CODEWITHSUNDEEP
carrayspointersmemcpy
user2626445
user2626445

Reputation: 1021

How to copy data of one array to another in C?

I have to copy Mac address stored as an array with a pointer reference to another. I want to perform a swap but, i dont know if i am copying right.

uint8_t * custom_mac_store;
memcpy(custom_mac_store, ehdr->ether_shost, sizeof(ehdr->ether_shost));
memcpy(ehdr->ether_shost, ehdr->ether_dhost, sizeof(ehdr->ether_shost));
memcpy(ehdr->ether_dhost, custom_mac_store, sizeof(ehdr->ether_shost));

ehdr->ether_shost refers to a Mac address which is 48 bits long and stored in 6 arrays as 8 bits.

struct sr_ethernet_hdr
{
#ifndef ETHER_ADDR_LEN
#define ETHER_ADDR_LEN 6
#endif
    uint8_t  ether_dhost[ETHER_ADDR_LEN];    /* destination ethernet address */
    uint8_t  ether_shost[ETHER_ADDR_LEN];    /* source ethernet address */
    uint16_t ether_type;                     /* packet type ID */
} __attribute__ ((packed)) ;

Thanks in advance!!!

Upvotes: 1

Views: 2066

Answers (3)

Claudiordgz
Claudiordgz

Reputation: 3049

custom_mac_store is a pointer, not an array, you should allocate the memory doing the following:

uint8_t * custom_mac_store = malloc(sizeof(ehdr->ether_shost) * sizeof(uint8_t));

Then do the memcpy

This way you can erase or overwrite ehdr->ether_shost but you already copied your array to the new array.

@alk You are right you need to delete the memory

uint8_t * custom_mac_store = malloc(sizeof(ehdr->ether_shost) * sizeof(uint8_t));

memcpy(custom_mac_store, ehdr->ether_shost, sizeof(ehdr->ether_shost));
memcpy(ehdr->ether_shost, ehdr->ether_dhost, sizeof(ehdr->ether_shost));
memcpy(ehdr->ether_dhost, custom_mac_store, sizeof(ehdr->ether_shost));

free(custom_mac_store); //< Destroys everything in the array
custom_mac_store = NULL; //< NULL must be defined in a macro preferanly DEFINE NULL 0

or you could use c++

uint8_t * custom_mac_store = new uint8_t[sizeof(ehdr->ether_shost)];

std::copy ( ehdr->ether_shost, ehdr->ether_shost+sizeof(ehdr->ether_shost), custom_mac_store );
    std::copy ( ehdr->ether_dhost, ehdr->ether_dhost+sizeof(ehdr->ether_shost), ehdr->ether_shost );
    std::copy ( custom_mac_store, custom_mac_store+sizeof(ehdr->ether_shost), ehdr->ether_dhost );

delete[] custom_mac_store; //< Destroys everything in the array

or you could use the stack instead of the heap (this should be faster, just don't break the stack)

const std::size_t size_custom_mac_store = 48000;
uint8_t custom_mac_store[size_custom_mac_store]; //< The size must be either a const or a macro

std::copy ( ehdr->ether_shost, ehdr->ether_shost+sizeof(ehdr->ether_shost), custom_mac_store );
std::copy ( ehdr->ether_dhost, ehdr->ether_dhost+sizeof(ehdr->ether_shost), ehdr->ether_shost );
std::copy ( custom_mac_store, custom_mac_store+sizeof(ehdr->ether_shost), ehdr->ether_dhost );

delete[] custom_mac_store; //< Destroys everything in the array

Good luck.

PS: Love memory management, it's the only way

Upvotes: 2

FredP
FredP

Reputation: 170

It might work if some memory was allocated for your temporary buffer custom_mac_store. Like that it's just an uninitialized pointer waiting to cause a crash. You can either allocate explicitly some memory to it, or use directly a 6 byte array instead.

Upvotes: 2

Sunil Bojanapally
Sunil Bojanapally

Reputation: 12688

You can define ethernet length as 6 then making use of a temporary array, just perform memcpy as below,

#define ETH_ALEN        6
uint8_t tmp[ETH_ALEN];
memcpy(tmp, eth->ether_shost, ETH_ALEN);
memcpy(eth->ether_shost, eth->ether_dhost, ETH_ALEN);
memcpy(eth->ether_dhost, tmp, ETH_ALEN);

Upvotes: 2

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