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Author Topic: Need help understanding code  (Read 2491 times)

mr_kanister

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Need help understanding code
« on: August 05, 2013, 09:46:37 AM »
I've got a source file, but can't understand one line, i would be pleased if someone could explain me what exactly is happening here:

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int field = *(DWORD *)(packet + 0x08);
As far as i understood the code, packet is an array of bytes, which is filled with the data read by fpacket.
both are initilaized like this:

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BYTE* packet = NULL;
FILE *fpacket;

Here is where the bytes are written into packet:

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packet = malloc(filesize+1);
memset(packet, 0, filesize+1);
fread(packet, filesize, 1, fpacket);

Thank you in advance

Offline frankie

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Re: Need help understanding code
« Reply #1 on: August 05, 2013, 03:42:32 PM »
The line simply reads *edit* 4 bytes together (DWORD is 4 bytes) displaced of 8 bytes from beginning of the array. Is enough?  8)
OK let get a little deeper.
'packet ' is a pointer to a BYTE array.
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packet + 0x08points to 8 bytes forward (the eigthth byte from array begin)
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(DWORD *)This is a 'casting'. Crafted this way we are saying to compiler to consider the previous variable (packet + 0x08) no more a byte pointer, but a DWORD pointer.
The last asterisk, the first before assignment, tells to the compiler to get the DWORD value pointed by the pointer (and then assigning it to the variable 'field'.
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int field = [Get the DWORD pointed to]*[convert it to DWORD pointer](DWORD *)([displace 'packet pointer of 8 bytes] packet + 0x08);Hope is clear enough.....
« Last Edit: August 05, 2013, 04:18:49 PM by frankie »
"It is better to be hated for what you are than to be loved for what you are not." - Andre Gide

mr_kanister

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Re: Need help understanding code
« Reply #2 on: August 05, 2013, 03:57:05 PM »
That helped me a lot.

Thank you for your help  :D

Offline frankie

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Re: Need help understanding code
« Reply #3 on: August 05, 2013, 04:20:06 PM »
You're welcome.
I edited the former post because of a mistake on DWORD size (a DWORD is 4 bytes long).
I can also add that the code:
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int field = *(DWORD *)(packet + 0x08);have one more intrinsic casting from DWORD to int.
It would have been better:
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int field = *(int *)(packet + 0x08);
« Last Edit: August 05, 2013, 04:23:31 PM by frankie »
"It is better to be hated for what you are than to be loved for what you are not." - Andre Gide