struct and union

[grcunning]

we are using a struct and a union to map a 32bit microcode instruction to its corresponding bit fields. We have used this method before and it worked fine, except we did it in C++ before, and this one has to be in C.I've removed everything from the program except for the struct for testing purposes, and for some reasone it keeps telling me in Visual Studio:
error C2061: syntax error : identifier 'microFields'
or in Linux:
error: expected specifier-qualifier-list before 'microFields'
and
error: two or more data types in declaration specifiers

#include <stdio.h>


struct microFields
{
	unsigned int addr:9;
	unsigned int cond:2;
	unsigned int wr:1;
	unsigned int rd:1;
	unsigned int mar:1;
	unsigned int alu:3;
	unsigned int b:5;
	unsigned int a:5;
	unsigned int c:5;
};
union micro
{
	unsigned int microCode;
	microFields code;
};

int main(int argc, char* argv[])
{
	micro test;

	return 0;
} 

am i missing an include or something? The syntax is exactly what we used before, i can't figure out why it won't compile.

[helios]

In C++, unions can only contain pointers.

[pjwarez]

If I remember correctly, I thought you either had to typedef a struct/union or explicitly say what type it was, like this...

typedef struct
{
	unsigned int addr:9;
	unsigned int cond:2;
	unsigned int wr:1;
	unsigned int rd:1;
	unsigned int mar:1;
	unsigned int alu:3;
	unsigned int b:5;
	unsigned int a:5;
	unsigned int c:5;
}microFields;

typedef union
{
	unsigned int microCode;
	microFields code;
}micro;

int main(int argc, char* argv[])
{
	micro test;

	return 0;
} 

If not, then you'd have to do something like this..

struct microFields
{
	unsigned int addr:9;
	unsigned int cond:2;
	unsigned int wr:1;
	unsigned int rd:1;
	unsigned int mar:1;
	unsigned int alu:3;
	unsigned int b:5;
	unsigned int a:5;
	unsigned int c:5;
};
union micro
{
	unsigned int microCode;
	struct microFields code;
};

int main(int argc, char* argv[])
{
	union micro test;

	return 0;
} 

Try it... I might be wrong. I just had a midterm and my brain is tired!

[firedraco]

^That is only in C. In C++, it creates the typedef for you automatically.

[chrisname]

@pjwarez;

You can also do

typedef struct myStruct myStruct_t;

/* in main() */
    myStruct_t* pMyStruct;

but I think a simple
struct myStruct stMyStruct is more readable anyway.

[jsmith]

myStruct stMyStruct;

is all that C++ requires. Prefixing with "struct" (or "union") is a requirement in C. In C++ it's just annoying.

[pjwarez]

Prefixing with "struct" (or "union") is a requirement in C. In C++ it's just annoying.

Yes... that's the whole point. In grcunning's original post, he's trying to compile some old code in C, not C++

grcunning

we did it in C++ before, and this one has to be in C

At least that what it looks like to me.

[Duthomhas]

As yet, no one has mentioned that the thing the OP is trying to do is dangerous.

The order of bitfields in a structure, in both C and C++, is (you'd better sit down) not defined. That's right, upgrade your compiler and your program could break. The whole union thing is therefore a mistake.

You are much better off writing a couple of routines to encode and decode the bitfields:

unsigned int micro_encode( struct microFields fields )
  {
  unsigned int result = 0;
  result |= (fields.addr & 0x1FF) << 23;
  result |= (fields.cond & 0x003) << 21;
  result |= (fields.wr   & 0x001) << 20;
  result |= (fields.rd   & 0x001) << 19;
  result |= (fields.mar  & 0x001) << 18;
  result |= (fields.alu  & 0x007) << 15;
  result |= (fields.b    & 0x01F) << 10;
  result |= (fields.a    & 0x01F) << 5;
  result |= (fields.c    & 0x01F);
  return result;
  }

struct microFields micro_decode( unsigned int code )
  {
  ...
  }

It costs you nothing more than a few lines of code and saves you against fragile code that will break with your next compiler upgrade, etc.

BTW, I don't know anything about microcode. I could very well have ordered the bitfields the wrong way in the above example. That's easy enough for you to fix.

Hope this helps.