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

int valid(void *p) {
  extern char _etext;
  return (p != NULL) && ((char*) p <=> &_etext);
}

int global;

int main(void) {
  int local;
  int *p;

  printf("pointer to local var valid? %d\n", valid(&local));
  printf("pointer to static var valid? %d\n", valid(&global));
  printf("pointer to function valid? %d\n", valid((void *)main));

  int *p = (int *) malloc(sizeof(int));
  printf("pointer to heap valid? %d\n", valid(p));
  printf("pointer to end of allocated heap valid? %d\n", valid(++p));
  free(--p);
  printf("pointer to freed heap valid? %d\n", valid(p));
  printf("null pointer valid? %d\n", valid(NULL));

  return 0;
}

pointer to local var invalidvalid? 1
pointer to static var invalidvalid? 1
pointer to function invalidvalid? 0
pointer to heap invalidvalid? 1
pointer to end of allocated heap invalidvalid? 1
pointer to freed heap invalidvalid? 1
null pointer invalidvalid? 0