putty/doc/udp.but

\# This file is so named for tradition's sake: it contains what we
\# always used to refer to, before they were written down, as
\# PuTTY's `unwritten design principles'. It has nothing to do with
\# the User Datagram Protocol.

\A{udp} PuTTY hacking guide

This appendix lists a selection of the design principles applying to
the PuTTY source code. If you are planning to send code
contributions, you should read this first.

\H{udp-portability} Cross-OS portability

Despite Windows being its main area of fame, PuTTY is no longer a
Windows-only application suite. It has a working Unix port; a Mac
port is in progress; more ports may or may not happen at a later
date.

Therefore, embedding Windows-specific code in core modules such as
\cw{ssh.c} is not acceptable. We went to great lengths to \e{remove}
all the Windows-specific stuff from our core modules, and to shift
it out into Windows-specific modules. Adding large amounts of
Windows-specific stuff in parts of the code that should be portable
is almost guaranteed to make us reject a contribution.

The PuTTY source base is divided into platform-specific modules and
platform-generic modules. The Unix-specific modules are all in the
\c{unix} subdirectory; the Windows-specific modules are in the
\c{windows} subdirectory.

All the modules in the main source directory and other
subdirectories - notably \e{all} of the code for the various back
ends - are platform-generic. We want to keep them that way.

This also means you should stick to the C semantics guaranteed by the
C standard: try not to make assumptions about the precise size of
basic types such as \c{int} and \c{long int}; don't use pointer casts
to do endianness-dependent operations, and so on.

(Even \e{within} a platform front end you should still be careful of
some of these portability issues. The Windows front end compiles on
both 32- and 64-bit x86 and also Arm.)

Our current choice of C standards version is \e{mostly} C99. With a
couple of exceptions, you can assume that C99 features are available
(in particular \cw{<stdint.h>}, \cw{<stdbool.h>} and \c{inline}), but
you shouldn't use things that are new in C11 (such as \cw{<uchar.h>}
or \cw{_Generic}).

The exceptions to that rule are due to the need for Visual Studio
compatibility:

\b Don't use variable-length arrays. Visual Studio doesn't support
them even now that it's adopted the rest of C99. We use \cw{-Wvla}
when building with gcc and clang, to make it easier to avoid
accidentally breaking that rule.

\b For historical reasons, we still build with one older VS version
which lacks \cw{<inttypes.h>}. So that file is included centrally in
\c{defs.h}, and has a set of workaround definitions for the
\cw{PRIx64}-type macros we use. If you need to use another one of
those macros, you need to add a workaround definition in \c{defs.h},
and don't casually re-include \cw{<inttypes.h>} anywhere else in the
source file.

Here are a few portability assumptions that we \e{do} currently allow
(because we'd already have to thoroughly vet the existing code if they
ever needed to change, and it doesn't seem worth doing that unless we
really have to):

\b You can assume \c{int} is \e{at least} 32 bits wide. (We've never
tried to port PuTTY to a platform with 16-bit \cw{int}, and it doesn't
look likely to be necessary in future.)

\b Similarly, you can assume \c{char} is exactly 8 bits. (Exceptions
to that are even less likely to be relevant to us than short
\cw{int}.)

\b You can assume that using \c{memset} to write zero bytes over a
whole structure will have the effect of setting all its pointer fields
to \cw{NULL}. (The standard itself guarantees this for \e{integer}
fields, but not for pointers.)

\b You can assume that \c{time_t} has POSIX semantics, i.e. that it
represents an integer number of non-leap seconds since 1970-01-01
00:00:00 UTC. (Times in this format are used in X authorisation, but
we could work around that by carefully distinguishing local \c{time_t}
from time values used in the wire protocol; but these semantics of
\c{time_t} are also baked into the shared library API used by the
GSSAPI authentication code, which would be much harder to change.)

\b You can assume that the execution character encoding is a superset
of the printable characters of ASCII. (In particular, it's fine to do
arithmetic on a \c{char} value representing a Latin alphabetic
character, without bothering to allow for EBCDIC or other
non-consecutive encodings of the alphabet.)

On the other hand, here are some particular things \e{not} to assume:

\b Don't assume anything about the \e{signedness} of \c{char}. In
particular, you \e{must} cast \c{char} values to \c{unsigned char}
before passing them to any \cw{<ctype.h>} function (because those
expect a non-negative character value, or \cw{EOF}). If you need a
particular signedness, explicitly specify \c{signed char} or
\c{unsigned char}, or use C99 \cw{int8_t} or \cw{uint8_t}.

\b From past experience with MacOS, we're still a bit nervous about
\cw{'\\n'} and \cw{'\\r'} potentially having unusual meanings on a
given platform. So it's fine to say \c{\\n} in a string you're passing
to \c{printf}, but in any context where those characters appear in a
standardised wire protocol or a binary file format, they should be
spelled \cw{'\\012'} and \cw{'\\015'} respectively.

\H{udp-multi-backend} Multiple backends treated equally

PuTTY is not an SSH client with some other stuff tacked on the side.
PuTTY is a generic, multiple-backend, remote VT-terminal client
which happens to support one backend which is larger, more popular
and more useful than the rest. Any extra feature which can possibly
be general across all backends should be so: localising features
unnecessarily into the SSH back end is a design error. (For example,
we had several code submissions for proxy support which worked by
hacking \cw{ssh.c}. Clearly this is completely wrong: the
\cw{network.h} abstraction is the place to put it, so that it will
apply to all back ends equally, and indeed we eventually put it
there after another contributor sent a better patch.)

The rest of PuTTY should try to avoid knowing anything about
specific back ends if at all possible. To support a feature which is
only available in one network protocol, for example, the back end
interface should be extended in a general manner such that \e{any}
back end which is able to provide that feature can do so. If it so
happens that only one back end actually does, that's just the way it
is, but it shouldn't be relied upon by any code.

\H{udp-globals} Multiple sessions per process on some platforms

Some ports of PuTTY - notably the in-progress Mac port - are
constrained by the operating system to run as a single process
potentially managing multiple sessions.

Therefore, the platform-independent parts of PuTTY never use global
variables to store per-session data. The global variables that do
exist are tolerated because they are not specific to a particular
login session. The random number state in \cw{sshrand.c}, the timer
list in \cw{timing.c} and the queue of top-level callbacks in
\cw{callback.c} serve all sessions equally. But most data is specific
to a particular network session, and is therefore stored in
dynamically allocated data structures, and pointers to these
structures are passed around between functions.

Platform-specific code can reverse this decision if it likes. The
Windows code, for historical reasons, stores most of its data as
global variables. That's OK, because \e{on Windows} we know there is
only one session per PuTTY process, so it's safe to do that. But
changes to the platform-independent code should avoid introducing
global variables, unless they are genuinely cross-session.

\H{udp-pure-c} C, not C++

PuTTY is written entirely in C, not in C++.

We have made \e{some} effort to make it easy to compile our code
using a C++ compiler: notably, our \c{snew}, \c{snewn} and
\c{sresize} macros explicitly cast the return values of \cw{malloc}
and \cw{realloc} to the target type. (This has type checking
advantages even in C: it means you never accidentally allocate the
wrong size piece of memory for the pointer type you're assigning it
to. C++ friendliness is really a side benefit.)

We want PuTTY to continue being pure C, at least in the
platform-independent parts and the currently existing ports. Patches
which switch the Makefiles to compile it as C++ and start using
classes will not be accepted.

The one exception: a port to a new platform may use languages other
than C if they are necessary to code on that platform. If your
favourite PDA has a GUI with a C++ API, then there's no way you can
do a port of PuTTY without using C++, so go ahead and use it. But
keep the C++ restricted to that platform's subdirectory; if your
changes force the Unix or Windows ports to be compiled as C++, they
will be unacceptable to us.

\H{udp-security} Security-conscious coding

PuTTY is a network application and a security application. Assume
your code will end up being fed deliberately malicious data by
attackers, and try to code in a way that makes it unlikely to be a
security risk.

In particular, try not to use fixed-size buffers for variable-size
data such as strings received from the network (or even the user).
We provide functions such as \cw{dupcat} and \cw{dupprintf}, which
dynamically allocate buffers of the right size for the string they
construct. Use these wherever possible.

\H{udp-multi-compiler} Independence of specific compiler

Windows PuTTY can currently be compiled with any of three Windows
compilers: MS Visual C, the Cygwin / \cw{mingw32} GNU tools, and
\cw{clang} (in MS compatibility mode).

This is a really useful property of PuTTY, because it means people
who want to contribute to the coding don't depend on having a
specific compiler; so they don't have to fork out money for MSVC if
they don't already have it, but on the other hand if they \e{do}
have it they also don't have to spend effort installing \cw{gcc}
alongside it. They can use whichever compiler they happen to have
available, or install whichever is cheapest and easiest if they
don't have one.

Therefore, we don't want PuTTY to start depending on which compiler
you're using. Using GNU extensions to the C language, for example,
would ruin this useful property (not that anyone's ever tried it!);
and more realistically, depending on an MS-specific library function
supplied by the MSVC C library (\cw{_snprintf}, for example) is a
mistake, because that function won't be available under the other
compilers. Any function supplied in an official Windows DLL as part
of the Windows API is fine, and anything defined in the C library
standard is also fine, because those should be available
irrespective of compilation environment. But things in between,
available as non-standard library and language extensions in only
one compiler, are disallowed.

(\cw{_snprintf} in particular should be unnecessary, since we
provide \cw{dupprintf}; see \k{udp-security}.)

Compiler independence should apply on all platforms, of course, not
just on Windows.

\H{udp-small} Small code size

PuTTY is tiny, compared to many other Windows applications. And it's
easy to install: it depends on no DLLs, no other applications, no
service packs or system upgrades. It's just one executable. You
install that executable wherever you want to, and run it.

We want to keep both these properties - the small size, and the ease
of installation - if at all possible. So code contributions that
depend critically on external DLLs, or that add a huge amount to the
code size for a feature which is only useful to a small minority of
users, are likely to be thrown out immediately.

We do vaguely intend to introduce a DLL plugin interface for PuTTY,
whereby seriously large extra features can be implemented in plugin
modules. The important thing, though, is that those DLLs will be
\e{optional}; if PuTTY can't find them on startup, it should run
perfectly happily and just won't provide those particular features.
A full installation of PuTTY might one day contain ten or twenty
little DLL plugins, which would cut down a little on the ease of
installation - but if you really needed ease of installation you
\e{could} still just install the one PuTTY binary, or just the DLLs
you really needed, and it would still work fine.

Depending on \e{external} DLLs is something we'd like to avoid if at
all possible (though for some purposes, such as complex SSH
authentication mechanisms, it may be unavoidable). If it can't be
avoided, the important thing is to follow the same principle of
graceful degradation: if a DLL can't be found, then PuTTY should run
happily and just not supply the feature that depended on it.

\H{udp-single-threaded} Single-threaded code

PuTTY and its supporting tools, or at least the vast majority of
them, run in only one OS thread.

This means that if you're devising some piece of internal mechanism,
there's no need to use locks to make sure it doesn't get called by
two threads at once. The only way code can be called re-entrantly is
by recursion.

That said, most of Windows PuTTY's network handling is triggered off
Windows messages requested by \cw{WSAAsyncSelect()}, so if you call
\cw{MessageBox()} deep within some network event handling code you
should be aware that you might be re-entered if a network event
comes in and is passed on to our window procedure by the
\cw{MessageBox()} message loop.

Also, the front ends can use multiple threads if they like. For
example, the Windows front-end code spawns subthreads to deal with
bidirectional blocking I/O on non-network streams such as Windows
pipes. However, it keeps tight control of its auxiliary threads, and
uses them only for that one purpose, as a form of \cw{select()}.
Pretty much all the code outside \cw{windows/handle-io.c} is \e{only}
ever called from the one primary thread; the others just loop round
blocking on file handles, and signal the main thread (via Windows
event objects) when some real work needs doing. This is not considered
a portability hazard because that code is already Windows-specific and
needs rewriting on other platforms.

One important consequence of this: PuTTY has only one thread in
which to do everything. That \q{everything} may include managing
more than one login session (\k{udp-globals}), managing multiple
data channels within an SSH session, responding to GUI events even
when nothing is happening on the network, and responding to network
requests from the server (such as repeat key exchange) even when the
program is dealing with complex user interaction such as the
re-configuration dialog box. This means that \e{almost none} of the
PuTTY code can safely block.

\H{udp-keystrokes} Keystrokes sent to the server wherever possible

In almost all cases, PuTTY sends keystrokes to the server. Even
weird keystrokes that you think should be hot keys controlling
PuTTY. Even Alt-F4 or Alt-Space, for example. If a keystroke has a
well-defined escape sequence that it could usefully be sending to
the server, then it should do so, or at the very least it should be
configurably able to do so.

To unconditionally turn a key combination into a hot key to control
PuTTY is almost always a design error. If a hot key is really truly
required, then try to find a key combination for it which \e{isn't}
already used in existing PuTTYs (either it sends nothing to the
server, or it sends the same thing as some other combination). Even
then, be prepared for the possibility that one day that key
combination might end up being needed to send something to the
server - so make sure that there's an alternative way to invoke
whatever PuTTY feature it controls.

\H{udp-640x480} 640\u00D7{x}480 friendliness in configuration panels

There's a reason we have lots of tiny configuration panels instead
of a few huge ones, and that reason is that not everyone has a
1600\u00D7{x}1200 desktop. 640\u00D7{x}480 is still a viable
resolution for running Windows (and indeed it's still the default if
you start up in safe mode), so it's still a resolution we care
about.

Accordingly, the PuTTY configuration box, and the PuTTYgen control
window, are deliberately kept just small enough to fit comfortably
on a 640\u00D7{x}480 display. If you're adding controls to either of
these boxes and you find yourself wanting to increase the size of
the whole box, \e{don't}. Split it into more panels instead.

\H{udp-ssh-coroutines} Coroutines in protocol code

Large parts of the code in modules implementing wire protocols
(mainly SSH) are structured using a set of macros that implement
(something close to) Donald Knuth's \q{coroutines} concept in C.

Essentially, the purpose of these macros are to arrange that a
function can call \cw{crReturn()} to return to its caller, and the
next time it is called control will resume from just after that
\cw{crReturn} statement.

This means that any local (automatic) variables declared in such a
function will be corrupted every time you call \cw{crReturn}. If you
need a variable to persist for longer than that, you \e{must} make it
a field in some appropriate structure containing the persistent state
of the coroutine \dash typically the main state structure for a
protocol layer.

See
\W{https://www.chiark.greenend.org.uk/~sgtatham/coroutines.html}\c{https://www.chiark.greenend.org.uk/~sgtatham/coroutines.html}
for a more in-depth discussion of what these macros are for and how
they work.

Another caveat: most of these coroutines are not \e{guaranteed} to run
to completion, because the SSH connection (or whatever) that they're
part of might be interrupted at any time by an unexpected network
event or user action. So whenever a coroutine-managed variable refers
to a resource that needs releasing, you should also ensure that the
cleanup function for its containing state structure can reliably
release it even if the coroutine is aborted at an arbitrary point.

For example, if an SSH packet protocol layer has to have a field that
sometimes points to a piece of allocated memory, then you should
ensure that when you free that memory you reset the pointer field to
\cw{NULL}. Then, no matter when the protocol layer's cleanup function
is called, it can reliably free the memory if there is any, and not
crash if there isn't.

\H{udp-traits} Explicit vtable structures to implement traits

A lot of PuTTY's code is written in a style that looks structurally
rather like an object-oriented language, in spite of PuTTY being a
pure C program.

For example, there's a single data type called \cw{ssh_hash}, which is
an abstraction of a secure hash function, and a bunch of functions
called things like \cw{ssh_hash_}\e{foo} that do things with those
data types. But in fact, PuTTY supports many different hash functions,
and each one has to provide its own implementation of those functions.

In C++ terms, this is rather like having a single abstract base class,
and multiple concrete subclasses of it, each of which fills in all the
pure virtual methods in a way that's compatible with the data fields
of the subclass. The implementation is more or less the same, as well:
in C, we do explicitly in the source code what the C++ compiler will
be doing behind the scenes at compile time.

But perhaps a closer analogy in functional terms is the Rust concept
of a \q{trait}, or the Java idea of an \q{interface}. C++ supports a
multi-level hierarchy of inheritance, whereas PuTTY's system \dash
like traits or interfaces \dash has only two levels, one describing a
generic object of a type (e.g. a hash function) and another describing
a specific implementation of that type (e.g. SHA-256).

The PuTTY code base has a standard idiom for doing this in C, as
follows.

Firstly, we define two \cw{struct} types for our trait. One of them
describes a particular \e{kind} of implementation of that trait, and
it's full of (mostly) function pointers. The other describes a
specific \e{instance} of an implementation of that trait, and it will
contain a pointer to a \cw{const} instance of the first type. For
example:

\c typedef struct MyAbstraction MyAbstraction;
\c typedef struct MyAbstractionVtable MyAbstractionVtable;
\c
\c struct MyAbstractionVtable {
\c     MyAbstraction *(*new)(const MyAbstractionVtable *vt);
\c     void (*free)(MyAbstraction *);
\c     void (*modify)(MyAbstraction *, unsigned some_parameter);
\c     unsigned (*query)(MyAbstraction *, unsigned some_parameter);
\c };
\c
\c struct MyAbstraction {
\c     const MyAbstractionVtable *vt;
\c };

Here, we imagine that \cw{MyAbstraction} might be some kind of object
that contains mutable state. The associated vtable structure shows
what operations you can perform on a \cw{MyAbstraction}: you can
create one (dynamically allocated), free one you already have, or call
the example methods \q{modify} (to change the state of the object in
some way) and \q{query} (to return some value derived from the
object's current state).

(In most cases, the vtable structure has a name ending in \cq{vtable}.
But for historical reasons a lot of the crypto primitives that use
this scheme \dash ciphers, hash functions, public key methods and so
on \dash instead have names ending in \cq{alg}, on the basis that the
primitives they implement are often referred to as \q{encryption
algorithms}, \q{hash algorithms} and so forth.)

Now, to define a concrete instance of this trait, you'd define a
\cw{struct} that contains a \cw{MyAbstraction} field, plus any other
data it might need:

\c struct MyImplementation {
\c     unsigned internal_data[16];
\c     SomeOtherType *dynamic_subthing;
\c
\c     MyAbstraction myabs;
\c };

Next, you'd implement all the necessary methods for that
implementation of the trait, in this kind of style:

\c static MyAbstraction *myimpl_new(const MyAbstractionVtable *vt)
\c {
\c     MyImplementation *impl = snew(MyImplementation);
\c     memset(impl, 0, sizeof(*impl));
\c     impl->dynamic_subthing = allocate_some_other_type();
\c     impl->myabs.vt = vt;
\c     return &impl->myabs;
\c }
\c
\c static void myimpl_free(MyAbstraction *myabs)
\c {
\c     MyImplementation *impl = container_of(myabs, MyImplementation, myabs);
\c     free_other_type(impl->dynamic_subthing);
\c     sfree(impl);
\c }
\c
\c static void myimpl_modify(MyAbstraction *myabs, unsigned param)
\c {
\c     MyImplementation *impl = container_of(myabs, MyImplementation, myabs);
\c     impl->internal_data[param] += do_something_with(impl->dynamic_subthing);
\c }
\c
\c static unsigned myimpl_query(MyAbstraction *myabs, unsigned param)
\c {
\c     MyImplementation *impl = container_of(myabs, MyImplementation, myabs);
\c     return impl->internal_data[param];
\c }

Having defined those methods, now we can define a \cw{const} instance
of the vtable structure containing pointers to them:

\c const MyAbstractionVtable MyImplementation_vt = {
\c     .new = myimpl_new,
\c     .free = myimpl_free,
\c     .modify = myimpl_modify,
\c     .query = myimpl_query,
\c };

\e{In principle}, this is all you need. Client code can construct a
new instance of a particular implementation of \cw{MyAbstraction} by
digging out the \cw{new} method from the vtable and calling it (with
the vtable itself as a parameter), which returns a \cw{MyAbstraction
*} pointer that identifies a newly created instance, in which the
\cw{vt} field will contain a pointer to the same vtable structure you
passed in. And once you have an instance object, say \cw{MyAbstraction
*myabs}, you can dig out one of the other method pointers from the
vtable it points to, and call that, passing the object itself as a
parameter.

But in fact, we don't do that, because it looks pretty ugly at all the
call sites. Instead, what we generally do in this code base is to
write a set of \cw{static inline} wrapper functions in the same header
file that defined the \cw{MyAbstraction} structure types, like this:

\c static inline MyAbstraction *myabs_new(const MyAbstractionVtable *vt)
\c { return vt->new(vt); }
\c static inline void myabs_free(MyAbstraction *myabs)
\c { myabs->vt->free(myabs); }
\c static inline void myimpl_modify(MyAbstraction *myabs, unsigned param)
\c { myabs->vt->modify(myabs, param); }
\c static inline unsigned myimpl_query(MyAbstraction *myabs, unsigned param)
\c { return myabs->vt->query(myabs, param); }

And now call sites can use those reasonably clean-looking wrapper
functions, and shouldn't ever have to directly refer to the \cw{vt}
field inside any \cw{myabs} object they're holding. For example, you
might write something like this:

\c MyAbstraction *myabs = myabs_new(&MyImplementation_vtable);
\c myabs_update(myabs, 10);
\c unsigned output = myabs_query(myabs, 2);
\c myabs_free(myabs);

and then all this code can use a different implementation of the same
abstraction by just changing which vtable pointer it passed in in the
first line.

Some things to note about this system:

\b The implementation instance type (here \cq{MyImplementation}
contains the abstraction type (\cq{MyAbstraction}) as one of its
fields. But that field is not necessarily at the start of the
structure. So you can't just \e{cast} pointers back and forth between
the two types. Instead:

\lcont{

\b You \q{up-cast} from implementation to abstraction by taking the
address of the \cw{MyAbstraction} field. You can see the example
\cw{new} method above doing this, returning \cw{&impl->myabs}. All
\cw{new} methods do this on return.

\b Going in the other direction, each method that was passed a generic
\cw{MyAbstraction *myabs} parameter has to recover a pointer to the
specific implementation type \cw{MyImplementation *impl}. The idiom
for doing that is to use the \cq{container_of} macro, also seen in the
Linux kernel code. Generally, \cw{container_of(p, Type, field)} says:
\q{I'm confident that the pointer value \cq{p} is pointing to the
field called \cq{field} within a larger \cw{struct} of type \cw{Type}.
Please return me the pointer to the containing structure.} So in this
case, we take the \cq{myabs} pointer passed to the function, and
\q{down-cast} it into a pointer to the larger and more specific
structure type \cw{MyImplementation}, by adjusting the pointer value
based on the offset within that structure of the field called
\cq{myabs}.

This system is flexible enough to permit \q{multiple inheritance}, or
rather, multiple \e{implementation}: having one object type implement
more than one trait. For example, the \cw{ProxySocket} type implements
both the \cw{Socket} trait and the \cw{Plug} trait that connects to it,
because it has to act as an adapter between another instance of each
of those types.

It's also perfectly possible to have the same object implement the
\e{same} trait in two different ways. At the time of writing this I
can't think of any case where we actually do this, but a theoretical
example might be if you needed to support a trait like \cw{Comparable}
in two ways that sorted by different criteria. There would be no
difficulty doing this in the PuTTY system: simply have your
implementation \cw{struct} contain two (or more) fields of the same
abstraction type. The fields will have different names, which makes it
easy to explicitly specify which one you're returning a pointer to
during up-casting, or which one you're down-casting from using
\cw{container_of}. And then both sets of implementation methods can
recover a pointer to the same containing structure.

}

\b Unlike in C++, all objects in PuTTY that use this system are
dynamically allocated. The \q{constructor} functions (whether they're
virtualised across the whole abstraction or specific to each
implementation) always allocate memory and return a pointer to it. The
\q{free} method (our analogue of a destructor) always expects the
input pointer to be dynamically allocated, and frees it. As a result,
client code doesn't need to know how large the implementing object
type is, because it will never need to allocate it (on the stack or
anywhere else).

\b Unlike in C++, the abstraction's \q{vtable} structure does not only
hold methods that you can call on an instance object. It can also
hold several other kinds of thing:

\lcont{

\b Methods that you can call \e{without} an instance object, given
only the vtable structure identifying a particular implementation of
the trait. You might think of these as \q{static methods}, as in C++,
except that they're \e{virtual} \dash the same code can call the
static method of a different \q{class} given a different vtable
pointer. So they're more like \q{virtual static methods}, which is a
concept C++ doesn't have. An example is the \cw{pubkey_bits} method in
\cw{ssh_keyalg}.

\b The most important case of a \q{virtual static method} is the
\cw{new} method that allocates and returns a new object. You can think
of it as a \q{virtual constructor} \dash another concept C++ doesn't
have. (However, not all types need one of these: see below.)

\b The vtable can also contain constant data relevant to the class as
a whole \dash \q{virtual constant data}. For example, a cryptographic
hash function will contain an integer field giving the length of the
output hash, and most crypto primitives will contain a string field
giving the identifier used in the SSH protocol that describes that
primitive.

The effect of all of this is that you can make other pieces of code
able to use any instance of one of these types, by passing it an
actual vtable as a parameter. For example, the \cw{hash_simple}
function takes an \cw{ssh_hashalg} vtable pointer specifying any hash
algorithm you like, and internally, it creates an object of that type,
uses it, and frees it. In C++, you'd probably do this using a
template, which would mean you had multiple specialisations of
\cw{hash_simple} \dash and then it would be much more difficult to
decide \e{at run time} which one you needed to use. Here,
\cw{hash_simple} is still just one function, and you can decide as
late as you like which vtable to pass to it.

}

\b The abstract \e{instance} structure can also contain publicly
visible data fields (this time, usually treated as mutable) which are
common to all implementations of the trait. For example,
\cw{BinaryPacketProtocol} has lots of these.

\b Not all abstractions of this kind want virtual constructors. It
depends on how different the implementations are.

\lcont{

With a crypto primitive like a hash algorithm, the constructor call
looks the same for every implementing type, so it makes sense to have
a standardised virtual constructor in the vtable and a
\cw{ssh_hash_new} wrapper function which can make an instance of
whatever vtable you pass it. And then you make all the vtable objects
themselves globally visible throughout the source code, so that any
module can call (for example) \cw{ssh_hash_new(&ssh_sha256)}.

But with other kinds of object, the constructor for each implementing
type has to take a different set of parameters. For example,
implementations of \cw{Socket} are not generally interchangeable at
construction time, because constructing different kinds of socket
require totally different kinds of address parameter. In that
situation, it makes more sense to keep the vtable structure itself
private to the implementing source file, and instead, publish an
ordinary constructing function that allocates and returns an instance
of that particular subtype, taking whatever parameters are appropriate
to that subtype.

}

\b If you do have virtual constructors, you can choose whether they
take a vtable pointer as a parameter (as shown above), or an
\e{existing} instance object. In the latter case, they can refer to
the object itself as well as the vtable. For example, you could have a
trait come with a virtual constructor called \q{clone}, meaning
\q{Make a copy of this object, no matter which implementation it is.}

\b Sometimes, a single vtable structure type can be shared between two
completely different object types, and contain all the methods for
both. For example, \cw{ssh_compression_alg} contains methods to
create, use and free \cw{ssh_compressor} and \cw{ssh_decompressor}
objects, which are not interchangeable \dash but putting their methods
in the same vtable means that it's easy to create a matching pair of
objects that are compatible with each other.

\b Passing the vtable itself as an argument to the \cw{new} method is
not compulsory: if a given \cw{new} implementation is only used by a
single vtable, then that function can simply hard-code the vtable
pointer that it writes into the object it constructs. But passing the
vtable is more flexible, because it allows a single constructor
function to be shared between multiple slightly different object
types. For example, SHA-384 and SHA-512 share the same \cw{new} method
and the same implementation data type, because they're very nearly the
same hash algorithm \dash but a couple of the other methods in their
vtables are different, because the \q{reset} function has to set up
the initial algorithm state differently, and the \q{digest} method has
to write out a different amount of data.

\lcont{

One practical advantage of having the \cw{myabs_}\e{foo} family of
inline wrapper functions in the header file is that if you change your
mind later about whether the vtable needs to be passed to \cw{new},
you only have to update the \cw{myabs_new} wrapper, and then the
existing call sites won't need changing.

}

\b Another piece of \q{stunt object orientation} made possible by this
scheme is that you can write two vtables that both use the same
structure layout for the implementation object, and have an object
\e{transform from one to the other} part way through its lifetime, by
overwriting its own vtable pointer field. For example, the
\cw{sesschan} type that handles the server side of an SSH terminal
session will sometimes transform in mid-lifetime into an SCP or SFTP
file-transfer channel in this way, at the point where the client sends
an \cq{exec} or \cq{subsystem} request that indicates that that's what
it wants to do with the channel.

\lcont{

This concept would be difficult to arrange in C++. In Rust, it
wouldn't even \e{make sense}, because in Rust, objects implementing a
trait don't even contain a vtable pointer at all \dash instead, the
\q{trait object} type (identifying a specific instance of some
implementation of a given trait) consists of a pair of pointers, one
to the object itself and one to the vtable. In that model, the only
way you could make an existing object turn into a different trait
would be to know where all the pointers to it were stored elsewhere in
the program, and persuade all their owners to rewrite them.

}

\b Another stunt you can do is to have a vtable that doesn't have a
corresponding implementation structure at all, because the only
methods implemented in it are the constructors, and they always end up
returning an implementation of some other vtable. For example, some of
PuTTY's crypto primitives have a hardware-accelerated version and a
pure software version, and decide at run time which one to use (based
on whether the CPU they're running on supports the necessary
acceleration instructions). So, for example, there are vtables for
\cw{ssh_sha256_sw} and \cw{ssh_sha256_hw}, each of which has its own
data layout and its own implementations of all the methods; and then
there's a top-level vtable \cw{ssh_sha256}, which only provides the
\q{new} method, and implements it by calling the \q{new} method on one
or other of the subtypes depending on what it finds out about the
machine it's running on. That top-level selector vtable is nearly
always the one used by client code. (Except for the test suite, which
has to instantiate both of the subtypes in order to make sure they
both pass the tests.)

\lcont{

As a result, the top-level selector vtable \cw{ssh_sha256} doesn't
need to implement any method that takes an \cw{ssh_cipher *}
parameter, because no \cw{ssh_cipher} object is ever constructed whose
\cw{vt} field points to \cw{&ssh_sha256}: they all point to one of the
other two full implementation vtables.

}

\H{udp-perfection} Do as we say, not as we do

The current PuTTY code probably does not conform strictly to \e{all}
of the principles listed above. There may be the occasional
SSH-specific piece of code in what should be a backend-independent
module, or the occasional dependence on a non-standard X library
function under Unix.

This should not be taken as a licence to go ahead and violate the
rules. Where we violate them ourselves, we're not happy about it,
and we would welcome patches that fix any existing problems. Please
try to help us make our code better, not worse!