Segment fault on trying to access head while inserting first value into linked list

Question

I have been working all day on not just this but 3 other assignments(for the same class) and this is the last issue I cannot figure out on my own, I am new with templates so not so sure as to how they work 100%.

Without templates this code runs perfectly, but with templates I am receiving a segmentation fault in the prioqueueUNS file at the if(head == NULL) and I cannot figure out why this is happening because I am defaulting head to NULL in the constructor, so any help would be very appreciated

int main

#include "node.h" 
#include "prioqueueUNS.cpp"
int main() { 
    PrioQueueUNS<int> list; 
    list.insertItem(1);
}

node.h

#ifndef node_h
#define node_h
using namespace std; 
template<class Type> 
struct node { 
    Type data; 
    node<Type> *next; 
}; 
#endif

prioqueueUNS.cpp

#ifndef prioqueueUNS_cpp
#define prioqueueUNS_cpp
#include "node.h
using namespace std; 

template<class Type> 
class PrioQueueUNS { 
private: 
    node<Type> *head; 
    node<type> *tail;
    int sizee; 
    int size;
    int min; 

public: 
    PrioQueueUNS() { 
        head = NULL;
        tail = NULL;
    } 

    PrioQueueUNS(Type *dataArray, int n) { 
        head = NULL;
        tail = NULL;
    } 

    void insertItem(Type n) { 
        node<Type> *temp;
        temp->data = n;
        temp->next = NULL;

        if (head == NULL) { //<-- segment faulting when trying to access head
            head = temp;
            tail = temp;
            min = n; 
        } 
     }
 };

Possible duplicate of [What is a debugger and how can it help me diagnose problems?](https://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems) — Raedwald, Apr 20 '18 at 06:53

score 3 · Accepted Answer · answered Apr 20 '18 at 06:33

3

node<Type> *temp;
temp->data = n;

You create a pointer (temp) but it points to nothing, so temp->data tries to access a data field of something that does not exist.

You could fix this by using new but this would require you to destroy the object afterwards.

answered Apr 20 '18 at 06:33

Jerome Reinländer

1,227
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so with this, my seg fault is not coming from temp, its coming from if(head == NULL), if i put a cout after all the temp stuff, it will still run through and seg fault on head – Colby Grappe Apr 20 '18 at 06:56
but it fixed the issue i think – Colby Grappe Apr 20 '18 at 06:57
if you see this, do you have any clue why my linked list can take in roughly 1.4 million values before it seg faults now – Colby Grappe Apr 20 '18 at 07:11
@ColbyGrappe Sorry, can't help you with that. My first intuition was that you may be running out of memory (have you checked that?), but I am not sure if this can cause a segmentation fault and 1.4 million values should still be in the mega byte range, which should be fine, I guess. You might want to open a new question with this very specific question and information. – Jerome Reinländer Apr 20 '18 at 07:39

Abhijit Pritam Dutta · Answer 2 · 2018-04-20T06:51:50.833

0

You never allocate memory for your node. Also declaration of pointer temp is also not correct:-

 node<Type> *temp;// temp is a pointer only you have to allocate memory first
 temp->data = n;
 temp->next = NULL;

Correct it like below:-

For an integer:-

node<int> *temp = new node<int>();

For a flot:-

 node<flot> *temp = new node<flot>();

Also correct below declaragtion:-

node<type> *tail;  to node<Type> *tail;

edited Apr 20 '18 at 06:51

answered Apr 20 '18 at 06:37

Abhijit Pritam Dutta

5,521
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Segment fault on trying to access head while inserting first value into linked list

2 Answers2