scc

 1#include <stdio.h>
 2
 3/**
 4 * ungetc() - push back one character to a input stream
 5 *
 6 * Context: A stream just opened has rp, wp, lp and buf pointing to NULL,
 7 *          in the same way that a closed stream. In both cases we have to
 8 *          return EOF, so the check fp->rp == fp->buf detects both cases.
 9 *          An input stream can be either a read only input or a read and
10 *          write input in read state, and we can detect the write state
11 *          when wp does not point to the beginning of buf. _IOSTRG is used
12 *          in sprintf/sscanf functions, where it is possible rp points to
13 *          a constant string, so we cannot write back c, but it is safe
14 *          because in those functions we are going to push back always
15 *          the same character that the one contained in the string.
16 */
17int
18ungetc(int c, FILE *fp)
19{
20	if (c == EOF)
21		return EOF;
22
23	if ((fp->flags & _IOWRITE) != 0)
24		return EOF;
25
26	if (fp->rp == fp->buf || fp->wp != fp->buf)
27		return EOF;
28
29	--fp->rp;
30	if ((fp->flags & _IOSTRG) == 0)
31		*fp->rp = c;
32	fp->flags &= ~_IOEOF;
33
34	return c;
35}