life span of file descriptors?

[Torsten Fink <tnfink@inf.fu-berlin.de>]

Hi,

to learn a little bit about system calls I tried to hack some
kind of monitoring system. Each system call of a user should
be logged in some log file.

I added a system call to activate monitoring of a specific user.
In the correspending service routine I opened a file (mode
O_CREAT | O_APPEND) and store the file descriptor in some table.

Now, every time a system call is executed a log message is
appended to this file. To do this I hacked the 
 execute_syscall()
function.

This works now fine for the first time a system call is executed.
But after the 2nd time the stored file descriptor seems
to be invalid. A write request (with sys_write()) always returns 
the errorcode -9 (i.e. EBADF).

If I changed the execute_syscall to 
- open the file, 
- write the message, 
- and close the file
everything works fine.

But my guess is that opening and closing a file is quite expensive
and the first approach would be preferrable.

So the question is: Why can't I write more than once to the
same file descriptor in the kernel from different system calls?

Any ideas?

(I used the UML system for the implementation and I did not try
this on a "real" kernel.)

-- Torsten
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