path: root/doc/overview.txt

Contents
========

* Basic MuPDF usage example
* Common function arguments
* Error Handling
* Multi-threading

Basic MuPDF usage example
=========================

For an example of how to use MuPDF in the most basic way, see
doc/example.c. 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
=========================

Many functions in MuPDFs 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)

Other functions in MuPDF's interface take arguments such as document,
stream and device which contain state for each type of object. Those
arguments each have a reference to a context and therefore act as
proxies for a context.

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 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 an 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:

a)	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);

b)	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.

c)	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.

d)	We assume the standard C convention that it is safe to destroy
	NULL things.

Multi-threading
===============

First off, study the basic usage example in doc/example.c and make
sure you understand how it works as it will be the referenced in this
section too.

There are two variations of how to create multi-threaded applications:

1)	lock-less operation -- in which one thread is requesting pages
	to be drawn and responding to user interface actions, while
	another thread is dedicated to drawing pages. In this scenario
	only one thread owns and manipulates the context and document
	at any one time.

2)	using locking -- where one thread is requesting pages to be
	draw and responding to user interface actions, while several
	threads may be drawing pages. In this scenario each thread has
	its own context but they share some global state, for example
	the resource store. An additional constraint

The usage example starts by getting a context from fz_new_context with
standard memory allocation functions, default resource store size and,
crucially, no locking. 

In a multi-threaded application every thread must have a context. Or
more specifically, each context can only be used from one thread at a
time. When starting another thread, do NOT call fz_new_context again;
instead call fz_clone_context. This creates a context sharing the
memory allocator, resource store etc.




så utan lås: en gui-tråd som visa progress, en tråd som renderar
med lås: en gui-tråd som request:ar och flera trådar som renderar

having fitz level display list objects created in 
one thread, consumed as read-only in multiple threads, with locked access 
around a few shared caches