Allocating dynamic array within a function of a structure

Question

I have a BST structure:

struct bst {
 int *data;
 int max;
}; 

And I have a function to create a bst initially:

struct bst *create_bst(int max) {
 struct bst *b;
 b->data = malloc(pow(2, max) * sizeof(int));
 return b;
}

But I'm getting error at the line where I'm allocating memory to data.
Am I doing something wrong?

• T.V.: You getting an error because you have not allocated memory for b since you defined it as a pointer to a struct bst
• Werner Henze: What is the value of max when the malloc fails?
• BLUEPIXY: struct bst *b; --> struct bst *b = malloc(sizeof(*b));
• Choice: @T.V. Do I just add b = malloc(sizeof(struct bst)); ?
• BLUEPIXY: @T.V. you misunderstand *b. *b is not pointer.

Answer 1

You are not allocating data for the struct itself, just one of its members. This should help:

struct bst *create_bst(int max) {
 struct bst *b;
 if ((b = calloc((size_t)1, sizeof(struct bst))) == NULL) {
 printf("Allocation error\n");
 return NULL;
 }
 if ((b->data = calloc((size_t)1<<max, sizeof(int))) == NULL) {
 printf("Allocation error\n");
 free(b);
 return NULL;
 }
 return b;
}

Later on in some other part of your code, you'll need to clean this memory up. ie: free(b->data); free(b).

Also, remember that pow doesn't work quite how you think it does. You could get something like pow(5,2) == 24.999999..., and when you assign this value to an integer variable, it gets truncated to 24. Never mix and match int and float logic unless you know exactly what you're doing.