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Date: Tue, 25 Feb 2014 05:22:25 +1100
Subject: Re: Can global variable be passed into Python function?
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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On Tue, Feb 25, 2014 at 5:05 AM,  <j.e.haque@gmail.com> wrote:
> typedef struct {
>   int value;
> } Number;
>
>   Number *o;
>   o = malloc(sizeof(*o));
>   o->value=3;
>   printf("o<%p>, o->value<%p>\n", o, &o->value);
>
> o<0x9fe5008>, o->value<0x9fe5008>
>
> Is the compiler borked?

No, because a structure in C is simply a laying-out of its members. A
pointer to the structure is exactly the same as a pointer to its first
member, same as a pointer to an array is really just a pointer to its
first element (with other elements laid out sequentially, possibly
with padding for alignment). In Python, a complex object is a thing in
its own right; it has an identity and a set of members, each of which
has its own identity. In C, the structure is an invisible piece of
nothingness that surrounds an aggregation of other objects. For
instance:

typedef struct {
    int val1;
    int val2;
} NumberPair;

NumberPair five_pairs[5];
int *ten_ints = (int *)five_pairs;

This is considered naughty (pointer aliasing), and can make a mess of
optimization, but as far as I know, it's never going to be a problem
(I don't think it's possible for there to be any padding between two
ints; it's possible the struct will demand coarser alignment than the
ints would, which is why I've written them in that order). You can
access ten integers from the aliased pointer, and five pairs of
integers through the original structure array, and they're all exactly
the same. If you print out those two pointers, they will have the same
value in them, and &five_pairs[3]->val2 will be the same as
&ten_ints[7] (and the same as ten_ints+7).

For in C, neither the structure nor the non-structure is anything. The
only thing that counts is code expressing itself through love.
(Galatians 5:6, K&R translation. Or something like that.)

ChrisA