Thread: poniters will kill me

Halo all,
First of all, I would like to say that this thing called "pointers" is making me sick. Each time I see '*' I get totally confused as to what should I do. So please help me, you can say me a prey of pointers. I want them to understand like you guys here on this forum have done. But still I'm sure I dont know even 0.0001% of it. Here's a code which I cant understand

Code:

#include<stdio.h>
#include<string.h>
void f(int *a[])
{
	printf("%d",(*++a)[0]);
	printf("\n%p",(void *)(*++a[0]));
}
int main(void)
{
	int i=10,j=20,k=40,l=50;
	int *r[4];
	r[0]=&i;
	r[1]=&j;
	r[2]=&k;
	r[3]=&l;
	f(r);
}

In this code except for knowing that r is an array of 4 int pointers, I dont know anything, specially when it's passed to the function. Now my questions are:
1. what is a[] in the function?
2.How are the outputs coming out to be 20 and CCCCCCCC respectively?
3.When I try to initialize r like this

Code:

int *r[4]={&i,&j,&k,&l};

I get warnings(j,k,l also)

Warning 2 warning C4221: nonstandard extension used : 'r' : cannot be initialized using address of automatic variable 'i'

I also want to say that this code wasn't given anywhere but I was exprimenting with array of pointers and suddenly made the code.
Waiting in anticipation
Thanks

Originally Posted by BEN10

1. what is a[] in the function?

Notation that hints that the argument is supposed to be a pointer to the first element of an array, in this case an array of pointers. You could have written without this hint:

Code:

void f(int **a)

Originally Posted by BEN10

2.How are the outputs coming out to be 20 and CCCCCCCC respectively?

When control enters f, a is a pointer to the first pointer in r from main. The result of ++a is therefore a pointer to the second pointer in r from main. Hence, the result of *++a is the second pointer in r from main. This means that (*++a)[0] (equivalent to *((*++a) + 0)) is what the second pointer in r from main points to, i.e., j from main. Since j has a value of 20, 20 is printed.

Now, a is a pointer to the second pointer in r from main. The result of a[0] is therefore what a points to, i.e., the second pointer in r from main. On the next step, we have undefined behaviour: ++a[0] is then the next pointer in some array that the second pointer in r is supposed to point into, but the second pointer in r points to j, which is a variable by itself. Chances are, what you will end up with is the address of a variable adjacent on the stack that j is on, e.g., you might end up with the address of i, or maybe that of k. Of course, then *++a[0] is the corresponding value, e.g., 10 or 40 (but remember: we are in the realm of undefined behaviour). So, the cast just casts an int value to void*, then prints it as if it were an address.

Thanks laserlight for your explanatory response. Now I understand it. But what about my third question. I repeat it here. When I initialize the array of pointers I get warnings:

Code:

Warning	1	warning C4221: nonstandard extension used : 'r' : cannot be initialized using address of automatic variable 'i'		
Warning	2	warning C4221: nonstandard extension used : 'r' : cannot be initialized using address of automatic variable 'j'		
Warning	3	warning C4221: nonstandard extension used : 'r' : cannot be initialized using address of automatic variable 'k'		
Warning	4	warning C4221: nonstandard extension used : 'r' : cannot be initialized using address of automatic variable 'l'

Originally Posted by BEN10

But what about my third question.

I've never tried to do that but evidently you can't, which is a bit of a drag. There are other little no-no's about variable initialization like that, I imagine it has to do with how the compiler works.

Originally Posted by tabstop

In other words, you are asking the compiler (at compile time) to know what addresses those four variables will have (at runtime), which of course it can't.

Thanks tabstop that makes sense. Now I get the whole thing.
Now something off topic.
How do all of you professionals are so genious in programming? All of the answers and concepts are on your tips. Be it a toughest of tough question you are there to crack it. I still just imagine what it takes to become a professional in coding. Seriously, all of you people in this forum are genious. Hats off.

Originally Posted by BEN10

Seriously, all of you people in this forum are genious. Hats off.

Someone please give BEN10 a job hiring programmers, maybe at a nice company in the Bahamas...

Most here have been programming for years. That's how they seem to know everything.

Originally Posted by BEN10

Thanks tabstop that makes sense. Now I get the whole thing.
Now something off topic.
How do all of you professionals are so genious in programming? All of the answers and concepts are on your tips. Be it a toughest of tough question you are there to crack it. I still just imagine what it takes to become a professional in coding. Seriously, all of you people in this forum are genious. Hats off.

Because we encountered the same problems you have, and put an unreasonable amount of time into finding a solution

Originally Posted by BEN10

Thanks laserlight for your explanatory response. Now I understand it. But what about my third question. I repeat it here. When I initialize the array of pointers I get warnings:

Code:

Warning	1	warning C4221: nonstandard extension used : 'r' : cannot be initialized using address of automatic variable 'i'		
Warning	2	warning C4221: nonstandard extension used : 'r' : cannot be initialized using address of automatic variable 'j'		
Warning	3	warning C4221: nonstandard extension used : 'r' : cannot be initialized using address of automatic variable 'k'		
Warning	4	warning C4221: nonstandard extension used : 'r' : cannot be initialized using address of automatic variable 'l'

What compiler are you using? I can't reproduce that error

Visual C++.
And it's a warning. Albeit a non-standard code warning.

Originally Posted by Elysia

Most here have been programming for years. That's how they seem to know everything.

For how many years are you programming? And what approach did you take for difficult to understand topics specially pointers?

Originally Posted by ಠ_ಠ

Because we encountered the same problems you have, and put an unreasonable amount of time into finding a solution

I also devote time in thinking but to no avail. If I would have not joined this forum I would be still using void main(). Seriously.

Originally Posted by BEN10

And what approach did you take for difficult to understand topics specially pointers?

I'm still not Elysia but my #1 recommendation for understanding pointers is linked lists. IMO you are right to hang around cboard. Sometimes it seems like a lot of nitpicking (like the void main thing) but all those nits picked add up to "one big ball of wax". Also, you get a chorus of opinions, which you won't find in a book written by only one or two people.

Originally Posted by MK27

I'm still not Elysia but my #1 recommendation for understanding pointers is linked lists.

Yeah I was referring this question in general to all. Ok I was thinking of going to data structure soon after finishing my C basics but everytime I get stuck at one point or the other. Thanks, now after your reply I think I'll soon switch to linked lists.

Originally Posted by tabstop

In other words, you are asking the compiler (at compile time) to know what addresses those four variables will have (at runtime), which of course it can't.

Of course it can, though. The addresses of those local variables are just offsets from the current base pointer and the instructions to produce those addresses can be generated at compile time.

I have no idea why that initialization is non-standard. It isn't like it's hard for the compiler to implement it.