從 Mailslot 讀取

建立 mailslot 的程式可以使用 ReadFile 函式呼叫中的 mailslot 控制碼，從中讀取郵件。 下列範例會呼叫 GetMailslotInfo 函式，以判斷 mailslot 中是否有訊息。 如果訊息正在等候，則會隨著剩餘要讀取的訊息數目一起顯示。

Mailslot 會存在，直到所有開啟的伺服器控制碼呼叫 CloseHandle 函式，或直到擁有 mailslot 控制碼的所有伺服器進程結束為止。 在這兩種情況下，都會從 mailslot 刪除任何未讀取的郵件、關閉 mailslot 的所有用戶端控制碼，並從記憶體中刪除 mailslot 本身。

#include <windows.h>
#include <tchar.h>
#include <stdio.h>
#include <strsafe.h>

HANDLE hSlot;
LPCTSTR SlotName = TEXT("\\\\.\\mailslot\\sample_mailslot");

BOOL ReadSlot() 
{ 
    DWORD cbMessage, cMessage, cbRead; 
    BOOL fResult; 
    LPTSTR lpszBuffer; 
    TCHAR achID[80]; 
    DWORD cAllMessages; 
    HANDLE hEvent;
    OVERLAPPED ov;
 
    cbMessage = cMessage = cbRead = 0; 

    hEvent = CreateEvent(NULL, FALSE, FALSE, TEXT("ExampleSlot"));
    if( NULL == hEvent )
        return FALSE;
    ov.Offset = 0;
    ov.OffsetHigh = 0;
    ov.hEvent = hEvent;
 
    fResult = GetMailslotInfo( hSlot, // mailslot handle 
        (LPDWORD) NULL,               // no maximum message size 
        &cbMessage,                   // size of next message 
        &cMessage,                    // number of messages 
        (LPDWORD) NULL);              // no read time-out 
 
    if (!fResult) 
    { 
        printf("GetMailslotInfo failed with %d.\n", GetLastError()); 
        return FALSE; 
    } 
 
    if (cbMessage == MAILSLOT_NO_MESSAGE) 
    { 
        printf("Waiting for a message...\n"); 
        return TRUE; 
    } 
 
    cAllMessages = cMessage; 
 
    while (cMessage != 0)  // retrieve all messages
    { 
        // Create a message-number string. 
 
        StringCchPrintf((LPTSTR) achID, 
            80,
            TEXT("\nMessage #%d of %d\n"), 
            cAllMessages - cMessage + 1, 
            cAllMessages); 

        // Allocate memory for the message. 
 
        lpszBuffer = (LPTSTR) GlobalAlloc(GPTR, 
            lstrlen((LPTSTR) achID)*sizeof(TCHAR) + cbMessage); 
        if( NULL == lpszBuffer )
            return FALSE;
        lpszBuffer[0] = '\0'; 
 
        fResult = ReadFile(hSlot, 
            lpszBuffer, 
            cbMessage, 
            &cbRead, 
            &ov); 
 
        if (!fResult) 
        { 
            printf("ReadFile failed with %d.\n", GetLastError()); 
            GlobalFree((HGLOBAL) lpszBuffer); 
            return FALSE; 
        } 
 
        // Concatenate the message and the message-number string. 
 
        StringCbCat(lpszBuffer, 
                    lstrlen((LPTSTR) achID)*sizeof(TCHAR)+cbMessage, 
                    (LPTSTR) achID); 
 
        // Display the message. 
 
        _tprintf(TEXT("Contents of the mailslot: %s\n"), lpszBuffer); 
 
        GlobalFree((HGLOBAL) lpszBuffer); 
 
        fResult = GetMailslotInfo(hSlot,  // mailslot handle 
            (LPDWORD) NULL,               // no maximum message size 
            &cbMessage,                   // size of next message 
            &cMessage,                    // number of messages 
            (LPDWORD) NULL);              // no read time-out 
 
        if (!fResult) 
        { 
            printf("GetMailslotInfo failed (%d)\n", GetLastError());
            return FALSE; 
        } 
    } 
    CloseHandle(hEvent);
    return TRUE; 
}

BOOL WINAPI MakeSlot(LPCTSTR lpszSlotName) 
{ 
    hSlot = CreateMailslot(lpszSlotName, 
        0,                             // no maximum message size 
        MAILSLOT_WAIT_FOREVER,         // no time-out for operations 
        (LPSECURITY_ATTRIBUTES) NULL); // default security
 
    if (hSlot == INVALID_HANDLE_VALUE) 
    { 
        printf("CreateMailslot failed with %d\n", GetLastError());
        return FALSE; 
    } 
    return TRUE; 
}

void main()
{
   MakeSlot(SlotName);

   while(TRUE)
   {
      ReadSlot();
      Sleep(3000);
   }
}

以下是使用 寫入至 Mailslot中顯示的 mailslot 用戶端來執行此範例時所顯示的輸出。

Waiting for a message...
Waiting for a message...
Contents of the mailslot: Message one for mailslot.
Message #1 of 2

Contents of the mailslot: Message two for mailslot.
Message #2 of 2

Waiting for a message...
Contents of the mailslot: Message three for mailslot.
Message #1 of 1

Waiting for a message...
Waiting for a message...
Waiting for a message...
^C