c++ - TransmitFile 一次又一次地发送相同的字节

Question

我正在使用 TransmitFile功能，它正在做一些与我预期不同的事情。

最小的例子:

#include <mswsock.h>

int main(void)
{
    WSADATA wsa;
    SOCKET ListenSocket, ClientSocket;
    struct sockaddr_in server;

    if (WSAStartup(MAKEWORD(2,2),&wsa) != 0)
        return 1;

    if ((ListenSocket = socket(AF_INET , SOCK_STREAM , 0 )) == INVALID_SOCKET)
        return 2;

    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(1234);

    if (bind(ListenSocket, (struct sockaddr *)&server, sizeof(server)) == SOCKET_ERROR)
        return 3;

    listen(ListenSocket, 1);

    if ((ClientSocket = accept(ListenSocket, NULL, NULL)) == INVALID_SOCKET)
        return 4;

    HANDLE hFile = CreateFile("alphabet.txt", GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

    do {
        if (!TransmitFile(ClientSocket, hFile, 3, 0, NULL, NULL, 0))
            return 5;
        Sleep(1000);
    } while (1);
}

使用 MinGW 构建:g++ main.cpp -lws2_32 -lmswsock

alphabet.txt 文件必须存在并包含:

abcdefghijklmnopqrstuvwxyz

当我运行程序并连接(例如使用 PuTTY 或 netcat)到端口 1234 时，我得到:

abcdefdefdefdefdef

我希望:

abcdefghijklmnopqr

Answer 1

使用您的确切代码，我能够重现该问题。使用 SetFilePointer()在每次循环迭代中，我可以看到文件位置在第一次发送后按预期前进到偏移量 3，但在后续发送时从未超过偏移量 3，因此一遍又一遍地重新传输相同的 3 个字节。为什么会这样，我不知道。

我能够得到 TransmitFile()通过传递 TransmitFile() 来发送正确的文件 block (无论传递给 OVERLAPPED 的标志如何)结构为 TransmitFile() (在 FILE_FLAG_OVERLAPPED 上指定 CreateFile() 与否，没有区别)，根据 documentation :

You can use the lpOverlapped parameter to specify a 64-bit offset within the file at which to start the file data transfer by setting the Offset and OffsetHigh member of the OVERLAPPED structure.

尝试这样的事情:

#include <mswsock.h>

int main(void)
{
    WSADATA wsa;
    SOCKET ListenSocket, ClientSocket;
    struct sockaddr_in server = {};
    HANDLE hFile;

    if (WSAStartup(MAKEWORD(2,2), &wsa) != 0)
        return 1;

    if ((ListenSocket = socket(AF_INET, SOCK_STREAM, 0)) == INVALID_SOCKET)
        return 2;

    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(1234);

    if (bind(ListenSocket, (struct sockaddr *)&server, sizeof(server)) == SOCKET_ERROR)
        return 3;

    if (listen(ListenSocket, 1) == SOCKET_ERROR)
        return 4;

    if ((ClientSocket = accept(ListenSocket, NULL, NULL)) == INVALID_SOCKET)
        return 5;

    if ((hFile = CreateFileA("alphabet.txt", GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL)) == INVALID_HANDLE_VALUE)
        return 6;

    ULARGE_INTEGER ul;
    ul.LowPart = GetFileSize(hFile, &ul.HighPart);

    if ((ul.LowPart == INVALID_FILE_SIZE) && (GetLastError() != 0))
        return 7;

    OVERLAPPED ov = {};
    ov.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
    if (!ov.hEvent)
        return 8;

    unsigned __int64 uiPos = 0;
    unsigned __int64 uiRemaining = ul.QuadPart;

    while (uiRemaining > 0)
    {
        ul.QuadPart = uiPos;

        ov.Offset = ul.LowPart;
        ov.OffsetHigh = ul.HighPart;

        DWORD dwNumToSend = (uiRemaining >= 3) ? 3 : (DWORD)uiRemaining;

        if (!TransmitFile(ClientSocket, hFile, dwNumToSend, 0, &ov, NULL, 0))
        {
            if ((GetLastError() != ERROR_IO_PENDING) && (WSAGetLastError() != WSA_IO_PENDING))
                break;

            WaitForSingleObject(ov.hEvent, INFINITE);
        }

        uiPos += dwNumToSend;
        uiRemaining -= dwNumToSend;

        Sleep(1000);
    }

    closesocket(ClientSocket);
    CloseHandle(ov.hEvent);
    CloseHandle(hFile);

    return 0;
}

或者，不使用 OVERLAPPED ，我也是为了让代码正常工作，只需在每次发送后手动推进文件位置，再次根据文档:

If lpOverlapped is a NULL pointer, the transmission of data always starts at the current byte offset in the file.

#include <mswsock.h>

int main(void)
{
    WSADATA wsa;
    SOCKET ListenSocket, ClientSocket;
    struct sockaddr_in server = {};
    HANDLE hFile;

    if (WSAStartup(MAKEWORD(2,2), &wsa) != 0)
        return 1;

    if ((ListenSocket = socket(AF_INET, SOCK_STREAM, 0)) == INVALID_SOCKET)
        return 2;

    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(1234);

    if (bind(ListenSocket, (struct sockaddr *)&server, sizeof(server)) == SOCKET_ERROR)
        return 3;

    if (listen(ListenSocket, 1) == SOCKET_ERROR)
        return 4;

    if ((ClientSocket = accept(ListenSocket, NULL, NULL)) == INVALID_SOCKET)
        return 5;

    if ((hFile = CreateFileA("alphabet.txt", GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL)) == INVALID_HANDLE_VALUE)
        return 6;

    ULARGE_INTEGER ul;
    ul.LowPart = GetFileSize(hFile, &ul.HighPart);

    if ((ul.LowPart == INVALID_FILE_SIZE) && (GetLastError() != 0))
        return 7;

    unsigned __int64 uiPos = 0;
    unsigned __int64 uiRemaining = ul.QuadPart;

    while (uiRemaining > 0)
    {
        ul.QuadPart = uiPos;
        if ((SetFilePointer(hFile, (LONG)ul.LowPart, (PLONG)&ul.HighPart, FILE_BEGIN) == INVALID_SET_FILE_POINTER) && (GetLastError() != 0))
            break;

        DWORD dwNumToSend = (uiRemaining >= 3) ? 3 : (DWORD)uiRemaining;

        if (!TransmitFile(ClientSocket, hFile, dwNumToSend, 0, NULL, NULL, 0))
            break;

        uiPos += dwNumToSend;
        uiRemaining -= dwNumToSend;

        Sleep(1000);
    }

    closesocket(ClientSocket);
    CloseHandle(hFile);

    return 0;
}