Insert a node to last position in a LinkedList

Question

I need to insert a node to the last position of a linked list. This is what I came up with:

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

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

struct node* makenode(float item){
    struct node* p=(struct node*)malloc(sizeof (struct node));
    if(p) p->data = item;
    return p;
}
void init (struct node **p){
    *p=0;
}
int addlast(struct node **ptr, float item){
    struct node* p=makenode(item);
    if(!p) return 0;
    struct node* temp=*ptr;
    while(temp->next)temp = temp->next;
    p->next=0;
    temp->next=p;
    return 1;
}      
float delfirst(struct node **ptr){
    struct node* p =*ptr;
    *ptr=(*ptr)->next;
    float temp=p->data;
    free(p);
    return temp;
}

void main(){
    struct node *list,*list2;
    init (&list);
    int i;

    for(i=0;i<10;i++)addlast(&list,i);

    while(list)printf("%4.2f\t",delfirst(&list));
    getchar();

}

but when I compile my code, its keep getting crashed and the error is in the addlast function. but I can't find where I got wrong. Can anyone please tell me where I got wrong in addlast function?

Answer 1

Your makenode function is flawed, it doesn't initialize all elements in the structure.

Your addlast is also flawed, in that when you're adding the first node then *ptr is NULL and you're dereferencing this NULL pointer in temp->next , leading to undefined behavior .

Answer 2

struct node* temp=*ptr;
while(temp->next)temp = temp->next;

When you call addlast() API your pass a pointer which is NULL and you use this pointer to initialize temp and start using temp.

Accessing/Dereferencing NULL pointers will lead to undefined behavior and hence the crash.

Answer 3

在使用temp->next之前，测试temp是否为NULL。