为什么我的代码中出现分段错误（核心转储）错误？

Question

This code is for creating a graph using adjacency-list.
node is the structure for horizontal linked list for every element of the vertical linked list,
whereas graphnode is the structure for vertical linked list.

The code is compiling properly, but when the first input is given, it shows segmentation fault after it.

#include<iostream>
using namespace std;
struct node        //struct
{
    struct node *next;
    char value;
};
struct graphnode
{
    struct node *start;
    struct graphnode *down;
    char value;
    int count;
};
graphnode * creategraphnode(char val)
{
    struct graphnode *newnode=new graphnode;
    newnode->start=NULL;
    newnode->down=NULL;
    newnode->value=val;
    newnode->count=0;
    return newnode;
}
node *createnode(char val)
{
    struct node *newnode=new node;
    newnode->next=NULL;
    newnode->value=val;
}
void insertgraphnode(char value,graphnode *graph)
{
    struct graphnode *newnode=creategraphnode(value);
    if(graph==NULL)
    {
        graph=newnode;
    }
    else
    {
        graphnode *temp=graph;
        while(temp->down)
            temp=temp->down;
        temp->down=newnode;
    }
}
void insertnode(graphnode *graph)
{
    char val;
    struct node *temp=graph->start;
    cout<<"What is the outdegree of "<<graph->value;
    cin>>graph->count;
    cout<<"\nEnter"<<graph->count<<" nodes separated by space:";
    for(int i=1;i<=graph->count;i++)
    {
        cin>>val;
        node* newnode=createnode(val);
        temp=newnode;
        temp=temp->next;
    }
}
void display(struct graphnode *graph)
{
    if(graph==NULL)
    {
        cout<<"\nNo data to display";
        return;
    }
    struct graphnode *temp=graph;
    while(temp)
    {
        struct node *temp1=temp->start;
        cout<<temp->value<<"=>> ";
        if(temp1==NULL)
        {
            continue;
        }
        else
        {
            while(temp1)
            {
                cout<<temp1->value<<"-> ";
                temp1=temp1->next;
            }
        }
        cout<<"/\n";
    }
}
int main()
{
    struct graphnode *graph=NULL;
    int totalnodes;
    char val;
    cout<<"\nHow many nodes does the graph contain? : ";
    cin>>totalnodes;
    cout<<"\nEnter "<<totalnodes<<" space separated nodes : ";
    for(int i=1;i<=totalnodes;i++)
    {
        cin>>val;
        insertgraphnode(val,graph);
    }
    struct graphnode *temp=graph;
    while(temp->down)
    {
        insertnode(temp);
        temp=temp->down;
    }
    display(graph);
    return 0;
}

Answer 1

If I'm not wrong, your code has at least three problems.

1) your createnode() don't return the created node; add return newnode;

2) your insertgraphnode() don't change the second parameter in the calling function; so, when you call it in main()

insertgraphnode(val,graph);

graph is NULL ; inside insertgraphnode() you change the value but outside the function graph remain NULL ; ever. This cause the segmentation fault.

You can solve this changing insertgraphnode() so the second parameter is a pointer-to-pointer and passing &graph .

Another solution is modify insertgraphnode() to return the graph value

graphnode *  insertgraphnode(char value,graphnode *graph)
{
    struct graphnode *newnode=creategraphnode(value);
    if(graph==NULL)
    {
        graph=newnode;
    }
    else
    {
        graphnode *temp=graph;
        while(temp->down)
            temp=temp->down;
        temp->down=newnode;
    }
    return graph;
}

and calling it in this way

graph = insertgraphnode(val,graph);

3) the while in display() goes in loop because you don't change the value of temp