diff options
Diffstat (limited to 'docs/overview.txt')
-rw-r--r-- | docs/overview.txt | 268 |
1 files changed, 268 insertions, 0 deletions
diff --git a/docs/overview.txt b/docs/overview.txt new file mode 100644 index 00000000..2d4e25d7 --- /dev/null +++ b/docs/overview.txt @@ -0,0 +1,268 @@ +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 the data structures manipulated +there will be refered 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. + +2) "The document is bound to the context with which it is created." + + All subsequent accesses to the document implicitly use the same + context; this means that only 1 thread can ever be accessing + the document at once. This does not mean that the document can + only ever be used from one thread, though in many cases this + is the simplest structure overall. + +3) "Any device is bound to the context with which it is created." + + All subsequent uses of a device implicitly use the context with + which it was created; this means that if a device is used with + a document, it should be created with the same context as that + document was. This does not mean that the device can only ever + be used from one thread, though in many cases this is the + simplest structure overall. + +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. + +To open a document, call fz_open_document as usual, passing a context +and a filename; this context is bound to the document. All future +calls to access the document will use this context internally. + +Only one thread at a time can therefore 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 doc/multi-threaded.c +which has a main thread and one rendering thread per page. |