Fix some typos.

Get rid of the paragraph about C++-style comments at the beginning of
the document, as it also appears in section 1.1.1 "Comments".

Add a section on how to extract data from packets, which explains the
"pd" and "offset" arguments to a dissector, and notes that you should
not just blithely cast pointers into the packet data to 2-byte or 4-byte
integral types and dereference them, as the pointer may not be aligned,
and the field may not have the same byte order as the processor on which
Ethereal is running (in fact, it's probably *guaranteed* not to on at
least one machine, as Ethereal runs on both big-endian and little-endian
platforms...).

svn path=/trunk/; revision=1710

doc/README.developer

@@ -1,4 +1,4 @@
-$Id: README.developer,v 1.8 2000/03/09 19:32:31 oabad Exp $
+$Id: README.developer,v 1.9 2000/03/10 08:57:05 guy Exp $
 
 This file is a HOWTO for Ethereal developers. It describes how to start coding
 a protocol dissector and the use some of the important functions and variables 
@@ -7,12 +7,6 @@ in Ethereal.
 See also the "proto_tree" file for a more detailed description of the
 protocol tree construction functions.
 
-NOTE: please don't use C++-style comments (comments beginning with "//"
-and running to the end of the line); Ethereal's dissectors are written
-in C, and thus run through C rather than C++ compilers, and not all C
-compilers support C++-style comments (GCC does, but IBM's C compiler for
-AIX, for example, doesn't do so by default).
-
 1. Setting up your protocol dissector code.
 
 This section provides skeleton code for a protocol dissector. It also explains
@@ -33,9 +27,8 @@ rather than C++ compilers, and not all C compilers support C++-style comments
 Ethereal uses the underscore_convention rather than the InterCapConvention for
 function names, so new code should probably use underscores rather than
 intercaps for functions and variable names. This is especially important if you
-are writting code that will be called from outside your code.  We are just
-trying to keep thing consistant for other users.
-
+are writing code that will be called from outside your code.  We are just
+trying to keep thing consistent for other users.
 
 1.2 Skeleton code.
 
@@ -64,7 +57,7 @@ code inside
 
 is needed only if you are using the "snprintf()" function.
 
-The "$Id: README.developer,v 1.8 2000/03/09 19:32:31 oabad Exp $" in the comment will be updated by CVS when the file is
+The "$Id: README.developer,v 1.9 2000/03/10 08:57:05 guy Exp $" in the comment will be updated by CVS when the file is
 checked in; it will allow the RCS "ident" command to report which
 version of the file is currently checked out.
 
@@ -73,7 +66,7 @@ version of the file is currently checked out.
  * Routines for PROTONAME dissection
  * Copyright 2000, YOUR_NAME <YOUR_EMAIL_ADDRESS>
  *
- * $Id: README.developer,v 1.8 2000/03/09 19:32:31 oabad Exp $
+ * $Id: README.developer,v 1.9 2000/03/10 08:57:05 guy Exp $
  *
  * Ethereal - Network traffic analyzer
  * By Gerald Combs <gerald@unicom.net>
@@ -227,12 +220,60 @@ FIELDDESCR	A brief description of the field.
 
 1.4 The dissector and the data it receives.
 
-1.4.1 The dissector has the following header which must be placed into
+1.4.1 Header file.
+
+The dissector has the following header which must be placed into
 packet-PROTOABBREV.h.
 
 void
 dissect_PROTOABBREV(const u_char *pd, int offset, frame_data *fd, proto_tree *tree); 
 
+1.4.2 Extracting data from packets.
+
+The "pd" argument to a dissector points to a buffer containing the raw
+data for the frame; the "offset" argument is the offset into that buffer
+of the first byte belonging to the protocol to be dissected by that
+dissector.
+
+One must not assume that the data to be dissected is aligned on a
+particular boundary in memory, and therefore one must not, for example,
+extract a 4-byte unsigned integer by doing
+
+	foo = *(guint32 *)pd[offset + 8];
+
+as, if "pd + offset" is not aligned on a 4-byte boundary in memory
+(because, for example, "pd" is so aligned but "offset" is not a multiple
+of 4), that line of code may, when executed, cause Ethereal to crash.
+
+In addition, one must not assume that Ethereal is running on a machine
+with a particular byte order (big-endian or little-endian); if the field
+one is dissecting is, for example, big-endian, code such as the example
+above, even if "pd + offset" is aligned on a 4-byte boundary or if the
+processor on which Ethereal is running doesn't require addresses to be
+aligned on appropriate boundaries, will not work correctly on
+little-endian systems such as IBM-compatible PCs or most systems with
+Alpha processors - and the same problem would exist if the field were
+little-endian and the code were running on big-endian systems such as
+SPARC machines.
+
+One should, instead, use the "pntohs()", "pntohl()", "pletohs()", or
+"pletohl()" macros:
+
+	foo = pntohl(&pd[offset + 8]);
+
+which will extract a 4-byte big-endian value from the field starting at
+an offset of "offset + 8" from the beginning of the frame.
+
+The macros in question extract:
+
+	pntohs()		2-byte, big-endian quantity
+
+	pntohl()		4-byte, big-endian quantity
+
+	pletohs()		2-byte, little-endian quantity
+
+	pletohl()		4-byte, little-endian quantity
+
 1.5 Functions to handle columns in the traffic summary window.
 
 The topmost pane of the main window is a list of the packets in the