What's the difference between intializating a struct as pointer or not?

Question

Hi,

I have the following for my HashTable structure:

typedef char *HashKey;
typedef int HashValue;

typedef struct sHashElement {
    HashKey key;
    HashValue value;
} HashElement;

typedef struct sHashTable {
    HashElement *items;
    float loadFactor;
} HashTable;

I never really thought about it until now but I just realized there's two ways how I can use this:

Alternative 1:

void hashInitialize(HashTable *table, int tabSize) {
    table->items = malloc(sizeof(HashElement) * tabSize);

    if(!table->items) {
        perror("malloc");
        exit(1);
    }

    table->items[0].key = "AAA";
    table->items[0].value = 45;
    table->items[1].key = "BBB";
    table->items[1].value = 82;

    table->loadFactor = (float)2 / tabSize;
}


int main(void) {
    HashTable t1;
    int i;

    hashInitialize(&t1, HASHSIZE);

    for(i = 0; i < HASHSIZE - 1; i++) {
        printf("PAIR(%d): %s, %d\n", i+1, t1.items[i].key, t1.items[i].value);
    }

    printf("LOAD FACTOR: %.2f\n", t1.loadFactor);

    return 0;
}

Alternative 2:

void hashInitialize(HashTable **table, int tabSize) {
    *table = malloc(sizeof(HashTable));

    if(!*table) {
        perror("malloc");
        exit(1);
    }

    (*table)->items = malloc(sizeof(HashElement) * tabSize);

    if(!(*table)->items) {
        perror("malloc");
        exit(1);
    }

    (*table)->items[0].key = "AAA";
    (*table)->items[0].value = 45;
    (*table)->items[1].key = "BBB";
    (*table)->items[1].value = 82;

    (*table)->loadFactor = (float)2 / tabSize;
}


int main(void) {
    HashTable *t1 = NULL;
    int i;

    hashInitialize(&t1, HASHSIZE);

    for(i = 0; i < HASHSIZE - 1; i++) {
        printf("PAIR(%d): %s, %d\n", i+1, t1->items[i].key, t1->items[i].value);
    }

    printf("LOAD FACTOR: %.2f\n", t1->loadFactor);

    return 0;
}

Question 1: They both seem to produce the same result. On main, both examples print the right key/value pair. So, what exactly is the different between them besides the syntax change (using (*table) instead of just table), the extra code to allocate memory for the HashTable structure and the declaration of HashTable pointer?

I've been writing a few data structures lately like stacks, linked lists, binary search trees and now hash tables. And for all of them, I've always used the alternative 2. But now I'm thinking if I could have used alternative 1 and simplify the code, removing most of the * and & that are all over the place.

But I'm asking this question to understand the differences between the two methods and if, and also why, I should use on over the other.

Question 2: As you can see in the structures code, HashKey is a pointer. However, I'm not using strdup nor malloc to allocate space for that string. How and why is this working? Is this OK to do? I've always used malloc or strdup where appropriate when handling dynamic strings or I would get lots of segmentation faults. But this code is not giving me any segmentation faults and I don't understand why and if I should do it like this.

Answer 1

The only difference is where the memory comes from -- local variables are typically on the stack whereas mallocs typically come from the heap.

Answer 2

In alternative 1, the caller would allocate table but your function would allocate the contents thereof, which is not always a good idea in terms of memory management. Alternative 2 keeps all allocations in the same place.

Answer 3

As answered previously, the differences between the two alternatives is memory management. In alternative 1 you expect the caller to allocate the memory for table prior to the call; whereas, in alternative 2 just a pointer declaration is required to give you a place to put the memory after you've created it.

To question 2, the simple answer is that you are assigning a constant to the string. According to the following site the assignment is set up at compile time, not runtime.

http://publications.gbdirect.co.uk/c_book/chapter6/initialization.html

Answer 4

First both solutions are perfectly right !

Alternative 1 :

Your HashTable is declared in the main, which means the struct is somewhere in the call stack. The struct will be destroy if you leave the scope. Note : In your case that can't happen because the declaration is in the main so the scope ends on process exit.

Alternative 2:

You've got a HashTable* (pointer) in the call stack so you need to allocate the memory for the struct. To do so you use malloc.

In both case your struct is correctly allocated. The main difference will be on performances. It's far more performant to allocate on the stack but you can't do dynamic allocation. To do so you need to use malloc. So, some times, you have to use malloc but try to avoid mallocing a lot if you want to do a high performance application.

Is that clear enough? :)

Answer 5

for question 2: (*table)->items[0].key = "AAA";

actually puts "AAA" in read only parts of memory and char *key points to it, contents pointed by key cannot be changed.

(*table)->items[0].key[0]='a' gives and error

Here you can find further discussion about it.

http://stackoverflow.com/questions/1704407/what-is-the-difference-between-char-s-and-char-s-in-c