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?

You getting an error because you have not allocated memory for `b` since you defined it as a pointer to a `struct bst` — T.V., Nov 17 '14 at 13:31
Note: An alternative `pow(2, max)` here is `(size_t)1 << max)`. — chux - Reinstate Monica, Nov 17 '14 at 13:35
@BLUEPIXY After adding that line, it still fails at malloc of a->data saying there were memory leaks :/ — Choice, Nov 17 '14 at 13:37
Suggest `struct bst *b = malloc(sizeof *b); b->data = malloc( ((size_t)1 << max) * sizeof *(b->data)); b->max = max;` — chux - Reinstate Monica, Nov 17 '14 at 13:39
@Haxify `a->data` ?? It does not take this case memory leak here(`b->data = ...`). — BLUEPIXY, Nov 17 '14 at 13:41
@chux `malloc(sizeof *b)` is not allocating the correct amount of memory, it's only allocating the size of a pointer. @Haxify please refer to @Dogbert's answer, his solution should work and handle's error if allocation fails. — T.V., Nov 17 '14 at 13:42
@T.V. `malloc(sizeof *b)` _is_ the correct amount of memory. `sizeof of *b` is the size of the structure pointed to by `b`. Try compiling it. — chux - Reinstate Monica, Nov 17 '14 at 13:48
@BLUEPIXY You're right, my mistake. Haven't written C code in a long time. — T.V., Nov 17 '14 at 13:49

score 3 · Accepted Answer · edited May 23 '17 at 10:26

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.

Allocating dynamic array within a function of a structure

1 Answers1