10.临界区

线程安全问题

• 每个线程都有自己的栈，而局部变量是存储在栈中的，这就意味着每个线程都有一份自己的“局部变量”，如果线程仅仅使用“局部变量”，那么就不存在线程安全问题。如果多个线程公用一个全局变量呢？
• 线程锁

#include <handleapi.h>
#include <minwinbase.h>
#include <minwindef.h>
#include <processthreadsapi.h>
#include <stdio.h>
#include <synchapi.h>
#include <windows.h>
#include <winnt.h>

int g_dwTickets = 10;
CRITICAL_SECTION cs;

DWORD WINAPI MyFirstThreadProc(LPVOID IpParameter) {
    while (1) {
        EnterCriticalSection(&cs);
        if (g_dwTickets <= 0) {
            LeaveCriticalSection(&cs);
            break;
        }
        printf("还有%d张票\n", g_dwTickets);
        g_dwTickets--;
        printf("卖出去一张，现在还有%d张\n", g_dwTickets);
        LeaveCriticalSection(&cs);
    }
    return 0;
}

int main(int argc, char *argv[]) {
    DWORD dwR1,dwR2;
    HANDLE aThreadHandles[2];

    InitializeCriticalSection(&cs);
    aThreadHandles[0] = CreateThread(NULL, 0, MyFirstThreadProc, NULL, 0, NULL);
    aThreadHandles[1] = CreateThread(NULL, 0, MyFirstThreadProc, NULL, 0, NULL);
    WaitForMultipleObjects(2, aThreadHandles, TRUE, INFINITE);

    GetExitCodeThread(aThreadHandles[0], &dwR1);
    GetExitCodeThread(aThreadHandles[1], &dwR2);
    CloseHandle(aThreadHandles[0]);
    CloseHandle(aThreadHandles[1]);
    DeleteCriticalSection(&cs);
	return 0;
}