Problem using mandatory locks (other apps can read/delete etc)

Hi,

I'm trying to solve a problem using mandatory locks but am having some difficulty in doing so. (if there's a more appropriate (non-devel) place for discussing this please ignore the rest of this post. pointers to that place would be appreciated ;) )

my problem:

when I lock a file with a mandatory write lock (ie. fcntl, +s-x bits and mand mount option. for code see below) it is still possible:

- for me to rm the file in question (i can see some reasoning behind this, but in my case I lock the file because everybody else should not be able to touch it at all)

- for the file to be read by an other process (as I understand it F_WRLCK sets an exclusive lock and fcntl can set only 1 lock anyway, so why are reads not blocked?)

It's not possible to cat to the file or cp another file over it (as expected). For my application I want a locked file to be completely locked/protected from other processes.

If I retry the same without setting the +s-x bits the cp & cat succeed, so something special is actually being done in the first case.

According to all docs (i can find) mandatory locks should block both the read and write system calls (i cant find anything in it regarding unlink though...). In my case however it seems as if only write calls are blocked but not read calls?

If anyone could shed any light on this it would be much appreciated.

Esger

system details:

linux 2.4.17 partially suse & xfs patched kernel
filesystem is reiserfs
i386 dual pIII450

test app code:

int main ()

{
    int fd ;


    fd = open ("image.jpg", O_RDWR) ;

    if (fd == -1)
    {
        printf ("error %d while opening file\n", errno) ;
        exit (1) ;
}

struct flock lock ;

    lock.l_type = F_WRLCK ;
    lock.l_whence = SEEK_SET ;
    lock.l_start = 0 ;
    lock.l_len = 0 ;
    lock.l_pid = 0 ; // ignored

    int err = fcntl (fd, F_SETLK, &lock) ;

    if (err == -1)
    {
        printf ("error %d while locking file\n", errno) ;
        exit (1) ;
}
else
        printf ("file locked\n") ;


    while (1)
        sleep (1) ;
}

NeoMail - Webmail that doesn't suck... as much.
http://neomail.sourceforge.net