/* $FreeBSD: src/lib/libstand/read.c,v 1.1.1.1.6.1 2000/09/10 01:32:06 ps Exp $ */
/* $DragonFly: src/lib/libstand/read.c,v 1.2 2003/06/17 04:26:51 dillon Exp $ */
/*	$NetBSD: read.c,v 1.8 1997/01/22 00:38:12 cgd Exp $	*/

/*-
 * Copyright (c) 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * The Mach Operating System project at Carnegie-Mellon University.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *	@(#)read.c	8.1 (Berkeley) 6/11/93
 *  
 *
 * Copyright (c) 1989, 1990, 1991 Carnegie Mellon University
 * All Rights Reserved.
 *
 * Author: Alessandro Forin
 * 
 * Permission to use, copy, modify and distribute this software and its
 * documentation is hereby granted, provided that both the copyright
 * notice and this permission notice appear in all copies of the
 * software, derivative works or modified versions, and any portions
 * thereof, and that both notices appear in supporting documentation.
 * 
 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR
 * ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
 * 
 * Carnegie Mellon requests users of this software to return to
 * 
 *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
 *  School of Computer Science
 *  Carnegie Mellon University
 *  Pittsburgh PA 15213-3890
 * 
 * any improvements or extensions that they make and grant Carnegie the
 * rights to redistribute these changes.
 */

#include <sys/param.h>
#include "stand.h"

ssize_t
read(int fd, void *dest, size_t bcount)
{
    struct open_file	*f = &files[fd];
    size_t		resid;

    if ((unsigned)fd >= SOPEN_MAX || !(f->f_flags & F_READ)) {
	errno = EBADF;
	return (-1);
    }
    if (f->f_flags & F_RAW) {
	twiddle();
	errno = (f->f_dev->dv_strategy)(f->f_devdata, F_READ,
					btodb(f->f_offset), bcount, dest, &resid);
	if (errno)
	    return (-1);
	f->f_offset += resid;
	return (resid);
    } 

    /*
     * Optimise reads from regular files using a readahead buffer.
     * If the request can't be satisfied from the current buffer contents,
     * check to see if it should be bypassed, or refill the buffer and complete
     * the request.
     */
    resid = bcount;
    for (;;) {
	size_t	ccount, cresid;
	/* how much can we supply? */
	ccount = imin(f->f_ralen, resid);
	if (ccount > 0) {
	    bcopy(f->f_rabuf + f->f_raoffset, dest, ccount);
	    f->f_raoffset += ccount;
	    f->f_ralen -= ccount;
	    resid -= ccount;
	    if (resid == 0)
		return(bcount);
	    dest += ccount;
	}

	/* will filling the readahead buffer again not help? */
	if (resid >= SOPEN_RASIZE) {
	    /* bypass the rest of the request and leave the buffer empty */
	    if ((errno = (f->f_ops->fo_read)(f, dest, resid, &cresid)))
		return(bcount - resid);
	    return(bcount - cresid);
	}

	/* fetch more data */
	if ((errno = (f->f_ops->fo_read)(f, f->f_rabuf, SOPEN_RASIZE, &cresid)))
	    return(bcount - resid);	/* behave like fread() */
	f->f_raoffset = 0;
	f->f_ralen = SOPEN_RASIZE - cresid;
	/* no more data, return what we had */
	if (f->f_ralen == 0)
	    return(bcount - resid);
    }	
}