The ungetchar function stores the character c back into the input stream. The next call to getchar or any other stream input function returns c. Only one character may be passed to unget between calls to getchar.

The ungetchar function returns the character c if successful. If ungetchar is called more than once between function calls that read from the input stream, EOF is returned indicating an error condition.

_getkey, getchar, putchar

#include <stdio.h>
#include <ctype.h>  /* for isdigit */

void tst_ungetchar (void) {
  char k;

  while (isdigit (k = getchar ())) {
    /* stay in the loop as long as k is a digit */
  }

  ungetchar (k);
}