How to create a Singleton in C?

Question

What's the best way to create a singleton in C? A concurrent solution would be nice.

I am aware that C isn't the first language you would use for a singleton.

Answer 1

First, C is not suitable for OO programming. You'd be fighting all the way if you do. Secondly, singletons are just static variables with some encapsulation. So you can use a static global variable. However, global variables typically have far too many ills associated with them. You could otherwise use a function local static variable, like this:

 int *SingletonInt() {      static int instance = 42;      return &instance;  } 

or a smarter macro:

#define SINGLETON(t, inst, init) t* Singleton_##t() { \                  static t inst = init;               \                  return &inst;                       \                 }  #include <stdio.h>    /* actual definition */ SINGLETON(float, finst, 4.2);  int main() {     printf("%f\n", *(Singleton_float()));     return 0; } 

And finally, remember, that singletons are mostly abused. It is difficult to get them right, especially under multi-threaded environments...