj has a copy and your other code has an independent copy. (Bloating the final executable with multiple copies of the code).

b) static member function - what Object Orientation terms a static method. Lives inside a class. You call this with the class rather than through an object instance.

These two different static function definitions are completely different. Be careful - here be dragons.

回答7:

static function definitions will mark this symbol as internal. So it will not be visible for linking from outside, but only to functions in the same compilation unit, usually the same file.

回答8:

A static function is one that can be called on the class itself, as opposed to an instance of the class.

For example a non-static would be:

Person* tom = new Person();
tom->setName(\"Tom\");

This method works on an instance of the class, not the class itself. However you can have a static method that can work without having an instance. This is sometimes used in the Factory pattern:

Person* tom = Person::createNewPerson();

回答9:

The answer to static function depends on the language:

1) In languages without OOPS like C, it means that the function is accessible only within the file where its defined.

2)In languages with OOPS like C++ , it means that the function can be called directly on the class without creating an instance of it.

回答10:

Minor nit: static functions are visible to a translation unit, which for most practical cases is the file the function is defined in. The error you are getting is commonly referred to as violation of the One Definition Rule.

The standard probably says something like:

\"Every program shall contain exactly one definition of every noninline function or object that is used in that program; no diagnostic required.\"

That is the C way of looking at static functions. This is deprecated in C++ however.

In C++, additionally, you can declare member functions static. These are mostly metafunctions i.e. they do not describe/modify a particular object\'s behavior/state but act on the whole class itself. Also, this means that you do not need to create an object to call a static member function. Further, this also means, you only get access to static member variables from within such a function.

I\'d add to Parrot\'s example the Singleton pattern which is based on this sort of a static member function to get/use a single object throughout the lifetime of a program.