[Deque]

I already provided an example how to use c functions in assembly and stated differences between 32 bit and 64 bit systems: http://www.hackcommunity.com/Thread-ASM-...chitecture

It is pretty much the same with command line arguments. main is just a function like every function else. You treat it the same way. The declaration is the following:

Code:

int main(int argc, char** argv)

For those who don't know C:
int argc - contains the number of command line arguments
char** argv - contains addresses to the command line argument strings

You can imagine it like that:

Code:

argv | address1 | address2 | address 3 | / | \ / | \ | arg string 1 | | arg string 2 | | arg string 3 |

On a 32 bit system you can jump to the next command line argument address by adding 4 bytes (=32 bit) to the previous address (i.e. jumping from address1 to address2).
A 64 bit system needs 8 byte to jump to the next address that argv contains.

I will show how to get and print command line arguments on the screen.

The following is an example I found on http://www.cs.lmu.edu/~ray/notes/nasmexamples/
It is written for 32 bit Windows. Here you get the arguments argv and argc from the stack.

Code:

; ---------------------------------------------------------------------------- ; echo.asm ; ; NASM implementation of a program that displays its commandline arguments, ; one per line. ; ---------------------------------------------------------------------------- global _main extern _printf section .text _main: mov ecx, [esp+4] ; argc mov edx, [esp+8] ; argv top: push ecx ; save registers that printf wastes push edx push dword [edx] ; the argument string to display push format ; the format string call _printf add esp, 8 ; remove the two parameters pop edx ; restore registers printf used pop ecx add edx, 4 ; point to next argument dec ecx ; count down jnz top ; if not done counting keep going ret format: db '%s', 10, 0

To use the same in 32 bit Linux you just have to remove the underscores:

Code:

; create executable this way: ; nasm -f elf -g -F stabs echo.asm ; gcc -o echo echo.o ; global main extern printf section .text main: mov ecx, [esp+4] ; argc mov edx, [esp+8] ; argv top: push ecx ; save registers that printf wastes push edx push dword [edx] ; the argument string to display push format ; the format string call printf add esp, 8 ; remove the two parameters pop edx ; restore registers printf used pop ecx add edx, 4 ; point to next argument dec ecx ; count down jnz top ; if not done counting keep going ret format: db '%s', 10, 0

In a 64 bit system the arguments are not in the stack but in the general purpose registers. So the code above would not work. The order of the registers is the following: %rdi, %rsi, %rdx, %rcx, %r8 and %r9
That means argc is in %rdi and argv in %rsi. Here is my 64 bit version of the same program (linux). It uses the 64 bit registers and adds 8 byte to jump to the next address of the next command line argument.

Code:

; create executable this way: ; nasm -f elf64 -g -F stabs echo.asm ; gcc -o echo echo.o ; extern printf section .data format: db '%s', 10, 0 section .text global main main: mov rcx, rdi ; argc mov r8, 0 ; offset repeat: mov rdx, qword [rsi+r8] ; argv push rcx ; save registers that printf wastes push rdx push rsi push r8 mov rdi, format ; first parameter for printf mov rsi, rdx ; second parameter for printf mov rax, 0 ; no floating point register used call printf ; call to printf pop r8 ; restore registers pop rsi pop rdx pop rcx add r8, 8 ; point to next argument dec rcx ; count down jnz repeat ; if not done counting keep going mov rax, 1 ; stop the program mov rbx, 0 int 80h

An example output:

[deque@Desolate echo]$ ./echo bla la ka2
./echo
bla
la
ka2

Here is a makefile for the 64 bit version, so you only need to type "make" to create an executable file:

Code:

echo: echo.o gcc -o echo echo.o echo.o: echo.asm nasm -f elf64 -g -F stabs echo.asm

Deque

[bluedog.tar.gz]

No responds yet?
Anyway very nice thread.

I never done ASM and I am fairly a 'noobie' in C.
I might try this out sometime.

Thanks

[H4R0015K]

very useful For me!

thanks for sharing!

[shortcircuit]

Nice tutorial, although ASM is harder to use for tasks that could be implemented easily in C/C++, so for everyday usage I never find myself needing command-line arguments.

[hacxx]

ASM? ChatGPT is becoming very good in replacing programmers with AI. I got an idea a few months ago to convert TSRH ASM Keygens from ASM to JS and ChatGPT was able to convert half of the code, it wasn't enough to get a full keygen. Something for the future.