1/*	$OpenBSD: fseek.c,v 1.14 2022/05/14 05:06:32 guenther Exp $ */
2/*-
3 * Copyright (c) 1990, 1993
4 *	The Regents of the University of California.  All rights reserved.
5 *
6 * This code is derived from software contributed to Berkeley by
7 * Chris Torek.
8 *
9 * Redistribution and use in source and binary forms, with or without
10 * modification, are permitted provided that the following conditions
11 * are met:
12 * 1. Redistributions of source code must retain the above copyright
13 *    notice, this list of conditions and the following disclaimer.
14 * 2. Redistributions in binary form must reproduce the above copyright
15 *    notice, this list of conditions and the following disclaimer in the
16 *    documentation and/or other materials provided with the distribution.
17 * 3. Neither the name of the University nor the names of its contributors
18 *    may be used to endorse or promote products derived from this software
19 *    without specific prior written permission.
20 *
21 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
22 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
23 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
24 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
25 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
26 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
27 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
28 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
29 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
30 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
31 * SUCH DAMAGE.
32 */
33
34#include <sys/types.h>
35#include <sys/stat.h>
36#include <fcntl.h>
37#include <stdio.h>
38#include <stdlib.h>
39#include <errno.h>
40#include "local.h"
41
42#define	POS_ERR	(-(fpos_t)1)
43
44/*
45 * Seek the given file to the given offset.
46 * `Whence' must be one of the three SEEK_* macros.
47 */
48int
49fseeko(FILE *fp, off_t offset, int whence)
50{
51	fpos_t (*seekfn)(void *, fpos_t, int);
52	fpos_t target, curoff;
53	size_t n;
54	struct stat st;
55	int havepos;
56
57	/* make sure stdio is set up */
58	if (!__sdidinit)
59		__sinit();
60
61	/*
62	 * Have to be able to seek.
63	 */
64	if ((seekfn = fp->_seek) == NULL) {
65		errno = ESPIPE;			/* historic practice */
66		return (EOF);
67	}
68
69	/*
70	 * Change any SEEK_CUR to SEEK_SET, and check `whence' argument.
71	 * After this, whence is either SEEK_SET or SEEK_END.
72	 */
73	FLOCKFILE(fp);
74	switch (whence) {
75
76	case SEEK_CUR:
77		/*
78		 * In order to seek relative to the current stream offset,
79		 * we have to first find the current stream offset a la
80		 * ftell (see ftell for details).
81		 */
82		__sflush(fp);	/* may adjust seek offset on append stream */
83		if (fp->_flags & __SOFF)
84			curoff = fp->_offset;
85		else {
86			curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR);
87			if (curoff == (fpos_t)-1) {
88				FUNLOCKFILE(fp);
89				return (EOF);
90			}
91		}
92		if (fp->_flags & __SRD) {
93			curoff -= fp->_r;
94			if (HASUB(fp))
95				curoff -= fp->_ur;
96		} else if (fp->_flags & __SWR && fp->_p != NULL)
97			curoff += fp->_p - fp->_bf._base;
98
99		offset += curoff;
100		whence = SEEK_SET;
101		havepos = 1;
102		break;
103
104	case SEEK_SET:
105	case SEEK_END:
106		curoff = 0;		/* XXX just to keep gcc quiet */
107		havepos = 0;
108		break;
109
110	default:
111		FUNLOCKFILE(fp);
112		errno = EINVAL;
113		return (EOF);
114	}
115
116	/*
117	 * Can only optimise if:
118	 *	reading (and not reading-and-writing);
119	 *	not unbuffered; and
120	 *	this is a `regular' Unix file (and hence seekfn==__sseek).
121	 * We must check __NBF first, because it is possible to have __NBF
122	 * and __SOPT both set.
123	 */
124	if (fp->_bf._base == NULL)
125		__smakebuf(fp);
126	if (fp->_flags & (__SWR | __SRW | __SNBF | __SNPT))
127		goto dumb;
128	if ((fp->_flags & __SOPT) == 0) {
129		if (seekfn != __sseek ||
130		    fp->_file < 0 || fstat(fp->_file, &st) == -1 ||
131		    (st.st_mode & S_IFMT) != S_IFREG) {
132			fp->_flags |= __SNPT;
133			goto dumb;
134		}
135		fp->_blksize = st.st_blksize;
136		fp->_flags |= __SOPT;
137	}
138
139	/*
140	 * We are reading; we can try to optimise.
141	 * Figure out where we are going and where we are now.
142	 */
143	if (whence == SEEK_SET)
144		target = offset;
145	else {
146		if (fstat(fp->_file, &st) == -1)
147			goto dumb;
148		target = st.st_size + offset;
149	}
150
151	if (!havepos) {
152		if (fp->_flags & __SOFF)
153			curoff = fp->_offset;
154		else {
155			curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR);
156			if (curoff == POS_ERR)
157				goto dumb;
158		}
159		curoff -= fp->_r;
160		if (HASUB(fp))
161			curoff -= fp->_ur;
162	}
163
164	/*
165	 * Compute the number of bytes in the input buffer (pretending
166	 * that any ungetc() input has been discarded).  Adjust current
167	 * offset backwards by this count so that it represents the
168	 * file offset for the first byte in the current input buffer.
169	 */
170	if (HASUB(fp)) {
171		curoff += fp->_r;	/* kill off ungetc */
172		n = fp->_up - fp->_bf._base;
173		curoff -= n;
174		n += fp->_ur;
175	} else {
176		n = fp->_p - fp->_bf._base;
177		curoff -= n;
178		n += fp->_r;
179	}
180
181	/*
182	 * If the target offset is within the current buffer,
183	 * simply adjust the pointers, clear EOF, undo ungetc(),
184	 * and return.  (If the buffer was modified, we have to
185	 * skip this; see fgetln.c.)
186	 */
187	if ((fp->_flags & __SMOD) == 0 &&
188	    target >= curoff && target < curoff + n) {
189		int o = target - curoff;
190
191		fp->_p = fp->_bf._base + o;
192		fp->_r = n - o;
193		if (HASUB(fp))
194			FREEUB(fp);
195		fp->_flags &= ~__SEOF;
196		FUNLOCKFILE(fp);
197		return (0);
198	}
199
200	/*
201	 * The place we want to get to is not within the current buffer,
202	 * but we can still be kind to the kernel copyout mechanism.
203	 * By aligning the file offset to a block boundary, we can let
204	 * the kernel use the VM hardware to map pages instead of
205	 * copying bytes laboriously.  Using a block boundary also
206	 * ensures that we only read one block, rather than two.
207	 */
208	curoff = target & ~(fp->_blksize - 1);
209	if ((*seekfn)(fp->_cookie, curoff, SEEK_SET) == POS_ERR)
210		goto dumb;
211	fp->_r = 0;
212 	fp->_p = fp->_bf._base;
213	if (HASUB(fp))
214		FREEUB(fp);
215	fp->_flags &= ~__SEOF;
216	n = target - curoff;
217	if (n) {
218		if (__srefill(fp) || fp->_r < n)
219			goto dumb;
220		fp->_p += n;
221		fp->_r -= n;
222	}
223	FUNLOCKFILE(fp);
224	return (0);
225
226	/*
227	 * We get here if we cannot optimise the seek ... just
228	 * do it.  Allow the seek function to change fp->_bf._base.
229	 */
230dumb:
231	if (__sflush(fp) ||
232	    (*seekfn)(fp->_cookie, (fpos_t)offset, whence) == POS_ERR) {
233		FUNLOCKFILE(fp);
234		return (EOF);
235	}
236	/* success: clear EOF indicator and discard ungetc() data */
237	if (HASUB(fp))
238		FREEUB(fp);
239	fp->_p = fp->_bf._base;
240	fp->_r = 0;
241	/* fp->_w = 0; */	/* unnecessary (I think...) */
242	fp->_flags &= ~__SEOF;
243	FUNLOCKFILE(fp);
244	return (0);
245}
246DEF_WEAK(fseeko);
247
248int
249fseek(FILE *fp, long offset, int whence)
250{
251	return (fseeko(fp, offset, whence));
252}
253