RE: [avr-gcc-list] Struct problem

[Brian Cuthie]

Yeah, but the problem is that he can't dynamically change the buffer size
this way, which is I think what he was trying to do.

Eric, I think you're learning the hard way about the nature of embedded
development, especially when using very small processors with limited
resources. There's often a way you'd *like* to do something, and then
there's the way you end up having to do it to avoid eating up too much
memory.

My advice is that you use fixed sized buffers (as Dave suggests). You can
either make them part of your structs directly (like your original design,
except you'll need to specify the size of the arrays) or you can do as Dave
has and have the memory blocks dynamically assigned to pointers in the
structs. Which makes the most sense depends on what you're really trying to
accomplish, which right now is still somewhat of a mystery.

Good luck.

-brian

> -----Original Message-----
> From: address@hidden 
> [mailto:address@hidden On Behalf Of Dave Hylands
> Sent: Saturday, January 31, 2004 1:17 AM
> To: Eric Fu; address@hidden
> Subject: RE: [avr-gcc-list] Struct problem
> 
> You don't need to use malloc. You could just put the pointers 
> in you UART struct and still use global variables.
> 
> byte rxBuf[ RX_BUFFER_SIZE0];
> byte txBuf[ TX_BUFFER_SIZE0];
> 
> uart0.pRxBuf = rxBuf;
> uart0.pTxBuf = txBuf;
> 
> If you don't need to use malloc, then this will save you the 
> 500 bytes.
> 
> *(uart0.pRxBuf+TmpTail) is the same as uart0.pRxBuf[ TmpTail ];
> 
> Personally, I find the 2nd form easier to read.
> 
> --
> Dave Hylands
> Vancouver, BC, Canada
> http://www.DaveHylands.com/
> 
> 
> > -----Original Message-----
> > From: Eric Fu [mailto:address@hidden
> > Sent: Friday, January 30, 2004 9:44 PM
> > To: address@hidden
> > Subject: Re: [avr-gcc-list] Struct problem
> > 
> > 
> > It works. That's great.
> > But it added an unbelievable 500Bytes! It seems the code size under 
> > AVR GCC varies a lot for different implementations of the 
> same task. 
> > In my case, what I changed was to access  UART buffer from 
> arrays to 
> > pointers with malloc. I redefined:
> >     ...
> >   byte *pTxBuf,*pRxBuf;
> >      ...
> > Allocated memory as:
> > uart0.pRxBuf = malloc(RX_BUFFER_SIZE0);  uart0.pTxBuf = 
> > malloc(TX_BUFFER_SIZE0);
> > 
> > Used such as
> > return *(uart0.pRxBuf+TmpTail);           // Return data
> > There are no other changes apart from the change to access 
> the buffer. 
> > Is this normal?
> > 
> > Eric Fu
> > 
> > ----- Original Message -----
> > From: "Brian Cuthie" <address@hidden>
> > To: "'Eric Fu'" <address@hidden>; <address@hidden>
> > Sent: Saturday, January 31, 2004 1:59 PM
> > Subject: RE: [avr-gcc-list] Struct problem
> > 
> > 
> > >
> > > You can use malloc() to create your buffers and they'll still be 
> > > globally accessible so long as the pointers you store them in are 
> > > global.
> > >
> > > -brian
> > >
> > > > -----Original Message-----
> > > > From: Eric Fu [mailto:address@hidden
> > > > Sent: Friday, January 30, 2004 9:52 PM
> > > > To: address@hidden; address@hidden
> > > > Subject: Re: [avr-gcc-list] Struct problem
> > > >
> > > > 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
> > > >
> > > >
> > > > ----- 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
> > > > > >
> > > > >
> > > > >
> > > >
> > > >
> > >
> > >
> > 
> > 
> > 
> > _______________________________________________
> > avr-gcc-list mailing list
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> > http://www.avr1.org/mailman/listinfo/avr-gcc-> list
> > 
> > 
> 
> 
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