Re: [C400-L] New to C-coding on the AS/400

[Simon Coulter <shc@xxxxxxxxxxxxxxxxx>]

On 10/07/2004, at 6:22 AM, Peter Grace wrote:

> I've been fiddling with C/C++ on the AS/400 and am suffering from a
> lack of rudimentary knowledge on how you do simple things (like,
> return character arrays back to an RPG program)...

C doesn't have the concept of return parameters. C operates expecting 
the pointers specified in argv to be input parameters. The only return 
from a C program is via the return value. The only return value from a 
C *PGM object is an integer via the return statement (e.g., return(8)).

However, because all parameters are are passed to OS/400 *PGM objects 
via reference (convention rather than requirement) you can de-reference 
any of the argv pointers and copy the required data into your caller's 
storage. Not an intuitive C-like way of doing things but perfectly 
understandable to an OS/400 programmer.

Thus:
	*argv[1] = 'N';
or
	argv[1][0] = 'N';
or
	memcpy( argv[2], "NO", 2 ); /* No null-terminator rubbish */
or
	int i;
	i = *(int*)arvg[3]; /* Presuming the caller passed an integer */
	i++;
	*(int*)argv[3] = i; /* Presuming the caller expects an integer 
returned */

As long as you know what the call interface is then you can accept and 
return any data type.

> Are there any redbooks or documentation on the internet relating to
> "How to program in ILE-C for dummies!"

There used to be a couple of books written from a 400 perspective:

Common Sense C by Paul Conte
C for RPG Programmers by Jennifer Hamilton

I think they are both out of print now. The IBM porting web site has 
some information that may prove helpful.

> I've got several years of C programming experience so I know the
> syntax and highlights.. I know there must be some kind of extensions
> (through external API's and such) that make tasks like creating
> subfile programs and such a breeze...

No such luck. In C you get to do everything the hard way (just like C 
programmers). You can call the external APIs listed in the a API 
section of the Information Centre (just remember that an API definition 
of CHAR(*) is not a char pointer but rather a pointer to a buffer) to 
get access to OS/400 objects, jobs, spooled files, and much other 
stuff. ILE C has record I/O extensions to handle native I/O. See the 
various _Rxxxx functions (e.g., _Ropen, _Rread, etc.).

For subfiles you would use:
        dbf = _Ropen( database file )
        dspf = _Ropen( display file )
        _Rformat( dspf, subfile )
        for (i=1; 1<= rcdCount; i++)
                _Rreadn( dbf, &record, rcdLen, .... )
                _Rwrited( dspf, &record, rcdLen, i )
        end-for
        _Rformat( dspf, subfile control )
        _Rwrite( dspf )
        _Rclose( dspf )
        _Rclose( dbf )

Modify the structure as appropriate for load-all, page-at-a-time, and 
page=size subfiles.

> Any help/links to docs would be greatly appreciated!

Only the C Programmer's Guide, C Language Reference, ILE C for OS/400 
Run-Time Reference, and the MI Library Reference.

They are very helpful but much of it makes sense only if you also know 
how the 400 operates.

Regards,
Simon Coulter.
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