现在本人知道C/C++多线程同步有互斥锁、信号量、Event等几种方式，但是以前本人不知道线程同步概念的时候，用全局变量控制了两个线程的执行顺序，就是线程1->线程2->线程1->线程2……
请大家看下这种方式是不是有漏洞，举个简单例子。
请大家看下这种方式是不是有漏洞，举个简单例子。
//MultiThread
#include <iostream>
#include <cstdlib>
#include <windows.h>
using namespace std;
#define MUTELOCK
DWORD WINAPI Fun1Proc(LPVOID lpParameter);
DWORD WINAPI Fun2Proc(LPVOID lpParameter);
int tickets = 200;
HANDLE hMutex;
int fun1over = 0;
int fun2over = 1;
int main()
{
hMutex = CreateMutex(NULL, FALSE, NULL);
HANDLE hThread_1 = CreateThread(NULL, 0, Fun1Proc, NULL, 0, NULL);
HANDLE hThread_2 = CreateThread(NULL, 0, Fun2Proc, NULL, 0, NULL);
CloseHandle(hThread_1);
cout << "close handle1" << endl;
CloseHandle(hThread_2);
cout << "close handle2" << endl;
system("pause");
return 0;
}
DWORD WINAPI Fun1Proc(LPVOID lpParameter)
{
while (true)
{
#ifdef MUTELOCK
WaitForSingleObject(hMutex, INFINITE);
if (tickets > 0)
{
cout << "Thread 1 sell ticket : "<<tickets--<<endl;
}
else
break;
ReleaseMutex(hMutex);
#else
if (fun2over)
{
fun2over = 0;
if (tickets > 0)
{
cout << "Thread 1 sell ticket : "<<tickets--<<endl;
}
else
break;
fun1over = 1;
}
#endif
}
return 0;
}
DWORD WINAPI Fun2Proc(LPVOID lpParameter)
{
while (true)
{
#ifdef MUTELOCK
WaitForSingleObject(hMutex, INFINITE);
if (tickets > 0)
{
cout << "Thread 2 sell ticket : "<<tickets--<<endl;
}
else
break;
ReleaseMutex(hMutex);
#else
if (fun1over)
{
fun1over = 0;
if (tickets > 0)
{
cout << "Thread 2 sell ticket : "<<tickets--<<endl;
}
else
break;
fun2over = 1;
}
#endif
}
return 0;
}
打开和关闭MUTELOCK，运行结果是没有区别的。
解决方案
8
这一点在《Windows核心编程》第五版有讲，全局变量肯定不合理
4
各个线程读到的 全局变量的值又不是一致的, 你的代码当然是错的…
这代码看起来正常仅仅是原因是 cout IO时内部是加了锁的, 这刷新了CPU的高速缓存, 而其他代码运行时间相对于IO , 几乎可以忽略.. 你把中间处理的代码换成一个不用加锁的处理代码 就可以发现问题了…
这代码看起来正常仅仅是原因是 cout IO时内部是加了锁的, 这刷新了CPU的高速缓存, 而其他代码运行时间相对于IO , 几乎可以忽略.. 你把中间处理的代码换成一个不用加锁的处理代码 就可以发现问题了…
4
仅供参考：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#ifdef WIN32
#include <windows.h>
#include <io.h>
#include <process.h>
#define MYVOID void
#else
#include <unistd.h>
#include <sys/time.h>
#include <pthread.h>
#define CRITICAL_SECTION pthread_mutex_t
#define _vsnprintf vsnprintf
#define MYVOID void *
#endif
//Log{
#define MAXLOGSIZE 20000000
#define ARRSIZE(x) (sizeof(x)/sizeof(x[0]))
#include <time.h>
#include <sys/timeb.h>
#include <stdarg.h>
char logfilename1[]="MyLog1.log";
char logfilename2[]="MyLog2.log";
char logstr[16000];
char datestr[16];
char timestr[16];
char mss[4];
CRITICAL_SECTION cs_log;
FILE *flog;
#ifdef WIN32
void Lock(CRITICAL_SECTION *l) {
EnterCriticalSection(l);
}
void Unlock(CRITICAL_SECTION *l) {
LeaveCriticalSection(l);
}
void sleep_ms(int ms) {
Sleep(ms);
}
#else
void Lock(CRITICAL_SECTION *l) {
pthread_mutex_lock(l);
}
void Unlock(CRITICAL_SECTION *l) {
pthread_mutex_unlock(l);
}
void sleep_ms(int ms) {
usleep(ms*1000);
}
#endif
void LogV(const char *pszFmt,va_list argp) {
struct tm *now;
struct timeb tb;
if (NULL==pszFmt||0==pszFmt[0]) return;
if (-1==_vsnprintf(logstr,ARRSIZE(logstr),pszFmt,argp)) logstr[ARRSIZE(logstr)-1]=0;
ftime(&tb);
now=localtime(&tb.time);
sprintf(datestr,"%04d-%02d-%02d",now->tm_year+1900,now->tm_mon+1,now->tm_mday);
sprintf(timestr,"%02d:%02d:%02d",now->tm_hour ,now->tm_min ,now->tm_sec );
sprintf(mss,"%03d",tb.millitm);
printf("%s %s.%s %s",datestr,timestr,mss,logstr);
flog=fopen(logfilename1,"a");
if (NULL!=flog) {
fprintf(flog,"%s %s.%s %s",datestr,timestr,mss,logstr);
if (ftell(flog)>MAXLOGSIZE) {
fclose(flog);
if (rename(logfilename1,logfilename2)) {
remove(logfilename2);
rename(logfilename1,logfilename2);
}
flog=fopen(logfilename1,"a");
if (NULL==flog) return;
}
fclose(flog);
}
}
void Log(const char *pszFmt,...) {
va_list argp;
Lock(&cs_log);
va_start(argp,pszFmt);
LogV(pszFmt,argp);
va_end(argp);
Unlock(&cs_log);
}
//Log}
static int volatile No_Loop=0;
static int volatile gLock = 0;
static int volatile gCounter = 0;
MYVOID testThread(void *pcn) {
int n,i;
n=(int)pcn;
while (1) {
if (No_Loop==2) {
for (i = 0; i < 1000000; ) {
//以下代码无效
// if ( 0 == gLock ) {
// gLock = n;
// if( gLock == n ) {
// ++i;
// ++gCounter;
// //if (i%100000==0) Log("%d %d\n",n,gCounter);
// gLock = 0;
// //if (i%100000==0) sleep_ms(10*(rand()%50));
// continue;
// } else sleep_ms(100);
// } else sleep_ms(100);
//应改为以下代码
InterlockedCompareExchange((void **)&gLock, (void *)n, 0);
if( gLock == n ) {
++i;
++gCounter;
if (i%100000==0) Log("%d %d\n",n,gCounter);
InterlockedExchange((long *)&gLock, 0);
if (i%100000==0) sleep_ms(10*(rand()%50));
continue;
} else sleep_ms(100);
}
sleep_ms(1000);
No_Loop=1;
}
}
}
int main(int argc,char * argv[]) {
int i;
srand(time(NULL));
#ifdef WIN32
InitializeCriticalSection(&cs_log);
#else
pthread_mutex_init(&cs_log,NULL);
pthread_t threads[4];
// void *thread_result;
int threadsN;
int rc;
#endif
Log("=========BEGIN==================\n");
#ifdef WIN32
_beginthread((void(__cdecl *)(void *))testThread,0,(void *)1);
_beginthread((void(__cdecl *)(void *))testThread,0,(void *)2);
_beginthread((void(__cdecl *)(void *))testThread,0,(void *)3);
_beginthread((void(__cdecl *)(void *))testThread,0,(void *)4);
#else
threadsN=0;
rc=pthread_create(&(threads[threadsN++]),NULL,testThread,(void *)1);if (rc) Log("%d=pthread_create %d error!\n",rc,threadsN-1);
rc=pthread_create(&(threads[threadsN++]),NULL,testThread,(void *)2);if (rc) Log("%d=pthread_create %d error!\n",rc,threadsN-1);
rc=pthread_create(&(threads[threadsN++]),NULL,testThread,(void *)3);if (rc) Log("%d=pthread_create %d error!\n",rc,threadsN-1);
rc=pthread_create(&(threads[threadsN++]),NULL,testThread,(void *)4);if (rc) Log("%d=pthread_create %d error!\n",rc,threadsN-1);
#endif
sleep_ms(1000);
No_Loop=2;
i=0;
while (1) {
sleep_ms(1000);
if (No_Loop==1) break;//
}
sleep_ms(1000);
Log("Result: %d\n",gCounter);
Log("=========END====================\n");
#ifdef WIN32
DeleteCriticalSection(&cs_log);
#else
pthread_mutex_destroy(&cs_log);
#endif
return 0;
}
4
那个是在 单核 单CPU 时代的问题, 到了 多核/多CPU的时候, 即使 ++, — , += -= 啥的都用单条指令实现( incl, decl, xadd …) 一样是错误的, 这条指令只是修改了当前核心 L1 Cache 中的内容, 在高速缓存刷新周期内, 其他 CPU , 其他核心根本就不能发现变量的值已经发生了改变 …
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