[Sdcc-user] a better way to define a pointer to structure ?

[roelof 't Hooft]

Hi all,

It is me again with a pointer issue :-(
(that is an ongoing battle with me)

I would like to know if there is a better way to do
what I have done in the next source code :


extern.h :

#define _cast_message (volatile __xdata struct _message *)

typedef struct _message
{
        uint8_t address;
        uint8_t trx_bytes;
        uint8_t rcv_bytes;
        uint8_t control;
        uint8_t buffer[16];
};
extern volatile __xdata struct _message *message;

uint16_t externe_init(uint8_t);



extern.c :

volatile __xdata struct _message *message;
volatile __xdata uint8_t memoryspace[16 + 4];

uint16_t externe_init(uint8_t databyte)
{
        message = _cast_message &memoryspace[0];
        ...;
        message->address = databyte;
        message->buffer[10] = 0x96;
        ...;
        return (uint16_t)&message->address;
}



main.c :

void main(void)
{
        message = _cast_message externe_init(0x30);
        ...;
}



I have a structure which I would like to access in other c files.
To have an actual piece of RAM I needed to create some with the
memoryspace buffer and get that address in the pointer message
(a poor man's malloc function).
After some hacking I also got the address exported to the main
function. So far so good.
I know this is not good coding but I am unable to find a better
way of doing this.

If I do not get the address of memoryspace into message it will
be zero. I can understand that but I defined the memory as 0x1000
bytes large starting at 0x7000. What is going on there ?
I expected the compiler/linker to complain about that.

sdcc version :

SDCC : mcs51 3.2.1 #8035 (11 Jul 2012) (Linux)


command line to compile :

sdcc -D_87c591 -c --model-small --code-size 0x7000 --xram-size 0x1000
--xram-loc 0x7000 --iram-size 0xff extern.c
sdcc -D_87c591 -c --model-small --code-size 0x7000 --xram-size 0x1000
--xram-loc 0x7000 --iram-size 0xff main.c
sdcc --code-size 0x7000 --xram-size 0x1000 --xram-loc 0x7000 --iram-size
0xff main.rel extern.rel
packihx main.ihx > main.hex


Thanks,

roelof


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