Similar Synchronization Functions: Read-Write Locks

Read-write locks allow simultaneous read access by many threads while restricting write access to only one thread at a time. This section discusses the following topics:

• Initializing a readers/writer lock

• Acquiring a read lock

• Trying to acquire a read lock

• Acquiring a write lock

• Trying to acquire a write

• Unlocking a readers/writer lock

• Destroying readers/writer lock state

When any thread holds the lock for reading, other threads can also acquire the lock for reading but must wait to acquire the lock for writing. If one thread holds the lock for writing, or is waiting to acquire the lock for writing, other threads must wait to acquire the lock for either reading or writing.

Read-write locks are slower than mutexes. But read-write locks can improve performance when the locks protect data not frequently written but are read by many concurrent threads.

Use read-write locks to synchronize threads in this process and other processes. Allocate read-write locks in memory that is writable and shared among the cooperating processes. See themmap(2) man page for information about mapping read-write locks for this behavior.

By default, the acquisition order is not defined when multiple threads are waiting for a read-write lock. However, to avoid writer starvation, the Oracle Solaris threads package tends to favor writers over readers of equal priority.

Read-write locks must be initialized before use.

Initialize a Read-Write Lock

Use rwlock_init(3C) to initialize the read-write lock pointed to by rwlp and to set the lock state to unlocked.

rwlock_init Syntax

#include <synch.h>  (or #include <thread.h>)

int rwlock_init(rwlock_t *rwlp, int type, void * 
arg);

type can be one of the following values:

• USYNC_PROCESS The read-write lock can be used to synchronize threads in this process and other processes. arg is ignored.

• USYNC_THREAD The read-write lock can be used to synchronize threads in this process only. arg is ignored.

Multiple threads must not initialize the same read-write lock simultaneously. Read-write locks can also be initialized by allocation in zeroed memory, in which case a type of USYNC_THREAD is assumed. A read-write lock must not be reinitialized while other threads might be using the lock.

For POSIX threads, see pthread_rwlock_init Syntax .

Initializing Read-Write Locks With Intraprocess Scope

#include <thread.h> 
rwlock_t rwlp; 
int ret; 
/* to be used within this process only */ 
ret = rwlock_init(&rwlp, USYNC_THREAD, 0); 

Initializing Read-Write Locks With Interprocess Scope

#include <thread.h> 
rwlock_t rwlp; 
int ret; 
/* to be used among all processes */ 
ret = rwlock_init(&rwlp, USYNC_PROCESS, 0); 

rwlock_init Return Values

rwlock_init() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.

EINVAL

Description: Invalid argument.

EFAULT

Description: rwlp or arg points to an illegal address.

Acquiring a Read Lock

Use rw_rdlock(3C) to acquire a read lock on the read-write lock pointed to by rwlp.

rw_rdlock Syntax

#include <synch.h> (or #include <thread.h>)

int rw_rdlock(rwlock_t *rwlp);

When the read-write lock is already locked for writing, the calling thread blocks until the write lock is released. Otherwise, the read lock is acquired. For POSIX threads, see pthread_rwlock_rdlock Syntax.

rw_rdlock Return Values

rw_rdlock() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.

EINVAL

Description: Invalid argument.

EFAULT

Description: rwlp points to an illegal address.

Trying to Acquire a Read Lock

Use rw_tryrdlock(3C) to attempt to acquire a read lock on the read-write lock pointed to by rwlp.

rw_tryrdlock Syntax

#include <synch.h>  (or #include <thread.h>)

int rw_tryrdlock(rwlock_t *rwlp);

When the read-write lock is already locked for writing, rw_tryrdlock() returns an error. Otherwise, the read lock is acquired. For POSIX threads, see pthread_rwlock_tryrdlock Syntax.

rw_tryrdlock Return Values

rw_tryrdlock() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.

EINVAL

Description: Invalid argument.

EFAULT

Description: rwlp points to an illegal address.

EBUSY

Description: The read-write lock pointed to by rwlp was already locked.

Acquiring a Write Lock

Use rw_wrlock(3C) to acquire a write lock on the read-write lock pointed to by rwlp.

rw_wrlock Syntax

#include <synch.h>  (or #include <thread.h>)

int rw_wrlock(rwlock_t *rwlp);

When the read-write lock is already locked for reading or writing, the calling thread blocks until all read locks and write locks are released. Only one thread at a time can hold a write lock on a read-write lock. For POSIX threads, see pthread_rwlock_wrlock Syntax.

rw_wrlock Return Values

rw_wrlock() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.

EINVAL

Description: Invalid argument.

EFAULT

Description: rwlp points to an illegal address.

Trying to Acquire a Write Lock

Use rw_trywrlock(3C) to attempt to acquire a write lock on the read-write lock pointed to by rwlp.

rw_trywrlock Syntax

#include <synch.h>  (or #include <thread.h>)

int rw_trywrlock(rwlock_t *rwlp);

When the read-write lock is already locked for reading or writing, rw_trywrlock() returns an error. For POSIX threads, see pthread_rwlock_trywrlock Syntax.

rw_trywrlock Return Values

rw_trywrlock() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.

EINVAL

Description: Invalid argument.

EFAULT

Description: rwlp points to an illegal address.

EBUSY

Description: The read-write lock pointed to by rwlp was already locked.

Unlock a Read-Write Lock

Use rw_unlock(3C) to unlock a read-write lock pointed to by rwlp.

rw_unlock Syntax

#include <synch.h>  (or #include <thread.h>)

int rw_unlock(rwlock_t *rwlp);

The read-write lock must be locked, and the calling thread must hold the lock either for reading or writing. When any other threads are waiting for the read-write lock to become available, one of the threads is unblocked. For POSIX threads, see pthread_rwlock_unlock Syntax.

rw_unlock Return Values

rw_unlock() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.

EINVAL

Description: Invalid argument.

EFAULT

Description: rwlp points to an illegal address.

Destroying the Read-Write Lock State

Use rwlock_destroy(3C) to destroy any state that is associated with the read-write lock pointed to by rlwp.

rwlock_destroy Syntax

#include <synch.h>  (or #include <thread.h>)

int rwlock_destroy(rwlock_t *rwlp);

The space for storing the read-write lock is not freed. For POSIX threads, see pthread_rwlock_destroy Syntax.

Example 6-1 uses a bank account to demonstrate read-write locks. While the program could allow multiple threads to have concurrent read-only access to the account balance, only a single writer is allowed. Note that the get_balance() function needs the lock to ensure that the addition of the checking and saving balances occurs atomically.

Example 6-1 Read-Write Bank Account

rwlock_t account_lock;
float checking_balance = 100.0;
float saving_balance = 100.0;
...
rwlock_init(&account_lock, 0, NULL);
...

float
get_balance() {
    float bal;

    rw_rdlock(&account_lock);
    bal = checking_balance + saving_balance;
    rw_unlock(&account_lock);
    return(bal);
}

void
transfer_checking_to_savings(float amount) {
    rw_wrlock(&account_lock);
    checking_balance = checking_balance - amount;
    saving_balance = saving_balance + amount;
    rw_unlock(&account_lock);
}

rwlock_destroy Return Values

rwlock_destroy() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.

EINVAL

Description: Invalid argument.

EFAULT

Description: rwlp points to an illegal address.