把ptr作为reference传递

2.c

#include <stdio.h>

int foo(void *addr, void **ptr) {
  *ptr = addr;
}

int main() {
  int x = 42;
  int y = 66;

  int *p;
  p = &x;

  foo(&y, &p);
  printf("*p: %d\n", *p);
  return 0;
}