1PTHREAD_MUTEX_LOCK(3P) POSIX Programmer's Manual PTHREAD_MUTEX_LOCK(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock - lock
13 and unlock a mutex
14
16 #include <pthread.h>
17 int pthread_mutex_lock(pthread_mutex_t *mutex);
19 int pthread_mutex_trylock(pthread_mutex_t *mutex);
20 int pthread_mutex_unlock(pthread_mutex_t *mutex);
21
24 The mutex object referenced by mutex shall be locked by calling
25 pthread_mutex_lock(). If the mutex is already locked, the calling
26 thread shall block until the mutex becomes available. This operation
27 shall return with the mutex object referenced by mutex in the locked
28 state with the calling thread as its owner.
29 If the mutex type is PTHREAD_MUTEX_NORMAL, deadlock detection shall not
31 be provided. Attempting to relock the mutex causes deadlock. If a
32 thread attempts to unlock a mutex that it has not locked or a mutex
33 which is unlocked, undefined behavior results.
34 If the mutex type is PTHREAD_MUTEX_ERRORCHECK, then error checking
36 shall be provided. If a thread attempts to relock a mutex that it has
37 already locked, an error shall be returned. If a thread attempts to
38 unlock a mutex that it has not locked or a mutex which is unlocked, an
39 error shall be returned.
40 If the mutex type is PTHREAD_MUTEX_RECURSIVE, then the mutex shall
42 maintain the concept of a lock count. When a thread successfully
43 acquires a mutex for the first time, the lock count shall be set to
44 one. Every time a thread relocks this mutex, the lock count shall be
45 incremented by one. Each time the thread unlocks the mutex, the lock
46 count shall be decremented by one. When the lock count reaches zero,
47 the mutex shall become available for other threads to acquire. If a
48 thread attempts to unlock a mutex that it has not locked or a mutex
49 which is unlocked, an error shall be returned.
50 If the mutex type is PTHREAD_MUTEX_DEFAULT, attempting to recursively
52 lock the mutex results in undefined behavior. Attempting to unlock the
53 mutex if it was not locked by the calling thread results in undefined
54 behavior. Attempting to unlock the mutex if it is not locked results in
55 undefined behavior.
56 The pthread_mutex_trylock() function shall be equivalent to
58 pthread_mutex_lock(), except that if the mutex object referenced by
59 mutex is currently locked (by any thread, including the current
60 thread), the call shall return immediately. If the mutex type is
61 PTHREAD_MUTEX_RECURSIVE and the mutex is currently owned by the calling
62 thread, the mutex lock count shall be incremented by one and the
63 pthread_mutex_trylock() function shall immediately return success.
64 The pthread_mutex_unlock() function shall release the mutex object ref‐
66 erenced by mutex. The manner in which a mutex is released is dependent
67 upon the mutex's type attribute. If there are threads blocked on the
68 mutex object referenced by mutex when pthread_mutex_unlock() is called,
69 resulting in the mutex becoming available, the scheduling policy shall
70 determine which thread shall acquire the mutex.
71 (In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex shall become
73 available when the count reaches zero and the calling thread no longer
74 has any locks on this mutex.)
75 If a signal is delivered to a thread waiting for a mutex, upon return
77 from the signal handler the thread shall resume waiting for the mutex
78 as if it was not interrupted.
79
81 If successful, the pthread_mutex_lock() and pthread_mutex_unlock()
82 functions shall return zero; otherwise, an error number shall be
83 returned to indicate the error.
84 The pthread_mutex_trylock() function shall return zero if a lock on the
86 mutex object referenced by mutex is acquired. Otherwise, an error num‐
87 ber is returned to indicate the error.
88
90 The pthread_mutex_lock() and pthread_mutex_trylock() functions shall
91 fail if:
92 EINVAL The mutex was created with the protocol attribute having the
94 value PTHREAD_PRIO_PROTECT and the calling thread's priority is
95 higher than the mutex's current priority ceiling.
96 The pthread_mutex_trylock() function shall fail if:
99 EBUSY The mutex could not be acquired because it was already locked.
101 The pthread_mutex_lock(), pthread_mutex_trylock(), and
104 pthread_mutex_unlock() functions may fail if:
105 EINVAL The value specified by mutex does not refer to an initialized
107 mutex object.
108 EAGAIN The mutex could not be acquired because the maximum number of
110 recursive locks for mutex has been exceeded.
111 The pthread_mutex_lock() function may fail if:
114 EDEADLK
116 The current thread already owns the mutex.
117 The pthread_mutex_unlock() function may fail if:
120 EPERM The current thread does not own the mutex.
122 These functions shall not return an error code of [EINTR].
125 The following sections are informative.
127
129 None.
130
132 None.
133
135 Mutex objects are intended to serve as a low-level primitive from which
136 other thread synchronization functions can be built. As such, the
137 implementation of mutexes should be as efficient as possible, and this
138 has ramifications on the features available at the interface.
139 The mutex functions and the particular default settings of the mutex
141 attributes have been motivated by the desire to not preclude fast,
142 inlined implementations of mutex locking and unlocking.
143 For example, deadlocking on a double-lock is explicitly allowed behav‐
145 ior in order to avoid requiring more overhead in the basic mechanism
146 than is absolutely necessary. (More "friendly" mutexes that detect
147 deadlock or that allow multiple locking by the same thread are easily
148 constructed by the user via the other mechanisms provided. For example,
149 pthread_self() can be used to record mutex ownership.) Implementations
150 might also choose to provide such extended features as options via spe‐
151 cial mutex attributes.
152 Since most attributes only need to be checked when a thread is going to
154 be blocked, the use of attributes does not slow the (common) mutex-
155 locking case.
156 Likewise, while being able to extract the thread ID of the owner of a
158 mutex might be desirable, it would require storing the current thread
159 ID when each mutex is locked, and this could incur unacceptable levels
160 of overhead. Similar arguments apply to a mutex_tryunlock operation.
161
163 None.
164
166 pthread_mutex_destroy(), pthread_mutex_timedlock(), the Base Defini‐
167 tions volume of IEEE Std 1003.1-2001, <pthread.h>
168
170 Portions of this text are reprinted and reproduced in electronic form
171 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
172 -- Portable Operating System Interface (POSIX), The Open Group Base
173 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
174 Electrical and Electronics Engineers, Inc and The Open Group. In the
175 event of any discrepancy between this version and the original IEEE and
176 The Open Group Standard, the original IEEE and The Open Group Standard
177 is the referee document. The original Standard can be obtained online
178 at http://www.opengroup.org/unix/online.html .
179IEEE/The Open Group 2003 PTHREAD_MUTEX_LOCK(3P)