// 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.
//
// ---
// Author: Craig Silverstein.
//
// A simple mutex wrapper, supporting locks and read-write locks.
// You should assume the locks are *not* re-entrant.
//
// To use: you should define the following macros in your configure.ac:
// ACX_PTHREAD
// AC_RWLOCK
// The latter is defined in ../autoconf.
//
// 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.
#ifndef GOOGLE_MUTEX_H_
#define GOOGLE_MUTEX_H_
#include "config.h" // to figure out pthreads support
#if defined(NO_THREADS)
typedef int MutexType; // to keep a lock-count
#elif defined(_WIN32) || defined(__CYGWIN32__) || defined(__CYGWIN64__)
# ifndef WIN32_LEAN_AND_MEAN
# define WIN32_LEAN_AND_MEAN // We only need minimal includes
# 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
// To avoid macro definition of ERROR.
# ifndef NOGDI
# define NOGDI
# endif
// To avoid macro definition of min/max.
# ifndef NOMINMAX
# define NOMINMAX
# 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__
# ifndef _XOPEN_SOURCE // Some other header might have already set it for us.
# 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
// We need to include these header files after defining _XOPEN_SOURCE
// as they may define the _XOPEN_SOURCE macro.
#include <assert.h>
#include <stdlib.h> // for abort()
#define MUTEX_NAMESPACE glog_internal_namespace_
namespace MUTEX_NAMESPACE {
class Mutex {
public:
// Create a Mutex that is not held by anybody. This constructor is
// typically used for Mutexes allocated on the heap or the stack.
// See below for a recommendation for constructing global Mutex
// objects.
inline Mutex();
// 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()
// TODO(hamaji): Do nothing, implement correctly.
inline void AssertHeld() {}
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_;
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() { 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(_WIN32) || defined(__CYGWIN32__) || defined(__CYGWIN64__)
Mutex::Mutex() { InitializeCriticalSection(&mutex_); SetIsSafe(); }
Mutex::~Mutex() { 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() {
SetIsSafe();
if (is_safe_ && pthread_rwlock_init(&mutex_, NULL) != 0) abort();
}
Mutex::~Mutex() { 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() {
SetIsSafe();
if (is_safe_ && pthread_mutex_init(&mutex_, NULL) != 0) abort();
}
Mutex::~Mutex() { 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 GOOGLE_MUTEX_H__ */
