CODEWITHSUNDEEP
c++cmemoryunions
Simon Schernthanner
Simon Schernthanner

Reputation: 11

Issues with union and memory aligment

i'm currently working on a Port of Embedded Code (on a Freescale S12) so GNU and i hava a issue with unions. i have the following union 

typedef signed short        sint16;
typedef signed long         sint32;

typedef union
{
    sint32 Akku;
    sint16 AkkuHigh;
    sint16 AkkuLow;
} akku_type;

and i want to access the highest 2 byte of the union. The Problem is, that both AkkuHigh and AkkuLow have the same starting adress as Akku. It seems to be compiler specific. My Questions are: Is the there a Compiler Flag which changes the behaviour of the union? Can atribute((align (2))) help me?

Thank you in Advance

Upvotes: 0

Views: 121

Answers (2)

axiac
axiac

Reputation: 72425

The correct definition of the union can be found in this answer.

atribute(align(2)) will definitely help you if you compile this on a 32bit or 64bit architecture. Also, on 64bit sizeof(sint32) is 8 (64 bits).

Depending on the endian-ness of the architecture, you may need to swap AkkuHigh and AkkuLow.

Upvotes: 0

Pascal Cuoq
Pascal Cuoq

Reputation: 80355

Yes, all of Akku, AkkuHigh, AkkuLow have the same address. This is how unions work in C. By the look of it, you intended to make an union with a 32-bit member and a member that is a struct of two 16-bit members instead. What you wrote is not the way to achieve it. Try instead:

typedef union
{
    sint32 Akku;
    struct s {
      sint16 AkkuHigh;
      sint16 AkkuLow;
    } representation;
} akku_type;

Upvotes: 3

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