Re: [xml] libxml2 on OpenVMS 8

["Craig A. Berry" <craigberry mac com>]

On Dec 7, 2009, at 11:25 AM, Lopez, Jesus wrote:

> Hi all,
> I’m facing some trouble with the use of libxml2 with COBOL on  
> OpenVMS v8. We want to process some XML calls (SOA) from Cobol, but  
> I’d need some clues.
> I’ve compiled libxml on OpenVMS with apparent success, but I don’t  
> know how to use them with my COBOL program.

There are a number of ways to skin this cat.  Pretty much all of them  
involve some basic knowledge of the OpenVMS calling standard,  
documented in, of all places, the HP OpenVMS Calling Standard  
reference manual at:

<http://h71000.www7.hp.com/doc/82final/5973/5973pro.html>

And you'll want to be familiar with the section in your COBOL user  
manual entitled, "Using HP COBOL in the Alpha, I64, or VAX Common  
Language Environment":

<http://h71000.www7.hp.com/doc/82final/6297/6297pro_095.html#com_lang_env_chap 
>

though this latter is not as helpful as it might be for your specific  
situation of calling C libraries from COBOL.

But the gist of it is that you can call anything from anything as long  
as you specify the correct argument passing mechanisms and make sure  
the data you are passing comply with the requirements of the language  
you are calling, such as counted strings for Pascal, null-terminated  
strings for C, etc.

So then you have to ask yourself whether you really want to be moving  
a null byte to the end of every string before passing it to a C  
function (or whether every one of your COBOL programmers will do that  
correctly and consistently), and whether there isn't a way to create a  
set of wrapper routines with a translated and possibly simplified  
version of the API that is more COBOL-friendly.  There is an excellent  
blog article on one way to do that here:

<http://www.eight-cubed.com/blog/archives/001147.html>

That post proposes using string descriptors, which is a tried-and-true  
method for passing strings around in VMS (and the same method you'd  
use for calling system routines).   A somewhat less elegant method  
that might be slightly more COBOL-friendly would be creating an extra  
argument with the string length for each string argument.  Sort of  
ugly but might be easier to grasp for the average COBOL programmer  
(though I haven't done any COBOL in 16-18 years so take this comment  
with a huge grain of salt).

What the best solution is depends highly on your environment, e.g.,  
COBOL programmers calling C, C programmers maintaining COBOL, etc.

________________________________________
Craig A. Berry
mailto:craigberry mac com

"... getting out of a sonnet is much more
 difficult than getting in."
                 Brad Leithauser