CODEWITHSUNDEEP
csegmentation-faultmallochashtable
Highway62
Highway62

Reputation: 800

How do I properly initialise this hash table implementation in C? (Getting segmentation fault)

I am unsure what it is I am doing wrong when initialising this hash table. Can anyone help? I am getting a segfault at the initialisation of the bucket list in the for loop, where I have labelled it.

#define BUCKETMAX 20
#define TABLESIZE 100 

//Node to hold entry
typedef struct node{
    MEntry *entry;
    struct node *next;
}MListNode;

//Bucket linked list to hold Nodes
typedef struct bucket{
    int size;
    MListNode *top;
}BucketList;

//Hash table to hold Bucket Lists
typedef struct mlist{
    int capacity;
    BucketList **bList;
}MList;

MList *ml_create(void){

    //Initialising the hash table
    MList *ml;
    //Check if memory if available to be allocated before assigning capacity
    if((ml = (MList *) malloc(sizeof(BucketList **) * TABLESIZE)) == NULL){
        fprintf(stderr, "Hash Table Could Not be Created (Not Enough Memory)\n");
        exit(EXIT_FAILURE);
    }else{
        ml->capacity = TABLESIZE;
        //Initialise the bucket lists
        int i;
        for(i =0; i<TABLESIZE;i++){
            /****************************************************************
            SEGFAULT IS HERE */
            if((ml->bList[i] = (BucketList *) malloc(sizeof(MListNode *) * BUCKETMAX)) ==  NULL){
            /****************************************************************/
                fprintf(stderr, "Bucket List Could Not be Created (Not Enough Memory)\n");
                exit(EXIT_FAILURE);
            }else{
                ml->bList[i]->size = 0;
                ml->bList[i]->top = (MListNode *) NULL; 
            }
        }
    }

    return ml;
}

Any help would be appreciated, thanks.

Upvotes: 0

Views: 94

Answers (1)

jlahd
jlahd

Reputation: 6293

You cannot allocate memory for structures the way you do.

ml = (MList *) malloc(sizeof(BucketList **) * TABLESIZE)

That line allocates memory to the MList structure ml, but not to its member ml->bListwhich you are then filling in the loop at the point of the segfault.

You need to allocate memory for both purposes separately:

ml = malloc(sizeof(MList));
ml->bList = malloc(sizeof(BucketList*) * TABLESIZE);

The same applies to the latter memory allocation for ml->bList[i].

Upvotes: 1

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