RHCE课程-RH253Linux服务器架设笔记五-APACHE服务器配置(4)

lock Statement

The lock keyword marks a statement block as a critical section by obtaining the mutual-exclusion lock for a given object, executing a statement, and then releasing the lock. This statement takes the following form:

lock(expression) statement_block

Remarks

lock ensures that one thread does not enter a critical section while another thread is in the critical section of code. If another thread attempts to enter a locked code, it will wait (block) until the object is released.

Example 1

The following sample shows a simple use of threads in C#.

// statements_lock.cs
using System;
using System.Threading;
class ThreadTest
{
public void runme()
{
Console.WriteLine("runme called");
}
public static void Main()
{
ThreadTest b = new ThreadTest();
Thread t = new Thread(new ThreadStart(b.runme));
t.Start();
}
}

Example 2

The following sample uses threads and lock. As long as the lock statement is present, the statement block is a critical section and

balance

will never become a negative number.

// statements_lock2.cs
using System;
using System.Threading;
class Account
{
int balance;
Random r = new Random();
public Account(int initial)
{
balance = initial;
}
int Withdraw(int amount)
{
// This condition will never be true unless the lock statement
// is commented out:
if (balance < 0)
{
throw new Exception("Negative Balance");
}
// Comment out the next line to see the effect of leaving out
// the lock keyword:
lock (this)
{
if (balance >= amount)
{
Console.WriteLine("Balance before Withdrawal :  " + balance);
Console.WriteLine("Amount to Withdraw        : -" + amount);
balance = balance - amount;
Console.WriteLine("Balance after Withdrawal  :  " + balance);
return amount;
}
else
{
return 0; // transaction rejected
}
}
}
public void DoTransactions()
{
for (int i = 0; i < 100; i++)
{
Withdraw(r.Next(1, 100));
}
}
}
class Test
{
public static void Main()
{
Thread[] threads = new Thread[10];
Account acc = new Account (1000);
for (int i = 0; i < 10; i++)
{
Thread t = new Thread(new ThreadStart(acc.DoTransactions));
threads[i] = t;
}
for (int i = 0; i < 10; i++)
{
threads[i].Start();
}
}
}