winapi 中的 MailSlot 问题，ReadFile 不从 mailslot 读取值

Question

我正在尝试设置一个邮槽，我可以写入我的值并从中读取。就我而言，我有两个相互配合的进程。第一个进程从用户那里获取信息并将信息发送到另一个进程以计算结果。第二个进程应该从邮槽读取数据，但是当调用函数 ReadFile() 时进程被冻结并且我什么也没得到。 我试图找到有关邮槽的一些信息，但没有得到我想要的。谁知道这个邮槽并且可以帮助我？

第一个过程：

int main()
{
    HANDLE m = CreateMutex(NULL, FALSE, L"MyMutex");
    if (m == 0)
        return -1;

    STARTUPINFO si;
    PROCESS_INFORMATION pi;
    ZeroMemory(&si, sizeof(si));
    si.cb = sizeof(si);
    ZeroMemory(&pi, sizeof(pi));

    if (CreateProcess(L"c:\\Users\\user\\Desktop\\Lab3\\Procces1\\Debug\\Procces1.exe", NULL,
        NULL, NULL, FALSE, NULL, NULL, NULL, &si, &pi) == TRUE) {

        bool flagEnd = true;
        double a,b,c;

        HANDLE file = CreateFile(L"\\\\.\\mailslot\\c:\\Users\\user\\Desktop\\Lab3\\mail", GENERIC_WRITE|GENERIC_READ,
            FILE_SHARE_READ|FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
        if (file == 0)
            return -1;

        while (true) {
            WaitForSingleObject(m, INFINITE);
            cout << "Please enter A and B values\n";
            cout << "A = ";
            cin >> a;
            cout << "B = ";
            cin >> b;
            WriteFile(file, &a, sizeof(double), NULL, NULL);
            WriteFile(file, &b, sizeof(double), NULL, NULL);
            ReleaseMutex(m);
        }

        CloseHandle(pi.hProcess);
        CloseHandle(pi.hThread);
        CloseHandle(m);
    }
    else {
        cout << "Process has not been created\n";
    }
    return 0;
}

第二道工序：

int main()
{
    HANDLE mut = OpenMutex(MUTEX_ALL_ACCESS, TRUE, L"MyMutex");
    if (mut == 0)
        return -1;

    HANDLE mail = CreateMailslot(L"\\\\.\\mailslot\\c:\\Users\\user\\Desktop\\Lab3\\mail", 0, MAILSLOT_WAIT_FOREVER, NULL);
    if (mail == 0)
        return -1;

    double a, b, c;
    DWORD byte;
    while (true) {
        WaitForSingleObject(mut, INFINITE);
        cout << "Reading\n"; //this works
        ReadFile(mail, &a, sizeof(double), &byte, NULL);  
        ReadFile(mail, &b, sizeof(double), &byte, NULL);  
        cout << "I have read " << a << " " << b << endl;  //this message doesn't send
        ReleaseMutex(mut);                                //and therefore releasemutex wont be called
    }
    return 0;
}

Answer 1

注意：使用错误的值进行错误检查。使用if (file == INVALID_HANDLE_VALUE) 而不是if (file == 0)。 （如果你还没有做，建议为所有函数调用添加错误检查。ReadFile，WriteFile等）

并删除Procces1.exe（我在这里没有看到它的用法）部分。先启动第二个进程（创建并读取Mailslot进程），再启动第一个进程（写入mailslot进程），运行，得到如下结果：

修改后的代码：

第一个过程：

int main()
{
    STARTUPINFO si;
    PROCESS_INFORMATION pi;
    ZeroMemory(&si, sizeof(si));
    si.cb = sizeof(si);
    ZeroMemory(&pi, sizeof(pi));

    HANDLE m = CreateMutex(NULL, FALSE, L"MyMutex");
    if (m == 0)
        return -1;

    bool flagEnd = true;
    double a, b, c;

    HANDLE file = CreateFile(L"\\\\.\\mailslot\\c:\\Users\\user\\Desktop\\Lab3\\mail", GENERIC_WRITE | GENERIC_READ,
        FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
    if (file == INVALID_HANDLE_VALUE)
        return -1;

    while (true) {
        cout << "Please enter A and B values\n";
        cout << "A = ";
        cin >> a;
        cout << "B = ";
        cin >> b;
        WriteFile(file, &a, sizeof(double), NULL, NULL);
        WriteFile(file, &b, sizeof(double), NULL, NULL);
        ReleaseMutex(m);
    }

    CloseHandle(pi.hProcess);
    CloseHandle(pi.hThread);
    CloseHandle(m);

    return 0;
}

第二道工序：

int main()
{
    HANDLE mail = CreateMailslot(L"\\\\.\\mailslot\\c:\\Users\\user\\Desktop\\Lab3\\mail", 0, MAILSLOT_WAIT_FOREVER, NULL);
    if (mail == INVALID_HANDLE_VALUE)
        return -1;

    HANDLE mut = NULL;

    cout << "Waiting for another process write something to mailslot\n"; //this works
    while (NULL == mut)
    {
        mut = OpenMutex(MUTEX_ALL_ACCESS, TRUE, L"MyMutex");
        Sleep(10);
        cout << ".";
    }
    cout << "\n";

    double a, b, c;
    DWORD byte;
    while (true) {
        WaitForSingleObject(mut, INFINITE);
        cout << "Reading\n"; //this works
        ReadFile(mail, &a, sizeof(double), &byte, NULL);
        ReadFile(mail, &b, sizeof(double), &byte, NULL);
        cout << "I have read " << a << " " << b << endl;  //this message doesn't send
    }
    return 0;
}

更多参考：“Reading from a Mailslot”和“Writing to a Mailslot”的官方示例。