Using a pointer-to-pointer to access a pointer-to-pointer in a structure

Question

I have a pointer to a pointer to a structure and I am trying to access a pointer-to-pointer pointer within that structure. I keep getting an error that reads: "request for member 'buckets' in something not a structure or union." I commented next to the error. So my question is, how do I properly access buckets and allocate memory for it.

typedef struct bucket {
  char *key;
  void *value;
  struct bucket *next;
} Bucket;

typedef struct {
  int key_count;
  int table_size;
  void (*free_value)(void *);
  Bucket **buckets;
} 


int create_table(Table ** table, int table_size, void (*free_value)(void *)){
  int iterate = 0;
  *table = malloc(sizeof(Table));

  if(table && table_size != 0) {
    (*table)->key_count = 0;
    (*table)->table_size = table_size;
    (*table)->free_value = free_value;
    (*table)->buckets = malloc(table_size * sizeof(Bucket));  /* Error is here */

    while(iterate < table_size)
      *table->buckets[iterate++] = NULL;
    return SUCC;
  }
  return FAIL;
}

Stian Svedenborg · Accepted Answer · 2014-07-13 20:30:24Z

3

By the look of your allocation:

(*table)->buckets = malloc(table_size * sizeof(Bucket));

It looks as though you are attempting to create space for table_size buckets and not table_size pointers to bucket. It the allocation should read:

(*table)->buckets = malloc(table_size * sizeof(Bucket*));

The error, I believe, is further down in the while loop. Operator -> has precedence over [] which has precedence over *, therefore you are actually saying *(table->buckets[iterate++]), and table is a pointer-pointer and does therefore not have a member called bucket.

answered Jul 13, 2014 at 20:30

Stian Svedenborg

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1 Comment

M.M Over a year ago

Note, the malloc error could have been avoided by using the idiom ptr = malloc( N * sizeof *ptr );

Nisse Engström · Accepted Answer · 2016-03-07 03:58:40Z

(Note: You're missing a Table; in your second typedef.

int create_table(Table ** table, int table_size, void (*free_value)(void *)){
  int iterate = 0;
  *table = malloc(sizeof(Table));

  if(table && table_size != 0) {

This isn't right. You should test table before you allocate memory, and then test *table.

    (*table)->key_count = 0;
    (*table)->table_size = table_size;
    (*table)->free_value = free_value;
    (*table)->buckets = malloc(table_size * sizeof(Bucket));  /* Error is here */

Here, you are allocating space for table_size number of Bucket, but assigning the memory to a pointer to pointer to Bucket. It looks like you want to allocate table_size number of pointers to Bucket.

    while(iterate < table_size)
      *table->buckets[iterate++] = NULL;

This is the same as *(table->buckets[iterate++]) which is obviously wrong. table is not a pointer to struct, *table is. This is where your real error is.

I would probably write something like this:

typedef struct {
  int key_count;
  int table_size;
  void (*free_value)(void *);
  Bucket **buckets;
} Table;

int create_table(Table **table, int table_size, void (*free_value)(void *))
{
  if (!table || table_size == 0) {
    return FAIL;
  }
  *table = malloc(sizeof(Table));
  if (*table) {
    (*table)->key_count = 0;
    (*table)->table_size = table_size;
    (*table)->free_value = free_value;
    (*table)->buckets = malloc(table_size * sizeof(Bucket *));
    if ((*table)->buckets) {
      int iterate = 0;
      while(iterate < table_size)
        (*table)->buckets[iterate++] = NULL;
      return SUCC;
    }
  }

  if (*table) {
    free ((*table)->buckets);
  }
  free (*table);
  return FAIL;
}

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