fusl/src/exit/atexit.c

#include <stdlib.h>
#include <stdint.h>
#include "libc.h"

/* Ensure that at least 32 atexit handlers can be registered without malloc */
#define COUNT 32

static struct fl {
  struct fl* next;
  void (*f[COUNT])(void*);
  void* a[COUNT];
} builtin, *head;

static int slot;
static volatile int lock[2];

void __funcs_on_exit() {
  void (*func)(void*), *arg;
  LOCK(lock);
  for (; head; head = head->next, slot = COUNT)
    while (slot-- > 0) {
      func = head->f[slot];
      arg = head->a[slot];
      UNLOCK(lock);
      func(arg);
      LOCK(lock);
    }
}

void __cxa_finalize(void* dso) {}

int __cxa_atexit(void (*func)(void*), void* arg, void* dso) {
  LOCK(lock);

  /* Defer initialization of head so it can be in BSS */
  if (!head)
    head = &builtin;

  /* If the current function list is full, add a new one */
  if (slot == COUNT) {
    struct fl* new_fl = calloc(sizeof(struct fl), 1);
    if (!new_fl) {
      UNLOCK(lock);
      return -1;
    }
    new_fl->next = head;
    head = new_fl;
    slot = 0;
  }

  /* Append function to the list. */
  head->f[slot] = func;
  head->a[slot] = arg;
  slot++;

  UNLOCK(lock);
  return 0;
}

static void call(void* p) {
  ((void (*)(void))(uintptr_t)p)();
}

int atexit(void (*func)(void)) {
  return __cxa_atexit(call, (void*)(uintptr_t)func, 0);
}