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