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