Trying to free reallocated memory gives free(): invalid pointer

Question

I can't figure out why I am getting the error free(): invalid pointer when running this code. I thought p is going to point to a block of memory set aside for the structures, so I am trying to free pointers beginning from the p e.g. p+1, but this is no good. What am I missing here?

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

struct s {
    int x;
    
};
int main()
{
    struct s *p = NULL;
    for (int i = 0; i < 3; i++) {
        if ((p = realloc(p, (i+1) * sizeof(struct s))) != NULL) {
            struct s x = {.x=i*10};
            *(p+i) = x;
            
        } else exit(EXIT_FAILURE);
    }
    for (int i=0;i<3;i++) {printf("%d ", (p+i)->x);}
    //free(p);
    free(p+1);
    //free(p+2);
    return 0;
}

Answer 1

Before the loop you declared the pointer p

struct s *p = NULL;

So after the loop it will store the address of the last reallocated memory.

To free the allocated memory you need just to write

free( p );

The expression p + 1 points to the second element of the dynamically allocated array that was not allocated dynamically. It is the whole array that was allocated dynamically.