Thread: SD card write to sector

Hi all,

I've been programming in Visual Studio 2015 - Windows Forms Managed C.

What I'd like to be able to do now is write to an SD card by sector i.e. not just create files using the simple method. The reason for this is that I have created a small data logger which reads a command string from the SD card that instructs it which sensors/frequencies to use, which the user has to put into a small text file manually. I'd instead like an app to create the command string on the SD card directly by writing this to a specific sector which the PIC can access simply.

I found a link on NERDKITS website, but I can't register or login there due to some web-safety issue (according to Chrome)

They provide some nice simple code that shows how to read a sector and write a sector. Reading works fine, but writing doesn't, although no error is produced at all. Note that this is writing to the volume, not to a specific file. I adjusted the code to create a file rather than writing to the volume directly and that worked fine, it just won't write to a specific sector.

Here's the code that's just not doing anything - If I have a text file with it's content sat at that exact offset (262144), the text file content appears in the console. It should modify to first 5 characters to "hello", save, flush buffer, close the file, and then re-read and display what's there. No change to the file content. If I open the file in Windows, that too shows no change to the file.

Code:

    char buf[1024];
    long long offset = 262144;
    FILE *volume;
    volume = fopen("\\\\.\\j:", "r");
    if (volume)
    {
        setbuf(volume, NULL);


        if (_fseeki64(volume, offset, SEEK_SET) == 0)
        {
            size_t cb = fread(buf, sizeof(*buf), _countof(buf), volume);
            System::String^ bufd = gcnew System::String(buf);
            Console::WriteLine(bufd);
        }
        fclose(volume);
    }

    buf[0] = 'h';
    buf[1] = 'e';
    buf[2] = 'l';
    buf[3] = 'l';
    buf[4] = 'o';

    volume = fopen("\\\\.\\j:", "a");
    if (!volume)
    {
        Console::WriteLine("failed volume");
    }
    else
    {
        setbuf(volume, NULL);


        if (_fseeki64(volume, offset, SEEK_SET) == 0)
        {
            size_t cb = fwrite(buf, sizeof(*buf), _countof(buf), volume);
            fflush(volume);
        }
        else
        {
            Console::WriteLine("failed");
        }


        fclose(volume);
    }

    volume = fopen("\\\\.\\j:", "r");
    if (volume)
    {
        setbuf(volume, NULL);


        if (_fseeki64(volume, offset, SEEK_SET) == 0)
        {
            size_t cb = fread(buf, sizeof(*buf), _countof(buf), volume);
            System::String^ bufd = gcnew System::String(buf);
            Console::WriteLine(bufd);
            // Process the data
        }
        fclose(volume);
    }

This code sits within a larger application which is executed at the click of a button.

The application is run in Administrator Mode, which I hoped would solve the problem but...

Any clue as to why no change is made to the file?

Note the "lock" is off on the sd card :P

Best,

Jimbo

Hi Salem,

Many thanks for getting back to me quickly on this.

Aye, I've noticed that before, that things don't happen and there's no error. Annoying! :-)

The opening in "a" mode is what they showed on the website (I forgot to send the link earlier: NerdKits - C program to read and write raw data on SD card (Everything Else) )

I did try various other options after looking up fwrite syntax etc. including r+, but they fail to fopen the volume, resulting in the console reported "failed" or "failed volume".

The bit of code you highlighted; it doesn't really matter too much about that, I'm just using that to view what it has written - it's after the fail, but surprisingly, it does work i.e. if the fopen mode is "a", it does print the first 1024 chars present at that offset.

Open to further suggestions...

Best,

Jimbo

I've since read that, since Vista/Windows 7, files/volumes must be locked (exclusive?) before they can be written to, else some other app' could potentially write to the same file/sector as my app. Not sure if this is the case as I did manage to create a new file with a modified version of the code above, but I've not written to an existing file (amending / "a").

Does anyone have any idea of how to implement this into my code (including unlocking it at the end...) if this is the case?

Best,

Jimbo

To UNLOCK the drive, I simply had to close the file handle.

Code:

    CloseHandle(hFile);

Hope the above helps someone...