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# 11.9 Command Line Arguments

Our hex program will be more useful if it can read the names of an input and output file from its command line, i.e., if it can process the command line arguments. But... Where are they?

Before a UNIX® system starts a program, it `push`es some data on the stack, then jumps at the `_start` label of the program. Yes, I said jumps, not calls. That means the data can be accessed by reading `[esp+offset]`, or by simply `pop`ping it.

The value at the top of the stack contains the number of command line arguments. It is traditionally called `argc`, for "argument count."

Command line arguments follow next, all `argc` of them. These are typically referred to as `argv`, for "argument value(s)." That is, we get `argv[0]`, `argv[1]`, `...`, `argv[argc-1]`. These are not the actual arguments, but pointers to arguments, i.e., memory addresses of the actual arguments. The arguments themselves are NUL-terminated character strings.

The `argv` list is followed by a NULL pointer, which is simply a `0`. There is more, but this is enough for our purposes right now.

> Note: If you have come from the MS-DOS® programming environment, the main difference is that each argument is in a separate string. The second difference is that there is no practical limit on how many arguments there can be.

Armed with this knowledge, we are almost ready for the next version of `hex.asm`. First, however, we need to add a few lines to `system.inc`:

First, we need to add two new entries to our list of system call numbers:

    %define	SYS_open	5
    %define	SYS_close	6

Then we add two new macros at the end of the file:

    %macro	sys.open	0
    	system	SYS_open
    %endmacro

    %macro	sys.close	0
    	system	SYS_close
    %endmacro

Here, then, is our modified source code:

    %include	'system.inc'

    %define	BUFSIZE	2048

    section	.data
    fd.in	dd	stdin
    fd.out	dd	stdout
    hex	db	'0123456789ABCDEF'

    section .bss
    ibuffer	resb	BUFSIZE
    obuffer	resb	BUFSIZE

    section	.text
    align 4
    err:
    	push	dword 1		; return failure
    	sys.exit

    align 4
    global	_start
    _start:
    	add	esp, byte 8	; discard argc and argv[0]

    	pop	ecx
    	jecxz	.init		; no more arguments

    	; ECX contains the path to input file
    	push	dword 0		; O_RDONLY
    	push	ecx
    	sys.open
    	jc	err		; open failed

    	add	esp, byte 8
    	mov	[fd.in], eax

    	pop	ecx
    	jecxz	.init		; no more arguments

    	; ECX contains the path to output file
    	push	dword 420	; file mode (644 octal)
    	push	dword 0200h | 0400h | 01h
    	; O_CREAT | O_TRUNC | O_WRONLY
    	push	ecx
    	sys.open
    	jc	err

    	add	esp, byte 12
    	mov	[fd.out], eax

    .init:
    	sub	eax, eax
    	sub	ebx, ebx
    	sub	ecx, ecx
    	mov	edi, obuffer

    .loop:
    	; read a byte from input file or stdin
    	call	getchar

    	; convert it to hex
    	mov	dl, al
    	shr	al, 4
    	mov	al, [hex+eax]
    	call	putchar

    	mov	al, dl
    	and	al, 0Fh
    	mov	al, [hex+eax]
    	call	putchar

    	mov	al, ' '
    	cmp	dl, 0Ah
    	jne	.put
    	mov	al, dl

    .put:
    	call	putchar
    	cmp	al, dl
    	jne	.loop
    	call	write
    	jmp	short .loop

    align 4
    getchar:
    	or	ebx, ebx
    	jne	.fetch

    	call	read

    .fetch:
    	lodsb
    	dec	ebx
    	ret

    read:
    	push	dword BUFSIZE
    	mov	esi, ibuffer
    	push	esi
    	push	dword [fd.in]
    	sys.read
    	add	esp, byte 12
    	mov	ebx, eax
    	or	eax, eax
    	je	.done
    	sub	eax, eax
    	ret

    align 4
    .done:
    	call	write		; flush output buffer

    	; close files
    	push	dword [fd.in]
    	sys.close

    	push	dword [fd.out]
    	sys.close

    	; return success
    	push	dword 0
    	sys.exit

    align 4
    putchar:
    	stosb
    	inc	ecx
    	cmp	ecx, BUFSIZE
    	je	write
    	ret

    align 4
    write:
    	sub	edi, ecx	; start of buffer
    	push	ecx
    	push	edi
    	push	dword [fd.out]
    	sys.write
    	add	esp, byte 12
    	sub	eax, eax
    	sub	ecx, ecx	; buffer is empty now
    	ret

In our `.data` section we now have two new variables, `fd.in` and `fd.out`. We store the input and output file descriptors here.

In the `.text` section we have replaced the references to `stdin` and `stdout` with `[fd.in]` and `[fd.out]`.

The `.text` section now starts with a simple error handler, which does nothing but exit the program with a return value of `1`. The error handler is before `_start` so we are within a short distance from where the errors occur.

Naturally, the program execution still begins at `_start`. First, we remove `argc` and `argv[0]` from the stack: They are of no interest to us (in this program, that is).

We pop `argv[1]` to `ECX`. This register is particularly suited for pointers, as we can handle NULL pointers with `jecxz`. If `argv[1]` is not NULL, we try to open the file named in the first argument. Otherwise, we continue the program as before: Reading from `stdin`, writing to `stdout`. If we fail to open the input file (e.g., it does not exist), we jump to the error handler and quit.

If all went well, we now check for the second argument. If it is there, we open the output file. Otherwise, we send the output to `stdout`. If we fail to open the output file (e.g., it exists and we do not have the write permission), we, again, jump to the error handler.

The rest of the code is the same as before, except we close the input and output files before exiting, and, as mentioned, we use `[fd.in]` and `[fd.out]`.

Our executable is now a whopping 768 bytes long.

Can we still improve it? Of course! Every program can be improved. Here are a few ideas of what we could do:

- Have our error handler print a message to `stderr`.

- Add error handlers to the `read` and `write` functions.

- Close `stdin` when we open an input file, `stdout` when we open an output file.

- Add command line switches, such as `-i` and `-o`, so we can list the input and output files in any order, or perhaps read from `stdin` and write to a file.

- Print a usage message if command line arguments are incorrect.

I shall leave these enhancements as an exercise to the reader: You already know everything you need to know to implement them.

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