sizeof() function error ???

Quest101 · 03-09-2005, 08:31 PM

Hi

I am using the size of function to get the size of a struct

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

Dave Kelly · 03-09-2005, 08:52 PM

Quoting for K & R: Section 6.4 Pointers to Structures.

Quote:

Don't assume, however, that the size of a structure is the sum of the sizes of its members. Because of alignment requirements of different objects, there may be unnamed "holes" in a structure. Thus, for instance, if a char is one byte and an int four byte, the structure

Code:

structure {
               char c;
                int i;
               };

might well require eight bytes, not five. The sizeof operator returns the proper value.

Quest101 · 03-09-2005, 09:03 PM

Hey Dave,
That pretty much settles that , thanx for the reference. Dont know if you would have a suggestion on something else i could use to store the values ??

The struct is actual the header for packets that i am sending , i store the header/struct in a buffer which contains the payload and send it off as a packet using winsock.

Dave Kelly · 03-09-2005, 09:23 PM

Store which values?

Quest101 · 03-09-2005, 09:34 PM

the header structure is suppose to store two values,

u_int32_t timestamp and
u_int16_t sequencenumber

itsme86 · 03-09-2005, 10:16 PM

Is 2 bytes really so painful? I mean, it would take more than 500,000 instances of the struct just to take up 1MB. What is it that you're using this for?

If size is really an issue you could do something like:

Code:

char *header = malloc(sizeof(u_int32_t) + sizeof(u_int16_t));
*(long long *)header = timestamp << 16;
*(long long *)header |= sequencenumber & 0x1F;

Now you've got both numbers in a single 6-byte buffer.

Quest101 · 03-09-2005, 10:23 PM

Hey ,
under normal situations i would be thinking the exact samething , why the big deal over 2 Bytes

however this project is about packet header compression , the the smaller i get the the header the , the more successfull my header compression will be .

thanx alot for the sample code , seems like it should work , I will try and implement it now

Regards

Dave Kelly · 03-10-2005, 09:28 PM

The word 'packet' has creeped into the conversation. That could make this a different animal.

Does the original assignment require you to use a structure?

aluser · 03-10-2005, 09:34 PM

If you can depend on gcc extensions, you could use gcc's packed attribute. example:

Code:

#include <stdio.h>
#include <stdint.h>

struct foo {
        uint32_t a;
        uint16_t b;
};

struct bar {
        uint32_t a;
        uint16_t b;
} __attribute((packed))__;

int main()
{
        printf("%u %u\n", sizeof(struct foo), sizeof (struct bar));
        return 0;
}

This outputs "8 6".

aluser · 03-10-2005, 09:35 PM

but if you do that, be very careful about performing operations on members of the struct. They may fail with something like "bus error", IIRC.