Path: utzoo!utgpu!attcan!lsuc!ncrcan!ziebmef!ross
From: ross@ziebmef.uucp (Ross Ridge)
Newsgroups: comp.sys.cbm
Subject: Re: C Language for Commodore 64/128
Summary: zero-page variables
Keywords: Looking for C compiler
Message-ID: <1988Oct8.021541.10093@ziebmef.uucp>
Date: 8 Oct 88 06:15:40 GMT
References: <1520@nunki.usc.edu> <1825@uokmax.UUCP> <6135@watcgl.waterloo.edu>
Reply-To: ross@watcsc.uucp (Ross Ridge)
Organization: Ziebmef Public Access BBS/Unix
Lines: 62

In article <6135@watcgl.waterloo.edu> bphollander@watcgl.waterloo.edu (Brian Hollander) writes:

>When a new function is invoked it clears out however many bytes of
>zero page it needs by pushing the old stuff to the C stack, not the CPU
>stack.  If you pass the address of a zero page stored variable to one
>of YOUR functions (library routines do not clear out that part of zero
>page) and that function uses enough zero page variables to push the 
>old one to the stack, then the address is no longer valid.  It still
>points to the zero page address where it used to be.
>
>This example should demonstrate the problem.
>
>main()
>{
>  int value;
>
>  value = 5;
>  foo(&value);
>  printf("value = %d\n",value);
>}
>
>foo(value)
>int *value;
>{
>  int var1,var2,var3; /* a bunch of locals only one is needed here */
>
>  *value = 20;
>}
>
>When this is run value should be 20 but it will be 5.
>
>I have not tested this specific example but others that are similar.  This
>means that this example will work fine on someone else's computer because:
>1 - They are using a version where this bug has been fixed.
>2 - My C64 was having a bad day (occurs very often)
>3 - I'm seeing things.

This realy isn't so much a bug as an undocumentated feature. C Power (all
versions) has in effect a new storage class (which I like to call) "zero-page".
By default the first 32 bytes of (what would be automatic) variables
in a function have this class. "Zero-page" storage-class is sort of
cross between register and automatic storage classes. If you think of these
varibles this way, you can go along way in avoiding problems

>If anyone else comes across useful information (undocumented functions,
>hard to find bugs) PLEASE post it to the net.  It may save me some long
>sleepless nights!

Two solutions to the above problem:

       Declare the varible explicitly as automatic (ie. auto int a;)
       Compile with "cc -a file.c"


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