C Having Trouble Resizing a Hash Table - c

Question

I'll post snippets of the code here which (I think) are relevant to the problem, but I can pastebin if necessary. Probably posting more than enough code already :P
My program includes a hash table which needs to double when a certain hash bucket reaches 20 entries. Although I believe the logic to be good, and it compiles like a charm, it throws up a Segmentation Fault. The code runs like a charm when not resizing, but resizing messes things up.
Thanks for any help :)
Error
Program received signal SIGSEGV, Segmentation fault.
0x0000000000401012 in ml_add (ml=0x7fffffffe528, me=0x75a5a0) at mlist.c:74
74 while((cursorNode->next) != NULL){
Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.80.el6_3.5.x86_64
(gdb) backtrace
#0 0x0000000000401012 in ml_add (ml=0x7fffffffe528, me=0x75a5a0) at mlist.c:74
#1 0x0000000000401554 in main (argc=1, argv=0x7fffffffe638) at finddupl.c:39
Structure of Hash Table
typedef struct bN { //linked list node containing data and next
MEntry *nestedEntry;
struct bN *next;
} bucketNode;
typedef struct bL { // bucket as linked list
struct bN *first;
int bucketSize;
} bucket;
struct mlist {
struct bL *currentTable; //bucket array
};
Add Function
int ml_add(MList **ml, MEntry *me){
MList *tempList;
tempList = *ml;
bucketNode *tempNode = (bucketNode *)malloc(sizeof(bucketNode));
tempNode->nestedEntry = me;
tempNode->next = NULL;
unsigned long currentHash = me_hash(me, tableSize);
if((tempList->currentTable[currentHash].bucketSize) == 0) {
tempList->currentTable[currentHash].first = tempNode;
tempList->currentTable[currentHash].bucketSize = (tempList->currentTable[currentHash].bucketSize) + 1;
}
else if((tempList->currentTable[currentHash].bucketSize) == 20){
printf("About to resize");
printf("About to resize");
tempList = ml_resize(&tempList, (tableSize * 2));
tableSize = tableSize * 2;
ml_add(&tempList,me);
}
else{
bucketNode *cursorNode;
cursorNode = tempList->currentTable[currentHash].first;
while((cursorNode->next) != NULL){
cursorNode = cursorNode->next;
}
cursorNode->next = tempNode;
tempList->currentTable[currentHash].bucketSize = (tempList->currentTable[currentHash].bucketSize) + 1;
return 1;
}
return 1;
}
Resize Function
MList *ml_resize(MList **ml, int newSize){
MList *oldList;
oldList = *ml;
MList *newList;
if ((newList = (MList *)malloc(sizeof(MList))) != NULL){
newList->currentTable = (bucket *)malloc(newSize * sizeof(bucket));
int i;
for(i = 0; i < newSize; i++){
newList->currentTable[i].first = NULL;
newList->currentTable[i].bucketSize = 0;
}
}
int j;
for(j = 0; j < tableSize; j++){
bucketNode *cursorNode = oldList->currentTable[j].first;
bucketNode *nextNode;
while(cursorNode != NULL){
nextNode = cursorNode->next;
ml_transfer(&newList, cursorNode, newSize);
cursorNode = nextNode;
}
}
free(oldList);
return newList;
}
Transfer to new list function
void ml_transfer(MList **ml, bucketNode *insertNode, int newSize){
MList *newList;
newList = *ml;
bucketNode *tempNode = insertNode;
tempNode->next = NULL;
unsigned long currentHash = me_hash((tempNode->nestedEntry), newSize);
if((newList->currentTable[currentHash].bucketSize) == 0) {
newList->currentTable[currentHash].first = tempNode;
newList->currentTable[currentHash].bucketSize = (newList->currentTable[currentHash].bucketSize) + 1;
}
else{
bucketNode *cursorNode;
cursorNode = newList->currentTable[currentHash].first;
while((cursorNode->next) != NULL){
cursorNode = cursorNode->next;
}
cursorNode->next = tempNode;
newList->currentTable[currentHash].bucketSize = (newList->currentTable[currentHash].bucketSize) + 1;
}
}

Answer 1

The problem most probably lies on the fact that the ml_add() function is failing to update the MList** ml parameter node whenever the hashtable is resized.
When the hashtable is resized, the old hashtable is destroyed (inside, ml_resize()), but the pointer to the resized, new hashtable is just updated in the tempList variable, that is just a local copy of *ml. You should also update *ml in order to modify the variable that is keeeping reference of the hashTable outside of the function, otherwise, it is left pointing to the deleted, invalid Hashtable. Try the following modification:
...
else if((tempList->currentTable[currentHash].bucketSize) == 20){
printf("About to resize");
printf("About to resize");
tempList = ml_resize(&tempList, (tableSize * 2));
tableSize = tableSize * 2;
ml_add(&tempList,me);
*ml = tempList; // this is necesary to fix the pointer outside the
// function, that still points to the hashtable
// memory freed by the resize function
}
...
Also please note the comments I made about two memory leaks existing in your code, and I would also take into account what #hexist pointed out that it is not necessary to insert at the end of the liked list at the head, simplifying the code and making it faster.