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 😛

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.