Fat pointer of pascal string

fatptr.c

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

struct Pstr {
    int size;
    char* str;
};

char** pstr_new()
{
    struct Pstr* p = malloc(sizeof(struct Pstr));
    p->size = 11;
    p->str = "Hello world";

    return &(p->str);
}

int main()
{
    char** pstr = pstr_new();
    int size = *(int*)((unsigned long)pstr - 8);  // only for LP64 arch

    printf("pstr->str: %s - pstr->size: %d", *pstr, size);

    return 0;
}