传输文件一次又一次地发送相同的字节

TransmitFile sending the same bytes again and again

本文关键字：字节 文件 一次又一次 传输      更新时间：2023-10-16

我正在使用TransmitFile函数，它正在做一些与我预期不同的事情。

最小示例：

#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);
}

用MinGW建造：g++ main.cpp -lws2_32 -lmswsock

文件alphabet.txt必须存在并包含：

abcdefghijklmnopqrstuvwxyz

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

abcdefdefdefdefdef

我期望：

abcdefghijklmnopqr

使用您的确切代码，我能够重现该问题。 在每次循环迭代中使用SetFilePointer()，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，但在后续发送时永远不会超过偏移量 3，因此相同的 3 个字节会一遍又一遍地重新传输。 为什么它以这种方式工作，我不知道。

我能够通过将OVERLAPPED结构传递给TransmitFile()(指定FILE_FLAG_OVERLAPPEDCreateFile()与否，没有区别(来TransmitFile()发送正确的文件块(无论传递给TransmitFile()的标志如何(，根据文档：

可以使用lpOverlapped参数指定文件中的 64 位偏移量，通过设置OVERLAPPED结构的Offset和OffsetHigh成员，从该偏移量开始文件数据传输。

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，我不是使用OVERLAPPED，我还要在每次发送后简单地手动推进文件位置，再次根据文档，使代码正常工作：

如果lpOverlapped是 NULL 指针，则数据传输始终从文件中的当前字节偏移量开始。

#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;
}
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