Thread: Linked List Structs

I'm getting a segmentation fault when I run this. It is supposed to take in a pointer to an array of structures which contain a weight variable with varying amounts in them. listAddInOrder is supposed to store the weights in increasing weight starting at the struct element in the array with the lowest weight and going up storing them in one linked list. A portion of my linked list function was originally written for strings, so I'm having difficulty modifying it to work with an array of structs. This is in my linkedlist.c file, the last chunk of code attached. Any help would be greatly appreciated.

hcompress.h

Code:

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

    typedef struct tnode {
        double weight;
        int c;
        struct tnode* left;
        struct tnode* right;
        struct tnode* parent;
    } singleNode;
    
    //prototypes
    singleNode* createFreqTable(char* filename);
    singleNode* createHuffmanTree(singleNode* leafNodes);

linkedlist.h

Code:

#include "hcompress.h"

typedef struct node {
  singleNode* value;
  struct node* next;
} LinkedList;

//function prototypes
LinkedList* llCreate();
void listAddInOrder(LinkedList** ll, singleNode* value);

linkedlist.c

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "linkedlist.h"

int main() {
    LinkedList* list = (LinkedList*)malloc(sizeof(LinkedList));
    singleNode* first;
    singleNode* second;
    singleNode* third;
    first->weight = 3.0;
    second->weight = 7.0;
    third->weight = 12.0;
   
    listAddInOrder(&list, first);
    listAddInOrder(&list, second);
    listAddInOrder(&list, third);
   
   
    printf("weight: %f\n", first->weight);
   // llDisplay(list);
}

void listAddInOrder(LinkedList** ll, singleNode* value){
//add in order function - needs work
LinkedList* nn = (LinkedList*)malloc(sizeof(LinkedList));
nn->value = value;

if (*ll == NULL) {
   *ll = nn;
} else {
   // General case - find the end
   LinkedList* p = *ll;
   while (p->next != NULL) {
    p = p->next;
   }
   p->next = nn;
}
singleNode temp;
for (int i = 0; i < 127; i++) {
       for (int j = i + 1; j < 127; j++) {
           if ((*(value+i)).weight > (*(value+j)).weight) {
               temp = value[i];
               value[i] = value[j];
               value[j] = temp;
           }
       }
   }
}



LinkedList* llCreate() {
  return NULL;
}

Think about what you are doing here:

Code:

singleNode* first;
singleNode* second;
singleNode* third;
first->weight = 3.0;
second->weight = 7.0;
third->weight = 12.0;

You're assigning values to uninitalized pointers.

Originally Posted by johngoodman

I'm getting a segmentation fault when I run this.

Isn't it time to learn to use a debugger which will help you to find the errors in your program yourself?

At least you would be able to tell us where the program crashes instead of telling us that it doesn't work.

Even just compiling with warnings should have told you that you are using the pointers uninitialised:

Code:

$ cat foo.c
#include<stdio.h>

int main(void)
{
    struct test {int a;} *t;
    t->a = 1;

    return 0;
}
$ gcc -Wall foo.c
foo.c: In function ‘main’:
foo.c:6:10: warning: ‘t’ is used uninitialised in this function [-Wuninitialized]

Bye, Andreas