Question

I’m writing a program that uses a Hashtable of linked lists to count the frequency of words. The program will count all the words that I enter along with the frequency however after printing the hashtable, I get a segmentation fault (core dumped) error. When I valgrind my program, it shows that I get errors in three different places that are Invalid read of size 8. I’m not sure how to fix them though. Here are the three different places:

 void freeTable(HashTablePtr table) {
     int i;
     ListPtr list;

     if (table == NULL)
         return;

     for (i = 0; i < table->size; i++) {
         list = table->table[i];
         freeList(list);
     }

     free(table->table);
     free(table);
 }


 HashTablePtr createTable(int tableSize) {

     int i;
     HashTablePtr table = (HashTablePtr) malloc(sizeof(HashTablePtr));
     table->table = (ListPtr *) malloc(sizeof(ListPtr) * tableSize);
     table->size = tableSize;

     for (i = 0; i < table->size; i++) {
         table->table[i] = createList();
     }

     return table;
 }


 void printTable(HashTablePtr table) {

     ListPtr tempList;
     NodePtr tempNode;
     HashObjectPtr obj;
     int i;

     for (i = 1; i < table->size; i++) {
         tempList = table->table[i];
         if (tempList->size != 0) {
             tempNode = tempList->head;
             obj = tempNode->HashObject;
             printf("%s\n\n", toString(obj));
         }
     }
 }

I think that the error has to due with using these lines:

tempList = table->table[i];

table->table[i] = createList();

but I’m not sure how to fix it.

Edit:

 typedef struct hashtable HashTable;
 typedef struct hashtable * HashTablePtr;

 struct hashtable {
     int size;
     ListPtr *table;
 };

Valgrind errors:

999 errors in context 5 of 9:

==73795== Invalid read of size 8

==73795== at 0x400B7D: printTable (HashTable.c:96)

==73795== by 0x400766: main (wf.c:16)

==73795== Address 0x4c34048 is 0 bytes after a block of size 8 alloc’d
==73795== at 0x4A0515D: malloc (vg_replace_malloc.c:195)
==73795== by 0x400D05: createTable (HashTable.c:17)
==73795== by 0x400753: main (wf.c:14)
==73795==
==73795==
==73795== 1000 errors in context 6 of 9:
==73795== Invalid read of size 8
==73795== at 0x400B2B: freeTable (HashTable.c:128)
==73795== by 0x40076E: main (wf.c:17)
==73795== Address 0x4c34048 is 0 bytes after a block of size 8 alloc’d
==73795== at 0x4A0515D: malloc (vg_replace_malloc.c:195)
==73795== by 0x400D05: createTable (HashTable.c:17)
==73795== by 0x400753: main (wf.c:14)
==73795==
==73795==
==73795== 1000 errors in context 7 of 9:
==73795== Invalid read of size 8
==73795== at 0x400D4C: createTable (HashTable.c:25)
==73795== by 0x400753: main (wf.c:14)
==73795== Address 0x4c34048 is 0 bytes after a block of size 8 alloc’d
==73795== at 0x4A0515D: malloc (vg_replace_malloc.c:195)
==73795== by 0x400D05: createTable (HashTable.c:17)
==73795== by 0x400753: main (wf.c:14)

 ListPtr createList() {
     ListPtr list;
     list = (ListPtr) malloc(sizeof(List));
     list->size = 0;
     list->head = NULL;
     list->tail = NULL;
     return list;
 }

Answer 1

The HashTablePtr table = (HashTablePtr) malloc(sizeof(HashTablePtr)); is almost certainly wrong. You want to allocate enough storage for a HashTable, but it seems you’re allocating storage for just a pointer to a HashTable(your HashTablePtr)

If you drop the habit of typedef’ing pointer and instead follow the approach of allocating in the following form, you won’t get into that sort of problems:

HashTable *table = malloc(sizeof *table);