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.