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
23 The mutex object referenced by mutex shall be locked by a call to
24 pthread_mutex_lock() that returns zero or [EOWNERDEAD]. If the mutex
25 is already locked by another thread, the calling thread shall block
26 until the mutex becomes available. This operation shall return with the
27 mutex object referenced by mutex in the locked state with the calling
28 thread as its owner. If a thread attempts to relock a mutex that it has
29 already locked, pthread_mutex_lock() shall behave as described in the
30 Relock column of the following table. If a thread attempts to unlock a
31 mutex that it has not locked or a mutex which is unlocked,
32 pthread_mutex_unlock() shall behave as described in the Unlock When Not
33 Owner column of the following table.
34 ┌───────────┬────────────┬────────────────┬───────────────────────┐
36 │Mutex Type │ Robustness │ Relock │ Unlock When Not Owner │
37 ├───────────┼────────────┼────────────────┼───────────────────────┤
38 │NORMAL │ non-robust │ deadlock │ undefined behavior │
39 ├───────────┼────────────┼────────────────┼───────────────────────┤
40 │NORMAL │ robust │ deadlock │ error returned │
41 ├───────────┼────────────┼────────────────┼───────────────────────┤
42 │ERRORCHECK │ either │ error returned │ error returned │
43 ├───────────┼────────────┼────────────────┼───────────────────────┤
44 │RECURSIVE │ either │ recursive │ error returned │
45 │ │ │ (see below) │ │
46 ├───────────┼────────────┼────────────────┼───────────────────────┤
47 │DEFAULT │ non-robust │ undefined │ undefined behavior† │
48 │ │ │ behavior† │ │
49 ├───────────┼────────────┼────────────────┼───────────────────────┤
50 │DEFAULT │ robust │ undefined │ error returned │
51 │ │ │ behavior† │ │
52 └───────────┴────────────┴────────────────┴───────────────────────┘
53 † If the mutex type is PTHREAD_MUTEX_DEFAULT, the behavior of
54 pthread_mutex_lock() may correspond to one of the three other
55 standard mutex types as described in the table above. If it does
56 not correspond to one of those three, the behavior is undefined
57 for the cases marked †.
58 Where the table indicates recursive behavior, the mutex shall maintain
60 the concept of a lock count. When a thread successfully acquires a
61 mutex for the first time, the lock count shall be set to one. Every
62 time a thread relocks this mutex, the lock count shall be incremented
63 by one. Each time the thread unlocks the mutex, the lock count shall be
64 decremented by one. When the lock count reaches zero, the mutex shall
65 become available for other threads to acquire.
66 The pthread_mutex_trylock() function shall be equivalent to
68 pthread_mutex_lock(), except that if the mutex object referenced by
69 mutex is currently locked (by any thread, including the current
70 thread), the call shall return immediately. If the mutex type is
71 PTHREAD_MUTEX_RECURSIVE and the mutex is currently owned by the calling
72 thread, the mutex lock count shall be incremented by one and the
73 pthread_mutex_trylock() function shall immediately return success.
74 The pthread_mutex_unlock() function shall release the mutex object ref‐
76 erenced by mutex. The manner in which a mutex is released is dependent
77 upon the mutex's type attribute. If there are threads blocked on the
78 mutex object referenced by mutex when pthread_mutex_unlock() is called,
79 resulting in the mutex becoming available, the scheduling policy shall
80 determine which thread shall acquire the mutex.
81 (In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex shall become
83 available when the count reaches zero and the calling thread no longer
84 has any locks on this mutex.)
85 If a signal is delivered to a thread waiting for a mutex, upon return
87 from the signal handler the thread shall resume waiting for the mutex
88 as if it was not interrupted.
89 If mutex is a robust mutex and the process containing the owning thread
91 terminated while holding the mutex lock, a call to pthread_mutex_lock()
92 shall return the error value [EOWNERDEAD]. If mutex is a robust mutex
93 and the owning thread terminated while holding the mutex lock, a call
94 to pthread_mutex_lock() may return the error value [EOWNERDEAD] even if
95 the process in which the owning thread resides has not terminated. In
96 these cases, the mutex is locked by the thread but the state it pro‐
97 tects is marked as inconsistent. The application should ensure that the
98 state is made consistent for reuse and when that is complete call
99 pthread_mutex_consistent(). If the application is unable to recover
100 the state, it should unlock the mutex without a prior call to
101 pthread_mutex_consistent(), after which the mutex is marked permanently
102 unusable.
103 If mutex does not refer to an initialized mutex object, the behavior of
105 pthread_mutex_lock(), pthread_mutex_trylock(), and
106 pthread_mutex_unlock() is undefined.
107
109 If successful, the pthread_mutex_lock(), pthread_mutex_trylock(), and
110 pthread_mutex_unlock() functions shall return zero; otherwise, an error
111 number shall be returned to indicate the error.
112
114 The pthread_mutex_lock() and pthread_mutex_trylock() functions shall
115 fail if:
116 EAGAIN The mutex could not be acquired because the maximum number of
118 recursive locks for mutex has been exceeded.
119 EINVAL The mutex was created with the protocol attribute having the
121 value PTHREAD_PRIO_PROTECT and the calling thread's priority is
122 higher than the mutex's current priority ceiling.
123 ENOTRECOVERABLE
125 The state protected by the mutex is not recoverable.
126 EOWNERDEAD
128 The mutex is a robust mutex and the process containing the pre‐
129 vious owning thread terminated while holding the mutex lock. The
130 mutex lock shall be acquired by the calling thread and it is up
131 to the new owner to make the state consistent.
132 The pthread_mutex_lock() function shall fail if:
134 EDEADLK
136 The mutex type is PTHREAD_MUTEX_ERRORCHECK and the current
137 thread already owns the mutex.
138 The pthread_mutex_trylock() function shall fail if:
140 EBUSY The mutex could not be acquired because it was already locked.
142 The pthread_mutex_unlock() function shall fail if:
144 EPERM The mutex type is PTHREAD_MUTEX_ERRORCHECK or
146 PTHREAD_MUTEX_RECURSIVE, or the mutex is a robust mutex, and the
147 current thread does not own the mutex.
148 The pthread_mutex_lock() and pthread_mutex_trylock() functions may fail
150 if:
151 EOWNERDEAD
153 The mutex is a robust mutex and the previous owning thread ter‐
154 minated while holding the mutex lock. The mutex lock shall be
155 acquired by the calling thread and it is up to the new owner to
156 make the state consistent.
157 The pthread_mutex_lock() function may fail if:
159 EDEADLK
161 A deadlock condition was detected.
162 These functions shall not return an error code of [EINTR].
164 The following sections are informative.
166
168 None.
169
171 Applications that have assumed that non-zero return values are errors
172 will need updating for use with robust mutexes, since a valid return
173 for a thread acquiring a mutex which is protecting a currently incon‐
174 sistent state is [EOWNERDEAD]. Applications that do not check the
175 error returns, due to ruling out the possibility of such errors aris‐
176 ing, should not use robust mutexes. If an application is supposed to
177 work with normal and robust mutexes it should check all return values
178 for error conditions and if necessary take appropriate action.
179
181 Mutex objects are intended to serve as a low-level primitive from which
182 other thread synchronization functions can be built. As such, the
183 implementation of mutexes should be as efficient as possible, and this
184 has ramifications on the features available at the interface.
185 The mutex functions and the particular default settings of the mutex
187 attributes have been motivated by the desire to not preclude fast,
188 inlined implementations of mutex locking and unlocking.
189 Since most attributes only need to be checked when a thread is going to
191 be blocked, the use of attributes does not slow the (common) mutex-
192 locking case.
193 Likewise, while being able to extract the thread ID of the owner of a
195 mutex might be desirable, it would require storing the current thread
196 ID when each mutex is locked, and this could incur unacceptable levels
197 of overhead. Similar arguments apply to a mutex_tryunlock operation.
198 For further rationale on the extended mutex types, see the Rationale
200 (Informative) volume of POSIX.1‐2017, Threads Extensions.
201 If an implementation detects that the value specified by the mutex
203 argument does not refer to an initialized mutex object, it is recom‐
204 mended that the function should fail and report an [EINVAL] error.
205
207 None.
208
210 pthread_mutex_consistent(), pthread_mutex_destroy(),
211 pthread_mutex_timedlock(), pthread_mutexattr_getrobust()
212 The Base Definitions volume of POSIX.1‐2017, Section 4.12, Memory Syn‐
214 chronization, <pthread.h>
215
217 Portions of this text are reprinted and reproduced in electronic form
218 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
219 table Operating System Interface (POSIX), The Open Group Base Specifi‐
220 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
221 Electrical and Electronics Engineers, Inc and The Open Group. In the
222 event of any discrepancy between this version and the original IEEE and
223 The Open Group Standard, the original IEEE and The Open Group Standard
224 is the referee document. The original Standard can be obtained online
225 at http://www.opengroup.org/unix/online.html .
226 Any typographical or formatting errors that appear in this page are
228 most likely to have been introduced during the conversion of the source
229 files to man page format. To report such errors, see https://www.ker‐
230 nel.org/doc/man-pages/reporting_bugs.html .
231IEEE/The Open Group 2017 PTHREAD_MUTEX_LOCK(3P)