[src] cvs commit: src/include unistd.h src/lib/libc/sys readlink.2 src/sys/compat/freebsd32 syscalls.master src/sys/kern syscalls.master vfs_syscalls.c src/sys/sys syscallsubr.h

Daniel O'Connor doconnor at gsoft.com.au
Mon Feb 18 04:43:59 UTC 2008


On Mon, 18 Feb 2008, Giorgos Keramidas wrote:
> >>> However, you still keep the file around which can be rather space
> >>> consuming :(
> >>
> >> Yes, but it also means you can do offline analysis later. :)
> >> Tradeoffs either way.
> >
> > Yes, but being able to specify stdout to ktrace would be really,
> > really nice..
>
> Specifying stdout may be a bit tricky, since the traced application
> may be using the same stream to print output.  The same is possible
> with stderr, but may be a tiny bit less likely.

I didn't realise that the file descriptor used to write tracing data out 
was 'owned' by the process being traced, I always thought ktrace did.

> It is probably easy to add an -F flag to ktrace/kdump which would
> inhibit the check for a `regular' file, so you could then write:
>
> 	( ktrace -aF -f /dev/stdout ls ) | \
> 	  kdump -F -f /dev/stdin
>
> 	( ktrace -aF -f /dev/stderr ls >/dev/null ) 2>&1 | \
> 	  kdump -F -f /dev/stdin
>
> But the first will probably fail when kdump tries to parse the output
> of ls(1), and the second may fail in a similar manner when kdump
> tries to parse an error message like a ktrace record.
>
> This sort of difficulty in separating the output of the traced
> process and the ktrace records themselves is probably at least part
> of the reason why nobody has done it yet.

I did have a look at the source and the file opening etc is handled by 
the kernel but I am not sure who 'owns' that file descriptor.

If, as you suggest, it is the process being traced then yes it would 
cause problems.

I guess it couldn't be moved to ktrace without rearchitecting how 
ktracing works so the ktrace process sticks around writing stuff out to 
disk.

-- 
Daniel O'Connor software and network engineer
for Genesis Software - http://www.gsoft.com.au
"The nice thing about standards is that there
are so many of them to choose from."
  -- Andrew Tanenbaum
GPG Fingerprint - 5596 B766 97C0 0E94 4347 295E E593 DC20 7B3F CE8C
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