TransmitFile sending the same bytes again and again

Question

I'm using the TransmitFile function and it's doing something different than I would expect.

Minimal example:

#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);
}

Built with MinGW: g++ main.cpp -lws2_32 -lmswsock

A file alphabet.txt must exist and contain:

abcdefghijklmnopqrstuvwxyz

When I run the program and connect (e.g. with PuTTY or netcat) to port 1234, I get:

abcdefdefdefdefdef

I would expect:

abcdefghijklmnopqr

Answer 1

Using your exact code, I was able to reproduce the issue. Using SetFilePointer() on each loop iteration, I can see that the file position advances to offset 3 after the first send as expected, but never advances beyond offset 3 on subsequent sends, so the same 3 bytes gets re-transmitted over and over. Why it works this way, I have no idea.

I was able to get TransmitFile() to send the correct file blocks (regardless of the flags passed to TransmitFile()) by passing an OVERLAPPED struct to TransmitFile() (specifying FILE_FLAG_OVERLAPPED on CreateFile() or not, it makes no difference), per the 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.

Try something like this:

#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;
}

Alternatively, instead of using OVERLAPPED, I was was also to make the code work correctly by simply advancing the file position manually after each send, again per the documentation:

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;
}