1PTHREAD_MUTEX_LOCK(P) POSIX Programmer's Manual PTHREAD_MUTEX_LOCK(P)
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6 pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock - lock
7 and unlock a mutex
8
10 #include <pthread.h>
11 int pthread_mutex_lock(pthread_mutex_t *mutex);
13 int pthread_mutex_trylock(pthread_mutex_t *mutex);
14 int pthread_mutex_unlock(pthread_mutex_t *mutex);
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18 The mutex object referenced by mutex shall be locked by calling
19 pthread_mutex_lock(). If the mutex is already locked, the calling
20 thread shall block until the mutex becomes available. This operation
21 shall return with the mutex object referenced by mutex in the locked
22 state with the calling thread as its owner.
23 If the mutex type is PTHREAD_MUTEX_NORMAL, deadlock detection shall not
25 be provided. Attempting to relock the mutex causes deadlock. If a
26 thread attempts to unlock a mutex that it has not locked or a mutex
27 which is unlocked, undefined behavior results.
28 If the mutex type is PTHREAD_MUTEX_ERRORCHECK, then error checking
30 shall be provided. If a thread attempts to relock a mutex that it has
31 already locked, an error shall be returned. If a thread attempts to
32 unlock a mutex that it has not locked or a mutex which is unlocked, an
33 error shall be returned.
34 If the mutex type is PTHREAD_MUTEX_RECURSIVE, then the mutex shall
36 maintain the concept of a lock count. When a thread successfully
37 acquires a mutex for the first time, the lock count shall be set to
38 one. Every time a thread relocks this mutex, the lock count shall be
39 incremented by one. Each time the thread unlocks the mutex, the lock
40 count shall be decremented by one. When the lock count reaches zero,
41 the mutex shall become available for other threads to acquire. If a
42 thread attempts to unlock a mutex that it has not locked or a mutex
43 which is unlocked, an error shall be returned.
44 If the mutex type is PTHREAD_MUTEX_DEFAULT, attempting to recursively
46 lock the mutex results in undefined behavior. Attempting to unlock the
47 mutex if it was not locked by the calling thread results in undefined
48 behavior. Attempting to unlock the mutex if it is not locked results in
49 undefined behavior.
50 The pthread_mutex_trylock() function shall be equivalent to
52 pthread_mutex_lock(), except that if the mutex object referenced by
53 mutex is currently locked (by any thread, including the current
54 thread), the call shall return immediately. If the mutex type is
55 PTHREAD_MUTEX_RECURSIVE and the mutex is currently owned by the calling
56 thread, the mutex lock count shall be incremented by one and the
57 pthread_mutex_trylock() function shall immediately return success.
58 The pthread_mutex_unlock() function shall release the mutex object ref‐
60 erenced by mutex. The manner in which a mutex is released is depen‐
61 dent upon the mutex's type attribute. If there are threads blocked on
62 the mutex object referenced by mutex when pthread_mutex_unlock() is
63 called, resulting in the mutex becoming available, the scheduling pol‐
64 icy shall determine which thread shall acquire the mutex.
65 (In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex shall become
67 available when the count reaches zero and the calling thread no longer
68 has any locks on this mutex.)
69 If a signal is delivered to a thread waiting for a mutex, upon return
71 from the signal handler the thread shall resume waiting for the mutex
72 as if it was not interrupted.
73
75 If successful, the pthread_mutex_lock() and pthread_mutex_unlock()
76 functions shall return zero; otherwise, an error number shall be
77 returned to indicate the error.
78 The pthread_mutex_trylock() function shall return zero if a lock on the
80 mutex object referenced by mutex is acquired. Otherwise, an error num‐
81 ber is returned to indicate the error.
82
84 The pthread_mutex_lock() and pthread_mutex_trylock() functions shall
85 fail if:
86 EINVAL The mutex was created with the protocol attribute having the
88 value PTHREAD_PRIO_PROTECT and the calling thread's priority is
89 higher than the mutex's current priority ceiling.
90 The pthread_mutex_trylock() function shall fail if:
93 EBUSY The mutex could not be acquired because it was already locked.
95 The pthread_mutex_lock(), pthread_mutex_trylock(), and
98 pthread_mutex_unlock() functions may fail if:
99 EINVAL The value specified by mutex does not refer to an initialized
101 mutex object.
102 EAGAIN The mutex could not be acquired because the maximum number of
104 recursive locks for mutex has been exceeded.
105 The pthread_mutex_lock() function may fail if:
108 EDEADLK
110 The current thread already owns the mutex.
111 The pthread_mutex_unlock() function may fail if:
114 EPERM The current thread does not own the mutex.
116 These functions shall not return an error code of [EINTR].
119 The following sections are informative.
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123 None.
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126 None.
127
129 Mutex objects are intended to serve as a low-level primitive from which
130 other thread synchronization functions can be built. As such, the
131 implementation of mutexes should be as efficient as possible, and this
132 has ramifications on the features available at the interface.
133 The mutex functions and the particular default settings of the mutex
135 attributes have been motivated by the desire to not preclude fast,
136 inlined implementations of mutex locking and unlocking.
137 For example, deadlocking on a double-lock is explicitly allowed behav‐
139 ior in order to avoid requiring more overhead in the basic mechanism
140 than is absolutely necessary. (More "friendly" mutexes that detect
141 deadlock or that allow multiple locking by the same thread are easily
142 constructed by the user via the other mechanisms provided. For example,
143 pthread_self() can be used to record mutex ownership.) Implementations
144 might also choose to provide such extended features as options via spe‐
145 cial mutex attributes.
146 Since most attributes only need to be checked when a thread is going to
148 be blocked, the use of attributes does not slow the (common) mutex-
149 locking case.
150 Likewise, while being able to extract the thread ID of the owner of a
152 mutex might be desirable, it would require storing the current thread
153 ID when each mutex is locked, and this could incur unacceptable levels
154 of overhead. Similar arguments apply to a mutex_tryunlock operation.
155
157 None.
158
160 pthread_mutex_destroy() , pthread_mutex_timedlock() , the Base Defini‐
161 tions volume of IEEE Std 1003.1-2001, <pthread.h>
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164 Portions of this text are reprinted and reproduced in electronic form
165 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
166 -- Portable Operating System Interface (POSIX), The Open Group Base
167 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
168 Electrical and Electronics Engineers, Inc and The Open Group. In the
169 event of any discrepancy between this version and the original IEEE and
170 The Open Group Standard, the original IEEE and The Open Group Standard
171 is the referee document. The original Standard can be obtained online
172 at http://www.opengroup.org/unix/online.html .
173IEEE/The Open Group 2003 PTHREAD_MUTEX_LOCK(P)