Function pointer confusion

Question

I'm trying to use function pointers and Abstract data types in c. This is my first time using it and I'm really confused. Anyways when I tried to compile this code I gave me an error. The first time I ran it worked. But when I change the arguments by switch a and b. It gave me the old answer and never updated.

typedef struct data_{
  void *data;
  struct data_ *next;
}data;

typedef struct buckets_{
  void *key;
}buckets;

typedef struct hash_table_ {
  /* structure definition goes here */
  int (*hash_func)(char *);
  int (*comp_func)(void*, void*);
  buckets **buckets_array;
} hash_table, *Phash_table;

main(){

  Phash_table table_p;
  char word[20]= "Hellooooo";
  int a;
  a = 5;
  int b;
  b = 10;
  /*Line 11*/
  table_p = new_hash(15, *print_test(word), *comp_func(&a, &b)); 

}

int *print_test(char *word){
  printf("%s", word);
}

int *comp_func(int *i,int *j){

  if(*i < *j){
    printf("yeeeeeeee");
  }else{
    printf("yeaaaaaaaaaaaaa");
  }
}

Phash_table new_hash(int size, int (*hash_func)(char *), int (*cmp_func)(void *, void *)){
  int i;
  Phash_table table_p;
  buckets *buckets_p;
  hash_table hash_table;

  table_p = (void *)malloc(sizeof(hash_table));

  /*Setting function pointers*/
  table_p -> hash_func = hash_func;
  table_p -> comp_func = cmp_func;

  /*Create buckets array and set to null*/
  table_p -> buckets_array = (buckets **)malloc(sizeof(buckets_p)*(size+1));

  for(i = 0; i < size; i++){
    table_p -> buckets_array[i] = NULL;
  }

  return table_p;
}

This is the error message:

functions.c: In function 'main':
functions.c:11:26: error: invalid type argument of unary '*' (have 'int')
functions.c:11:45: error: invalid type argument of unary '*' (have 'int')
Helloyeaaaaaaaaaaaaa

New error:

functions.c:11:3: warning: passing argument 2 of 'new_hash' makes pointer from integer without a cast [enabled by default]
hash.h:29:13: note: expected 'int (*)(char *)' but argument is of type 'int'
functions.c:11:3: warning: passing argument 3 of 'new_hash' makes pointer from integer without a cast [enabled by default]
hash.h:29:13: note: expected 'int (*)(void *, void *)' but argument is of type 'int'

Answer 1

If you want to pass a function as a function-pointer, simply provide the name:

new_hash(15, print_test, comp_func);

or alternatively (and equivalently), use the & symbol to make it clear what's going on:

new_hash(15, &print_test, &comp_func);

Answer 2

You should declare function before using it. If you don't do this, the compiler assumes that it returns int, and gives you an error when you try to dereference it (since it's impossibole to dereference int).

EDIT: you may also misunderstood the concept of function pointers. you should not pass the result of print_test(word) to new_hash - you should pass print_test itself. (also, change its return type)