cs50 speller keeps prompting free(): double free detected in tcache 2 - c

Question

I have been working on this problem set for quite a time and the code seems to be wrong but I couldn't find the solution. I have been comparing my code and other people's code but I still don't know where I got wrong. Really appreciate all your help if you can provide me with some ways to solve this problem. It keeps prompting me free(): double free detected in tcache 2 but I can't seem to find my mistake.
// Implements a dictionary's functionality
#include <stdbool.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>
#include <strings.h>
#include <stdlib.h>
#include "dictionary.h"
// Represents a node in a hash table
typedef struct node
{
char word[LENGTH + 1];
struct node *next;
}
node;
// Number of buckets in hash table
const unsigned int N = 50;
// Hash table
node *table[N];
//word count
int count = 0;
// Returns true if word is in dictionary, else false
bool check(const char *word)
{
// TODO
bool found = false;
node *current = table[hash(word)];
while (current != NULL)
{
if (strcasecmp(current -> word, word) == 0)
{
found = true;
}
else if(current -> next != NULL)
{
current = current -> next;
}
else
{
return false;
}
}
return found;
}
// Hashes word to a number
unsigned int hash(const char *word)
{
// TODO
unsigned long hash = 5381;
int c;
while ((c = toupper(*word++)))
{
hash = ((hash << 5) + hash) + c; /* hash * 33 + c */
}
return hash % N;
}
// Loads dictionary into memory, returning true if successful, else false
bool load(const char *dictionary)
{
// TODO
FILE *infile = fopen(dictionary, "r");
if (infile == NULL)
{
return false;
}
char buffer[LENGTH+1];
while (fscanf(infile, "%s", buffer) != EOF)
{
node *n = malloc(sizeof(node));
strcpy(n -> word, buffer);
n -> next = table[hash(buffer)];
table[hash(buffer)] = n;
count++;
free(n);
}
fclose(infile);
return true;
}
// Returns number of words in dictionary if loaded, else 0 if not yet loaded
unsigned int size(void)
{
// TODO
return count;
}
// Unloads dictionary from memory, returning true if successful, else false
bool unload(void)
{
// TODO
int num = count;
for (int i = 0; i < N ; i++)
{
node *current = table[i];
while (current != NULL)
{
node *temp = current;
current = current -> next;
free(temp);
num--;
}
}
if (num == 0)
{
return true;
}
else
{
return false;
}
}

Answer 1

The calls of free in this while loop
while (fscanf(infile, "%s", buffer) != EOF)
{
node *n = malloc(sizeof(node));
strcpy(n -> word, buffer);
n -> next = table[hash(buffer)];
table[hash(buffer)] = n;
count++;
free(n);
}
does not make a sense. You deleted at once (an object of the type node using the pointer n) what you was trying to add to the table (a valid address to an allocated object of the type node). As a result the element of the table at the position hash(buffer) that is set like
table[hash(buffer)] = n;
has an invalid value because it is the address of the already deleted node in this statement
free(n);
So in the function unload this invalid address will be again used to free already freed memory within the function load.
Pay attention to that you did not allocate memory as you wrote in a comment "for node n". n is just a pointer to the allocated unnamed object of the type node. So you are not freeing the pointer n itself in this statement
free(n);
You are freeing the allocated object of the type node using the pointer n. Thus all pointers that pointed to the allocated object of the type node become invalid.