beginners problem

[Lars]

Hi all
If this is the wrong list for beginners trouble I apologize.
please refer to me the correct list in such case.

I am trying to create a simple linked list but I keep getting segmentation
errors and I dont know why. I realize that I do not free the allocated
memory but the segfault happpens (it seems) with the insert function.
Can any one explain ?

Regards Lars

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

typedef struct node {
  struct node * next;
  int data;
} Node;


Node * alloc_node(int n);

void insert(Node * head, int n);

int main(){
  Node * nod = NULL;
  insert(nod,4);
  printf("%d\n",n od->data);
  printf("%10p\n" , nod);
  printf("%10p\n" , nod->next);
  return 0;
}

Node * alloc_node(int n){
  Node * tmp;
  tmp = (Node *) malloc(sizeof(N ode));
  tmp->data = n;
  tmp->next = NULL;
  return tmp;
}

void insert(Node * head, int n) {
  Node * tmp;
  if (head==NULL) {
    head = alloc_node(n);
  }
  else {  
    tmp = alloc_node(n);
    head->next = tmp;
  }
}

[Ian Collins]

Re: beginners problem

Lars wrote:

Hi all
If this is the wrong list for beginners trouble I apologize.
please refer to me the correct list in such case.
>
I am trying to create a simple linked list but I keep getting segmentation
errors and I dont know why. I realize that I do not free the allocated
memory but the segfault happpens (it seems) with the insert function.
Can any one explain ?
>
Regards Lars
>
#include <stdio.h>
#include <stdlib.h>
>
typedef struct node {
struct node * next;
int data;
} Node;
>
>
Node * alloc_node(int n);
>
void insert(Node * head, int n);
>
int main(){
Node * nod = NULL;
insert(nod,4);
printf("%d\n",n od->data);

There's your problem, nod is NULL.

Node * alloc_node(int n){
Node * tmp;
tmp = (Node *) malloc(sizeof(N ode));

You don't have to (and shouldn't) cast the return value of malloc.

tmp = malloc(sizeof *tmp);

--
Ian Collins.

[Lars]

Re: beginners problem

Ian Collins wrote:

Lars wrote:

>Hi all
>If this is the wrong list for beginners trouble I apologize.
>please refer to me the correct list in such case.
>>
>I am trying to create a simple linked list but I keep getting
>segmentation errors and I dont know why. I realize that I do not free the
>allocated memory but the segfault happpens (it seems) with the insert
>function. Can any one explain ?
>>
>Regards Lars
>>
>#include <stdio.h>
>#include <stdlib.h>
>>
>typedef struct node {
> struct node * next;
> int data;
>} Node;
>>
>>
>Node * alloc_node(int n);
>>
>void insert(Node * head, int n);
>>
>int main(){
> Node * nod = NULL;
> insert(nod,4);
> printf("%d\n",n od->data);

>
There's your problem, nod is NULL.
>

>Node * alloc_node(int n){
> Node * tmp;
> tmp = (Node *) malloc(sizeof(N ode));

>
You don't have to (and shouldn't) cast the return value of malloc.
>
tmp = malloc(sizeof *tmp);
>

Thanks for your reply. But is this not handled by the the insert functio
through this ?
if (head==NULL) {
    head = alloc_node(n);
  }

Regards
Lars

[Ian Collins]

Re: beginners problem

Lars wrote:

Ian Collins wrote:

>>>
>>Node * alloc_node(int n);
>>>
>>void insert(Node * head, int n);
>>>
>>int main(){
>> Node * nod = NULL;
>> insert(nod,4);
>> printf("%d\n",n od->data);

>There's your problem, nod is NULL.
>>

>
Thanks for your reply. But is this not handled by the the insert functio
through this ?
if (head==NULL) {
head = alloc_node(n);
}
>

No, ant changes to head are local to the function, heat is a pointer, if
you wanted the change to be visible to the caller, it would have to be a
pointer to a pointer.

void insert(Node** head, int n);

--
Ian Collins.

[Lars]

Re: beginners problem

Ian Collins wrote:

Lars wrote:

>Ian Collins wrote:

>>>>
>>>Node * alloc_node(int n);
>>>>
>>>void insert(Node * head, int n);
>>>>
>>>int main(){
>>> Node * nod = NULL;
>>> insert(nod,4);
>>> printf("%d\n",n od->data);
>>There's your problem, nod is NULL.
>>>

>>
>Thanks for your reply. But is this not handled by the the insert functio
>through this ?
> if (head==NULL) {
> head = alloc_node(n);
> }
>>

No, ant changes to head are local to the function, heat is a pointer, if
you wanted the change to be visible to the caller, it would have to be a
pointer to a pointer.
>
void insert(Node** head, int n);
>

Thats it!!!
I completely overlooked that.
Thank you very much.
Best regards
Lars

[Ian Collins]

Re: beginners problem

Richard Bos wrote:

Lars <a@nondisclosur e.dkwrote:
>

>If this is the wrong list for beginners trouble I apologize.

>
This is not a list, it's a newsgroup. And if you post questions about
ISO C, not about, oh, I dunno, M$VC#++4.6 or Ganuck 4.77.35.88.55-alpha,
you should be fine here whether you're a beginner or not. Your question
is perfectly on-topic. However...
>

>#include <stdio.h>
>#include <stdlib.h>
>>
>typedef struct node {
>Â struct node * next;
>Â int data;

>
....you might want to check what is inserting these weird characters into
your code, and...
>

Your client? They don't appear to be in the message source.

--
Ian Collins.

[Randy Howard]

Re: beginners problem

On Mon, 11 Feb 2008 02:47:11 -0600, Ian Collins wrote
(in article <61agcgF1ua3ccU 9@mid.individua l.net>):

Richard Bos wrote:

>Lars <a@nondisclosur e.dkwrote:
>>

>>If this is the wrong list for beginners trouble I apologize.

>>
>This is not a list, it's a newsgroup. And if you post questions about
>ISO C, not about, oh, I dunno, M$VC#++4.6 or Ganuck 4.77.35.88.55-alpha,
>you should be fine here whether you're a beginner or not. Your question
>is perfectly on-topic. However...
>>

>>#include <stdio.h>
>>#include <stdlib.h>
>>>
>>typedef struct node {
>>Â struct node * next;
>>Â int data;

>>
>....you might want to check what is inserting these weird characters into
>your code, and...
>>

Your client? They don't appear to be in the message source.

I saw them as well. I'm pretty sure it's some UTF/Unicode weirdness
with tab expansion or perhaps newline encoding somewhere along the way,
and we're both using different clients, on different operating systems.
Being that the OP is apparently in Denmark, it's a safe bet that it
something along those lines.


--
Randy Howard (2reply remove FOOBAR)
"The power of accurate observation is called cynicism by those
who have not got it." - George Bernard Shaw