January 30, 2003 PTHREAD_RWLOCK_RDLOCK 3

### NAME

pthread_rwlock_rdlock, pthread_rwlock_timedrdlock, pthread_rwlock_tryrdlock - acquire a read/write lock for reading

### LIBRARY

POSIX Thread Library (libpthread, -lpthread)

### SYNOPSIS

```int` pthread_rwlock_rdlock(```pthread_rwlock_t *lock`)
```int` pthread_rwlock_timedrdlock(```pthread_rwlock_t * restrict lock``, const struct timespec * restrict abstime`)
```int` pthread_rwlock_tryrdlock(```pthread_rwlock_t *lock`)

### DESCRIPTION

The pthread_rwlock_rdlock(``) function acquires a read lock on ```lock` provided that ```lock` is not presently held for writing and no writer threads are presently blocked on the lock. If the read lock cannot be immediately acquired, the calling thread blocks until it can acquire the lock.

The pthread_rwlock_timedrdlock(``) performs the same action, but will not wait beyond ```abstime` to obtain the lock before returning.

The pthread_rwlock_tryrdlock(``) function performs the same action as pthread_rwlock_rdlock(``), but does not block if the lock cannot be immediately obtained (i.e., the lock is held for writing or there are waiting writers).

A thread may hold multiple concurrent read locks. If so, pthread_rwlock_unlock(``) must be called once for each lock obtained.

The results of acquiring a read lock while the calling thread holds a write lock are undefined.

### RETURN VALUES

If successful, the pthread_rwlock_rdlock(``), pthread_rwlock_timedrdlock(``), and pthread_rwlock_tryrdlock(``) functions will return zero. Otherwise an error number will be returned to indicate the error.

### ERRORS

The pthread_rwlock_tryrdlock(``) function shall fail if:

[```EBUSY`]
  The lock could not be acquired because a writer holds the lock or was blocked on it.

The pthread_rwlock_timedrdlock(``) function shall fail if:

[```ETIMEDOUT`]
  The time specified by ```abstime` was reached before the lock could be obtained.

The pthread_rwlock_rdlock(``), pthread_rwlock_timedrdlock(``), and pthread_rwlock_tryrdlock(``) functions may fail if:

[```EAGAIN`]
  The lock could not be acquired because the maximum number of read locks against ```lock` has been exceeded.

[```EDEADLK`]
  The current thread already owns ```lock` for writing.

[```EINVAL`]
  The value specified by ```lock` is invalid.

### SEE ALSO

pthread_rwlock_destroy(3), pthread_rwlock_init(3), pthread_rwlock_unlock(3), pthread_rwlock_wrlock(3)

### STANDARDS

pthread_rwlock_rdlock(``), pthread_rwlock_timedrdlock(``), and pthread_rwlock_tryrdlock(``) conform to ISO/IEC 9945-1:1996 (``POSIX.1'') .