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
13 pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock — lock
14 and unlock a mutex
15
17 #include <pthread.h>
18 int pthread_mutex_lock(pthread_mutex_t *mutex);
20 int pthread_mutex_trylock(pthread_mutex_t *mutex);
21 int pthread_mutex_unlock(pthread_mutex_t *mutex);
22
24 The mutex object referenced by mutex shall be locked by a call to
25 pthread_mutex_lock() that returns zero or [EOWNERDEAD]. If the mutex
26 is already locked by another thread, the calling thread shall block
27 until the mutex becomes available. This operation shall return with the
28 mutex object referenced by mutex in the locked state with the calling
29 thread as its owner. If a thread attempts to relock a mutex that it has
30 already locked, pthread_mutex_lock() shall behave as described in the
31 Relock column of the following table. If a thread attempts to unlock a
32 mutex that it has not locked or a mutex which is unlocked,
33 pthread_mutex_unlock() shall behave as described in the Unlock When Not
34 Owner column of the following table.
35 ┌───────────┬────────────┬────────────────┬───────────────────────┐
37 │Mutex Type │ Robustness │ Relock │ Unlock When Not Owner │
38 ├───────────┼────────────┼────────────────┼───────────────────────┤
39 │NORMAL │ non-robust │ deadlock │ undefined behavior │
40 ├───────────┼────────────┼────────────────┼───────────────────────┤
41 │NORMAL │ robust │ deadlock │ error returned │
42 ├───────────┼────────────┼────────────────┼───────────────────────┤
43 │ERRORCHECK │ either │ error returned │ error returned │
44 ├───────────┼────────────┼────────────────┼───────────────────────┤
45 │RECURSIVE │ either │ recursive │ error returned │
46 │ │ │ (see below) │ │
47 ├───────────┼────────────┼────────────────┼───────────────────────┤
48 │DEFAULT │ non-robust │ undefined │ undefined behavior† │
49 │ │ │ behavior† │ │
50 ├───────────┼────────────┼────────────────┼───────────────────────┤
51 │DEFAULT │ robust │ undefined │ error returned │
52 │ │ │ behavior† │ │
53 └───────────┴────────────┴────────────────┴───────────────────────┘
54 † If the mutex type is PTHREAD_MUTEX_DEFAULT, the behavior of
55 pthread_mutex_lock() may correspond to one of the three other
56 standard mutex types as described in the table above. If it does
57 not correspond to one of those three, the behavior is undefined
58 for the cases marked †.
59 Where the table indicates recursive behavior, the mutex shall maintain
61 the concept of a lock count. When a thread successfully acquires a
62 mutex for the first time, the lock count shall be set to one. Every
63 time a thread relocks this mutex, the lock count shall be incremented
64 by one. Each time the thread unlocks the mutex, the lock count shall be
65 decremented by one. When the lock count reaches zero, the mutex shall
66 become available for other threads to acquire.
67 The pthread_mutex_trylock() function shall be equivalent to
69 pthread_mutex_lock(), except that if the mutex object referenced by
70 mutex is currently locked (by any thread, including the current
71 thread), the call shall return immediately. If the mutex type is
72 PTHREAD_MUTEX_RECURSIVE and the mutex is currently owned by the calling
73 thread, the mutex lock count shall be incremented by one and the
74 pthread_mutex_trylock() function shall immediately return success.
75 The pthread_mutex_unlock() function shall release the mutex object ref‐
77 erenced by mutex. The manner in which a mutex is released is dependent
78 upon the mutex's type attribute. If there are threads blocked on the
79 mutex object referenced by mutex when pthread_mutex_unlock() is called,
80 resulting in the mutex becoming available, the scheduling policy shall
81 determine which thread shall acquire the mutex.
82 (In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex shall become
84 available when the count reaches zero and the calling thread no longer
85 has any locks on this mutex.)
86 If a signal is delivered to a thread waiting for a mutex, upon return
88 from the signal handler the thread shall resume waiting for the mutex
89 as if it was not interrupted.
90 If mutex is a robust mutex and the process containing the owning thread
92 terminated while holding the mutex lock, a call to pthread_mutex_lock()
93 shall return the error value [EOWNERDEAD]. If mutex is a robust mutex
94 and the owning thread terminated while holding the mutex lock, a call
95 to pthread_mutex_lock() may return the error value [EOWNERDEAD] even if
96 the process in which the owning thread resides has not terminated. In
97 these cases, the mutex is locked by the thread but the state it pro‐
98 tects is marked as inconsistent. The application should ensure that the
99 state is made consistent for reuse and when that is complete call
100 pthread_mutex_consistent(). If the application is unable to recover
101 the state, it should unlock the mutex without a prior call to
102 pthread_mutex_consistent(), after which the mutex is marked permanently
103 unusable.
104 If mutex does not refer to an initialized mutex object, the behavior of
106 pthread_mutex_lock(), pthread_mutex_trylock(), and
107 pthread_mutex_unlock() is undefined.
108
110 If successful, the pthread_mutex_lock(), pthread_mutex_trylock(), and
111 pthread_mutex_unlock() functions shall return zero; otherwise, an error
112 number shall be returned to indicate the error.
113
115 The pthread_mutex_lock() and pthread_mutex_trylock() functions shall
116 fail if:
117 EAGAIN The mutex could not be acquired because the maximum number of
119 recursive locks for mutex has been exceeded.
120 EINVAL The mutex was created with the protocol attribute having the
122 value PTHREAD_PRIO_PROTECT and the calling thread's priority is
123 higher than the mutex's current priority ceiling.
124 ENOTRECOVERABLE
126 The state protected by the mutex is not recoverable.
127 EOWNERDEAD
129 The mutex is a robust mutex and the process containing the pre‐
130 vious owning thread terminated while holding the mutex lock. The
131 mutex lock shall be acquired by the calling thread and it is up
132 to the new owner to make the state consistent.
133 The pthread_mutex_lock() function shall fail if:
135 EDEADLK
137 The mutex type is PTHREAD_MUTEX_ERRORCHECK and the current
138 thread already owns the mutex.
139 The pthread_mutex_trylock() function shall fail if:
141 EBUSY The mutex could not be acquired because it was already locked.
143 The pthread_mutex_unlock() function shall fail if:
145 EPERM The mutex type is PTHREAD_MUTEX_ERRORCHECK or
147 PTHREAD_MUTEX_RECURSIVE, or the mutex is a robust mutex, and the
148 current thread does not own the mutex.
149 The pthread_mutex_lock() and pthread_mutex_trylock() functions may fail
151 if:
152 EOWNERDEAD
154 The mutex is a robust mutex and the previous owning thread ter‐
155 minated while holding the mutex lock. The mutex lock shall be
156 acquired by the calling thread and it is up to the new owner to
157 make the state consistent.
158 The pthread_mutex_lock() function may fail if:
160 EDEADLK
162 A deadlock condition was detected.
163 These functions shall not return an error code of [EINTR].
165 The following sections are informative.
167
169 None.
170
172 Applications that have assumed that non-zero return values are errors
173 will need updating for use with robust mutexes, since a valid return
174 for a thread acquiring a mutex which is protecting a currently incon‐
175 sistent state is [EOWNERDEAD]. Applications that do not check the
176 error returns, due to ruling out the possibility of such errors aris‐
177 ing, should not use robust mutexes. If an application is supposed to
178 work with normal and robust mutexes it should check all return values
179 for error conditions and if necessary take appropriate action.
180
182 Mutex objects are intended to serve as a low-level primitive from which
183 other thread synchronization functions can be built. As such, the
184 implementation of mutexes should be as efficient as possible, and this
185 has ramifications on the features available at the interface.
186 The mutex functions and the particular default settings of the mutex
188 attributes have been motivated by the desire to not preclude fast,
189 inlined implementations of mutex locking and unlocking.
190 Since most attributes only need to be checked when a thread is going to
192 be blocked, the use of attributes does not slow the (common) mutex-
193 locking case.
194 Likewise, while being able to extract the thread ID of the owner of a
196 mutex might be desirable, it would require storing the current thread
197 ID when each mutex is locked, and this could incur unacceptable levels
198 of overhead. Similar arguments apply to a mutex_tryunlock operation.
199 For further rationale on the extended mutex types, see the Rationale
201 (Informative) volume of POSIX.1‐2008, Threads Extensions.
202 If an implementation detects that the value specified by the mutex
204 argument does not refer to an initialized mutex object, it is recom‐
205 mended that the function should fail and report an [EINVAL] error.
206
208 None.
209
211 pthread_mutex_consistent(), pthread_mutex_destroy(),
212 pthread_mutex_timedlock(), pthread_mutexattr_getrobust()
213 The Base Definitions volume of POSIX.1‐2008, Section 4.11, Memory Syn‐
215 chronization, <pthread.h>
216
218 Portions of this text are reprinted and reproduced in electronic form
219 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
220 -- Portable Operating System Interface (POSIX), The Open Group Base
221 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
222 cal and Electronics Engineers, Inc and The Open Group. (This is
223 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
224 event of any discrepancy between this version and the original IEEE and
225 The Open Group Standard, the original IEEE and The Open Group Standard
226 is the referee document. The original Standard can be obtained online
227 at http://www.unix.org/online.html .
228 Any typographical or formatting errors that appear in this page are
230 most likely to have been introduced during the conversion of the source
231 files to man page format. To report such errors, see https://www.ker‐
232 nel.org/doc/man-pages/reporting_bugs.html .
233IEEE/The Open Group 2013 PTHREAD_MUTEX_LOCK(3P)