Sizeof returning wrong packet size ??

Quest101 · 03-09-2005, 08:36 PM

Hi

I am using the size of function to get the size of a struct, this size assists me in sending packets over a network

struct header
{
u_int32_t a
u_int16_t b

}

now the sizeof operator when returns a size of 8 bytes for the above header

ie sizeof( header ) == 8 Bytes

now if you do the math you will realise that the actual size should be

32bits + 16bits = 48bits which is actually 6 Bytes
****************************************************************

Now when i remove variable b from the struct

struct header
{
u_int32_t a
}

the sizeof operator now returns a correct value of 4 Bytes,

Any help in at least know what causes this error would be greatly appreciated

Thanx in Advance

YetAnotherDave · 03-10-2005, 08:37 AM

This is not an error. The compiler ( gcc ? ) will add extra bytes to a struct to make it fill a multiple of 4 bytes. My guess is that this is so that when you have an array of these structs each one begins on a 4 byte boundary. If you are using gcc, you can use pragmas to disable this padding as shown below.

Code:

#include <iostream>

using namespace std;

#pragma pack(push,1)
struct packed
{
    int a;
    short b;
};
#pragma pack(pop)

struct padded
{
    int a;
    short b;
};

main()
{
    cout << "sizeof(packed)=" << sizeof(packed) << endl;
    cout << "sizeof(padded)=" << sizeof(padded) << endl;
    return 0;
}

Output:
sizeof(packed)=6
sizeof(padded)=8