1PTHREAD_RWLOCK_RDLOCK(3P) POSIX Programmer's Manual PTHREAD_RWLOCK_RDLOCK(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.
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12 pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock a read-write
13 lock object for reading
14
16 #include <pthread.h>
17 int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
19 int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);
20
23 The pthread_rwlock_rdlock() function shall apply a read lock to the
24 read-write lock referenced by rwlock. The calling thread acquires the
25 read lock if a writer does not hold the lock and there are no writers
26 blocked on the lock.
27 If the Thread Execution Scheduling option is supported, and the threads
29 involved in the lock are executing with the scheduling policies
30 SCHED_FIFO or SCHED_RR, the calling thread shall not acquire the lock
31 if a writer holds the lock or if writers of higher or equal priority
32 are blocked on the lock; otherwise, the calling thread shall acquire
33 the lock.
34 If the Threads Execution Scheduling option is supported, and the
36 threads involved in the lock are executing with the SCHED_SPORADIC
37 scheduling policy, the calling thread shall not acquire the lock if a
38 writer holds the lock or if writers of higher or equal priority are
39 blocked on the lock; otherwise, the calling thread shall acquire the
40 lock.
41 If the Thread Execution Scheduling option is not supported, it is
43 implementation-defined whether the calling thread acquires the lock
44 when a writer does not hold the lock and there are writers blocked on
45 the lock. If a writer holds the lock, the calling thread shall not
46 acquire the read lock. If the read lock is not acquired, the calling
47 thread shall block until it can acquire the lock. The calling thread
48 may deadlock if at the time the call is made it holds a write lock.
49 A thread may hold multiple concurrent read locks on rwlock (that is,
51 successfully call the pthread_rwlock_rdlock() function n times). If so,
52 the application shall ensure that the thread performs matching unlocks
53 (that is, it calls the pthread_rwlock_unlock() function n times).
54 The maximum number of simultaneous read locks that an implementation
56 guarantees can be applied to a read-write lock shall be implementation-
57 defined. The pthread_rwlock_rdlock() function may fail if this maximum
58 would be exceeded.
59 The pthread_rwlock_tryrdlock() function shall apply a read lock as in
61 the pthread_rwlock_rdlock() function, with the exception that the func‐
62 tion shall fail if the equivalent pthread_rwlock_rdlock() call would
63 have blocked the calling thread. In no case shall the
64 pthread_rwlock_tryrdlock() function ever block; it always either
65 acquires the lock or fails and returns immediately.
66 Results are undefined if any of these functions are called with an
68 uninitialized read-write lock.
69 If a signal is delivered to a thread waiting for a read-write lock for
71 reading, upon return from the signal handler the thread resumes waiting
72 for the read-write lock for reading as if it was not interrupted.
73
75 If successful, the pthread_rwlock_rdlock() function shall return zero;
76 otherwise, an error number shall be returned to indicate the error.
77 The pthread_rwlock_tryrdlock() function shall return zero if the lock
79 for reading on the read-write lock object referenced by rwlock is
80 acquired. Otherwise, an error number shall be returned to indicate the
81 error.
82
84 The pthread_rwlock_tryrdlock() function shall fail if:
85 EBUSY The read-write lock could not be acquired for reading because a
87 writer holds the lock or a writer with the appropriate priority
88 was blocked on it.
89 The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions
92 may fail if:
93 EINVAL The value specified by rwlock does not refer to an initialized
95 read-write lock object.
96 EAGAIN The read lock could not be acquired because the maximum number
98 of read locks for rwlock has been exceeded.
99 The pthread_rwlock_rdlock() function may fail if:
102 EDEADLK
104 The current thread already owns the read-write lock for writing.
105 These functions shall not return an error code of [EINTR].
108 The following sections are informative.
110
112 None.
113
115 Applications using these functions may be subject to priority inver‐
116 sion, as discussed in the Base Definitions volume of
117 IEEE Std 1003.1-2001, Section 3.285, Priority Inversion.
118
120 None.
121
123 None.
124
126 pthread_rwlock_destroy(), pthread_rwlock_timedrdlock(),
127 pthread_rwlock_timedwrlock(), pthread_rwlock_trywrlock(),
128 pthread_rwlock_unlock(), pthread_rwlock_wrlock(), the Base Definitions
129 volume of IEEE Std 1003.1-2001, <pthread.h>
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132 Portions of this text are reprinted and reproduced in electronic form
133 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
134 -- Portable Operating System Interface (POSIX), The Open Group Base
135 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
136 Electrical and Electronics Engineers, Inc and The Open Group. In the
137 event of any discrepancy between this version and the original IEEE and
138 The Open Group Standard, the original IEEE and The Open Group Standard
139 is the referee document. The original Standard can be obtained online
140 at http://www.opengroup.org/unix/online.html .
141IEEE/The Open Group 2003 PTHREAD_RWLOCK_RDLOCK(3P)