Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Wed, 20 Jul 2005 10:12:39 -0500

Kay-Uwe Kasemir wrote:

do you know any example where you actually need to destructively

update a string,

once you have it constructed?

I think Ben is right here.

I disagree, or maybe I don't fully understand the suggestion.

There is a record type called the stringCalc record, which is used

extensively by the BCDA group's synApps software (among other things, it

permits you to interface to new ASCII devices without having to write

device support). The VAL field of a stringCalc is string, which is

regenerated every time the record processes. I don't know how you would

implement this record type if all strings become read-only once created.

Similarly any device support that is driving a message-based device has

to be able to create a new message every time it processes; if we don't

support mutable strings, these device supports would have to use char*

buffers for generating their messages in.

I think we need to concentrate on the 99% where the string
is a short, contiguous (char *),
instead of the 1% where the string
is 1MB long, stored as non-contiguous 10 byte segments.

As I understand it, one of the main purposes of the segmented string

requirement is to permit Jeff to create a complete string out of the

characters that appear in the CA network input buffer, without having to

copy the character data at all. The chances that even a short string

would span two (or more) buffers are not that small, and the

implications of his having to copy the data are not just the time taken

to do the copy - if we force him to copy it out of the buffer before

presenting the string to the application, he has to temporarily allocate

memory for the destination string, and then manage the lifetime of that

memory allocation.

Sorry guys, I really can't support the development of an API that only

supports non-modifyable strings.

- Andrew
--
Podiabombastic: The tendency to shoot oneself in the foot.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System