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All Rights Reserved. 45 .\" Copyright (c) 2008, Sun Microsystems, Inc. All Rights Reserved. 46 .\" 47 .TH PTHREAD_RWLOCK_RDLOCK 3C "Mar 23, 2005" 48 .SH NAME 49 pthread_rwlock_rdlock, pthread_rwlock_tryrdlock \- lock or attempt to lock 50 read-write lock object for reading 51 .SH SYNOPSIS 52 .LP 53 .nf 54 cc -mt [ \fIflag\fR... ] \fIfile\fR... -lpthread [ \fIlibrary\fR... ] 55 #include <pthread.h> 56 57 \fBint\fR \fBpthread_rwlock_rdlock\fR(\fBpthread_rwlock_t\fR \fI*rwlock\fR); 58 .fi 59 60 .LP 61 .nf 62 \fBint\fR \fBpthread_rwlock_tryrdlock\fR(\fBpthread_rwlock_t\fR \fI*rwlock\fR); 63 .fi 64 65 .SH DESCRIPTION 66 .LP 67 The \fBpthread_rwlock_rdlock()\fR function applies a read lock to the 68 read-write lock referenced by \fIrwlock\fR. The calling thread acquires the 69 read lock if a writer does not hold the lock and there are no writers blocked 70 on the lock. 71 .sp 72 .LP 73 The calling thread does not acquire the lock if a writer holds the lock or if 74 writers of higher or equal priority are blocked on the lock; otherwise, the 75 calling thread acquires the lock. If the read lock is not acquired, the calling 76 thread blocks until it can acquire the lock. 77 .sp 78 .LP 79 A thread can hold multiple concurrent read locks on \fIrwlock\fR (that is, 80 successfully call the \fBpthread_rwlock_rdlock()\fR function \fIn\fR times). If 81 so, the thread must perform matching unlocks (that is, it must call the 82 \fBpthread_rwlock_unlock()\fR function \fIn\fR times). 83 .sp 84 .LP 85 The maximum number of concurrent read locks that a thread can hold on one 86 read-write lock is currently set at 100,000, though this number could change in 87 a future release. There is no imposed limit on the number of different threads 88 that can apply a read lock to one read-write lock. 89 .sp 90 .LP 91 The \fBpthread_rwlock_tryrdlock()\fR function applies a read lock like the 92 \fBpthread_rwlock_rdlock()\fR function, with the exception that the function 93 fails if the equivalent \fBpthread_rwlock_rdlock()\fR call would have blocked 94 the calling thread. In no case will the \fBpthread_rwlock_tryrdlock()\fR 95 function ever block. It always either acquires the lock or fails and returns 96 immediately. 97 .sp 98 .LP 99 Results are undefined if any of these functions are called with an 100 uninitialized read-write lock. 101 .sp 102 .LP 103 If a signal is delivered to a thread waiting for a read-write lock for reading, 104 upon return from the signal handler the thread resumes waiting for the 105 read-write lock for reading as if it was not interrupted. 106 .SH RETURN VALUES 107 .LP 108 If successful, the \fBpthread_rwlock_rdlock()\fR function returns \fB0\fR. 109 Otherwise, an error number is returned to indicate the error. 110 .sp 111 .LP 112 The \fBpthread_rwlock_tryrdlock()\fR function returns \fB0\fR if the lock for 113 reading on the read-write lock object referenced by \fIrwlock\fR is acquired. 114 Otherwise an error number is returned to indicate the error. 115 .SH ERRORS 116 .LP 117 The \fBpthread_rwlock_rdlock()\fR and \fBpthread_rwlock_tryrdlock()\fR 118 functions will fail if: 119 .sp 120 .ne 2 121 .na 122 \fB\fBEAGAIN\fR\fR 123 .ad 124 .RS 10n 125 The read lock could not be acquired because the maximum number of read locks by 126 the current thread for \fIrwlock\fR has been exceeded. 127 .RE 128 129 .sp 130 .LP 131 The \fBpthread_rwlock_rdlock()\fR function will fail if: 132 .sp 133 .ne 2 134 .na 135 \fB\fBEDEADLK\fR\fR 136 .ad 137 .RS 11n 138 The current thread already owns the read-write lock for writing. 139 .RE 140 141 .sp 142 .LP 143 The \fBpthread_rwlock_tryrdlock()\fR function will fail if: 144 .sp 145 .ne 2 146 .na 147 \fB\fBEBUSY\fR\fR 148 .ad 149 .RS 9n 150 The read-write lock could not be acquired for reading because a writer holds 151 the lock or a writer with the appropriate priority was blocked on it. 152 .RE 153 154 .SH ATTRIBUTES 155 .LP 156 See \fBattributes\fR(5) for descriptions of the following attributes: 157 .sp 158 159 .sp 160 .TS 161 box; 162 c | c 163 l | l . 164 ATTRIBUTE TYPE ATTRIBUTE VALUE 165 _ 166 Interface Stability Standard 167 _ 168 MT-Level MT-Safe 169 .TE 170 171 .SH SEE ALSO 172 .LP 173 \fBpthread_rwlock_init\fR(3C), \fBpthread_rwlock_wrlock\fR(3C), 174 \fBpthread_rwlockattr_init\fR(3C), \fBpthread_rwlock_unlock\fR(3C), 175 \fBattributes\fR(5), \fBstandards\fR(5)