beginners problem

L

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",nod->data);
  printf("%10p\n", nod);
  printf("%10p\n", nod->next);
  return 0;
}

Node * alloc_node(int n){
  Node * tmp;
  tmp = (Node *) malloc(sizeof(Node));
  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;
  }
}
 
I

Ian Collins

Lars said:
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",nod->data);
Click to expand...

There's your problem, nod is NULL.
Node * alloc_node(int n){
Node * tmp;
tmp = (Node *) malloc(sizeof(Node));
Click to expand...

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

tmp = malloc(sizeof *tmp);
 
L

Lars

Ian said:
There's your problem, nod is NULL.


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

tmp = malloc(sizeof *tmp);
Click to expand...

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
 
I

Ian Collins

Lars said:
Thanks for your reply. But is this not handled by the the insert functio
through this ?
if (head==NULL) {
head = alloc_node(n);
}
Click to expand...
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);
 
L

Lars

Ian said:
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);
Click to expand...
Thats it!!!
I completely overlooked that.
Thank you very much.
Best regards
Lars
 
R

Richard Bos

Lars said:
If this is the wrong list for beginners trouble I apologize.
Click to expand...

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 {
  struct node * next;
  int data;
Click to expand...

....you might want to check what is inserting these weird characters into
your code, and...
int main(){
  Node * nod = NULL;
  insert(nod,4);
Click to expand...

....you might want to read the FAQ, in particular question 4.8:
<http://c-faq.com/ptrs/passptrinit.html>. The rest of that section, and
the rest of the FAQ, is instructive, too.

Richard
 
M

Martin Ambuhl

Lars said:
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 ?
Click to expand...

Below is a modified copy of your program. Not everything that might be
improved has been noted, but enough to suggest where your main problems
might lie apart from not handling allocation failure and not freeing
allocated memory. Consider especially the lines marked with /* $ */ and
think about what is different from your code and why.

There are several questions in the FAQ that are relevant to your code.
You might want to start with <http://c-faq.com/ptrs/passptrinit.html>,
"Question 4.8 Q: I have a function which accepts, and is supposed to
initialize, a pointer:"


#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", nod->data);
printf("%10p\n", (void *) nod); /* $ */
printf("%10p\n", (void *) nod->next); /* $ */
return 0;
}

Node *alloc_node(int n)
{
Node *tmp;
tmp = malloc(sizeof *tmp);
if (tmp) {
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; /* $ */
}
}
 
I

Ian Collins

Richard said:
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...


....you might want to check what is inserting these weird characters into
your code, and...
Click to expand...
Your client? They don't appear to be in the message source.
 
L

Lars

Thanks for the pointers :)
Regards
Lars

Martin said:
Below is a modified copy of your program. Not everything that might be
improved has been noted, but enough to suggest where your main problems
might lie apart from not handling allocation failure and not freeing
allocated memory. Consider especially the lines marked with /* $ */ and
think about what is different from your code and why.

There are several questions in the FAQ that are relevant to your code.
You might want to start with <http://c-faq.com/ptrs/passptrinit.html>,
"Question 4.8 Q: I have a function which accepts, and is supposed to
initialize, a pointer:"


#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", nod->data);
printf("%10p\n", (void *) nod); /* $ */
printf("%10p\n", (void *) nod->next); /* $ */
return 0;
}

Node *alloc_node(int n)
{
Node *tmp;
tmp = malloc(sizeof *tmp);
if (tmp) {
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; /* $ */
}
}
Click to expand...
 
R

Randy Howard

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

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.
 
S

SM Ryan

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

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

I prefer an applicative style. An equivalent to the above would be
...
nod = insert(nod,4);
...
Node *insert(Node * head, int n)
{
Node *tmp;
if (head == NULL) { /* $ */
head = alloc_node(n); /* $ */
}
else {
tmp = alloc_node(n);
head->next = tmp; /* $ */
}
return head;
}

There may be a marginal difference that an applicative style is
easier for the compiler to optimise, but overall it is question
of which is easier for you to program in.

For example if you were to implement a stack, one might do pushes
push(&stack,a);
push(&stack,b);
...
or
stack = push(stack,a);
stack = push(stack,b);
...
or
stack = push(push(push(stack,a),b),...);
 

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