Mercurial > hgrepos > Python2 > PyMuPDF

annotate mupdf-source/docs/reference/c/overview.md @ 22:d77477b4e151

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Let _int_rc() also handle (i.e. ignore) a local version suffix
author Franz Glasner <fzglas.hg@dom66.de>
date Fri, 19 Sep 2025 12:05:57 +0200
parents b50eed0cc0ef
children
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1 # Overview
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3 ## Basic MuPDF usage example
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5 To limit the complexity and give an easier introduction
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6 this code has no error handling at all, but any serious piece of code
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7 using MuPDF should use the error handling strategies described below.
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9 ## Common function arguments
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11 Most functions in MuPDF's interface take a context argument.
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13 A context contains global state used by MuPDF inside functions when
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14 parsing or rendering pages of the document. It contains for example:
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16 - an exception stack (see error handling below),
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17 - a memory allocator (allowing for custom allocators)
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18 - a resource store (for caching of images, fonts, etc.)
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19 - a set of locks and (un-)locking functions (for multi-threading)
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21 Without the set of locks and accompanying functions the context and
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22 its proxies may only be used in a single-threaded application.
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24 ## Error handling
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26 MuPDF uses a set of exception handling macros to simplify error return
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27 and cleanup. Conceptually, they work a lot like C++'s try/catch
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28 system, but do not require any special compiler support.
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30 The basic formulation is as follows:
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32 fz_try(ctx)
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33 {
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34 // Try to perform a task. Never 'return', 'goto' or
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35 // 'longjmp' out of here. 'break' may be used to
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36 // safely exit (just) the try block scope.
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37 }
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38 fz_always(ctx)
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39 {
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40 // Any code here is always executed, regardless of
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41 // whether an exception was thrown within the try or
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42 // not. Never 'return', 'goto' or longjmp out from
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43 // here. 'break' may be used to safely exit (just) the
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44 // always block scope.
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45 }
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46 fz_catch(ctx)
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47 {
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48 // This code is called (after any always block) only
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49 // if something within the fz_try block (including any
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50 // functions it called) threw an exception. The code
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51 // here is expected to handle the exception (maybe
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52 // record/report the error, cleanup any stray state
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53 // etc) and can then either exit the block, or pass on
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54 // the exception to a higher level (enclosing) fz_try
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55 // block (using fz_throw, or fz_rethrow).
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56 }
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58 The fz_always block is optional, and can safely be omitted.
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60 The macro based nature of this system has 3 main limitations:
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61
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62 1.
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63 Never return from within try (or 'goto' or longjmp out of it).
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64 This upsets the internal housekeeping of the macros and will
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65 cause problems later on. The code will detect such things
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66 happening, but by then it is too late to give a helpful error
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67 report as to where the original infraction occurred.
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69 2.
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70 The try/always/catch statement is not one atomic C statement.
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71
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72 The `fz_try(ctx) { ... } fz_always(ctx) { ... } fz_catch(ctx) { ... }` is not one atomic C statement.
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73 That is to say, if you do:
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75 if (condition)
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76 fz_try(ctx) { ... }
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77 fz_catch(ctx) { ... }
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78
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79 then you will not get what you want. Use the following instead:
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81 if (condition) {
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82 fz_try(ctx) { ... }
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83 fz_catch(ctx) { ... }
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84 }
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85
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86 3.
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87 The macros are implemented using setjmp and longjmp, and so
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88 the standard C restrictions on the use of those functions
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89 apply to fz_try/fz_catch too.
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90
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91 In particular, any "truly local"
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92 variable that is set between the start of fz_try and something
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93 in fz_try throwing an exception may become undefined as part
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94 of the process of throwing that exception.
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96 As a way of mitigating this problem, we provide a fz_var()
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97 macro that tells the compiler to ensure that that variable is
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98 not unset by the act of throwing the exception.
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99
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100 A model piece of code using these macros then might be:
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101
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102 house build_house(plans *p)
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103 {
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104 material m = NULL;
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105 walls w = NULL;
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106 roof r = NULL;
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107 house h = NULL;
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108 tiles t = make_tiles();
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109
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110 fz_var(w);
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111 fz_var(r);
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112 fz_var(h);
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113
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114 fz_try(ctx)
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115 {
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116 fz_try(ctx)
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117 {
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118 m = make_bricks();
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119 }
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120 fz_catch(ctx)
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121 {
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122 // No bricks available, make do with straw?
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123 m = make_straw();
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124 }
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125 w = make_walls(m, p);
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126 r = make_roof(m, t);
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127 // Note, NOT: return combine(w,r);
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128 h = combine(w, r);
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129 }
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130 fz_always(ctx)
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131 {
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132 drop_walls(w);
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133 drop_roof(r);
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134 drop_material(m);
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135 drop_tiles(t);
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136 }
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137 fz_catch(ctx)
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138 {
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139 fz_throw(ctx, "build_house failed");
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140 }
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141 return h;
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142 }
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143
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144 Things to note about this:
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145
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146 1. If make_tiles throws an exception, this will immediately be
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147 handled by some higher level exception handler. If it
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148 succeeds, t will be set before fz_try starts, so there is no
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149 need to fz_var(t);
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150
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151 2. We try first off to make some bricks as our building material.
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152 If this fails, we fall back to straw. If this fails, we'll end
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153 up in the fz_catch, and the process will fail neatly.
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154
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155 3. We assume in this code that combine takes new reference to
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156 both the walls and the roof it uses, and therefore that w and
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157 r need to be cleaned up in all cases.
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158
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159 4. We assume the standard C convention that it is safe to destroy
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160 NULL things.
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161
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162 ## Multi-threading
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163
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164 First off, study the basic usage example in `docs/examples/example.c`
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165 and make sure you understand how it works as the data structures manipulated
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166 there will be referred to in this section too.
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167
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168 MuPDF can usefully be built into a multi-threaded application without
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169 the library needing to know anything threading at all. If the library
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170 opens a document in one thread, and then sits there as a 'server'
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171 requesting pages and rendering them for other threads that need them,
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172 then the library is only ever being called from this one thread.
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173
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174 Other threads can still be used to handle UI requests etc, but as far
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175 as MuPDF is concerned it is only being used in a single threaded way.
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176 In this instance, there are no threading issues with MuPDF at all,
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177 and it can safely be used without any locking, as described in the
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178 previous sections.
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179
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180 This section will attempt to explain how to use MuPDF in the more
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181 complex case; where we genuinely want to call the MuPDF library
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182 concurrently from multiple threads within a single application.
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183
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184 MuPDF can be invoked with a user supplied set of locking functions.
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185 It uses these to take mutexes around operations that would conflict
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186 if performed concurrently in multiple threads. By leaving the
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187 exact implementation of locks to the caller MuPDF remains threading
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188 library agnostic.
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189
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190 The following simple rules should be followed to ensure that
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191 multi-threaded operations run smoothly:
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192
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193 1.
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194 "No simultaneous calls to MuPDF in different threads are
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195 allowed to use the same context."
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196
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197 Most of the time it is simplest to just use a different
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198 context for every thread; just create a new context at the
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199 same time as you create the thread. For more details see
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200 "Cloning the context" below.
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201
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202 2.
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203 "No simultaneous calls to MuPDF in different threads are
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204 allowed to use the same document."
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205
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206 Only one thread can be accessing a document at a time, but
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207 once display lists are created from that document, multiple
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208 threads at a time can operate on them.
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209
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210 The document can be used from several different threads as
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211 long as there are safeguards in place to prevent the usages
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212 being simultaneous.
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213
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214 3.
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215 "No simultaneous calls to MuPDF in different threads are
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216 allowed to use the same device."
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217
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218 Calling a device simultaneously from different threads will
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219 cause it to get confused and may crash. Calling a device from
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220 several different threads is perfectly acceptable as long as
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221 there are safeguards in place to prevent the calls being
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222 simultaneous.
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223
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224 4.
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225 "An fz_locks_context must be supplied at context creation time,
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226 unless MuPDF is to be used purely in a single thread at a time."
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227
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228 MuPDF needs to protect against unsafe access to certain structures/
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229 resources/libraries from multiple threads. It does this by using
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230 the user supplied locking functions. This holds true even when
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231 using completely separate instances of MuPDF.
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232
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233 5.
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234 "All contexts in use must share the same fz_locks_context (or
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235 the underlying locks thereof)."
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236
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237 We strongly recommend that fz_new_context is called just once,
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238 and fz_clone_context is called to generate new contexts from
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239 that. This will automatically ensure that the same locking
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240 mechanism is used in all MuPDF instances. For now, we do support
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241 multiple completely independent contexts being created using
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242 repeated calls to fz_new_context, but these MUST share the
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243 same fz_locks_context (or at least depend upon the same underlying
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244 locks). The facility to create different independent contexts
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245 may be removed in future.
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246
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247 So, how does a multi-threaded example differ from a non-multithreaded
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248 one?
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249
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250 Firstly, when we create the first context, we call fz_new_context
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251 as before, but the second argument should be a pointer to a set
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252 of locking functions.
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253
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254 The calling code should provide FZ_LOCK_MAX mutexes, which will be
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255 locked/unlocked by MuPDF calling the lock/unlock function pointers
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256 in the supplied structure with the user pointer from the structure
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257 and the lock number, i (0 &lt;= i &lt; FZ_LOCK_MAX). These mutexes can
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258 safely be recursive or non-recursive as MuPDF only calls in a non-
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259 recursive style.
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260
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261 To make subsequent contexts, the user should NOT call fz_new_context
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262 again (as this will fail to share important resources such as the
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263 store and glyphcache), but should rather call fz_clone_context.
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parents:
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264 Each of these cloned contexts can be freed by fz_free_context as
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265 usual. They will share the important data structures (like store,
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parents:
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266 glyph cache etc) with the original context, but will have their
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267 own exception stacks.
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268
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269 To open a document, call fz_open_document as usual, passing a context
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270 and a filename. It is important to realise that only one thread at a
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Franz Glasner <fzglas.hg@dom66.de>
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diff changeset
271 time can be accessing the documents itself.
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272
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273 This means that only one thread at a time can perform operations such
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parents:
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274 as fetching a page, or rendering that page to a display list. Once a
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Franz Glasner <fzglas.hg@dom66.de>
parents:
diff changeset
275 display list has been obtained however, it can be rendered from any
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diff changeset
276 other thread (or even from several threads simultaneously, giving
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parents:
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277 banded rendering).
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278
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279 This means that an implementer has 2 basic choices when constructing
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280 an application to use MuPDF in multi-threaded mode. Either he can
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281 construct it so that a single nominated thread opens the document
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parents:
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282 and then acts as a 'server' creating display lists for other threads
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parents:
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283 to render, or he can add his own mutex around calls to mupdf that
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Franz Glasner <fzglas.hg@dom66.de>
parents:
diff changeset
284 use the document. The former is likely to be far more efficient in
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parents:
diff changeset
285 the long run.
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286
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287 For an example of how to do multi-threading see
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288 `docs/examples/multi-threaded.c`
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289 which has a main thread and one rendering thread per page.
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290
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291 ## Cloning the context
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292
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293 As described above, every context contains an exception stack which is
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Franz Glasner <fzglas.hg@dom66.de>
parents:
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294 manipulated during the course of nested fz_try/fz_catches. For obvious
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Franz Glasner <fzglas.hg@dom66.de>
parents:
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295 reasons the same exception stack cannot be used from more than one
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parents:
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296 thread at a time.
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297
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parents:
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298 If, however, we simply created a new context (using fz_new_context) for
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Franz Glasner <fzglas.hg@dom66.de>
parents:
diff changeset
299 every thread, we would end up with separate stores/glyph caches etc,
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Franz Glasner <fzglas.hg@dom66.de>
parents:
diff changeset
300 which is not (generally) what is desired. MuPDF therefore provides a
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Franz Glasner <fzglas.hg@dom66.de>
parents:
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301 mechanism for "cloning" a context. This creates a new context that
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Franz Glasner <fzglas.hg@dom66.de>
parents:
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302 shares everything with the given context, except for the exception
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Franz Glasner <fzglas.hg@dom66.de>
parents:
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303 stack.
b50eed0cc0ef ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4.
Franz Glasner <fzglas.hg@dom66.de>
parents:
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304
b50eed0cc0ef ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4.
Franz Glasner <fzglas.hg@dom66.de>
parents:
diff changeset
305 A commonly used general scheme is therefore to create a 'base' context
b50eed0cc0ef ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4.
Franz Glasner <fzglas.hg@dom66.de>
parents:
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306 at program start up, and to clone this repeatedly to get new contexts
b50eed0cc0ef ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4.
Franz Glasner <fzglas.hg@dom66.de>
parents:
diff changeset
307 that can be used on new threads.