xref: /freebsd/share/man/man3/pthread_rwlock_rdlock.3 (revision e8d8bef961a50d4dc22501cde4fb9fb0be1b2532)
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25.\" $FreeBSD$
26.\"
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
43The
44.Fn pthread_rwlock_rdlock
45function acquires a read lock on
46.Fa lock
47provided that
48.Fa lock
49is not presently held for writing and no writer threads are
50presently blocked on the lock.
51If the read lock cannot be
52immediately acquired, the calling thread blocks until it can
53acquire the lock.
54.Pp
55The
56.Fn pthread_rwlock_tryrdlock
57function performs the same action, but does not block if the lock
58cannot be immediately obtained (i.e., the lock is held for writing
59or there are waiting writers).
60.Pp
61A thread may hold multiple concurrent read locks.
62If so,
63.Fn pthread_rwlock_unlock
64must be called once for each lock obtained.
65.Pp
66The results of acquiring a read lock while the calling thread holds
67a write lock are undefined.
68.Sh IMPLEMENTATION NOTES
69To prevent writer starvation, writers are favored over readers.
70.Sh RETURN VALUES
71If successful, the
72.Fn pthread_rwlock_rdlock
73and
74.Fn pthread_rwlock_tryrdlock
75functions will return zero.
76Otherwise an error number will be returned
77to indicate the error.
78.Sh ERRORS
79The
80.Fn pthread_rwlock_tryrdlock
81function will fail if:
82.Bl -tag -width Er
83.It Bq Er EBUSY
84The lock could not be acquired because a writer holds the lock or
85was blocked on it.
86.El
87.Pp
88The
89.Fn pthread_rwlock_rdlock
90and
91.Fn pthread_rwlock_tryrdlock
92functions may fail if:
93.Bl -tag -width Er
94.It Bq Er EAGAIN
95The lock could not be acquired because the maximum number of read locks
96against
97.Fa lock
98has been exceeded.
99.It Bq Er EDEADLK
100The current thread already owns
101.Fa lock
102for writing.
103.It Bq Er EINVAL
104The value specified by
105.Fa lock
106is invalid.
107.It Bq Er ENOMEM
108Insufficient memory exists to initialize the lock (applies to
109statically 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
117The
118.Fn pthread_rwlock_rdlock
119and
120.Fn pthread_rwlock_tryrdlock
121functions are expected to conform to
122.St -susv2 .
123.Sh HISTORY
124The
125.Fn pthread_rwlock_rdlock
126function first appeared in
127.Fx 3.0 .
128