CS50 PSET5 cannot free memory, seg fault

Question

It seems that my program, though it compiles and begins to run, seg faults due to a memory leak acording to valgrind:

==4602== HEAP SUMMARY:
==4602==     in use at exit: 4,007,248 bytes in 71,558 blocks
==4602==   total heap usage: 71,564 allocs, 6 frees, 4,017,464 bytes allocated
==4602== 
==4602== 336 bytes in 6 blocks are still reachable in loss record 1 of 4
==4602==    at 0x307307F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==4602==    by 0x4019C1: check (dictionary.c:37)
==4602==    by 0x40160B: main (speller.c:113)
==4602== 
==4602== 336 bytes in 6 blocks are definitely lost in loss record 2 of 4
==4602==    at 0x307307F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==4602==    by 0x4019F5: check (dictionary.c:44)
==4602==    by 0x40160B: main (speller.c:113)
==4602== 
==4602== 510,048 bytes in 9,108 blocks are definitely lost in loss record 3 of 4
==4602==    at 0x307307F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==4602==    by 0x401BFA: load (dictionary.c:144)
==4602==    by 0x4012CE: main (speller.c:40)
==4602== 
==4602== 3,496,528 bytes in 62,438 blocks are still reachable in loss record 4 of 4
==4602==    at 0x307307F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==4602==    by 0x401BFA: load (dictionary.c:144)
==4602==    by 0x4012CE: main (speller.c:40)
==4602== 
==4602== LEAK SUMMARY:
==4602==    definitely lost: 510,384 bytes in 9,114 blocks
==4602==    indirectly lost: 0 bytes in 0 blocks
==4602==      possibly lost: 0 bytes in 0 blocks
==4602==    still reachable: 3,496,864 bytes in 62,444 blocks
==4602==         suppressed: 0 bytes in 0 blocks

here are the check and load funtions in which the leaks seem to be occuring, particularly at the lines in which memory is allocated:

bool check(const char *word)
{
    // Hash word to check
    unsigned int h = hash(word);
    table[h] = malloc(sizeof(node));
    if (table[h] == NULL)
    {
        return false;
    }

    // Set up cursor
    node *cursor = malloc(sizeof(node));
    if (cursor == NULL)
    {
        return false;
    }

    // Copy word to be checked into cursor
    strcpy(cursor->word, word);

    // Set cursor to point at first element of the linked list
    cursor = table[h];

    // Traverse linked list to check for word's presence
    while (cursor->next != NULL)
    {
        if (strcasecmp(cursor->word, table[h]->next->word) == 0)
        {
            return true;

            // Free allocated memory
            while (cursor != NULL || table[h] != NULL)
            {
                node *tmp = cursor->next;
                free(cursor);
                cursor = tmp;

                node *tmp2 = table[h]->next;
                free(table[h]);
                table[h] = tmp2;

            }

        }
        else
        {
            // Move cursor to next node
            cursor = cursor->next;
        }
    }

    return false;
}

bool load(const char *dictionary)
{
    // Open dictionary file
    FILE *file = fopen(dictionary, "r");

    // Check if opening was successful
    if (file == NULL)
    {
        return false;
    }

    // Malloc new node
    node *string = malloc(sizeof(node));

    // Set up buffer, remember to initialize it, otherwise the compiler won't know what it points at
    char read_file[LENGTH + 1];

    //Read every word
    while (fscanf(file, "%s", read_file) != EOF)
    {
        // Scan word into read_file
        fscanf(file, "%s", read_file);

        // Check if memory was allocated correctly
        if (string == NULL)
        {
            return false;
        }

        // Copy string from read file to node
        strcpy(string->word, read_file);

        // Hash the word and malloc
        unsigned int h = hash(string->word);
        table[h] = malloc(sizeof(node));

        // Insert node into hash table
        string->next = table[h]->next;
        table[h]->next = string;

        counter++;
    }

    // Close file
    fclose(file);

    // Free memory
    free(string);


    return true;
}

// Returns number of words in dictionary if loaded, else 0 if not yet loaded
unsigned int size(void)
{
    return counter;
}

// Unloads dictionary from memory, returning true if successful, else false
bool unload(void)
{
    // TODO
    for (int i = 0; i < N; i++)
    {
        node *cursor = NULL;

        while (cursor != NULL)
        {
            cursor = table[i]->next;
            node *tmp = cursor;
            cursor = cursor->next;
            free(tmp);
        }
    }
    return true;
}

I will be happy to provide further information if it is needed.

Answer 1

Just considering the allocations and the assignment:

table[h] = malloc();
cursor = malloc();
cursor = table[h]; // here you lose the only link to the newly allocated node!
                   // -> memory leak

Additionally you do not check if there's already a node in table[h] then you lose the link to it as you just overwrite without checking.

My personal approach would be first looking for the insertion point, then create the new node:

node** cursor = &table[h];
while(*cursor)
{
    cursor = &(*cursor)->next;
}
*cursor = malloc(sizeof(**cursor));