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Comments:
<0> gn
<1> Hey guys
<1> is anybody around to help me with some relativly simple code?
<2> don't ask to ask, guy. it's, like, in the topic.
<1> point taken
<1> im doing an asm ***ignment for a college ***ignment
<1> and have ran into some trouble figuring out why a certain buffer seems to get nulled out
<1> i will post the code on a paste bin now
<2> okay.
<1> http://wraith.pastebin.ca/339335
<1> on here, i wish to print out the variable called buffer
<1> in option2
<1> but it seems to be nulled out
<1> but when i want to print it inside option1, it prints perfectly
<1> any help will be very appreciated
<2> eh. fine.
<2> i'll do it rogerian-like.
<2> what's wrong?
<2> the program seems to be working
<1> if you run the program, then enter option a, feed it a string, then it will print it back to you, you can then enter another option b, which should also print back the buffer but does not.
<2> why do you think option b doesn't print back the buffer correctly? it works.
<2> it prints current contents of buffer.
<1> it doesnt seem to for me, it does successfully print it back to after it states String loaded into buffer
<1> but thats not where i wanted it, was there just for a test
<1> after it has printed the buffer, input b
<1> and it prints out "printing out buffer" but is just a blank line
<2> mhm. correct. because the buffer is empty.
<1> Can you tell me how it has become empty?
<2> what does getoption do?
<1> gets the choice letter, moves into into ebx and compares it, depending on the outcome jmps to that label
<2> how does it get the choice letter
<2> ?
<2> [btw, try this: run your program and type 'blah' into it. maybe you'll get it.]
<1> it has outputted just lah. So it has used the b for the option choice and outputted the ramaindor
<2> indeed.
<2> how does getoption get the choice letter?
<1> it moves the lea of the variable that is to contain the data, calls a sub that uses this information in loop fashion to getchar and then stores it in the variable?
<2> what variable?
<2> and what is exactly stored in it?
<1> the vairable called choice, and what is stored inside of it is the hex equavilent of the inputted letter?
<1> oh. does it append a null?
<2> mhm.
<2> how big is variable choice?
<2> and how big is the stuff stored into it?
<1> 1 byte? and it stores 1byte?
<1> so theres no room for a null?
<2> choice is 1byte.
<2> but it stored more than one byte in it.
<2> which is bad.
<1> so data is overflowing on the stack?
<2> it is overflowing.
<2> but not on the stack.
<2> it is overflowing inside your data segment
<2> the next variable that happens to be there is buffer.
<2> the null byte ends up in buffer
<1> as the first byte?
<2> and that's why our program prints nothing.
<1> Thank you :]
<1> now its working fine.
<2> out of curiosity... what sort of OS is this?
<2> using GNU as and names beginning with underscore...
<1> cygwin
<2> ugh. suspected so.
<1> dont have X on linux
<1> and we need to use insight
<1> wont allow CL gdb
<3> how can they decide what programs you can use? ;)
<1> I dont think its strict but they ask for evidence of use of insight inside of labs
<2> speaking of which... why didn't you just debug this thing with insight and set a watch on buffer?
<1> I guess I didnt think of that
<1> Lesson learnt
<1> Thank you for all your help :]
<4> anyone mind helping me out with some very basic MIPS code? I'm trying to swap elements in an array... "abcdef" -> "badcfe" - http://pastebin.perl.se/0b0a8c7e6c/
<5> could someone explain why right after the function prologue gcc is anding -16 with the stack pointer?
<5> that is actually the start of a sequence of 3 instructions that i don't understand what gcc is doing
<6> where it shifts 16 bits one way and then the other?
<5> andl $-16, %esp
<5> movl $0, %eax
<5> subl %eax, %esp
<5> i should add that this only seems to appear in main()
<6> well, the first part is padding the stack
<6> but yeah, those prologs always make me have to look at them a while
<5> could you explain what you mean by padding the stack?
<6> well, -16 in 2's compliment is like 11111111111111111111111111110000
<5> right
<6> so, anding that with the stack pointer ensures the stack pointer is at some multiple of 4
<5> ohh
<6> lik,e making room for some data types or whatever
<5> hmm
<6> i'm totally a n00b-, but i've seen wierd prologs like that right before variable ***ignment
<6> plenty of times
<6> and it's always something like -16, -8, -24....
<5> before that it: 1) pushes the ebp 2) moves the esp to ebp 3) subtracts space for the amount of local variables from esp
<6> right
<5> so my understanding was the subtraction aboce was when it made room for the data
<5> s/aboce/above
<6> right
<5> hmm i guess i'll have to think about it some more
<6> what about a return value
<6> is there a function call later int he code?
<5> yeah. i just have a super simple main that calls another super simple function
<5> (basically wanted to see what p***ing structs on the stack looked like)
<5> what did you want to know about the return value?
<6> my inclination is that whenever you see the anding with a small negative number, the compiler is padding the stack to line up some data with the addresses properly
<5> i see. that sounds logical, but i'll have to let it rattle a round in my head a bit before i can get it :)
<6> it's really important when you're talking about arrays or strings
<6> though, usually a string is referenced off the heap
<5> it sounds like it does that to avoid unaligned memory access
<6> right
<7> Hey, anyone know how to get IDA Pro to dis***emble 8051? Trying to write a port of something
<3> 8051?
<7> 8051 processor
<7> doesnt seem to be on the processor list :S
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