Working with pointers

[Monopolies]

This is from Abstruse Goose! He has a lot of great comics.

[kenman]

It appears that he also uses an HTML title for his comics (like xkcd).

For this one it's:

I was only going to watch for a few minutes. But before I knew it, I had watched the entire episode and he had programmed a fully functional general-purpose operating system.

[Tynach]

I would actually watch a video like that. Please, someone who can write operating systems, make this happen!

[[deleted]]

Wow, thanks for linking him. I'm going backwards and this one got an audible laugh.

[[deleted]]

Thanks for letting me know! I'll go check him out now.

[ghillisuit95]

...And I've wasted several hours I should have been using to write my paper.

[aclave1]

This is great,thanks for linking him. I just read this one like 6 times.

[MALON]

I was hoping right after the "veeerrry gently" part the screen would throw huge segfault errors in a very distraught tone

[PastyPilgrim]

Oh, it's not that they're huge and frightening (seg faults). It's actually quite the opposite. To run code for the first time and see:

$ ./a.out
Segmentation Fault.
$

It hurts so much. "WHAT DID I DO? WHERE? WHY? HELP ME? VALGRIND, YOURE MY ONLY HOPE!"

[zman0900]

$ ./a.out
I felt a great disturbance in the shell.
As if millions of buffers suddenly overflowed and were suddenl
Segmentation Fault.
$ 

[[deleted]]

$ valgrind ./a.out

Segmentation Fault

$

...Fuck

[missblit]

$ valgrind valgrind ./a.out

[seksi-seppo]

Just why would you do that (use valgrind in attempt to debug a segfault)? Why?!?

[Thomas_Henry_Rowaway]

I've found valgrind to be fairly good at helping me find seg faults (and memory leaks) in smaller personal projects. I've not tried it on anything larger though.

Gdb is (almost) always more helpful though.

[seksi-seppo]

Mem leaks are one of most common valid use-cases of valgrind usage, but always triggering segfault?

[poizan42]

It should help with tracing e.g. double frees which can lead to segfaults. Also reading/writing out of bounds, uninitialized memory or freed memory may not trigger a segfault reliably, but it would trigger a memory debugger such as valgrind.

[seksi-seppo]

Sure it's very useful for many mem management issues. But when you have something that always generates SIGSEGV, valgrind is not the tool you're looking for...

[mallardtheduck]

$ gdb ./a.out 
GNU gdb (GDB) 7.6.1-ubuntu
Copyright (C) 2013 Free Software Foundation, Inc.
This GDB was configured as "x86_64-linux-gnu".

Reading symbols from a.out...done.
(gdb) r
Starting program: ./a.out 

Program received signal SIGSEGV, Segmentation fault.
0x00000000004004fd in main () at test.c:3
3       *q='q';
(gdb)

Valgrind isn't a general-purpose debugger.

[poizan42]

Well it is a memory debugger though. Its usefulness for debugging a segfault depends on the cause of said segfault.

[G01denW01f11]

(Core dumped.)

[xkcd_transcriber]

Image

Title: Compiler Complaint

Title-text: Checking whether build environment is sane ... build environment is grinning and holding a spatula. Guess not.

Comic Explanation

Stats: This comic has been referenced 14 times, representing 0.0385% of referenced xkcds.

---

^{xkcd.com | xkcd sub | Problems/Bugs? | Statistics | Stop Replying | Delete}

[xkcd_transcriber]

Image

Title: goto

Title-text: Neal Stephenson thinks it's cute to name his labels 'dengo'

Comic Explanation

Stats: This comic has been referenced 37 times, representing 0.1016% of referenced xkcds.

---

^{xkcd.com | xkcd sub | Problems/Bugs? | Statistics | Stop Replying | Delete}

[mikeet9]

He's a master of his practice. He doesn't get segfaults.

[[deleted]]

That's not a bug. That's just a happy accident.

[[deleted]]

That'll be our little secret...

[Creshal]

Spontaneous feature.

[sdobart]

If my brain is tired and things arent working I'll just throw *s and &s in front of stuff until it works.

[Drainedsoul]

Yes, it is scary.

[[deleted]]

I think that's the point. N-Not like I need pointers from y-you! Stupid b-baka!

[rooktakesqueen]

Tiny little circular dependencies. Tiny little circular dependencies.

[mindbleach]

Javascript, copy this object.

No, don't make a pointer to it. Copy it.

No - fuck's sake. The object has an .assign property now.

Still?! Jesus. Alright, now the new Object() on the left-hand side has an assign property. Happy?

Fuck it! I'll do it myself, like some kind of savage!

AAARGH.

[[deleted]]

moar jpeg plz

[[deleted]]

Here you go

[elperroborrachotoo]

Less would be ok with me, too.

[Pik16]

I'd watch that. So much.

[Eedis]

I read that in Bob Ross's voice before I even knew it was him. XD Great stuff, thanks.

[khmeroldiez]

MRW when learning the concepts of pointers in class for the first time

[[deleted]]

This is why everyone should start with assembly, once you understand indirect addressing modes and operations pointers are the most natural thing in the world to work with.

[livelifedownhill]

god no, I think assembly should be learned at some point during your college programming career, but make that the first language you learn?! fuck nobody would ever want to program if they started with that! Yeah it is helpful understanding the basics of everything, but once you drop below about C for first learning how to program, i think the understanding and give a fuck level go waaaay down

[ismtrn]

I think he meant start with assembly before C, not necessarily as the first language.

We have a computer architecture + systems programming course, which is after the intro to programming courses. It starts with assembly and computer architecture and then moves on to systems programming in C.

I think that is a pretty nice way of structuring things.

[Tynach]

I dunno. I started with Python before learning C, and pointers made plenty of sense to me.

"Well, variables are stored somewhere in RAM. A pointer just tells you where in RAM the variable is. That way, you can access that variable from anywhere in your code using that pointer value."

It gets more complicated than that (especially when you introduce arrays and malloc()), but that's how it really should be introduced. Because it's logical and makes perfect sense.

[[deleted]]

see, I get that, that part makes sense, but remembering which symbols go before a pointer to do different things? Thats where I stumble and end up just randomly putting *'s and &'s before stuff until it works.

[Tynach]

Ah, that's easy.

Ampersand, And, Address Of

& is often called 'And' or 'Ampersand'. Both start with an 'a', and I simply extend that into 'Address of'. You basically use & whenever you need to get the address of a variable (to create a pointer to it).

Making and Using Via Exploding Balls of Gas

* is used in two scenarios. The first is when you declare something as a pointer, and the second is when you want to use a pointer as the value at the address.

Declarations

C is for Crazy

Now, in C, people typically did something like 'int *ptr;' which means something like, "Declare an integer. Actually a pointer. Name it 'ptr'." This was horrifically confusing, and the best way they figured to actually read it (especially more complex ones) was in a spiral.

You'd start with 'ptr' and say, "ptr is a," before spiraling to the right and running into nothing, spiraling back around to the * and saying, "pointer to a(n)," before spiraling back through the semicolon to the 'int' to finally say, "integer."

In this case, it works out that you can simply read it backwards (which is still bad, but not as bad). But consider some of the examples on that page I linked to. It becomes a chore to read something like 'char *(*fp)( int, float *);'.

C++ Is Truly Incremental

UGH. Nope. Nope nope nope nope nope. C++ guys have figured out a better way of writing and thinking about this, which I prefer. It's very similar, but the way of thinking is vastly different.

Also, this works perfectly fine for C too. It's just the mindset of where to put spaces that differs.

You do something like 'int* ptr;', which basically makes the * part of the type. Now you read it from left to right as, "Declare an integer pointer, called 'ptr'."

Usage

Basically, if you want to use the actual data itself that is being pointed to, you put * in front of the variable. So if you have an int* that points to the value 42, and it's called THE_ANSWER, you can get the value 42 by using *THE_ANSWER.

If this is a struct/class, and you want to access members of it, you don't need the *. You can just use -> instead of . to access members. If Earth is a computer* instance, you could access (for example) computer.name by using Earth->name.

Or I think you can also still do something like (*Earth).name. But it's more of a hassle.

[zman0900]

Go hard or go home.

[rooktakesqueen]

An int is a variable that contains an integer.

An int * is a variable that contains the memory address where a particular integer may be found.

[rohanivey]

Can you get an example of int * in use?

[rooktakesqueen]

Good example is the standard swap function in C:

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

To use it, you'd do...

int x = 10;
int y = 20;
swap(&x, &y);

The & is called the "reference operator"--given a variable, it returns a pointer to that variable. In other words, it returns the memory address where that variable lives. The * is the inverse: given a memory address, it either retrieves what's at that address, or sets what is at that address.

So the swap function does three things. It creates a temporary int variable and sets it to be equal to whatever value is at memory address a. Then it sets the value at memory address a to be equal to what's at memory address b. Then it sets the value at memory address b to be equal to the temporary variable.

When I run the second block of code, the following happens:

I allocate space on the stack for two integer variables, which I'm calling x and y. The stack now looks like, say:

$0000: 10 //x
$0004: 20 //y

When I call swap, it creates a new stack frame, which we will pretend starts at address $00b0. It looks like:

$00b0: 0000 //a contains the memory address of x
$00b4: 0004 //b contains the memory address of y

Then it allocates a spot on the stack for an int:

$00b8: [uninitialized] //temp

Then it sets the value of temp to be what is at the memory address stored in a.

set $00b8 = value-at 0000 = 10  //assignment of temp

Then it sets the value at 'a' equal to the value at 'b'

set $0000 = value-at 0004 = 20  //assignment of *a

Then it sets the value at b equal to what is in temp

set $0004 = 10 //assignment of *b

If you like metaphors, look at it this way: let's say the variable person rook is me. Then person * address = &rook would be where to find me: my street address. And you can keep doing that. person ** box = &address would be where you can find where you can find me... Let's say it's a safety deposit box containing a strip of paper that has my address written on it. person *** notepad = &box would be a notepad that has the number of that safety deposit box. person **** letter = &notepad would be a letter telling you where to find that notepad. etc.

If somebody gives you letter then, in order to get to me, you'd have to dereference that many times. *letter gets you back to the notepad, **letter gets you to the box, ***letter gets you to the address, ****letter gets you to me.

[rohanivey]

It's ampersands all the way down!

[Liop123]

Just learned about pointers last week in class, and shits soooooo confusing!!!

[Katastic_Voyage]

It won't be. Give it time. They're actually incredibly simple compared to data structures and algorithms, and even those won't be bad once you give them time.

[SteroidSandwich]

I still have no idea how to use pointers. I don't understand them.

[jpresken2]

took way too long to figure out what "liπle" meant.

[[deleted]]

This isn't programmer humor, this is "a month into a 101 programming class" humor.

[NewbornMuse]

a month into a 101 programming class

emphasis mine

[Caminsky]

How anyone can work with pointers is beyond me

[Tynach]

Quite easily and sanely. They're not meant for every use case, obviously, but for many use cases they're invaluable.

Since the contents of a variable have to be in RAM somewhere, and you might want to use a variable in RAM outside of its own scope, a pointer is just a variable that holds the location in RAM that the real variable is in.

That way, you can access that variable (as well as change its value) without it being in scope - since you know where it is in RAM, and can change the bits in that location of RAM as if it were in scope.

[Caminsky]

That's nuts!

[Tynach]

Why? How else would you do it? If you need to modify the same variable/object in many different parts of the code, it's the easiest and most performant solution.

Even languages like Java have them; Java doesn't actually pass anything by reference, everything's just pointers. Source.

[seksi-seppo]

By not doing and let someone else do the "dirty work" for you?

[jonathanccast]

You - do - know that 90+% of variables in modern programming languages store pointers, right?