read.c revision 65470 
1/* $FreeBSD: head/lib/libstand/read.c 65470 2000-09-05 09:52:50Z msmith $ */
2/*	$NetBSD: read.c,v 1.8 1997/01/22 00:38:12 cgd Exp $	*/
3
4/*-
5 * Copyright (c) 1993
6 *	The Regents of the University of California.  All rights reserved.
7 *
8 * This code is derived from software contributed to Berkeley by
9 * The Mach Operating System project at Carnegie-Mellon University.
10 *
11 * Redistribution and use in source and binary forms, with or without
12 * modification, are permitted provided that the following conditions
13 * are met:
14 * 1. Redistributions of source code must retain the above copyright
15 *    notice, this list of conditions and the following disclaimer.
16 * 2. Redistributions in binary form must reproduce the above copyright
17 *    notice, this list of conditions and the following disclaimer in the
18 *    documentation and/or other materials provided with the distribution.
19 * 3. All advertising materials mentioning features or use of this software
20 *    must display the following acknowledgement:
21 *	This product includes software developed by the University of
22 *	California, Berkeley and its contributors.
23 * 4. Neither the name of the University nor the names of its contributors
24 *    may be used to endorse or promote products derived from this software
25 *    without specific prior written permission.
26 *
27 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
28 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
29 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
30 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
31 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
32 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
33 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
34 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
35 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
36 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
37 * SUCH DAMAGE.
38 *
39 *	@(#)read.c	8.1 (Berkeley) 6/11/93
40 *
41 *
42 * Copyright (c) 1989, 1990, 1991 Carnegie Mellon University
43 * All Rights Reserved.
44 *
45 * Author: Alessandro Forin
46 *
47 * Permission to use, copy, modify and distribute this software and its
48 * documentation is hereby granted, provided that both the copyright
49 * notice and this permission notice appear in all copies of the
50 * software, derivative works or modified versions, and any portions
51 * thereof, and that both notices appear in supporting documentation.
52 *
53 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
54 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR
55 * ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
56 *
57 * Carnegie Mellon requests users of this software to return to
58 *
59 *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
60 *  School of Computer Science
61 *  Carnegie Mellon University
62 *  Pittsburgh PA 15213-3890
63 *
64 * any improvements or extensions that they make and grant Carnegie the
65 * rights to redistribute these changes.
66 */
67
68#include <sys/param.h>
69#include "stand.h"
70
71ssize_t
72read(int fd, void *dest, size_t bcount)
73{
74    struct open_file	*f = &files[fd];
75    size_t		resid;
76
77    if ((unsigned)fd >= SOPEN_MAX || !(f->f_flags & F_READ)) {
78	errno = EBADF;
79	return (-1);
80    }
81    if (f->f_flags & F_RAW) {
82	twiddle();
83	errno = (f->f_dev->dv_strategy)(f->f_devdata, F_READ,
84					btodb(f->f_offset), bcount, dest, &resid);
85	if (errno)
86	    return (-1);
87	f->f_offset += resid;
88	return (resid);
89    }
90
91    /*
92     * Optimise reads from regular files using a readahead buffer.
93     * If the request can't be satisfied from the current buffer contents,
94     * check to see if it should be bypassed, or refill the buffer and complete
95     * the request.
96     */
97    resid = bcount;
98    for (;;) {
99	size_t	ccount, cresid;
100	/* how much can we supply? */
101	ccount = imin(f->f_ralen, resid);
102	if (ccount > 0) {
103	    bcopy(f->f_rabuf + f->f_raoffset, dest, ccount);
104	    f->f_raoffset += ccount;
105	    f->f_ralen -= ccount;
106	    resid -= ccount;
107	    if (resid == 0)
108		return(bcount);
109	    dest += ccount;
110	}
111
112	/* will filling the readahead buffer again not help? */
113	if (resid >= SOPEN_RASIZE) {
114	    /* bypass the rest of the request and leave the buffer empty */
115	    if ((errno = (f->f_ops->fo_read)(f, dest, resid, &cresid)))
116		return(bcount - resid);
117	    return(bcount - cresid);
118	}
119
120	/* fetch more data */
121	if ((errno = (f->f_ops->fo_read)(f, f->f_rabuf, SOPEN_RASIZE, &cresid)))
122	    return(bcount - resid);	/* behave like fread() */
123	f->f_raoffset = 0;
124	f->f_ralen = SOPEN_RASIZE - cresid;
125	/* no more data, return what we had */
126	if (f->f_ralen == 0)
127	    return(bcount - resid);
128    }
129}
130