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From: johnsone@uxh.cso.uiuc.edu
Newsgroups: comp.sys.mac.programmer
Subject: (THINK) C string/Str255 help?
Message-ID: <53100015@uxh.cso.uiuc.edu>
Date: 15 Apr 90 00:38:57 GMT
Lines: 63
Nf-ID: #N:uxh.cso.uiuc.edu:53100015:000:2695
Nf-From: uxh.cso.uiuc.edu!johnsone    Apr 14 00:57:00 1990


Hi.  I'm very new to the Mac programming scene and my C is a bit rusty,
so I have been a bit confused.  Here is a section of code that I have been
working on (THINK C v4.0) and I don't understand exactly what's happening.
The exact strings in the STR# resource that this reads in are irrelevant
(they are about 20 characters long, if that makes any difference).

 > main()
 >   {Str255  myString;
 >     /* initialize toolbox, quickdraw, etc.; set up a window to draw in */
 >     GetIndString( &myString, 400, 1 );             /* Location (1) */
 >       MoveTo( 0, 20 );
 >     DrawString( &myString );                       /* Location (2) */
 >     MyChangeString( myString );                    /* Location (3) */
 >       MoveTo( 0, 60 );
 >     DrawString( myString ); }                      /* Location (4) */
 > 
 > MyChangeString( aString )
 >   Str255 aString;
 >   { GetIndString( aString, 400, 2 );               /* Location (5) */
 >       MoveTo( 0, 40 );
 >     DrawString( aString ); }                       /* Location (6) */

In the first call to GetIndString (1), it will accept the statement either
with or without the '&', and the first DrawString (2) then draws the
correct string, again with or without the '&'.

It doesn't seem to matter whether MyChangeString is called with the '&'
before myString or without it (3)

In the subroutine MyChangeString, in order for that DrawString to output
the correct string (6), the '&' must be in both (or neither) of the
GetIndString statement (5) and the DrawString statement (6).

But if the '&' is in both (5) and (6), then the final DrawString (4)
draws a corrupted string, whether or not a '&' is there.

My confusion lies especially in the fact that since the GetIndString is
defined in Inside Mac (I-468) as

    PRODEDURE GetIndString (VAR string: Str255, [etc.]

then (according to THINK C v4.0 manual page 120) the string should be
passed _with_ the '&' to send a pointer.

But then why does it not matter sometimes if the '&' is there?  Isn't
Str255 a pointer anyway?  Is it true to say, " Str255 = char * " ?

Similarly, how can one copy one Str255 to another?  Is it necessary to
use a function like strcpy?

Also, when I write my C subroutines, how should I pass Str255's when I
want to "pass by value"? and how should the routine (or function) be
declared?  What if I want to "pass by reference" (that is, like the
Pascal VAR pass)?  If anyone has some examples of code that demonstrate
this, I would greatly appreciate seeing it.

Thanks much.


Erik A. Johnson, Graduate Student        \ Internet:  johnsone@uxh.cso.uiuc.edu
Aeronautical & Astronautical Engineering  \
University of Illinois at Urbana-Champaign \