C question

From: Sean 'Captain Napalm' Conner <spc_at_conman.org>
Date: Thu Aug 1 17:16:00 2002

It was thus said that the Great Cini, Richard once stated:
>
> Hello, all:
>
> I have another C issue that I can't seem to see the answer for, so I
> thought I'd throw it out to the masses.
>
> I'm deep into the Altair 680b emulation project (base code done,
> working on telnet access) and I'm experiencing problems reading a file
> stream into the memory array. Here's snippets of the code:
>
> <SNIP>

  First off, ``//'' is not a valid ANSI C-89 comment marker. I think it is
now allowed under ANSI C-99 but I haven't read the spec yet. I'll assume
your compiler allows it, since you are using it, but just forwarned---you
can't assume an ANSI-C compiler supports it (and yes, I've had problems with
that before).

> byte *ppmem ; // pointer into memory array
> byte ucMem[3*65536] ; // actual memory-see defines below for usage

  Technically this should probably be:

  byte ucMem[3ul * 65536ul];

> // roms[] is an array of structures defining start-time loaded ROMs. Only
> one ROM now
> //(the monitor PROM), but flexible enough to allow bank-switched ROMs (not applicable
> //in the 680b but a leftover from the 6800 processor emulation code that was borrowed
> //for the project)
> //
> //MEM_xxx are defines for the offset into the ucMem array to various areas
> #define MEM_RAM 0
> #define MEM_ROM 0x10000
> #define MEM_FLAGS 0x20000

#define MEM_RAM 0ul
#define MEM_ROM 0x10000ul
#define MEM_FLAGS 0x2000ul

  (sorry to be so pedantic, but it helps narrow down the problems)

> // Read ROM directly to memory. This works.
> ppmem = &ucMem[roms[i].iROMStart + MEM_ROM] ;
> fread(ppmem, sizeof(byte), roms[i].iROMLen, pFH) ;
>
> //this doesn't work...
> //fread(&ucMem[roms[i].iROMStart + MEM_ROM], sizeof(byte), roms[i].iROMLen, pFH) ;
>
> //...nor does this
> //fread((char *)ucMem[roms[i].iROMStart + MEM_ROM], sizeof(byte), roms[i].iROMLen, pFH) ;
>
> So here's where I'm missing it. Under all three fread scenarios, the
> compiler doesn't throw a warning...they all compile cleanly. But only the
> first works.
>
> This sounds like one of those "What's the Bug" ads from DDJ. What am
> I missing?

  Make sure you are including stdio.h

        #include <stdio.h>

  in the file; that will bring in the prototype for fread():

        size_t fread(const void *ptr,size_t size,size_t nobj,FILE *stream);

  What type are

        roms[].iROMStart
        roms[].iROMLen

  If they aren't of type size_t, then you might want to change them to that,
and see if they work then. I might be tempted to say you are triggering a
compiler bug but you would have to check the generated code to make sure.
You don't need the cast to (char *) in the third fread() example (since the
pointer to fread() is void *) since the conversion will go:

        (void *((char *)ucMem...))

  If my suggestions don't help, see if you can see the code its generating.

  -spc (So far, have found one compiler bug (IRIX ANSI C compiler under IRIX
        4.0.x) and one Standard C library bug (GLIBC 2.mumble for the
        DEC Alpha architecture))
Received on Thu Aug 01 2002 - 17:16:00 BST