Undefined behaviour when setting sFlowCpInterval to zero

From: andy kitchingman <andy.kitchingman@alliedtelesis.co.nz>
Date: 02/11/10
Message-Id: <4B7542840200005D000177DA@gwia.alliedtelesyn.co.nz>


In the poller function:

sfl_poller_set_sFlowCpInterval (SFLPoller * poller, u_int32_t sFlowCpInterval)
  poller->sFlowCpInterval = sFlowCpInterval;
  /* Set the countersCountdown to be a randomly selected value between 1 and
     sFlowCpInterval. That way the counter polling would be desynchronised
     (on a 200-port switch, polling all the counters in one second could be
harmful). */
  poller->countersCountdown = 1 + (random () % sFlowCpInterval); <--- MAY NOT
DO WHAT YOU THINK IF sFlowCpInterval == 0!!

The subexpression "random () % sFlowCpInterval" results in undefined behaviour
according to the C and C++ standards, if sFlowCpInterval is zero.

Depending on what platform you compile this on, you get different results.

For example, on the i686, I got a floating point error at runtime.

On a PowerPC, the expression evaluated to 1 + random().

Zero is a valid value for sFlowCpInterval according to the sFlow specification
V5, so shouldn't poller->countersCountdown be set to zero in this case?



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Received on Thu Feb 11 15:00:04 2010

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