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+// Copyright (c) 2007, Google Inc.
+// 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.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+// 
+// 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
+// OWNER 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.
+// 
+// ---
+//
+// A simple mutex wrapper, supporting locks and read-write locks.
+// You should assume the locks are *not* re-entrant.
+//
+// This class is meant to be internal-only and should be wrapped by an
+// internal namespace.  Before you use this module, please give the
+// name of your internal namespace for this module.  Or, if you want
+// to expose it, you'll want to move it to the Google namespace.  We
+// cannot put this class in global namespace because there can be some
+// problems when we have multiple versions of Mutex in each shared object.
+//
+// NOTE: by default, we have #ifdef'ed out the TryLock() method.
+//       This is for two reasons:
+// 1) TryLock() under Windows is a bit annoying (it requires a
+//    #define to be defined very early).
+// 2) TryLock() is broken for NO_THREADS mode, at least in NDEBUG
+//    mode.
+// If you need TryLock(), and either these two caveats are not a
+// problem for you, or you're willing to work around them, then
+// feel free to #define GMUTEX_TRYLOCK, or to remove the #ifdefs
+// in the code below.
+//
+// CYGWIN NOTE: Cygwin support for rwlock seems to be buggy:
+//    http://www.cygwin.com/ml/cygwin/2008-12/msg00017.html
+// Because of that, we might as well use windows locks for
+// cygwin.  They seem to be more reliable than the cygwin pthreads layer.
+//
+// TRICKY IMPLEMENTATION NOTE:
+// This class is designed to be safe to use during
+// dynamic-initialization -- that is, by global constructors that are
+// run before main() starts.  The issue in this case is that
+// dynamic-initialization happens in an unpredictable order, and it
+// could be that someone else's dynamic initializer could call a
+// function that tries to acquire this mutex -- but that all happens
+// before this mutex's constructor has run.  (This can happen even if
+// the mutex and the function that uses the mutex are in the same .cc
+// file.)  Basically, because Mutex does non-trivial work in its
+// constructor, it's not, in the naive implementation, safe to use
+// before dynamic initialization has run on it.
+//
+// The solution used here is to pair the actual mutex primitive with a
+// bool that is set to true when the mutex is dynamically initialized.
+// (Before that it's false.)  Then we modify all mutex routines to
+// look at the bool, and not try to lock/unlock until the bool makes
+// it to true (which happens after the Mutex constructor has run.)
+//
+// This works because before main() starts -- particularly, during
+// dynamic initialization -- there are no threads, so a) it's ok that
+// the mutex operations are a no-op, since we don't need locking then
+// anyway; and b) we can be quite confident our bool won't change
+// state between a call to Lock() and a call to Unlock() (that would
+// require a global constructor in one translation unit to call Lock()
+// and another global constructor in another translation unit to call
+// Unlock() later, which is pretty perverse).
+//
+// That said, it's tricky, and can conceivably fail; it's safest to
+// avoid trying to acquire a mutex in a global constructor, if you
+// can.  One way it can fail is that a really smart compiler might
+// initialize the bool to true at static-initialization time (too
+// early) rather than at dynamic-initialization time.  To discourage
+// that, we set is_safe_ to true in code (not the constructor
+// colon-initializer) and set it to true via a function that always
+// evaluates to true, but that the compiler can't know always
+// evaluates to true.  This should be good enough.
+//
+// A related issue is code that could try to access the mutex
+// after it's been destroyed in the global destructors (because
+// the Mutex global destructor runs before some other global
+// destructor, that tries to acquire the mutex).  The way we
+// deal with this is by taking a constructor arg that global
+// mutexes should pass in, that causes the destructor to do no
+// work.  We still depend on the compiler not doing anything
+// weird to a Mutex's memory after it is destroyed, but for a
+// static global variable, that's pretty safe.
+
+#ifndef GFLAGS_MUTEX_H_
+#define GFLAGS_MUTEX_H_
+
+#include "gflags_declare.h"     // to figure out pthreads support
+
+#if defined(NO_THREADS)
+  typedef int MutexType;        // to keep a lock-count
+#elif defined(OS_WINDOWS)
+# ifndef WIN32_LEAN_AND_MEAN
+#   define WIN32_LEAN_AND_MEAN  // We only need minimal includes
+# endif
+# ifndef NOMINMAX
+#   define NOMINMAX             // Don't want windows to override min()/max()
+# endif
+# ifdef GMUTEX_TRYLOCK
+  // We need Windows NT or later for TryEnterCriticalSection().  If you
+  // don't need that functionality, you can remove these _WIN32_WINNT
+  // lines, and change TryLock() to assert(0) or something.
+#   ifndef _WIN32_WINNT
+#     define _WIN32_WINNT 0x0400
+#   endif
+# endif
+# include <windows.h>
+  typedef CRITICAL_SECTION MutexType;
+#elif defined(HAVE_PTHREAD) && defined(HAVE_RWLOCK)
+  // Needed for pthread_rwlock_*.  If it causes problems, you could take it
+  // out, but then you'd have to unset HAVE_RWLOCK (at least on linux -- it
+  // *does* cause problems for FreeBSD, or MacOSX, but isn't needed
+  // for locking there.)
+# ifdef __linux__
+#   if _XOPEN_SOURCE < 500      // including not being defined at all
+#     undef _XOPEN_SOURCE
+#     define _XOPEN_SOURCE 500  // may be needed to get the rwlock calls
+#   endif
+# endif
+# include <pthread.h>
+  typedef pthread_rwlock_t MutexType;
+#elif defined(HAVE_PTHREAD)
+# include <pthread.h>
+  typedef pthread_mutex_t MutexType;
+#else
+# error Need to implement mutex.h for your architecture, or #define NO_THREADS
+#endif
+
+#include <assert.h>
+#include <stdlib.h>      // for abort()
+
+#define MUTEX_NAMESPACE gflags_mutex_namespace
+
+namespace MUTEX_NAMESPACE {
+
+class Mutex {
+ public:
+  // This is used for the single-arg constructor
+  enum LinkerInitialized { LINKER_INITIALIZED };
+
+  // Create a Mutex that is not held by anybody.  This constructor is
+  // typically used for Mutexes allocated on the heap or the stack.
+  inline Mutex();
+  // This constructor should be used for global, static Mutex objects.
+  // It inhibits work being done by the destructor, which makes it
+  // safer for code that tries to acqiure this mutex in their global
+  // destructor.
+  inline Mutex(LinkerInitialized);
+
+  // Destructor
+  inline ~Mutex();
+
+  inline void Lock();    // Block if needed until free then acquire exclusively
+  inline void Unlock();  // Release a lock acquired via Lock()
+#ifdef GMUTEX_TRYLOCK
+  inline bool TryLock(); // If free, Lock() and return true, else return false
+#endif
+  // Note that on systems that don't support read-write locks, these may
+  // be implemented as synonyms to Lock() and Unlock().  So you can use
+  // these for efficiency, but don't use them anyplace where being able
+  // to do shared reads is necessary to avoid deadlock.
+  inline void ReaderLock();   // Block until free or shared then acquire a share
+  inline void ReaderUnlock(); // Release a read share of this Mutex
+  inline void WriterLock() { Lock(); }     // Acquire an exclusive lock
+  inline void WriterUnlock() { Unlock(); } // Release a lock from WriterLock()
+
+ private:
+  MutexType mutex_;
+  // We want to make sure that the compiler sets is_safe_ to true only
+  // when we tell it to, and never makes assumptions is_safe_ is
+  // always true.  volatile is the most reliable way to do that.
+  volatile bool is_safe_;
+  // This indicates which constructor was called.
+  bool destroy_;
+
+  inline void SetIsSafe() { is_safe_ = true; }
+
+  // Catch the error of writing Mutex when intending MutexLock.
+  Mutex(Mutex* /*ignored*/) {}
+  // Disallow "evil" constructors
+  Mutex(const Mutex&);
+  void operator=(const Mutex&);
+};
+
+// Now the implementation of Mutex for various systems
+#if defined(NO_THREADS)
+
+// When we don't have threads, we can be either reading or writing,
+// but not both.  We can have lots of readers at once (in no-threads
+// mode, that's most likely to happen in recursive function calls),
+// but only one writer.  We represent this by having mutex_ be -1 when
+// writing and a number > 0 when reading (and 0 when no lock is held).
+//
+// In debug mode, we assert these invariants, while in non-debug mode
+// we do nothing, for efficiency.  That's why everything is in an
+// assert.
+
+Mutex::Mutex() : mutex_(0) { }
+Mutex::Mutex(Mutex::LinkerInitialized) : mutex_(0) { }
+Mutex::~Mutex()            { assert(mutex_ == 0); }
+void Mutex::Lock()         { assert(--mutex_ == -1); }
+void Mutex::Unlock()       { assert(mutex_++ == -1); }
+#ifdef GMUTEX_TRYLOCK
+bool Mutex::TryLock()      { if (mutex_) return false; Lock(); return true; }
+#endif
+void Mutex::ReaderLock()   { assert(++mutex_ > 0); }
+void Mutex::ReaderUnlock() { assert(mutex_-- > 0); }
+
+#elif defined(OS_WINDOWS)
+
+Mutex::Mutex() : destroy_(true) {
+  InitializeCriticalSection(&mutex_);
+  SetIsSafe();
+}
+Mutex::Mutex(LinkerInitialized) : destroy_(false) {
+  InitializeCriticalSection(&mutex_);
+  SetIsSafe();
+}
+Mutex::~Mutex()            { if (destroy_) DeleteCriticalSection(&mutex_); }
+void Mutex::Lock()         { if (is_safe_) EnterCriticalSection(&mutex_); }
+void Mutex::Unlock()       { if (is_safe_) LeaveCriticalSection(&mutex_); }
+#ifdef GMUTEX_TRYLOCK
+bool Mutex::TryLock()      { return is_safe_ ?
+                                 TryEnterCriticalSection(&mutex_) != 0 : true; }
+#endif
+void Mutex::ReaderLock()   { Lock(); }      // we don't have read-write locks
+void Mutex::ReaderUnlock() { Unlock(); }
+
+#elif defined(HAVE_PTHREAD) && defined(HAVE_RWLOCK)
+
+#define SAFE_PTHREAD(fncall)  do {   /* run fncall if is_safe_ is true */  \
+  if (is_safe_ && fncall(&mutex_) != 0) abort();                           \
+} while (0)
+
+Mutex::Mutex() : destroy_(true) {
+  SetIsSafe();
+  if (is_safe_ && pthread_rwlock_init(&mutex_, NULL) != 0) abort();
+}
+Mutex::Mutex(Mutex::LinkerInitialized) : destroy_(false) {
+  SetIsSafe();
+  if (is_safe_ && pthread_rwlock_init(&mutex_, NULL) != 0) abort();
+}
+Mutex::~Mutex()       { if (destroy_) SAFE_PTHREAD(pthread_rwlock_destroy); }
+void Mutex::Lock()         { SAFE_PTHREAD(pthread_rwlock_wrlock); }
+void Mutex::Unlock()       { SAFE_PTHREAD(pthread_rwlock_unlock); }
+#ifdef GMUTEX_TRYLOCK
+bool Mutex::TryLock()      { return is_safe_ ?
+                               pthread_rwlock_trywrlock(&mutex_) == 0 : true; }
+#endif
+void Mutex::ReaderLock()   { SAFE_PTHREAD(pthread_rwlock_rdlock); }
+void Mutex::ReaderUnlock() { SAFE_PTHREAD(pthread_rwlock_unlock); }
+#undef SAFE_PTHREAD
+
+#elif defined(HAVE_PTHREAD)
+
+#define SAFE_PTHREAD(fncall)  do {   /* run fncall if is_safe_ is true */  \
+  if (is_safe_ && fncall(&mutex_) != 0) abort();                           \
+} while (0)
+
+Mutex::Mutex() : destroy_(true) {
+  SetIsSafe();
+  if (is_safe_ && pthread_mutex_init(&mutex_, NULL) != 0) abort();
+}
+Mutex::Mutex(Mutex::LinkerInitialized) : destroy_(false) {
+  SetIsSafe();
+  if (is_safe_ && pthread_mutex_init(&mutex_, NULL) != 0) abort();
+}
+Mutex::~Mutex()       { if (destroy_) SAFE_PTHREAD(pthread_mutex_destroy); }
+void Mutex::Lock()         { SAFE_PTHREAD(pthread_mutex_lock); }
+void Mutex::Unlock()       { SAFE_PTHREAD(pthread_mutex_unlock); }
+#ifdef GMUTEX_TRYLOCK
+bool Mutex::TryLock()      { return is_safe_ ?
+                                 pthread_mutex_trylock(&mutex_) == 0 : true; }
+#endif
+void Mutex::ReaderLock()   { Lock(); }
+void Mutex::ReaderUnlock() { Unlock(); }
+#undef SAFE_PTHREAD
+
+#endif
+
+// --------------------------------------------------------------------------
+// Some helper classes
+
+// MutexLock(mu) acquires mu when constructed and releases it when destroyed.
+class MutexLock {
+ public:
+  explicit MutexLock(Mutex *mu) : mu_(mu) { mu_->Lock(); }
+  ~MutexLock() { mu_->Unlock(); }
+ private:
+  Mutex * const mu_;
+  // Disallow "evil" constructors
+  MutexLock(const MutexLock&);
+  void operator=(const MutexLock&);
+};
+
+// ReaderMutexLock and WriterMutexLock do the same, for rwlocks
+class ReaderMutexLock {
+ public:
+  explicit ReaderMutexLock(Mutex *mu) : mu_(mu) { mu_->ReaderLock(); }
+  ~ReaderMutexLock() { mu_->ReaderUnlock(); }
+ private:
+  Mutex * const mu_;
+  // Disallow "evil" constructors
+  ReaderMutexLock(const ReaderMutexLock&);
+  void operator=(const ReaderMutexLock&);
+};
+
+class WriterMutexLock {
+ public:
+  explicit WriterMutexLock(Mutex *mu) : mu_(mu) { mu_->WriterLock(); }
+  ~WriterMutexLock() { mu_->WriterUnlock(); }
+ private:
+  Mutex * const mu_;
+  // Disallow "evil" constructors
+  WriterMutexLock(const WriterMutexLock&);
+  void operator=(const WriterMutexLock&);
+};
+
+// Catch bug where variable name is omitted, e.g. MutexLock (&mu);
+#define MutexLock(x) COMPILE_ASSERT(0, mutex_lock_decl_missing_var_name)
+#define ReaderMutexLock(x) COMPILE_ASSERT(0, rmutex_lock_decl_missing_var_name)
+#define WriterMutexLock(x) COMPILE_ASSERT(0, wmutex_lock_decl_missing_var_name)
+
+}  // namespace MUTEX_NAMESPACE
+
+using namespace MUTEX_NAMESPACE;
+
+#undef MUTEX_NAMESPACE
+
+#endif  /* #define GFLAGS_MUTEX_H__ */