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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 .sp 67 .LP 68 The \fBpthread_rwlock_rdlock()\fR function applies a read lock to the 69 read-write lock referenced by \fIrwlock\fR. The calling thread acquires the 70 read lock if a writer does not hold the lock and there are no writers blocked 71 on the lock. 72 .sp 73 .LP 74 The calling thread does not acquire the lock if a writer holds the lock or if 75 writers of higher or equal priority are blocked on the lock; otherwise, the 76 calling thread acquires the lock. If the read lock is not acquired, the calling 77 thread blocks until it can acquire the lock. 78 .sp 79 .LP 80 A thread can hold multiple concurrent read locks on \fIrwlock\fR (that is, 81 successfully call the \fBpthread_rwlock_rdlock()\fR function \fIn\fR times). If 82 so, the thread must perform matching unlocks (that is, it must call the 83 \fBpthread_rwlock_unlock()\fR function \fIn\fR times). 84 .sp 85 .LP 86 The maximum number of concurrent read locks that a thread can hold on one 87 read-write lock is currently set at 100,000, though this number could change in 88 a future release. There is no imposed limit on the number of different threads 89 that can apply a read lock to one read-write lock. 90 .sp 91 .LP 92 The \fBpthread_rwlock_tryrdlock()\fR function applies a read lock like the 93 \fBpthread_rwlock_rdlock()\fR function, with the exception that the function 94 fails if the equivalent \fBpthread_rwlock_rdlock()\fR call would have blocked 95 the calling thread. In no case will the \fBpthread_rwlock_tryrdlock()\fR 96 function ever bloc. It always either acquires the lock or fails and returns 97 immediately. 98 .sp 99 .LP 100 Results are undefined if any of these functions are called with an 101 uninitialized read-write lock. 102 .sp 103 .LP 104 If a signal is delivered to a thread waiting for a read-write lock for reading, 105 upon return from the signal handler the thread resumes waiting for the 106 read-write lock for reading as if it was not interrupted. 107 .SH RETURN VALUES 108 .sp 109 .LP 110 If successful, the \fBpthread_rwlock_rdlock()\fR function returns \fB0\fR. 111 Otherwise, an error number is returned to indicate the error. 112 .sp 113 .LP 114 The \fBpthread_rwlock_tryrdlock()\fR function returns \fB0\fR if the lock for 115 reading on the read-write lock object referenced by \fIrwlock\fR is acquired. 116 Otherwise an error number is returned to indicate the error. 117 .SH ERRORS 118 .sp 119 .LP 120 The \fBpthread_rwlock_rdlock()\fR and \fBpthread_rwlock_tryrdlock()\fR 121 functions will fail if: 122 .sp 123 .ne 2 124 .na 125 \fB\fBEAGAIN\fR\fR 126 .ad 127 .RS 10n 128 The read lock could not be acquired because the maximum number of read locks by 129 the current thread for \fIrwlock\fR has been exceeded. 130 .RE 131 132 .sp 133 .LP 134 The \fBpthread_rwlock_rdlock()\fR function will fail if: 135 .sp 136 .ne 2 137 .na 138 \fB\fBEDEADLK\fR\fR 139 .ad 140 .RS 11n 141 The current thread already owns the read-write lock for writing. 142 .RE 143 144 .sp 145 .LP 146 The \fBpthread_rwlock_tryrdlock()\fR function will fail if: 147 .sp 148 .ne 2 149 .na 150 \fB\fBEBUSY\fR\fR 151 .ad 152 .RS 9n 153 The read-write lock could not be acquired for reading because a writer holds 154 the lock or a writer with the appropriate priority was blocked on it. 155 .RE 156 157 .SH ATTRIBUTES 158 .sp 159 .LP 160 See \fBattributes\fR(5) for descriptions of the following attributes: 161 .sp 162 163 .sp 164 .TS 165 box; 166 c | c 167 l | l . 168 ATTRIBUTE TYPE ATTRIBUTE VALUE 169 _ 170 Interface Stability Standard 171 _ 172 MT-Level MT-Safe 173 .TE 174 175 .SH SEE ALSO 176 .sp 177 .LP 178 \fBpthread_rwlock_init\fR(3C), \fBpthread_rwlock_wrlock\fR(3C), 179 \fBpthread_rwlockattr_init\fR(3C), \fBpthread_rwlock_unlock\fR(3C), 180 \fBattributes\fR(5), \fBstandards\fR(5)