371 lines
11 KiB
HTML
371 lines
11 KiB
HTML
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<!DOCTYPE html>
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<html>
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<head>
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<title>MuPDF Overview</title>
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<link rel="stylesheet" href="style.css" type="text/css">
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</head>
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<body>
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<header>
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<h1>MuPDF Overview</h1>
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</header>
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<article>
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<h2>Contents</h2>
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<ul>
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<li><a href="#basic-mupdf-usage-example">Basic MuPDF usage example</a>
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<li><a href="#common-function-arguments">Common function arguments</a>
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<li><a href="#error-handling">Error Handling</a>
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<li><a href="#multi-threading">Multi-threading</a>
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<li><a href="#cloning-the-context">Cloning the context</a>
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</ul>
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<h2><a id="basic-mupdf-usage-example"></a>
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Basic MuPDF usage example
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</h2>
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<p>
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For an example of how to use MuPDF in the most basic way, see
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<a href="examples/example.c">docs/examples/example.c</a>.
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To limit the complexity and give an easier introduction
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this code has no error handling at all, but any serious piece of code
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using MuPDF should use the error handling strategies described below.
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<h2><a id="common-function-arguments"></a>
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Common function arguments
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</h2>
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<p>
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Most functions in MuPDF's interface take a context argument.
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<p>
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A context contains global state used by MuPDF inside functions when
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parsing or rendering pages of the document. It contains for example:
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<ul>
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<li> an exception stack (see error handling below),
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<li> a memory allocator (allowing for custom allocators)
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<li> a resource store (for caching of images, fonts, etc.)
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<li> a set of locks and (un-)locking functions (for multi-threading)
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</ul>
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<p>
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Without the set of locks and accompanying functions the context and
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its proxies may only be used in a single-threaded application.
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<h2><a id="error-handling"></a>
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Error handling
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</h2>
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<p>
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MuPDF uses a set of exception handling macros to simplify error return
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and cleanup. Conceptually, they work a lot like C++'s try/catch
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system, but do not require any special compiler support.
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<p>
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The basic formulation is as follows:
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<pre>
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fz_try(ctx)
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{
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// Try to perform a task. Never 'return', 'goto' or
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// 'longjmp' out of here. 'break' may be used to
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// safely exit (just) the try block scope.
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}
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fz_always(ctx)
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{
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// Any code here is always executed, regardless of
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// whether an exception was thrown within the try or
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// not. Never 'return', 'goto' or longjmp out from
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// here. 'break' may be used to safely exit (just) the
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// always block scope.
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}
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fz_catch(ctx)
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{
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// This code is called (after any always block) only
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// if something within the fz_try block (including any
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// functions it called) threw an exception. The code
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// here is expected to handle the exception (maybe
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// record/report the error, cleanup any stray state
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// etc) and can then either exit the block, or pass on
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// the exception to a higher level (enclosing) fz_try
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// block (using fz_throw, or fz_rethrow).
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}
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</pre>
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<p>
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The fz_always block is optional, and can safely be omitted.
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<p>
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The macro based nature of this system has 3 main limitations:
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<ol>
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<li> <p>
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Never return from within try (or 'goto' or longjmp out of it).
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This upsets the internal housekeeping of the macros and will
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cause problems later on. The code will detect such things
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happening, but by then it is too late to give a helpful error
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report as to where the original infraction occurred.
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<li> <p>
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The fz_try(ctx) { ... } fz_always(ctx) { ... } fz_catch(ctx) { ... }
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is not one atomic C statement. That is to say, if you do:
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<pre>
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if (condition)
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fz_try(ctx) { ... }
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fz_catch(ctx) { ... }
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</pre>
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then you will not get what you want. Use the following instead:
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<pre>
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if (condition) {
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fz_try(ctx) { ... }
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fz_catch(ctx) { ... }
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}
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</pre>
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<li> <p>
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The macros are implemented using setjmp and longjmp, and so
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the standard C restrictions on the use of those functions
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apply to fz_try/fz_catch too. In particular, any "truly local"
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variable that is set between the start of fz_try and something
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in fz_try throwing an exception may become undefined as part
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of the process of throwing that exception.
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<p>
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As a way of mitigating this problem, we provide a fz_var()
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macro that tells the compiler to ensure that that variable is
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not unset by the act of throwing the exception.
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</ol>
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<p>
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A model piece of code using these macros then might be:
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<pre>
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house build_house(plans *p)
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{
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material m = NULL;
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walls w = NULL;
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roof r = NULL;
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house h = NULL;
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tiles t = make_tiles();
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fz_var(w);
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fz_var(r);
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fz_var(h);
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fz_try(ctx)
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{
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fz_try(ctx)
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{
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m = make_bricks();
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}
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fz_catch(ctx)
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{
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// No bricks available, make do with straw?
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m = make_straw();
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}
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w = make_walls(m, p);
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r = make_roof(m, t);
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// Note, NOT: return combine(w,r);
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h = combine(w, r);
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}
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fz_always(ctx)
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{
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drop_walls(w);
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drop_roof(r);
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drop_material(m);
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drop_tiles(t);
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}
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fz_catch(ctx)
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{
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fz_throw(ctx, "build_house failed");
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}
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return h;
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}
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</pre>
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<p>
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Things to note about this:
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<ol>
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<li> If make_tiles throws an exception, this will immediately be
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handled by some higher level exception handler. If it
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succeeds, t will be set before fz_try starts, so there is no
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need to fz_var(t);
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<li> We try first off to make some bricks as our building material.
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If this fails, we fall back to straw. If this fails, we'll end
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up in the fz_catch, and the process will fail neatly.
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<li> We assume in this code that combine takes new reference to
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both the walls and the roof it uses, and therefore that w and
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r need to be cleaned up in all cases.
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<li> We assume the standard C convention that it is safe to destroy
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NULL things.
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</ol>
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<h2><a id="multi-threading"></a>
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Multi-threading
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</h2>
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<p>
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First off, study the basic usage example in
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<a href="examples/example.c">docs/examples/example.c</a>
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and make sure you understand how it works as the data structures manipulated
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there will be referred to in this section too.
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<p>
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MuPDF can usefully be built into a multi-threaded application without
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the library needing to know anything threading at all. If the library
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opens a document in one thread, and then sits there as a 'server'
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requesting pages and rendering them for other threads that need them,
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then the library is only ever being called from this one thread.
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<p>
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Other threads can still be used to handle UI requests etc, but as far
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as MuPDF is concerned it is only being used in a single threaded way.
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In this instance, there are no threading issues with MuPDF at all,
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and it can safely be used without any locking, as described in the
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previous sections.
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<p>
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This section will attempt to explain how to use MuPDF in the more
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complex case; where we genuinely want to call the MuPDF library
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concurrently from multiple threads within a single application.
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<p>
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MuPDF can be invoked with a user supplied set of locking functions.
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It uses these to take mutexes around operations that would conflict
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if performed concurrently in multiple threads. By leaving the
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exact implementation of locks to the caller MuPDF remains threading
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library agnostic.
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<p>
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The following simple rules should be followed to ensure that
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multi-threaded operations run smoothly:
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<ol>
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<li> <p>
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"No simultaneous calls to MuPDF in different threads are
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allowed to use the same context."
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<p>
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Most of the time it is simplest to just use a different
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context for every thread; just create a new context at the
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same time as you create the thread. For more details see
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"Cloning the context" below.
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<li> <p>
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"No simultaneous calls to MuPDF in different threads are
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allowed to use the same document."
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<p>
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Only one thread can be accessing a document at a time, but
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once display lists are created from that document, multiple
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threads at a time can operate on them.
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<p>
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The document can be used from several different threads as
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long as there are safeguards in place to prevent the usages
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being simultaneous.
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<li> <p>
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"No simultaneous calls to MuPDF in different threads are
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allowed to use the same device."
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<p>
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Calling a device simultaneously from different threads will
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cause it to get confused and may crash. Calling a device from
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several different threads is perfectly acceptable as long as
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there are safeguards in place to prevent the calls being
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simultaneous.
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</ol>
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<p>
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So, how does a multi-threaded example differ from a non-multithreaded
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one?
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<p>
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Firstly, when we create the first context, we call fz_new_context
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as before, but the second argument should be a pointer to a set
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of locking functions.
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<p>
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The calling code should provide FZ_LOCK_MAX mutexes, which will be
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locked/unlocked by MuPDF calling the lock/unlock function pointers
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in the supplied structure with the user pointer from the structure
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and the lock number, i (0 <= i < FZ_LOCK_MAX). These mutexes can
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safely be recursive or non-recursive as MuPDF only calls in a non-
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recursive style.
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<p>
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To make subsequent contexts, the user should NOT call fz_new_context
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again (as this will fail to share important resources such as the
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store and glyphcache), but should rather call fz_clone_context.
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Each of these cloned contexts can be freed by fz_free_context as
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usual. They will share the important data structures (like store,
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glyph cache etc) with the original context, but will have their
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own exception stacks.
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<p>
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To open a document, call fz_open_document as usual, passing a context
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and a filename. It is important to realise that only one thread at a
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time can be accessing the documents itself.
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<p>
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This means that only one thread at a time can perform operations such
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as fetching a page, or rendering that page to a display list. Once a
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display list has been obtained however, it can be rendered from any
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other thread (or even from several threads simultaneously, giving
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banded rendering).
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<p>
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This means that an implementer has 2 basic choices when constructing
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an application to use MuPDF in multi-threaded mode. Either he can
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construct it so that a single nominated thread opens the document
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and then acts as a 'server' creating display lists for other threads
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to render, or he can add his own mutex around calls to mupdf that
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use the document. The former is likely to be far more efficient in
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the long run.
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<p>
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For an example of how to do multi-threading see
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<a href="examples/multi-threaded.c">docs/examples/multi-threaded.c</a>
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which has a main thread and one rendering thread per page.
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<h2><a id="cloning-the-context"></a>
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Cloning the context
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</h2>
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<p>
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As described above, every context contains an exception stack which is
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manipulated during the course of nested fz_try/fz_catches. For obvious
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reasons the same exception stack cannot be used from more than one
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thread at a time.
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<p>
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If, however, we simply created a new context (using fz_new_context) for
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every thread, we would end up with separate stores/glyph caches etc,
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which is not (generally) what is desired. MuPDF therefore provides a
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mechanism for "cloning" a context. This creates a new context that
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shares everything with the given context, except for the exception
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stack.
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<p>
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A commonly used general scheme is therefore to create a 'base' context
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at program start up, and to clone this repeatedly to get new contexts
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that can be used on new threads.
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</article>
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<footer>
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<a href="http://www.artifex.com/"><img src="artifex-logo.png" align="right"></a>
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Copyright © 2006-2018 Artifex Software Inc.
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</footer>
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</body>
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</html>
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