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1 | =head1 NAME |
2 | |
3 | perlcall - Perl calling conventions from C |
4 | |
5 | =head1 DESCRIPTION |
6 | |
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7 | The purpose of this document is to show you how to call Perl subroutines |
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8 | directly from C, i.e., how to write I<callbacks>. |
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9 | |
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10 | Apart from discussing the C interface provided by Perl for writing |
11 | callbacks the document uses a series of examples to show how the |
12 | interface actually works in practice. In addition some techniques for |
13 | coding callbacks are covered. |
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14 | |
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15 | Examples where callbacks are necessary include |
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16 | |
17 | =over 5 |
18 | |
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19 | =item * An Error Handler |
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20 | |
21 | You have created an XSUB interface to an application's C API. |
22 | |
23 | A fairly common feature in applications is to allow you to define a C |
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24 | function that will be called whenever something nasty occurs. What we |
25 | would like is to be able to specify a Perl subroutine that will be |
26 | called instead. |
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27 | |
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28 | =item * An Event Driven Program |
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29 | |
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30 | The classic example of where callbacks are used is when writing an |
31 | event driven program like for an X windows application. In this case |
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32 | you register functions to be called whenever specific events occur, |
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33 | e.g., a mouse button is pressed, the cursor moves into a window or a |
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34 | menu item is selected. |
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35 | |
36 | =back |
37 | |
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38 | Although the techniques described here are applicable when embedding |
39 | Perl in a C program, this is not the primary goal of this document. |
40 | There are other details that must be considered and are specific to |
41 | embedding Perl. For details on embedding Perl in C refer to |
42 | L<perlembed>. |
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43 | |
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44 | Before you launch yourself head first into the rest of this document, |
45 | it would be a good idea to have read the following two documents - |
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46 | L<perlxs> and L<perlguts>. |
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47 | |
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48 | =head1 THE CALL_ FUNCTIONS |
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49 | |
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50 | Although this stuff is easier to explain using examples, you first need |
51 | be aware of a few important definitions. |
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52 | |
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53 | Perl has a number of C functions that allow you to call Perl |
54 | subroutines. They are |
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55 | |
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56 | I32 call_sv(SV* sv, I32 flags); |
57 | I32 call_pv(char *subname, I32 flags); |
58 | I32 call_method(char *methname, I32 flags); |
59 | I32 call_argv(char *subname, I32 flags, register char **argv); |
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60 | |
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61 | The key function is I<call_sv>. All the other functions are |
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62 | fairly simple wrappers which make it easier to call Perl subroutines in |
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63 | special cases. At the end of the day they will all call I<call_sv> |
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64 | to invoke the Perl subroutine. |
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65 | |
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66 | All the I<call_*> functions have a C<flags> parameter which is |
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67 | used to pass a bit mask of options to Perl. This bit mask operates |
68 | identically for each of the functions. The settings available in the |
69 | bit mask are discussed in L<FLAG VALUES>. |
70 | |
71 | Each of the functions will now be discussed in turn. |
72 | |
73 | =over 5 |
74 | |
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75 | =item call_sv |
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76 | |
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77 | I<call_sv> takes two parameters, the first, C<sv>, is an SV*. |
d1b91892 |
78 | This allows you to specify the Perl subroutine to be called either as a |
79 | C string (which has first been converted to an SV) or a reference to a |
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80 | subroutine. The section, I<Using call_sv>, shows how you can make |
81 | use of I<call_sv>. |
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82 | |
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83 | =item call_pv |
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84 | |
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85 | The function, I<call_pv>, is similar to I<call_sv> except it |
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86 | expects its first parameter to be a C char* which identifies the Perl |
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87 | subroutine you want to call, e.g., C<call_pv("fred", 0)>. If the |
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88 | subroutine you want to call is in another package, just include the |
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89 | package name in the string, e.g., C<"pkg::fred">. |
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90 | |
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91 | =item call_method |
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92 | |
4929bf7b |
93 | The function I<call_method> is used to call a method from a Perl |
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94 | class. The parameter C<methname> corresponds to the name of the method |
95 | to be called. Note that the class that the method belongs to is passed |
96 | on the Perl stack rather than in the parameter list. This class can be |
97 | either the name of the class (for a static method) or a reference to an |
98 | object (for a virtual method). See L<perlobj> for more information on |
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99 | static and virtual methods and L<Using call_method> for an example |
100 | of using I<call_method>. |
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101 | |
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102 | =item call_argv |
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103 | |
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104 | I<call_argv> calls the Perl subroutine specified by the C string |
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105 | stored in the C<subname> parameter. It also takes the usual C<flags> |
106 | parameter. The final parameter, C<argv>, consists of a NULL terminated |
107 | list of C strings to be passed as parameters to the Perl subroutine. |
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108 | See I<Using call_argv>. |
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109 | |
110 | =back |
111 | |
112 | All the functions return an integer. This is a count of the number of |
113 | items returned by the Perl subroutine. The actual items returned by the |
114 | subroutine are stored on the Perl stack. |
115 | |
116 | As a general rule you should I<always> check the return value from |
117 | these functions. Even if you are expecting only a particular number of |
118 | values to be returned from the Perl subroutine, there is nothing to |
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119 | stop someone from doing something unexpected--don't say you haven't |
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120 | been warned. |
121 | |
122 | =head1 FLAG VALUES |
123 | |
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124 | The C<flags> parameter in all the I<call_*> functions is a bit mask |
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125 | which can consist of any combination of the symbols defined below, |
126 | OR'ed together. |
127 | |
128 | |
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129 | =head2 G_VOID |
130 | |
131 | Calls the Perl subroutine in a void context. |
132 | |
133 | This flag has 2 effects: |
134 | |
135 | =over 5 |
136 | |
137 | =item 1. |
138 | |
139 | It indicates to the subroutine being called that it is executing in |
140 | a void context (if it executes I<wantarray> the result will be the |
141 | undefined value). |
142 | |
143 | =item 2. |
144 | |
145 | It ensures that nothing is actually returned from the subroutine. |
146 | |
147 | =back |
148 | |
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149 | The value returned by the I<call_*> function indicates how many |
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150 | items have been returned by the Perl subroutine - in this case it will |
151 | be 0. |
152 | |
153 | |
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154 | =head2 G_SCALAR |
155 | |
156 | Calls the Perl subroutine in a scalar context. This is the default |
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157 | context flag setting for all the I<call_*> functions. |
d1b91892 |
158 | |
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159 | This flag has 2 effects: |
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160 | |
161 | =over 5 |
162 | |
163 | =item 1. |
164 | |
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165 | It indicates to the subroutine being called that it is executing in a |
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166 | scalar context (if it executes I<wantarray> the result will be false). |
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167 | |
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168 | =item 2. |
169 | |
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170 | It ensures that only a scalar is actually returned from the subroutine. |
d1b91892 |
171 | The subroutine can, of course, ignore the I<wantarray> and return a |
172 | list anyway. If so, then only the last element of the list will be |
173 | returned. |
174 | |
175 | =back |
176 | |
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177 | The value returned by the I<call_*> function indicates how many |
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178 | items have been returned by the Perl subroutine - in this case it will |
179 | be either 0 or 1. |
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180 | |
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181 | If 0, then you have specified the G_DISCARD flag. |
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182 | |
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183 | If 1, then the item actually returned by the Perl subroutine will be |
184 | stored on the Perl stack - the section I<Returning a Scalar> shows how |
185 | to access this value on the stack. Remember that regardless of how |
186 | many items the Perl subroutine returns, only the last one will be |
187 | accessible from the stack - think of the case where only one value is |
188 | returned as being a list with only one element. Any other items that |
189 | were returned will not exist by the time control returns from the |
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190 | I<call_*> function. The section I<Returning a list in a scalar |
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191 | context> shows an example of this behavior. |
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192 | |
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193 | |
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194 | =head2 G_ARRAY |
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195 | |
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196 | Calls the Perl subroutine in a list context. |
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197 | |
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198 | As with G_SCALAR, this flag has 2 effects: |
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199 | |
200 | =over 5 |
201 | |
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202 | =item 1. |
203 | |
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204 | It indicates to the subroutine being called that it is executing in a |
205 | list context (if it executes I<wantarray> the result will be true). |
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206 | |
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207 | |
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208 | =item 2. |
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209 | |
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210 | It ensures that all items returned from the subroutine will be |
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211 | accessible when control returns from the I<call_*> function. |
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212 | |
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213 | =back |
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214 | |
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215 | The value returned by the I<call_*> function indicates how many |
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216 | items have been returned by the Perl subroutine. |
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217 | |
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218 | If 0, then you have specified the G_DISCARD flag. |
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219 | |
d1b91892 |
220 | If not 0, then it will be a count of the number of items returned by |
221 | the subroutine. These items will be stored on the Perl stack. The |
222 | section I<Returning a list of values> gives an example of using the |
223 | G_ARRAY flag and the mechanics of accessing the returned items from the |
224 | Perl stack. |
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225 | |
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226 | =head2 G_DISCARD |
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227 | |
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228 | By default, the I<call_*> functions place the items returned from |
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229 | by the Perl subroutine on the stack. If you are not interested in |
230 | these items, then setting this flag will make Perl get rid of them |
231 | automatically for you. Note that it is still possible to indicate a |
232 | context to the Perl subroutine by using either G_SCALAR or G_ARRAY. |
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233 | |
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234 | If you do not set this flag then it is I<very> important that you make |
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235 | sure that any temporaries (i.e., parameters passed to the Perl |
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236 | subroutine and values returned from the subroutine) are disposed of |
237 | yourself. The section I<Returning a Scalar> gives details of how to |
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238 | dispose of these temporaries explicitly and the section I<Using Perl to |
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239 | dispose of temporaries> discusses the specific circumstances where you |
240 | can ignore the problem and let Perl deal with it for you. |
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241 | |
d1b91892 |
242 | =head2 G_NOARGS |
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243 | |
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244 | Whenever a Perl subroutine is called using one of the I<call_*> |
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245 | functions, it is assumed by default that parameters are to be passed to |
246 | the subroutine. If you are not passing any parameters to the Perl |
247 | subroutine, you can save a bit of time by setting this flag. It has |
248 | the effect of not creating the C<@_> array for the Perl subroutine. |
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249 | |
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250 | Although the functionality provided by this flag may seem |
251 | straightforward, it should be used only if there is a good reason to do |
252 | so. The reason for being cautious is that even if you have specified |
253 | the G_NOARGS flag, it is still possible for the Perl subroutine that |
254 | has been called to think that you have passed it parameters. |
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255 | |
d1b91892 |
256 | In fact, what can happen is that the Perl subroutine you have called |
257 | can access the C<@_> array from a previous Perl subroutine. This will |
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258 | occur when the code that is executing the I<call_*> function has |
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259 | itself been called from another Perl subroutine. The code below |
260 | illustrates this |
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261 | |
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262 | sub fred |
263 | { print "@_\n" } |
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264 | |
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265 | sub joe |
266 | { &fred } |
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267 | |
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268 | &joe(1,2,3); |
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269 | |
270 | This will print |
271 | |
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272 | 1 2 3 |
273 | |
274 | What has happened is that C<fred> accesses the C<@_> array which |
275 | belongs to C<joe>. |
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276 | |
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277 | |
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278 | =head2 G_EVAL |
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279 | |
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280 | It is possible for the Perl subroutine you are calling to terminate |
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281 | abnormally, e.g., by calling I<die> explicitly or by not actually |
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282 | existing. By default, when either of these events occurs, the |
283 | process will terminate immediately. If you want to trap this |
d1b91892 |
284 | type of event, specify the G_EVAL flag. It will put an I<eval { }> |
285 | around the subroutine call. |
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286 | |
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287 | Whenever control returns from the I<call_*> function you need to |
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288 | check the C<$@> variable as you would in a normal Perl script. |
289 | |
4929bf7b |
290 | The value returned from the I<call_*> function is dependent on |
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291 | what other flags have been specified and whether an error has |
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292 | occurred. Here are all the different cases that can occur: |
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293 | |
294 | =over 5 |
295 | |
296 | =item * |
297 | |
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298 | If the I<call_*> function returns normally, then the value |
d1b91892 |
299 | returned is as specified in the previous sections. |
300 | |
301 | =item * |
302 | |
303 | If G_DISCARD is specified, the return value will always be 0. |
304 | |
305 | =item * |
306 | |
307 | If G_ARRAY is specified I<and> an error has occurred, the return value |
308 | will always be 0. |
309 | |
310 | =item * |
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311 | |
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312 | If G_SCALAR is specified I<and> an error has occurred, the return value |
313 | will be 1 and the value on the top of the stack will be I<undef>. This |
314 | means that if you have already detected the error by checking C<$@> and |
315 | you want the program to continue, you must remember to pop the I<undef> |
316 | from the stack. |
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317 | |
318 | =back |
319 | |
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320 | See I<Using G_EVAL> for details on using G_EVAL. |
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321 | |
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322 | =head2 G_KEEPERR |
323 | |
324 | You may have noticed that using the G_EVAL flag described above will |
325 | B<always> clear the C<$@> variable and set it to a string describing |
326 | the error iff there was an error in the called code. This unqualified |
327 | resetting of C<$@> can be problematic in the reliable identification of |
328 | errors using the C<eval {}> mechanism, because the possibility exists |
329 | that perl will call other code (end of block processing code, for |
330 | example) between the time the error causes C<$@> to be set within |
331 | C<eval {}>, and the subsequent statement which checks for the value of |
332 | C<$@> gets executed in the user's script. |
333 | |
334 | This scenario will mostly be applicable to code that is meant to be |
335 | called from within destructors, asynchronous callbacks, signal |
336 | handlers, C<__DIE__> or C<__WARN__> hooks, and C<tie> functions. In |
337 | such situations, you will not want to clear C<$@> at all, but simply to |
338 | append any new errors to any existing value of C<$@>. |
339 | |
340 | The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in |
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341 | I<call_*> functions that are used to implement such code. This flag |
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342 | has no effect when G_EVAL is not used. |
343 | |
344 | When G_KEEPERR is used, any errors in the called code will be prefixed |
345 | with the string "\t(in cleanup)", and appended to the current value |
d1f347d7 |
346 | of C<$@>. an error will not be appended if that same error string is |
347 | already at the end of C<$@>. |
348 | |
349 | In addition, a warning is generated using the appended string. This can be |
350 | disabled using C<no warnings 'misc'>. |
c07a80fd |
351 | |
352 | The G_KEEPERR flag was introduced in Perl version 5.002. |
353 | |
354 | See I<Using G_KEEPERR> for an example of a situation that warrants the |
355 | use of this flag. |
356 | |
54310121 |
357 | =head2 Determining the Context |
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358 | |
359 | As mentioned above, you can determine the context of the currently |
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360 | executing subroutine in Perl with I<wantarray>. The equivalent test |
361 | can be made in C by using the C<GIMME_V> macro, which returns |
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362 | C<G_ARRAY> if you have been called in a list context, C<G_SCALAR> if |
7a2e2cd6 |
363 | in a scalar context, or C<G_VOID> if in a void context (i.e. the |
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364 | return value will not be used). An older version of this macro is |
365 | called C<GIMME>; in a void context it returns C<G_SCALAR> instead of |
366 | C<G_VOID>. An example of using the C<GIMME_V> macro is shown in |
367 | section I<Using GIMME_V>. |
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368 | |
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369 | =head1 EXAMPLES |
370 | |
371 | Enough of the definition talk, let's have a few examples. |
372 | |
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373 | Perl provides many macros to assist in accessing the Perl stack. |
374 | Wherever possible, these macros should always be used when interfacing |
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375 | to Perl internals. We hope this should make the code less vulnerable |
d1b91892 |
376 | to any changes made to Perl in the future. |
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377 | |
d1b91892 |
378 | Another point worth noting is that in the first series of examples I |
4929bf7b |
379 | have made use of only the I<call_pv> function. This has been done |
d1b91892 |
380 | to keep the code simpler and ease you into the topic. Wherever |
4929bf7b |
381 | possible, if the choice is between using I<call_pv> and |
382 | I<call_sv>, you should always try to use I<call_sv>. See |
383 | I<Using call_sv> for details. |
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384 | |
d1b91892 |
385 | =head2 No Parameters, Nothing returned |
a0d0e21e |
386 | |
d1b91892 |
387 | This first trivial example will call a Perl subroutine, I<PrintUID>, to |
388 | print out the UID of the process. |
a0d0e21e |
389 | |
84f709e7 |
390 | sub PrintUID |
391 | { |
4358a253 |
392 | print "UID is $<\n"; |
a0d0e21e |
393 | } |
394 | |
d1b91892 |
395 | and here is a C function to call it |
a0d0e21e |
396 | |
d1b91892 |
397 | static void |
a0d0e21e |
398 | call_PrintUID() |
399 | { |
4358a253 |
400 | dSP; |
a0d0e21e |
401 | |
4358a253 |
402 | PUSHMARK(SP); |
403 | call_pv("PrintUID", G_DISCARD|G_NOARGS); |
a0d0e21e |
404 | } |
405 | |
d1b91892 |
406 | Simple, eh. |
a0d0e21e |
407 | |
d1b91892 |
408 | A few points to note about this example. |
a0d0e21e |
409 | |
410 | =over 5 |
411 | |
d1b91892 |
412 | =item 1. |
a0d0e21e |
413 | |
924508f0 |
414 | Ignore C<dSP> and C<PUSHMARK(SP)> for now. They will be discussed in |
d1b91892 |
415 | the next example. |
a0d0e21e |
416 | |
417 | =item 2. |
418 | |
d1b91892 |
419 | We aren't passing any parameters to I<PrintUID> so G_NOARGS can be |
420 | specified. |
a0d0e21e |
421 | |
d1b91892 |
422 | =item 3. |
a0d0e21e |
423 | |
424 | We aren't interested in anything returned from I<PrintUID>, so |
5f05dabc |
425 | G_DISCARD is specified. Even if I<PrintUID> was changed to |
a0d0e21e |
426 | return some value(s), having specified G_DISCARD will mean that they |
4929bf7b |
427 | will be wiped by the time control returns from I<call_pv>. |
a0d0e21e |
428 | |
d1b91892 |
429 | =item 4. |
a0d0e21e |
430 | |
4929bf7b |
431 | As I<call_pv> is being used, the Perl subroutine is specified as a |
d1b91892 |
432 | C string. In this case the subroutine name has been 'hard-wired' into the |
433 | code. |
a0d0e21e |
434 | |
435 | =item 5. |
436 | |
d1b91892 |
437 | Because we specified G_DISCARD, it is not necessary to check the value |
4929bf7b |
438 | returned from I<call_pv>. It will always be 0. |
a0d0e21e |
439 | |
440 | =back |
441 | |
d1b91892 |
442 | =head2 Passing Parameters |
a0d0e21e |
443 | |
d1b91892 |
444 | Now let's make a slightly more complex example. This time we want to |
19799a22 |
445 | call a Perl subroutine, C<LeftString>, which will take 2 parameters--a |
446 | string ($s) and an integer ($n). The subroutine will simply |
447 | print the first $n characters of the string. |
a0d0e21e |
448 | |
d1b91892 |
449 | So the Perl subroutine would look like this |
a0d0e21e |
450 | |
84f709e7 |
451 | sub LeftString |
452 | { |
4358a253 |
453 | my($s, $n) = @_; |
454 | print substr($s, 0, $n), "\n"; |
a0d0e21e |
455 | } |
456 | |
457 | The C function required to call I<LeftString> would look like this. |
458 | |
459 | static void |
460 | call_LeftString(a, b) |
4358a253 |
461 | char * a; |
462 | int b; |
a0d0e21e |
463 | { |
4358a253 |
464 | dSP; |
a0d0e21e |
465 | |
4358a253 |
466 | ENTER; |
467 | SAVETMPS; |
9b6570b4 |
468 | |
4358a253 |
469 | PUSHMARK(SP); |
a0d0e21e |
470 | XPUSHs(sv_2mortal(newSVpv(a, 0))); |
471 | XPUSHs(sv_2mortal(newSViv(b))); |
4358a253 |
472 | PUTBACK; |
a0d0e21e |
473 | |
4929bf7b |
474 | call_pv("LeftString", G_DISCARD); |
9b6570b4 |
475 | |
4358a253 |
476 | FREETMPS; |
477 | LEAVE; |
a0d0e21e |
478 | } |
479 | |
a0d0e21e |
480 | Here are a few notes on the C function I<call_LeftString>. |
481 | |
482 | =over 5 |
483 | |
d1b91892 |
484 | =item 1. |
a0d0e21e |
485 | |
d1b91892 |
486 | Parameters are passed to the Perl subroutine using the Perl stack. |
487 | This is the purpose of the code beginning with the line C<dSP> and |
1e62ac33 |
488 | ending with the line C<PUTBACK>. The C<dSP> declares a local copy |
924508f0 |
489 | of the stack pointer. This local copy should B<always> be accessed |
490 | as C<SP>. |
a0d0e21e |
491 | |
d1b91892 |
492 | =item 2. |
a0d0e21e |
493 | |
494 | If you are going to put something onto the Perl stack, you need to know |
19799a22 |
495 | where to put it. This is the purpose of the macro C<dSP>--it declares |
d1b91892 |
496 | and initializes a I<local> copy of the Perl stack pointer. |
a0d0e21e |
497 | |
498 | All the other macros which will be used in this example require you to |
d1b91892 |
499 | have used this macro. |
a0d0e21e |
500 | |
d1b91892 |
501 | The exception to this rule is if you are calling a Perl subroutine |
502 | directly from an XSUB function. In this case it is not necessary to |
19799a22 |
503 | use the C<dSP> macro explicitly--it will be declared for you |
d1b91892 |
504 | automatically. |
a0d0e21e |
505 | |
d1b91892 |
506 | =item 3. |
a0d0e21e |
507 | |
508 | Any parameters to be pushed onto the stack should be bracketed by the |
d1b91892 |
509 | C<PUSHMARK> and C<PUTBACK> macros. The purpose of these two macros, in |
5f05dabc |
510 | this context, is to count the number of parameters you are |
511 | pushing automatically. Then whenever Perl is creating the C<@_> array for the |
d1b91892 |
512 | subroutine, it knows how big to make it. |
513 | |
514 | The C<PUSHMARK> macro tells Perl to make a mental note of the current |
515 | stack pointer. Even if you aren't passing any parameters (like the |
516 | example shown in the section I<No Parameters, Nothing returned>) you |
517 | must still call the C<PUSHMARK> macro before you can call any of the |
4929bf7b |
518 | I<call_*> functions--Perl still needs to know that there are no |
d1b91892 |
519 | parameters. |
520 | |
521 | The C<PUTBACK> macro sets the global copy of the stack pointer to be |
4929bf7b |
522 | the same as our local copy. If we didn't do this I<call_pv> |
19799a22 |
523 | wouldn't know where the two parameters we pushed were--remember that |
d1b91892 |
524 | up to now all the stack pointer manipulation we have done is with our |
525 | local copy, I<not> the global copy. |
526 | |
527 | =item 4. |
528 | |
a0d0e21e |
529 | Next, we come to XPUSHs. This is where the parameters actually get |
d1b91892 |
530 | pushed onto the stack. In this case we are pushing a string and an |
531 | integer. |
a0d0e21e |
532 | |
54310121 |
533 | See L<perlguts/"XSUBs and the Argument Stack"> for details |
d1b91892 |
534 | on how the XPUSH macros work. |
a0d0e21e |
535 | |
087fe227 |
536 | =item 5. |
a0d0e21e |
537 | |
9b6570b4 |
538 | Because we created temporary values (by means of sv_2mortal() calls) |
539 | we will have to tidy up the Perl stack and dispose of mortal SVs. |
540 | |
541 | This is the purpose of |
542 | |
4358a253 |
543 | ENTER; |
544 | SAVETMPS; |
9b6570b4 |
545 | |
546 | at the start of the function, and |
547 | |
4358a253 |
548 | FREETMPS; |
549 | LEAVE; |
9b6570b4 |
550 | |
551 | at the end. The C<ENTER>/C<SAVETMPS> pair creates a boundary for any |
552 | temporaries we create. This means that the temporaries we get rid of |
553 | will be limited to those which were created after these calls. |
554 | |
555 | The C<FREETMPS>/C<LEAVE> pair will get rid of any values returned by |
556 | the Perl subroutine (see next example), plus it will also dump the |
557 | mortal SVs we have created. Having C<ENTER>/C<SAVETMPS> at the |
558 | beginning of the code makes sure that no other mortals are destroyed. |
559 | |
560 | Think of these macros as working a bit like using C<{> and C<}> in Perl |
561 | to limit the scope of local variables. |
562 | |
563 | See the section I<Using Perl to dispose of temporaries> for details of |
564 | an alternative to using these macros. |
565 | |
087fe227 |
566 | =item 6. |
9b6570b4 |
567 | |
087fe227 |
568 | Finally, I<LeftString> can now be called via the I<call_pv> function. |
569 | The only flag specified this time is G_DISCARD. Because we are passing |
570 | 2 parameters to the Perl subroutine this time, we have not specified |
571 | G_NOARGS. |
a0d0e21e |
572 | |
573 | =back |
574 | |
d1b91892 |
575 | =head2 Returning a Scalar |
a0d0e21e |
576 | |
d1b91892 |
577 | Now for an example of dealing with the items returned from a Perl |
578 | subroutine. |
a0d0e21e |
579 | |
5f05dabc |
580 | Here is a Perl subroutine, I<Adder>, that takes 2 integer parameters |
d1b91892 |
581 | and simply returns their sum. |
a0d0e21e |
582 | |
84f709e7 |
583 | sub Adder |
584 | { |
4358a253 |
585 | my($a, $b) = @_; |
586 | $a + $b; |
a0d0e21e |
587 | } |
588 | |
5f05dabc |
589 | Because we are now concerned with the return value from I<Adder>, the C |
d1b91892 |
590 | function required to call it is now a bit more complex. |
a0d0e21e |
591 | |
592 | static void |
593 | call_Adder(a, b) |
4358a253 |
594 | int a; |
595 | int b; |
a0d0e21e |
596 | { |
4358a253 |
597 | dSP; |
598 | int count; |
a0d0e21e |
599 | |
4358a253 |
600 | ENTER; |
a0d0e21e |
601 | SAVETMPS; |
602 | |
4358a253 |
603 | PUSHMARK(SP); |
a0d0e21e |
604 | XPUSHs(sv_2mortal(newSViv(a))); |
605 | XPUSHs(sv_2mortal(newSViv(b))); |
4358a253 |
606 | PUTBACK; |
a0d0e21e |
607 | |
4929bf7b |
608 | count = call_pv("Adder", G_SCALAR); |
a0d0e21e |
609 | |
4358a253 |
610 | SPAGAIN; |
a0d0e21e |
611 | |
d1b91892 |
612 | if (count != 1) |
4358a253 |
613 | croak("Big trouble\n"); |
a0d0e21e |
614 | |
4358a253 |
615 | printf ("The sum of %d and %d is %d\n", a, b, POPi); |
a0d0e21e |
616 | |
4358a253 |
617 | PUTBACK; |
618 | FREETMPS; |
619 | LEAVE; |
a0d0e21e |
620 | } |
621 | |
a0d0e21e |
622 | Points to note this time are |
623 | |
624 | =over 5 |
625 | |
54310121 |
626 | =item 1. |
a0d0e21e |
627 | |
d1b91892 |
628 | The only flag specified this time was G_SCALAR. That means the C<@_> |
629 | array will be created and that the value returned by I<Adder> will |
4929bf7b |
630 | still exist after the call to I<call_pv>. |
a0d0e21e |
631 | |
a0d0e21e |
632 | =item 2. |
633 | |
a0d0e21e |
634 | The purpose of the macro C<SPAGAIN> is to refresh the local copy of the |
635 | stack pointer. This is necessary because it is possible that the memory |
68dc0745 |
636 | allocated to the Perl stack has been reallocated whilst in the |
4929bf7b |
637 | I<call_pv> call. |
a0d0e21e |
638 | |
d1b91892 |
639 | If you are making use of the Perl stack pointer in your code you must |
54310121 |
640 | always refresh the local copy using SPAGAIN whenever you make use |
4929bf7b |
641 | of the I<call_*> functions or any other Perl internal function. |
a0d0e21e |
642 | |
9b6570b4 |
643 | =item 3. |
a0d0e21e |
644 | |
d1b91892 |
645 | Although only a single value was expected to be returned from I<Adder>, |
4929bf7b |
646 | it is still good practice to check the return code from I<call_pv> |
d1b91892 |
647 | anyway. |
a0d0e21e |
648 | |
d1b91892 |
649 | Expecting a single value is not quite the same as knowing that there |
650 | will be one. If someone modified I<Adder> to return a list and we |
651 | didn't check for that possibility and take appropriate action the Perl |
652 | stack would end up in an inconsistent state. That is something you |
5f05dabc |
653 | I<really> don't want to happen ever. |
a0d0e21e |
654 | |
9b6570b4 |
655 | =item 4. |
a0d0e21e |
656 | |
d1b91892 |
657 | The C<POPi> macro is used here to pop the return value from the stack. |
658 | In this case we wanted an integer, so C<POPi> was used. |
a0d0e21e |
659 | |
660 | |
d1b91892 |
661 | Here is the complete list of POP macros available, along with the types |
662 | they return. |
a0d0e21e |
663 | |
d1b91892 |
664 | POPs SV |
665 | POPp pointer |
666 | POPn double |
667 | POPi integer |
668 | POPl long |
a0d0e21e |
669 | |
9b6570b4 |
670 | =item 5. |
a0d0e21e |
671 | |
d1b91892 |
672 | The final C<PUTBACK> is used to leave the Perl stack in a consistent |
673 | state before exiting the function. This is necessary because when we |
674 | popped the return value from the stack with C<POPi> it updated only our |
675 | local copy of the stack pointer. Remember, C<PUTBACK> sets the global |
676 | stack pointer to be the same as our local copy. |
a0d0e21e |
677 | |
678 | =back |
679 | |
680 | |
d1b91892 |
681 | =head2 Returning a list of values |
a0d0e21e |
682 | |
d1b91892 |
683 | Now, let's extend the previous example to return both the sum of the |
684 | parameters and the difference. |
a0d0e21e |
685 | |
d1b91892 |
686 | Here is the Perl subroutine |
a0d0e21e |
687 | |
84f709e7 |
688 | sub AddSubtract |
689 | { |
4358a253 |
690 | my($a, $b) = @_; |
691 | ($a+$b, $a-$b); |
a0d0e21e |
692 | } |
693 | |
a0d0e21e |
694 | and this is the C function |
695 | |
696 | static void |
697 | call_AddSubtract(a, b) |
4358a253 |
698 | int a; |
699 | int b; |
a0d0e21e |
700 | { |
4358a253 |
701 | dSP; |
702 | int count; |
a0d0e21e |
703 | |
4358a253 |
704 | ENTER; |
a0d0e21e |
705 | SAVETMPS; |
706 | |
4358a253 |
707 | PUSHMARK(SP); |
a0d0e21e |
708 | XPUSHs(sv_2mortal(newSViv(a))); |
709 | XPUSHs(sv_2mortal(newSViv(b))); |
4358a253 |
710 | PUTBACK; |
a0d0e21e |
711 | |
4929bf7b |
712 | count = call_pv("AddSubtract", G_ARRAY); |
a0d0e21e |
713 | |
4358a253 |
714 | SPAGAIN; |
a0d0e21e |
715 | |
d1b91892 |
716 | if (count != 2) |
4358a253 |
717 | croak("Big trouble\n"); |
a0d0e21e |
718 | |
4358a253 |
719 | printf ("%d - %d = %d\n", a, b, POPi); |
720 | printf ("%d + %d = %d\n", a, b, POPi); |
a0d0e21e |
721 | |
4358a253 |
722 | PUTBACK; |
723 | FREETMPS; |
724 | LEAVE; |
a0d0e21e |
725 | } |
726 | |
d1b91892 |
727 | If I<call_AddSubtract> is called like this |
728 | |
4358a253 |
729 | call_AddSubtract(7, 4); |
d1b91892 |
730 | |
731 | then here is the output |
732 | |
733 | 7 - 4 = 3 |
734 | 7 + 4 = 11 |
a0d0e21e |
735 | |
736 | Notes |
737 | |
738 | =over 5 |
739 | |
740 | =item 1. |
741 | |
90fdbbb7 |
742 | We wanted list context, so G_ARRAY was used. |
a0d0e21e |
743 | |
744 | =item 2. |
745 | |
d1b91892 |
746 | Not surprisingly C<POPi> is used twice this time because we were |
747 | retrieving 2 values from the stack. The important thing to note is that |
748 | when using the C<POP*> macros they come off the stack in I<reverse> |
749 | order. |
a0d0e21e |
750 | |
751 | =back |
752 | |
d1b91892 |
753 | =head2 Returning a list in a scalar context |
754 | |
755 | Say the Perl subroutine in the previous section was called in a scalar |
756 | context, like this |
757 | |
758 | static void |
759 | call_AddSubScalar(a, b) |
4358a253 |
760 | int a; |
761 | int b; |
d1b91892 |
762 | { |
4358a253 |
763 | dSP; |
764 | int count; |
765 | int i; |
d1b91892 |
766 | |
4358a253 |
767 | ENTER; |
d1b91892 |
768 | SAVETMPS; |
769 | |
4358a253 |
770 | PUSHMARK(SP); |
d1b91892 |
771 | XPUSHs(sv_2mortal(newSViv(a))); |
772 | XPUSHs(sv_2mortal(newSViv(b))); |
4358a253 |
773 | PUTBACK; |
d1b91892 |
774 | |
4929bf7b |
775 | count = call_pv("AddSubtract", G_SCALAR); |
d1b91892 |
776 | |
4358a253 |
777 | SPAGAIN; |
d1b91892 |
778 | |
4358a253 |
779 | printf ("Items Returned = %d\n", count); |
d1b91892 |
780 | |
4358a253 |
781 | for (i = 1; i <= count; ++i) |
782 | printf ("Value %d = %d\n", i, POPi); |
d1b91892 |
783 | |
4358a253 |
784 | PUTBACK; |
785 | FREETMPS; |
786 | LEAVE; |
d1b91892 |
787 | } |
788 | |
789 | The other modification made is that I<call_AddSubScalar> will print the |
790 | number of items returned from the Perl subroutine and their value (for |
791 | simplicity it assumes that they are integer). So if |
792 | I<call_AddSubScalar> is called |
793 | |
4358a253 |
794 | call_AddSubScalar(7, 4); |
d1b91892 |
795 | |
796 | then the output will be |
797 | |
798 | Items Returned = 1 |
799 | Value 1 = 3 |
800 | |
801 | In this case the main point to note is that only the last item in the |
54310121 |
802 | list is returned from the subroutine, I<AddSubtract> actually made it back to |
d1b91892 |
803 | I<call_AddSubScalar>. |
804 | |
805 | |
806 | =head2 Returning Data from Perl via the parameter list |
a0d0e21e |
807 | |
808 | It is also possible to return values directly via the parameter list - |
809 | whether it is actually desirable to do it is another matter entirely. |
810 | |
d1b91892 |
811 | The Perl subroutine, I<Inc>, below takes 2 parameters and increments |
812 | each directly. |
a0d0e21e |
813 | |
84f709e7 |
814 | sub Inc |
815 | { |
4358a253 |
816 | ++ $_[0]; |
817 | ++ $_[1]; |
a0d0e21e |
818 | } |
819 | |
820 | and here is a C function to call it. |
821 | |
822 | static void |
823 | call_Inc(a, b) |
4358a253 |
824 | int a; |
825 | int b; |
a0d0e21e |
826 | { |
4358a253 |
827 | dSP; |
828 | int count; |
829 | SV * sva; |
830 | SV * svb; |
a0d0e21e |
831 | |
4358a253 |
832 | ENTER; |
a0d0e21e |
833 | SAVETMPS; |
834 | |
4358a253 |
835 | sva = sv_2mortal(newSViv(a)); |
836 | svb = sv_2mortal(newSViv(b)); |
a0d0e21e |
837 | |
4358a253 |
838 | PUSHMARK(SP); |
a0d0e21e |
839 | XPUSHs(sva); |
840 | XPUSHs(svb); |
4358a253 |
841 | PUTBACK; |
a0d0e21e |
842 | |
4929bf7b |
843 | count = call_pv("Inc", G_DISCARD); |
a0d0e21e |
844 | |
845 | if (count != 0) |
d1b91892 |
846 | croak ("call_Inc: expected 0 values from 'Inc', got %d\n", |
4358a253 |
847 | count); |
a0d0e21e |
848 | |
4358a253 |
849 | printf ("%d + 1 = %d\n", a, SvIV(sva)); |
850 | printf ("%d + 1 = %d\n", b, SvIV(svb)); |
a0d0e21e |
851 | |
4358a253 |
852 | FREETMPS; |
853 | LEAVE; |
a0d0e21e |
854 | } |
855 | |
d1b91892 |
856 | To be able to access the two parameters that were pushed onto the stack |
4929bf7b |
857 | after they return from I<call_pv> it is necessary to make a note |
19799a22 |
858 | of their addresses--thus the two variables C<sva> and C<svb>. |
a0d0e21e |
859 | |
d1b91892 |
860 | The reason this is necessary is that the area of the Perl stack which |
861 | held them will very likely have been overwritten by something else by |
4929bf7b |
862 | the time control returns from I<call_pv>. |
a0d0e21e |
863 | |
864 | |
865 | |
866 | |
d1b91892 |
867 | =head2 Using G_EVAL |
a0d0e21e |
868 | |
d1b91892 |
869 | Now an example using G_EVAL. Below is a Perl subroutine which computes |
870 | the difference of its 2 parameters. If this would result in a negative |
871 | result, the subroutine calls I<die>. |
a0d0e21e |
872 | |
84f709e7 |
873 | sub Subtract |
874 | { |
4358a253 |
875 | my ($a, $b) = @_; |
a0d0e21e |
876 | |
4358a253 |
877 | die "death can be fatal\n" if $a < $b; |
a0d0e21e |
878 | |
4358a253 |
879 | $a - $b; |
a0d0e21e |
880 | } |
881 | |
882 | and some C to call it |
883 | |
884 | static void |
885 | call_Subtract(a, b) |
4358a253 |
886 | int a; |
887 | int b; |
a0d0e21e |
888 | { |
4358a253 |
889 | dSP; |
890 | int count; |
a0d0e21e |
891 | |
4358a253 |
892 | ENTER; |
a0d0e21e |
893 | SAVETMPS; |
894 | |
4358a253 |
895 | PUSHMARK(SP); |
a0d0e21e |
896 | XPUSHs(sv_2mortal(newSViv(a))); |
897 | XPUSHs(sv_2mortal(newSViv(b))); |
4358a253 |
898 | PUTBACK; |
a0d0e21e |
899 | |
4929bf7b |
900 | count = call_pv("Subtract", G_EVAL|G_SCALAR); |
a0d0e21e |
901 | |
4358a253 |
902 | SPAGAIN; |
d1b91892 |
903 | |
904 | /* Check the eval first */ |
9cde0e7f |
905 | if (SvTRUE(ERRSV)) |
d1b91892 |
906 | { |
2d8e6c8d |
907 | STRLEN n_a; |
4358a253 |
908 | printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)); |
909 | POPs; |
d1b91892 |
910 | } |
911 | else |
912 | { |
913 | if (count != 1) |
914 | croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n", |
4358a253 |
915 | count); |
a0d0e21e |
916 | |
4358a253 |
917 | printf ("%d - %d = %d\n", a, b, POPi); |
d1b91892 |
918 | } |
a0d0e21e |
919 | |
4358a253 |
920 | PUTBACK; |
921 | FREETMPS; |
922 | LEAVE; |
a0d0e21e |
923 | } |
924 | |
925 | If I<call_Subtract> is called thus |
926 | |
d1b91892 |
927 | call_Subtract(4, 5) |
a0d0e21e |
928 | |
929 | the following will be printed |
930 | |
d1b91892 |
931 | Uh oh - death can be fatal |
a0d0e21e |
932 | |
933 | Notes |
934 | |
935 | =over 5 |
936 | |
937 | =item 1. |
938 | |
d1b91892 |
939 | We want to be able to catch the I<die> so we have used the G_EVAL |
940 | flag. Not specifying this flag would mean that the program would |
941 | terminate immediately at the I<die> statement in the subroutine |
942 | I<Subtract>. |
a0d0e21e |
943 | |
944 | =item 2. |
945 | |
54310121 |
946 | The code |
a0d0e21e |
947 | |
9cde0e7f |
948 | if (SvTRUE(ERRSV)) |
d1b91892 |
949 | { |
2d8e6c8d |
950 | STRLEN n_a; |
4358a253 |
951 | printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)); |
952 | POPs; |
d1b91892 |
953 | } |
a0d0e21e |
954 | |
d1b91892 |
955 | is the direct equivalent of this bit of Perl |
a0d0e21e |
956 | |
4358a253 |
957 | print "Uh oh - $@\n" if $@; |
a0d0e21e |
958 | |
9cde0e7f |
959 | C<PL_errgv> is a perl global of type C<GV *> that points to the |
960 | symbol table entry containing the error. C<ERRSV> therefore |
c07a80fd |
961 | refers to the C equivalent of C<$@>. |
962 | |
d1b91892 |
963 | =item 3. |
a0d0e21e |
964 | |
d1b91892 |
965 | Note that the stack is popped using C<POPs> in the block where |
9cde0e7f |
966 | C<SvTRUE(ERRSV)> is true. This is necessary because whenever a |
4929bf7b |
967 | I<call_*> function invoked with G_EVAL|G_SCALAR returns an error, |
5f05dabc |
968 | the top of the stack holds the value I<undef>. Because we want the |
d1b91892 |
969 | program to continue after detecting this error, it is essential that |
970 | the stack is tidied up by removing the I<undef>. |
a0d0e21e |
971 | |
972 | =back |
973 | |
974 | |
c07a80fd |
975 | =head2 Using G_KEEPERR |
976 | |
977 | Consider this rather facetious example, where we have used an XS |
978 | version of the call_Subtract example above inside a destructor: |
979 | |
980 | package Foo; |
84f709e7 |
981 | sub new { bless {}, $_[0] } |
54310121 |
982 | sub Subtract { |
84f709e7 |
983 | my($a,$b) = @_; |
4358a253 |
984 | die "death can be fatal" if $a < $b; |
84f709e7 |
985 | $a - $b; |
c07a80fd |
986 | } |
84f709e7 |
987 | sub DESTROY { call_Subtract(5, 4); } |
988 | sub foo { die "foo dies"; } |
c07a80fd |
989 | |
990 | package main; |
991 | eval { Foo->new->foo }; |
992 | print "Saw: $@" if $@; # should be, but isn't |
993 | |
994 | This example will fail to recognize that an error occurred inside the |
995 | C<eval {}>. Here's why: the call_Subtract code got executed while perl |
5f05dabc |
996 | was cleaning up temporaries when exiting the eval block, and because |
4929bf7b |
997 | call_Subtract is implemented with I<call_pv> using the G_EVAL |
c07a80fd |
998 | flag, it promptly reset C<$@>. This results in the failure of the |
999 | outermost test for C<$@>, and thereby the failure of the error trap. |
1000 | |
4929bf7b |
1001 | Appending the G_KEEPERR flag, so that the I<call_pv> call in |
c07a80fd |
1002 | call_Subtract reads: |
1003 | |
4929bf7b |
1004 | count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR); |
c07a80fd |
1005 | |
1006 | will preserve the error and restore reliable error handling. |
1007 | |
4929bf7b |
1008 | =head2 Using call_sv |
a0d0e21e |
1009 | |
d1b91892 |
1010 | In all the previous examples I have 'hard-wired' the name of the Perl |
1011 | subroutine to be called from C. Most of the time though, it is more |
1012 | convenient to be able to specify the name of the Perl subroutine from |
1013 | within the Perl script. |
a0d0e21e |
1014 | |
1015 | Consider the Perl code below |
1016 | |
84f709e7 |
1017 | sub fred |
1018 | { |
4358a253 |
1019 | print "Hello there\n"; |
d1b91892 |
1020 | } |
1021 | |
4358a253 |
1022 | CallSubPV("fred"); |
d1b91892 |
1023 | |
1024 | Here is a snippet of XSUB which defines I<CallSubPV>. |
1025 | |
1026 | void |
1027 | CallSubPV(name) |
1028 | char * name |
1029 | CODE: |
4358a253 |
1030 | PUSHMARK(SP); |
1031 | call_pv(name, G_DISCARD|G_NOARGS); |
a0d0e21e |
1032 | |
54310121 |
1033 | That is fine as far as it goes. The thing is, the Perl subroutine |
5f05dabc |
1034 | can be specified as only a string. For Perl 4 this was adequate, |
d1b91892 |
1035 | but Perl 5 allows references to subroutines and anonymous subroutines. |
4929bf7b |
1036 | This is where I<call_sv> is useful. |
d1b91892 |
1037 | |
1038 | The code below for I<CallSubSV> is identical to I<CallSubPV> except |
1039 | that the C<name> parameter is now defined as an SV* and we use |
4929bf7b |
1040 | I<call_sv> instead of I<call_pv>. |
d1b91892 |
1041 | |
1042 | void |
1043 | CallSubSV(name) |
1044 | SV * name |
1045 | CODE: |
4358a253 |
1046 | PUSHMARK(SP); |
1047 | call_sv(name, G_DISCARD|G_NOARGS); |
a0d0e21e |
1048 | |
5f05dabc |
1049 | Because we are using an SV to call I<fred> the following can all be used |
a0d0e21e |
1050 | |
4358a253 |
1051 | CallSubSV("fred"); |
1052 | CallSubSV(\&fred); |
1053 | $ref = \&fred; |
1054 | CallSubSV($ref); |
1055 | CallSubSV( sub { print "Hello there\n" } ); |
a0d0e21e |
1056 | |
4929bf7b |
1057 | As you can see, I<call_sv> gives you much greater flexibility in |
d1b91892 |
1058 | how you can specify the Perl subroutine. |
1059 | |
1060 | You should note that if it is necessary to store the SV (C<name> in the |
1061 | example above) which corresponds to the Perl subroutine so that it can |
5f05dabc |
1062 | be used later in the program, it not enough just to store a copy of the |
d1b91892 |
1063 | pointer to the SV. Say the code above had been like this |
1064 | |
4358a253 |
1065 | static SV * rememberSub; |
d1b91892 |
1066 | |
1067 | void |
1068 | SaveSub1(name) |
1069 | SV * name |
1070 | CODE: |
4358a253 |
1071 | rememberSub = name; |
d1b91892 |
1072 | |
1073 | void |
1074 | CallSavedSub1() |
1075 | CODE: |
4358a253 |
1076 | PUSHMARK(SP); |
1077 | call_sv(rememberSub, G_DISCARD|G_NOARGS); |
a0d0e21e |
1078 | |
d1b91892 |
1079 | The reason this is wrong is that by the time you come to use the |
1080 | pointer C<rememberSub> in C<CallSavedSub1>, it may or may not still refer |
1081 | to the Perl subroutine that was recorded in C<SaveSub1>. This is |
1082 | particularly true for these cases |
a0d0e21e |
1083 | |
4358a253 |
1084 | SaveSub1(\&fred); |
1085 | CallSavedSub1(); |
a0d0e21e |
1086 | |
4358a253 |
1087 | SaveSub1( sub { print "Hello there\n" } ); |
1088 | CallSavedSub1(); |
a0d0e21e |
1089 | |
d1b91892 |
1090 | By the time each of the C<SaveSub1> statements above have been executed, |
54310121 |
1091 | the SV*s which corresponded to the parameters will no longer exist. |
d1b91892 |
1092 | Expect an error message from Perl of the form |
a0d0e21e |
1093 | |
d1b91892 |
1094 | Can't use an undefined value as a subroutine reference at ... |
a0d0e21e |
1095 | |
d1b91892 |
1096 | for each of the C<CallSavedSub1> lines. |
a0d0e21e |
1097 | |
54310121 |
1098 | Similarly, with this code |
a0d0e21e |
1099 | |
4358a253 |
1100 | $ref = \&fred; |
1101 | SaveSub1($ref); |
1102 | $ref = 47; |
1103 | CallSavedSub1(); |
a0d0e21e |
1104 | |
54310121 |
1105 | you can expect one of these messages (which you actually get is dependent on |
1106 | the version of Perl you are using) |
a0d0e21e |
1107 | |
d1b91892 |
1108 | Not a CODE reference at ... |
1109 | Undefined subroutine &main::47 called ... |
a0d0e21e |
1110 | |
19799a22 |
1111 | The variable $ref may have referred to the subroutine C<fred> |
d1b91892 |
1112 | whenever the call to C<SaveSub1> was made but by the time |
5f05dabc |
1113 | C<CallSavedSub1> gets called it now holds the number C<47>. Because we |
d1b91892 |
1114 | saved only a pointer to the original SV in C<SaveSub1>, any changes to |
19799a22 |
1115 | $ref will be tracked by the pointer C<rememberSub>. This means that |
d1b91892 |
1116 | whenever C<CallSavedSub1> gets called, it will attempt to execute the |
1117 | code which is referenced by the SV* C<rememberSub>. In this case |
1118 | though, it now refers to the integer C<47>, so expect Perl to complain |
1119 | loudly. |
a0d0e21e |
1120 | |
d1b91892 |
1121 | A similar but more subtle problem is illustrated with this code |
a0d0e21e |
1122 | |
4358a253 |
1123 | $ref = \&fred; |
1124 | SaveSub1($ref); |
1125 | $ref = \&joe; |
1126 | CallSavedSub1(); |
a0d0e21e |
1127 | |
d1b91892 |
1128 | This time whenever C<CallSavedSub1> get called it will execute the Perl |
54310121 |
1129 | subroutine C<joe> (assuming it exists) rather than C<fred> as was |
d1b91892 |
1130 | originally requested in the call to C<SaveSub1>. |
a0d0e21e |
1131 | |
d1b91892 |
1132 | To get around these problems it is necessary to take a full copy of the |
1133 | SV. The code below shows C<SaveSub2> modified to do that |
a0d0e21e |
1134 | |
4358a253 |
1135 | static SV * keepSub = (SV*)NULL; |
d1b91892 |
1136 | |
1137 | void |
1138 | SaveSub2(name) |
1139 | SV * name |
1140 | CODE: |
1141 | /* Take a copy of the callback */ |
1142 | if (keepSub == (SV*)NULL) |
1143 | /* First time, so create a new SV */ |
4358a253 |
1144 | keepSub = newSVsv(name); |
d1b91892 |
1145 | else |
1146 | /* Been here before, so overwrite */ |
4358a253 |
1147 | SvSetSV(keepSub, name); |
d1b91892 |
1148 | |
1149 | void |
1150 | CallSavedSub2() |
1151 | CODE: |
4358a253 |
1152 | PUSHMARK(SP); |
1153 | call_sv(keepSub, G_DISCARD|G_NOARGS); |
d1b91892 |
1154 | |
5f05dabc |
1155 | To avoid creating a new SV every time C<SaveSub2> is called, |
d1b91892 |
1156 | the function first checks to see if it has been called before. If not, |
1157 | then space for a new SV is allocated and the reference to the Perl |
1158 | subroutine, C<name> is copied to the variable C<keepSub> in one |
1159 | operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called |
1160 | the existing SV, C<keepSub>, is overwritten with the new value using |
1161 | C<SvSetSV>. |
1162 | |
4929bf7b |
1163 | =head2 Using call_argv |
d1b91892 |
1164 | |
1165 | Here is a Perl subroutine which prints whatever parameters are passed |
1166 | to it. |
1167 | |
84f709e7 |
1168 | sub PrintList |
1169 | { |
4358a253 |
1170 | my(@list) = @_; |
d1b91892 |
1171 | |
84f709e7 |
1172 | foreach (@list) { print "$_\n" } |
d1b91892 |
1173 | } |
1174 | |
4929bf7b |
1175 | and here is an example of I<call_argv> which will call |
d1b91892 |
1176 | I<PrintList>. |
1177 | |
4358a253 |
1178 | static char * words[] = {"alpha", "beta", "gamma", "delta", NULL}; |
d1b91892 |
1179 | |
1180 | static void |
1181 | call_PrintList() |
1182 | { |
4358a253 |
1183 | dSP; |
d1b91892 |
1184 | |
4358a253 |
1185 | call_argv("PrintList", G_DISCARD, words); |
d1b91892 |
1186 | } |
1187 | |
1188 | Note that it is not necessary to call C<PUSHMARK> in this instance. |
4929bf7b |
1189 | This is because I<call_argv> will do it for you. |
d1b91892 |
1190 | |
4929bf7b |
1191 | =head2 Using call_method |
a0d0e21e |
1192 | |
1193 | Consider the following Perl code |
1194 | |
d1b91892 |
1195 | { |
4358a253 |
1196 | package Mine; |
84f709e7 |
1197 | |
1198 | sub new |
1199 | { |
4358a253 |
1200 | my($type) = shift; |
84f709e7 |
1201 | bless [@_] |
1202 | } |
1203 | |
1204 | sub Display |
1205 | { |
4358a253 |
1206 | my ($self, $index) = @_; |
1207 | print "$index: $$self[$index]\n"; |
84f709e7 |
1208 | } |
1209 | |
1210 | sub PrintID |
1211 | { |
4358a253 |
1212 | my($class) = @_; |
1213 | print "This is Class $class version 1.0\n"; |
84f709e7 |
1214 | } |
d1b91892 |
1215 | } |
1216 | |
5f05dabc |
1217 | It implements just a very simple class to manage an array. Apart from |
d1b91892 |
1218 | the constructor, C<new>, it declares methods, one static and one |
5f05dabc |
1219 | virtual. The static method, C<PrintID>, prints out simply the class |
d1b91892 |
1220 | name and a version number. The virtual method, C<Display>, prints out a |
1221 | single element of the array. Here is an all Perl example of using it. |
1222 | |
4358a253 |
1223 | $a = new Mine ('red', 'green', 'blue'); |
1224 | $a->Display(1); |
84f709e7 |
1225 | PrintID Mine; |
a0d0e21e |
1226 | |
d1b91892 |
1227 | will print |
a0d0e21e |
1228 | |
d1b91892 |
1229 | 1: green |
54310121 |
1230 | This is Class Mine version 1.0 |
a0d0e21e |
1231 | |
d1b91892 |
1232 | Calling a Perl method from C is fairly straightforward. The following |
1233 | things are required |
a0d0e21e |
1234 | |
d1b91892 |
1235 | =over 5 |
1236 | |
1237 | =item * |
1238 | |
1239 | a reference to the object for a virtual method or the name of the class |
1240 | for a static method. |
1241 | |
1242 | =item * |
1243 | |
1244 | the name of the method. |
1245 | |
1246 | =item * |
1247 | |
1248 | any other parameters specific to the method. |
1249 | |
1250 | =back |
1251 | |
1252 | Here is a simple XSUB which illustrates the mechanics of calling both |
1253 | the C<PrintID> and C<Display> methods from C. |
1254 | |
1255 | void |
1256 | call_Method(ref, method, index) |
1257 | SV * ref |
1258 | char * method |
1259 | int index |
1260 | CODE: |
924508f0 |
1261 | PUSHMARK(SP); |
d1b91892 |
1262 | XPUSHs(ref); |
4358a253 |
1263 | XPUSHs(sv_2mortal(newSViv(index))); |
d1b91892 |
1264 | PUTBACK; |
1265 | |
4358a253 |
1266 | call_method(method, G_DISCARD); |
d1b91892 |
1267 | |
1268 | void |
1269 | call_PrintID(class, method) |
1270 | char * class |
1271 | char * method |
1272 | CODE: |
924508f0 |
1273 | PUSHMARK(SP); |
4358a253 |
1274 | XPUSHs(sv_2mortal(newSVpv(class, 0))); |
d1b91892 |
1275 | PUTBACK; |
1276 | |
4358a253 |
1277 | call_method(method, G_DISCARD); |
d1b91892 |
1278 | |
1279 | |
1280 | So the methods C<PrintID> and C<Display> can be invoked like this |
1281 | |
4358a253 |
1282 | $a = new Mine ('red', 'green', 'blue'); |
1283 | call_Method($a, 'Display', 1); |
1284 | call_PrintID('Mine', 'PrintID'); |
d1b91892 |
1285 | |
1286 | The only thing to note is that in both the static and virtual methods, |
19799a22 |
1287 | the method name is not passed via the stack--it is used as the first |
4929bf7b |
1288 | parameter to I<call_method>. |
d1b91892 |
1289 | |
54310121 |
1290 | =head2 Using GIMME_V |
d1b91892 |
1291 | |
54310121 |
1292 | Here is a trivial XSUB which prints the context in which it is |
d1b91892 |
1293 | currently executing. |
1294 | |
1295 | void |
1296 | PrintContext() |
1297 | CODE: |
54310121 |
1298 | I32 gimme = GIMME_V; |
1299 | if (gimme == G_VOID) |
4358a253 |
1300 | printf ("Context is Void\n"); |
54310121 |
1301 | else if (gimme == G_SCALAR) |
4358a253 |
1302 | printf ("Context is Scalar\n"); |
d1b91892 |
1303 | else |
4358a253 |
1304 | printf ("Context is Array\n"); |
d1b91892 |
1305 | |
1306 | and here is some Perl to test it |
1307 | |
4358a253 |
1308 | PrintContext; |
1309 | $a = PrintContext; |
1310 | @a = PrintContext; |
d1b91892 |
1311 | |
1312 | The output from that will be |
1313 | |
54310121 |
1314 | Context is Void |
d1b91892 |
1315 | Context is Scalar |
1316 | Context is Array |
1317 | |
1318 | =head2 Using Perl to dispose of temporaries |
1319 | |
1320 | In the examples given to date, any temporaries created in the callback |
4929bf7b |
1321 | (i.e., parameters passed on the stack to the I<call_*> function or |
d1b91892 |
1322 | values returned via the stack) have been freed by one of these methods |
1323 | |
1324 | =over 5 |
1325 | |
1326 | =item * |
1327 | |
4929bf7b |
1328 | specifying the G_DISCARD flag with I<call_*>. |
d1b91892 |
1329 | |
1330 | =item * |
1331 | |
1332 | explicitly disposed of using the C<ENTER>/C<SAVETMPS> - |
1333 | C<FREETMPS>/C<LEAVE> pairing. |
1334 | |
1335 | =back |
1336 | |
1337 | There is another method which can be used, namely letting Perl do it |
1338 | for you automatically whenever it regains control after the callback |
1339 | has terminated. This is done by simply not using the |
1340 | |
4358a253 |
1341 | ENTER; |
1342 | SAVETMPS; |
d1b91892 |
1343 | ... |
4358a253 |
1344 | FREETMPS; |
1345 | LEAVE; |
d1b91892 |
1346 | |
1347 | sequence in the callback (and not, of course, specifying the G_DISCARD |
1348 | flag). |
1349 | |
1350 | If you are going to use this method you have to be aware of a possible |
1351 | memory leak which can arise under very specific circumstances. To |
1352 | explain these circumstances you need to know a bit about the flow of |
1353 | control between Perl and the callback routine. |
1354 | |
1355 | The examples given at the start of the document (an error handler and |
1356 | an event driven program) are typical of the two main sorts of flow |
1357 | control that you are likely to encounter with callbacks. There is a |
1358 | very important distinction between them, so pay attention. |
1359 | |
1360 | In the first example, an error handler, the flow of control could be as |
1361 | follows. You have created an interface to an external library. |
1362 | Control can reach the external library like this |
1363 | |
1364 | perl --> XSUB --> external library |
1365 | |
1366 | Whilst control is in the library, an error condition occurs. You have |
1367 | previously set up a Perl callback to handle this situation, so it will |
1368 | get executed. Once the callback has finished, control will drop back to |
1369 | Perl again. Here is what the flow of control will be like in that |
1370 | situation |
1371 | |
1372 | perl --> XSUB --> external library |
1373 | ... |
1374 | error occurs |
1375 | ... |
4929bf7b |
1376 | external library --> call_* --> perl |
d1b91892 |
1377 | | |
4929bf7b |
1378 | perl <-- XSUB <-- external library <-- call_* <----+ |
d1b91892 |
1379 | |
4929bf7b |
1380 | After processing of the error using I<call_*> is completed, |
d1b91892 |
1381 | control reverts back to Perl more or less immediately. |
1382 | |
1383 | In the diagram, the further right you go the more deeply nested the |
1384 | scope is. It is only when control is back with perl on the extreme |
1385 | left of the diagram that you will have dropped back to the enclosing |
1386 | scope and any temporaries you have left hanging around will be freed. |
1387 | |
1388 | In the second example, an event driven program, the flow of control |
1389 | will be more like this |
1390 | |
1391 | perl --> XSUB --> event handler |
1392 | ... |
4929bf7b |
1393 | event handler --> call_* --> perl |
d1b91892 |
1394 | | |
4929bf7b |
1395 | event handler <-- call_* <----+ |
d1b91892 |
1396 | ... |
4929bf7b |
1397 | event handler --> call_* --> perl |
d1b91892 |
1398 | | |
4929bf7b |
1399 | event handler <-- call_* <----+ |
d1b91892 |
1400 | ... |
4929bf7b |
1401 | event handler --> call_* --> perl |
d1b91892 |
1402 | | |
4929bf7b |
1403 | event handler <-- call_* <----+ |
d1b91892 |
1404 | |
1405 | In this case the flow of control can consist of only the repeated |
1406 | sequence |
1407 | |
4929bf7b |
1408 | event handler --> call_* --> perl |
d1b91892 |
1409 | |
54310121 |
1410 | for practically the complete duration of the program. This means that |
1411 | control may I<never> drop back to the surrounding scope in Perl at the |
1412 | extreme left. |
d1b91892 |
1413 | |
1414 | So what is the big problem? Well, if you are expecting Perl to tidy up |
1415 | those temporaries for you, you might be in for a long wait. For Perl |
5f05dabc |
1416 | to dispose of your temporaries, control must drop back to the |
d1b91892 |
1417 | enclosing scope at some stage. In the event driven scenario that may |
1418 | never happen. This means that as time goes on, your program will |
1419 | create more and more temporaries, none of which will ever be freed. As |
1420 | each of these temporaries consumes some memory your program will |
19799a22 |
1421 | eventually consume all the available memory in your system--kapow! |
d1b91892 |
1422 | |
19799a22 |
1423 | So here is the bottom line--if you are sure that control will revert |
d1b91892 |
1424 | back to the enclosing Perl scope fairly quickly after the end of your |
5f05dabc |
1425 | callback, then it isn't absolutely necessary to dispose explicitly of |
d1b91892 |
1426 | any temporaries you may have created. Mind you, if you are at all |
1427 | uncertain about what to do, it doesn't do any harm to tidy up anyway. |
1428 | |
1429 | |
1430 | =head2 Strategies for storing Callback Context Information |
1431 | |
1432 | |
1433 | Potentially one of the trickiest problems to overcome when designing a |
1434 | callback interface can be figuring out how to store the mapping between |
1435 | the C callback function and the Perl equivalent. |
1436 | |
1437 | To help understand why this can be a real problem first consider how a |
1438 | callback is set up in an all C environment. Typically a C API will |
1439 | provide a function to register a callback. This will expect a pointer |
1440 | to a function as one of its parameters. Below is a call to a |
1441 | hypothetical function C<register_fatal> which registers the C function |
1442 | to get called when a fatal error occurs. |
1443 | |
4358a253 |
1444 | register_fatal(cb1); |
d1b91892 |
1445 | |
1446 | The single parameter C<cb1> is a pointer to a function, so you must |
1447 | have defined C<cb1> in your code, say something like this |
1448 | |
1449 | static void |
1450 | cb1() |
1451 | { |
4358a253 |
1452 | printf ("Fatal Error\n"); |
1453 | exit(1); |
d1b91892 |
1454 | } |
1455 | |
1456 | Now change that to call a Perl subroutine instead |
1457 | |
1458 | static SV * callback = (SV*)NULL; |
1459 | |
1460 | static void |
1461 | cb1() |
1462 | { |
4358a253 |
1463 | dSP; |
d1b91892 |
1464 | |
4358a253 |
1465 | PUSHMARK(SP); |
d1b91892 |
1466 | |
1467 | /* Call the Perl sub to process the callback */ |
4358a253 |
1468 | call_sv(callback, G_DISCARD); |
d1b91892 |
1469 | } |
1470 | |
1471 | |
1472 | void |
1473 | register_fatal(fn) |
1474 | SV * fn |
1475 | CODE: |
1476 | /* Remember the Perl sub */ |
1477 | if (callback == (SV*)NULL) |
4358a253 |
1478 | callback = newSVsv(fn); |
d1b91892 |
1479 | else |
4358a253 |
1480 | SvSetSV(callback, fn); |
d1b91892 |
1481 | |
1482 | /* register the callback with the external library */ |
4358a253 |
1483 | register_fatal(cb1); |
d1b91892 |
1484 | |
1485 | where the Perl equivalent of C<register_fatal> and the callback it |
1486 | registers, C<pcb1>, might look like this |
1487 | |
1488 | # Register the sub pcb1 |
4358a253 |
1489 | register_fatal(\&pcb1); |
d1b91892 |
1490 | |
84f709e7 |
1491 | sub pcb1 |
1492 | { |
4358a253 |
1493 | die "I'm dying...\n"; |
d1b91892 |
1494 | } |
1495 | |
1496 | The mapping between the C callback and the Perl equivalent is stored in |
1497 | the global variable C<callback>. |
1498 | |
5f05dabc |
1499 | This will be adequate if you ever need to have only one callback |
d1b91892 |
1500 | registered at any time. An example could be an error handler like the |
1501 | code sketched out above. Remember though, repeated calls to |
1502 | C<register_fatal> will replace the previously registered callback |
1503 | function with the new one. |
1504 | |
1505 | Say for example you want to interface to a library which allows asynchronous |
1506 | file i/o. In this case you may be able to register a callback whenever |
1507 | a read operation has completed. To be of any use we want to be able to |
1508 | call separate Perl subroutines for each file that is opened. As it |
1509 | stands, the error handler example above would not be adequate as it |
1510 | allows only a single callback to be defined at any time. What we |
1511 | require is a means of storing the mapping between the opened file and |
1512 | the Perl subroutine we want to be called for that file. |
1513 | |
1514 | Say the i/o library has a function C<asynch_read> which associates a C |
19799a22 |
1515 | function C<ProcessRead> with a file handle C<fh>--this assumes that it |
d1b91892 |
1516 | has also provided some routine to open the file and so obtain the file |
1517 | handle. |
1518 | |
1519 | asynch_read(fh, ProcessRead) |
1520 | |
1521 | This may expect the C I<ProcessRead> function of this form |
1522 | |
1523 | void |
1524 | ProcessRead(fh, buffer) |
4358a253 |
1525 | int fh; |
1526 | char * buffer; |
d1b91892 |
1527 | { |
54310121 |
1528 | ... |
d1b91892 |
1529 | } |
1530 | |
1531 | To provide a Perl interface to this library we need to be able to map |
1532 | between the C<fh> parameter and the Perl subroutine we want called. A |
1533 | hash is a convenient mechanism for storing this mapping. The code |
1534 | below shows a possible implementation |
1535 | |
4358a253 |
1536 | static HV * Mapping = (HV*)NULL; |
a0d0e21e |
1537 | |
d1b91892 |
1538 | void |
1539 | asynch_read(fh, callback) |
1540 | int fh |
1541 | SV * callback |
1542 | CODE: |
1543 | /* If the hash doesn't already exist, create it */ |
1544 | if (Mapping == (HV*)NULL) |
4358a253 |
1545 | Mapping = newHV(); |
d1b91892 |
1546 | |
1547 | /* Save the fh -> callback mapping */ |
4358a253 |
1548 | hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0); |
d1b91892 |
1549 | |
1550 | /* Register with the C Library */ |
4358a253 |
1551 | asynch_read(fh, asynch_read_if); |
d1b91892 |
1552 | |
1553 | and C<asynch_read_if> could look like this |
1554 | |
1555 | static void |
1556 | asynch_read_if(fh, buffer) |
4358a253 |
1557 | int fh; |
1558 | char * buffer; |
d1b91892 |
1559 | { |
4358a253 |
1560 | dSP; |
1561 | SV ** sv; |
d1b91892 |
1562 | |
1563 | /* Get the callback associated with fh */ |
4358a253 |
1564 | sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE); |
d1b91892 |
1565 | if (sv == (SV**)NULL) |
4358a253 |
1566 | croak("Internal error...\n"); |
d1b91892 |
1567 | |
4358a253 |
1568 | PUSHMARK(SP); |
1569 | XPUSHs(sv_2mortal(newSViv(fh))); |
1570 | XPUSHs(sv_2mortal(newSVpv(buffer, 0))); |
1571 | PUTBACK; |
d1b91892 |
1572 | |
1573 | /* Call the Perl sub */ |
4358a253 |
1574 | call_sv(*sv, G_DISCARD); |
d1b91892 |
1575 | } |
1576 | |
1577 | For completeness, here is C<asynch_close>. This shows how to remove |
1578 | the entry from the hash C<Mapping>. |
1579 | |
1580 | void |
1581 | asynch_close(fh) |
1582 | int fh |
1583 | CODE: |
1584 | /* Remove the entry from the hash */ |
4358a253 |
1585 | (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD); |
a0d0e21e |
1586 | |
d1b91892 |
1587 | /* Now call the real asynch_close */ |
4358a253 |
1588 | asynch_close(fh); |
a0d0e21e |
1589 | |
d1b91892 |
1590 | So the Perl interface would look like this |
1591 | |
84f709e7 |
1592 | sub callback1 |
1593 | { |
4358a253 |
1594 | my($handle, $buffer) = @_; |
d1b91892 |
1595 | } |
a0d0e21e |
1596 | |
d1b91892 |
1597 | # Register the Perl callback |
4358a253 |
1598 | asynch_read($fh, \&callback1); |
a0d0e21e |
1599 | |
4358a253 |
1600 | asynch_close($fh); |
d1b91892 |
1601 | |
1602 | The mapping between the C callback and Perl is stored in the global |
1603 | hash C<Mapping> this time. Using a hash has the distinct advantage that |
1604 | it allows an unlimited number of callbacks to be registered. |
1605 | |
1606 | What if the interface provided by the C callback doesn't contain a |
1607 | parameter which allows the file handle to Perl subroutine mapping? Say |
1608 | in the asynchronous i/o package, the callback function gets passed only |
1609 | the C<buffer> parameter like this |
1610 | |
1611 | void |
1612 | ProcessRead(buffer) |
4358a253 |
1613 | char * buffer; |
d1b91892 |
1614 | { |
1615 | ... |
1616 | } |
a0d0e21e |
1617 | |
d1b91892 |
1618 | Without the file handle there is no straightforward way to map from the |
1619 | C callback to the Perl subroutine. |
a0d0e21e |
1620 | |
54310121 |
1621 | In this case a possible way around this problem is to predefine a |
d1b91892 |
1622 | series of C functions to act as the interface to Perl, thus |
1623 | |
1624 | #define MAX_CB 3 |
1625 | #define NULL_HANDLE -1 |
4358a253 |
1626 | typedef void (*FnMap)(); |
d1b91892 |
1627 | |
1628 | struct MapStruct { |
4358a253 |
1629 | FnMap Function; |
1630 | SV * PerlSub; |
1631 | int Handle; |
1632 | }; |
d1b91892 |
1633 | |
4358a253 |
1634 | static void fn1(); |
1635 | static void fn2(); |
1636 | static void fn3(); |
d1b91892 |
1637 | |
1638 | static struct MapStruct Map [MAX_CB] = |
1639 | { |
1640 | { fn1, NULL, NULL_HANDLE }, |
1641 | { fn2, NULL, NULL_HANDLE }, |
1642 | { fn3, NULL, NULL_HANDLE } |
4358a253 |
1643 | }; |
d1b91892 |
1644 | |
1645 | static void |
1646 | Pcb(index, buffer) |
4358a253 |
1647 | int index; |
1648 | char * buffer; |
d1b91892 |
1649 | { |
4358a253 |
1650 | dSP; |
d1b91892 |
1651 | |
4358a253 |
1652 | PUSHMARK(SP); |
1653 | XPUSHs(sv_2mortal(newSVpv(buffer, 0))); |
1654 | PUTBACK; |
d1b91892 |
1655 | |
1656 | /* Call the Perl sub */ |
4358a253 |
1657 | call_sv(Map[index].PerlSub, G_DISCARD); |
d1b91892 |
1658 | } |
1659 | |
1660 | static void |
1661 | fn1(buffer) |
4358a253 |
1662 | char * buffer; |
d1b91892 |
1663 | { |
4358a253 |
1664 | Pcb(0, buffer); |
d1b91892 |
1665 | } |
1666 | |
1667 | static void |
1668 | fn2(buffer) |
4358a253 |
1669 | char * buffer; |
d1b91892 |
1670 | { |
4358a253 |
1671 | Pcb(1, buffer); |
d1b91892 |
1672 | } |
1673 | |
1674 | static void |
1675 | fn3(buffer) |
4358a253 |
1676 | char * buffer; |
d1b91892 |
1677 | { |
4358a253 |
1678 | Pcb(2, buffer); |
d1b91892 |
1679 | } |
1680 | |
1681 | void |
1682 | array_asynch_read(fh, callback) |
1683 | int fh |
1684 | SV * callback |
1685 | CODE: |
4358a253 |
1686 | int index; |
1687 | int null_index = MAX_CB; |
d1b91892 |
1688 | |
1689 | /* Find the same handle or an empty entry */ |
4358a253 |
1690 | for (index = 0; index < MAX_CB; ++index) |
d1b91892 |
1691 | { |
1692 | if (Map[index].Handle == fh) |
4358a253 |
1693 | break; |
d1b91892 |
1694 | |
1695 | if (Map[index].Handle == NULL_HANDLE) |
4358a253 |
1696 | null_index = index; |
d1b91892 |
1697 | } |
1698 | |
1699 | if (index == MAX_CB && null_index == MAX_CB) |
4358a253 |
1700 | croak ("Too many callback functions registered\n"); |
d1b91892 |
1701 | |
1702 | if (index == MAX_CB) |
4358a253 |
1703 | index = null_index; |
d1b91892 |
1704 | |
1705 | /* Save the file handle */ |
4358a253 |
1706 | Map[index].Handle = fh; |
d1b91892 |
1707 | |
1708 | /* Remember the Perl sub */ |
1709 | if (Map[index].PerlSub == (SV*)NULL) |
4358a253 |
1710 | Map[index].PerlSub = newSVsv(callback); |
d1b91892 |
1711 | else |
4358a253 |
1712 | SvSetSV(Map[index].PerlSub, callback); |
d1b91892 |
1713 | |
4358a253 |
1714 | asynch_read(fh, Map[index].Function); |
d1b91892 |
1715 | |
1716 | void |
1717 | array_asynch_close(fh) |
1718 | int fh |
1719 | CODE: |
4358a253 |
1720 | int index; |
d1b91892 |
1721 | |
1722 | /* Find the file handle */ |
4358a253 |
1723 | for (index = 0; index < MAX_CB; ++ index) |
d1b91892 |
1724 | if (Map[index].Handle == fh) |
4358a253 |
1725 | break; |
d1b91892 |
1726 | |
1727 | if (index == MAX_CB) |
4358a253 |
1728 | croak ("could not close fh %d\n", fh); |
d1b91892 |
1729 | |
4358a253 |
1730 | Map[index].Handle = NULL_HANDLE; |
1731 | SvREFCNT_dec(Map[index].PerlSub); |
1732 | Map[index].PerlSub = (SV*)NULL; |
d1b91892 |
1733 | |
4358a253 |
1734 | asynch_close(fh); |
d1b91892 |
1735 | |
5f05dabc |
1736 | In this case the functions C<fn1>, C<fn2>, and C<fn3> are used to |
d1b91892 |
1737 | remember the Perl subroutine to be called. Each of the functions holds |
4a6725af |
1738 | a separate hard-wired index which is used in the function C<Pcb> to |
d1b91892 |
1739 | access the C<Map> array and actually call the Perl subroutine. |
1740 | |
1741 | There are some obvious disadvantages with this technique. |
1742 | |
1743 | Firstly, the code is considerably more complex than with the previous |
1744 | example. |
1745 | |
4a6725af |
1746 | Secondly, there is a hard-wired limit (in this case 3) to the number of |
d1b91892 |
1747 | callbacks that can exist simultaneously. The only way to increase the |
1748 | limit is by modifying the code to add more functions and then |
54310121 |
1749 | recompiling. None the less, as long as the number of functions is |
d1b91892 |
1750 | chosen with some care, it is still a workable solution and in some |
1751 | cases is the only one available. |
1752 | |
1753 | To summarize, here are a number of possible methods for you to consider |
1754 | for storing the mapping between C and the Perl callback |
1755 | |
1756 | =over 5 |
1757 | |
1758 | =item 1. Ignore the problem - Allow only 1 callback |
1759 | |
1760 | For a lot of situations, like interfacing to an error handler, this may |
1761 | be a perfectly adequate solution. |
1762 | |
1763 | =item 2. Create a sequence of callbacks - hard wired limit |
1764 | |
1765 | If it is impossible to tell from the parameters passed back from the C |
1766 | callback what the context is, then you may need to create a sequence of C |
1767 | callback interface functions, and store pointers to each in an array. |
1768 | |
1769 | =item 3. Use a parameter to map to the Perl callback |
1770 | |
1771 | A hash is an ideal mechanism to store the mapping between C and Perl. |
1772 | |
1773 | =back |
a0d0e21e |
1774 | |
a0d0e21e |
1775 | |
1776 | =head2 Alternate Stack Manipulation |
1777 | |
a0d0e21e |
1778 | |
d1b91892 |
1779 | Although I have made use of only the C<POP*> macros to access values |
1780 | returned from Perl subroutines, it is also possible to bypass these |
8e07c86e |
1781 | macros and read the stack using the C<ST> macro (See L<perlxs> for a |
d1b91892 |
1782 | full description of the C<ST> macro). |
1783 | |
1784 | Most of the time the C<POP*> macros should be adequate, the main |
1785 | problem with them is that they force you to process the returned values |
1786 | in sequence. This may not be the most suitable way to process the |
1787 | values in some cases. What we want is to be able to access the stack in |
1788 | a random order. The C<ST> macro as used when coding an XSUB is ideal |
1789 | for this purpose. |
1790 | |
1791 | The code below is the example given in the section I<Returning a list |
1792 | of values> recoded to use C<ST> instead of C<POP*>. |
1793 | |
1794 | static void |
1795 | call_AddSubtract2(a, b) |
4358a253 |
1796 | int a; |
1797 | int b; |
d1b91892 |
1798 | { |
4358a253 |
1799 | dSP; |
1800 | I32 ax; |
1801 | int count; |
d1b91892 |
1802 | |
4358a253 |
1803 | ENTER; |
d1b91892 |
1804 | SAVETMPS; |
1805 | |
4358a253 |
1806 | PUSHMARK(SP); |
d1b91892 |
1807 | XPUSHs(sv_2mortal(newSViv(a))); |
1808 | XPUSHs(sv_2mortal(newSViv(b))); |
4358a253 |
1809 | PUTBACK; |
d1b91892 |
1810 | |
4929bf7b |
1811 | count = call_pv("AddSubtract", G_ARRAY); |
d1b91892 |
1812 | |
4358a253 |
1813 | SPAGAIN; |
1814 | SP -= count; |
1815 | ax = (SP - PL_stack_base) + 1; |
d1b91892 |
1816 | |
1817 | if (count != 2) |
4358a253 |
1818 | croak("Big trouble\n"); |
a0d0e21e |
1819 | |
4358a253 |
1820 | printf ("%d + %d = %d\n", a, b, SvIV(ST(0))); |
1821 | printf ("%d - %d = %d\n", a, b, SvIV(ST(1))); |
d1b91892 |
1822 | |
4358a253 |
1823 | PUTBACK; |
1824 | FREETMPS; |
1825 | LEAVE; |
d1b91892 |
1826 | } |
1827 | |
1828 | Notes |
1829 | |
1830 | =over 5 |
1831 | |
1832 | =item 1. |
1833 | |
1834 | Notice that it was necessary to define the variable C<ax>. This is |
1835 | because the C<ST> macro expects it to exist. If we were in an XSUB it |
1836 | would not be necessary to define C<ax> as it is already defined for |
1837 | you. |
1838 | |
1839 | =item 2. |
1840 | |
1841 | The code |
1842 | |
4358a253 |
1843 | SPAGAIN; |
1844 | SP -= count; |
1845 | ax = (SP - PL_stack_base) + 1; |
d1b91892 |
1846 | |
1847 | sets the stack up so that we can use the C<ST> macro. |
1848 | |
1849 | =item 3. |
1850 | |
1851 | Unlike the original coding of this example, the returned |
1852 | values are not accessed in reverse order. So C<ST(0)> refers to the |
54310121 |
1853 | first value returned by the Perl subroutine and C<ST(count-1)> |
d1b91892 |
1854 | refers to the last. |
1855 | |
1856 | =back |
a0d0e21e |
1857 | |
8f183262 |
1858 | =head2 Creating and calling an anonymous subroutine in C |
1859 | |
4929bf7b |
1860 | As we've already shown, C<call_sv> can be used to invoke an |
c2611fb3 |
1861 | anonymous subroutine. However, our example showed a Perl script |
1862 | invoking an XSUB to perform this operation. Let's see how it can be |
8f183262 |
1863 | done inside our C code: |
1864 | |
8f183262 |
1865 | ... |
1866 | |
4929bf7b |
1867 | SV *cvrv = eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE); |
8f183262 |
1868 | |
1869 | ... |
1870 | |
4929bf7b |
1871 | call_sv(cvrv, G_VOID|G_NOARGS); |
8f183262 |
1872 | |
4929bf7b |
1873 | C<eval_pv> is used to compile the anonymous subroutine, which |
1874 | will be the return value as well (read more about C<eval_pv> in |
4a4eefd0 |
1875 | L<perlapi/eval_pv>). Once this code reference is in hand, it |
8f183262 |
1876 | can be mixed in with all the previous examples we've shown. |
1877 | |
9850bf21 |
1878 | =head1 LIGHTWEIGHT CALLBACKS |
1879 | |
1880 | Sometimes you need to invoke the same subroutine repeatedly. |
1881 | This usually happens with a function that acts on a list of |
1882 | values, such as Perl's built-in sort(). You can pass a |
1883 | comparison function to sort(), which will then be invoked |
1884 | for every pair of values that needs to be compared. The first() |
1885 | and reduce() functions from L<List::Util> follow a similar |
1886 | pattern. |
1887 | |
1888 | In this case it is possible to speed up the routine (often |
1889 | quite substantially) by using the lightweight callback API. |
1890 | The idea is that the calling context only needs to be |
1891 | created and destroyed once, and the sub can be called |
1892 | arbitrarily many times in between. |
1893 | |
1894 | It is usual to pass parameters using global variables -- typically |
1895 | $_ for one parameter, or $a and $b for two parameters -- rather |
1896 | than via @_. (It is possible to use the @_ mechanism if you know |
1897 | what you're doing, though there is as yet no supported API for |
1898 | it. It's also inherently slower.) |
1899 | |
1900 | The pattern of macro calls is like this: |
1901 | |
82f35e8b |
1902 | dMULTICALL; /* Declare local variables */ |
9850bf21 |
1903 | I32 gimme = G_SCALAR; /* context of the call: G_SCALAR, |
1904 | * G_LIST, or G_VOID */ |
1905 | |
82f35e8b |
1906 | PUSH_MULTICALL(cv); /* Set up the context for calling cv, |
1907 | and set local vars appropriately */ |
9850bf21 |
1908 | |
1909 | /* loop */ { |
1910 | /* set the value(s) af your parameter variables */ |
1911 | MULTICALL; /* Make the actual call */ |
1912 | } /* end of loop */ |
1913 | |
1914 | POP_MULTICALL; /* Tear down the calling context */ |
1915 | |
1916 | For some concrete examples, see the implementation of the |
1917 | first() and reduce() functions of List::Util 1.18. There you |
1918 | will also find a header file that emulates the multicall API |
1919 | on older versions of perl. |
1920 | |
a0d0e21e |
1921 | =head1 SEE ALSO |
1922 | |
8e07c86e |
1923 | L<perlxs>, L<perlguts>, L<perlembed> |
a0d0e21e |
1924 | |
1925 | =head1 AUTHOR |
1926 | |
0536e0eb |
1927 | Paul Marquess |
a0d0e21e |
1928 | |
d1b91892 |
1929 | Special thanks to the following people who assisted in the creation of |
1930 | the document. |
a0d0e21e |
1931 | |
c07a80fd |
1932 | Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy |
1933 | and Larry Wall. |
a0d0e21e |
1934 | |
1935 | =head1 DATE |
1936 | |
137443ea |
1937 | Version 1.3, 14th Apr 1997 |