Re: problem when run tcp/udp probing script

[Mark Wielaard <mjw at redhat dot com>]

On Mon, Mar 19, 2012 at 01:25:06PM +0800, ch huang wrote:
> i write the following script ( from book 'Instrumenting the Linux
> Kernel for Analyzing Performance and Functional Problems )
> 
> probe udp.recvmsg {
> printf("%s: UDP: Receiving message. Socket %d. Size %d\n", execname(),
> sock, size)
> }
> [...] 
> but why the value of socket is negative?

sock is a pointer of type struct sock*.
So it is better to print it as an hex value with:

  printf("%s: UDP: Receiving message. Socket ptr 0x%x. Size %d\n", execname(),
         sock, size);

When you print is as %d, it is interpreted as an signed long value and so
can also be negative.

If you are interested in the actual struct sock value you can also try pretty
printing it using @cast and adding $ at the end. e.g.

probe tcp.recvmsg {
  printf("%s: TCP: Receiving message %s.\n", execname(),
         @cast(sock, "struct sock")$)
}

Cheers,

Mark