Windows 平台下的同步机制（4）– 信号量(Semaphore)

技术2022-07-04 134

Semaphore是旗语的意思，在Windows中，Semaphore对象用来控制对资源的并发访问数。Semaphore对象具有一个计数值，当值大于0时，Semaphore被置信号，当计数值等于0时，Semaphore被清除信号。每次针对Semaphore的wait functions返回时，计数值被减1，调用ReleaseSemaphore可以将计数值增加 lReleaseCount 参数值指定的值。

CreateSemaphore函数用于创建一个Semaphore

HANDLE CreateSemaphore( LPSECURITY_ATTRIBUTES lpSemaphoreAttributes, LONG lInitialCount, LONG lMaximumCount, LPCTSTR lpName );

lpSemaphoreAttributes为安全属性， lInitialCount为Semaphore的初始值， lMaximumCount为最大值， lpName为Semaphore对象的名字，NULL表示创建匿名Semaphore

此外还可以调用OpenSemaphore来打开已经创建的非匿名Semaphore

HANDLE OpenSemaphore( DWORD dwDesiredAccess, BOOL bInheritHandle, LPCTSTR lpName );

调用ReleaseSemaphore增加Semaphore计算值

BOOL ReleaseSemaphore( HANDLE hSemaphore, LONG lReleaseCount, LPLONG lpPreviousCount );

lpReleaseCount参数表示要增加的数值， lpPreviousCount参数用于返回之前的计算值，如果不需要可以设置为NULL

比如我们要控制到服务器的连接数不超过10个，可以创建一个Semaphore，初值为10，每当要连接到服务器时，使用WaitForSingleObject请求Semaphore，当成功返回后再尝试连接到服务器，当连接失败或连接使用完后释放时，调用ReleaseSemaphore增加Semaphore计数值。

看个例子，popo现在好像在本机只能运行三个实例，mutex可以让程序只是运行一个实例，下面我通过信号量机制让程序像popo一样运行三个实例。

#include "stdafx.h" #include #include using namespace std;

const int MAX_RUNNUM = 3; //最多运行实例个数 void PrintInfo() { char c; cout << "run program" << endl; cout << "input s to exit program!" << endl; while (1) { cin >> c; if (c == ‘s’) { break; } Sleep(10); } } int main(int argc, char* argv[]) { HANDLE hSe = CreateSemaphore(NULL, MAX_RUNNUM, MAX_RUNNUM, "semaphore_test"); DWORD ret = 0; if (hSe == NULL) { cout << "createsemaphore failed with code: " << GetLastError() << endl; return -1; } ret = WaitForSingleObject(hSe, 1000); if (ret == WAIT_TIMEOUT) { cout << "you have runned " << MAX_RUNNUM << " program!" << endl; ret = WaitForSingleObject(hSe, INFINITE); } PrintInfo(); ReleaseSemaphore(hSe, 1, NULL); CloseHandle(hSe); return 0; }

From MSDN:

Using Semaphore Objects

The following example uses a semaphore object to limit the number of threads that can perform a particular task. First, it uses the CreateSemaphore function to create the semaphore and to specify initial and maximum counts, then it uses the CreateThreadfunction to create the threads.

Before a thread attempts to perform the task, it uses the WaitForSingleObject function to determine whether the semaphore’s current count permits it to do so. The wait function’s time-out parameter is set to zero, so the function returns immediately if the semaphore is in the nonsignaled state. WaitForSingleObject decrements the semaphore’s count by one.

When a thread completes the task, it uses the ReleaseSemaphore function to increment the semaphore’s count, thus enabling another waiting thread to perform the task.

Copy

#include #include #define MAX_SEM_COUNT 10 #define THREADCOUNT 12 HANDLE ghSemaphore; DWORD WINAPI ThreadProc( LPVOID ); void main() { HANDLE aThread[THREADCOUNT]; DWORD ThreadID; int i; // Create a semaphore with initial and max counts of MAX_SEM_COUNT ghSemaphore = CreateSemaphore( NULL, // default security attributes MAX_SEM_COUNT, // initial count MAX_SEM_COUNT, // maximum count NULL); // unnamed semaphore if (ghSemaphore == NULL) { printf("CreateSemaphore error: %dn", GetLastError()); return; } // Create worker threads for( i=0; i < THREADCOUNT; i++ ) { aThread[i] = CreateThread( NULL, // default security attributes 0, // default stack size (LPTHREAD_START_ROUTINE) ThreadProc, NULL, // no thread function arguments 0, // default creation flags &ThreadID); // receive thread identifier if( aThread[i] == NULL ) { printf("CreateThread error: %dn", GetLastError()); return; } } // Wait for all threads to terminate WaitForMultipleObjects(THREADCOUNT, aThread, TRUE, INFINITE); // Close thread and semaphore handles for( i=0; i < THREADCOUNT; i++ ) CloseHandle(aThread[i]); CloseHandle(ghSemaphore); } DWORD WINAPI ThreadProc( LPVOID lpParam ) { DWORD dwWaitResult; BOOL bContinue=TRUE; while(bContinue) { // Try to enter the semaphore gate. dwWaitResult = WaitForSingleObject( ghSemaphore, // handle to semaphore 0L); // zero-second time-out interval switch (dwWaitResult) { // The semaphore object was signaled. case WAIT_OBJECT_0: // TODO: Perform task printf("Thread %d: wait succeededn", GetCurrentThreadId()); bContinue=FALSE; // Simulate thread spending time on task Sleep(5); // Release the semaphore when task is finished if (!ReleaseSemaphore( ghSemaphore, // handle to semaphore 1, // increase count by one NULL) ) // not interested in previous count { printf("ReleaseSemaphore error: %dn", GetLastError()); } break; // The semaphore was nonsignaled, so a time-out occurred. case WAIT_TIMEOUT: printf("Thread %d: wait timed outn", GetCurrentThreadId()); break; } } return TRUE; }

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Windows 平台下的同步机制 （4）– 信号量(Semaphore)

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Windows 平台下的同步机制（4）– 信号量(Semaphore)