[Solved] Help!! Structure is bigger than expected!

[Dave007]

Hello Qt Community!

I want to program a simple program that sends raw arp-packets through the network to find out wich ip numbers are in unse and wich not.
My problem is, that after i compile my project the array 'target_mac' is 2 bytes bigger than i defined it.
When I define its leght as 6 bytes 2 where add.
When I define it with a lenght of 5, 3 bytes were add after the array.
And with a lenght of 4 the array is then 4 bytes big.

I think I have to add or remove a compiler option, but I have no Idea since I haven't had this problem yet.

Here is the ARP Srtucture:
@struct eth_hdr {
unsigned char dest[6];
unsigned char src[6];
unsigned short type;
};

typedef struct _arp_hdr {
struct eth_hdr ethhdr; // Ethernet header
unsigned short hwtype; // Hardware type
unsigned short proto; // Protocol type
unsigned char hlen; // Hardware Address Length
unsigned char plen; // Protocol Address Length
unsigned short opcode; // Opcode
unsigned char sender_mac[6]; // Source hardware address
unsigned long sender_ip; // Source Ip address
unsigned char target_mac[6]; // Destination hardware address
unsigned long target_ip; // Destination Ip address
} arp_hdr;@

And here is a Picture of the memory while executing the program.
The 2 bytes wich were added are marked red. The Green marked Bytes are the char array of the target mac:

!http://www.mediafire.com/view/cvoqe6ck6ik9u4j/ArpPacket.JPG(Memory Dump)!
(Link of the Picture: "Memory Dump of ArpPacket-structure":http://www.mediafire.com/view/cvoqe6ck6ik9u4j ArpPacket.JPG )

I use the Qt Version 5.3 with the MinGW 4.8.2 Compiler for 32 Bit.

Thank you for every help!

[alexwolf]

Which compiler are you used?

[Dave007]

Sry I have edited my Post.
I use the Qt Version 5.3 with the MinGW 4.8.2 Compiler for 32 Bit.

[alexwolf]

What about adding @ attribute ((packed))@ to structure?

[Dave007]

No, that don't solved the Problem..

[JKSH]

Hi,

Your compiler padded your struct for data alignment. See http://msdn.microsoft.com/en-us/library/ms253949(VS.80).aspx for a detailed explanation.

To disable this, you need to use compiler-specific options. Many compilers use "#pragma pack":http://www.cplusplus.com/forum/general/14659/

[Dave007]

Yes, thank u all very much!

I've now added "# pragma pack (1)" befor the structure definition and a "# pragma pack ( )" after it and it is now working! :)