Re: What other languages use the same data model as Python?

From	harrismh777 <harrismh777@charter.net>
Newsgroups	comp.lang.python
Subject	Re: What other languages use the same data model as Python?
References	(6 earlier) <ipscbo$j6v$1@reader1.panix.com> <WEjwp.18479$uh5.14097@newsfe02.iad> <ipsi49$cb5$1@reader1.panix.com> <GOmwp.13554$Vp.9996@newsfe14.iad> <ph54s6t8me8gnpp9l2sc4j9kk6s0djtra5@4ax.com>
Message-ID	<f5qwp.642$xo2.335@newsfe07.iad> (permalink)
Date	2011-05-04 23:55 -0500

Tim Roberts wrote:
> The fact that the parameter "a"
> in BumpMe happens to be an address is completely irrelevent to the
> definition of the parameter passing mechanism.
>
> C has pass-by-value, exclusively.  End of story.

Yeah, Tim, I know... but that's my entire point in a nut-shell... 
whether the language is pass-by-value or pass-by-reference has less to 
do with how it is 'defined' (its mechanism--- indirection and stack) and 
more to do with how it is routinely used with the standard features it 
provides--- in this case memory indirection--- as pointers.

Something new here, just for fun...

... I ran my hello.c program through the gcc compiler and intercepted 
its assembly source output. Some folks on the list may not know that the 
gcc compiler used to generate CPython (at least on *nix systems) does 
not generate object or machine code directly, but generates an 
intermediate assembly source in AT&T syntax.   The assembly code is 
interesting for the discussion, if you've never seen it. If you have, 
blow this off.

    Anyway, I built a small wrapper called 'say()' around the printf 
function, so that I could pass a string var to say(), and I then called 
it a couple of times in main(). The assembly source code is listed at 
the end of this post. The thing to notice here is two things:
    1) assembly code is actually being used to generate the machine 
code, not 'C' (and this is true for Python as well, compiled from 
sources) In other words, Python interpreter does not do anything more 
(nor less) than what can be done with assembler (nor machine code for 
that matter).  And,
    2) The strings I am 'passing' to the say() function don't get 
'passed' anywhere. (I know that most of you know this, bear with me) The 
strings are set in memory, and through memory indirection pointers (the 
parenthesis 'references') the string's memory addresses are placed on 
the stack. The called routine say() has full access to the original 
strings in memory (via their memory addresses) if necessary. The main 
point being that the say() function has a 'reference' to the original, 
and not a local copy of the 'value', and it can change it!  The string's 
addresses are .LC0 and .LC1/

Here is the assembly of my hello.c  saved as hello.s with:

    gcc -Wall -S -o hello.s hello.c


	.file	"hello.c"
	.section	.rodata
.LC0:
	.string	"\nhello, world!\n"
	.align 4
.LC1:
	.string	"...and again I say, hello there, world!\n\n"
	.text
.globl main
	.type	main, @function
main:
	pushl	%ebp
	movl	%esp, %ebp
	andl	$-16, %esp
	subl	$32, %esp
	movl	$0, 28(%esp)
	movl	$.LC0, (%esp)
	call	say
	movl	$.LC1, (%esp)
	call	say
	movl	28(%esp), %eax
	leave
	ret
	.size	main, .-main
	.section	.rodata
.LC2:
	.string	"%s"
	.text
.globl say
	.type	say, @function
say:
	pushl	%ebp
	movl	%esp, %ebp
	subl	$24, %esp
	movl	$.LC2, %eax
	movl	8(%ebp), %edx
	movl	%edx, 4(%esp)
	movl	%eax, (%esp)
	call	printf
	leave
	ret
	.size	say, .-say
	.ident	"GCC: (Ubuntu 4.4.1-4ubuntu9) 4.4.1"
	.section	.note.GNU-stack,"",@progbits

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