man/pthread_rwlock_rdlock.3

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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 SYNOPSIS
  35 .In pthread.h
  36 .Ft int
  37 .Fo pthread_rwlock_rdlock
  38 .Fa "pthread_rwlock_t *rwlock"
  39 .Fc
  40 .Ft int
  41 .Fo pthread_rwlock_tryrdlock
  42 .Fa "pthread_rwlock_t *rwlock"
  43 .Fc
  44 .Sh DESCRIPTION
  45 The
  46 .Fn pthread_rwlock_rdlock
  47 function acquires a read lock on
  48 .Fa rwlock ,
  49 provided that
  50 .Fa rwlock
  51 is not presently held for writing and no writer threads are
  52 presently blocked on the lock.  If the read lock cannot be
  53 immediately acquired, the calling thread blocks until it can
  54 acquire the lock.
  55 .Pp
  56 The
  57 .Fn pthread_rwlock_tryrdlock
  58 function performs the same action, but does not block if the lock
  59 cannot be immediately obtained (i.e., the lock is held for writing
  60 or there are waiting writers).
  61 .Pp
  62 A thread may hold multiple concurrent read locks.  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 to indicate the error.
  77 .Sh SEE ALSO
  78 .Xr pthread_rwlock_init 3 ,
  79 .Xr pthread_rwlock_trywrlock 3 ,
  80 .Xr pthread_rwlock_unlock 3 ,
  81 .Xr pthread_rwlock_wrlock 3
  82 .Sh STANDARDS
  83 The
  84 .Fn pthread_rwlock_rdlock
  85 and
  86 .Fn pthread_rwlock_tryrdlock
  87 functions are expected to conform to
  88 .St -susv2 .
  89 .Sh ERRORS
  90 The
  91 .Fn pthread_rwlock_tryrdlock
  92 function will fail if:
  93 .Bl -tag -width Er
  94 .It Bq Er EBUSY
  95 The lock could not be acquired, because a writer holds the lock or
  96 was blocked on it.
  97 .El
  98 .Pp
  99 The
 100 .Fn pthread_rwlock_rdlock
 101 and
 102 .Fn pthread_rwlock_tryrdlock
 103 functions may fail if:
 104 .Bl -tag -width Er
 105 .It Bq Er EAGAIN
 106 The lock could not be acquired, because the maximum number of read locks
 107 against
 108 .Fa lock
 109 has been exceeded.
 110 .It Bq Er EDEADLK
 111 The current thread already owns
 112 .Fa rwlock
 113 for writing.
 114 .It Bq Er EINVAL
 115 The value specified by
 116 .Fa rwlock
 117 is invalid.
 118 .It Bq Er ENOMEM
 119 Insufficient memory exists to initialize the lock (applies to
 120 statically initialized locks only).
 121 .El
 122 .Sh HISTORY
 123 The
 124 .Fn pthread_rwlock_rdlock
 125 function first appeared in
 126 .Fx 3.0 .