Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!att!cbnewsk!eob
From: eob@cbnewsk.ATT.COM (eamonn.j.o'brien)
Newsgroups: comp.arch
Subject: Re: Self-modifying code
Summary: pointers to functions
Message-ID: <1247@cbnewsk.ATT.COM>
Date: 21 Oct 89 19:27:58 GMT
References: <1080@mipos3.intel.com> <23062@cup.portal.com>
Organization: AT&T Bell Laboratories
Lines: 57

In article <23062@cup.portal.com>, cliffhanger@cup.portal.com (Cliff C Heyer) writes:
> in general you 
> have no way of knowing [program] address locations that your C 
> compiler will create

... except for pointers to functions.


The following is my attempt at self modifying C code:

--------------------------------------------------

foo()
{
        printf( "foo()\n" );
}

endfoo(){}

bar()
{
        printf( "bar()\n" );
}

endbar(){}
        
main()
{
        char* foo_ptr    = foo;
        char* endfoo_ptr = endfoo;
        char* bar_ptr    = bar;
        char* endbar_ptr = endbar;
        
        printf("before:\n");
        foo();
        bar();
        
        printf("modifying:\n"); 
        for( ; bar_ptr < endbar_ptr; ++foo_ptr,++bar_ptr )
                *foo_ptr = *bar_ptr;
                
        printf("after:\n");             
        foo();
        bar();
        
}

----------------------------------------------------

It calls foo() and bar(), overwrites foo() with bar(), and calls
foo() and bar() again. However on my system (AT&T Unix PC --
68000) it coredumps after printing "modifying:". Perhaps the
program memory is read-only.
-- 
Eamonn O'Brien          ohm!eob      eob@ohm.att.com
~~~~~~~~~~~~~~
I'm not speaking for the company, I'm just losing my mind.