Problem with pointers, require some help

[david]

Hello, I am getting some problems with C and how it handles the
pointers. I will tell more about the situation: I have created my own
structure for making one way linked list. I have procedure int
createList(child *root) [child is that structure] and before that I
create pointer child *rootA, and then I invoke createList(rootA)
[*rootA holds the value, and rootA is the pointer, holds the address
as I remember]. And this function creates dynamic list, but the
problem is that rootA does not point to it. After creating list
function returns the length of it.

The question is, how should I send to function a pointer, create a
list in heap and then make that pointer to point to it?

It would be something like this:
int main {
int *item;
func(*item);
printf("%d", *item);
return 0;
}

func(int *num) {
num = malloc(sizeof(int));
*num = 8;
printf("%d", *num);


printf in func gives "8", but item still does not point to the same
memory where that number is located. But I want it to point. How?
}

 

[david]

Small mistaking rewriting, should be "func(item);" in program. Any
ideas how to make it point to what I want?

 

[fnegroni]

Use a pointer to pointer.

You can read about it in any C programming manual.

 

[Malcolm McLean]

int main {
int *item;
func(*item);
printf("%d", *item);
return 0;
}

func(int *num) {
num = malloc(sizeof(int));
*num = 8;
printf("%d", *num);


printf in func gives "8", but item still does not point to the same
memory where that number is located. But I want it to point. How?
}

C always passes parameters by value. So when you set num to the return value
of malloc(), you are setting a temporary copy.

The way round this is to pass the address of a variable. This is one use of
pointers.
Let's give a slightly more realistic example. You want to clamp a pair of x,
y coordinates to the width and height of an image.

/*
clamp x, y coordiantes to edges of image
Params: width - image width
height - image height
x (in / out) x coordinate
y (in / out) y coordinate
Returns: 0 if point withing image, 1 if clamped
*/
int clamp(int width, int height, int *x, int *y)
{
/* check if we are within the image */
if(*x >= 0 && *x < width && *y >= 0 && *y < height)
return 0;
/* we're outside it, so adjust coordinates to nearest edge */
if(*x < 0)
*x = 0;
if(*x >= width)
*x = width-1;
if(*y < 0)
*y = 0;
if(*y >= height)
*y = height -1;
return 1;
}

 

[david]

Thanks for the help, I used pointer to pointer as a few people
suggested.

It is my first day with C programming language and I still collecting
books, websites and etc. with the most detailed explanation how
everything here works. Maybe you could recommend some good material?

And for ASM looks a lot easier comparing to C, but it looks that in a
week I will manage to write some good or at least better code than I
do now.

Thanks again.

 

[santosh]

Thanks for the help, I used pointer to pointer as a few people
suggested.

It is my first day with C programming language and I still collecting
books, websites and etc. with the most detailed explanation how
everything here works. Maybe you could recommend some good material?

The C Programming Language (Second Edition) by Kernighan & Ritchie
C: A Reference Manual (Fifth Edition) by Harbison & Steele
C Programming: A Modern Approach by K.N. King

Steve Summit's "notes" on C
<http://www.eskimo.com/~scs/cclass/cclass.html>

C tutorial by Tom Torf
<http://cprog.tomsweb.net/>

Latest draft of the evolving C Standard:

And for ASM looks a lot easier comparing to C, [ ... ]

The clinching advantage of C is that you needn't rewrite everything for
every platform and chip that you target.

 

[Default User]

Thanks for the help, I used pointer to pointer as a few people
suggested.

It is my first day with C programming language and I still collecting
books, websites and etc. with the most detailed explanation how
everything here works. Maybe you could recommend some good material?

Then this is not how you should approach learning. Get a basic tutorial
book. Read through it, working the exercises in each chapter. Then
start creating simple programs.





Brian

 

[CBFalconer]

Small mistaking rewriting, should be "func(item);" in program. Any
ideas how to make it point to what I want?

See sig below.

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[Richard Bos]

See sig below.

What's so interesting about that?

Richard

 

[Flash Gordon]

santosh wrote, On 05/02/08 19:54:

The C Programming Language (Second Edition) by Kernighan & Ritchie

<snip good suggestions>

Also the comp.lang.c FAQ which would have answered the OPs question. If
can be found at http://c-faq.com/