1 files changed, 148 insertions, 0 deletions
diff --git a/lib/pthread-once.c b/lib/pthread-once.c
new file mode 100644
index 00000000..4b4a18d2
--- /dev/null
+++ b/lib/pthread-once.c
@@ -0,0 +1,148 @@
+/* POSIX once-only control.
+   Copyright (C) 2019-2024 Free Software Foundation, Inc.
+
+   This file is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   This file is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2019.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include <pthread.h>
+
+#if (defined _WIN32 && ! defined __CYGWIN__) && USE_WINDOWS_THREADS
+# include "windows-once.h"
+#endif
+
+#if (defined _WIN32 && ! defined __CYGWIN__) && USE_WINDOWS_THREADS
+/* Use Windows threads.  */
+
+int
+pthread_once (pthread_once_t *once_control, void (*initfunction) (void))
+{
+  glwthread_once (once_control, initfunction);
+  return 0;
+}
+
+#elif HAVE_PTHREAD_H
+/* Provide workarounds for POSIX threads.  */
+
+# if defined __CYGWIN__
+
+#  include <stdlib.h>
+
+int
+pthread_once (pthread_once_t *once_control, void (*initfunction) (void))
+{
+  /* In this implementation, we reuse the type
+       typedef struct { pthread_mutex_t mutex; int state; } pthread_once_t;
+       #define PTHREAD_ONCE_INIT { PTHREAD_MUTEX_INITIALIZER, 0 }
+     while assigning the following meaning to the state:
+       state = (<number of waiting threads> << 16) + <1 if done>
+     In other words:
+       state = { unsigned int num_threads : 16; unsigned int done : 16; }
+   */
+  struct actual_state
+    {
+      _Atomic unsigned short num_threads;
+      /* done == 0: initial state
+         done == 1: initfunction executed, lock still active
+         done == 2: initfunction executed, lock no longer usable */
+      _Atomic unsigned short done;
+    };
+  struct actual_state *state_p = (struct actual_state *) &once_control->state;
+  /* This test is not necessary.  It's only an optimization, to establish
+     a fast path for the common case that the 'done' word is already > 0.  */
+  if (state_p->done == 0)
+    {
+      /* Increment num_threads (atomically), to indicate that this thread will
+         possibly take the lock.  */
+      state_p->num_threads += 1;
+      /* Test the 'done' word.  */
+      if (state_p->done == 0)
+        {
+          /* The 'done' word is still zero.  Now take the lock.  */
+          pthread_mutex_lock (&once_control->mutex);
+          /* Test the 'done' word again.  */
+          if (state_p->done == 0)
+            {
+              /* Execute the initfunction.  */
+              (*initfunction) ();
+              /* Set the 'done' word to 1 (atomically).  */
+              state_p->done = 1;
+            }
+          /* Now the 'done' word is 1.  Release the lock.  */
+          pthread_mutex_unlock (&once_control->mutex);
+        }
+      /* Here, done is > 0.  */
+      /* Decrement num_threads (atomically).  */
+      if ((state_p->num_threads -= 1) == 0)
+        {
+          /* num_threads is now zero, and done is > 0.
+             No other thread will need to use the lock.
+             We can therefore destroy the lock, to free resources.  */
+          if (__sync_bool_compare_and_swap (&state_p->done, 1, 2))
+            pthread_mutex_destroy (&once_control->mutex);
+        }
+    }
+  /* Proof of correctness:
+     * num_threads is incremented and then decremented by some threads.
+       Therefore, num_threads always stays >= 0, and is == 0 at the end.
+     * The 'done' word, once > 0, stays > 0 (since it is never assigned 0).
+     * The 'done' word is changed from == 0 to > 0 only while the lock
+       is taken. Therefore, only the first thread that succeeds in taking
+       the lock executes the initfunction and sets the 'done' word to a
+       value > 0; the other threads that take the lock do no side effects
+       between taking and releasing the lock.
+     * The 'done' word does not change any more once it is 2.
+       Therefore, it can be changed from 1 to 2 only once.
+     * pthread_mutex_destroy gets invoked right after 'done' has been changed
+       from 1 to 2.  Therefore, pthread_mutex_destroy gets invoked only once.
+     * After a moment where num_threads was 0 and done was > 0, no thread can
+       reach the pthread_mutex_lock invocation. Proof:
+       - At such a moment, no thread is in the code range between
+           state_p->num_threads += 1
+         and
+           state_p->num_threads -= 1
+       - After such a moment, some thread can increment num_threads, but from
+         there they cannot reach the pthread_mutex_lock invocation, because the
+           if (state_p->done == 0)
+         test prevents that.
+     * From this it follows that:
+       - pthread_mutex_destroy cannot be executed while the lock is taken
+         (because pthread_mutex_destroy is only executed after a moment where
+         num_threads was 0 and done was > 0).
+       - Once pthread_mutex_destroy has been executed, the lock is not used any
+         more.
+   */
+  return 0;
+}
+
+# endif
+
+#else
+/* Provide a dummy implementation for single-threaded applications.  */
+
+int
+pthread_once (pthread_once_t *once_control, void (*initfunction) (void))
+{
+  if (*once_control == 0)
+    {
+      *once_control = ~ 0;
+      initfunction ();
+    }
+  return 0;
+}
+
+#endif