Previous | Contents | Index | Next

Appendix E: 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.

E.1 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 ssh.c is not acceptable. We went to great lengths to 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 unix subdirectory; the Windows-specific modules are in the windows subdirectory.

All the modules in the main source directory and other subdirectories - notably 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 int and long int; don't use pointer casts to do endianness-dependent operations, and so on.

(Even 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 mostly C99. With a couple of exceptions, you can assume that C99 features are available (in particular <stdint.h>, <stdbool.h> and inline), but you shouldn't use things that are new in C11 (such as <uchar.h> or _Generic).

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

Here are a few portability assumptions that we 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):

On the other hand, here are some particular things not to assume:

E.2 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 ssh.c. Clearly this is completely wrong: the 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 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.

E.3 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 sshrand.c, the timer list in timing.c and the queue of top-level callbacks in 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 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.

E.4 C, not C++

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

We have made some effort to make it easy to compile our code using a C++ compiler: notably, our snew, snewn and sresize macros explicitly cast the return values of malloc and 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.

E.5 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 dupcat and dupprintf, which dynamically allocate buffers of the right size for the string they construct. Use these wherever possible.

E.6 Independence of specific compiler

Windows PuTTY can currently be compiled with any of three Windows compilers: MS Visual C, the Cygwin / mingw32 GNU tools, and 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 do have it they also don't have to spend effort installing 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 (_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.

(_snprintf in particular should be unnecessary, since we provide dupprintf; see section E.5.)

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

E.7 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 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 could still just install the one PuTTY binary, or just the DLLs you really needed, and it would still work fine.

Depending on 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.

E.8 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 WSAAsyncSelect(), so if you call 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 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 select(). Pretty much all the code outside windows/handle-io.c is 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 ‘everything’ may include managing more than one login session (section E.3), 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 almost none of the PuTTY code can safely block.

E.9 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 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.

E.10 640×480 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 1600×1200 desktop. 640×480 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 640×480 display. If you're adding controls to either of these boxes and you find yourself wanting to increase the size of the whole box, don't. Split it into more panels instead.

E.11 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 ‘coroutines’ concept in C.

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

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

See for a more in-depth discussion of what these macros are for and how they work.

Another caveat: most of these coroutines are not 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 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.

E.12 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 ssh_hash, which is an abstraction of a secure hash function, and a bunch of functions called things like ssh_hash_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 ‘trait’, or the Java idea of an ‘interface’. C++ supports a multi-level hierarchy of inheritance, whereas PuTTY's system – like traits or interfaces – 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 struct types for our trait. One of them describes a particular kind of implementation of that trait, and it's full of (mostly) function pointers. The other describes a specific instance of an implementation of that trait, and it will contain a pointer to a const instance of the first type. For example:

typedef struct MyAbstraction MyAbstraction;
typedef struct MyAbstractionVtable MyAbstractionVtable;

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

struct MyAbstraction {
    const MyAbstractionVtable *vt;

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

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

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

struct MyImplementation {
    unsigned internal_data[16];
    SomeOtherType *dynamic_subthing;

    MyAbstraction myabs;

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

static MyAbstraction *myimpl_new(const MyAbstractionVtable *vt)
    MyImplementation *impl = snew(MyImplementation);
    memset(impl, 0, sizeof(*impl));
    impl->dynamic_subthing = allocate_some_other_type();
    impl->myabs.vt = vt;
    return &impl->myabs;

static void myimpl_free(MyAbstraction *myabs)
    MyImplementation *impl = container_of(myabs, MyImplementation, myabs);

static void myimpl_modify(MyAbstraction *myabs, unsigned param)
    MyImplementation *impl = container_of(myabs, MyImplementation, myabs);
    impl->internal_data[param] += do_something_with(impl->dynamic_subthing);

static unsigned myimpl_query(MyAbstraction *myabs, unsigned param)
    MyImplementation *impl = container_of(myabs, MyImplementation, myabs);
    return impl->internal_data[param];

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

const MyAbstractionVtable MyImplementation_vt = {
    .new = myimpl_new,
    .free = myimpl_free,
    .modify = myimpl_modify,
    .query = myimpl_query,

In principle, this is all you need. Client code can construct a new instance of a particular implementation of MyAbstraction by digging out the new method from the vtable and calling it (with the vtable itself as a parameter), which returns a MyAbstraction * pointer that identifies a newly created instance, in which the vt field will contain a pointer to the same vtable structure you passed in. And once you have an instance object, say 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 static inline wrapper functions in the same header file that defined the MyAbstraction structure types, like this:

static inline MyAbstraction *myabs_new(const MyAbstractionVtable *vt)
{ return vt->new(vt); }
static inline void myabs_free(MyAbstraction *myabs)
{ myabs->vt->free(myabs); }
static inline void myimpl_modify(MyAbstraction *myabs, unsigned param)
{ myabs->vt->modify(myabs, param); }
static inline unsigned myimpl_query(MyAbstraction *myabs, unsigned param)
{ 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 vt field inside any myabs object they're holding. For example, you might write something like this:

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

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:

E.13 Do as we say, not as we do

The current PuTTY code probably does not conform strictly to 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!

If you want to provide feedback on this manual or on the PuTTY tools themselves, see the Feedback page.