pthreads/pthread_rwlock_rdlock.3

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  25 .\" $FreeBSD: src/lib/libc_r/man/pthread_rwlock_rdlock.3,v 1.4 2001/10/01 16:09:09 ru Exp $
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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 .Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock"
  38 .Ft int
  39 .Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock"
  40 .Sh DESCRIPTION
  41 The
  42 .Fn pthread_rwlock_rdlock
  43 function acquires a read lock on
  44 .Fa lock
  45 provided that
  46 .Fa lock
  47 is not presently held for writing and no writer threads are
  48 presently blocked on the lock.  If the read lock cannot be
  49 immediately acquired, the calling thread blocks until it can
  50 acquire the lock.
  51 .Pp
  52 The
  53 .Fn pthread_rwlock_tryrdlock
  54 function performs the same action, but does not block if the lock
  55 cannot be immediately obtained (i.e. the lock is held for writing
  56 or there are waiting writers).
  57 .Pp
  58 A thread may hold multiple concurrent read locks.  If so,
  59 .Fn pthread_rwlock_unlock
  60 must be called once for each lock obtained.
  61 .Pp
  62 The results of acquiring a read lock while the calling thread holds
  63 a write lock are undefined.
  64 .Sh IMPLEMENTATION NOTES
  65 To prevent writer starvation, writers are favored over readers.
  66 .Sh RETURN VALUES
  67 If successful, the
  68 .Fn pthread_rwlock_rdlock
  69 and
  70 .Fn pthread_rwlock_tryrdlock
  71 functions will return zero.  Otherwise an error number will be returned
  72 to indicate the error.
  73 .Sh SEE ALSO
  74 .Xr pthread_rwlock_init 3 ,
  75 .Xr pthread_rwlock_trywrlock 3 ,
  76 .Xr pthread_rwlock_unlock 3 ,
  77 .Xr pthread_rwlock_wrlock 3
  78 .Sh STANDARDS
  79 The
  80 .Fn pthread_rwlock_rdlock
  81 and
  82 .Fn pthread_rwlock_tryrdlock
  83 functions are expected to conform to
  84 .St -susv2 .
  85 .Sh ERRORS
  86 The
  87 .Fn pthread_rwlock_tryrdlock
  88 function will fail if:
  89 .Bl -tag -width Er
  90 .It Bq Er EBUSY
  91 The lock could not be acquired because a writer holds the lock or
  92 was blocked on it.
  93 .El
  94 .Pp
  95 The
  96 .Fn pthread_rwlock_rdlock
  97 and
  98 .Fn pthread_rwlock_tryrdlock
  99 functions may fail if:
 100 .Bl -tag -width Er
 101 .It Bq Er EAGAIN
 102 The lock could not be acquired because the maximum number of read locks
 103 against
 104 .Fa lock
 105 has been exceeded.
 106 .It Bq Er EDEADLK
 107 The current thread already owns
 108 .Fa lock
 109 for writing.
 110 .It Bq Er EINVAL
 111 The value specified by
 112 .Fa lock
 113 is invalid.
 114 .It Bq Er ENOMEM
 115 Insufficient memory exists to initialize the lock (applies to
 116 statically initialized locks only).
 117 .El
 118 .Sh HISTORY
 119 The
 120 .Fn pthread_rwlock_rdlock
 121 function first appeared in
 122 .Fx 3.0 .