array[1] doesn't work while array+1 works

Question

As far as I've understood this far array[1] and array+1 are practically two ways of writing the same thing. However I've been looking at void pointers and arrays recently and made this program to test my understanding of it.

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

int main(void){
    void** data;
    data = malloc(sizeof(int)*2);
    *((int*)data) = 5;
    *((int*)(data+1)) = 10;
    printf("%d\n", *((int*)data));
    printf("%d\n", *((int*)(data+1)));
    free(data);
    return 0;
}

That is the version of the program that works, for some reason however this version doesn't

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

int main(void){
    void** data;
    data = malloc(sizeof(int)*2);
    *((int*)data[0]) = 5;
    *((int*)data[1]) = 10;
    printf("%d\n", *((int*)data));
    printf("%d\n", *((int*)data1]));
    free(data);
    return 0;
}

I'm not exactly getting compiler errors but program simply stops running, I've compiled this on a win 10 machine using gcc with the following flags -pedantic-errors -Wall and like i said before, the program compiles but when run I get the classic Program.exe has stopped working error message and so far I really can't think of a single reason why one of those would work and the other wouldn't.

Answer 1

data+1 is not valid C. You cannot do pointer arithmetic on void pointers, since that wouldn't make any sense.

So it would seem that you are using gcc in non-standard crap mode (default setting), which translates void pointer arithmetic to character arithmetic and therefore the program compiles, but as non-standard C. data+1 would then mean +1 byte, not +1 int.

Use gcc as a a standard C compiler instead -std=c11 -pedantic-errors. Then change the code to (int*)data+1.

Also the void** makes no sense, should be a void*. Please note that (int*)data[0] means "do pointer arithmetic on void** type, then cast the result to int*. This is an operator precedence bug. [] has higher precedence than the cast () operator.

Just toss that whole code out and use this:

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

int main(void){
    void* data;
    data = malloc( sizeof(int[2]) );
    ((int*)data)[0] = 5;
    ((int*)data)[1] = 10;
    printf("%d\n", *(int*)data );
    printf("%d\n", *((int*)data+1) );
    free(data);
    return 0;
}

Answer 2

Both your examples are not correct.

void** data = malloc(sizeof(int)*2);

is allocating 2 int integers, but data if of type void**.If you wish to still use this, which is not recommended, you need to allocate 2 void* pointers. This would then be:

void** data = malloc(sizeof(void*)*2);

Having said this, using void** is not needed here. You can just use void* as pointed out in @Lundin's post. Your code would be optimal if you use int *data instead though, as it doesn't really make sense to use void* pointers here. If you decide to do this, your code can just be:

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

int main(void){
    int *data;
    data = malloc(sizeof(int)*2);
    data[0] = 5;
    data[1] = 10;
    printf("%d\n", data[0]);
    printf("%d\n", data[1]);
    free(data);
    return 0;
}

Which is more straightforward, and skips the complications that void* pointers bring in code.

Note: You must check return of malloc() always, as it can return NULL on failure.

Answer 3

first: please change the type of data to int* or at least to void*. don't mess with void** unless you need to pass to a function a pointer.

second: change data[0/1] to &data[0/1].data[] is an argument and &data[] is his pointer. if you still using the void* choose *((int*)data+?). if yopu changed to int* use data[?].

third: why to use pointers in this function? this is not a function that need pointers.

fourth: i would suggest in this case to use an array instead of the pointers. if you already know the type and size of your argument so you better use arrays. more comfortable.