Experimental Physics and
Industrial Control System

Michael Davidsaver <[email protected]> · Tue, 17 Apr 2018 11:40:43 -0700

On 04/17/2018 06:18 AM, Jörn Dreyer wrote:
> ~ $caget -d 2 HIBEF:XFEL:image1:ArrayData | more
> HIBEF:XFEL:image1:ArrayData
>     Native data type: DBF_FLOAT
>     Request type:     DBR_FLOAT
>     Element count:    1000
>     Value:            2.02038e-39 0 ...

Don't ask me why, but I was annoyed at seeing (again) this garbage element
in what should be a zero'd array.  So I traced it back.  The issue is in
camessage.c in read_reply().  Specifically, that

> dbr_size_n(X,0) == dbr_size_n(X,1)

as the underlying dbr_size[] includes the size of meta-data and one value element.

So dbChannel_get_count() correctly sets item_count to zero.  Then read_reply()
zeros part, but all, of the buffer.  In this case data_size includes the first
value element, which isn't set by dbChannel_get_count(), and not isn't zeroed
here.

> ca_uint32_t data_size = dbr_size_n(pevext->msg.m_dataType, item_count);
> ...
> memset((char *) pPayload + data_size, 0, payload_size - data_size);

This is only in the !autosize case (get or non-dynamic monitor).

I'm not sure where to fix this, as there would seem to be several possibilities,
depending on whether or not dbr_size_n() should change.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System