Re: Python/CA

[Noboru Yamamoto <noboru.yamamoto@kek.jp>]

Janousch Markus wrote:
> 
> Noboru Yamamoto wrote:
> >
> > Janousch Markus wrote:
> > >
> > >
> > ca_test.Simple class gives a chance to CA library using pendio() to establish
> > connection, however, it does not check it before get() call.
> >
> > >def wait_conn(self,wait=5,dt=0.05):
> > As you see, wait_conn() has default values of wait=5,dt=0.05. If network
> > condition is
> > too bad or IOC is working very busy, even wait_conn() may raise ECA_BADCHID.
> >
> > > class Simple(ca.channel,Frame):
> > >       def __init__(self,name,master=None,*cnf):
> > >               Frame.__init__(self,master)
> > >               self.pack(expand=1,fill='both')
> > >               ca.channel.__init__(self,name);self.pendio(1.0)
> >                                          ^^^^^^^^^^^^^^^^^
> > >               self.get();self.pend_event(1.0)
> > >               .....
> >
>  Strange enough, the above call to self.pendio(1.0) before get() in the
> class Simple does not seem to work. Either I have to replace it with a
> call to wait_conn(), or with a call to pend_event(1.0). The latter two
> always seem to work. Also, when the call to pend_event(1.0) after the
> call to get() is replaced by pendio(1.0) it does not work anymore.

Now I know what happens.
"EPICS Channel Access Reference" says,
" ca_pend_io() flushes the send buffer and then wait until outstanding
queries complete
or the specified time out expires. At this time queries include calls to
ca_get() and
call to ca_search() WHICH DON'T specify a connection handler."

The creation method of ca.channel , ca.channel.__init__, set a
connection callback 
by default. So, pend_io will not wait for the completion of search_and_connect.

.get() also uses ca_array_get_callback() function internally.  You need
to call pend_event()
rather than pend_io().

In conclusion, I should have replace pend_io() call in "class Simple" by
pend_event() call.
Thank you for pointed it out.  
> 
> > __init__ has a optional argument cb. A function , which has no
> > arugument, will be called when channel connection complete. Because it
> > use CA library, you need to call
> > pend_event() or pendio() for this callback to happen. (Tk mainloop
> > calles pend_event()
> > automatically if you use -DWITH_TK option to compile _ca.c)
> >
>  When does this automatic call happen?

In _ca.c, Py_ca_fd_register() is registered as a file descriptor
registration function
using ca_add_fd_registraion() call in EPICS-CA library.
This file descriptor registration function will be called whenever CA
library creates
a new socket with a file descriptor of the new socket. 
Py_ca_fd_register will register this file descriptor to the Tk-event
loop. 
Tk event loop checks if this file descriptor has data for input/output and
calls Py_ca_service() when data is available.

> static void Py_ca_fd_register(void *pfdctx, int fd, int condition)
> {
> #if defined(WITH_TK) && ! defined(WITH_THREAD)
>   if(condition){
>     Tk_CreateFileHandler(fd, TK_READABLE, Py_ca_service, pfdctx);
>   }
>   else{
>     Tk_DeleteFileHandler(fd);
>   }
> #endif
> }

> 
> > I'm reading Python/C API reference to make _ca.c work with Python thread.
> >
>  I stay tuned and good luck!
Although Python supports threads, it interpreter is not 100% thread safe.
Tcl/Tk was not 100% thread safe before, which is also supported by Python.
CA is not thread safe on Unix (may be I am wrong).

So I need to put may Lock/Unlock statements to protect these three libraries.
I have managed to compile Python/CA with Tkinter on the CPU supporting thread(
Digital Unix v4). However, I cannot use Tk_CreateFileHandler/
Tk_DeleteFileHandler in C-level.
I need to register these function in Python level, as show below.

========== ========== ========== ========== ========== ==========
def ca_service(*arg,**kw):
    ca.pend_event(0.0001)

def ca_fd_register(arg, fd, condition):
    if condition:
        Tkinter.tkinter.createfilehandler(fd,Tkinter.READABLE, ca_service)
    else:
        Tkinter.tkinter.deletefilehandler(fd)

ca.add_fd_registration(ca_fd_register, "ca_register")
========== ========== ========== ========== ========== ==========

> 
>   Markus

Thank you for listening.

Noboru