Newsgroups: comp.sys.apple2.programmer
Path: blue.weeg.uiowa.edu!news.uiowa.edu!uunet!munnari.oz.au!labtam!news
From: philip@labtam.oz.au (Philip Stephens)
Subject: Re: Question about ORCA/C, ORCA/M subroutines,
Message-ID: <1994Sep1.004708.18587@labtam.labtam.oz.au>
Sender: news@labtam.labtam.oz.au (Net News system)
Reply-To: philip@labtam.oz.au
Organization: Labtam Australia Pty. Ltd.
References: <hexmanCvELJD.DF8@netcom.com>
Date: Thu, 1 Sep 1994 00:47:08 GMT
Lines: 218

Tony Morales writes:

>I have a question dealing with ORCA/C, ORCA/M subroutines, and strings,
>and why I cannot get these three things to get along with each other.
>Let's say the following is an assembly subroutine which I will link in
>with my main C source file:
>
>StringData	START
>String1	ENTRY
>		str 'This is String #1'
>String2 	ENTRY
>		str 'This is String #2'
>
>StringTable 	ENTRY
> 		DC I4'String1' ;point this to the first string
>		END
>
>OK, this seems fine from assembly.  Now, in my C program, I do this:
>
>extern char String1, String2; 	/* these are pointers to char, but */
>	 			/* work as pointers to strings */
>extern void *StringTable; 	/* define as pointer to pointer */

You have made several mistakes in the above code!

1. You _cannot_ use the 'str' directive to create a string for C.
   Strings in C do not start with a length byte; they are instead
   terminated with a zero byte.  Hence you should have written:

	String1 ENTRY
		DC c'This is String #1',h'00'
	String2 ENTRY
		DC c'This is String #2',h'00'

2. If you want StringTable to be an _array_ of pointers to strings,
   then you better allocate space for more than one pointer in that
   array!!!

3. String1 and String2 are an array of characters.  Hence you must
   declare them in C as:

	extern char String1[], String2[];

   What you declared in your code above were two character variables,
   not two character strings!!!

4. If StringTable is to be an array of pointers to strings, you must
   declare it as:

	extern char *StringTable[];

   What you declared in your code above was a pointer to a void data
   type i.e. a pointer to something, but you don't know _what_ that
   something is.

5. If you haven't do so already, you must tell ORCA/M that the labels
   you are declaring in your assembler program are case sensitive.
   i.e. you must write at the top of your file:

	CASE ON

   If you don't do this, ORCA/M will convert String1 to STRING1 and
   so on, and because C is case sensitive, it won't find the labels
   you declared in your assembler source!

			* * * * * * * *

It seems to me that you're confused about the concept of pointers
and strings in C, since that's where most of your problems lie.  Let
me see if I can clear up the confusion for you.

			* * * * * * * *

The C declaration:

	char Character;

means that Character is a character.  It is _not_ a pointer to a
character.  In assembler, the equivilant declaration would be:

	Character DS 1

i.e. you are reserving one byte of space to hold a character.

If you assign an initial value to a character variable in C:

	char Character = 'A';

then the equivilant in assembler would be:

	Character DC c'A'

			* * * * * * * *

The C declaration:

	char String[];

means that String is an _array_ of characters of unknown length.  If you
assign a value to this array:

	char String[] = "This is a sample string";

then in assembler you would write:

	String DC c'This is a sample string',h'00';

Now, when you write:

	String[1]

you are asking for the first character in the array:

	'T'

But when you write:

	String

you get a _pointer_ to the character array!  This is often a point of
confusion to people trying to learn C.
	 
			* * * * * * * *

The C declaration:

	char *String_ptr;

means that String_ptr is a _pointer_ to a character.  It is also often
used as a pointer to an _array_ of characters (a string).  But the point
is that String_ptr is a 4-byte integer that holds the address of the
character or string that it is pointing to.  In assembler, the equivilant
declaration would be:

	String_ptr DS 4

If you declare this in C:

	char *String_ptr = "This is a sample string";

Then in assembler you would do this:

	String	   DC c'This is a sample string',h'00'
	String_ptr DC i4'String'

			* * * * * * * *

The C declaration:

	char **String_table;

means that String_table is an _array_ of character/string _pointers_.
It can also be written:

	char *String_table[];

to make it clear that it is array of pointers.  If you then declare in
C:

	char *String_table[] = {"first string", "second string"};

then in assembler you would write:

	String1	     DC c'first string',h'00'
	String2	     DC c'second string',h'00'
	String_table DC i4'String1, String2'

			* * * * * * * * 

>What I want to do, is first define a number of strings in assembly,
>and then define a table of pointers to these strings, (also in assembly).

  Okay, you should be able to do this now based on what I've told you
above, but let's show another example just to make things crystal clear.
Here's an example of a number of C strings defined in assembly, with a
array of pointers to each of these strings:

  In assembly:

			CASE ON
	
	StringData	START

	String1		ENTRY
			DC c'This is string #1',h'00'
	String2		ENTRY
			DC c'This is string #2',h'00'
		
	StringTable	ENTRY
			DC i4'String1, String2'
			END

  In C:

	extern char String1[];
	extern char String2[];

	extern char *StringTable[2];

  Now beware that StringTable has being declared to have _exactly_ two
  elements in this example.  If you want StringTable to be bigger, you
  must reserve space for it in the assembler code:

	StringTable	ENTRY
			DC i4'String1, String2'
			DS 4*number_of_additional_elements

  Meaning in C you'd have:

	extern char *StringTable[2 + number_of_additional_elements];



--
=========== Philip Stephens, Systems Programmer, Labtam Australia ============
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