Question

Say I have a C structure like:

typedef struct {
UINT8  nRow;
UINT8  nCol;
UINT16 nData; } tempStruct;

Is there a way to put all of those 3 members of the struct into a single 32-bit word, yet still be able to access them individually?

Answer 1

What about just referring to it as a UINT32? It’s not like C is type-safe.

tempStruct t;
t.nRow = 0x01;
t.nCol = 0x02;
t.nData = 0x04;

//put a reference to the struct as a pointer to a UINT32
UINT32* word = (UINT32 *) &t;

printf("%x", *word);

You can then get the value of the struct as a 32-bit word by dereferencing the pointer. The specifics of your system may matter, though…if I run this on my machine, the value of word is 0x00040201—that is, the fields are in reverse order. I don’t think that’s necessarily going to be the case if you’re trying to serialize this to another system, so it’s not portable.

If you want to actually store it as a 32-bit integer and then refer to the fields individually, why not

UINT32 word = 0x01020004;

and then somewhere else…

UINT8* row(UINT32 word) {
    return (UINT8 *) &word + 3;
}

UINT8* col(UINT32 word) {
    return ((UINT8 *) &word) + 2;
}

UINT16* data(UINT32 word) {
    return ((UINT16 *) &word);
}

Macros will facilitate portable endianness.