我最近一直致力于一个需要与服务器建立FTP连接并从中下载/上传文件的应用程序。出于性能原因，我想一次下载多个文件。出于这个原因，我尝试使用Wininet函数以及InternetOpen标志以及INTERNET_FLAG_ASYNC函数在InternetSetStatusCallback API上实现异步操作。以下是我的代码示例，我希望以递归方式列出远程服务器主目录中的所有文件：
/*Global variables*/
HANDLE MayContinue=0;
DWORD LatestResult=1;
/*Prototypes*/
void CALLBACK CallbackFunction(HINTERNET,DWORD_PTR,DWORD,LPVOID,DWORD);
//Iteration function called by main()
void FTPIterate()
{
WIN32_FIND_DATAA data;
HINTERNET Internet;
INTERNET_STATUS_CALLBACK call;
HINTERNET h_file;
MayContinue = ::CreateEvent (NULL, FALSE, FALSE, NULL);
iconnect=InternetOpen(NULL,INTERNET_OPEN_TYPE_PROXY,proxy_url,NULL,INTERNET_FLAG_ASYNC);
call=InternetSetStatusCallback(iconnect,(INTERNET_STATUS_CALLBACK)CallbackFunction);
while(f[FLAG_FTP_ITERATE])
{
MayContinue = ::CreateEvent(NULL,FALSE,FALSE,NULL);
InternetConnect(iconnect,ftp_url,INTERNET_DEFAULT_FTP_PORT,ftp_user,ftp_pass,INTERNET_SERVICE_FTP,NULL,LatestResult);
WaitForSingleObject (MayContinue, INFINITE);
server=(HINTERNET)LatestResult;
printf("Server handle: %i\n",(int)server);
printf("Server Error: %i\n",GetLastError());
SetLastError(0);
MayContinue = ::CreateEvent(NULL,FALSE,FALSE,NULL);
FtpFindFirstFile(server,ftp_base,&data,INTERNET_FLAG_NO_CACHE_WRITE,LatestResult);
WaitForSingleObject(MayContinue,INFINITE);
h_file=(HINTERNET)LatestResult;
//do stuff
printf("FindFirstFile handle: %i\n",(int)h_File);
while((MayContinue = ::CreateEvent(NULL,FALSE,FALSE,NULL)) && InternetFindNextFileA(h_file,&data))
{
WaitForSingleObject(MayContinue,INFINITE);
//do stuff
}
printf("FindNextFile Error: %i\n",GetLastError()); //loop is never executed
InternetCloseHandle(server);
}
}
void CALLBACK CallbackFunction(HINTERNET hInternet,DWORD_PTR dwContext,DWORD dwInternetStatus,LPVOID lpvStatusInformation,DWORD dwStatusInformationLength)
{
if (dwInternetStatus == INTERNET_STATUS_REQUEST_COMPLETE)
{
LatestResult = ((LPINTERNET_ASYNC_RESULT)lpvStatusInformation)->dwResult;
SetEvent (MayContinue);
}
}
我的代码基于Stack Overflow上的this帖子。当我运行它时，我首先在调用InternetConnect后发现错误，即ERROR_IO_PENDING。根据WinAPI参考，这意味着仍有一些操作正在执行。调用WaitForSingleObject不应该阻止这种情况发生吗？ （实际上，HINTERNET返回的InternetConnect句柄似乎是有效的。
当我调用FtpFindFirstFile函数时，它正确地检索了第一个文件，但是当我使用它在InternetFindNextFile函数中返回的HINTERNET句柄（它似乎也是有效的）时，它失败并出现错误{ {1}}。
编辑：在使用Remy的代码时，我会解决这些错误：
INVALID_HANDLE_VALUE有人可以帮我找到错误吗？ 提前谢谢。
答案 0 :(得分：2)
ERROR_IO_PENDING错误来自InternetOpen()本身。由于WaitForSingleObject()成功，因此不会覆盖GetLastError()（它只会出错，就像大多数API那样），因此错误将从InternetOpen()的结果中继承。这是不使用GetLastError()的正确方法。假设所有API都覆盖GetLastError()（如果记录为完全使用GetLastError()），并确保仅在API失败时才将其称为 （除非记录为在使用期间使用）成功条件）。
您的代码正在执行的是 NOT 异步！您正在发出异步API调用，但您正在等待其结果，这会破坏目的。您的代码是同步执行的，就像您省略INTERNAL_FLAG_ASYNC标志和WaitForSingleObject()调用一样（更不用说您通过不必要地调用CreateEvent()来泄漏事件资源），例如：< / p>
void LogError(const char *Op)
{
DWORD err = GetLastError();
if (err == ERROR_INTERNET_EXTENDED_ERROR)
{
LPSTR szBuffer;
DWORD dwLength = 0;
InternetGetLastResponseInfoA(&err, NULL, &dwLength);
if (GetLastError() != INSUFFICIENT_BUFFER)
{
printf("%s. Unknown Inet Error. OS Error: %u", Op, GetLastError());
return;
}
szBuffer = new char[dwLength+1];
InternetGetLastResponseInfoA(&err, szBuffer, &dwLength);
szBuffer[dwLength] = 0;
printf("%s. Inet Error: %u %s", Op, err, szBuffer);
delete[] szBuffer;
}
else
{
LPSTR lpBuffer = NULL;
DWORD dwLen = FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_ARGUMENT_ARRAY | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL, err, 0, (LPSTR)&lpBuffer, 0, NULL);
if (lpBuffer)
{
printf("%s. OS Error: %u %s", Op, err, lpBuffer);
LocalFree(lpBuffer);
}
else
printf("%s. OS Error: %u", Op, err);
}
printf("\n");
}
void FTPIterate()
{
WIN32_FIND_DATAA data;
HINTERNET hConnect;
HINTERNET hServer;
HINTERNET hFile;
hConnect = InternetOpen(NULL, INTERNET_OPEN_TYPE_PROXY, proxy_url, NULL, 0);
if (hConnect == NULL)
{
LogError("Unable to Open Internet");
return;
}
printf("Connect handle: %p\n", hConnect);
while (f[FLAG_FTP_ITERATE])
{
printf("Connecting to Server\n");
hServer = InternetConnect(hConnect, ftp_url, INTERNET_DEFAULT_FTP_PORT, ftp_user, ftp_pass, INTERNET_SERVICE_FTP, NULL, 0);
if (hServer == NULL)
{
LogError("Unable to connect to Server");
continue;
}
printf("Connected to Server. Server handle: %p\n", hServer);
printf("Finding first file\n");
hFile = FtpFindFirstFileA(hServer, ftp_base, &data, INTERNET_FLAG_NO_CACHE_WRITE, 0);
if (hFile == NULL)
{
if (GetLastError() == ERROR_NO_MORE_FILES)
printf("No files were found\n");
else
LogError("Unable to find first file");
}
else
{
printf("Find handle: %p\n", hFile);
do
{
//do stuff
printf("Finding next file\n");
if (!InternetFindNextFileA(hFile, &data))
{
if (GetLastError() == ERROR_NO_MORE_FILES)
printf("No more files were found\n");
else
LogError("Unable to find next file")
break;
}
}
while (true);
InternetCloseHandle(hFile);
}
InternetCloseHandle(hServer);
}
InternetCloseHandle(hConnect);
}
为了使这段代码异步运行，摆脱所有等待并实现一个状态机，你的回调根据需要进行，例如：
enum FTPState {ftpConnect, ftpWaitingForConnect, ftpFindFirstFile, ftpWaitingForFirstFind, ftpFindNextFile, ftpWaitingForNextFind, ftpProcessFile, ftpDisconnect};
struct REQ_CONTEXT
{
FTPState State;
WIN32_FIND_DATAA data;
HINTERNET hConnect;
HINTERNET hServer;
HINTERNET hFile;
HANDLE hDoneEvent;
};
void LogError(const char *Op, DWORD err)
{
if (err == ERROR_INTERNET_EXTENDED_ERROR)
{
LPSTR szBuffer;
DWORD dwLength = 0;
InternetGetLastResponseInfoA(&err, NULL, &dwLength);
if (GetLastError() != ERROR_INSUFFICIENT_BUFFER)
{
printf("%s. Unknown Inet Error. OS Error: %u", Op, GetLastError());
return;
}
szBuffer = new char[dwLength+1];
InternetGetLastResponseInfoA(&err, szBuffer, &dwLength);
szBuffer[dwLength] = 0;
printf("%s. Inet Error: %u %s", Op, err, szBuffer);
delete[] szBuffer;
}
else
{
LPSTR lpBuffer = NULL;
DWORD dwLen = FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_ARGUMENT_ARRAY | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL, err, 0, (LPSTR)&lpBuffer, 0, NULL);
if (lpBuffer)
{
printf("%s. OS Error: %u %s", Op, err, lpBuffer);
LocalFree(lpBuffer);
}
else
printf("%s. OS Error: %u", Op, err);
}
printf("\n");
}
void LogError(const char *Op)
{
LogError(Op, GetLastError());
}
void DoNextStep(REQ_CONTEXT *ctx)
{
do
{
if ((ctx->State == ftpConnect) && (!f[FLAG_FTP_ITERATE]))
{
printf("Done!\n");
SetEvent(ctx->hDoneEvent);
return;
}
switch (ctx->State)
{
case ftpConnect:
{
printf("Connecting to Server\n");
HINTERNET hServer = InternetConnect(ctx->hConnect, ftp_url, INTERNET_DEFAULT_FTP_PORT, ftp_user, ftp_pass, INTERNET_SERVICE_FTP, NULL, (DWORD_PTR)ctx);
if (hServer != NULL)
{
if (ctx->hServer == NULL)
{
ctx->hServer = hServer;
printf("Server handle: %p\n", ctx->hServer);
}
printf("Connected to Server\n");
ctx->State = ftpFindFirstFile;
}
else if (GetLastError() == ERROR_IO_PENDING)
{
if (ctx->hServer == NULL)
printf("Waiting for Server handle\n");
printf("Waiting for Server connect to complete\n");
ctx->State = ftpWaitingForConnect;
}
else
LogError("Unable to connect to Server");
break;
}
case ftpWaitingForConnect:
return;
case ftpFindFirstFile:
{
printf("Finding first file\n");
HINTERNET hFile = FtpFindFirstFileA(ctx->hServer, ftp_base, &ctx->data, INTERNET_FLAG_NO_CACHE_WRITE, (DWORD_PTR)ctx);
if (hFile != NULL)
{
if (ctx->hFile == NULL)
{
ctx->hFile = hFile;
printf("Find handle: %p\n", ctx->hFile);
}
ctx->State = ftpProcessFile;
}
else if (GetLastError() == ERROR_IO_PENDING)
{
if (ctx->hFile == NULL)
printf("Waiting for Find handle\n");
printf("Waiting for Find to complete\n");
ctx->State = ftpWaitingForFirstFind;
}
else
{
if (GetLastError() == ERROR_NO_MORE_FILES)
printf("No files were found\n");
else
LogError("Unable to find first file");
ctx->State = ftpDisconnect;
}
break;
}
case ftpWaitingForFirstFind:
case ftpWaitingForNextFind:
return;
case ftpProcessFile:
{
//do stuff
printf("Finding next file\n");
if (!InternetFindNextFileA(ctx->hFile, &ctx->data))
{
if (GetLastError() == ERROR_IO_PENDING)
{
printf("Waiting for next file to complete\n");
ctx->State = ftpWaitingForNextFind;
}
else
{
if (GetLastError() == ERROR_NO_MORE_FILES)
printf("No more files were found\n");
else
LogError("Unable to find next file");
ctx->State = ftpDisconnect;
}
}
break;
}
case ftpDisconnect:
{
printf("Disconnecting\n");
if (ctx->hFile != NULL)
{
InternetCloseHandle(ctx->hFile);
ctx->hFile = NULL;
}
if (ctx->hServer != NULL)
{
InternetCloseHandle(ctx->hServer);
ctx->hServer = NULL;
}
ctx->State = ftpConnect;
break;
}
}
}
while (true);
}
void CALLBACK CallbackFunction(HINTERNET hInternet, DWORD_PTR dwContext, DWORD dwInternetStatus, LPVOID lpvStatusInformation, DWORD dwStatusInformationLength)
{
REQ_CONTEXT *ctx = (REQ_CONTEXT*) dwContext;
switch (dwInternetStatus)
{
case INTERNET_STATUS_HANDLE_CREATED:
{
LPINTERNET_ASYNC_RESULT Result = (LPINTERNET_ASYNC_RESULT) lpvStatusInformation;
switch (ctx->State)
{
case ftpConnect:
case ftpWaitingForConnect:
ctx->hServer = (HINTERNET) Result->dwResult;
printf("Server handle: %p\n", ctx->hServer);
break;
case ftpFindFirstFile:
case ftpWaitingForFirstFind:
ctx->hFile = (HINTERNET) Result->dwResult;
printf("Find handle: %p\n", ctx->hFile);
break;
}
break;
}
case INTERNET_STATUS_REQUEST_COMPLETE:
{
LPINTERNET_ASYNC_RESULT Result = (LPINTERNET_ASYNC_RESULT) lpvStatusInformation;
switch (ctx->State)
{
case ftpWaitingForConnect:
{
if (!Result->dwResult)
{
LogError("Unable to connect to Server", Result->dwError);
ctx->State = ftpDisconnect;
}
else
{
printf("Connected to Server\n");
ctx->State = ftpFindFirstFile;
}
break;
}
case ftpWaitingForFirstFind:
case ftpWaitingForNextFind:
{
if (!Result->dwResult)
{
if (Result->dwError == ERROR_NO_MORE_FILES)
printf("No %sfiles were found\n", (ctx->State == ftpWaitingForNextFind) ? "more " : "");
else if (ctx->State == ftpWaitingForFirstFind)
LogError("Unable to find first file", Result->dwError);
else
LogError("Unable to find next file", Result->dwError);
ctx->State = ftpDisconnect;
}
else
ctx->State = ftpProcessFile;
break;
}
}
DoNextStep(ctx);
break;
}
}
}
void FTPIterate()
{
REQ_CONTEXT ctx = {0};
ctx.State = ftpConnect;
ctx.hDoneEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if (ctx.hDoneEvent == NULL)
{
LogError("Unable to Create Done Event");
return;
}
ctx.hConnect = InternetOpen(NULL, INTERNET_OPEN_TYPE_PROXY, proxy_url, NULL, INTERNET_FLAG_ASYNC);
if (ctx.hConnect == NULL)
{
LogError("Unable to Open Internet");
CloseHandle(ctx.hDoneEvent);
return;
}
printf("Connect handle: %p\n", ctx.hConnect);
InternetSetStatusCallback(ctx.hConnect, &CallbackFunction);
DoNextStep(&ctx);
WaitForSingleObject(ctx.hDoneEvent, INFINITE);
InternetCloseHandle(ctx.hConnect);
CloseHandle(ctx.hDoneEvent);
}