/share/man/man3/pthread_rwlock_rdlock.3

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  1. .\" Copyright (c) 1998 Alex Nash
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  27. .Dd August 4, 1998
  28. .Dt PTHREAD_RWLOCK_RDLOCK 3
  29. .Os
  30. .Sh NAME
  31. .Nm pthread_rwlock_rdlock ,
  32. .Nm pthread_rwlock_tryrdlock
  33. .Nd acquire a read/write lock for reading
  34. .Sh LIBRARY
  35. .Lb libpthread
  36. .Sh SYNOPSIS
  37. .In pthread.h
  38. .Ft int
  39. .Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock"
  40. .Ft int
  41. .Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock"
  42. .Sh DESCRIPTION
  43. The
  44. .Fn pthread_rwlock_rdlock
  45. function acquires a read lock on
  46. .Fa lock
  47. provided that
  48. .Fa lock
  49. is not presently held for writing and no writer threads are
  50. presently blocked on the lock.
  51. If the read lock cannot be
  52. immediately acquired, the calling thread blocks until it can
  53. acquire the lock.
  54. .Pp
  55. The
  56. .Fn pthread_rwlock_tryrdlock
  57. function performs the same action, but does not block if the lock
  58. cannot be immediately obtained (i.e., the lock is held for writing
  59. or there are waiting writers).
  60. .Pp
  61. A thread may hold multiple concurrent read locks.
  62. If so,
  63. .Fn pthread_rwlock_unlock
  64. must be called once for each lock obtained.
  65. .Pp
  66. The results of acquiring a read lock while the calling thread holds
  67. a write lock are undefined.
  68. .Sh IMPLEMENTATION NOTES
  69. To prevent writer starvation, writers are favored over readers.
  70. .Sh RETURN VALUES
  71. If successful, the
  72. .Fn pthread_rwlock_rdlock
  73. and
  74. .Fn pthread_rwlock_tryrdlock
  75. functions will return zero.
  76. Otherwise an error number will be returned
  77. to indicate the error.
  78. .Sh ERRORS
  79. The
  80. .Fn pthread_rwlock_tryrdlock
  81. function will fail if:
  82. .Bl -tag -width Er
  83. .It Bq Er EBUSY
  84. The lock could not be acquired because a writer holds the lock or
  85. was blocked on it.
  86. .El
  87. .Pp
  88. The
  89. .Fn pthread_rwlock_rdlock
  90. and
  91. .Fn pthread_rwlock_tryrdlock
  92. functions may fail if:
  93. .Bl -tag -width Er
  94. .It Bq Er EAGAIN
  95. The lock could not be acquired because the maximum number of read locks
  96. against
  97. .Fa lock
  98. has been exceeded.
  99. .It Bq Er EDEADLK
  100. The current thread already owns
  101. .Fa lock
  102. for writing.
  103. .It Bq Er EINVAL
  104. The value specified by
  105. .Fa lock
  106. is invalid.
  107. .It Bq Er ENOMEM
  108. Insufficient memory exists to initialize the lock (applies to
  109. statically initialized locks only).
  110. .El
  111. .Sh SEE ALSO
  112. .Xr pthread_rwlock_init 3 ,
  113. .Xr pthread_rwlock_trywrlock 3 ,
  114. .Xr pthread_rwlock_unlock 3 ,
  115. .Xr pthread_rwlock_wrlock 3
  116. .Sh STANDARDS
  117. The
  118. .Fn pthread_rwlock_rdlock
  119. and
  120. .Fn pthread_rwlock_tryrdlock
  121. functions are expected to conform to
  122. .St -susv2 .
  123. .Sh HISTORY
  124. The
  125. .Fn pthread_rwlock_rdlock
  126. function first appeared in
  127. .Fx 3.0 .