Experimental Physics and
Industrial Control System

Mark Rivers via Tech-talk <tech-talk at aps.anl.gov> · Fri, 29 Jan 2021 23:52:17 +0000

I ran into a program running a Python program that uses both PyEpics and pvaPy.  Here I describe the problem and the solution.

It works fine on Linux:

corvette:iocBoot/iocTomoScan_13BM_PSO/autosave>conda activate base
(base) corvette:iocBoot/iocTomoScan_13BM_PSO/autosave>python
Python 3.7.3 (default, Mar 27 2019, 22:11:17)
[GCC 7.3.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.
Imported history from: '/home/epics/.pyhist'
>>> import epics
>>> import pvaccess
>>> epics.caget('13BMD:m1')
-5.250001587314

On Windows it works fine to run PyEpics by itself:

(base) C:\Users\tomo_user>python
Python 3.7.4 (default, Aug  9 2019, 18:34:13) [MSC v.1915 64 bit (AMD64)] :: Anaconda, Inc. on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import epics
>>> epics.caget('13BMD:m1')
-5.250001587314

However, if I also load pvaccess I get this error:

(base) C:\Users\tomo_user>python
Python 3.7.4 (default, Aug  9 2019, 18:34:13) [MSC v.1915 64 bit (AMD64)] :: Anaconda, Inc. on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import epics
>>> import pvaccess
>>> epics.caget('13BMD:m1')
Traceback (most recent call last):
  File "C:\ProgramData\Anaconda3\lib\site-packages\epics\ca.py", line 373, in initialize_libca
    libca = load_dll(find_libca())
  File "C:\ProgramData\Anaconda3\lib\ctypes\__init__.py", line 442, in LoadLibrary
    return self._dlltype(name)
  File "C:\ProgramData\Anaconda3\lib\ctypes\__init__.py", line 364, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: [WinError 127] The specified procedure could not be found
Traceback (most recent call last):
  File "C:\ProgramData\Anaconda3\lib\site-packages\epics\ca.py", line 373, in initialize_libca
    libca = load_dll(find_libca())
  File "C:\ProgramData\Anaconda3\lib\ctypes\__init__.py", line 442, in LoadLibrary
    return self._dlltype(name)
  File "C:\ProgramData\Anaconda3\lib\ctypes\__init__.py", line 364, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: [WinError 127] The specified procedure could not be found

[cid:292fe37f-72a4-462e-a05f-674aed70857d]

Matt Newville told me the problem is because PyEpics and pvaPy must load the same version of ca.dll.  pvaccess loads this version at import time:

C:\ProgramData\Anaconda3\Lib\site-packages\pvaccess\ca.dll

PyEpics by default loads this version upon the first "real connection":

C:\ProgramData\Anaconda3\Lib\site-packages\epics\clibs\win64\ca.dll

They are not the same and so the error occurs.

However, PyEpics allows setting the environment variable PYEPICS_LIBCA to define the location of ca.dll.

I set it to the same as pvaccess:

set PYEPICS_LIBCA=C:\ProgramData\Anaconda3\Lib\site-packages\pvaccess\ca.dll

and now I can run both PyEpics and pvaPy at the same time:

(base) C:\ProgramData>python
Python 3.7.4 (default, Aug  9 2019, 18:34:13) [MSC v.1915 64 bit (AMD64)] :: Anaconda, Inc. on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import epics
>>> import pvaccess
>>> epics.caget('13BMD:m1')
-5.250001587314

I thought others might find this useful some day.

Mark

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System