Question

I’m a new guy on the website here, so forgive me if I do anything wrong in this post.

I’m working with WinINet and trying to download a binary file from the internet but, for some reason when I get to the point of actually downloading that file using InternetReadFile(), it returns reading nothing at all (0 bytes of information read). Running the Visual Studio 2012 debugger revealed that detail to me, because the HINTERNET handler that I feed into the API call definitely has data for sure. I just don’t understand what I’m doing wrong. Maybe you guys can help?

The basic gist of my program is that I’m downloading a binary file from the net, and saving it into a temporary file that’s in a temporary directory. After copying the contents into the temporary file, I transfer that temp file’s binary data contents into another local file (this time within a valid directory). Here’s what I have so far. Hopefully with the logic breakdown I provide, you guys will be able to follow it despite the length of code…

#include "httpfileretrieval.h"    // contains all handlers (hInstance, etc.)

bool downloadFile(const char* lpszServer, const char* lpszUrl, const char* destPath) 
{
    FILE *tempFile  = NULL;
    FILE *localFile = NULL;

    const int bufsize = 4096;
    DWORD tempDirBytes;
    DWORD dwSize = 4096;           // experiment - ignore the fact this is the same as bufsize
    DWORD dwRead = 0;

    char lpszDataBuffer[bufsize];
    lpszDataBuffer[bufsize] = '\0';

    char tempPath[MAX_PATH];
    char tempFileName[bufsize];  // will hold the FULL temp file path

    std::string srcPath;
    srcPath.append(lpszServer);
    srcPath.append(lpszUrl);    // http://www.domain.com/url into srcPath

    hInstance = InternetOpen("httpfret", 
                         INTERNET_OPEN_TYPE_PRECONFIG,
                         NULL,
                         NULL,
                         INTERNET_FLAG_ASYNC); // ASYNC Flag

    if (!hInstance)
    {
        DWORD errorNum = GetLastError();
        fprintf(stderr, "InternetOpen Failed! Windows Error %d\n", errorNum);
        return false;
    }

    // Setup callback function due to INTERNET_FLAG_ASYNC
    if (InternetSetStatusCallback(hInstance,(INTERNET_STATUS_CALLBACK)&Callback) 
    == INTERNET_INVALID_STATUS_CALLBACK)
    {
        DWORD errorNum = GetLastError();
        fprintf(stderr, "InternetSetStatusCallback Failed! Windows Error %d\n", errorNum);
        return false;
    }

    // First call that will actually complete asynchronously even though 
    // there is no network traffic
    hConnect = InternetConnect(hInstance, 
                           lpszServer, 
                           INTERNET_DEFAULT_HTTP_PORT,
                           NULL,
                           NULL,hg
                           INTERNET_SERVICE_HTTP,
                           0,
                           1); // Connection handle's Context
    if (!hConnect)
    {
        if (GetLastError() != ERROR_IO_PENDING)
        {
            DWORD errorNum = GetLastError();
            fprintf(stderr, "InternetConnect Failed! Windows Error %d\n", errorNum);
            InternetCloseHandle(hInstance);
            return false;
        }
        // Wait until we get the connection handle
        WaitForSingleObject(hConnectedEvent, INFINITE);
    }


    // Open the request
    hRequest = HttpOpenRequest(hConnect, "GET", lpszUrl, NULL, NULL, NULL,
                           INTERNET_FLAG_RELOAD | INTERNET_FLAG_NO_CACHE_WRITE,
                           2);  // Request handle's context 
    if (!hRequest)
    {
        if (GetLastError() != ERROR_IO_PENDING)
        {
            DWORD errorNum = GetLastError();
            fprintf(stderr, "HttpOpenRequest Failed! Windows Error %d\n", errorNum);
            InternetCloseHandle(hConnect);
            InternetCloseHandle(hInstance);h
            return false;
        }
        // Wait until we get the request handle
        WaitForSingleObject(hRequestOpenedEvent, INFINITE);
    }

    // Send the request
    if (!HttpSendRequest(hRequest, NULL, 0, NULL, 0))
    {
        if (GetLastError() != ERROR_IO_PENDING)
        {
            DWORD errorNum = GetLastError();
            fprintf(stderr, "HttpSendRequest Failed! Windows Error %d\n", errorNum);
            InternetCloseHandle(hRequest);
            InternetCloseHandle(hConnect);
            InternetCloseHandle(hInstance);
            return false;
        }
    }

    if (bVerbose)
    {
        printf("HttpSendRequest called successfully\n");
    }

    WaitForSingleObject(hRequestCompleteEvent, INFINITE);

    // Before downloading file...
    // 1. Get the temp directory
    if (!(tempDirBytes = GetTempPathA(MAX_PATH, tempPath)))
    {
        fprintf(stderr, "Could not get temporary directory\n");
        InternetCloseHandle(hRequest);
        InternetCloseHandle(hConnect);
        InternetCloseHandle(hInstance);
        return false;
    }

    // 2. Get temp file name (full name: tempPath\temp.tmp)
    srand(GetTickCount());
    sprintf(tempFileName, "%s\\%08X.tmp", tempPath, rand());

    // Error check the end of temp file name for ending double slash
    if (tempFileName[bufsize] == '\\')
        tempFileName[bufsize] = '\0';

    // 3. Create temp file
    printf("Creating temp file %s\nto store %s\n", tempFileName, srcPath.c_str());
    tempFile = fopen(tempFileName, "wb");       // Open the file for writing
    if (!tempFile)
    {
        DWORD errorNum = GetLastError();
        fprintf(stderr, "Could not create temp file! Error %d\n", errorNum);
        InternetCloseHandle(hRequest);
        InternetCloseHandle(hConnect);
        InternetCloseHandle(hInstance);
        return false;
    }
    printf("Done!\n\n");

    printf("------------------- Read the response -------------------\n");

    unsigned long n = 0;
    unsigned long sum = 0;

    printf("Copying %s\n to %s\n", srcPath.c_str(), tempFileName);


    // WHERE THE MAGIC HAPPENS - AND WHERE EVERYTHING FAILS!
    while ( InternetReadFile(hRequest, lpszDataBuffer, dwSize, &dwRead) && !(bAllDone) )
    {
        if (dwRead != 0)
        {
            sum = 0;
            fwrite(lpszDataBuffer, 1, dwRead, tempFile);
            for (unsigned long i = 0; i < dwRead; ++i)
            {
                sum += lpszDataBuffer[i];
                sum %= 0xFFFF;
            }
            printf("Received 4KB block %d. Sum %04X\r", n++, sum);
        }
        else
        {
            bAllDone = TRUE;
            printf("\n");
            break;
        }
    }

    printf("\n\n------------------- Request Complete ----------------\n");



    fclose(tempFile);               // Done writing to file
    tempFile = fopen(tempFileName, "rb");   // Reopen for reading


    //Create the local file
    printf("Creating local file %s\n", destPath);
    localFile = fopen(destPath, "wb");
    if (!localFile)
    {
        DWORD errorNum = GetLastError();
        fprintf(stderr, "Could not create local file! Windows Error %d\n", errorNum);
        fclose(tempFile);
        remove(tempFileName);       // delete temporary file from machine
        InternetCloseHandle(hRequest);
        InternetCloseHandle(hConnect);
        InternetCloseHandle(hInstance);
        return false;
    }
    printf("Done!\n\n");


    // Copy the contents from the temp file to the local file
    printf("Copying temp file %s contents\nto local file %s\n", tempFileName, destPath);
    if (!copyFile(tempFile, localFile))
    {
        DWORD errorNum = GetLastError();
        fprintf(stderr, "Could not copy temp file to local directory! Windows Error\n", errorNum);
        fclose(tempFile);
        remove(tempFileName);       // delete temporary file from machine
        fclose(localFile);
        InternetCloseHandle(hRequest);
        InternetCloseHandle(hConnect);
        InternetCloseHandle(hInstance);
        return false;
    }
    printf("Done!\n\n");






    // end of logic housekeeping
    fclose(tempFile);

    printf("Deleting temp file %s\n", tempFileName);
    remove(tempFileName);   // delete temporary file from machine
    printf("Done!\n\n");

    fclose(localFile);

    printf("Ending Internet Session\n");
    InternetCloseHandle(hRequest);
    InternetCloseHandle(hConnect);
    InternetCloseHandle(hInstance);
    printf("Done!\n");

    printf("Press Enter to continue\n");
    std::cin.get();


    return true;
}

Answer 1

Your code works for me. Are you sure that the server is returning a non-empty response? You can use a tool like Fiddler2 to check. There are quite a few problems in this code, including a buffer overrun here: lpszDataBuffer[bufsize] = '\0';. In addition, you’re using asynchronous mode, but you don’t have any asynchronous handling in your read loop. I recommend that you post the code for review here: https://codereview.stackexchange.com/.

One final note. If you’re just going to wait for each operation to complete anyway, then there’s no benefit to doing things asynchronously. You can leave off the INTERNET_FLAG_ASYNC flag. This will make your function much simpler.