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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 PERL_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 | |
56 | I32 perl_call_sv(SV* sv, I32 flags) ; |
57 | I32 perl_call_pv(char *subname, I32 flags) ; |
58 | I32 perl_call_method(char *methname, I32 flags) ; |
59 | I32 perl_call_argv(char *subname, I32 flags, register char **argv) ; |
60 | |
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61 | The key function is I<perl_call_sv>. All the other functions are |
62 | fairly simple wrappers which make it easier to call Perl subroutines in |
63 | special cases. At the end of the day they will all call I<perl_call_sv> |
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64 | to invoke the Perl subroutine. |
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65 | |
66 | All the I<perl_call_*> functions have a C<flags> parameter which is |
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 | |
75 | =item B<perl_call_sv> |
76 | |
77 | I<perl_call_sv> takes two parameters, the first, C<sv>, is an SV*. |
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 |
80 | subroutine. The section, I<Using perl_call_sv>, shows how you can make |
81 | use of I<perl_call_sv>. |
82 | |
83 | =item B<perl_call_pv> |
84 | |
85 | The function, I<perl_call_pv>, is similar to I<perl_call_sv> except it |
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<perl_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 | |
91 | =item B<perl_call_method> |
92 | |
93 | The function I<perl_call_method> is used to call a method from a Perl |
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 |
99 | static and virtual methods and L<Using perl_call_method> for an example |
100 | of using I<perl_call_method>. |
101 | |
102 | =item B<perl_call_argv> |
103 | |
104 | I<perl_call_argv> calls the Perl subroutine specified by the C string |
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. |
108 | See I<Using perl_call_argv>. |
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 |
119 | stop someone from doing something unexpected - don't say you haven't |
120 | been warned. |
121 | |
122 | =head1 FLAG VALUES |
123 | |
124 | The C<flags> parameter in all the I<perl_call_*> functions is a bit mask |
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 | |
149 | The value returned by the I<perl_call_*> function indicates how many |
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 |
157 | context flag setting for all the I<perl_call_*> functions. |
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. |
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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<perl_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 |
190 | I<perl_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 an |
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205 | array 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<perl_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<perl_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 | |
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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<perl_call_*> functions place the items returned from |
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 | |
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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<perl_call_*> |
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 | |
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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 |
258 | occur when the code that is executing the I<perl_call_*> function has |
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 of events occurs, the |
283 | process will terminate immediately. If though, you want to trap this |
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 | |
287 | Whenever control returns from the I<perl_call_*> function you need to |
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288 | check the C<$@> variable as you would in a normal Perl script. |
289 | |
290 | The value returned from the I<perl_call_*> function is dependent on |
291 | what other flags have been specified and whether an error has |
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292 | occurred. Here are all the different cases that can occur: |
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293 | |
294 | =over 5 |
295 | |
296 | =item * |
297 | |
298 | If the I<perl_call_*> function returns normally, then the value |
299 | returned is as specified in the previous sections. |
300 | |
301 | =item * |
302 | |
303 | If G_DISCARD is specified, the return value will always be 0. |
304 | |
305 | =item * |
306 | |
307 | If G_ARRAY is specified I<and> an error has occurred, the return value |
308 | will always be 0. |
309 | |
310 | =item * |
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311 | |
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312 | If G_SCALAR is specified I<and> an error has occurred, the return value |
313 | will be 1 and the value on the top of the stack will be I<undef>. This |
314 | means that if you have already detected the error by checking C<$@> 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 |
341 | I<perl_call_*> functions that are used to implement such code. This flag |
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 |
346 | of C<$@>. |
347 | |
348 | The G_KEEPERR flag was introduced in Perl version 5.002. |
349 | |
350 | See I<Using G_KEEPERR> for an example of a situation that warrants the |
351 | use of this flag. |
352 | |
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353 | =head2 Determining the Context |
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354 | |
355 | As mentioned above, you can determine the context of the currently |
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356 | executing subroutine in Perl with I<wantarray>. The equivalent test |
357 | can be made in C by using the C<GIMME_V> macro, which returns |
358 | C<G_ARRAY> if you have been called in an array context, C<G_SCALAR> if |
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359 | in a scalar context, or C<G_VOID> if in a void context (i.e. the |
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360 | return value will not be used). An older version of this macro is |
361 | called C<GIMME>; in a void context it returns C<G_SCALAR> instead of |
362 | C<G_VOID>. An example of using the C<GIMME_V> macro is shown in |
363 | section I<Using GIMME_V>. |
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364 | |
365 | =head1 KNOWN PROBLEMS |
366 | |
367 | This section outlines all known problems that exist in the |
368 | I<perl_call_*> functions. |
369 | |
370 | =over 5 |
371 | |
372 | =item 1. |
373 | |
374 | If you are intending to make use of both the G_EVAL and G_SCALAR flags |
375 | in your code, use a version of Perl greater than 5.000. There is a bug |
376 | in version 5.000 of Perl which means that the combination of these two |
377 | flags will not work as described in the section I<FLAG VALUES>. |
378 | |
379 | Specifically, if the two flags are used when calling a subroutine and |
380 | that subroutine does not call I<die>, the value returned by |
381 | I<perl_call_*> will be wrong. |
382 | |
383 | |
384 | =item 2. |
385 | |
386 | In Perl 5.000 and 5.001 there is a problem with using I<perl_call_*> if |
387 | the Perl sub you are calling attempts to trap a I<die>. |
388 | |
389 | The symptom of this problem is that the called Perl sub will continue |
390 | to completion, but whenever it attempts to pass control back to the |
391 | XSUB, the program will immediately terminate. |
392 | |
393 | For example, say you want to call this Perl sub |
394 | |
395 | sub fred |
396 | { |
397 | eval { die "Fatal Error" ; } |
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398 | print "Trapped error: $@\n" |
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399 | if $@ ; |
400 | } |
401 | |
402 | via this XSUB |
403 | |
404 | void |
405 | Call_fred() |
406 | CODE: |
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407 | PUSHMARK(SP) ; |
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408 | perl_call_pv("fred", G_DISCARD|G_NOARGS) ; |
409 | fprintf(stderr, "back in Call_fred\n") ; |
410 | |
411 | When C<Call_fred> is executed it will print |
412 | |
413 | Trapped error: Fatal Error |
414 | |
415 | As control never returns to C<Call_fred>, the C<"back in Call_fred"> |
416 | string will not get printed. |
417 | |
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418 | To work around this problem, you can either upgrade to Perl 5.002 or |
419 | higher, or use the G_EVAL flag with I<perl_call_*> as shown below |
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420 | |
421 | void |
422 | Call_fred() |
423 | CODE: |
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424 | PUSHMARK(SP) ; |
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425 | perl_call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ; |
426 | fprintf(stderr, "back in Call_fred\n") ; |
427 | |
428 | =back |
429 | |
430 | |
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431 | |
432 | =head1 EXAMPLES |
433 | |
434 | Enough of the definition talk, let's have a few examples. |
435 | |
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436 | Perl provides many macros to assist in accessing the Perl stack. |
437 | Wherever possible, these macros should always be used when interfacing |
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438 | to Perl internals. We hope this should make the code less vulnerable |
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439 | to any changes made to Perl in the future. |
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440 | |
d1b91892 |
441 | Another point worth noting is that in the first series of examples I |
442 | have made use of only the I<perl_call_pv> function. This has been done |
443 | to keep the code simpler and ease you into the topic. Wherever |
444 | possible, if the choice is between using I<perl_call_pv> and |
445 | I<perl_call_sv>, you should always try to use I<perl_call_sv>. See |
446 | I<Using perl_call_sv> for details. |
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447 | |
d1b91892 |
448 | =head2 No Parameters, Nothing returned |
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449 | |
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450 | This first trivial example will call a Perl subroutine, I<PrintUID>, to |
451 | print out the UID of the process. |
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452 | |
453 | sub PrintUID |
454 | { |
455 | print "UID is $<\n" ; |
456 | } |
457 | |
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458 | and here is a C function to call it |
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459 | |
d1b91892 |
460 | static void |
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461 | call_PrintUID() |
462 | { |
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463 | dSP ; |
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464 | |
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465 | PUSHMARK(SP) ; |
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466 | perl_call_pv("PrintUID", G_DISCARD|G_NOARGS) ; |
467 | } |
468 | |
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469 | Simple, eh. |
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470 | |
d1b91892 |
471 | A few points to note about this example. |
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472 | |
473 | =over 5 |
474 | |
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475 | =item 1. |
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476 | |
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477 | Ignore C<dSP> and C<PUSHMARK(SP)> for now. They will be discussed in |
d1b91892 |
478 | the next example. |
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479 | |
480 | =item 2. |
481 | |
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482 | We aren't passing any parameters to I<PrintUID> so G_NOARGS can be |
483 | specified. |
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484 | |
d1b91892 |
485 | =item 3. |
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486 | |
487 | We aren't interested in anything returned from I<PrintUID>, so |
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488 | G_DISCARD is specified. Even if I<PrintUID> was changed to |
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489 | return some value(s), having specified G_DISCARD will mean that they |
490 | will be wiped by the time control returns from I<perl_call_pv>. |
491 | |
d1b91892 |
492 | =item 4. |
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493 | |
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494 | As I<perl_call_pv> is being used, the Perl subroutine is specified as a |
495 | C string. In this case the subroutine name has been 'hard-wired' into the |
496 | code. |
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497 | |
498 | =item 5. |
499 | |
d1b91892 |
500 | Because we specified G_DISCARD, it is not necessary to check the value |
501 | returned from I<perl_call_pv>. It will always be 0. |
a0d0e21e |
502 | |
503 | =back |
504 | |
d1b91892 |
505 | =head2 Passing Parameters |
a0d0e21e |
506 | |
d1b91892 |
507 | Now let's make a slightly more complex example. This time we want to |
508 | call a Perl subroutine, C<LeftString>, which will take 2 parameters - a |
509 | string (C<$s>) and an integer (C<$n>). The subroutine will simply |
510 | print the first C<$n> characters of the string. |
a0d0e21e |
511 | |
d1b91892 |
512 | So the Perl subroutine would look like this |
a0d0e21e |
513 | |
514 | sub LeftString |
515 | { |
516 | my($s, $n) = @_ ; |
517 | print substr($s, 0, $n), "\n" ; |
518 | } |
519 | |
520 | The C function required to call I<LeftString> would look like this. |
521 | |
522 | static void |
523 | call_LeftString(a, b) |
524 | char * a ; |
525 | int b ; |
526 | { |
527 | dSP ; |
528 | |
924508f0 |
529 | PUSHMARK(SP) ; |
a0d0e21e |
530 | XPUSHs(sv_2mortal(newSVpv(a, 0))); |
531 | XPUSHs(sv_2mortal(newSViv(b))); |
532 | PUTBACK ; |
533 | |
534 | perl_call_pv("LeftString", G_DISCARD); |
535 | } |
536 | |
a0d0e21e |
537 | Here are a few notes on the C function I<call_LeftString>. |
538 | |
539 | =over 5 |
540 | |
d1b91892 |
541 | =item 1. |
a0d0e21e |
542 | |
d1b91892 |
543 | Parameters are passed to the Perl subroutine using the Perl stack. |
544 | This is the purpose of the code beginning with the line C<dSP> and |
1e62ac33 |
545 | ending with the line C<PUTBACK>. The C<dSP> declares a local copy |
924508f0 |
546 | of the stack pointer. This local copy should B<always> be accessed |
547 | as C<SP>. |
a0d0e21e |
548 | |
d1b91892 |
549 | =item 2. |
a0d0e21e |
550 | |
551 | If you are going to put something onto the Perl stack, you need to know |
d1b91892 |
552 | where to put it. This is the purpose of the macro C<dSP> - it declares |
553 | and initializes a I<local> copy of the Perl stack pointer. |
a0d0e21e |
554 | |
555 | All the other macros which will be used in this example require you to |
d1b91892 |
556 | have used this macro. |
a0d0e21e |
557 | |
d1b91892 |
558 | The exception to this rule is if you are calling a Perl subroutine |
559 | directly from an XSUB function. In this case it is not necessary to |
5f05dabc |
560 | use the C<dSP> macro explicitly - it will be declared for you |
d1b91892 |
561 | automatically. |
a0d0e21e |
562 | |
d1b91892 |
563 | =item 3. |
a0d0e21e |
564 | |
565 | Any parameters to be pushed onto the stack should be bracketed by the |
d1b91892 |
566 | C<PUSHMARK> and C<PUTBACK> macros. The purpose of these two macros, in |
5f05dabc |
567 | this context, is to count the number of parameters you are |
568 | pushing automatically. Then whenever Perl is creating the C<@_> array for the |
d1b91892 |
569 | subroutine, it knows how big to make it. |
570 | |
571 | The C<PUSHMARK> macro tells Perl to make a mental note of the current |
572 | stack pointer. Even if you aren't passing any parameters (like the |
573 | example shown in the section I<No Parameters, Nothing returned>) you |
574 | must still call the C<PUSHMARK> macro before you can call any of the |
575 | I<perl_call_*> functions - Perl still needs to know that there are no |
576 | parameters. |
577 | |
578 | The C<PUTBACK> macro sets the global copy of the stack pointer to be |
579 | the same as our local copy. If we didn't do this I<perl_call_pv> |
580 | wouldn't know where the two parameters we pushed were - remember that |
581 | up to now all the stack pointer manipulation we have done is with our |
582 | local copy, I<not> the global copy. |
583 | |
584 | =item 4. |
585 | |
5f05dabc |
586 | The only flag specified this time is G_DISCARD. Because we are passing 2 |
d1b91892 |
587 | parameters to the Perl subroutine this time, we have not specified |
588 | G_NOARGS. |
a0d0e21e |
589 | |
590 | =item 5. |
591 | |
592 | Next, we come to XPUSHs. This is where the parameters actually get |
d1b91892 |
593 | pushed onto the stack. In this case we are pushing a string and an |
594 | integer. |
a0d0e21e |
595 | |
54310121 |
596 | See L<perlguts/"XSUBs and the Argument Stack"> for details |
d1b91892 |
597 | on how the XPUSH macros work. |
a0d0e21e |
598 | |
599 | =item 6. |
600 | |
d1b91892 |
601 | Finally, I<LeftString> can now be called via the I<perl_call_pv> |
602 | function. |
a0d0e21e |
603 | |
604 | =back |
605 | |
d1b91892 |
606 | =head2 Returning a Scalar |
a0d0e21e |
607 | |
d1b91892 |
608 | Now for an example of dealing with the items returned from a Perl |
609 | subroutine. |
a0d0e21e |
610 | |
5f05dabc |
611 | Here is a Perl subroutine, I<Adder>, that takes 2 integer parameters |
d1b91892 |
612 | and simply returns their sum. |
a0d0e21e |
613 | |
614 | sub Adder |
615 | { |
616 | my($a, $b) = @_ ; |
617 | $a + $b ; |
618 | } |
619 | |
5f05dabc |
620 | Because we are now concerned with the return value from I<Adder>, the C |
d1b91892 |
621 | function required to call it is now a bit more complex. |
a0d0e21e |
622 | |
623 | static void |
624 | call_Adder(a, b) |
625 | int a ; |
626 | int b ; |
627 | { |
628 | dSP ; |
629 | int count ; |
630 | |
631 | ENTER ; |
632 | SAVETMPS; |
633 | |
924508f0 |
634 | PUSHMARK(SP) ; |
a0d0e21e |
635 | XPUSHs(sv_2mortal(newSViv(a))); |
636 | XPUSHs(sv_2mortal(newSViv(b))); |
637 | PUTBACK ; |
638 | |
639 | count = perl_call_pv("Adder", G_SCALAR); |
640 | |
641 | SPAGAIN ; |
642 | |
d1b91892 |
643 | if (count != 1) |
644 | croak("Big trouble\n") ; |
a0d0e21e |
645 | |
d1b91892 |
646 | printf ("The sum of %d and %d is %d\n", a, b, POPi) ; |
a0d0e21e |
647 | |
648 | PUTBACK ; |
649 | FREETMPS ; |
650 | LEAVE ; |
651 | } |
652 | |
a0d0e21e |
653 | Points to note this time are |
654 | |
655 | =over 5 |
656 | |
54310121 |
657 | =item 1. |
a0d0e21e |
658 | |
d1b91892 |
659 | The only flag specified this time was G_SCALAR. That means the C<@_> |
660 | array will be created and that the value returned by I<Adder> will |
661 | still exist after the call to I<perl_call_pv>. |
a0d0e21e |
662 | |
663 | |
664 | |
665 | =item 2. |
666 | |
d1b91892 |
667 | Because we are interested in what is returned from I<Adder> we cannot |
668 | specify G_DISCARD. This means that we will have to tidy up the Perl |
669 | stack and dispose of any temporary values ourselves. This is the |
670 | purpose of |
a0d0e21e |
671 | |
d1b91892 |
672 | ENTER ; |
673 | SAVETMPS ; |
a0d0e21e |
674 | |
675 | at the start of the function, and |
676 | |
d1b91892 |
677 | FREETMPS ; |
678 | LEAVE ; |
679 | |
680 | at the end. The C<ENTER>/C<SAVETMPS> pair creates a boundary for any |
681 | temporaries we create. This means that the temporaries we get rid of |
682 | will be limited to those which were created after these calls. |
a0d0e21e |
683 | |
d1b91892 |
684 | The C<FREETMPS>/C<LEAVE> pair will get rid of any values returned by |
54310121 |
685 | the Perl subroutine, plus it will also dump the mortal SVs we have |
d1b91892 |
686 | created. Having C<ENTER>/C<SAVETMPS> at the beginning of the code |
687 | makes sure that no other mortals are destroyed. |
a0d0e21e |
688 | |
d1b91892 |
689 | Think of these macros as working a bit like using C<{> and C<}> in Perl |
690 | to limit the scope of local variables. |
691 | |
692 | See the section I<Using Perl to dispose of temporaries> for details of |
693 | an alternative to using these macros. |
a0d0e21e |
694 | |
695 | =item 3. |
696 | |
697 | The purpose of the macro C<SPAGAIN> is to refresh the local copy of the |
698 | stack pointer. This is necessary because it is possible that the memory |
68dc0745 |
699 | allocated to the Perl stack has been reallocated whilst in the |
d1b91892 |
700 | I<perl_call_pv> call. |
a0d0e21e |
701 | |
d1b91892 |
702 | If you are making use of the Perl stack pointer in your code you must |
54310121 |
703 | always refresh the local copy using SPAGAIN whenever you make use |
a0d0e21e |
704 | of the I<perl_call_*> functions or any other Perl internal function. |
705 | |
d1b91892 |
706 | =item 4. |
a0d0e21e |
707 | |
d1b91892 |
708 | Although only a single value was expected to be returned from I<Adder>, |
709 | it is still good practice to check the return code from I<perl_call_pv> |
710 | anyway. |
a0d0e21e |
711 | |
d1b91892 |
712 | Expecting a single value is not quite the same as knowing that there |
713 | will be one. If someone modified I<Adder> to return a list and we |
714 | didn't check for that possibility and take appropriate action the Perl |
715 | stack would end up in an inconsistent state. That is something you |
5f05dabc |
716 | I<really> don't want to happen ever. |
a0d0e21e |
717 | |
718 | =item 5. |
719 | |
d1b91892 |
720 | The C<POPi> macro is used here to pop the return value from the stack. |
721 | In this case we wanted an integer, so C<POPi> was used. |
a0d0e21e |
722 | |
723 | |
d1b91892 |
724 | Here is the complete list of POP macros available, along with the types |
725 | they return. |
a0d0e21e |
726 | |
d1b91892 |
727 | POPs SV |
728 | POPp pointer |
729 | POPn double |
730 | POPi integer |
731 | POPl long |
a0d0e21e |
732 | |
733 | =item 6. |
734 | |
d1b91892 |
735 | The final C<PUTBACK> is used to leave the Perl stack in a consistent |
736 | state before exiting the function. This is necessary because when we |
737 | popped the return value from the stack with C<POPi> it updated only our |
738 | local copy of the stack pointer. Remember, C<PUTBACK> sets the global |
739 | stack pointer to be the same as our local copy. |
a0d0e21e |
740 | |
741 | =back |
742 | |
743 | |
d1b91892 |
744 | =head2 Returning a list of values |
a0d0e21e |
745 | |
d1b91892 |
746 | Now, let's extend the previous example to return both the sum of the |
747 | parameters and the difference. |
a0d0e21e |
748 | |
d1b91892 |
749 | Here is the Perl subroutine |
a0d0e21e |
750 | |
751 | sub AddSubtract |
752 | { |
753 | my($a, $b) = @_ ; |
754 | ($a+$b, $a-$b) ; |
755 | } |
756 | |
a0d0e21e |
757 | and this is the C function |
758 | |
759 | static void |
760 | call_AddSubtract(a, b) |
761 | int a ; |
762 | int b ; |
763 | { |
764 | dSP ; |
765 | int count ; |
766 | |
767 | ENTER ; |
768 | SAVETMPS; |
769 | |
924508f0 |
770 | PUSHMARK(SP) ; |
a0d0e21e |
771 | XPUSHs(sv_2mortal(newSViv(a))); |
772 | XPUSHs(sv_2mortal(newSViv(b))); |
773 | PUTBACK ; |
774 | |
775 | count = perl_call_pv("AddSubtract", G_ARRAY); |
776 | |
777 | SPAGAIN ; |
778 | |
d1b91892 |
779 | if (count != 2) |
780 | croak("Big trouble\n") ; |
a0d0e21e |
781 | |
d1b91892 |
782 | printf ("%d - %d = %d\n", a, b, POPi) ; |
783 | printf ("%d + %d = %d\n", a, b, POPi) ; |
a0d0e21e |
784 | |
785 | PUTBACK ; |
786 | FREETMPS ; |
787 | LEAVE ; |
788 | } |
789 | |
d1b91892 |
790 | If I<call_AddSubtract> is called like this |
791 | |
792 | call_AddSubtract(7, 4) ; |
793 | |
794 | then here is the output |
795 | |
796 | 7 - 4 = 3 |
797 | 7 + 4 = 11 |
a0d0e21e |
798 | |
799 | Notes |
800 | |
801 | =over 5 |
802 | |
803 | =item 1. |
804 | |
d1b91892 |
805 | We wanted array context, so G_ARRAY was used. |
a0d0e21e |
806 | |
807 | =item 2. |
808 | |
d1b91892 |
809 | Not surprisingly C<POPi> is used twice this time because we were |
810 | retrieving 2 values from the stack. The important thing to note is that |
811 | when using the C<POP*> macros they come off the stack in I<reverse> |
812 | order. |
a0d0e21e |
813 | |
814 | =back |
815 | |
d1b91892 |
816 | =head2 Returning a list in a scalar context |
817 | |
818 | Say the Perl subroutine in the previous section was called in a scalar |
819 | context, like this |
820 | |
821 | static void |
822 | call_AddSubScalar(a, b) |
823 | int a ; |
824 | int b ; |
825 | { |
826 | dSP ; |
827 | int count ; |
828 | int i ; |
829 | |
830 | ENTER ; |
831 | SAVETMPS; |
832 | |
924508f0 |
833 | PUSHMARK(SP) ; |
d1b91892 |
834 | XPUSHs(sv_2mortal(newSViv(a))); |
835 | XPUSHs(sv_2mortal(newSViv(b))); |
836 | PUTBACK ; |
837 | |
838 | count = perl_call_pv("AddSubtract", G_SCALAR); |
839 | |
840 | SPAGAIN ; |
841 | |
842 | printf ("Items Returned = %d\n", count) ; |
843 | |
844 | for (i = 1 ; i <= count ; ++i) |
845 | printf ("Value %d = %d\n", i, POPi) ; |
846 | |
847 | PUTBACK ; |
848 | FREETMPS ; |
849 | LEAVE ; |
850 | } |
851 | |
852 | The other modification made is that I<call_AddSubScalar> will print the |
853 | number of items returned from the Perl subroutine and their value (for |
854 | simplicity it assumes that they are integer). So if |
855 | I<call_AddSubScalar> is called |
856 | |
857 | call_AddSubScalar(7, 4) ; |
858 | |
859 | then the output will be |
860 | |
861 | Items Returned = 1 |
862 | Value 1 = 3 |
863 | |
864 | In this case the main point to note is that only the last item in the |
54310121 |
865 | list is returned from the subroutine, I<AddSubtract> actually made it back to |
d1b91892 |
866 | I<call_AddSubScalar>. |
867 | |
868 | |
869 | =head2 Returning Data from Perl via the parameter list |
a0d0e21e |
870 | |
871 | It is also possible to return values directly via the parameter list - |
872 | whether it is actually desirable to do it is another matter entirely. |
873 | |
d1b91892 |
874 | The Perl subroutine, I<Inc>, below takes 2 parameters and increments |
875 | each directly. |
a0d0e21e |
876 | |
877 | sub Inc |
878 | { |
879 | ++ $_[0] ; |
880 | ++ $_[1] ; |
881 | } |
882 | |
883 | and here is a C function to call it. |
884 | |
885 | static void |
886 | call_Inc(a, b) |
887 | int a ; |
888 | int b ; |
889 | { |
890 | dSP ; |
891 | int count ; |
892 | SV * sva ; |
893 | SV * svb ; |
894 | |
895 | ENTER ; |
896 | SAVETMPS; |
897 | |
898 | sva = sv_2mortal(newSViv(a)) ; |
899 | svb = sv_2mortal(newSViv(b)) ; |
900 | |
924508f0 |
901 | PUSHMARK(SP) ; |
a0d0e21e |
902 | XPUSHs(sva); |
903 | XPUSHs(svb); |
904 | PUTBACK ; |
905 | |
906 | count = perl_call_pv("Inc", G_DISCARD); |
907 | |
908 | if (count != 0) |
d1b91892 |
909 | croak ("call_Inc: expected 0 values from 'Inc', got %d\n", |
910 | count) ; |
a0d0e21e |
911 | |
912 | printf ("%d + 1 = %d\n", a, SvIV(sva)) ; |
913 | printf ("%d + 1 = %d\n", b, SvIV(svb)) ; |
914 | |
915 | FREETMPS ; |
d1b91892 |
916 | LEAVE ; |
a0d0e21e |
917 | } |
918 | |
d1b91892 |
919 | To be able to access the two parameters that were pushed onto the stack |
920 | after they return from I<perl_call_pv> it is necessary to make a note |
921 | of their addresses - thus the two variables C<sva> and C<svb>. |
a0d0e21e |
922 | |
d1b91892 |
923 | The reason this is necessary is that the area of the Perl stack which |
924 | held them will very likely have been overwritten by something else by |
925 | the time control returns from I<perl_call_pv>. |
a0d0e21e |
926 | |
927 | |
928 | |
929 | |
d1b91892 |
930 | =head2 Using G_EVAL |
a0d0e21e |
931 | |
d1b91892 |
932 | Now an example using G_EVAL. Below is a Perl subroutine which computes |
933 | the difference of its 2 parameters. If this would result in a negative |
934 | result, the subroutine calls I<die>. |
a0d0e21e |
935 | |
936 | sub Subtract |
937 | { |
d1b91892 |
938 | my ($a, $b) = @_ ; |
a0d0e21e |
939 | |
940 | die "death can be fatal\n" if $a < $b ; |
941 | |
d1b91892 |
942 | $a - $b ; |
a0d0e21e |
943 | } |
944 | |
945 | and some C to call it |
946 | |
947 | static void |
948 | call_Subtract(a, b) |
949 | int a ; |
950 | int b ; |
951 | { |
952 | dSP ; |
953 | int count ; |
a0d0e21e |
954 | |
955 | ENTER ; |
956 | SAVETMPS; |
957 | |
924508f0 |
958 | PUSHMARK(SP) ; |
a0d0e21e |
959 | XPUSHs(sv_2mortal(newSViv(a))); |
960 | XPUSHs(sv_2mortal(newSViv(b))); |
961 | PUTBACK ; |
962 | |
963 | count = perl_call_pv("Subtract", G_EVAL|G_SCALAR); |
964 | |
d1b91892 |
965 | SPAGAIN ; |
966 | |
967 | /* Check the eval first */ |
c07a80fd |
968 | if (SvTRUE(GvSV(errgv))) |
d1b91892 |
969 | { |
c07a80fd |
970 | printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ; |
d1b91892 |
971 | POPs ; |
972 | } |
973 | else |
974 | { |
975 | if (count != 1) |
976 | croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n", |
977 | count) ; |
a0d0e21e |
978 | |
d1b91892 |
979 | printf ("%d - %d = %d\n", a, b, POPi) ; |
980 | } |
a0d0e21e |
981 | |
982 | PUTBACK ; |
983 | FREETMPS ; |
984 | LEAVE ; |
a0d0e21e |
985 | } |
986 | |
987 | If I<call_Subtract> is called thus |
988 | |
d1b91892 |
989 | call_Subtract(4, 5) |
a0d0e21e |
990 | |
991 | the following will be printed |
992 | |
d1b91892 |
993 | Uh oh - death can be fatal |
a0d0e21e |
994 | |
995 | Notes |
996 | |
997 | =over 5 |
998 | |
999 | =item 1. |
1000 | |
d1b91892 |
1001 | We want to be able to catch the I<die> so we have used the G_EVAL |
1002 | flag. Not specifying this flag would mean that the program would |
1003 | terminate immediately at the I<die> statement in the subroutine |
1004 | I<Subtract>. |
a0d0e21e |
1005 | |
1006 | =item 2. |
1007 | |
54310121 |
1008 | The code |
a0d0e21e |
1009 | |
c07a80fd |
1010 | if (SvTRUE(GvSV(errgv))) |
d1b91892 |
1011 | { |
c07a80fd |
1012 | printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ; |
d1b91892 |
1013 | POPs ; |
1014 | } |
a0d0e21e |
1015 | |
d1b91892 |
1016 | is the direct equivalent of this bit of Perl |
a0d0e21e |
1017 | |
d1b91892 |
1018 | print "Uh oh - $@\n" if $@ ; |
a0d0e21e |
1019 | |
c07a80fd |
1020 | C<errgv> is a perl global of type C<GV *> that points to the |
1021 | symbol table entry containing the error. C<GvSV(errgv)> therefore |
1022 | refers to the C equivalent of C<$@>. |
1023 | |
d1b91892 |
1024 | =item 3. |
a0d0e21e |
1025 | |
d1b91892 |
1026 | Note that the stack is popped using C<POPs> in the block where |
c07a80fd |
1027 | C<SvTRUE(GvSV(errgv))> is true. This is necessary because whenever a |
d1b91892 |
1028 | I<perl_call_*> function invoked with G_EVAL|G_SCALAR returns an error, |
5f05dabc |
1029 | the top of the stack holds the value I<undef>. Because we want the |
d1b91892 |
1030 | program to continue after detecting this error, it is essential that |
1031 | the stack is tidied up by removing the I<undef>. |
a0d0e21e |
1032 | |
1033 | =back |
1034 | |
1035 | |
c07a80fd |
1036 | =head2 Using G_KEEPERR |
1037 | |
1038 | Consider this rather facetious example, where we have used an XS |
1039 | version of the call_Subtract example above inside a destructor: |
1040 | |
1041 | package Foo; |
1042 | sub new { bless {}, $_[0] } |
54310121 |
1043 | sub Subtract { |
c07a80fd |
1044 | my($a,$b) = @_; |
1045 | die "death can be fatal" if $a < $b ; |
1046 | $a - $b; |
1047 | } |
1048 | sub DESTROY { call_Subtract(5, 4); } |
1049 | sub foo { die "foo dies"; } |
1050 | |
1051 | package main; |
1052 | eval { Foo->new->foo }; |
1053 | print "Saw: $@" if $@; # should be, but isn't |
1054 | |
1055 | This example will fail to recognize that an error occurred inside the |
1056 | C<eval {}>. Here's why: the call_Subtract code got executed while perl |
5f05dabc |
1057 | was cleaning up temporaries when exiting the eval block, and because |
c07a80fd |
1058 | call_Subtract is implemented with I<perl_call_pv> using the G_EVAL |
1059 | flag, it promptly reset C<$@>. This results in the failure of the |
1060 | outermost test for C<$@>, and thereby the failure of the error trap. |
1061 | |
1062 | Appending the G_KEEPERR flag, so that the I<perl_call_pv> call in |
1063 | call_Subtract reads: |
1064 | |
1065 | count = perl_call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR); |
1066 | |
1067 | will preserve the error and restore reliable error handling. |
1068 | |
d1b91892 |
1069 | =head2 Using perl_call_sv |
a0d0e21e |
1070 | |
d1b91892 |
1071 | In all the previous examples I have 'hard-wired' the name of the Perl |
1072 | subroutine to be called from C. Most of the time though, it is more |
1073 | convenient to be able to specify the name of the Perl subroutine from |
1074 | within the Perl script. |
a0d0e21e |
1075 | |
1076 | Consider the Perl code below |
1077 | |
d1b91892 |
1078 | sub fred |
1079 | { |
1080 | print "Hello there\n" ; |
1081 | } |
1082 | |
1083 | CallSubPV("fred") ; |
1084 | |
1085 | Here is a snippet of XSUB which defines I<CallSubPV>. |
1086 | |
1087 | void |
1088 | CallSubPV(name) |
1089 | char * name |
1090 | CODE: |
924508f0 |
1091 | PUSHMARK(SP) ; |
d1b91892 |
1092 | perl_call_pv(name, G_DISCARD|G_NOARGS) ; |
a0d0e21e |
1093 | |
54310121 |
1094 | That is fine as far as it goes. The thing is, the Perl subroutine |
5f05dabc |
1095 | can be specified as only a string. For Perl 4 this was adequate, |
d1b91892 |
1096 | but Perl 5 allows references to subroutines and anonymous subroutines. |
1097 | This is where I<perl_call_sv> is useful. |
1098 | |
1099 | The code below for I<CallSubSV> is identical to I<CallSubPV> except |
1100 | that the C<name> parameter is now defined as an SV* and we use |
1101 | I<perl_call_sv> instead of I<perl_call_pv>. |
1102 | |
1103 | void |
1104 | CallSubSV(name) |
1105 | SV * name |
1106 | CODE: |
924508f0 |
1107 | PUSHMARK(SP) ; |
d1b91892 |
1108 | perl_call_sv(name, G_DISCARD|G_NOARGS) ; |
a0d0e21e |
1109 | |
5f05dabc |
1110 | Because we are using an SV to call I<fred> the following can all be used |
a0d0e21e |
1111 | |
d1b91892 |
1112 | CallSubSV("fred") ; |
1113 | CallSubSV(\&fred) ; |
1114 | $ref = \&fred ; |
1115 | CallSubSV($ref) ; |
1116 | CallSubSV( sub { print "Hello there\n" } ) ; |
a0d0e21e |
1117 | |
d1b91892 |
1118 | As you can see, I<perl_call_sv> gives you much greater flexibility in |
1119 | how you can specify the Perl subroutine. |
1120 | |
1121 | You should note that if it is necessary to store the SV (C<name> in the |
1122 | example above) which corresponds to the Perl subroutine so that it can |
5f05dabc |
1123 | be used later in the program, it not enough just to store a copy of the |
d1b91892 |
1124 | pointer to the SV. Say the code above had been like this |
1125 | |
1126 | static SV * rememberSub ; |
1127 | |
1128 | void |
1129 | SaveSub1(name) |
1130 | SV * name |
1131 | CODE: |
1132 | rememberSub = name ; |
1133 | |
1134 | void |
1135 | CallSavedSub1() |
1136 | CODE: |
924508f0 |
1137 | PUSHMARK(SP) ; |
d1b91892 |
1138 | perl_call_sv(rememberSub, G_DISCARD|G_NOARGS) ; |
a0d0e21e |
1139 | |
d1b91892 |
1140 | The reason this is wrong is that by the time you come to use the |
1141 | pointer C<rememberSub> in C<CallSavedSub1>, it may or may not still refer |
1142 | to the Perl subroutine that was recorded in C<SaveSub1>. This is |
1143 | particularly true for these cases |
a0d0e21e |
1144 | |
d1b91892 |
1145 | SaveSub1(\&fred) ; |
1146 | CallSavedSub1() ; |
a0d0e21e |
1147 | |
d1b91892 |
1148 | SaveSub1( sub { print "Hello there\n" } ) ; |
1149 | CallSavedSub1() ; |
a0d0e21e |
1150 | |
d1b91892 |
1151 | By the time each of the C<SaveSub1> statements above have been executed, |
54310121 |
1152 | the SV*s which corresponded to the parameters will no longer exist. |
d1b91892 |
1153 | Expect an error message from Perl of the form |
a0d0e21e |
1154 | |
d1b91892 |
1155 | Can't use an undefined value as a subroutine reference at ... |
a0d0e21e |
1156 | |
d1b91892 |
1157 | for each of the C<CallSavedSub1> lines. |
a0d0e21e |
1158 | |
54310121 |
1159 | Similarly, with this code |
a0d0e21e |
1160 | |
d1b91892 |
1161 | $ref = \&fred ; |
1162 | SaveSub1($ref) ; |
1163 | $ref = 47 ; |
1164 | CallSavedSub1() ; |
a0d0e21e |
1165 | |
54310121 |
1166 | you can expect one of these messages (which you actually get is dependent on |
1167 | the version of Perl you are using) |
a0d0e21e |
1168 | |
d1b91892 |
1169 | Not a CODE reference at ... |
1170 | Undefined subroutine &main::47 called ... |
a0d0e21e |
1171 | |
d1b91892 |
1172 | The variable C<$ref> may have referred to the subroutine C<fred> |
1173 | whenever the call to C<SaveSub1> was made but by the time |
5f05dabc |
1174 | C<CallSavedSub1> gets called it now holds the number C<47>. Because we |
d1b91892 |
1175 | saved only a pointer to the original SV in C<SaveSub1>, any changes to |
1176 | C<$ref> will be tracked by the pointer C<rememberSub>. This means that |
1177 | whenever C<CallSavedSub1> gets called, it will attempt to execute the |
1178 | code which is referenced by the SV* C<rememberSub>. In this case |
1179 | though, it now refers to the integer C<47>, so expect Perl to complain |
1180 | loudly. |
a0d0e21e |
1181 | |
d1b91892 |
1182 | A similar but more subtle problem is illustrated with this code |
a0d0e21e |
1183 | |
d1b91892 |
1184 | $ref = \&fred ; |
1185 | SaveSub1($ref) ; |
1186 | $ref = \&joe ; |
1187 | CallSavedSub1() ; |
a0d0e21e |
1188 | |
d1b91892 |
1189 | This time whenever C<CallSavedSub1> get called it will execute the Perl |
54310121 |
1190 | subroutine C<joe> (assuming it exists) rather than C<fred> as was |
d1b91892 |
1191 | originally requested in the call to C<SaveSub1>. |
a0d0e21e |
1192 | |
d1b91892 |
1193 | To get around these problems it is necessary to take a full copy of the |
1194 | SV. The code below shows C<SaveSub2> modified to do that |
a0d0e21e |
1195 | |
d1b91892 |
1196 | static SV * keepSub = (SV*)NULL ; |
1197 | |
1198 | void |
1199 | SaveSub2(name) |
1200 | SV * name |
1201 | CODE: |
1202 | /* Take a copy of the callback */ |
1203 | if (keepSub == (SV*)NULL) |
1204 | /* First time, so create a new SV */ |
1205 | keepSub = newSVsv(name) ; |
1206 | else |
1207 | /* Been here before, so overwrite */ |
1208 | SvSetSV(keepSub, name) ; |
1209 | |
1210 | void |
1211 | CallSavedSub2() |
1212 | CODE: |
924508f0 |
1213 | PUSHMARK(SP) ; |
d1b91892 |
1214 | perl_call_sv(keepSub, G_DISCARD|G_NOARGS) ; |
1215 | |
5f05dabc |
1216 | To avoid creating a new SV every time C<SaveSub2> is called, |
d1b91892 |
1217 | the function first checks to see if it has been called before. If not, |
1218 | then space for a new SV is allocated and the reference to the Perl |
1219 | subroutine, C<name> is copied to the variable C<keepSub> in one |
1220 | operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called |
1221 | the existing SV, C<keepSub>, is overwritten with the new value using |
1222 | C<SvSetSV>. |
1223 | |
1224 | =head2 Using perl_call_argv |
1225 | |
1226 | Here is a Perl subroutine which prints whatever parameters are passed |
1227 | to it. |
1228 | |
1229 | sub PrintList |
1230 | { |
1231 | my(@list) = @_ ; |
1232 | |
1233 | foreach (@list) { print "$_\n" } |
1234 | } |
1235 | |
1236 | and here is an example of I<perl_call_argv> which will call |
1237 | I<PrintList>. |
1238 | |
1239 | static char * words[] = {"alpha", "beta", "gamma", "delta", NULL} ; |
1240 | |
1241 | static void |
1242 | call_PrintList() |
1243 | { |
1244 | dSP ; |
1245 | |
1246 | perl_call_argv("PrintList", G_DISCARD, words) ; |
1247 | } |
1248 | |
1249 | Note that it is not necessary to call C<PUSHMARK> in this instance. |
1250 | This is because I<perl_call_argv> will do it for you. |
1251 | |
1252 | =head2 Using perl_call_method |
a0d0e21e |
1253 | |
1254 | Consider the following Perl code |
1255 | |
d1b91892 |
1256 | { |
1257 | package Mine ; |
1258 | |
1259 | sub new |
1260 | { |
1261 | my($type) = shift ; |
1262 | bless [@_] |
1263 | } |
1264 | |
1265 | sub Display |
1266 | { |
1267 | my ($self, $index) = @_ ; |
1268 | print "$index: $$self[$index]\n" ; |
1269 | } |
1270 | |
1271 | sub PrintID |
1272 | { |
1273 | my($class) = @_ ; |
1274 | print "This is Class $class version 1.0\n" ; |
1275 | } |
1276 | } |
1277 | |
5f05dabc |
1278 | It implements just a very simple class to manage an array. Apart from |
d1b91892 |
1279 | the constructor, C<new>, it declares methods, one static and one |
5f05dabc |
1280 | virtual. The static method, C<PrintID>, prints out simply the class |
d1b91892 |
1281 | name and a version number. The virtual method, C<Display>, prints out a |
1282 | single element of the array. Here is an all Perl example of using it. |
1283 | |
1284 | $a = new Mine ('red', 'green', 'blue') ; |
1285 | $a->Display(1) ; |
1286 | PrintID Mine; |
a0d0e21e |
1287 | |
d1b91892 |
1288 | will print |
a0d0e21e |
1289 | |
d1b91892 |
1290 | 1: green |
54310121 |
1291 | This is Class Mine version 1.0 |
a0d0e21e |
1292 | |
d1b91892 |
1293 | Calling a Perl method from C is fairly straightforward. The following |
1294 | things are required |
a0d0e21e |
1295 | |
d1b91892 |
1296 | =over 5 |
1297 | |
1298 | =item * |
1299 | |
1300 | a reference to the object for a virtual method or the name of the class |
1301 | for a static method. |
1302 | |
1303 | =item * |
1304 | |
1305 | the name of the method. |
1306 | |
1307 | =item * |
1308 | |
1309 | any other parameters specific to the method. |
1310 | |
1311 | =back |
1312 | |
1313 | Here is a simple XSUB which illustrates the mechanics of calling both |
1314 | the C<PrintID> and C<Display> methods from C. |
1315 | |
1316 | void |
1317 | call_Method(ref, method, index) |
1318 | SV * ref |
1319 | char * method |
1320 | int index |
1321 | CODE: |
924508f0 |
1322 | PUSHMARK(SP); |
d1b91892 |
1323 | XPUSHs(ref); |
1324 | XPUSHs(sv_2mortal(newSViv(index))) ; |
1325 | PUTBACK; |
1326 | |
1327 | perl_call_method(method, G_DISCARD) ; |
1328 | |
1329 | void |
1330 | call_PrintID(class, method) |
1331 | char * class |
1332 | char * method |
1333 | CODE: |
924508f0 |
1334 | PUSHMARK(SP); |
d1b91892 |
1335 | XPUSHs(sv_2mortal(newSVpv(class, 0))) ; |
1336 | PUTBACK; |
1337 | |
1338 | perl_call_method(method, G_DISCARD) ; |
1339 | |
1340 | |
1341 | So the methods C<PrintID> and C<Display> can be invoked like this |
1342 | |
1343 | $a = new Mine ('red', 'green', 'blue') ; |
1344 | call_Method($a, 'Display', 1) ; |
1345 | call_PrintID('Mine', 'PrintID') ; |
1346 | |
1347 | The only thing to note is that in both the static and virtual methods, |
1348 | the method name is not passed via the stack - it is used as the first |
1349 | parameter to I<perl_call_method>. |
1350 | |
54310121 |
1351 | =head2 Using GIMME_V |
d1b91892 |
1352 | |
54310121 |
1353 | Here is a trivial XSUB which prints the context in which it is |
d1b91892 |
1354 | currently executing. |
1355 | |
1356 | void |
1357 | PrintContext() |
1358 | CODE: |
54310121 |
1359 | I32 gimme = GIMME_V; |
1360 | if (gimme == G_VOID) |
1361 | printf ("Context is Void\n") ; |
1362 | else if (gimme == G_SCALAR) |
d1b91892 |
1363 | printf ("Context is Scalar\n") ; |
1364 | else |
1365 | printf ("Context is Array\n") ; |
1366 | |
1367 | and here is some Perl to test it |
1368 | |
54310121 |
1369 | PrintContext ; |
d1b91892 |
1370 | $a = PrintContext ; |
1371 | @a = PrintContext ; |
1372 | |
1373 | The output from that will be |
1374 | |
54310121 |
1375 | Context is Void |
d1b91892 |
1376 | Context is Scalar |
1377 | Context is Array |
1378 | |
1379 | =head2 Using Perl to dispose of temporaries |
1380 | |
1381 | In the examples given to date, any temporaries created in the callback |
5f05dabc |
1382 | (i.e., parameters passed on the stack to the I<perl_call_*> function or |
d1b91892 |
1383 | values returned via the stack) have been freed by one of these methods |
1384 | |
1385 | =over 5 |
1386 | |
1387 | =item * |
1388 | |
1389 | specifying the G_DISCARD flag with I<perl_call_*>. |
1390 | |
1391 | =item * |
1392 | |
1393 | explicitly disposed of using the C<ENTER>/C<SAVETMPS> - |
1394 | C<FREETMPS>/C<LEAVE> pairing. |
1395 | |
1396 | =back |
1397 | |
1398 | There is another method which can be used, namely letting Perl do it |
1399 | for you automatically whenever it regains control after the callback |
1400 | has terminated. This is done by simply not using the |
1401 | |
1402 | ENTER ; |
1403 | SAVETMPS ; |
1404 | ... |
1405 | FREETMPS ; |
1406 | LEAVE ; |
1407 | |
1408 | sequence in the callback (and not, of course, specifying the G_DISCARD |
1409 | flag). |
1410 | |
1411 | If you are going to use this method you have to be aware of a possible |
1412 | memory leak which can arise under very specific circumstances. To |
1413 | explain these circumstances you need to know a bit about the flow of |
1414 | control between Perl and the callback routine. |
1415 | |
1416 | The examples given at the start of the document (an error handler and |
1417 | an event driven program) are typical of the two main sorts of flow |
1418 | control that you are likely to encounter with callbacks. There is a |
1419 | very important distinction between them, so pay attention. |
1420 | |
1421 | In the first example, an error handler, the flow of control could be as |
1422 | follows. You have created an interface to an external library. |
1423 | Control can reach the external library like this |
1424 | |
1425 | perl --> XSUB --> external library |
1426 | |
1427 | Whilst control is in the library, an error condition occurs. You have |
1428 | previously set up a Perl callback to handle this situation, so it will |
1429 | get executed. Once the callback has finished, control will drop back to |
1430 | Perl again. Here is what the flow of control will be like in that |
1431 | situation |
1432 | |
1433 | perl --> XSUB --> external library |
1434 | ... |
1435 | error occurs |
1436 | ... |
1437 | external library --> perl_call --> perl |
1438 | | |
1439 | perl <-- XSUB <-- external library <-- perl_call <----+ |
1440 | |
1441 | After processing of the error using I<perl_call_*> is completed, |
1442 | control reverts back to Perl more or less immediately. |
1443 | |
1444 | In the diagram, the further right you go the more deeply nested the |
1445 | scope is. It is only when control is back with perl on the extreme |
1446 | left of the diagram that you will have dropped back to the enclosing |
1447 | scope and any temporaries you have left hanging around will be freed. |
1448 | |
1449 | In the second example, an event driven program, the flow of control |
1450 | will be more like this |
1451 | |
1452 | perl --> XSUB --> event handler |
1453 | ... |
54310121 |
1454 | event handler --> perl_call --> perl |
d1b91892 |
1455 | | |
54310121 |
1456 | event handler <-- perl_call <----+ |
d1b91892 |
1457 | ... |
54310121 |
1458 | event handler --> perl_call --> perl |
d1b91892 |
1459 | | |
54310121 |
1460 | event handler <-- perl_call <----+ |
d1b91892 |
1461 | ... |
54310121 |
1462 | event handler --> perl_call --> perl |
d1b91892 |
1463 | | |
54310121 |
1464 | event handler <-- perl_call <----+ |
d1b91892 |
1465 | |
1466 | In this case the flow of control can consist of only the repeated |
1467 | sequence |
1468 | |
1469 | event handler --> perl_call --> perl |
1470 | |
54310121 |
1471 | for practically the complete duration of the program. This means that |
1472 | control may I<never> drop back to the surrounding scope in Perl at the |
1473 | extreme left. |
d1b91892 |
1474 | |
1475 | So what is the big problem? Well, if you are expecting Perl to tidy up |
1476 | those temporaries for you, you might be in for a long wait. For Perl |
5f05dabc |
1477 | to dispose of your temporaries, control must drop back to the |
d1b91892 |
1478 | enclosing scope at some stage. In the event driven scenario that may |
1479 | never happen. This means that as time goes on, your program will |
1480 | create more and more temporaries, none of which will ever be freed. As |
1481 | each of these temporaries consumes some memory your program will |
1482 | eventually consume all the available memory in your system - kapow! |
1483 | |
1484 | So here is the bottom line - if you are sure that control will revert |
1485 | back to the enclosing Perl scope fairly quickly after the end of your |
5f05dabc |
1486 | callback, then it isn't absolutely necessary to dispose explicitly of |
d1b91892 |
1487 | any temporaries you may have created. Mind you, if you are at all |
1488 | uncertain about what to do, it doesn't do any harm to tidy up anyway. |
1489 | |
1490 | |
1491 | =head2 Strategies for storing Callback Context Information |
1492 | |
1493 | |
1494 | Potentially one of the trickiest problems to overcome when designing a |
1495 | callback interface can be figuring out how to store the mapping between |
1496 | the C callback function and the Perl equivalent. |
1497 | |
1498 | To help understand why this can be a real problem first consider how a |
1499 | callback is set up in an all C environment. Typically a C API will |
1500 | provide a function to register a callback. This will expect a pointer |
1501 | to a function as one of its parameters. Below is a call to a |
1502 | hypothetical function C<register_fatal> which registers the C function |
1503 | to get called when a fatal error occurs. |
1504 | |
1505 | register_fatal(cb1) ; |
1506 | |
1507 | The single parameter C<cb1> is a pointer to a function, so you must |
1508 | have defined C<cb1> in your code, say something like this |
1509 | |
1510 | static void |
1511 | cb1() |
1512 | { |
1513 | printf ("Fatal Error\n") ; |
1514 | exit(1) ; |
1515 | } |
1516 | |
1517 | Now change that to call a Perl subroutine instead |
1518 | |
1519 | static SV * callback = (SV*)NULL; |
1520 | |
1521 | static void |
1522 | cb1() |
1523 | { |
1524 | dSP ; |
1525 | |
924508f0 |
1526 | PUSHMARK(SP) ; |
d1b91892 |
1527 | |
1528 | /* Call the Perl sub to process the callback */ |
1529 | perl_call_sv(callback, G_DISCARD) ; |
1530 | } |
1531 | |
1532 | |
1533 | void |
1534 | register_fatal(fn) |
1535 | SV * fn |
1536 | CODE: |
1537 | /* Remember the Perl sub */ |
1538 | if (callback == (SV*)NULL) |
1539 | callback = newSVsv(fn) ; |
1540 | else |
1541 | SvSetSV(callback, fn) ; |
1542 | |
1543 | /* register the callback with the external library */ |
1544 | register_fatal(cb1) ; |
1545 | |
1546 | where the Perl equivalent of C<register_fatal> and the callback it |
1547 | registers, C<pcb1>, might look like this |
1548 | |
1549 | # Register the sub pcb1 |
1550 | register_fatal(\&pcb1) ; |
1551 | |
1552 | sub pcb1 |
1553 | { |
1554 | die "I'm dying...\n" ; |
1555 | } |
1556 | |
1557 | The mapping between the C callback and the Perl equivalent is stored in |
1558 | the global variable C<callback>. |
1559 | |
5f05dabc |
1560 | This will be adequate if you ever need to have only one callback |
d1b91892 |
1561 | registered at any time. An example could be an error handler like the |
1562 | code sketched out above. Remember though, repeated calls to |
1563 | C<register_fatal> will replace the previously registered callback |
1564 | function with the new one. |
1565 | |
1566 | Say for example you want to interface to a library which allows asynchronous |
1567 | file i/o. In this case you may be able to register a callback whenever |
1568 | a read operation has completed. To be of any use we want to be able to |
1569 | call separate Perl subroutines for each file that is opened. As it |
1570 | stands, the error handler example above would not be adequate as it |
1571 | allows only a single callback to be defined at any time. What we |
1572 | require is a means of storing the mapping between the opened file and |
1573 | the Perl subroutine we want to be called for that file. |
1574 | |
1575 | Say the i/o library has a function C<asynch_read> which associates a C |
1576 | function C<ProcessRead> with a file handle C<fh> - this assumes that it |
1577 | has also provided some routine to open the file and so obtain the file |
1578 | handle. |
1579 | |
1580 | asynch_read(fh, ProcessRead) |
1581 | |
1582 | This may expect the C I<ProcessRead> function of this form |
1583 | |
1584 | void |
1585 | ProcessRead(fh, buffer) |
1586 | int fh ; |
1587 | char * buffer ; |
1588 | { |
54310121 |
1589 | ... |
d1b91892 |
1590 | } |
1591 | |
1592 | To provide a Perl interface to this library we need to be able to map |
1593 | between the C<fh> parameter and the Perl subroutine we want called. A |
1594 | hash is a convenient mechanism for storing this mapping. The code |
1595 | below shows a possible implementation |
1596 | |
1597 | static HV * Mapping = (HV*)NULL ; |
a0d0e21e |
1598 | |
d1b91892 |
1599 | void |
1600 | asynch_read(fh, callback) |
1601 | int fh |
1602 | SV * callback |
1603 | CODE: |
1604 | /* If the hash doesn't already exist, create it */ |
1605 | if (Mapping == (HV*)NULL) |
1606 | Mapping = newHV() ; |
1607 | |
1608 | /* Save the fh -> callback mapping */ |
1609 | hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0) ; |
1610 | |
1611 | /* Register with the C Library */ |
1612 | asynch_read(fh, asynch_read_if) ; |
1613 | |
1614 | and C<asynch_read_if> could look like this |
1615 | |
1616 | static void |
1617 | asynch_read_if(fh, buffer) |
1618 | int fh ; |
1619 | char * buffer ; |
1620 | { |
1621 | dSP ; |
1622 | SV ** sv ; |
1623 | |
1624 | /* Get the callback associated with fh */ |
1625 | sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE) ; |
1626 | if (sv == (SV**)NULL) |
1627 | croak("Internal error...\n") ; |
1628 | |
924508f0 |
1629 | PUSHMARK(SP) ; |
d1b91892 |
1630 | XPUSHs(sv_2mortal(newSViv(fh))) ; |
1631 | XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ; |
1632 | PUTBACK ; |
1633 | |
1634 | /* Call the Perl sub */ |
1635 | perl_call_sv(*sv, G_DISCARD) ; |
1636 | } |
1637 | |
1638 | For completeness, here is C<asynch_close>. This shows how to remove |
1639 | the entry from the hash C<Mapping>. |
1640 | |
1641 | void |
1642 | asynch_close(fh) |
1643 | int fh |
1644 | CODE: |
1645 | /* Remove the entry from the hash */ |
1646 | (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD) ; |
a0d0e21e |
1647 | |
d1b91892 |
1648 | /* Now call the real asynch_close */ |
1649 | asynch_close(fh) ; |
a0d0e21e |
1650 | |
d1b91892 |
1651 | So the Perl interface would look like this |
1652 | |
1653 | sub callback1 |
1654 | { |
1655 | my($handle, $buffer) = @_ ; |
1656 | } |
a0d0e21e |
1657 | |
d1b91892 |
1658 | # Register the Perl callback |
1659 | asynch_read($fh, \&callback1) ; |
a0d0e21e |
1660 | |
d1b91892 |
1661 | asynch_close($fh) ; |
1662 | |
1663 | The mapping between the C callback and Perl is stored in the global |
1664 | hash C<Mapping> this time. Using a hash has the distinct advantage that |
1665 | it allows an unlimited number of callbacks to be registered. |
1666 | |
1667 | What if the interface provided by the C callback doesn't contain a |
1668 | parameter which allows the file handle to Perl subroutine mapping? Say |
1669 | in the asynchronous i/o package, the callback function gets passed only |
1670 | the C<buffer> parameter like this |
1671 | |
1672 | void |
1673 | ProcessRead(buffer) |
1674 | char * buffer ; |
1675 | { |
1676 | ... |
1677 | } |
a0d0e21e |
1678 | |
d1b91892 |
1679 | Without the file handle there is no straightforward way to map from the |
1680 | C callback to the Perl subroutine. |
a0d0e21e |
1681 | |
54310121 |
1682 | In this case a possible way around this problem is to predefine a |
d1b91892 |
1683 | series of C functions to act as the interface to Perl, thus |
1684 | |
1685 | #define MAX_CB 3 |
1686 | #define NULL_HANDLE -1 |
1687 | typedef void (*FnMap)() ; |
1688 | |
1689 | struct MapStruct { |
1690 | FnMap Function ; |
1691 | SV * PerlSub ; |
1692 | int Handle ; |
1693 | } ; |
1694 | |
1695 | static void fn1() ; |
1696 | static void fn2() ; |
1697 | static void fn3() ; |
1698 | |
1699 | static struct MapStruct Map [MAX_CB] = |
1700 | { |
1701 | { fn1, NULL, NULL_HANDLE }, |
1702 | { fn2, NULL, NULL_HANDLE }, |
1703 | { fn3, NULL, NULL_HANDLE } |
1704 | } ; |
1705 | |
1706 | static void |
1707 | Pcb(index, buffer) |
1708 | int index ; |
1709 | char * buffer ; |
1710 | { |
1711 | dSP ; |
1712 | |
924508f0 |
1713 | PUSHMARK(SP) ; |
d1b91892 |
1714 | XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ; |
1715 | PUTBACK ; |
1716 | |
1717 | /* Call the Perl sub */ |
1718 | perl_call_sv(Map[index].PerlSub, G_DISCARD) ; |
1719 | } |
1720 | |
1721 | static void |
1722 | fn1(buffer) |
1723 | char * buffer ; |
1724 | { |
1725 | Pcb(0, buffer) ; |
1726 | } |
1727 | |
1728 | static void |
1729 | fn2(buffer) |
1730 | char * buffer ; |
1731 | { |
1732 | Pcb(1, buffer) ; |
1733 | } |
1734 | |
1735 | static void |
1736 | fn3(buffer) |
1737 | char * buffer ; |
1738 | { |
1739 | Pcb(2, buffer) ; |
1740 | } |
1741 | |
1742 | void |
1743 | array_asynch_read(fh, callback) |
1744 | int fh |
1745 | SV * callback |
1746 | CODE: |
1747 | int index ; |
1748 | int null_index = MAX_CB ; |
1749 | |
1750 | /* Find the same handle or an empty entry */ |
1751 | for (index = 0 ; index < MAX_CB ; ++index) |
1752 | { |
1753 | if (Map[index].Handle == fh) |
1754 | break ; |
1755 | |
1756 | if (Map[index].Handle == NULL_HANDLE) |
1757 | null_index = index ; |
1758 | } |
1759 | |
1760 | if (index == MAX_CB && null_index == MAX_CB) |
1761 | croak ("Too many callback functions registered\n") ; |
1762 | |
1763 | if (index == MAX_CB) |
1764 | index = null_index ; |
1765 | |
1766 | /* Save the file handle */ |
1767 | Map[index].Handle = fh ; |
1768 | |
1769 | /* Remember the Perl sub */ |
1770 | if (Map[index].PerlSub == (SV*)NULL) |
1771 | Map[index].PerlSub = newSVsv(callback) ; |
1772 | else |
1773 | SvSetSV(Map[index].PerlSub, callback) ; |
1774 | |
1775 | asynch_read(fh, Map[index].Function) ; |
1776 | |
1777 | void |
1778 | array_asynch_close(fh) |
1779 | int fh |
1780 | CODE: |
1781 | int index ; |
1782 | |
1783 | /* Find the file handle */ |
1784 | for (index = 0; index < MAX_CB ; ++ index) |
1785 | if (Map[index].Handle == fh) |
1786 | break ; |
1787 | |
1788 | if (index == MAX_CB) |
1789 | croak ("could not close fh %d\n", fh) ; |
1790 | |
1791 | Map[index].Handle = NULL_HANDLE ; |
1792 | SvREFCNT_dec(Map[index].PerlSub) ; |
1793 | Map[index].PerlSub = (SV*)NULL ; |
1794 | |
1795 | asynch_close(fh) ; |
1796 | |
5f05dabc |
1797 | In this case the functions C<fn1>, C<fn2>, and C<fn3> are used to |
d1b91892 |
1798 | remember the Perl subroutine to be called. Each of the functions holds |
4a6725af |
1799 | a separate hard-wired index which is used in the function C<Pcb> to |
d1b91892 |
1800 | access the C<Map> array and actually call the Perl subroutine. |
1801 | |
1802 | There are some obvious disadvantages with this technique. |
1803 | |
1804 | Firstly, the code is considerably more complex than with the previous |
1805 | example. |
1806 | |
4a6725af |
1807 | Secondly, there is a hard-wired limit (in this case 3) to the number of |
d1b91892 |
1808 | callbacks that can exist simultaneously. The only way to increase the |
1809 | limit is by modifying the code to add more functions and then |
54310121 |
1810 | recompiling. None the less, as long as the number of functions is |
d1b91892 |
1811 | chosen with some care, it is still a workable solution and in some |
1812 | cases is the only one available. |
1813 | |
1814 | To summarize, here are a number of possible methods for you to consider |
1815 | for storing the mapping between C and the Perl callback |
1816 | |
1817 | =over 5 |
1818 | |
1819 | =item 1. Ignore the problem - Allow only 1 callback |
1820 | |
1821 | For a lot of situations, like interfacing to an error handler, this may |
1822 | be a perfectly adequate solution. |
1823 | |
1824 | =item 2. Create a sequence of callbacks - hard wired limit |
1825 | |
1826 | If it is impossible to tell from the parameters passed back from the C |
1827 | callback what the context is, then you may need to create a sequence of C |
1828 | callback interface functions, and store pointers to each in an array. |
1829 | |
1830 | =item 3. Use a parameter to map to the Perl callback |
1831 | |
1832 | A hash is an ideal mechanism to store the mapping between C and Perl. |
1833 | |
1834 | =back |
a0d0e21e |
1835 | |
a0d0e21e |
1836 | |
1837 | =head2 Alternate Stack Manipulation |
1838 | |
a0d0e21e |
1839 | |
d1b91892 |
1840 | Although I have made use of only the C<POP*> macros to access values |
1841 | returned from Perl subroutines, it is also possible to bypass these |
8e07c86e |
1842 | macros and read the stack using the C<ST> macro (See L<perlxs> for a |
d1b91892 |
1843 | full description of the C<ST> macro). |
1844 | |
1845 | Most of the time the C<POP*> macros should be adequate, the main |
1846 | problem with them is that they force you to process the returned values |
1847 | in sequence. This may not be the most suitable way to process the |
1848 | values in some cases. What we want is to be able to access the stack in |
1849 | a random order. The C<ST> macro as used when coding an XSUB is ideal |
1850 | for this purpose. |
1851 | |
1852 | The code below is the example given in the section I<Returning a list |
1853 | of values> recoded to use C<ST> instead of C<POP*>. |
1854 | |
1855 | static void |
1856 | call_AddSubtract2(a, b) |
1857 | int a ; |
1858 | int b ; |
1859 | { |
1860 | dSP ; |
1861 | I32 ax ; |
1862 | int count ; |
1863 | |
1864 | ENTER ; |
1865 | SAVETMPS; |
1866 | |
924508f0 |
1867 | PUSHMARK(SP) ; |
d1b91892 |
1868 | XPUSHs(sv_2mortal(newSViv(a))); |
1869 | XPUSHs(sv_2mortal(newSViv(b))); |
1870 | PUTBACK ; |
1871 | |
1872 | count = perl_call_pv("AddSubtract", G_ARRAY); |
1873 | |
1874 | SPAGAIN ; |
924508f0 |
1875 | SP -= count ; |
1876 | ax = (SP - stack_base) + 1 ; |
d1b91892 |
1877 | |
1878 | if (count != 2) |
1879 | croak("Big trouble\n") ; |
a0d0e21e |
1880 | |
d1b91892 |
1881 | printf ("%d + %d = %d\n", a, b, SvIV(ST(0))) ; |
1882 | printf ("%d - %d = %d\n", a, b, SvIV(ST(1))) ; |
1883 | |
1884 | PUTBACK ; |
1885 | FREETMPS ; |
1886 | LEAVE ; |
1887 | } |
1888 | |
1889 | Notes |
1890 | |
1891 | =over 5 |
1892 | |
1893 | =item 1. |
1894 | |
1895 | Notice that it was necessary to define the variable C<ax>. This is |
1896 | because the C<ST> macro expects it to exist. If we were in an XSUB it |
1897 | would not be necessary to define C<ax> as it is already defined for |
1898 | you. |
1899 | |
1900 | =item 2. |
1901 | |
1902 | The code |
1903 | |
1904 | SPAGAIN ; |
924508f0 |
1905 | SP -= count ; |
1906 | ax = (SP - stack_base) + 1 ; |
d1b91892 |
1907 | |
1908 | sets the stack up so that we can use the C<ST> macro. |
1909 | |
1910 | =item 3. |
1911 | |
1912 | Unlike the original coding of this example, the returned |
1913 | values are not accessed in reverse order. So C<ST(0)> refers to the |
54310121 |
1914 | first value returned by the Perl subroutine and C<ST(count-1)> |
d1b91892 |
1915 | refers to the last. |
1916 | |
1917 | =back |
a0d0e21e |
1918 | |
8f183262 |
1919 | =head2 Creating and calling an anonymous subroutine in C |
1920 | |
1921 | As we've already shown, L<perl_call_sv> can be used to invoke an |
1922 | anonymous subroutine. However, our example showed how Perl script |
1923 | invoking an XSUB to preform this operation. Let's see how it can be |
1924 | done inside our C code: |
1925 | |
8f183262 |
1926 | ... |
1927 | |
137443ea |
1928 | SV *cvrv = perl_eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE); |
8f183262 |
1929 | |
1930 | ... |
1931 | |
1932 | perl_call_sv(cvrv, G_VOID|G_NOARGS); |
1933 | |
137443ea |
1934 | L<perlguts/perl_eval_pv> is used to compile the anonymous subroutine, which |
1935 | will be the return value as well. Once this code reference is in hand, it |
8f183262 |
1936 | can be mixed in with all the previous examples we've shown. |
1937 | |
a0d0e21e |
1938 | =head1 SEE ALSO |
1939 | |
8e07c86e |
1940 | L<perlxs>, L<perlguts>, L<perlembed> |
a0d0e21e |
1941 | |
1942 | =head1 AUTHOR |
1943 | |
9607fc9c |
1944 | Paul Marquess <F<pmarquess@bfsec.bt.co.uk>> |
a0d0e21e |
1945 | |
d1b91892 |
1946 | Special thanks to the following people who assisted in the creation of |
1947 | the document. |
a0d0e21e |
1948 | |
c07a80fd |
1949 | Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy |
1950 | and Larry Wall. |
a0d0e21e |
1951 | |
1952 | =head1 DATE |
1953 | |
137443ea |
1954 | Version 1.3, 14th Apr 1997 |