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+# Overview
+
+## Basic MuPDF usage example
+
+To limit the complexity and give an easier introduction
+this code has no error handling at all, but any serious piece of code
+using MuPDF should use the error handling strategies described below.
+
+## Common function arguments
+
+Most functions in MuPDF's interface take a context argument.
+
+A context contains global state used by MuPDF inside functions when
+parsing or rendering pages of the document. It contains for example:
+
+- an exception stack (see error handling below),
+- a memory allocator (allowing for custom allocators)
+- a resource store (for caching of images, fonts, etc.)
+- a set of locks and (un-)locking functions (for multi-threading)
+
+Without the set of locks and accompanying functions the context and
+its proxies may only be used in a single-threaded application.
+
+## Error handling
+
+MuPDF uses a set of exception handling macros to simplify error return
+and cleanup. Conceptually, they work a lot like C++'s try/catch
+system, but do not require any special compiler support.
+
+The basic formulation is as follows:
+
+	fz_try(ctx)
+	{
+		// Try to perform a task. Never 'return', 'goto' or
+		// 'longjmp' out of here. 'break' may be used to
+		// safely exit (just) the try block scope.
+	}
+	fz_always(ctx)
+	{
+		// Any code here is always executed, regardless of
+		// whether an exception was thrown within the try or
+		// not. Never 'return', 'goto' or longjmp out from
+		// here. 'break' may be used to safely exit (just) the
+		// always block scope.
+	}
+	fz_catch(ctx)
+	{
+		// This code is called (after any always block) only
+		// if something within the fz_try block (including any
+		// functions it called) threw an exception. The code
+		// here is expected to handle the exception (maybe
+		// record/report the error, cleanup any stray state
+		// etc) and can then either exit the block, or pass on
+		// the exception to a higher level (enclosing) fz_try
+		// block (using fz_throw, or fz_rethrow).
+	}
+
+The fz_always block is optional, and can safely be omitted.
+
+The macro based nature of this system has 3 main limitations:
+
+1.
+	Never return from within try (or 'goto' or longjmp out of it).
+	This upsets the internal housekeeping of the macros and will
+	cause problems later on. The code will detect such things
+	happening, but by then it is too late to give a helpful error
+	report as to where the original infraction occurred.
+
+2.
+	The try/always/catch statement is not one atomic C statement.
+
+	The `fz_try(ctx) { ... } fz_always(ctx) { ... } fz_catch(ctx) { ... }` is not one atomic C statement.
+	That is to say, if you do:
+
+		if (condition)
+			fz_try(ctx) { ... }
+			fz_catch(ctx) { ... }
+
+	then you will not get what you want. Use the following instead:
+
+		if (condition) {
+			fz_try(ctx) { ... }
+			fz_catch(ctx) { ... }
+		}
+
+3.
+	The macros are implemented using setjmp and longjmp, and so
+	the standard C restrictions on the use of those functions
+	apply to fz_try/fz_catch too.
+
+	In particular, any "truly local"
+	variable that is set between the start of fz_try and something
+	in fz_try throwing an exception may become undefined as part
+	of the process of throwing that exception.
+
+As a way of mitigating this problem, we provide a fz_var()
+macro that tells the compiler to ensure that that variable is
+not unset by the act of throwing the exception.
+
+A model piece of code using these macros then might be:
+
+	house build_house(plans *p)
+	{
+		material m = NULL;
+		walls w = NULL;
+		roof r = NULL;
+		house h = NULL;
+		tiles t = make_tiles();
+
+		fz_var(w);
+		fz_var(r);
+		fz_var(h);
+
+		fz_try(ctx)
+		{
+			fz_try(ctx)
+			{
+				m = make_bricks();
+			}
+			fz_catch(ctx)
+			{
+				// No bricks available, make do with straw?
+				m = make_straw();
+			}
+			w = make_walls(m, p);
+			r = make_roof(m, t);
+			// Note, NOT: return combine(w,r);
+			h = combine(w, r);
+		}
+		fz_always(ctx)
+		{
+			drop_walls(w);
+			drop_roof(r);
+			drop_material(m);
+			drop_tiles(t);
+		}
+		fz_catch(ctx)
+		{
+			fz_throw(ctx, "build_house failed");
+		}
+		return h;
+	}
+
+Things to note about this:
+
+1. If make_tiles throws an exception, this will immediately be
+handled by some higher level exception handler. If it
+succeeds, t will be set before fz_try starts, so there is no
+need to fz_var(t);
+
+2. We try first off to make some bricks as our building material.
+If this fails, we fall back to straw. If this fails, we'll end
+up in the fz_catch, and the process will fail neatly.
+
+3. We assume in this code that combine takes new reference to
+both the walls and the roof it uses, and therefore that w and
+r need to be cleaned up in all cases.
+
+4. We assume the standard C convention that it is safe to destroy
+NULL things.
+
+## Multi-threading
+
+First off, study the basic usage example in `docs/examples/example.c`
+and make sure you understand how it works as the data structures manipulated
+there will be referred to in this section too.
+
+MuPDF can usefully be built into a multi-threaded application without
+the library needing to know anything threading at all. If the library
+opens a document in one thread, and then sits there as a 'server'
+requesting pages and rendering them for other threads that need them,
+then the library is only ever being called from this one thread.
+
+Other threads can still be used to handle UI requests etc, but as far
+as MuPDF is concerned it is only being used in a single threaded way.
+In this instance, there are no threading issues with MuPDF at all,
+and it can safely be used without any locking, as described in the
+previous sections.
+
+This section will attempt to explain how to use MuPDF in the more
+complex case; where we genuinely want to call the MuPDF library
+concurrently from multiple threads within a single application.
+
+MuPDF can be invoked with a user supplied set of locking functions.
+It uses these to take mutexes around operations that would conflict
+if performed concurrently in multiple threads. By leaving the
+exact implementation of locks to the caller MuPDF remains threading
+library agnostic.
+
+The following simple rules should be followed to ensure that
+multi-threaded operations run smoothly:
+
+1.
+	"No simultaneous calls to MuPDF in different threads are
+	allowed to use the same context."
+
+	Most of the time it is simplest to just use a different
+	context for every thread; just create a new context at the
+	same time as you create the thread. For more details see
+	"Cloning the context" below.
+
+2.
+	"No simultaneous calls to MuPDF in different threads are
+	allowed to use the same document."
+
+	Only one thread can be accessing a document at a time, but
+	once display lists are created from that document, multiple
+	threads at a time can operate on them.
+
+	The document can be used from several different threads as
+	long as there are safeguards in place to prevent the usages
+	being simultaneous.
+
+3.
+	"No simultaneous calls to MuPDF in different threads are
+	allowed to use the same device."
+
+	Calling a device simultaneously from different threads will
+	cause it to get confused and may crash. Calling a device from
+	several different threads is perfectly acceptable as long as
+	there are safeguards in place to prevent the calls being
+	simultaneous.
+
+4.
+	"An fz_locks_context must be supplied at context creation time,
+	unless MuPDF is to be used purely in a single thread at a time."
+
+	MuPDF needs to protect against unsafe access to certain structures/
+	resources/libraries from multiple threads. It does this by using
+	the user supplied locking functions. This holds true even when
+	using completely separate instances of MuPDF.
+
+5.
+	"All contexts in use must share the same fz_locks_context (or
+	the underlying locks thereof)."
+
+	We strongly recommend that fz_new_context is called just once,
+	and fz_clone_context is called to generate new contexts from
+	that. This will automatically ensure that the same locking
+	mechanism is used in all MuPDF instances. For now, we do support
+	multiple completely independent contexts being created using
+	repeated calls to fz_new_context, but these MUST share the
+	same fz_locks_context (or at least depend upon the same underlying
+	locks). The facility to create different independent contexts
+	may be removed in future.
+
+So, how does a multi-threaded example differ from a non-multithreaded
+one?
+
+Firstly, when we create the first context, we call fz_new_context
+as before, but the second argument should be a pointer to a set
+of locking functions.
+
+The calling code should provide FZ_LOCK_MAX mutexes, which will be
+locked/unlocked by MuPDF calling the lock/unlock function pointers
+in the supplied structure with the user pointer from the structure
+and the lock number, i (0 <= i < FZ_LOCK_MAX). These mutexes can
+safely be recursive or non-recursive as MuPDF only calls in a non-
+recursive style.
+
+To make subsequent contexts, the user should NOT call fz_new_context
+again (as this will fail to share important resources such as the
+store and glyphcache), but should rather call fz_clone_context.
+Each of these cloned contexts can be freed by fz_free_context as
+usual. They will share the important data structures (like store,
+glyph cache etc) with the original context, but will have their
+own exception stacks.
+
+To open a document, call fz_open_document as usual, passing a context
+and a filename. It is important to realise that only one thread at a
+time can be accessing the documents itself.
+
+This means that only one thread at a time can perform operations such
+as fetching a page, or rendering that page to a display list. Once a
+display list has been obtained however, it can be rendered from any
+other thread (or even from several threads simultaneously, giving
+banded rendering).
+
+This means that an implementer has 2 basic choices when constructing
+an application to use MuPDF in multi-threaded mode. Either he can
+construct it so that a single nominated thread opens the document
+and then acts as a 'server' creating display lists for other threads
+to render, or he can add his own mutex around calls to mupdf that
+use the document. The former is likely to be far more efficient in
+the long run.
+
+For an example of how to do multi-threading see
+`docs/examples/multi-threaded.c`
+which has a main thread and one rendering thread per page.
+
+## Cloning the context
+
+As described above, every context contains an exception stack which is
+manipulated during the course of nested fz_try/fz_catches. For obvious
+reasons the same exception stack cannot be used from more than one
+thread at a time.
+
+If, however, we simply created a new context (using fz_new_context) for
+every thread, we would end up with separate stores/glyph caches etc,
+which is not (generally) what is desired. MuPDF therefore provides a
+mechanism for "cloning" a context. This creates a new context that
+shares everything with the given context, except for the exception
+stack.
+
+A commonly used general scheme is therefore to create a 'base' context
+at program start up, and to clone this repeatedly to get new contexts
+that can be used on new threads.