Using a pointer to a struct without any definition.

struct.c

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

struct hoge * tmp_ptr;

void
foo( void )
{
	printf("%p\n", tmp_ptr);
}

int
main( )
{
	void * p = malloc(16*4);
	tmp_ptr = p;
	foo( );
	return 0;
}