[Bug 1881563] Re: Empty arrays have undefined behavior

[Dirk Zimoch via Core-talk <core-talk at aps.anl.gov>]

#3 Inddeed, sclar records reading from empty arrays do not get the value
0 but instead keep their old value (without setting any type of error).
This is clearly buggy behavior.

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https://bugs.launchpad.net/bugs/1881563

Title:
  Empty arrays have undefined behavior

Status in EPICS Base:
  In Progress
Status in EPICS Base 7.0 series:
  In Progress

Bug description:
  The behavior of arrays with 0 elements is inconsistent and seemingly
  undefined. I made the fllowing observations:

  1. Reading empty arrays with caget shows NELM values, all 0 except for
  the first which sometimes contains strange not-quite-random numbers
  (see bug report 1242919). I have observed values 0, 2.122e-314,
  2.50321e-308, 22, 18, 2.02038e-39, 1441792, 1179648, 3.05948e-308,
  also depending on the -d option (DBR type) of caget.

  2. Reading empty arrays with camonitor shows no values.

  3. A scalar input record reading INP or scalar output record reading
  DOL (with OMSL closed_loop) from an empty array (aai, aao or waveform)
  reliably reads the value 0 (tested with ai, bi, mbbi, mbbiDirect,
  longin, stringin, ao, bo, mbbo, mboDirect, longout, stringout). This
  is true for DB links as well as for CA, CP and CPP links.

  4. An array record (aai, waveform) reading from an empty array does
  not change its current value. It does not change NORD, it does not
  clear the arrays elements, it does not show an alarm.

  5. Writing 0 elements to an array with caput -a fails whith "Invalid
  element count requested".

  My opinion on this behavior is as follows:

  1. Reading arrays with caget should return NORD elements, not NELM
  elements. But if we put this side, the additional elements should all
  be set to 0. For empty arrays, this includes the the first one (index
  0).

  2. This is correct behavior.

  3. This is consistent but questionable. Converting an empy array to a
  scalar should fail as there is no value to convert. But that might
  break existing dbs. In that case this behavior should be documented.
  (Or is it?)

  4. This behavior is buggy. The receiving record should update its NORD
  to 0.

  5. It should be possible to write an empty array.

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