Add a "fork" of musl as //fusl.

fusl/src/exit/atexit.c

Index: fusl/src/exit/atexit.c
diff --git a/fusl/src/exit/atexit.c b/fusl/src/exit/atexit.c
new file mode 100644
index 0000000000000000000000000000000000000000..2b58b8bbf7416ad378f0068553417f9c50ff9c3f
--- /dev/null
+++ b/fusl/src/exit/atexit.c
@@ -0,0 +1,71 @@
+#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);
+}