NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/7684

Text File | 1996-08-05 | 3.9 KB | 76 lines

Path: keats.ugrad.cs.ubc.ca!not-for-mail
From: [e-mail] (Kazimir Kylheku)
Newsgroups: comp.lang.c
Subject: Re: Processing a structure- how?
Date: 26 Feb 1996 13:19:23 -0800
Organization: Computer Science, University of B.C., Vancouver, B.C., Canada
Message-ID: <[e-mail]>
References: <[e-mail]>
NNTP-Posting-Host: keats.ugrad.cs.ubc.ca

In article <[e-mail]>,
Mike Collins <[e-mail]> wrote:
>Folks,
>
>I want to write a generic routine that will print data contained
>in structures on the screen (of a DOS-based PC, but I don't think that
>comes into it). What I want is to be able to pass a structure  or
>structure pointer to the routine, which will then be able to find out what
>the contents of the structure are, so that if I pass a customer structure
>containing surname, forename, address, phone number, I will be able to
>print this. If I pass a different structure containing catalog data, I
>will be able to print Item, category, bin location, etc.
>
>Is there a way for a function to find out how many fields are in a
>structure and what kind of variable each is (int, char[], etc)? I reallise
>that this information is known to the compiler and not to the compiled
>program as such, but can the compiler be made to embed information of this
>kind into a program?

Yes and no. You need to develop your own system of augmenting a structure
declaration with additional info that can be used by special routines.

One way is to invent a data definition language, and write a data definition
compiler. You write your data object specifications in this language, and the
definition compiler will generate C language headers containing the requisite
struct declarations, and will perhaps declare some sort of meta-information
structures that will tell certain library routines extra information about the
structure fields. The compiler can even automatically generate C code for
filter routines for performing certain structure-specific operations on the
data (such as file input or output).

>The effect would be like a printf() statement (I don't mind sending it a
>formatting string), but with only a structure or structure pointer as a
>data argument.

An automatically generated filter routine would do this for you. The
challenging part is writing the processor. A robust data definition compiler
would parse a language that allows nested structure definitions.

I have worked with a number of such programs, designed for various purposes.
Sun's ``rpcgen'' interface compiler will generate C header files and
client/server skeleton/stub code from structure and function definitions. You
fill in the blanks and have a client/server system working. (SunSoft's latest
project, the Spring operating system, has a new interface compiler for
language-independent OO client/server programming). The rpcgen compiler
generates filter routines, for each structure, for serializing it into an
external, machine-independent representation, and for deserializing back from
that representation. A recent project of mine relies heavily on this function
of rpcgen---we have ported the XDR library to Windows/Winsock for the purpose
to support client software on that platform.

I've also worked with some database toolkits with such generators; for example
the Typhoon library, which  uses the data definition compiler to generate a
description of records, fields and their relationships which the database
engine understands. The database engine can then work with the resulting raw C
structures, which are directly written to the indexed database files. It's
fast, but I would not recommend this product except as an example of what you
can do with a little automatically-generated C.

There is probably freely available software that will do what you are looking
for, by the way, so unless you are skilled in the use of compiler construction
tools and can whip this thing out in less than a day, I'd do a little browsing
first.
--