Dereferencing structure instance element using structure instance pointer

Question

I have this function:

static pair_t * // not visible outside this file
pair_new(const int key)
{

  pair_t *pair = malloc(sizeof(pair_t));

  if (pair == NULL) {
    // error allocating memory for pair; return error
    return NULL;
  } else {


            printf("key is %d\n",key);

      *(pair->key) = key;
      *(pair->count) = 1;
      pair->next = NULL;
      return pair;

  }
}

but the way I am trying to dereference the key element of the instance of pair is giving me a seg fault. I think it is because 'pair' is a pointer.

the pair structure is defined as follows:

typedef struct pair {
  int *key;                // search key for this item
  int *count;                  // pointer to data for this item
  struct pair *next;   // children
} pair_t;

Any suggestions on how to properly change the value of key would be much appreciated

Sourav Ghosh · Accepted Answer · 2016-07-20 09:37:01Z

2

No, there's no issue with pair, it's with key (and the count, too). The pointer is used uninitialized which invokes undefined behavior.

At the point where you;re trying to dereference key

 *(pair->key) = key;

there's no valid memory allocated to key, so the dereference invokes UB. You need to allocate memory to key before you can use it. same goes for the count, too.

edited Jul 20, 2016 at 9:37

answered Jul 20, 2016 at 9:25

Sourav Ghosh

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2 Comments

dirtyb Over a year ago

thanks, i added : pair->key = malloc(sizeof(int)); pair->count = malloc(sizeof(int)); and it seems to work now

Sourav Ghosh Over a year ago

@dirtyb You're welcome. You can also consider accepting an answer that helped you.

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