Question

I have the following struct declaration and typedef in my code:

struct blockHeaderStruct {
    bool allocated;
    unsigned int length;
};
typedef struct blockHeaderStruct blockHeader;

When I do sizeof(blockheader), I get the value of 4 bytes back, but when I do sizeof(struct blockHeaderStruct), I get 8 bytes.

Why is this happening? Why am I not getting 5 back instead?

Answer 1

Looking at the definition of your struct, you have 1 byte value followed by 4 byte Integer. This integer needs to be allocated on 4 byte boundary, which will force compiler to insert a 3 byte padding after your 1 byte bool. Which makes the size of struct to 8 byte. To avoid this you can change order of elements in the struct.

Also for two sizeof calls returning different values, are you sure you do not have a typo here and you are not taking size of pointer or different type or some integer variable.