C Programming question, regarding structures/arrays

[Yohhan]

Question on C for anyone who can answer.

I have an array of unsigned chars representing bytes called memory[], and a structure called data. I want to be able to assign the address of the first item in memory[], to the structure so subsequent indeces (sp?) will be represented in fields. I've tried the following:

data *Ins_RR = memory;
data *Ins_RR = &memory[0];

But the compiler complains about both with the following error message:
-- RR *Ins_RR = &memory[0];
-- ^
-- cp.c(113) : Error: need explicit cast to convert
-- from: char (*)[1]
-- to : struct Register_to_Register*
-- --- errorlevel 1

So I try to cast:
data *Ins_RR = (data)memory;

But then it tells me:
-- RR *Ins_RR = (RR)memory;
-- ^
-- cp.c(113) : Error: illegal cast
-- from: char (*)[1]
-- to : struct Register_to_Register
-- --- errorlevel 1


Can anyone show me how to do this properly? Thanks ahead.

 

[BFG10K]

It won't work because your structure pointer is not the same type as the memory array. A data * pointer can only point to a data variable.

You need to make a pointer of the same type as memory inside the structure, eg:

struct data
{
unsigned char *pointer;
} data_instance;

And then

data_instance.pointer=memory;

Note that memory must already be defined as a static array, otherwise you'll need to use new if you want to create it dynamically as you assign it to the pointer in the structure.

I'm a little rusty but that should work OK.

 

[juiio]

data *Ins_RR = (data *)memory;

 

[Yohhan]

BFG10K:

If I use,
struct data
{
unsigned char *pointer;
unsigned char byteone;
unsigned char bytetwo;
// etc
} data_instance;

Will byteone and bytetwo be the first and second values of the array, if pointer is set to the array of unsigned characters?



juiio:
When using:
data *Ins_RR = (data *)memory;

It doesn't seem to let me access items in the structure, such as with:
Ins_RR->someobj

I get an error if I even try it. Why won't it let me access fields this way? How can I access fields?



Thanks!

 

[BFG10K]

Will byteone and bytetwo be the first and second values of the array, if pointer is set to the array of unsigned characters?

Absolutely not, unless you specifically say byteone = memory [0] and bytetwo = memory [1]. However if you do this then why not simply use two normal variables and forget about the structure and the pointers?

I must confess, I don't understand why you need a structure or pointers at all.

Also I'd be extremely careful in using the (data *) cast because even if it compiles it'll likely produce meaningless results. Casting usually only works reliably when you cast from a stronger variable to a weaker one (eg char to int or int to float). Casting the other way can cause a loss of data and casting to arbitrary types that you've invented can produce a wide range of different results.

 

[Yohhan]

It's a class project and we're making something of a simulated machine, with 3 instruction formats. Three different structures represent each of these formats, each unsigned char field specifying a byte in the instruction.

I read data representing instructions (in hex) from a file, and put them into an array called memory. Then I look at the first byte and determine what kind of instruction I'm dealing with. Once I know, I need to be able to point the appropriate structure to that location of the array, so I can access consecutive indices (bytes) as fields from within the structure.

ie one of my structures looks like this:
struct Register_to_Register {
unsigned char OC; // first byte (opcode)
unsigned char RS; // source register
unsigned char RD; // destination register
} RR;

I have two other structures with varying numbers of fields, representing the other two instructions. If the array contains:

FA
A9
15
B9
11

for example, then RR.OC would need to refer to hex FA, RR.RS to hex A9, RR.RD to hex 15. Another structure representing a different instruction may contain two more fields, and then use the next two bytes as well.

The problem is, I can't figure out how to make this work in C if I can't point a structure at an array.

Thanks.

 

[juiio]

Originally posted by: Yohhan

juiio:
When using:
data *Ins_RR = (data *)memory;

It doesn't seem to let me access items in the structure, such as with:
Ins_RR->someobj

I get an error if I even try it. Why won't it let me access fields this way? How can I access fields?

Thanks!

You will need to post more code. Was it a compile time error or a runtime error?

 

[Yohhan]

Strangely enough it's compiling now, and working. I have absolutely no idea what I changed. I think it's time to take a break... 🙂

Thanks for the help.