writing a file to a FAT partition

Cman · 01-14-2002, 01:22 PM

I am trying to create and write to file in a FAT32 partition. (I need to be able to see the file on the WIN2K side as well as Linux.)
using the following code ex.

ofstream wfile;

wfile.open ("/dosd/CSE_Raw/Heartbeat.txt", ofstream:

ut | ofstream::app);

wfile << ....

dosd is the mount point for the FAT32 partition.
But the file open fails. Any ideas? I am running this code from my home directory not the FAT partition if that makes a difference.

mtsinc · 01-15-2002, 03:14 PM

Well, there's a couple of things you can try. Although FAT has minimal file attribute bits compared to Linux, you may be being denied access because the FAT filesystem is blocking you.

Try:

ls -l /
ls -l /dosd
ls -l /dosd/CSE_Raw

to make sure than none of the directories of the traversal path is forbidden.

Also, do a quick command-line check:

echo Hello >/dosd/CSE_Raw/Heartbeat.txt
cat /dosd/CSE_Raw/Heartbeat.txt

If all of these work, you've coded the program incorrectly.

Cman · 01-17-2002, 08:47 AM

As root, the echo command works, and my program works. However when I try to chown the dosd (or anything under it) to my non-root account, I get a permission denied message. Same thing with chmod. I tried modifying the permissions through the WIN2K side but It won't allow me.
Is there anyway to be able to write to this directory not logged in as root? Or some other way to change the directory ownership or write permissions?

Thymox · 01-17-2002, 09:41 AM

Unfortunately none of the FAT types will allow you to have ownership. I think the only Windows compatible filesystem to have ownership is NTFS, which is not particularly well supported in Linux. If you want to be able to access r/w the files on the dos partition, you will need to look up the fstab properties for the mount-point (i.e. dosd).

man fstab.

I'm affraid I'm not at my Linux box at the mo and I can't remember the correct settings.

Cman · 01-17-2002, 10:22 AM

can I set uid=# in the fstab to force my FAT partition to be owned by someone other than root? If so, how do I find out the uid? I tried whoami, but it only tells me the name, not the number.

Thymox · 01-17-2002, 10:42 AM

No, no, I think you misunderstood me. Because there is no such thing as ownership under Windows (except NTFS) the Linux system defaults to everything being owned by Root. If you edit your fstab file you can add the option of 'user' which means that not only can any-old-user mount the partition, but can also write to it (presuming you've set the rw option aswell). The files will still be owned by root, though. If you read the man pages for mount and fstab respectively you should get the idea.

bluecadet · 01-17-2002, 10:42 AM

yeah, you can set it there, you can find the uid from looking at /etc/passwd. probably a command for it, but that's the text file it's stored in

Cman · 01-17-2002, 11:17 AM

appreciate all the help. I added uid= and rw to the fstab for the FAT partition and now I can write to it the way I want. Thymox, understand what you were suggesting but I prefer the partition be owned by the user, not root.

Thanks again

Gnomz · 09-21-2004, 04:25 PM

I think that is a weid way to make a FAT partition writeable.
Try adding umask=000 option this sets the default mask of permissions which are not present, thus allowing anyone to write, not just the owner. It is the same octal mask, it just does the opposite to chmod.
You can use fmask and dmask to separate rights for files and directories.

Check the man mount :-)