Re: Unpredictable behaviour with PyEpics

[Matt Newville via Tech-talk <tech-talk at aps.anl.gov>]

Jaroslav, Simon, 

On Tue, May 26, 2020 at 9:14 AM Jaroslav Adam via Tech-talk <tech-talk at aps.anl.gov> wrote:

Hi Simon,

verify what PYEPICS_LIBCA is pointing to. It is the library which PyEpics
is using to make connections.

From my experience with running PyEpics inside a PythonSoftIoc, the PYEPICS_LIBCA
should point to the same channel access library as the IOC is using.

Is Simon trying to use pyepics from a Python interpreter embedded in an IOC?  I do not read that from Simon's original message.

I hope that it is obvious to anyone trying to use pyepics from a Python interpreter embedded in an IOC that this definitely requires care in loading the Channel Access library.  The process running the IOC needs to have exactly one libca (and libCom) loaded.  That will be set before the Python interpreter is started, so the pyepics library (which loads libca as late as possible) would need to point to the same one that the IOC is linked against.

 

PYEPICS_LIBCA can be used to override the default CA library used by pyepics (which is distributed with pyepics, not some "system default"). 

Just for completeness, a similar situation of potential conflict between versions of libca/libCom can arise if using pyepics with PCASpy.  I believe that PCASpy loads libca on "import pcaspy", so that pyepics should be imported *after* PCASpy and ensure that PYEPICS_LIBCA points to the same libca file.

--Matt