Hvordan lese fil med buffered reader med Win32 APIet?

[siDDis]

Jeg prøver å lese en fil chunk by chunk.

 

 

    const size_t chunkSize = 4096;
    unsigned char *pBuffer = malloc(chunkSize);
    OVERLAPPED overlapped = { 0 };
    ULONGLONG total_read = 0;
    DWORD dwBytesRead = 0;
 
    LPCTSTR filePath = L"c:\\testdata\\pictures\\22085009751_5d9616c881_o.jpg";

    HANDLE readHandle = CreateFile(
        filePath,
        GENERIC_READ,
        FILE_SHARE_READ,
        NULL,
        OPEN_EXISTING,
        FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED,
        NULL);

    if (readHandle == INVALID_HANDLE_VALUE)
    {
        wprintf(TEXT(
            "Terminal failure: unable to open file \"%s\" for read.\n"
        ), filePath);
        return;
    }

    while (overlapped.Offset < file_size )
    {
        ReadFile(readHandle, &pBuffer, chunkSize, &dwBytesRead, &overlapped);
        dwBytesRead += dwBytesRead;
        overlapped.Offset += chunkSize;
        wprintf(L"Data read: [%d]\n", dwBytesRead);
        wprintf(L"Data read: [%lu]\n", overlapped.Offset);
    }

    CloseHandle(readHandle);

 

Problemet her er at dwBytesRead alltid blir 0. Jeg forstår ikke hva som er galt, dette er jo så enkelt å få til med "vanlig" c på Linux, pluss at der finnes det massevis av dokumentasjon. Men på Windows er det jo bare rot.

[Locrin]

slettet

Endret 10. august 2016 av Locrin

[Glutar]

Hvorfor kan du ikke bare bruke standard fopen fread, osc i C eller std::ifstream i c++?

[GeirGrusom]

If hFile was opened with FILE_FLAG_OVERLAPPED, the following conditions are in effect:

The lpOverlapped parameter must point to a valid and unique OVERLAPPED structure, otherwise the function can incorrectly report that the read operation is complete.

The lpNumberOfBytesRead parameter should be set to NULL. Use the GetOverlappedResult function to get the actual number of bytes read. If the hFile parameter is associated with an I/O completion port, you can also get the number of bytes read by calling the GetQueuedCompletionStatus function.

Så tror du trenger å sende nullptr istedet for dwBytesRead.

[siDDis]

Hmm, der blir eg bare meir forvirra. Ettersom eg forstår det så gjør eg dette heilt feil

 

Har funnet en metode som heter SetFilePointer, den er lik fseek. Skal sjå om eg får det til riktig då.

[siDDis]

Hehe, da fikk jeg det til.

 

GeirGrusom tok meg i riktig retning.

 

 

Komplett fungerende eksempel i Windows style

 

 

#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
#include <sys/stat.h>

void bin_to_strhex(unsigned char *bin, unsigned int binsz, char **result)
{
    char          hex_str[] = "0123456789abcdef";
    unsigned int  i;

    *result = (char *)malloc(binsz * 2 + 1);
    (*result)[binsz * 2] = 0;

    if (!binsz)
        return;

    for (i = 0; i < binsz; i++)
    {
        (*result)[i * 2 + 0] = hex_str[(bin[i] >> 4) & 0x0F];
        (*result)[i * 2 + 1] = hex_str[(bin[i]) & 0x0F];
    }
}

void
print_buffer(unsigned char *buffer, off_t buffer_size)
{
    char *str;
    bin_to_strhex((unsigned char *)buffer, buffer_size, &str);
    printf("%s\n", str);
}


int main()
{
    const size_t chunkSize = 4096;
    unsigned char *buf = malloc(chunkSize);

    ULONGLONG total_read = 0;
    OVERLAPPED overlapped = { 0 };

    const char* filePath = "C:\\testdata\\pictures\\someimage.jpg";

    HANDLE readHandle = CreateFile(
        filePath,
        GENERIC_READ,
        FILE_SHARE_READ,
        NULL,
        OPEN_EXISTING,
        FILE_ATTRIBUTE_NORMAL | FILE_FLAG_NO_BUFFERING,
        NULL);

    if (readHandle == INVALID_HANDLE_VALUE)
    {
        printf(TEXT("CreateFile"));
        printf(TEXT(
            "Terminal failure: unable to open file \"%s\" for read.\n"
            ), filePath);
        printf("Please press any key to exit the program ...\n");
        getchar();
        return;
    }

    ULONGLONG lpFileSize = 0;
    lpFileSize = GetFileSize(readHandle, NULL);
    printf(TEXT("File size: [%llu]\n"), lpFileSize);

    DWORD dwBytesRead = 0;

    while (ReadFile(readHandle, buf, chunkSize, &dwBytesRead, NULL))
    {
        if (dwBytesRead == 0) break;
        print_buffer(buf, chunkSize);
    }

    free(buf);

    CloseHandle(readHandle);

    printf("Please press any key to exit the program ...\n");
    getchar();
    return 0;
}