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byteorder.h
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/* Copyright (C) Andrew Tridgell 1992-1998
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*
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* SPDX-License-Identifier: GPL-2.0-or-later
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version 2
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* of the License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*/
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#ifndef _BYTEORDER_H
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#define _BYTEORDER_H
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/*
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This file implements macros for machine independent short and
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int manipulation
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Here is a description of this file that I emailed to the samba list once:
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> I am confused about the way that byteorder.h works in Samba. I have
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> looked at it, and I would have thought that you might make a distinction
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> between LE and BE machines, but you only seem to distinguish between 386
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> and all other architectures.
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>
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> Can you give me a clue?
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sure.
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The distinction between 386 and other architectures is only there as
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an optimisation. You can take it out completely and it will make no
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difference. The routines (macros) in byteorder.h are totally byteorder
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independent. The 386 optimsation just takes advantage of the fact that
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the x86 processors don't care about alignment, so we don't have to
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align ints on int boundaries etc. If there are other processors out
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there that aren't alignment sensitive then you could also define
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CAREFUL_ALIGNMENT=0 on those processors as well.
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Ok, now to the macros themselves. I'll take a simple example, say we
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want to extract a 2 byte integer from a SMB packet and put it into a
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type called uint16 that is in the local machines byte order, and you
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want to do it with only the assumption that uint16 is _at_least_ 16
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bits long (this last condition is very important for architectures
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that don't have any int types that are 2 bytes long)
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You do this:
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#define CVAL(buf,pos) (((unsigned char *)(buf))[pos])
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#define PVAL(buf,pos) ((unsigned)CVAL(buf,pos))
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#define SVAL(buf,pos) (PVAL(buf,pos)|PVAL(buf,(pos)+1)<<8)
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then to extract a uint16 value at offset 25 in a buffer you do this:
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char *buffer = foo_bar();
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uint16 xx = SVAL(buffer,25);
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We are using the byteoder independence of the ANSI C bitshifts to do
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the work. A good optimising compiler should turn this into efficient
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code, especially if it happens to have the right byteorder :-)
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I know these macros can be made a bit tidier by removing some of the
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casts, but you need to look at byteorder.h as a whole to see the
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reasoning behind them. byteorder.h defines the following macros:
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SVAL(buf,pos) - extract a 2 byte SMB value
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IVAL(buf,pos) - extract a 4 byte SMB value
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SVALS(buf,pos) signed version of SVAL()
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IVALS(buf,pos) signed version of IVAL()
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SSVAL(buf,pos,val) - put a 2 byte SMB value into a buffer
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SIVAL(buf,pos,val) - put a 4 byte SMB value into a buffer
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SSVALS(buf,pos,val) - signed version of SSVAL()
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SIVALS(buf,pos,val) - signed version of SIVAL()
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RSVAL(buf,pos) - like SVAL() but for NMB byte ordering
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RSVALS(buf,pos) - like SVALS() but for NMB byte ordering
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RIVAL(buf,pos) - like IVAL() but for NMB byte ordering
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RIVALS(buf,pos) - like IVALS() but for NMB byte ordering
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RSSVAL(buf,pos,val) - like SSVAL() but for NMB ordering
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RSIVAL(buf,pos,val) - like SIVAL() but for NMB ordering
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RSIVALS(buf,pos,val) - like SIVALS() but for NMB ordering
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it also defines lots of intermediate macros, just ignore those :-)
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*/
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#undef CAREFUL_ALIGNMENT
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/* we know that the 386 can handle misalignment and has the "right"
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byteorder */
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#ifdef __i386__
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#define CAREFUL_ALIGNMENT 0
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#endif
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#ifndef CAREFUL_ALIGNMENT
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#define CAREFUL_ALIGNMENT 1
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#endif
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#define CVAL(buf, pos) ((unsigned) (((const unsigned char *) (buf))[pos]))
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#define CVAL_NC(buf, pos) \
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(((unsigned char *) (buf))[pos])
/* Non-const version of CVAL */
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#define PVAL(buf, pos) (CVAL (buf, pos))
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#define SCVAL(buf, pos, val) (CVAL_NC (buf, pos) = (val))
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#if CAREFUL_ALIGNMENT
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#define SVAL(buf, pos) (PVAL (buf, pos) | PVAL (buf, (pos) + 1) << 8)
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#define IVAL(buf, pos) (SVAL (buf, pos) | SVAL (buf, (pos) + 2) << 16)
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#define SSVALX(buf, pos, val) \
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(CVAL_NC (buf, pos) = (unsigned char) ((val) &0xFF), \
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CVAL_NC (buf, pos + 1) = (unsigned char) ((val) >> 8))
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#define SIVALX(buf, pos, val) \
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(SSVALX (buf, pos, val & 0xFFFF), SSVALX (buf, pos + 2, val >> 16))
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#define SVALS(buf, pos) ((int16) SVAL (buf, pos))
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#define IVALS(buf, pos) ((int32) IVAL (buf, pos))
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#define SSVAL(buf, pos, val) SSVALX ((buf), (pos), ((uint16) (val)))
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#define SIVAL(buf, pos, val) SIVALX ((buf), (pos), ((uint32) (val)))
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#define SSVALS(buf, pos, val) SSVALX ((buf), (pos), ((int16) (val)))
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#define SIVALS(buf, pos, val) SIVALX ((buf), (pos), ((int32) (val)))
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#else
/* CAREFUL_ALIGNMENT */
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/* this handles things for architectures like the 386 that can handle
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alignment errors */
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/*
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WARNING: This section is dependent on the length of int16 and int32
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being correct
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*/
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/* get single value from an SMB buffer */
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#define SVAL(buf, pos) (*(const uint16 *) ((const char *) (buf) + (pos)))
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#define SVAL_NC(buf, pos) \
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(*(uint16 *) ((char *) (buf) + (pos)))
/* Non const version of above. */
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#define IVAL(buf, pos) (*(const uint32 *) ((const char *) (buf) + (pos)))
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#define IVAL_NC(buf, pos) \
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(*(uint32 *) ((char *) (buf) + (pos)))
/* Non const version of above. */
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#define SVALS(buf, pos) (*(const int16 *) ((const char *) (buf) + (pos)))
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#define SVALS_NC(buf, pos) \
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(*(int16 *) ((char *) (buf) + (pos)))
/* Non const version of above. */
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#define IVALS(buf, pos) (*(const int32 *) ((const char *) (buf) + (pos)))
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#define IVALS_NC(buf, pos) \
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(*(int32 *) ((char *) (buf) + (pos)))
/* Non const version of above. */
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/* store single value in an SMB buffer */
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#define SSVAL(buf, pos, val) SVAL_NC (buf, pos) = ((uint16) (val))
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#define SIVAL(buf, pos, val) IVAL_NC (buf, pos) = ((uint32) (val))
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#define SSVALS(buf, pos, val) SVALS_NC (buf, pos) = ((int16) (val))
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#define SIVALS(buf, pos, val) IVALS_NC (buf, pos) = ((int32) (val))
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#endif
/* CAREFUL_ALIGNMENT */
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/* now the reverse routines - these are used in nmb packets (mostly) */
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#define SREV(x) ((((x) &0xFF) << 8) | (((x) >> 8) & 0xFF))
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#define IREV(x) ((SREV (x) << 16) | (SREV ((x) >> 16)))
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#define RSVAL(buf, pos) SREV (SVAL (buf, pos))
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#define RSVALS(buf, pos) SREV (SVALS (buf, pos))
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#define RIVAL(buf, pos) IREV (IVAL (buf, pos))
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#define RIVALS(buf, pos) IREV (IVALS (buf, pos))
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#define RSSVAL(buf, pos, val) SSVAL (buf, pos, SREV (val))
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#define RSSVALS(buf, pos, val) SSVALS (buf, pos, SREV (val))
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#define RSIVAL(buf, pos, val) SIVAL (buf, pos, IREV (val))
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#define RSIVALS(buf, pos, val) SIVALS (buf, pos, IREV (val))
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/* Alignment macros. */
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#define ALIGN4(p, base) ((p) + ((4 - (PTR_DIFF ((p), (base)) & 3)) & 3))
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#define ALIGN2(p, base) ((p) + ((2 - (PTR_DIFF ((p), (base)) & 1)) & 1))
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#endif
/* _BYTEORDER_H */
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