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Hi,

I am using TMS320F28335 controller and i am compiling using code composer. I am tyring to use sizeof() in my code, and even after removing structure padding i am finding an unexpected behaviour of sizeof() operator.

I defined 3 structures as shown below.

Structure A:

typedef struct A_Tag
{                  // Var No      Add Loc
                   //----------------------------
    int   uVar1;     //1:          0
    int   uVar2;     //2:          1
    int   uVar3;     //3:          2
    int   uVar4;     //4:          3
    int   uVar5;     //5:          4 (because of structure padding float will not be allocated at 5)
    float floatVar6; //6:         6-7
}A_Struct;

Structure B:

typedef struct B_Tag
{                    // Var No      Add Loc
                     //----------------------------
    float floatVar1; //   1:         0 - 1
    int   uVar2;     //   2:          2
    int   uVar3;     //   3:          3
    int   uVar4;     //   4:          4
    int   uVar5;     //   5:          5
    int   uVar6;     //   6:          6 (Size should be 7, but its showing as 8)
}B_Struct;

Structure C:

typedef struct C_Tag
{              // Var No      Add Loc
               //----------------------------
    int uVar1; //   1:          0
    int uVar2; //   2:          1
    int uVar3; //   3:          2
    int uVar4; //   4:          3
    int uVar5; //   5:          4 (size of the structure is 5)
}C_Struct;

1. As shown in above example, because of padding/structure alignment, the size of structure A will be 8.
2. In structure B to avoid extra space, i rearranged the structure members. With this the members are allocated sequentially without any padding. So the size should be 7. But to my strange, the size of structure B is still 8. Extra variable is added as part of the structures. Please look into the attached document for more details.
3. To cross verify i created one more Structure C. All the members are allocated sequentially and the size of the structure is as expected and it is 5.

My Query is:

1. How come structure B is 8 instead of 7.
2. If structure B is 8 then structure C should also be 6, but it is as expected and it is 5.

For more details please see attached document and map file.

Detail snapshots:

7484.sizeof()_issues.doc

Map file

4265.rtl_test.txt

Thanks & Regards

Vishnu Beema