How to include a dynamic array INSIDE a struct in C?

I have looked around but have been unable to find a solution to what must be a well asked question. Here is the code I have:

 #include <stdlib.h>

struct my_struct {
    int n;
    char s[]
};

int main()
{
    struct my_struct ms;
    ms.s = malloc(sizeof(char*)*50);
}

and here is the error gcc gives me: error: invalid use of flexible array member

I can get it to compile if i declare the declaration of s inside the struct to be

char* s

and this is probably a superior implementation (pointer arithmetic is faster than arrays, yes?) but I thought in c a declaration of

char s[]

is the same as

char* s