Name to method?

[superheathen@yahoo.ca]

Hi

I'm reading from a database that stores information as an integer
representing a char array of ints, it is created in the following
way:

unsigned char examplearray[] = {4, 3, 2, 1};
unsigned int exampleint = *(unsigned int *)examplearray;

and back again using:

unsigned char examplearray2[4];
*(unsigned int*)examplearr ay2 = exampleint;

it works, I just have no clue how it works. Does this technique have a
name so I can look into it?

[Walter Roberson]

Re: Name to method?

In article <0cd27cba-0ba4-4163-906c-a25953bf50b5@u1 0g2000prn.googl egroups.com>,
<superheathen@y ahoo.cawrote:

>I'm reading from a database that stores information as an integer
>representing a char array of ints, it is created in the following
>way:

>unsigned char examplearray[] = {4, 3, 2, 1};
>unsigned int exampleint = *(unsigned int *)examplearray;

>and back again using:

>unsigned char examplearray2[4];
>*(unsigned int*)examplearr ay2 = exampleint;

>it works, I just have no clue how it works. Does this technique have a
>name so I can look into it?

It is sometimes called "type punning".
--
"Pray do not take the pains / To set me right. /
In vain my faults ye quote; / I wrote as others wrote /
On Sunium's hight." -- Walter Savage Landor

[superheathen@yahoo.ca]

Re: Name to method?

i'm still suck.

Maybe someone can explain the math in the (de)conversion?

[ts.death.angel@gmail.com]

Re: Name to method?

On Feb 11, 1:44 am, superheat...@ya hoo.ca wrote:

i'm still suck.
>
Maybe someone can explain the math in the (de)conversion?

There's not really much math involved, you're just swapping around
pointers. Here's a rewrite that may be easier to understand:
unsigned char examplearray[] = {4, 3, 2, 1};
unsigned int *pointerint = examplearray; /* pointerint points to
examplearray (which is 32-bits; the size of an int) */
unsigned int exampleint = *pointerint; /* sets new int exampleint to
what pointerint points to */

[superheathen@yahoo.ca]

Re: Name to method?

On Feb 10, 4:59 pm, ts.death.an...@ gmail.com wrote:

On Feb 11, 1:44 am, superheat...@ya hoo.ca wrote:
>

i'm still suck.

>

Maybe someone can explain the math in the (de)conversion?

>
There's not really much math involved, you're just swapping around
pointers. Here's a rewrite that may be easier to understand:
unsigned char examplearray[] = {4, 3, 2, 1};
unsigned int *pointerint = examplearray; /* pointerint points to
examplearray (which is 32-bits; the size of an int) */
unsigned int exampleint = *pointerint; /* sets new int exampleint to
what pointerint points to */

There's got to be a little, in the example I provided the integer
isn't a pointer or array. (though I tried using the example you
provided and got:
warning: initialization from incompatible pointer type) Using some
casting , it somehow takes the array and converts it to 16909060 (how
does it get this number?) and then using 16909060 is able to
reconstruct the array. To me it'd make more sense if it did use
pointers instead of the casting.

Sorry if I'm too dense to get what you're getting at.

[superheathen@yahoo.ca]

Re: Name to method?

On Feb 10, 5:27 pm, superheat...@ya hoo.ca wrote:

On Feb 10, 4:59 pm, ts.death.an...@ gmail.com wrote:On Feb 11, 1:44 am, superheat...@ya hoo.ca wrote:
>

i'm still suck.

>

Maybe someone can explain the math in the (de)conversion?

>

There's not really much math involved, you're just swapping around
pointers. Here's a rewrite that may be easier to understand:
unsigned char examplearray[] = {4, 3, 2, 1};
unsigned int *pointerint = examplearray; /* pointerint points to
examplearray (which is 32-bits; the size of an int) */
unsigned int exampleint = *pointerint; /* sets new int exampleint to
what pointerint points to */

>
There's got to be a little, in the example I provided the integer
isn't a pointer or array. (though I tried using the example you
provided and got:
warning: initialization from incompatible pointer type) Using some
casting , it somehow takes the array and converts it to 16909060 (how
does it get this number?) and then using 16909060 is able to
reconstruct the array. To me it'd make more sense if it did use
pointers instead of the casting.
>
Sorry if I'm too dense to get what you're getting at.

also, if it were some sort of memory location, wouldn't it be
subjected to change each compile, rendering it unable to read the
database?

[Jack Klein]

Re: Name to method?

On Sun, 10 Feb 2008 16:02:36 -0800 (PST), superheathen@ya hoo.ca wrote
in comp.lang.c:

Hi
>
I'm reading from a database that stores information as an integer
representing a char array of ints, it is created in the following
way:
>
unsigned char examplearray[] = {4, 3, 2, 1};
unsigned int exampleint = *(unsigned int *)examplearray;
>
and back again using:
>
unsigned char examplearray2[4];
*(unsigned int*)examplearr ay2 = exampleint;
>
it works, I just have no clue how it works. Does this technique have a
name so I can look into it?

It might happen to "work" for your expectation of "work" on the
particular platform where you are using it. The C standard makes no
such guarantee, because the behavior is undefined. On some platforms,
if "examplearr ay" does not have the proper alignment, trying to access
it as an unsigned int will generate a hardware trap.

What you have here is an example of poorly written code by a
programmer who isn't anywhere near as knowledgeable as he/she thinks.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++

http://www.club.cc.cmu.edu/~ajo/docs/FAQ-acllc.html

[Kenny McCormack]

Re: Name to method?

In article <1aivq312kd1ofj 0vg324232s2fqsa c4c6l@4ax.com>,
Jack Klein <jackklein@spam cop.netblathere d:
....

>What you have here is an example of poorly written code by a
>programmer who isn't anywhere near as knowledgeable as he/she thinks.

Yeah, that guy, Linus Torvalds, a real idiot. Probably lost his first
(and only) programming job - probably homeless and out on the street by now.

Yeah, I hear he used to do a lot of that sort of thing - type punning,
and god knows what else.