Initial Contribution
diff --git a/libc/stdlib/atexit.c b/libc/stdlib/atexit.c
new file mode 100644
index 0000000..97eac4a
--- /dev/null
+++ b/libc/stdlib/atexit.c
@@ -0,0 +1,133 @@
+/* $OpenBSD: atexit.c,v 1.12 2006/02/22 07:16:32 otto Exp $ */
+/*
+ * Copyright (c) 2002 Daniel Hartmeier
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ * - Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following
+ * disclaimer in the documentation and/or other materials provided
+ * with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include <sys/types.h>
+#include <sys/mman.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include "atexit.h"
+#include "thread_private.h"
+
+int __atexit_invalid = 1;
+struct atexit *__atexit;
+
+/*
+ * Function pointers are stored in a linked list of pages. The list
+ * is initially empty, and pages are allocated on demand. The first
+ * function pointer in the first allocated page (the last one in
+ * the linked list) is reserved for the cleanup function.
+ *
+ * Outside the following two functions, all pages are mprotect()'ed
+ * to prevent unintentional/malicious corruption.
+ */
+
+/*
+ * Register a function to be performed at exit.
+ */
+int
+atexit(void (*fn)(void))
+{
+ struct atexit *p;
+ int pgsize = getpagesize();
+ int ret = -1;
+
+ if (pgsize < (int)sizeof(*p))
+ return (-1);
+ _ATEXIT_LOCK();
+ p = __atexit;
+ if (p != NULL) {
+ if (p->ind + 1 >= p->max)
+ p = NULL;
+ else if (mprotect(p, pgsize, PROT_READ | PROT_WRITE))
+ goto unlock;
+ }
+ if (p == NULL) {
+ p = mmap(NULL, pgsize, PROT_READ | PROT_WRITE,
+ MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+ if (p == MAP_FAILED)
+ goto unlock;
+ if (__atexit == NULL) {
+ p->fns[0] = NULL;
+ p->ind = 1;
+ } else
+ p->ind = 0;
+ p->max = (pgsize - ((char *)&p->fns[0] - (char *)p)) /
+ sizeof(p->fns[0]);
+ p->next = __atexit;
+ __atexit = p;
+ if (__atexit_invalid)
+ __atexit_invalid = 0;
+ }
+ p->fns[p->ind++] = fn;
+ if (mprotect(p, pgsize, PROT_READ))
+ goto unlock;
+ ret = 0;
+unlock:
+ _ATEXIT_UNLOCK();
+ return (ret);
+}
+
+/*
+ * Register the cleanup function
+ */
+void
+__atexit_register_cleanup(void (*fn)(void))
+{
+ struct atexit *p;
+ int pgsize = getpagesize();
+
+ if (pgsize < (int)sizeof(*p))
+ return;
+ _ATEXIT_LOCK();
+ p = __atexit;
+ while (p != NULL && p->next != NULL)
+ p = p->next;
+ if (p == NULL) {
+ p = mmap(NULL, pgsize, PROT_READ | PROT_WRITE,
+ MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+ if (p == MAP_FAILED)
+ goto unlock;
+ p->ind = 1;
+ p->max = (pgsize - ((char *)&p->fns[0] - (char *)p)) /
+ sizeof(p->fns[0]);
+ p->next = NULL;
+ __atexit = p;
+ if (__atexit_invalid)
+ __atexit_invalid = 0;
+ } else {
+ if (mprotect(p, pgsize, PROT_READ | PROT_WRITE))
+ goto unlock;
+ }
+ p->fns[0] = fn;
+ mprotect(p, pgsize, PROT_READ);
+unlock:
+ _ATEXIT_UNLOCK();
+}