Best way to abstract away an init function?

Question

I am making a low level library that requires initialization to work properly which I implemented with a init function. I am wondering if there is a way to make the init call be called once the user calls a library function ideally without:

1. Any overhead
2. No repeated calls
3. No exposed global variables. (my current solution does this, which I don't quite like)

my current solution as per comment request:

bool isinit = 0;

void init()
{
  isinit = 1;
  // init code
}

void lib_function()
{
  if(!isinit) init();
  // function code
}

The compiler seems to be smart enough (using -0fast on gcc) to not make that comparison each time a lib_function is called, but this still exposes a global variable which I don't like.

Answer 1

Best way to abstract away an init function?

Surely your library has some state. Typically, a library exposes functions that work on a specific structure. Do not use global variables - do not write spaghetti code. Expose the structure that holds the state of your library, and make all functions of your library take a pointer to the structure as an argument. Use a namespace - prepend all exported symbols with a prefix. An init function is just like int lib_init(struct lib_the_struct *t); - it will be self-understandable that users need to initialize the structure with that function before use. For example: fopen(), pthread_create.

---

Write an init function in your library. Write clear documentation stating, that the user of your library has to call the function once before calling any other function. For example: https://curl.se/libcurl/c/curl_global_init.html .

Answer 2

If you're happy with a solution that is a common extension rather than part of the C standard, you can mark your init function with the constructor attribute, which ensures it will be called automatically during program initialization (or during shared library load if you eventually end up using that).