[RESOLVED] Aligning pointers to struct

Hi,
I have two structs and I would like to assign a pointer from t_TMSG to an address in t_TAG.
But the problem is i dont get the correct address.

typedef struct {
WORD tag;
WORD opr;
DWORD unit;
LPCTSTR text;
} t_TAG;

typedef struct {
long tag;
LPCTSTR text;
} t_TMSG;


t_TAG* pTag = SomeFunction(); //assuming pTAg is loaded with values
t_TMSG *pTMsg = (t_TMSG*)(pTag + sizeof(WORD) + sizeof(WORD) ); // i get the wrong address here.


Please help

Probably because of default alignment (8 byte boundaries by default I think). Try wrapping your structure definitions in "#pragma pack" directives:

Code:

---


#pragma pack (push,1)
typedef struct {
WORD tag;
WORD opr;
DWORD unit;
LPCTSTR text;
} t_TAG;

typedef struct {
long tag;
LPCTSTR text;
} t_TMSG;

#pragma pack(pop)

t_TAG* pTag = SomeFunction(); //assuming pTAg is loaded with values
t_TMSG *pTMsg = (t_TMSG*)(pTag + sizeof(WORD) + sizeof(WORD) ); // i get the wrong address here.

---

Quote:

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Originally Posted by dacky

t_TMSG *pTMsg = (t_TMSG*)(pTag + sizeof(WORD) + sizeof(WORD) ); // i get the wrong address here.

---

According to the rules of pointer arithmetic, pTag + 1 points to the NEXT struct t_TAG, effectively adding a sizeof(t_TAG) to the pointer’s value.
Is that your intention?

@hoxsiew
hi I tried your idea of the #pragma directive but it didn't work, the pointer jumped like 48 bytes.
And when i tried to just add 1 to the pointer
t_TMSG *pTMsg = (t_TMSG*)(pTag+1); // this will jump 12 bytes, but 8 bytes before without pragma
any more ideas? thanks

@VladimirF
Yes, you are correct when i do pTag + 1, it jumps to 8 bytes and i think it is the size of t_TMSG.
I want to the address of pTag->unit to be in pTMsg. Is this really possible?

Thanks

Of course you get the wrong address, what you are doing makes no sense because the types inside are different. You might want to use a void* instead, or it could be what you want is not something valid in the first place. You should probably say what you are really trying to achieve if you want more help.

what do you mean no sense? does it make any sense to align pTag->unit to pTMsg? The types may be different but they have the same size. What I am trying to achieve is that I only need pTag->unit and pTag-txt and I want starting from the address of pTag->unit to point to pTMsg which has the same allocated memory size of pTag->unit and pTag-txt. I hope this is clear enough. thanks hope you can help

How about trying this:

Code:

---

t_TMSG *pTMsg = (t_TMSG*)(&pTag->unit);

---

This assumes you want pTMsg to point to the unit member.

Although in your definitions, you have unit as a DWORD, and in the t_TMSG structure it's a long (signed/unsigned).

Hope that helps.

wow it works!!!
thanks krmed!!! why didn't i think of that...i have the correct address now..thanks again