sizeof gives wrong results?

[Null]

Hello, I have a question about sizeof() operator"

struct s1
{
short y;
int *p;
};

int main()
{

cout <<"size of ss1::y : "<<sizeof(s1::y)<<endl; // 2 bytes
cout <<"size of ss1::p : "<<sizeof(s1::p)<<endl; // 4 bytes
cout <<"size of struct : "<<sizeof(s1)<<endl;     // 2+4=8bytes?

}

Why sizeof(s1) is 8 bytes? Shouldn't it be 6 bytes?
Thanks for help.

[Disch]

padding.

structs are often padded to DWORD sizes to make indexing more efficient.

[Null]

Are these padding bytes placed at the beginning or at the end of structure?

[helios]

At the end.

[jsmith]

No, they are placed anywhere the compiler feels like putting them.

Most likely in your case the bytes are between y and p so that p is word-aligned.

[DrakeMagi]

That nice to know.

tried many other: i'm using gcc .

struct s1
{
    char x;
    int i;
};

size = 8

struct s2
{
    char x;
    int i;
    char z;
};

size = 12

struct s3
{
    char x;
    char z;
    int i;
};

size = 8

[maikel]

Because the compiler have to make sure, that the integer var lies on an address that is a multiple of 4 (32bit) or something like this.

[moorecm]

While you're at it, note that an empty class has a size of [at least] 1.

[kbw]

All compilers provide a #pragma to change the alignment.

[Null]

Thanks.