C++ to C -> malloc

[hit]

ive easily used new & delete in c++, but in c im getting confused

int size = 1;
int *arr = (int*)malloc(size*sizeof(int));

a dynamic int array that fits only 1 number its size = 0?
0 * anything = zero so don't use this in malloc?

i've seen code with simply

int *arr = (int*)malloc(size);

i assume this is incorrect because it also needs the size of the struct/etc?

here is a simple example I wrote:

int main(){
    int size = 10;
    int i=0;
    char *arr = (char*)malloc(size*sizeof(char));

    for(;i<size;i++)
        arr[i] = (rand() % ('Z' - 'A'))+'A';

    arr[size] = '\0';
    printf("%s",arr);
    free(arr);

    getchar();getchar();getchar();
    return 0;
}

why does this program crash when i add: free(arr); ?
when i change arr[size] = '\0'; to arr[size-1] = '\0'; it works

int main(){
    int size = 1;
    int i = 0;
    int *arr;
    int *temp;

    arr = (int*)malloc(size*sizeof(int));
    for(;i<size;i++){
        arr[i] = rand() % 256;
        printf("%d. %d",i,arr[i]);
    }
    ++size;
    temp = (int*)malloc(size*sizeof(int));
    for(i=0;i<size-1;i++){
        temp[i] = arr[i];
    }
    temp[size-1] = rand() % 256;

    free(arr);
    for(i=0;i<size;i++){
        printf("\n%d. %d",i,temp[i]);
    }
    free(temp);

    getchar();
    return 0;
}

do all dynamic arrays (int,structs,etc) hold an empty cell at the end?

[laserlight]

a dynamic int array that fits only 1 number its size = 0?
0 * anything = zero so don't use this in malloc?

Err... size == 1, so I am not sure what you are talking about. If you mean to ask what would happen if 0 was assigned to size instead of 1, then the answer is that you will either get a null pointer returned by malloc, or you get a non-null pointer that you cannot use. By the way, I suggest:

int *arr = malloc(size * sizeof(*arr));

You don't need to cast since this is not C++, and using sizeof(*arr) means that the code remains correct even if the type of arr changes.

i assume this is incorrect because it also needs the size of the struct/etc?

Not necessarily, e.g., sizeof(char) == 1.

why does this program crash when i add: free(arr); ?

Undefined behaviour. You write beyond the bounds of the dynamic array on this line:

arr[size] = '\0';

do all dynamic arrays (int,structs,etc) hold an empty cell at the end?

All dynamic arrays for which you designate an "empty cell" at the end and correctly implement it hold an empty cell at the end. All other dynamic arrays do not, except by coincidence.

EDIT:
Also, note that new and new[] may throw std::bad_alloc, but malloc, realloc and calloc may return null pointers. So, just as you would catch std::bad_alloc in more complete code, you should check for null pointers here.

[hit]

hmm i think i understand the problem, thanks