ungetc

int ungetc(int c, FILE *fp)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "local.h"

static int __submore(FILE *);
/*
 * Expand the ungetc buffer `in place'.  That is, adjust fp->_p when
 * the buffer moves, so that it points the same distance from the end,
 * and move the bytes in the buffer around as necessary so that they
 * are all at the end (stack-style).
 */
static int
__submore(FILE *fp)
{
	int i;
	unsigned char *p;

	if (_UB(fp)._base == fp->_ubuf) {
		/*
		 * Get a new buffer (rather than expanding the old one).
		 */
		if ((p = malloc((size_t)BUFSIZ)) == NULL)
			return (EOF);
		_UB(fp)._base = p;
		_UB(fp)._size = BUFSIZ;
		p += BUFSIZ - sizeof(fp->_ubuf);
		for (i = sizeof(fp->_ubuf); --i >= 0;)
			p[i] = fp->_ubuf[i];
		fp->_p = p;
		return (0);
	}
	i = _UB(fp)._size;
	p = realloc(_UB(fp)._base, i << 1);
	if (p == NULL)
		return (EOF);
	/* no overlap (hence can use memcpy) because we doubled the size */
	(void)memcpy((void *)(p + i), (void *)p, (size_t)i);
	fp->_p = p + i;
	_UB(fp)._base = p;
	_UB(fp)._size = i << 1;
	return (0);
}

int
ungetc(int c, FILE *fp)
{
	if (c == EOF)
		return (EOF);
	if (!__sdidinit)
		__sinit();
	_SET_ORIENTATION(fp, -1);
	if ((fp->_flags & __SRD) == 0) {
		/*
		 * Not already reading: no good unless reading-and-writing.
		 * Otherwise, flush any current write stuff.
		 */
		if ((fp->_flags & __SRW) == 0)
			return (EOF);
		if (fp->_flags & __SWR) {
			if (__sflush(fp))
				return (EOF);
			fp->_flags &= ~__SWR;
			fp->_w = 0;
			fp->_lbfsize = 0;
		}
		fp->_flags |= __SRD;
	}
	c = (unsigned char)c;

	/*
	 * If we are in the middle of ungetc'ing, just continue.
	 * This may require expanding the current ungetc buffer.
	 */
	if (HASUB(fp)) {
		if (fp->_r >= _UB(fp)._size && __submore(fp))
			return (EOF);
		*--fp->_p = c;
		fp->_r++;
		return (c);
	}
	fp->_flags &= ~__SEOF;

	/*
	 * If we can handle this by simply backing up, do so,
	 * but never replace the original character.
	 * (This makes sscanf() work when scanning `const' data.)
	 */
	if (fp->_bf._base != NULL && fp->_p > fp->_bf._base &&
	    fp->_p[-1] == c) {
		fp->_p--;
		fp->_r++;
		return (c);
	}

	/*
	 * Create an ungetc buffer.
	 * Initially, we will use the `reserve' buffer.
	 */
	fp->_ur = fp->_r;
	fp->_up = fp->_p;
	_UB(fp)._base = fp->_ubuf;
	_UB(fp)._size = sizeof(fp->_ubuf);
	fp->_ubuf[sizeof(fp->_ubuf) - 1] = c;
	fp->_p = &fp->_ubuf[sizeof(fp->_ubuf) - 1];
	fp->_r = 1;
	return (c);
}