C/C++: Static function in header file, what does it mean?

Question

I know what it means when declared in source file. I reading some code, find that static function in header files could be invoke in other files.

Answer 1

It will effectively create a separate static function with the same name inside every cpp file it is included into. The same applies to global variables.

Answer 2

Is the function defined in the header file? So that the actual code is given directly in the function, like this:

static int addTwo(int x)
{
  return x + 2;
}

Then that's just a way of providing a useful function to many different C files. Each C file that includes the header will get its own definition that it can call. This of course wastes memory, and is (in my opinion) a quite ugly thing to be doing, since having executable code in a header is generally not a good idea.

Remember that #include:ing a header basically just pastes the contents of the header (and any other headers included by it) into the C file as seen by the compiler. The compiler never knows that the one particular function definition came from a header file.

Answer 3

As others are saying, it has exactly the same meaning as a static function in the .c file itself. This is because there is no semantic difference between .c and .h files; there is only the compilation unit made up of the file actually passed to the compiler (usually named .c) with the contents of any and all files named in #include lines (usually named .h) inserted into the stream as they are seen by the preprocessor.

The convention that the C source is in a file named .c and public declarations are in files named .h is only a convention. But it is generally a good one. Under that convention, the only things that should appear in .h files are declarations so that you generally avoid having the same symbol defined more than once in a single program.

In this particular case, the static keyword makes the symbol be private to the module, so there isn't a multiple-definition conflict waiting to cause trouble. So in that one sense, it is safe to do. But in the absence of a guarantee that the function would be inlined, you take the risk that the function would be instantiated in every module that happened to #include that header file which at best is a waste of memory in the code segment.

I am not certain of what use cases would justify doing this at all in a generally available public header.

If the .h file is generated code and only included in a single .c file, then I would personally name the file something other than .h to emphasize that it isn't actually a public header at all. For example, a utility that converts a binary file into an initialized variable definition might write a file that is intended to be used via #include and could very well contain a static declaration of the variable, and possibly even static definitions of accessor or other related utility functions.

Answer 4

There is not semantic difference in defining in source file or header file, basically both means the same in plain C when using static keyword that, you are limiting the scope.

However, there is a problem in writing this in header file, this is because every time you include the header in a source file you'll have a copy of the function with same implementation which is much similar to have a normal function defined in header file. By adding the definition in header you are not achieving the what the static function is meant for.

Therefore, I suggest you should have your implementation only in your source file and not in header.