Initializing a variable sized array of booleans in C

Question

so I currently have a struct that looks like this:

typedef struct example {
    bool arr[]; //i would like this to be an array of booleans,
                //but i don't know the size right now; this is
                //probably wrong
} example_t;

I also have a create function that looks like this:

example_t *newExample(int SIZE){
    example_t *example = malloc(sizeof(example_t));
    //initialize arr to a bool array of size SIZE, ideally all false!
    example->arr = ???
    return example;
}

And from this, I would be able to do something like:

 example_t *example = newExample(MAX);
 if ( example->arr[10] )
      ....

Is this possible in C, to create a variable sized array of booleans?

For Reference: I need someway to map integers to either a char* or bool, so I can call arr[num] and be able to get either a string/word or a true/false value. Both ways, I'm not sure how to declare, and then initialize with a variable size. Thanks in advance!

Answer 1

In C99 you can use flexible array members where the last member can be an array without a given dimension, but there must be at least 2 members in the struct.

You can use a pointer (here I am using int instead of bool for brevity):

#include <stdlib.h>
#include <stdio.h>

typedef struct example {
    int *arr;
} example_t;

static example_t *newExample(size_t SIZE)
{
    example_t *example = malloc(sizeof(example_t) + sizeof(int) * SIZE);

    example->arr = (int *)(example + 1);
    example->arr[5] = 5;
    return example;
}

int main(void)
{
    example_t *x = newExample(10);

    printf("%d\n", x->arr[5]);
    free(x);
    return 0;
}

But this doesn't make much sense, why not add a first member containing the number of elements?

#include <stdlib.h>
#include <stdio.h>

typedef struct example {
    size_t size;
    int arr[];
} example_t;

static example_t *newExample(size_t SIZE)
{
    example_t *example = malloc(sizeof(example_t) + sizeof(int) * SIZE);

    example->size = SIZE;
    example->arr[5] = 5;
    return example;
}

int main(void)
{
    example_t *x = newExample(10);

    printf("%d\n", x->arr[5]);
    free(x);
    return 0;
}

Using this method haves an advantage: you can pass the object to any function without passing an extra parameter for the size.

Answer 2

You can do this by adding extra memory after example:

size_t boolsize = 10 * sizeof(bool);

struct example *ptr = malloc(sizeof *ptr + boolsize);

This way you call malloc only once and memory is layed out contiguously