segmentation fault -- cant understand the cause of the fault

Question

I dont understand the cause of the segmentation fault here. The code is:

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

void add(int a,struct node *lista)
{
    struct node *p;
    p=(struct node*)malloc(sizeof(struct node*));

    p->data=a;
    p->next=NULL;

    while(lista->next!=NULL)       <--- The segmentation fault is here. 
        lista=lista->next;                    
    lista->next=p;

    return lista;

}

int main(void)
{
    struct node *list=NULL;
    list_print(list);

    list=node123();
    list_print(list);

    add(7, &list);
    list_print(list);

    return 0;
}

the add function which adds a new node to the end of the list worked perfectly like this on a friends computer and setup. I get segmentation fault. i think the problem is the lista->next expression but I don't understand why. Any ideas?

Did you use a debugger like `gdb` on Linux? Did you initialize all fields and local variables? Did you asked the compiler for all warnings and debugging info (e.g. compile with `gcc -Wall -Wextra -g` on Linux)? — Basile Starynkevitch, Sep 05 '12 at 11:43
You didn't include the code for `node123` which could be the source of the error. — n. m. could be an AI, Sep 05 '12 at 11:46
add(7, &list); here i shoudnt use & before list and its working — user1648856, Sep 05 '12 at 11:47
[Do not cast the return of malloc](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — Eregrith, Sep 05 '12 at 12:19

score 2 · Answer 1 · answered Sep 05 '12 at 11:45

2

void add(int a,struct node *lista)... 2nd parameter is a struct node pointer.

struct node *list=NULL; -- list is a struct node pointer.

add(7, &list); -- &list is a struct node **; this is incorrect and likely to cause add()'s `while(lista->next!=NULL) to fail its dereference.

answered Sep 05 '12 at 11:45

mah

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+1 the only relevant answer (while others are true they do not answer the question) – giorashc Sep 05 '12 at 11:47

score 1 · Answer 2 · edited May 23 '17 at 11:56

1

p = (struct node*)malloc(sizeof(struct node*));

This is certainly wrong. You must not allocate memory sized as the pointer itself, but as big as the actual structure. Use

p = malloc(sizeof(struct node));

or even better

p = malloc(sizeof(*p));

And don't for the love of God cast the return value of malloc().

Also, you declare list as struct node *, and your add() function also expects a struct node * - so it's erronous to pass its address to the function. Instead of

add(7, &list);

write

add(7, list);

edited May 23 '17 at 11:56

Community

1
1

answered Sep 05 '12 at 11:45

Why the hint of not casting `malloc` returned value? I'll suggest to *always* cast it.... – Basile Starynkevitch Sep 05 '12 at 12:24
@BasileStarynkevitch because this is not C++. Haven't you read the SO question and answer behind that link? – Sep 05 '12 at 12:56

Hicham · Answer 3 · 2012-09-05T11:51:50.980

0

1 - you have to check if lista is not null before writhin lista->next

2 - There is an error in the malloc : p=(struct node*)malloc(sizeof(struct node));

the size to allocate is the size of a node, you allocated the size of a pointer struct node*.

3 - add(7 , lista) not add(7 , &lista) because lista is already a pointer.

edited Sep 05 '12 at 11:51

answered Sep 05 '12 at 11:46

Hicham

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score 0 · Answer 4 · answered Sep 05 '12 at 11:47

You're passing the address of list, but the funciton takes only a pointer, in order to pass 'list' by reference, You've got to chagne the decliration of add to:

void add(int a,struct node **lista);

and then use (*lista) instead of just 'list' . ex: (*lista)->next ...

score 0 · Answer 5 · answered Sep 05 '12 at 11:50

0

You declared 'add' to not return any type of data (void). but you're returning 'list'. either make the function work on a pointer to a pointer to 'list' (taking **list as a parameter instead of *list). or make it return a list type: struct list* add(

answered Sep 05 '12 at 11:50

Taher

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segmentation fault -- cant understand the cause of the fault

5 Answers5