Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!samsung!olivea!uunet!overload!dillon
From: dillon@overload.Berkeley.CA.US (Matthew Dillon)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: Dynamic struct's
Message-ID: <dillon.7453@overload.Berkeley.CA.US>
Date: 8 May 91 20:46:10 GMT
References: <52955@nigel.ee.udel.edu>
Organization: Not an Organization
Lines: 64

In article <52955@nigel.ee.udel.edu> DA3721A@acad.drake.edu writes:
>I am currently working on programming a fairly simple database in C. The
>program will allow the user to specify the number of fields in the database
>and their names. My question is: How does one dynamically declare a struct
>for accessing the correct number of fields? In other words, rather than doing
>
>	struct	{
>		int field1;
>		char field2;
>		int field3;
>	} record;
>
>I would like to do something like
>
>	struct	{
>		int field1;
>		char field2;
>		.
>		.
>		.
>		int fieldx;
>	} record;

    I generally do the following:

    struct {
	HeadInfo rc_Info;
	int rc_fields[XX];
    } record;

    Where XX can be 0, 1, or MAXFIELDS depending on how you implement
    the structure.  For example, if you use MAXFIELDS then you would
    allocate the structure like this:

	#include <stddef.h>

	struct record *rec = malloc(offsetof(struct record, rc_fields) + sizeof(rec->rc_fields[0]) * NumFields);

    If you use XX = 0 (int rc_fields[0]), assuming the compiler allows it,
    you can allocate the structure like this:

	struct record *rec = malloc(sizeof(struct record) + sizeof(rec->rc_fields[0]) * NumFields);

    If you use XX = 1 (int rc_fields[1]), which the compiler had better allow,
    you can allocate the structure like this:

	struct record *rec = malloc(sizeof(struct record) + sizeof(rec->rc_fields[0]) * (NumFields - 1));


    Once allocated, you can initialize/use up to NumFields of the array
    rc_field. I included the HeadInfo type just to show you how to allocate
    nominal structures with extensions.

>
>			Thanks in advance,
>			David Aschbrenner

					-Matt
--

    Matthew Dillon	    dillon@Overload.Berkeley.CA.US
    891 Regal Rd.	    uunet.uu.net!overload!dillon
    Berkeley, Ca. 94708
    USA