Array pointer always points to NULL

Question

Im trying create a linked list in an array of nodes. When I try to update the pointer for arrTab->h_table[index] to the address of newNode, The address points to newNodes address. But when I try to add to a list that exists in the array, the pointer always points to NULL instead of the previous value in memory. Basically the arrTab->h_table[index] head of the linked list does not update to the address of newNode.

typedef struct node {
  struct node* next;     
  int          hash;     
  s_type     symbol;
} node_t;


struct array {
  int      cap;    
  int      size;       
  n_type** h_table;
};



int add_to_array (array* arrTab, const char* name, int address) {
  if(s_search(arrTab, name, NULL, NULL) == NULL){
    s_type *symbol = (s_type*) calloc(1, sizeof(s_type));
    symbol->name = strdup(name);
    symbol->addr = addr;
    n_type  *newNode = (n_type*) calloc(1, sizeof(n_type));
    newNode->next = NULL;
    newNode->hash = nameHash(name);
    newNode->symbol = *symbol;
    int index = newNode->hash % arrTab->cap;
    if(arrTab->h_table[index] == NULL){
      arrTab->h_table[index] = newNode;
    } else {
      newNode->next = arrTab->h_table[index];
      arrTab->h_table[index] = newNode;
    }
    //
    arrTab->size++;
    return 1;
  }
  return 0;
}

struct node* s_search (array* arrTab, const char* name, int* hash, int* index) {
  int hashVal = nameHash(name);
  hash = &hashVal;
  int indexVal = *hash % arrTab->cap;
  index = &indexVal;
  s_type *symCopy = arrTab;
  while (symCopy->h_table[*index] != NULL){
    if(*hash == symCopy->h_table[*index]->hash){
      return symCopy->h_table[*index];
    }
    symCopy->h_table[*index] = symCopy->h_table[*index]->next;
  }
  return NULL;
}

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– Bhargav Rao, Commented Feb 18, 2019 at 7:10

user58697 · Accepted Answer · 2019-02-18 05:03:37Z

I cannot say for sure why the pointer always points to NULL; there is not enough code. Consider posting an MCVE.

The posted code however presents few problems to address.

First, it leaks memory like there is no tomorrow:

symbol_t *symbol = (symbol_t*) calloc(1, sizeof(symbol_t));

allocates some memory, and

newNode->symbol = *symbol;

copies the contents of that memory to the new location. The memory allocated still exists, and continues to exist after the function returns - but there's no way to get to it. I strongly recommend to not allocate symbol, and work directly with newNode->symbol:

newNode->symbol.name = strdup(name);
newNode->symbol.addr = addr;

The hash and index parameters to symbol_search seem to be planned as an out parameters. In that case, notice that the results of hash = &hashVal; and index = &indexVal; are invisible to the caller. You likely meant *hash = hashVal and *index = indexVal.

The biggest problem comes with sym_table_t *symCopy = symTab;.

symTab is a pointer. It points to an actual symbol table, a big piece of memory. After the assignment, symCopy points to the same piece of memory. Which means that

symCopy->hash_table[*index] = symCopy->hash_table[*index]->next;

modifies that piece of memory. Once the search is completed, the hash_table[index] is not the same as it was before the search. This could be a root of your problem. In any case, consider

node_t * cursor = symTab->hash_table[*index];

and work with this cursor instead.

As a side note, a search condition *hash == symCopy->hash_table[*index]->hash is strange. Every node in a given linked list has the same hash (check how you add them). The very first node would produce a match, even if the names are different.

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