target variables : doubts on usage.

[Om Narasimhan <om.turyx@xxxxxxxxx>]

Are the variables defined by probe aliases target variables? (accessible 
by $var?). I am getting different results in my machine.

$ uname -a
Linux testserv.home 2.6.23.17-88.fc7 #1 SMP Thu May 15 00:02:29 EDT 2008 
x86_64 x86_64 x86_64 GNU/Linux
$ stap -V
SystemTap translator/driver (version 0.6.2/0.127 built 2008-03-27)
Copyright (C) 2005-2008 Red Hat, Inc. and others
This is free software; see the source for copying conditions.

$ sudo stap -e 'probe vm.pagefault { printf("%#x->%d\n", $address, 
$write_access)}'
0x2aaaaf2d1000->1
0x2aaaaf2d1000->1
0x2aaaaf2d1000->1
0x3683adcfd0->0
0x40414d->0
^C

The corresponding function in tapset is,

$ grep vm.pagefault -A 5  /usr/share/systemtap/tapset/memory.stp

probe vm.pagefault = kernel.function("__handle_mm_fault@mm/memory.c") ?,
                     kernel.function("handle_mm_fault@mm/memory.c") ?
{
        write_access = $write_access
        address =  $address
}

Now I do the same operation using process.stp function `process_create`
$ sudo stap -e 'probe process.create { printf("%d\n", $new_pid) }'
semantic error: unable to find local 'new_pid' near pc 
0xffffffff81034327 (alternatives: clone_flags stack_start regs 
stack_size parent_tidptr child_tidptr pid retval p): identifier 
'$new_pid' at <input>:1:39
Pass 2: analysis failed.  Try again with more '-v' (verbose) options.

Corresponding code from process.stp is,
probe process.create = kernel.function("copy_process").return {
    task = $return
    new_pid = task_pid(task)
    if (_IS_ERR(task)) next
}

Despite new_pid is used(hence defined), I can't access it as a target 
variable. But if I omit $, it works fine.
Eg,
$ sudo stap -e 'probe process.create { printf("%d\n", new_pid) }'
10953
10953

(Though I don't understand how can it print same pid more than once)

So, my question is how to differentiate normal variables with target 
variables?

I would like to add that in the first example, address is accessible
without $variable too.

Thanks,
Om.

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