Make more fixes and improvements

- Remove PAGESIZE constant
- Fix realloc() documentation
- Fix ttyname_r() error reporting
- Make forking more reliable on Windows
- Make execvp() a few microseconds faster
- Make system() a few microseconds faster
- Tighten up the socket-related magic numbers
- Loosen restrictions on mmap() offset alignment
- Improve GetProgramExecutableName() with getenv("_")
- Use mkstemp() as basis for mktemp(), tmpfile(), tmpfd()
- Fix flakes in pthread_cancel_test, unix_test, fork_test
- Fix recently introduced futex stack overflow regression
- Let sockets be passed as stdio to subprocesses on Windows
- Improve security of bind() on Windows w/ SO_EXCLUSIVEADDRUSE

libc/intrin/cosmo_once.c

@@ -0,0 +1,62 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2023 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/cosmo.h"
+#include "libc/errno.h"
+#include "libc/intrin/atomic.h"
+#include "libc/thread/thread.h"
+
+#define INIT     0
+#define CALLING  1
+#define FINISHED 2
+
+/**
+ * Ensures initialization function is called exactly once.
+ *
+ * This is the same as `pthread_once` except that it always uses a tiny
+ * spinlock implementation and won't make any system calls. It's needed
+ * since this function is an upstream dependency of both pthread_once()
+ * and nsync_once(). Most code should favor calling those functions.
+ *
+ * @return 0 on success, or errno on error
+ */
+errno_t cosmo_once(_Atomic(uint32_t) *once, void init(void)) {
+  uint32_t old;
+  switch ((old = atomic_load_explicit(once, memory_order_relaxed))) {
+    case INIT:
+      if (atomic_compare_exchange_strong_explicit(once, &old, CALLING,
+                                                  memory_order_acquire,
+                                                  memory_order_relaxed)) {
+        init();
+        atomic_store_explicit(once, FINISHED, memory_order_release);
+        return 0;
+      }
+      // fallthrough
+    case CALLING:
+      for (;;) {
+        if (atomic_load_explicit(once, memory_order_acquire) != CALLING) {
+          break;
+        }
+      }
+      return 0;
+    case FINISHED:
+      return 0;
+    default:
+      return EINVAL;
+  }
+}