#include <string.h>
typedef struct {
  char name[32];
  char age;
} person_t;

void set_age(person_t * const p, int age) {
  p->age = age;  // this is OK
  //p++;         // this is not allowed!
}

int main() {
  person_t peter;
  strcpy(peter.name, "Peter Smith");
  set_age(&peter, 25);
}