1 // Copyright (c) 2007, Google Inc.
2 // All rights reserved.
4 // Redistribution and use in source and binary forms, with or without
5 // modification, are permitted provided that the following conditions are
8 // * Redistributions of source code must retain the above copyright
9 // notice, this list of conditions and the following disclaimer.
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11 // copyright notice, this list of conditions and the following disclaimer
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28 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
31 // Author: Craig Silverstein.
33 // A simple mutex wrapper, supporting locks and read-write locks.
34 // You should assume the locks are *not* re-entrant.
36 // To use: you should define the following macros in your configure.ac:
39 // The latter is defined in ../autoconf.
41 // This class is meant to be internal-only and should be wrapped by an
42 // internal namespace. Before you use this module, please give the
43 // name of your internal namespace for this module. Or, if you want
44 // to expose it, you'll want to move it to the Google namespace. We
45 // cannot put this class in global namespace because there can be some
46 // problems when we have multiple versions of Mutex in each shared object.
48 // NOTE: by default, we have #ifdef'ed out the TryLock() method.
49 // This is for two reasons:
50 // 1) TryLock() under Windows is a bit annoying (it requires a
51 // #define to be defined very early).
52 // 2) TryLock() is broken for NO_THREADS mode, at least in NDEBUG
54 // If you need TryLock(), and either these two caveats are not a
55 // problem for you, or you're willing to work around them, then
56 // feel free to #define GMUTEX_TRYLOCK, or to remove the #ifdefs
59 // CYGWIN NOTE: Cygwin support for rwlock seems to be buggy:
60 // http://www.cygwin.com/ml/cygwin/2008-12/msg00017.html
61 // Because of that, we might as well use windows locks for
62 // cygwin. They seem to be more reliable than the cygwin pthreads layer.
64 // TRICKY IMPLEMENTATION NOTE:
65 // This class is designed to be safe to use during
66 // dynamic-initialization -- that is, by global constructors that are
67 // run before main() starts. The issue in this case is that
68 // dynamic-initialization happens in an unpredictable order, and it
69 // could be that someone else's dynamic initializer could call a
70 // function that tries to acquire this mutex -- but that all happens
71 // before this mutex's constructor has run. (This can happen even if
72 // the mutex and the function that uses the mutex are in the same .cc
73 // file.) Basically, because Mutex does non-trivial work in its
74 // constructor, it's not, in the naive implementation, safe to use
75 // before dynamic initialization has run on it.
77 // The solution used here is to pair the actual mutex primitive with a
78 // bool that is set to true when the mutex is dynamically initialized.
79 // (Before that it's false.) Then we modify all mutex routines to
80 // look at the bool, and not try to lock/unlock until the bool makes
81 // it to true (which happens after the Mutex constructor has run.)
83 // This works because before main() starts -- particularly, during
84 // dynamic initialization -- there are no threads, so a) it's ok that
85 // the mutex operations are a no-op, since we don't need locking then
86 // anyway; and b) we can be quite confident our bool won't change
87 // state between a call to Lock() and a call to Unlock() (that would
88 // require a global constructor in one translation unit to call Lock()
89 // and another global constructor in another translation unit to call
90 // Unlock() later, which is pretty perverse).
92 // That said, it's tricky, and can conceivably fail; it's safest to
93 // avoid trying to acquire a mutex in a global constructor, if you
94 // can. One way it can fail is that a really smart compiler might
95 // initialize the bool to true at static-initialization time (too
96 // early) rather than at dynamic-initialization time. To discourage
97 // that, we set is_safe_ to true in code (not the constructor
98 // colon-initializer) and set it to true via a function that always
99 // evaluates to true, but that the compiler can't know always
100 // evaluates to true. This should be good enough.
102 #ifndef GOOGLE_MUTEX_H_
103 #define GOOGLE_MUTEX_H_
105 #include "config.h" // to figure out pthreads support
107 #if defined(NO_THREADS)
108 typedef int MutexType; // to keep a lock-count
109 #elif defined(_WIN32) || defined(__CYGWIN32__) || defined(__CYGWIN64__)
110 # ifndef WIN32_LEAN_AND_MEAN
111 # define WIN32_LEAN_AND_MEAN // We only need minimal includes
113 # ifdef GMUTEX_TRYLOCK
114 // We need Windows NT or later for TryEnterCriticalSection(). If you
115 // don't need that functionality, you can remove these _WIN32_WINNT
116 // lines, and change TryLock() to assert(0) or something.
117 # ifndef _WIN32_WINNT
118 # define _WIN32_WINNT 0x0400
121 // To avoid macro definition of ERROR.
125 // To avoid macro definition of min/max.
129 # include <windows.h>
130 typedef CRITICAL_SECTION MutexType;
131 #elif defined(HAVE_PTHREAD) && defined(HAVE_RWLOCK)
132 // Needed for pthread_rwlock_*. If it causes problems, you could take it
133 // out, but then you'd have to unset HAVE_RWLOCK (at least on linux -- it
134 // *does* cause problems for FreeBSD, or MacOSX, but isn't needed
135 // for locking there.)
137 # define _XOPEN_SOURCE 500 // may be needed to get the rwlock calls
139 # include <pthread.h>
140 typedef pthread_rwlock_t MutexType;
141 #elif defined(HAVE_PTHREAD)
142 # include <pthread.h>
143 typedef pthread_mutex_t MutexType;
145 # error Need to implement mutex.h for your architecture, or #define NO_THREADS
148 // We need to include these header files after defining _XOPEN_SOURCE
149 // as they may define the _XOPEN_SOURCE macro.
151 #include <stdlib.h> // for abort()
153 #define MUTEX_NAMESPACE glog_internal_namespace_
155 namespace MUTEX_NAMESPACE {
159 // Create a Mutex that is not held by anybody. This constructor is
160 // typically used for Mutexes allocated on the heap or the stack.
161 // See below for a recommendation for constructing global Mutex
168 inline void Lock(); // Block if needed until free then acquire exclusively
169 inline void Unlock(); // Release a lock acquired via Lock()
170 #ifdef GMUTEX_TRYLOCK
171 inline bool TryLock(); // If free, Lock() and return true, else return false
173 // Note that on systems that don't support read-write locks, these may
174 // be implemented as synonyms to Lock() and Unlock(). So you can use
175 // these for efficiency, but don't use them anyplace where being able
176 // to do shared reads is necessary to avoid deadlock.
177 inline void ReaderLock(); // Block until free or shared then acquire a share
178 inline void ReaderUnlock(); // Release a read share of this Mutex
179 inline void WriterLock() { Lock(); } // Acquire an exclusive lock
180 inline void WriterUnlock() { Unlock(); } // Release a lock from WriterLock()
182 // TODO(hamaji): Do nothing, implement correctly.
183 inline void AssertHeld() {}
187 // We want to make sure that the compiler sets is_safe_ to true only
188 // when we tell it to, and never makes assumptions is_safe_ is
189 // always true. volatile is the most reliable way to do that.
190 volatile bool is_safe_;
192 inline void SetIsSafe() { is_safe_ = true; }
194 // Catch the error of writing Mutex when intending MutexLock.
195 Mutex(Mutex* /*ignored*/) {}
196 // Disallow "evil" constructors
198 void operator=(const Mutex&);
201 // Now the implementation of Mutex for various systems
202 #if defined(NO_THREADS)
204 // When we don't have threads, we can be either reading or writing,
205 // but not both. We can have lots of readers at once (in no-threads
206 // mode, that's most likely to happen in recursive function calls),
207 // but only one writer. We represent this by having mutex_ be -1 when
208 // writing and a number > 0 when reading (and 0 when no lock is held).
210 // In debug mode, we assert these invariants, while in non-debug mode
211 // we do nothing, for efficiency. That's why everything is in an
214 Mutex::Mutex() : mutex_(0) { }
215 Mutex::~Mutex() { assert(mutex_ == 0); }
216 void Mutex::Lock() { assert(--mutex_ == -1); }
217 void Mutex::Unlock() { assert(mutex_++ == -1); }
218 #ifdef GMUTEX_TRYLOCK
219 bool Mutex::TryLock() { if (mutex_) return false; Lock(); return true; }
221 void Mutex::ReaderLock() { assert(++mutex_ > 0); }
222 void Mutex::ReaderUnlock() { assert(mutex_-- > 0); }
224 #elif defined(_WIN32) || defined(__CYGWIN32__) || defined(__CYGWIN64__)
226 Mutex::Mutex() { InitializeCriticalSection(&mutex_); SetIsSafe(); }
227 Mutex::~Mutex() { DeleteCriticalSection(&mutex_); }
228 void Mutex::Lock() { if (is_safe_) EnterCriticalSection(&mutex_); }
229 void Mutex::Unlock() { if (is_safe_) LeaveCriticalSection(&mutex_); }
230 #ifdef GMUTEX_TRYLOCK
231 bool Mutex::TryLock() { return is_safe_ ?
232 TryEnterCriticalSection(&mutex_) != 0 : true; }
234 void Mutex::ReaderLock() { Lock(); } // we don't have read-write locks
235 void Mutex::ReaderUnlock() { Unlock(); }
237 #elif defined(HAVE_PTHREAD) && defined(HAVE_RWLOCK)
239 #define SAFE_PTHREAD(fncall) do { /* run fncall if is_safe_ is true */ \
240 if (is_safe_ && fncall(&mutex_) != 0) abort(); \
245 if (is_safe_ && pthread_rwlock_init(&mutex_, NULL) != 0) abort();
247 Mutex::~Mutex() { SAFE_PTHREAD(pthread_rwlock_destroy); }
248 void Mutex::Lock() { SAFE_PTHREAD(pthread_rwlock_wrlock); }
249 void Mutex::Unlock() { SAFE_PTHREAD(pthread_rwlock_unlock); }
250 #ifdef GMUTEX_TRYLOCK
251 bool Mutex::TryLock() { return is_safe_ ?
252 pthread_rwlock_trywrlock(&mutex_) == 0 :
255 void Mutex::ReaderLock() { SAFE_PTHREAD(pthread_rwlock_rdlock); }
256 void Mutex::ReaderUnlock() { SAFE_PTHREAD(pthread_rwlock_unlock); }
259 #elif defined(HAVE_PTHREAD)
261 #define SAFE_PTHREAD(fncall) do { /* run fncall if is_safe_ is true */ \
262 if (is_safe_ && fncall(&mutex_) != 0) abort(); \
267 if (is_safe_ && pthread_mutex_init(&mutex_, NULL) != 0) abort();
269 Mutex::~Mutex() { SAFE_PTHREAD(pthread_mutex_destroy); }
270 void Mutex::Lock() { SAFE_PTHREAD(pthread_mutex_lock); }
271 void Mutex::Unlock() { SAFE_PTHREAD(pthread_mutex_unlock); }
272 #ifdef GMUTEX_TRYLOCK
273 bool Mutex::TryLock() { return is_safe_ ?
274 pthread_mutex_trylock(&mutex_) == 0 : true; }
276 void Mutex::ReaderLock() { Lock(); }
277 void Mutex::ReaderUnlock() { Unlock(); }
282 // --------------------------------------------------------------------------
283 // Some helper classes
285 // MutexLock(mu) acquires mu when constructed and releases it when destroyed.
288 explicit MutexLock(Mutex *mu) : mu_(mu) { mu_->Lock(); }
289 ~MutexLock() { mu_->Unlock(); }
292 // Disallow "evil" constructors
293 MutexLock(const MutexLock&);
294 void operator=(const MutexLock&);
297 // ReaderMutexLock and WriterMutexLock do the same, for rwlocks
298 class ReaderMutexLock {
300 explicit ReaderMutexLock(Mutex *mu) : mu_(mu) { mu_->ReaderLock(); }
301 ~ReaderMutexLock() { mu_->ReaderUnlock(); }
304 // Disallow "evil" constructors
305 ReaderMutexLock(const ReaderMutexLock&);
306 void operator=(const ReaderMutexLock&);
309 class WriterMutexLock {
311 explicit WriterMutexLock(Mutex *mu) : mu_(mu) { mu_->WriterLock(); }
312 ~WriterMutexLock() { mu_->WriterUnlock(); }
315 // Disallow "evil" constructors
316 WriterMutexLock(const WriterMutexLock&);
317 void operator=(const WriterMutexLock&);
320 // Catch bug where variable name is omitted, e.g. MutexLock (&mu);
321 #define MutexLock(x) COMPILE_ASSERT(0, mutex_lock_decl_missing_var_name)
322 #define ReaderMutexLock(x) COMPILE_ASSERT(0, rmutex_lock_decl_missing_var_name)
323 #define WriterMutexLock(x) COMPILE_ASSERT(0, wmutex_lock_decl_missing_var_name)
325 } // namespace MUTEX_NAMESPACE
327 using namespace MUTEX_NAMESPACE;
329 #undef MUTEX_NAMESPACE
331 #endif /* #define GOOGLE_MUTEX_H__ */