Dynamically and "correctly" allocating memory

[filia&sofia]

For example I would like to dynamically allocate memory for linked
list (or complex struct etc.) that has not maximum number of elements.
All that I can remember is that I have to have allocated variable to
point to the linked list in main program and that I have to have some
kind of a method that initializes the list (plus other methods to
manipulate list, of course). "Correctly" means that there should be
pointers involved, and possibly double or triple pointers. I have do
this long ago, but I just can't remember how to do this neatly... Any
ideas or links to help me rembember (and yes, I know malloc and
realloc basics)? Thx in advance.

 

[Nick Keighley]

please leave the subject in the body of your message as well
Subject: "Dynamically and "correctly" allocating memory"

For example I would like to dynamically allocate memory for linked
list (or complex struct etc.) that has not maximum number of elements.

perhaps look up linked lists in a data structures book. Or online.
Wiki probably has a good description.

All that I can remember is that I have to have allocated variable to
point to the linked list in main program

no. You have to "root" the linked list somewhere, but it doesn't have
to be the main program. It could be inside the library that implements
the linked list or global data. It could be static or dynamic.

and that I have to have some
kind of a method that initializes the list (plus other methods to
manipulate list, of course). "Correctly" means that there should be
pointers involved,

I'm nor sure how "correctly" implies pointers. Though I'd do it that
way.

and possibly double or triple pointers.
why?

I have do
this long ago, but I just can't remember how to do this neatly... Any
ideas or links to help me rembember (and yes, I know malloc and
realloc basics)? Thx in advance.

#include <stdlib.h>
#include <assert.h>

typedef struct node_tag
{
char* data; /* whatever data you want */
struct node_tag *next;
} Node;

typedef struct
{
Node *head;
Node *tail;
} List;

int main (void)
{
List list = {NULL, NULL};
Node *node;

node = create_node("red");
add (&list, node);
assert (find (&list "red"));
assert (!find (&list "blue"));

node = create_node("blue");
add (&list, node);
assert (find (&list "red"));
assert (find (&list "blue"));

node = remove (&list, "red");
destroy_node (node);
assert (!find (&list "red"));
assert (find (&list "blue"));

reurn 0;
}

now just implement the functions above. assert() halts if its
argument evaluates to false (zero).


--
Nick Keighley
"Of course I'm going to be in an aeroplane on 31st December 1999.
You scientists wouldn't be stupid enough to build things that don't
work.
Besides what have computers got to do with aeroplanes anyway?"
(Kerry Nov 1998)

 

[Malcolm McLean]

For example I would like to dynamically allocate memory for linked
list (or complex struct etc.) that has not maximum number of elements.
All that I can remember is that I have to have allocated variable to
point to the linked list in main program and that I have to have some
kind of a method that initializes the list (plus other methods to
manipulate list, of course). "Correctly" means that there should be
pointers involved, and possibly double or triple pointers. I have do
this long ago, but I just can't remember how to do this neatly... Any
ideas or links to help me rembember (and yes, I know malloc and
realloc basics)? Thx in advance.

typedef struct node
{
struct node *prev;
struct node *next;
void *ptr; /* hook to hang arbitrary data off */
} NODE;

Start with a node where prev and next are both zero. This is a list of one
item.

NODE *list = malloc(sizeof(NODE));
list->next = 0;
list->prev = 0;
list->ptr = strdup("item one");
(strdup() is a non-standard function that duplicates a string. If you don't
have it, write one and call it mystrdup to avoid name clashes).

To add

NODE *newnode = malloc(sizeof(NODE));
newnode->ptr = stdup("item two");
newnode->next = list;
list->prev = newnode;
list = newnode;

to print your list

void printlist(NDOE *head)
{
while(head)
{
printf("***%s***\n", (char *) head->ptr);
head = head->next;
}
}

Deletion is harder.

void del(NODE *node)
{
NODE *prev = node->prev;
NODE *next = node->next;

if(prev)
prev->next = next;
if(next)
next->prev = prev;
free(node->ptr);
free(node);
}

 

[August Karlstrom]

filia&sofia skrev:

For example I would like to dynamically allocate memory for linked
list (or complex struct etc.) that has not maximum number of elements.
All that I can remember is that I have to have allocated variable to
point to the linked list in main program and that I have to have some
kind of a method that initializes the list (plus other methods to
manipulate list, of course). "Correctly" means that there should be
pointers involved, and possibly double or triple pointers. I have do
this long ago, but I just can't remember how to do this neatly... Any
ideas or links to help me rembember (and yes, I know malloc and
realloc basics)? Thx in advance.

Below is a an example.


August


#include <stdio.h>
#include <stdlib.h>

typedef struct List {
int item;
struct List *next;
} List;

int main(void)
{
List *p, *q;
int i;

/* initialize the list */
p = NULL;
for (i = 0; i < 10; i++) {
q = malloc(sizeof *q); q->item = i; q->next = p;
p = q;
}

/* print the list */
q = p;
while (q != NULL) {
printf("%d\n", q->item);
q = q->next;
}

/* free the list */
while (p != NULL) {
q = p->next;
free(p);
p = q;
}

return 0;
}

 

[Richard Heathfield]

Malcolm McLean said:

typedef struct node
{
struct node *prev;
struct node *next;
void *ptr; /* hook to hang arbitrary data off */
} NODE;

Better: separate the type creation from the synonym creation.

struct node
{
struct node *prev;
struct node *next;
void *next;
};

typedef struct node NODE;

Start with a node where prev and next are both zero. This is a list of
one item.

NODE *list = malloc(sizeof(NODE));

Better: NODE *list = malloc(sizeof *list);

list->next = 0;

Don't write to the object pointed to by list until you're sure malloc
succeeded and that such an object therefore exists.

list->prev = 0;
list->ptr = strdup("item one");
(strdup() is a non-standard function that duplicates a string. If you
don't have it, write one and call it mystrdup to avoid name clashes).

Better: don't use it at all, even if you have it.

To add

NODE *newnode = malloc(sizeof(NODE));
newnode->ptr = stdup("item two");

What is stdup?

newnode->next = list;
list->prev = newnode;
list = newnode;

The same comments apply as above. Your adding algorithm fails to take
into account the possibility that the list is empty.

to print your list

void printlist(NDOE *head)

NDOE? Where did that come from? This gets worse and worse. In fact, it's
becoming ludicrous.

<snip>

 

[pete]

For example I would like to dynamically allocate memory for linked
list (or complex struct etc.) that has not maximum number of elements.
All that I can remember is that I have to have allocated variable to
point to the linked list in main program and that I have to have some
kind of a method that initializes the list (plus other methods to
manipulate list, of course). "Correctly" means that there should be
pointers involved, and possibly double or triple pointers. I have do
this long ago, but I just can't remember how to do this neatly... Any
ideas or links to help me rembember (and yes, I know malloc and
realloc basics)? Thx in advance.

realloc is not usually part of building a linked list.

This program shows one way to create a linked list:
http://www.mindspring.com/~pfilandr/C/get_line/get_delim.c

This is a similar one, but with more error handling:
http://www.mindspring.com/~pfilandr/C/get_line/get_line.c

 

[rsprawls]

Malcolm McLean said:


Better: separate the type creation from the synonym creation.

struct node
{
struct node *prev;
struct node *next;
void *next;

};

typedef struct node NODE;

Why is that better?

 

[Keith Thompson]

Why is that better?

Perhaps the point is that one declaration should do one thing.

In my opinion (and plenty of smart people disagree with me on this)
there's seldom a good reason to define a typedef for a struct type.
Just declare it as "struct node" and refer to it that way. Adding a
second name "NODE" for something that already has a perfectly good
name "struct node" is not helpful (it reduces typing, but that's
rarely a good enough reason to do something).

But if you want a typedef, why use a different identifier? Struct
tags and typedefs are in separate namespaces, since you can always
determine by context which one you're referring to. If you want a
one-word name for your type, you can declare it as:

typedef struct node {
struct node *prev;
struct node *next;
void *ptr;
} node;

 

[Richard Heathfield]

Keith Thompson said:

Malcolm McLean said:

typedef struct node
{ [...]>>> > } NODE;

Better: separate the type creation from the synonym creation.
[...]

typedef struct node NODE;

Why is that better?

Oh, how do people expect me to answer questions if they keep using
gmail? Sheesh.

Perhaps the point is that one declaration should do one thing.

Yes, it's to do with clarity. Too many C newbies have this idea that C
has a "typedef struct" concept. Separating the type creation from the
synonym creation makes the code clearer.

In my opinion (and plenty of smart people disagree with me on this)
there's seldom a good reason to define a typedef for a struct type.
Just declare it as "struct node" and refer to it that way. Adding a
second name "NODE" for something that already has a perfectly good
name "struct node" is not helpful (it reduces typing, but that's
rarely a good enough reason to do something).

Well, I must disagree with you there. Part of the reason for using C in
the first place (rather than assembly language) is that it reduces the
amount of typing we do. That's also why we use a loop to populate an
array of known size, rather than type out every assignment separately
(despite putative performance benefits concomitant with unrolling the
loop).

But if you want a typedef, why use a different identifier?

Because it helps out with some relatively brainless IDEs, such as Visual
Studio, by simplifying its task of interpreting the editor's "jump to
definition" command. (If you use the same name for type and tag, Visual
Studio doesn't know which you mean. The compiler does, of course! But
not the Intellisense thingy.)

I generally use a trailing underscore nowadays:

struct node_
{
whatever;
};
typedef struct node_ node;

which doesn't get in the way.

 

[Francine.Neary]

Keith Thompson said:
[...]>>> > } NODE;

Better: separate the type creation from the synonym creation.

[...]

typedef struct node NODE;
Why is that better?

Oh, how do people expect me to answer questions if they keep using
gmail? Sheesh.

Perhaps the point is that one declaration should do one thing.

Yes, it's to do with clarity. Too many C newbies have this idea that C
has a "typedef struct" concept. Separating the type creation from the
synonym creation makes the code clearer.

I don't see in what way it makes it clear. Refraining from using
idiomatic code on the off-chance that a newbie might see it and become
confused is surely going too far. (And perhaps the newbie will be
curious rather than confused, and read up to get clear in her mind
what the thing in question is doing.)

Well, I must disagree with you there. Part of the reason for using C in
the first place (rather than assembly language) is that it reduces the
amount of typing we do. That's also why we use a loop to populate an
array of known size, rather than type out every assignment separately
(despite putative performance benefits concomitant with unrolling the
loop).


Because it helps out with some relatively brainless IDEs, such as Visual
Studio, by simplifying its task of interpreting the editor's "jump to
definition" command. (If you use the same name for type and tag, Visual
Studio doesn't know which you mean. The compiler does, of course! But
not the Intellisense thingy.)

I generally use a trailing underscore nowadays:

struct node_
{
whatever;};

typedef struct node_ node;

which doesn't get in the way.

It doesn't get in the way, but undeniably it looks ugly. I go for
typedef struct node {
/* stuff */
} node_t;
which isn't beautiful but has a certain logic to it.

 

[August Karlstrom]

In my opinion (and plenty of smart people disagree with me on this)
there's seldom a good reason to define a typedef for a struct type.

In my opinion though, `struct T t' is harder to parse visually compared
to `T t'.


August

 

[rsprawls]

Keith Thompson said:
[...]>>> > } NODE;

Better: separate the type creation from the synonym creation.

[...]

typedef struct node NODE;
Why is that better?

Oh, how do people expect me to answer questions if they keep using
gmail? Sheesh.

Then don't. I'm not asking what bothers you in life, I'm asking for
your reasoning.

I use to to filter out nonsense, unfortunately it can't filter out
attitudes, stupid remarks and...well...

 

[rsprawls]

I don't see in what way it makes it clear. Refraining from using
idiomatic code on the off-chance that a newbie might see it and become
confused is surely going too far. (And perhaps the newbie will be
curious rather than confused, and read up to get clear in her mind
what the thing in question is doing.)

I agree. This is starting to sound more like a personal preference
rather than efficient coding.

 

[Richard Heathfield]

(e-mail address removed) said:

On Aug 31, 12:26 pm, Richard Heathfield <[email protected]> wrote:

I don't see in what way it makes it clear.

Well, this is obviously a matter of opinion, so I can't prove that it's
clearer with reference to the Standard. But I remember being misled
myself by such conflated constructs when I was learning C, and I have
no desire to inflict my own re-learning curve on other people - so
nowadays I always separate type creation from type renaming.

Refraining from using
idiomatic code on the off-chance that a newbie might see it and become
confused is surely going too far.

Um, yes and no. I think it depends very much on the idiom, and how
useful it is. I don't see the ability to create a type during the
process of renaming that very type as being particularly intuitive, and
I prefer to write my code in an intuitive way.

(And perhaps the newbie will be
curious rather than confused, and read up to get clear in her mind
what the thing in question is doing.)

Oh, very much so - but I'm afraid that this hypothetical newbie will
have to provoke her curiosity elsewhere. There's plenty of
curiosity-provoking code in the world. I see no reason, however, for me
to add my own contribution to it. I prefer my code to be mind-numbingly
obvious.

It doesn't get in the way, but undeniably it looks ugly.

Ugliosity is in the eye of the beholder.

I go for
typedef struct node {
/* stuff */
} node_t;
which isn't beautiful but has a certain logic to it.

It also violates the POSIX Standard, which reserves names ending with _t
for additional type names. Now, strictly speaking, that isn't an issue
here, since POSIX is not topical here - but I thought you might find it
thought-provoking nonetheless. (If you want to find out more about
that, comp.unix.programmer is the place to do so.)