void withdrawer(struct account* acct) {
    pthread_mutex_lock(&acct->mutex);
    while (acct->bal < 100) {
       pthread_cond_wait(&acct->cond,
                         &acct->mutex);
    }
    withdraw(acct, 100);
    pthread_mutex_unlock(&acct->mutex);
}

void depositer(struct account* acct) {
    pthread_mutex_lock(&acct->mutex);
    deposit(acct, 100);
    pthread_mutex_unlock(&acct->mutex);
    pthread_cond_signal(&acct->cond);
}