Why declare a variable or function static in C?

I understand what static does, but not why开发者_如何转开发 we use it. Is it just for keeping the abstraction layer?

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There are a few reasons to use static in C.

When used with functions, yes the intention is for creating abstraction. The original term for the scope of a C source code file was "translation unit." The static functions may only be reached from within the same translation unit. These static functions are similar to private methods in C++, liberally interpreted (in that analogy, a translation unit defines a class).

Static data at a global level is also not accessible from outside the translation unit, and this is also used for creating an abstraction. Additionally, all static data is initialized to zero, so static may be used to control initialization.

Static at the local ("automatic") variable level is used to abstract the implementation of the function which maintains state across calls, but avoids using a variable at translation unit scope. Again, the variables are initialized to zero due to static qualification.

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The keyword static has several uses; Outside of a function it simply limits the visibility of a function or variable to the compilation unit (.c file) the function or variable occurs in. That way the function or variable doesn't become global. This is a good thing, it promotes a kind of "need to know" principle (don't expose things that don't need to be exposed). Static variables of this type are zero initialized, but of course global variables are also zero initialized, so the static keyword is not responsible for zero initialization per se.

Variables can also be declared static inside a function. This feature means the variable is not automatic, i.e. allocated and freed on the stack with each invocation of the function. Instead the variable is allocated in the static data area, it is initialized to zero and persists for the life of the program. If the function modifies it during one invocation, the new modified value will be available at the next invocation. This sounds like a good thing, but there are good reasons "auto" is the default, and "static" variables within functions should be used sparingly. Briefly, auto variables are more memory efficient, and are essential if you want your function to be thread safe.

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static is used as both a storage class specifier and a linkage specifier. As a linkage specifier it restricts the scope of an otherwise global variable or function to a single compilation unit. This allows, for example a compilation unit to have variables and functions with the same identifier names as other compilation units but without causing a clash, since such identifiers are 'hidden' from the linker. This is useful if you are creating a library for example and need internal 'helper' functions that must not cause a conflict with user code.

As a storage class specifier applied to a local variable, it has different semantics entirely, but your question seems to imply that you are referring to static linkage.

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Static functions in C

In C, functions are global by default. The “static” keyword before a function name makes it static. For example, below function fun() is static.

static int fun(void)
{
  printf("I am a static function ");
}

Unlike global functions in C, access to static functions is restricted to the file where they are declared. Therefore, when we want to restrict access to functions, we make them static. Another reason for making functions static can be reuse of the same function name in other files.

For example, if we store following program in one file file1.c

/* Inside file1.c */
static void fun1(void)
{
  puts("fun1 called");
}

And store following program in another file file2.c

/* Iinside file2.c  */
int main(void)
{
  fun1(); 
  getchar();
  return 0;  
}

Now, if we compile the above code with command gcc file2.c file1.c, we get the error undefined reference to fun1. This is because fun1 is declared static in file1.c and cannot be used in file2.c. See also the explanation here, where the codes come from.