Temporary null termination of strings

This hack allows you to put a zero after a string temporarily, i.e., just long enough to use printf. Calling pushchar(p) is equivalent to writing *p = '\0', except that calling popchar() undoes the effect of the last pushchar. The stack is shallow, and the program halts with an assertion failure if it overflows. 

<header>= (U->)
extern void pushchar(char *p);
extern void popchar(void);

Here's the implementation. 

<*>=
#include <assert.h>
<header>
static char stack[3];
static char *pstack[3];
static int next = 0;
void pushchar(char *p) {
  assert(next < sizeof(stack));
  stack[next] = *p;
  pstack[next++] = p;
  *p = 0;
}
void popchar(void) {
  char *p;
  assert(next > 0);
  p = pstack[--next];
  *p = stack[next];
}
Defines next, popchar, pstack, pushchar, stack (links are to index).