Linked list crashes

[Wezel]

Hi there,

I was wondering... this code crashes when volg==NULL at (*p)->volg

But why does it crash? I need to check if this value is NULL
Any idea?

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

typedef struct record
{
	int getal;
	struct record *volg;
}ELT;

void toon_list(ELT** p)
{
	if(*p)
	{
		if((*p)->volg)
			toon_list((*p)->volg);
		printf("%d\n", (*p)->getal);

	}
}

[swoopy]

>void toon_list(ELT** p)
There's really no reason for this to be a double pointer. You could just as well use:

void toon_list(ELT* p)
{
	if(p)
	{
		if(p->volg)
			toon_list(p->volg);
		printf("%d\n", p->getal);

	}
}

[Wezel]

yeah, right, I knew that, but thank you anyway mate.

The assignment mentioned a pointer to pointer structure, and I was trying some stuff.

But still, it doesn't explain the exception error?

Any idea about that? I really need this to work.

Thanx swoopy!

Greetings

[Prelude]

>this code crashes when volg==NULL
Are you sure it's NULL? The error sounds more like you neglected to terminate your list properly and the last volg isn't actually a null pointer. Show us a complete program that crashes.

[swoopy]

>The assignment mentioned a pointer to pointer structure

> toon_list((*p)->volg);
Here the compiler should give you a warning about a type mismatch, as you're passing a pointer to ELT, but the function expects a double pointer, so you might try this:

			toon_list(&(*p)->volg);

[Wezel]

This is the code, I think I terminated it well, and I fixed that unneeded double pointer

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

typedef struct record
{
	int getal;
	struct record *volg;
}ELT;

void maak_list(ELT**, int*);
ELT* koppel(ELT *kop1, ELT *kop2);

void toon_list(ELT*);

int main(void)
{
	int tab1[]={1, 2, 3, 4, 5, 0};
	int tab2[]={6, 7, 8, 9, 0};

	ELT *kop1=NULL, *kop2=NULL, *kop;

	maak_list(&kop1, tab1);
	maak_list(&kop2, tab2);
	//kop=koppel(kop1, kop2);
	//toon_list(&kop);
	toon_list(kop1);
	toon_list(kop2);

	return 0;
}

void maak_list(ELT **pkop, int* tab)
{
	ELT *p, *vorig;
	int i;

	p=malloc(sizeof(ELT));
	p->getal=*tab;
	(*pkop)=p;
	vorig=pkop;

	for(i=1; tab[i]; i++)
	{
		p=malloc(sizeof(ELT));
		p->getal=tab[i];
		vorig->volg=p;
		vorig=p;
	}

	p->volg=NULL;
}


void toon_list(ELT* p)
{
	if(p)
	{
		if(p->volg)
			toon_list(p->volg);
		printf("%d\n", p->getal);

	}
}

[Wezel]

PLEAAAASE,really need some help on this one!!!! My exams are on monday! This problem happens on every exercise I make!

When debugging I can see that kop is still a value, and kop->volg=NULL, but WHY in eaerth does it crash????? It shouldn't crash should it???

Greetings, ANY reply would be GREATLY! appreciated, for I am getting into a state of panic since this sorta exercise WILL be my exam;
thanx!

[ZuK]

Shouting for help doesn't do much good.
It's better to listen to compiler warnings.
works this way

void maak_list(ELT **pkop, int* tab)
{
	ELT *p, *vorig;
	int i;

	p=malloc(sizeof(ELT));
	p->getal=*tab;
	(*pkop)=p;
	vorig=*pkop;

	for(i=1; tab[i]; i++)
	{
		p=malloc(sizeof(ELT));
		p->getal=tab[i];
		vorig->volg=p;
		vorig=p;
	}

	p->volg=NULL;
}

Kurt

[Wezel]

Dude I love you

Thank you I owe you