\# 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 Mac-specific modules are in the \c{mac}
subdirectory; the Windows-specific modules are in the \c{windows}
subdirectory.

All the modules in the main source directory - 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: \c{flags} defines properties that are expected to
apply equally to \e{all} the sessions run by a single PuTTY process,
the random number state in \cw{sshrand.c} and the timer list in
\cw{timing.c} serve all sessions equally, and so on. 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. Also, in particular, we disapprove of
\cw{//} comments, at least for the moment. (Perhaps once C99 becomes
genuinely widespread we might be more lenient.)

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 (in particular Windows Plink) can use multiple
threads if they like. However, Windows Plink keeps \e{very} tight
control of its auxiliary threads, and uses them pretty much
exclusively as a form of \cw{select()}. Pretty much all the code
outside \cw{windows/winplink.c} is \e{only} ever called from the one
primary thread; the others just loop round blocking on file handles
and send messages to the main thread when some real work needs
doing. This is not considered a portability hazard because that bit
of \cw{windows/winplink.c} will need rewriting on other platforms in
any case.

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-makefiles-auto} Automatically generated \cw{Makefile}s

PuTTY is intended to compile on multiple platforms, and with
multiple compilers. It would be horrifying to try to maintain a
single \cw{Makefile} which handled all possible situations, and just
as painful to try to directly maintain a set of matching
\cw{Makefile}s for each different compilation environment.

Therefore, we have moved the problem up by one level. In the PuTTY
source archive is a file called \c{Recipe}, which lists which source
files combine to produce which binaries; and there is also a script
called \cw{mkfiles.pl}, which reads \c{Recipe} and writes out the
real \cw{Makefile}s. (The script also reads all the source files and
analyses their dependencies on header files, so we get an extra
benefit from doing it this way, which is that we can supply correct
dependency information even in environments where it's difficult to
set up an automated \c{make depend} phase.)

You should \e{never} edit any of the PuTTY \cw{Makefile}s directly.
They are not stored in our source repository at all. They are
automatically generated by \cw{mkfiles.pl} from the file \c{Recipe}.

If you need to add a new object file to a particular binary, the
right thing to do is to edit \c{Recipe} and re-run \cw{mkfiles.pl}.
This will cause the new object file to be added in every tool that
requires it, on every platform where it matters, in every
\cw{Makefile} to which it is relevant, \e{and} to get all the
dependency data right.

If you send us a patch that modifies one of the \cw{Makefile}s, you
just waste our time, because we will have to convert it into a
change to \c{Recipe}. If you send us a patch that modifies \e{all}
of the \cw{Makefile}s, you will have wasted a lot of \e{your} time
as well!

(There is a comment at the top of every \cw{Makefile} in the PuTTY
source archive saying this, but many people don't seem to read it,
so it's worth repeating here.)

\H{udp-ssh-coroutines} Coroutines in the SSH code

Large parts of the code in the various SSH modules (in fact most of
the protocol layers) 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 an SSH
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-compile-once} Single compilation of each source file

The PuTTY build system for any given platform works on the following
very simple model:

\b Each source file is compiled precisely once, to produce a single
object file.

\b Each binary is created by linking together some combination of
those object files.

Therefore, if you need to introduce functionality to a particular
module which is only available in some of the tool binaries (for
example, a cryptographic proxy authentication mechanism which needs
to be left out of PuTTYtel to maintain its usability in
crypto-hostile jurisdictions), the \e{wrong} way to do it is by
adding \cw{#ifdef}s in (say) \cw{proxy.c}. This would require
separate compilation of \cw{proxy.c} for PuTTY and PuTTYtel, which
means that the entire \cw{Makefile}-generation architecture (see
\k{udp-makefiles-auto}) would have to be significantly redesigned.
Unless you are prepared to do that redesign yourself, \e{and}
guarantee that it will still port to any future platforms we might
decide to run on, you should not attempt this!

The \e{right} way to introduce a feature like this is to put the new
code in a separate source file, and (if necessary) introduce a
second new source file defining the same set of functions, but
defining them as stubs which don't provide the feature. Then the
module whose behaviour needs to vary (\cw{proxy.c} in this example)
can call the functions defined in these two modules, and it will
either provide the new feature or not provide it according to which
of your new modules it is linked with.

Of course, object files are never shared \e{between} platforms; so
it is allowable to use \cw{#ifdef} to select between platforms. This
happens in \cw{puttyps.h} (choosing which of the platform-specific
include files to use), and also in \cw{misc.c} (the Windows-specific
\q{Minefield} memory diagnostic system). It should be used
sparingly, though, if at all.

\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!
