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AndrewO_4840's avatar
AndrewO_4840
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Feb 12, 2009

How can this fail?

I'm using a variation of the request rate limiting iRule ( http://devcentral.f5.com/wiki/default.aspx/iRules/HTTPRequestThrottle.html ) - it's essentially the same without the whitelist or blacklist features.

The rule breaks on the line: 

  set elapsed_time [expr {$curr_time - $start_time}}

and throws the error:

> - can't use empty string as operand of "-" while executing "expr {$curr_time - $start_time}"

This rule contains the lines: 

    set curr_time [clock seconds]

and: 

set request_count [session lookup uie $reqkey]   
 if {$request_count > 0} { 
   set start_time [session lookup uie $timekey] 
   ...

Since $curr_time is defined, and $start_time is defined if the session lookup returns some value, how can either of them be 0 and cause my iRule to fail?
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  • L4L7_53191's avatar
    L4L7_53191
    Icon for Nimbostratus rankNimbostratus
    Feb 13, 2009
    It looks like the interpreter thinks you're dealing with a literal empty string, not a zero (or other number). This is from a normal TCL command line, not a BigIP, but it's pretty easy to replicate:

     

     

     
 % set one [clock seconds] 
 1234571258 
  
 % set two "" 
 % expr { $one - $two } 
 can't use empty string as operand of "-" 
 % 
 % set two 0 
 0 
 % expr { $one - $two } 
 1234571258 
 %

     

     

    So this hints that you may need to look back at the code and make sure that $start_time is correctly set to something other than an empty string.

     

     

    HTH,

     

    -Matt