source: azure_iot_hub/trunk/musl-1.1.18/src/exit/atexit.c@ 389

#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);
}