Re: command line arguments

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, Feb 23, 2006 at 05:42:18PM -0500, Frank Ch. Eigler wrote:
> Hi -
> 
> I committed a draft of command line argument support as per bug #1304.
> One part (plain substitution into scripts) is usable now.  Another
> part (initializing globals at module init time) is sort of dormant.
> 
> The way the first part works is by making any additional arguments 
> passed to "stap" available to the script for substitution as string
> or number literals:
> 
> # stap -e 'probe kernel.function(@1) { print($2) }' sys_open 4
> 
> The @ vs $ distinction encodes whether the numbered argument should be
> pasted as a string or number literal.  I considered guessing but
> heuristics don't seem to belong somewhere so critical.
> 
> I'm open to suggestions about better notation than $n and @n.  (#n is
> out because of comments; % and others because of arithmetic operators.)

The scenarios I'd like to use command line args in, are for tweaking
aspects of the data capture, say, sampling rate of a timer, or if I
was probing 'sys_read', some variable to control how often to log.
So I'd think named arguments would be clearer to use, possibly to set 
global variables. So for example

    global samplerate = 1000

    probe timer.jiffies(samplerate) {
       ...
    }

One could tweak the rate with

  # stap --define samplerate=50  stuff.stp

Or another exaple, if one wanted to watch all file opens under a particular
directory

    global prefix = "/"

    probe kernel.function("sys_open") {
      if ( isinstr(prefix, substr(user_string(filename),0,strlen(prefix)) ) {
         print("Pid " . pid() . " opens " . user_strin(filename))
      }
    }

Then one could restrict it to just files under /etc with

    # stap -D prefix=/etc  watchopen.stp

Dan.
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