CODEWITHSUNDEEP
windowsdriverhardwarediskwindows-kernel
user7225528
user7225528

Reputation:

Using SetFilePointer to change the location to write in the sector doesn't work?

I'm using SetFilePointer to rewrite the second half of the MBR with something, its a user-mode application and i opened a handle to PhysicalDrive

At first i tried to set the size parameter in WriteFile to 256 but the writefile gave the INVALID_PARAMETER error, as it turns out based on some search on other questions here it seems like this is because we are forced to write in multiplicand of the sector size when the handle is PhysicalDrive for some reason

then i tried to set the filePointer to 256, and Write 512 bytes, both of them return no error, but for some unknown reason it writes from the beginning of the sector! as if the SetFilePointer didn't work even tho the return value of SetFilePointer is OK and it returns 256

So my questions is :

  1. Why the write size have to be multiplicand of sector size when the handle is PhysicalDrive? which other device handles are like this?

  2. Why is this happening and when I set the file pointer to 256, WriteFile still writes from the start?

  3. isn't this really redundant, considering that even if I want to change 1 byte then I have to read the entire sector, change the one byte and then write it back, instead of just writing 1 byte, it seems like 10 times more overhead! isn't there a faster way to write a few bytes in a sector?

Upvotes: 0

Views: 388

Answers (1)

xord
xord

Reputation: 61

I think you are mixing the file system and the storage (block device). File system stays above storage device stack. If your code obtains a handle to a file system device, you can write byte by byte. But if you are accessing storage device stack, you can only write sector by sector (or block size).

Directly writing to block device is definitely slow as you discovered. However, in most cases, people just talk to file systems. Most file system drivers maintain cache and use algorithms for both read and write to improve performance.

Can't comment on file pointer based offset before seeing the actual code. But I guess it might be not sector aligned or it's not used at all.

Upvotes: 0

Related Questions