CODEWITHSUNDEEP
cstruct
janovak
janovak

Reputation: 1577

Size of a struct containing structs containing bitfields

I have a struct that looks something like the following:

typedef unsigned __int16 UINT16;
typedef unsigned __int64 UINT64;

struct Example {
    struct {
        UINT64 var1 : 5;
        UINT64 var2 : 2;
        UINT64 var3 : 29;
        UINT64 var4 : 23;
        UINT64      : 5;
    };
    struct {
        UINT16 var5 : 4;
        UINT16 var6 : 2;
        UINT16      : 10;
    };
};

I was expecting sizeof(struct Example) to return 10, but it returned 16. I have no clue why this is happening, and I would appreciate any input on the matter.

Upvotes: 1

Views: 179

Answers (1)

dbush
dbush

Reputation: 225507

This is due to alignment of the fields.

The first internal struct uses a uint64_t as the underlying type for the bitfield, while the second uses a uint16_t as the underlying type.

In order for the struct as a whole to be properly aligned, it must be aligned at the same offset as that required by the largest "base" internal field, which in this case is a uint64_t and therefore requires 8 byte alignment.

Without such alignment, if an array of this type is created then not all array elements will start on an 8 byte offset. So 6 padding bytes are added at the end to ensure proper alignment.

Upvotes: 6

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