Re: "Mastering C Pointers"....

From: Alan Connor (zzzzzz_at_xxx.yyy)
Date: 11/03/03 

Date: Mon, 03 Nov 2003 21:29:12 GMT

On Mon, 03 Nov 2003 05:50:46 GMT, Roose <nospam@nospam.nospam> wrote:
>
>
> Let me preface this with some meta-comments. If your goal is to learn C, by
> all means go ahead and dive right into the C language. You only learn by
> making mistakes. But is a common fact in computers that you really only
> master something when you have at least learned the level below it, i.e.
> _what it is abstracting_. Then you see where the abstraction came from.
>
> See these articles:
>
> http://www.joelonsoftware.com/articles/LeakyAbstractions.html
> http://biztech.ericsink.com/Abstraction_Pile.html
>

Will do.

> So if you configure OSes, it's good to learn some scripting. If you script,
> good to know some compiled programming. If you program C, good to know some
> assembly/computer architecture.
>

Yes!

> The idea was to give a simple concrete model that anyone can learn, but
> don't get bogged down if your real goal is to learn C. Basically what I'm
> describing is a von Neumann machine with some details (this is a very
> influential hardware architecture that created the descendants of PCs,
> roughly). Unfortunately with a cursory google search, I couldn't find any
> good links for that, maybe someone else can help out here.
>
>> > Basically you have a CPU which has a clock say every microsecond if
> you're
>> > on a gighertz machine.
>>
>> Wouldn't that be "nanosecond"?
>
> Yes, whoops.
>
>> > 1. load them both from memory to CPU registers
>> > 2. execute the add instruction and specify those two registers
>> > 3. store the result back to memory
>> >
>>
>> So memory is just dumb storage. I didn't realize that.
>
> Yes, you can think of it as inert. The CPU controls everything, the
> "brain". Memory is a separate unit which just stores bits. Same with the
> hard disk. Notice that as the access time increases, so does the size of
> the medium.
>
>> > Memory is just a big array of bytes. If you have 1 gig of memory,
> pretend
>> > it is addressed 0 .. 0x3FFFFFFF. This is 2^30-1, or 1 gig. 0xFFFFFFFF
> is >> > 2^32-1, which means 4 gigs. If you have heard Apple's stupid
> advertisements
>> > that you can have more than 4 gigs of RAM in their PCs, that is because
> they
>> > use more than 32 bits for pointers (i.e. the G5's 64-bit).
>>
>> Don't get THAT at all, but that's okay for now.
>
> The main point here was that a pointer is a C language abstraction (for an
> address in memory). A pointer at the hardware level _is an integer_.
> Memory is just a big array of bytes again. Say you have 1 meg of memory,
> then you can just number the bytes 0 .. 1,048,575 (2^20-1). Suppose you
> have a variable numFiles that is the number of files in a directory = 55.
> That variable must be stored somewhere. Say it is stored at the 200,005th
> byte. Then a pointer to numFiles would have the numeric value 200,004 (get
> used to indexing from 0 in C). That's it. Nothing else. It's like an
> array index if you think of the array as all of memory.
>
> e.g.
> int numFiles = 55;
> int* pNumFiles = &numFiles; // pNumFiles is a variable of pointer
> type, with the numeric value 200,004
>
> Of course, as always, there are details, but I'm not going to clutter up
> that simple concept with them. And this is very real, you can inspect
> pointers in a debugger and see this (which was my suggestion).
>
> Say you have a 32 bit register. After you learn binary/hex, you will see
> why the number of different integers you can store in 32-bits is 2^32 = 4
> gigs. A pointer can be thought of as an integer index, like I said. Thus
> if a pointer is 32-bits, then you can only address 4 gigs of memory.
>
>> In your previous post you recommended that I learn hexadecimal (which I
>> have a slight handle on: 0-9-F with one character to describe identify
>> a nibble.
>>
>> By this you mean learning to convert it to decimal, right?
>
> You don't actually NEED to in order to do the exercise I suggested, since
> many debuggers will display all values in decimal, but it helps to get into
> the mindset. It is pretty simple if you are even mildly mathematical. Hex
> is really a shorthand for binary, in some sense. There are probably plenty
> of tutorials on the web, but afterwards you should be able to understand
> these common equivalences:
>
> 2^8-1 = 255 = 0xFF = 1111 1111
> 2^8 = 256 = 0x100 = 1 0000 0000 = 0.25 K
> 2^16-1 = 65535 = 0xFFFF = 1111 1111 1111 1111 (spaces in binary added for
> clarity)
> 2^16 = 65536 = 0x10000 = 1 0000 0000 0000 0000 = 64 K
>
> 0x1 = 0001 b
> 0x2 = 0010 b
> 0x4 = 0100 b
> 0x8 = 1000 b
>
> 0x1 = 0001 b
> 0x3 = 0011 b
> 0x7 = 0111 b
> 0xF = 1111 b
>
>> > Also, you know that if's and goto's can be substituted for all for and
> while
>> > loops.
>>
>> I do?
>
> Yes, try taking a simple for loop or while loop and doing the exact same
> thing with if's and goto's, if you don't see it right away. Of course this
> is very bad programming practice, since loops make your logic more much
> clearer to the reader.
>
> (If you're not totally familiar with loops in general, you might want to try
> a higher-level language like Python/Java/C# before attempting C.)
>

Nah. I've written hundreds of for,while,until loops in sh.

>> Sounds like the b command in sed. (oops! The b command in sed must be
>> like the jump instruction in C :-)
>
> Well "jump" is an assembly term for specific CPU instructions. Goto is the
> equilavent in C. My point was that for and while loops compile down to
> jumps and tests (instruction that return true or false, basically).
>
>> Singe the hair off their balls: I'm getting a lot out of this.
>>
>> It's really hard to learn the basics of anything if you get too bogged
> down
>> in exactness.
>
> Good, I'm glad. This confirms an observation developed from some years of
> teaching. Exactness isn't necessarily the problem, but you just have to
> know WHAT you need to be exact about (i.e. not irrelevant details like
> rarely used terminology).
>
> Let me close with some more high level observations. What does C add on top
> of this model (besides nice syntax, I'm talking ideas here)?
>
> 1) Platform independence -- instead of writing in native assembly, you write
> in C, and then compilers for every platform translate your C program into a
> stream of instructions (just byte data)

Good. That's an important piece of the puzzle.

> 2) Constructs like functions, for and while loops, if's and switches, to
> organize this enormous stream of instructions

All of this is found in sh, some of it being called "flow control".

(except maybe "switches" do you mean options to the compiler/assembler?)

> 3) A strong type system -- this can be confusing at first

> This is why I wanted to emphasize that pointers are just integers in
> hardware. They're a C language construct. Same thing with characters/
> character strings. Now floats ARE actually different in hardware, but we
> won't get to that.
> 4) some other stuff which I don't care to think up now : )
>
> The point of the type system is to catch mistakes at an obvious level.
> Compiling a C program to see if all the types match up can catch a lot of
> mistakes. It's sort of a sanity check, and it helps you structure your
> program.
>

Don't really get that, but it *feels* important. Errr....Can you say
"feel" on a programming newsgroup? :-)

> But again, you can read all you want, but if you can pull off the exercise
> with the debugger I suggested, you will learn a whole lot.
>

It'll happen.

> Roose
>
>

Thank you again. Back to K&R and the Answer Book and I'll check out those
websites in due course.

Got really fed up with the Richard fellow. Killfiled his arrogant ass for
a while. 

-- 
Alan C    this post ends with    w
                                 q

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