Re: [avr-gcc-list] Struct problem

[David Brown]

> Thanks for the tip.
> The buffers need to be Global, since it needs to be accessible in
different
> modules. My understanding is that malloc is for allocating dynamic memory.
> Anyway, I will read my C book for clue. Thanks for pointing the direction.
>
> Eric Fu

As others have said, you can use malloc for allocating a globally accessible
lump of memory - basically you keep a pointer to it in your structure, and
if other modules can see the pointer, they can access the memory it points
to.  However, there are two rules regarding dynamic memory in small embedded
systems - 1) Don't use dynamic memory in small embedded systems, and 2) if
you really do have to use dynamic memory in small embedded systems, don't
use malloc() and free(), but write your own system.


The most common way to handle structures like your one is to have:

#define txBuffSize 16
#define rxBuffSize 16
typedef struct UART
{
    volatile byte RxHead;
    volatile byte RxTail;
    volatile byte TxHead;
    volatile byte TxTail;
    byte TxBuf[txBuffSize];
    byte RxBuf[rxBuffSize];
} UART;


You need a *very* good reason for an avr-based system to start needing
variable-sized buffers.

David



>
>
> ----- Original Message -----
> From: "Brian Cuthie" <address@hidden>
> To: "'Eric Fu'" <address@hidden>; <address@hidden>
> Sent: Saturday, January 31, 2004 12:22 PM
> Subject: RE: [avr-gcc-list] Struct problem
>
>
> >
> > The problem, quite simply, is that because the size of TxBuf[] isn't
> > specified, the offset within the struct to RxBuf[] can't be computed at
> > compile time (which it needs to be).
> >
> > To do what you want you'll need to change the struct so that instead of
> > arrays, TxBuf and RxBuf are pointers. Then malloc the memory you need
and
> > store the addresses in TxBuf and RxBuf respectively.
> >
> > You might want to re-read a few chapters in your C book.
> >
> > -brian
> >
> > > -----Original Message-----
> > > From: address@hidden
> > > [mailto:address@hidden On Behalf Of Eric Fu
> > > Sent: Friday, January 30, 2004 8:05 PM
> > > To: address@hidden
> > > Subject: [avr-gcc-list] Struct problem
> > >
> > > Hi All,
> > >
> > > I'm trying to declare a Structure for UART, so that it can be
> > > used for more than one UARTs such as:
> > > typedef struct UART
> > > {
> > >  volatile byte RxHead;
> > >  volatile byte RxTail;
> > >  volatile byte TxHead;
> > >  volatile byte TxTail;
> > >  byte TxBuf[],RxBuf[];
> > > }UART;
> > > extern UART uart0;
> > > extern UART uart1;
> > >
> > > By doing this way, I can assign different buffer size to
> > > different UART such as:
> > > byte uart0.RxBuf[RX_BUFFER_SIZE0];
> > > byte uart0.TxBuf[RX_BUFFER_SIZE0];
> > > byte uart1.RxBuf[RX_BUFFER_SIZE1];
> > > byte uart1.TxBuf[RX_BUFFER_SIZE1];
> > >
> > > However, I get the following compile error:
> > > UART.h:47: error: flexible array member not at end of struct
> > >
> > > Could anyone give me a hint on How to tell the compiler to do this?
> > > Thanks
> > >
> > > Eric Fu
> > > _______________________________________________
> > > avr-gcc-list mailing list
> > > address@hidden
> > > http://www.avr1.org/mailman/listinfo/avr-gcc-list
> > >
> >
> >
>
>
>
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