How to get write/read offset?

[David Smith]

On 03/10/2016 06:32 AM, Olausson, Bjoern wrote:
> Hello Systemtap users,
> 
> is there some straight forward way to get the offset for any read,
> write, pwrite etc. syscall?
> 
> So for example if I trace the the IO to a file and the program issues
> a "seek" and writes reads n bytes starting from this position, is
> there any way to get that start position (offset) except from probing
> for an "seek" before a write/read?
> 
> For example:
> 
> probe syscall.write.return {
>  if (pid() == target()) {
>   time_stamp = timestamp()
>   p = pid()
>   fd = $fd
>   bytes = $return
>   latency = gettimeofday_us() - @entry(gettimeofday_us())
>   printf("%d;%d;%s;%s;%d;%d;%d\n", p, fd, filehandles[p, fd], name,
> bytes, time_stamp, latency)
>  }
> }
> 
> If there is no straight forward way to get this information, what is
> the most native way to get the offset alongside with the time, latency
> and bytes_written/read?

To get the file offset, you'll have to probe 1 level down. syscall.read
(sys_read()) is basically a wrapper around vfs.read (vfs_read()).
syscall.write (sys_write()) is basically a wrapper around vfs.write
(vfs_write()).

So, you can (almost) replace syscall.write.return in your example above
with vfs.write.return. One of the differences are that you don't get $fd
anymore, instead you get a 'struct file' pointer.


-- 
David Smith
dsmith@redhat.com
Red Hat
http://www.redhat.com
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