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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 |
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 |
32 | your register functions to be called whenever specific events occur, |
33 | e.g. a mouse button is pressed, the cursor moves into a window or a |
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> |
64 | to actually invoke the Perl subroutine. |
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 |
87 | subroutine you want to call, e.g. C<perl_call_pv("fred", 0)>. If the |
88 | subroutine you want to call is in another package, just include the |
89 | package name in the string, e.g. C<"pkg::fred">. |
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 | |
129 | =head2 G_SCALAR |
130 | |
131 | Calls the Perl subroutine in a scalar context. This is the default |
132 | context flag setting for all the I<perl_call_*> functions. |
133 | |
134 | This flag has 2 effects |
135 | |
136 | =over 5 |
137 | |
138 | =item 1. |
139 | |
140 | it indicates to the subroutine being called that it is executing in a |
141 | scalar context (if it executes I<wantarray> the result will be false). |
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142 | |
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143 | |
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144 | =item 2. |
145 | |
146 | it ensures that only a scalar is actually returned from the subroutine. |
147 | The subroutine can, of course, ignore the I<wantarray> and return a |
148 | list anyway. If so, then only the last element of the list will be |
149 | returned. |
150 | |
151 | =back |
152 | |
153 | The value returned by the I<perl_call_*> function indicates how may |
154 | items have been returned by the Perl subroutine - in this case it will |
155 | be either 0 or 1. |
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156 | |
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157 | If 0, then you have specified the G_DISCARD flag. |
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158 | |
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159 | If 1, then the item actually returned by the Perl subroutine will be |
160 | stored on the Perl stack - the section I<Returning a Scalar> shows how |
161 | to access this value on the stack. Remember that regardless of how |
162 | many items the Perl subroutine returns, only the last one will be |
163 | accessible from the stack - think of the case where only one value is |
164 | returned as being a list with only one element. Any other items that |
165 | were returned will not exist by the time control returns from the |
166 | I<perl_call_*> function. The section I<Returning a list in a scalar |
167 | context> shows an example of this behaviour. |
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168 | |
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169 | |
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170 | =head2 G_ARRAY |
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171 | |
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172 | Calls the Perl subroutine in a list context. |
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173 | |
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174 | As with G_SCALAR, this flag has 2 effects |
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175 | |
176 | =over 5 |
177 | |
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178 | =item 1. |
179 | |
180 | it indicates to the subroutine being called that it is executing in an |
181 | array context (if it executes I<wantarray> the result will be true). |
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182 | |
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183 | |
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184 | =item 2. |
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185 | |
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186 | it ensures that all items returned from the subroutine will be |
187 | accessible when control returns from the I<perl_call_*> function. |
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188 | |
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189 | =back |
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190 | |
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191 | The value returned by the I<perl_call_*> function indicates how may |
192 | items have been returned by the Perl subroutine. |
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193 | |
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194 | If 0, the you have specified the G_DISCARD flag. |
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195 | |
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196 | If not 0, then it will be a count of the number of items returned by |
197 | the subroutine. These items will be stored on the Perl stack. The |
198 | section I<Returning a list of values> gives an example of using the |
199 | G_ARRAY flag and the mechanics of accessing the returned items from the |
200 | Perl stack. |
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201 | |
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202 | =head2 G_DISCARD |
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203 | |
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204 | By default, the I<perl_call_*> functions place the items returned from |
205 | by the Perl subroutine on the stack. If you are not interested in |
206 | these items, then setting this flag will make Perl get rid of them |
207 | automatically for you. Note that it is still possible to indicate a |
208 | context to the Perl subroutine by using either G_SCALAR or G_ARRAY. |
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209 | |
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210 | If you do not set this flag then it is I<very> important that you make |
211 | sure that any temporaries (i.e. parameters passed to the Perl |
212 | subroutine and values returned from the subroutine) are disposed of |
213 | yourself. The section I<Returning a Scalar> gives details of how to |
214 | explicitly dispose of these temporaries and the section I<Using Perl to |
215 | dispose of temporaries> discusses the specific circumstances where you |
216 | can ignore the problem and let Perl deal with it for you. |
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217 | |
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218 | =head2 G_NOARGS |
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219 | |
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220 | Whenever a Perl subroutine is called using one of the I<perl_call_*> |
221 | functions, it is assumed by default that parameters are to be passed to |
222 | the subroutine. If you are not passing any parameters to the Perl |
223 | subroutine, you can save a bit of time by setting this flag. It has |
224 | the effect of not creating the C<@_> array for the Perl subroutine. |
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225 | |
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226 | Although the functionality provided by this flag may seem |
227 | straightforward, it should be used only if there is a good reason to do |
228 | so. The reason for being cautious is that even if you have specified |
229 | the G_NOARGS flag, it is still possible for the Perl subroutine that |
230 | has been called to think that you have passed it parameters. |
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231 | |
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232 | In fact, what can happen is that the Perl subroutine you have called |
233 | can access the C<@_> array from a previous Perl subroutine. This will |
234 | occur when the code that is executing the I<perl_call_*> function has |
235 | itself been called from another Perl subroutine. The code below |
236 | illustrates this |
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237 | |
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238 | sub fred |
239 | { print "@_\n" } |
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240 | |
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241 | sub joe |
242 | { &fred } |
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243 | |
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244 | &joe(1,2,3) ; |
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245 | |
246 | This will print |
247 | |
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248 | 1 2 3 |
249 | |
250 | What has happened is that C<fred> accesses the C<@_> array which |
251 | belongs to C<joe>. |
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252 | |
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253 | |
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254 | =head2 G_EVAL |
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255 | |
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256 | It is possible for the Perl subroutine you are calling to terminate |
257 | abnormally, e.g. by calling I<die> explicitly or by not actually |
258 | existing. By default, when either of these of events occurs, the |
259 | process will terminate immediately. If though, you want to trap this |
260 | type of event, specify the G_EVAL flag. It will put an I<eval { }> |
261 | around the subroutine call. |
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262 | |
263 | Whenever control returns from the I<perl_call_*> function you need to |
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264 | check the C<$@> variable as you would in a normal Perl script. |
265 | |
266 | The value returned from the I<perl_call_*> function is dependent on |
267 | what other flags have been specified and whether an error has |
268 | occurred. Here are all the different cases that can occur |
269 | |
270 | =over 5 |
271 | |
272 | =item * |
273 | |
274 | If the I<perl_call_*> function returns normally, then the value |
275 | returned is as specified in the previous sections. |
276 | |
277 | =item * |
278 | |
279 | If G_DISCARD is specified, the return value will always be 0. |
280 | |
281 | =item * |
282 | |
283 | If G_ARRAY is specified I<and> an error has occurred, the return value |
284 | will always be 0. |
285 | |
286 | =item * |
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287 | |
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288 | If G_SCALAR is specified I<and> an error has occurred, the return value |
289 | will be 1 and the value on the top of the stack will be I<undef>. This |
290 | means that if you have already detected the error by checking C<$@> and |
291 | you want the program to continue, you must remember to pop the I<undef> |
292 | from the stack. |
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293 | |
294 | =back |
295 | |
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296 | See I<Using G_EVAL> for details of using G_EVAL. |
297 | |
298 | =head2 Determining the Context |
299 | |
300 | As mentioned above, you can determine the context of the currently |
301 | executing subroutine in Perl with I<wantarray>. The equivalent test can |
302 | be made in C by using the C<GIMME> macro. This will return C<G_SCALAR> |
303 | if you have been called in a scalar context and C<G_ARRAY> if in an |
304 | array context. An example of using the C<GIMME> macro is shown in |
305 | section I<Using GIMME>. |
306 | |
307 | =head1 KNOWN PROBLEMS |
308 | |
309 | This section outlines all known problems that exist in the |
310 | I<perl_call_*> functions. |
311 | |
312 | =over 5 |
313 | |
314 | =item 1. |
315 | |
316 | If you are intending to make use of both the G_EVAL and G_SCALAR flags |
317 | in your code, use a version of Perl greater than 5.000. There is a bug |
318 | in version 5.000 of Perl which means that the combination of these two |
319 | flags will not work as described in the section I<FLAG VALUES>. |
320 | |
321 | Specifically, if the two flags are used when calling a subroutine and |
322 | that subroutine does not call I<die>, the value returned by |
323 | I<perl_call_*> will be wrong. |
324 | |
325 | |
326 | =item 2. |
327 | |
328 | In Perl 5.000 and 5.001 there is a problem with using I<perl_call_*> if |
329 | the Perl sub you are calling attempts to trap a I<die>. |
330 | |
331 | The symptom of this problem is that the called Perl sub will continue |
332 | to completion, but whenever it attempts to pass control back to the |
333 | XSUB, the program will immediately terminate. |
334 | |
335 | For example, say you want to call this Perl sub |
336 | |
337 | sub fred |
338 | { |
339 | eval { die "Fatal Error" ; } |
340 | print "Trapped error: $@\n" |
341 | if $@ ; |
342 | } |
343 | |
344 | via this XSUB |
345 | |
346 | void |
347 | Call_fred() |
348 | CODE: |
349 | PUSHMARK(sp) ; |
350 | perl_call_pv("fred", G_DISCARD|G_NOARGS) ; |
351 | fprintf(stderr, "back in Call_fred\n") ; |
352 | |
353 | When C<Call_fred> is executed it will print |
354 | |
355 | Trapped error: Fatal Error |
356 | |
357 | As control never returns to C<Call_fred>, the C<"back in Call_fred"> |
358 | string will not get printed. |
359 | |
360 | To work around this problem, you can either upgrade to Perl 5.002 (or |
361 | later), or use the G_EVAL flag with I<perl_call_*> as shown below |
362 | |
363 | void |
364 | Call_fred() |
365 | CODE: |
366 | PUSHMARK(sp) ; |
367 | perl_call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ; |
368 | fprintf(stderr, "back in Call_fred\n") ; |
369 | |
370 | =back |
371 | |
372 | |
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373 | |
374 | =head1 EXAMPLES |
375 | |
376 | Enough of the definition talk, let's have a few examples. |
377 | |
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378 | Perl provides many macros to assist in accessing the Perl stack. |
379 | Wherever possible, these macros should always be used when interfacing |
380 | to Perl internals. Hopefully this should make the code less vulnerable |
381 | to any changes made to Perl in the future. |
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382 | |
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383 | Another point worth noting is that in the first series of examples I |
384 | have made use of only the I<perl_call_pv> function. This has been done |
385 | to keep the code simpler and ease you into the topic. Wherever |
386 | possible, if the choice is between using I<perl_call_pv> and |
387 | I<perl_call_sv>, you should always try to use I<perl_call_sv>. See |
388 | I<Using perl_call_sv> for details. |
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389 | |
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390 | =head2 No Parameters, Nothing returned |
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391 | |
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392 | This first trivial example will call a Perl subroutine, I<PrintUID>, to |
393 | print out the UID of the process. |
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394 | |
395 | sub PrintUID |
396 | { |
397 | print "UID is $<\n" ; |
398 | } |
399 | |
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400 | and here is a C function to call it |
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401 | |
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402 | static void |
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403 | call_PrintUID() |
404 | { |
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405 | dSP ; |
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406 | |
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407 | PUSHMARK(sp) ; |
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408 | perl_call_pv("PrintUID", G_DISCARD|G_NOARGS) ; |
409 | } |
410 | |
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411 | Simple, eh. |
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412 | |
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413 | A few points to note about this example. |
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414 | |
415 | =over 5 |
416 | |
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417 | =item 1. |
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418 | |
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419 | Ignore C<dSP> and C<PUSHMARK(sp)> for now. They will be discussed in |
420 | the next example. |
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421 | |
422 | =item 2. |
423 | |
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424 | We aren't passing any parameters to I<PrintUID> so G_NOARGS can be |
425 | specified. |
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426 | |
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427 | =item 3. |
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428 | |
429 | We aren't interested in anything returned from I<PrintUID>, so |
430 | G_DISCARD is specified. Even if I<PrintUID> was changed to actually |
431 | return some value(s), having specified G_DISCARD will mean that they |
432 | will be wiped by the time control returns from I<perl_call_pv>. |
433 | |
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434 | =item 4. |
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435 | |
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436 | As I<perl_call_pv> is being used, the Perl subroutine is specified as a |
437 | C string. In this case the subroutine name has been 'hard-wired' into the |
438 | code. |
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439 | |
440 | =item 5. |
441 | |
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442 | Because we specified G_DISCARD, it is not necessary to check the value |
443 | returned from I<perl_call_pv>. It will always be 0. |
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444 | |
445 | =back |
446 | |
d1b91892 |
447 | =head2 Passing Parameters |
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448 | |
d1b91892 |
449 | Now let's make a slightly more complex example. This time we want to |
450 | call a Perl subroutine, C<LeftString>, which will take 2 parameters - a |
451 | string (C<$s>) and an integer (C<$n>). The subroutine will simply |
452 | print the first C<$n> characters of the string. |
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453 | |
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454 | So the Perl subroutine would look like this |
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455 | |
456 | sub LeftString |
457 | { |
458 | my($s, $n) = @_ ; |
459 | print substr($s, 0, $n), "\n" ; |
460 | } |
461 | |
462 | The C function required to call I<LeftString> would look like this. |
463 | |
464 | static void |
465 | call_LeftString(a, b) |
466 | char * a ; |
467 | int b ; |
468 | { |
469 | dSP ; |
470 | |
471 | PUSHMARK(sp) ; |
472 | XPUSHs(sv_2mortal(newSVpv(a, 0))); |
473 | XPUSHs(sv_2mortal(newSViv(b))); |
474 | PUTBACK ; |
475 | |
476 | perl_call_pv("LeftString", G_DISCARD); |
477 | } |
478 | |
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479 | Here are a few notes on the C function I<call_LeftString>. |
480 | |
481 | =over 5 |
482 | |
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483 | =item 1. |
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484 | |
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485 | Parameters are passed to the Perl subroutine using the Perl stack. |
486 | This is the purpose of the code beginning with the line C<dSP> and |
487 | ending with the line C<PUTBACK>. |
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488 | |
489 | |
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490 | =item 2. |
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491 | |
492 | If you are going to put something onto the Perl stack, you need to know |
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493 | where to put it. This is the purpose of the macro C<dSP> - it declares |
494 | and initializes a I<local> copy of the Perl stack pointer. |
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495 | |
496 | All the other macros which will be used in this example require you to |
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497 | have used this macro. |
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498 | |
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499 | The exception to this rule is if you are calling a Perl subroutine |
500 | directly from an XSUB function. In this case it is not necessary to |
501 | explicitly use the C<dSP> macro - it will be declared for you |
502 | automatically. |
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503 | |
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504 | =item 3. |
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505 | |
506 | Any parameters to be pushed onto the stack should be bracketed by the |
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507 | C<PUSHMARK> and C<PUTBACK> macros. The purpose of these two macros, in |
508 | this context, is to automatically count the number of parameters you |
509 | are pushing. Then whenever Perl is creating the C<@_> array for the |
510 | subroutine, it knows how big to make it. |
511 | |
512 | The C<PUSHMARK> macro tells Perl to make a mental note of the current |
513 | stack pointer. Even if you aren't passing any parameters (like the |
514 | example shown in the section I<No Parameters, Nothing returned>) you |
515 | must still call the C<PUSHMARK> macro before you can call any of the |
516 | I<perl_call_*> functions - Perl still needs to know that there are no |
517 | parameters. |
518 | |
519 | The C<PUTBACK> macro sets the global copy of the stack pointer to be |
520 | the same as our local copy. If we didn't do this I<perl_call_pv> |
521 | wouldn't know where the two parameters we pushed were - remember that |
522 | up to now all the stack pointer manipulation we have done is with our |
523 | local copy, I<not> the global copy. |
524 | |
525 | =item 4. |
526 | |
527 | The only flag specified this time is G_DISCARD. Since we are passing 2 |
528 | parameters to the Perl subroutine this time, we have not specified |
529 | G_NOARGS. |
a0d0e21e |
530 | |
531 | =item 5. |
532 | |
533 | Next, we come to XPUSHs. This is where the parameters actually get |
d1b91892 |
534 | pushed onto the stack. In this case we are pushing a string and an |
535 | integer. |
a0d0e21e |
536 | |
d1b91892 |
537 | See the section L<perlguts/"XSUB'S and the Argument Stack"> for details |
538 | on how the XPUSH macros work. |
a0d0e21e |
539 | |
540 | =item 6. |
541 | |
d1b91892 |
542 | Finally, I<LeftString> can now be called via the I<perl_call_pv> |
543 | function. |
a0d0e21e |
544 | |
545 | =back |
546 | |
d1b91892 |
547 | =head2 Returning a Scalar |
a0d0e21e |
548 | |
d1b91892 |
549 | Now for an example of dealing with the items returned from a Perl |
550 | subroutine. |
a0d0e21e |
551 | |
d1b91892 |
552 | Here is a Perl subroutine, I<Adder>, which takes 2 integer parameters |
553 | and simply returns their sum. |
a0d0e21e |
554 | |
555 | sub Adder |
556 | { |
557 | my($a, $b) = @_ ; |
558 | $a + $b ; |
559 | } |
560 | |
d1b91892 |
561 | Since we are now concerned with the return value from I<Adder>, the C |
562 | function required to call it is now a bit more complex. |
a0d0e21e |
563 | |
564 | static void |
565 | call_Adder(a, b) |
566 | int a ; |
567 | int b ; |
568 | { |
569 | dSP ; |
570 | int count ; |
571 | |
572 | ENTER ; |
573 | SAVETMPS; |
574 | |
575 | PUSHMARK(sp) ; |
576 | XPUSHs(sv_2mortal(newSViv(a))); |
577 | XPUSHs(sv_2mortal(newSViv(b))); |
578 | PUTBACK ; |
579 | |
580 | count = perl_call_pv("Adder", G_SCALAR); |
581 | |
582 | SPAGAIN ; |
583 | |
d1b91892 |
584 | if (count != 1) |
585 | croak("Big trouble\n") ; |
a0d0e21e |
586 | |
d1b91892 |
587 | printf ("The sum of %d and %d is %d\n", a, b, POPi) ; |
a0d0e21e |
588 | |
589 | PUTBACK ; |
590 | FREETMPS ; |
591 | LEAVE ; |
592 | } |
593 | |
a0d0e21e |
594 | Points to note this time are |
595 | |
596 | =over 5 |
597 | |
598 | =item 1. |
599 | |
d1b91892 |
600 | The only flag specified this time was G_SCALAR. That means the C<@_> |
601 | array will be created and that the value returned by I<Adder> will |
602 | still exist after the call to I<perl_call_pv>. |
a0d0e21e |
603 | |
604 | |
605 | |
606 | =item 2. |
607 | |
d1b91892 |
608 | Because we are interested in what is returned from I<Adder> we cannot |
609 | specify G_DISCARD. This means that we will have to tidy up the Perl |
610 | stack and dispose of any temporary values ourselves. This is the |
611 | purpose of |
a0d0e21e |
612 | |
d1b91892 |
613 | ENTER ; |
614 | SAVETMPS ; |
a0d0e21e |
615 | |
616 | at the start of the function, and |
617 | |
d1b91892 |
618 | FREETMPS ; |
619 | LEAVE ; |
620 | |
621 | at the end. The C<ENTER>/C<SAVETMPS> pair creates a boundary for any |
622 | temporaries we create. This means that the temporaries we get rid of |
623 | will be limited to those which were created after these calls. |
a0d0e21e |
624 | |
d1b91892 |
625 | The C<FREETMPS>/C<LEAVE> pair will get rid of any values returned by |
626 | the Perl subroutine, plus it will also dump the mortal SV's we have |
627 | created. Having C<ENTER>/C<SAVETMPS> at the beginning of the code |
628 | makes sure that no other mortals are destroyed. |
a0d0e21e |
629 | |
d1b91892 |
630 | Think of these macros as working a bit like using C<{> and C<}> in Perl |
631 | to limit the scope of local variables. |
632 | |
633 | See the section I<Using Perl to dispose of temporaries> for details of |
634 | an alternative to using these macros. |
a0d0e21e |
635 | |
636 | =item 3. |
637 | |
638 | The purpose of the macro C<SPAGAIN> is to refresh the local copy of the |
639 | stack pointer. This is necessary because it is possible that the memory |
d1b91892 |
640 | allocated to the Perl stack has been re-allocated whilst in the |
641 | I<perl_call_pv> call. |
a0d0e21e |
642 | |
d1b91892 |
643 | If you are making use of the Perl stack pointer in your code you must |
644 | always refresh the your local copy using SPAGAIN whenever you make use |
a0d0e21e |
645 | of the I<perl_call_*> functions or any other Perl internal function. |
646 | |
d1b91892 |
647 | =item 4. |
a0d0e21e |
648 | |
d1b91892 |
649 | Although only a single value was expected to be returned from I<Adder>, |
650 | it is still good practice to check the return code from I<perl_call_pv> |
651 | anyway. |
a0d0e21e |
652 | |
d1b91892 |
653 | Expecting a single value is not quite the same as knowing that there |
654 | will be one. If someone modified I<Adder> to return a list and we |
655 | didn't check for that possibility and take appropriate action the Perl |
656 | stack would end up in an inconsistent state. That is something you |
657 | I<really> don't want to ever happen. |
a0d0e21e |
658 | |
659 | =item 5. |
660 | |
d1b91892 |
661 | The C<POPi> macro is used here to pop the return value from the stack. |
662 | In this case we wanted an integer, so C<POPi> was used. |
a0d0e21e |
663 | |
664 | |
d1b91892 |
665 | Here is the complete list of POP macros available, along with the types |
666 | they return. |
a0d0e21e |
667 | |
d1b91892 |
668 | POPs SV |
669 | POPp pointer |
670 | POPn double |
671 | POPi integer |
672 | POPl long |
a0d0e21e |
673 | |
674 | =item 6. |
675 | |
d1b91892 |
676 | The final C<PUTBACK> is used to leave the Perl stack in a consistent |
677 | state before exiting the function. This is necessary because when we |
678 | popped the return value from the stack with C<POPi> it updated only our |
679 | local copy of the stack pointer. Remember, C<PUTBACK> sets the global |
680 | stack pointer to be the same as our local copy. |
a0d0e21e |
681 | |
682 | =back |
683 | |
684 | |
d1b91892 |
685 | =head2 Returning a list of values |
a0d0e21e |
686 | |
d1b91892 |
687 | Now, let's extend the previous example to return both the sum of the |
688 | parameters and the difference. |
a0d0e21e |
689 | |
d1b91892 |
690 | Here is the Perl subroutine |
a0d0e21e |
691 | |
692 | sub AddSubtract |
693 | { |
694 | my($a, $b) = @_ ; |
695 | ($a+$b, $a-$b) ; |
696 | } |
697 | |
a0d0e21e |
698 | and this is the C function |
699 | |
700 | static void |
701 | call_AddSubtract(a, b) |
702 | int a ; |
703 | int b ; |
704 | { |
705 | dSP ; |
706 | int count ; |
707 | |
708 | ENTER ; |
709 | SAVETMPS; |
710 | |
711 | PUSHMARK(sp) ; |
712 | XPUSHs(sv_2mortal(newSViv(a))); |
713 | XPUSHs(sv_2mortal(newSViv(b))); |
714 | PUTBACK ; |
715 | |
716 | count = perl_call_pv("AddSubtract", G_ARRAY); |
717 | |
718 | SPAGAIN ; |
719 | |
d1b91892 |
720 | if (count != 2) |
721 | croak("Big trouble\n") ; |
a0d0e21e |
722 | |
d1b91892 |
723 | printf ("%d - %d = %d\n", a, b, POPi) ; |
724 | printf ("%d + %d = %d\n", a, b, POPi) ; |
a0d0e21e |
725 | |
726 | PUTBACK ; |
727 | FREETMPS ; |
728 | LEAVE ; |
729 | } |
730 | |
d1b91892 |
731 | If I<call_AddSubtract> is called like this |
732 | |
733 | call_AddSubtract(7, 4) ; |
734 | |
735 | then here is the output |
736 | |
737 | 7 - 4 = 3 |
738 | 7 + 4 = 11 |
a0d0e21e |
739 | |
740 | Notes |
741 | |
742 | =over 5 |
743 | |
744 | =item 1. |
745 | |
d1b91892 |
746 | We wanted array context, so G_ARRAY was used. |
a0d0e21e |
747 | |
748 | =item 2. |
749 | |
d1b91892 |
750 | Not surprisingly C<POPi> is used twice this time because we were |
751 | retrieving 2 values from the stack. The important thing to note is that |
752 | when using the C<POP*> macros they come off the stack in I<reverse> |
753 | order. |
a0d0e21e |
754 | |
755 | =back |
756 | |
d1b91892 |
757 | =head2 Returning a list in a scalar context |
758 | |
759 | Say the Perl subroutine in the previous section was called in a scalar |
760 | context, like this |
761 | |
762 | static void |
763 | call_AddSubScalar(a, b) |
764 | int a ; |
765 | int b ; |
766 | { |
767 | dSP ; |
768 | int count ; |
769 | int i ; |
770 | |
771 | ENTER ; |
772 | SAVETMPS; |
773 | |
774 | PUSHMARK(sp) ; |
775 | XPUSHs(sv_2mortal(newSViv(a))); |
776 | XPUSHs(sv_2mortal(newSViv(b))); |
777 | PUTBACK ; |
778 | |
779 | count = perl_call_pv("AddSubtract", G_SCALAR); |
780 | |
781 | SPAGAIN ; |
782 | |
783 | printf ("Items Returned = %d\n", count) ; |
784 | |
785 | for (i = 1 ; i <= count ; ++i) |
786 | printf ("Value %d = %d\n", i, POPi) ; |
787 | |
788 | PUTBACK ; |
789 | FREETMPS ; |
790 | LEAVE ; |
791 | } |
792 | |
793 | The other modification made is that I<call_AddSubScalar> will print the |
794 | number of items returned from the Perl subroutine and their value (for |
795 | simplicity it assumes that they are integer). So if |
796 | I<call_AddSubScalar> is called |
797 | |
798 | call_AddSubScalar(7, 4) ; |
799 | |
800 | then the output will be |
801 | |
802 | Items Returned = 1 |
803 | Value 1 = 3 |
804 | |
805 | In this case the main point to note is that only the last item in the |
806 | list returned from the subroutine, I<Adder> actually made it back to |
807 | I<call_AddSubScalar>. |
808 | |
809 | |
810 | =head2 Returning Data from Perl via the parameter list |
a0d0e21e |
811 | |
812 | It is also possible to return values directly via the parameter list - |
813 | whether it is actually desirable to do it is another matter entirely. |
814 | |
d1b91892 |
815 | The Perl subroutine, I<Inc>, below takes 2 parameters and increments |
816 | each directly. |
a0d0e21e |
817 | |
818 | sub Inc |
819 | { |
820 | ++ $_[0] ; |
821 | ++ $_[1] ; |
822 | } |
823 | |
824 | and here is a C function to call it. |
825 | |
826 | static void |
827 | call_Inc(a, b) |
828 | int a ; |
829 | int b ; |
830 | { |
831 | dSP ; |
832 | int count ; |
833 | SV * sva ; |
834 | SV * svb ; |
835 | |
836 | ENTER ; |
837 | SAVETMPS; |
838 | |
839 | sva = sv_2mortal(newSViv(a)) ; |
840 | svb = sv_2mortal(newSViv(b)) ; |
841 | |
842 | PUSHMARK(sp) ; |
843 | XPUSHs(sva); |
844 | XPUSHs(svb); |
845 | PUTBACK ; |
846 | |
847 | count = perl_call_pv("Inc", G_DISCARD); |
848 | |
849 | if (count != 0) |
d1b91892 |
850 | croak ("call_Inc: expected 0 values from 'Inc', got %d\n", |
851 | count) ; |
a0d0e21e |
852 | |
853 | printf ("%d + 1 = %d\n", a, SvIV(sva)) ; |
854 | printf ("%d + 1 = %d\n", b, SvIV(svb)) ; |
855 | |
856 | FREETMPS ; |
d1b91892 |
857 | LEAVE ; |
a0d0e21e |
858 | } |
859 | |
d1b91892 |
860 | To be able to access the two parameters that were pushed onto the stack |
861 | after they return from I<perl_call_pv> it is necessary to make a note |
862 | of their addresses - thus the two variables C<sva> and C<svb>. |
a0d0e21e |
863 | |
d1b91892 |
864 | The reason this is necessary is that the area of the Perl stack which |
865 | held them will very likely have been overwritten by something else by |
866 | the time control returns from I<perl_call_pv>. |
a0d0e21e |
867 | |
868 | |
869 | |
870 | |
d1b91892 |
871 | =head2 Using G_EVAL |
a0d0e21e |
872 | |
d1b91892 |
873 | Now an example using G_EVAL. Below is a Perl subroutine which computes |
874 | the difference of its 2 parameters. If this would result in a negative |
875 | result, the subroutine calls I<die>. |
a0d0e21e |
876 | |
877 | sub Subtract |
878 | { |
d1b91892 |
879 | my ($a, $b) = @_ ; |
a0d0e21e |
880 | |
881 | die "death can be fatal\n" if $a < $b ; |
882 | |
d1b91892 |
883 | $a - $b ; |
a0d0e21e |
884 | } |
885 | |
886 | and some C to call it |
887 | |
888 | static void |
889 | call_Subtract(a, b) |
890 | int a ; |
891 | int b ; |
892 | { |
893 | dSP ; |
894 | int count ; |
d1b91892 |
895 | SV * sv ; |
a0d0e21e |
896 | |
897 | ENTER ; |
898 | SAVETMPS; |
899 | |
900 | PUSHMARK(sp) ; |
901 | XPUSHs(sv_2mortal(newSViv(a))); |
902 | XPUSHs(sv_2mortal(newSViv(b))); |
903 | PUTBACK ; |
904 | |
905 | count = perl_call_pv("Subtract", G_EVAL|G_SCALAR); |
906 | |
d1b91892 |
907 | SPAGAIN ; |
908 | |
909 | /* Check the eval first */ |
a0d0e21e |
910 | sv = GvSV(gv_fetchpv("@", TRUE, SVt_PV)); |
911 | if (SvTRUE(sv)) |
d1b91892 |
912 | { |
a0d0e21e |
913 | printf ("Uh oh - %s\n", SvPV(sv, na)) ; |
d1b91892 |
914 | POPs ; |
915 | } |
916 | else |
917 | { |
918 | if (count != 1) |
919 | croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n", |
920 | count) ; |
a0d0e21e |
921 | |
d1b91892 |
922 | printf ("%d - %d = %d\n", a, b, POPi) ; |
923 | } |
a0d0e21e |
924 | |
925 | PUTBACK ; |
926 | FREETMPS ; |
927 | LEAVE ; |
a0d0e21e |
928 | } |
929 | |
930 | If I<call_Subtract> is called thus |
931 | |
d1b91892 |
932 | call_Subtract(4, 5) |
a0d0e21e |
933 | |
934 | the following will be printed |
935 | |
d1b91892 |
936 | Uh oh - death can be fatal |
a0d0e21e |
937 | |
938 | Notes |
939 | |
940 | =over 5 |
941 | |
942 | =item 1. |
943 | |
d1b91892 |
944 | We want to be able to catch the I<die> so we have used the G_EVAL |
945 | flag. Not specifying this flag would mean that the program would |
946 | terminate immediately at the I<die> statement in the subroutine |
947 | I<Subtract>. |
a0d0e21e |
948 | |
949 | =item 2. |
950 | |
951 | The code |
952 | |
d1b91892 |
953 | sv = GvSV(gv_fetchpv("@", TRUE, SVt_PV)); |
954 | if (SvTRUE(sv)) |
955 | { |
956 | printf ("Uh oh - %s\n", SvPVx(sv, na)) ; |
957 | POPs ; |
958 | } |
a0d0e21e |
959 | |
d1b91892 |
960 | is the direct equivalent of this bit of Perl |
a0d0e21e |
961 | |
d1b91892 |
962 | print "Uh oh - $@\n" if $@ ; |
a0d0e21e |
963 | |
d1b91892 |
964 | =item 3. |
a0d0e21e |
965 | |
d1b91892 |
966 | Note that the stack is popped using C<POPs> in the block where |
967 | C<SvTRUE(sv)> is true. This is necessary because whenever a |
968 | I<perl_call_*> function invoked with G_EVAL|G_SCALAR returns an error, |
969 | the top of the stack holds the value I<undef>. Since we want the |
970 | program to continue after detecting this error, it is essential that |
971 | the stack is tidied up by removing the I<undef>. |
a0d0e21e |
972 | |
973 | =back |
974 | |
975 | |
d1b91892 |
976 | =head2 Using perl_call_sv |
a0d0e21e |
977 | |
d1b91892 |
978 | In all the previous examples I have 'hard-wired' the name of the Perl |
979 | subroutine to be called from C. Most of the time though, it is more |
980 | convenient to be able to specify the name of the Perl subroutine from |
981 | within the Perl script. |
a0d0e21e |
982 | |
983 | Consider the Perl code below |
984 | |
d1b91892 |
985 | sub fred |
986 | { |
987 | print "Hello there\n" ; |
988 | } |
989 | |
990 | CallSubPV("fred") ; |
991 | |
992 | Here is a snippet of XSUB which defines I<CallSubPV>. |
993 | |
994 | void |
995 | CallSubPV(name) |
996 | char * name |
997 | CODE: |
998 | PUSHMARK(sp) ; |
999 | perl_call_pv(name, G_DISCARD|G_NOARGS) ; |
a0d0e21e |
1000 | |
d1b91892 |
1001 | That is fine as far as it goes. The thing is, the Perl subroutine |
1002 | can be specified only as a string. For Perl 4 this was adequate, |
1003 | but Perl 5 allows references to subroutines and anonymous subroutines. |
1004 | This is where I<perl_call_sv> is useful. |
1005 | |
1006 | The code below for I<CallSubSV> is identical to I<CallSubPV> except |
1007 | that the C<name> parameter is now defined as an SV* and we use |
1008 | I<perl_call_sv> instead of I<perl_call_pv>. |
1009 | |
1010 | void |
1011 | CallSubSV(name) |
1012 | SV * name |
1013 | CODE: |
1014 | PUSHMARK(sp) ; |
1015 | perl_call_sv(name, G_DISCARD|G_NOARGS) ; |
a0d0e21e |
1016 | |
d1b91892 |
1017 | Since we are using an SV to call I<fred> the following can all be used |
a0d0e21e |
1018 | |
d1b91892 |
1019 | CallSubSV("fred") ; |
1020 | CallSubSV(\&fred) ; |
1021 | $ref = \&fred ; |
1022 | CallSubSV($ref) ; |
1023 | CallSubSV( sub { print "Hello there\n" } ) ; |
a0d0e21e |
1024 | |
d1b91892 |
1025 | As you can see, I<perl_call_sv> gives you much greater flexibility in |
1026 | how you can specify the Perl subroutine. |
1027 | |
1028 | You should note that if it is necessary to store the SV (C<name> in the |
1029 | example above) which corresponds to the Perl subroutine so that it can |
1030 | be used later in the program, it not enough to just store a copy of the |
1031 | pointer to the SV. Say the code above had been like this |
1032 | |
1033 | static SV * rememberSub ; |
1034 | |
1035 | void |
1036 | SaveSub1(name) |
1037 | SV * name |
1038 | CODE: |
1039 | rememberSub = name ; |
1040 | |
1041 | void |
1042 | CallSavedSub1() |
1043 | CODE: |
1044 | PUSHMARK(sp) ; |
1045 | perl_call_sv(rememberSub, G_DISCARD|G_NOARGS) ; |
a0d0e21e |
1046 | |
d1b91892 |
1047 | The reason this is wrong is that by the time you come to use the |
1048 | pointer C<rememberSub> in C<CallSavedSub1>, it may or may not still refer |
1049 | to the Perl subroutine that was recorded in C<SaveSub1>. This is |
1050 | particularly true for these cases |
a0d0e21e |
1051 | |
d1b91892 |
1052 | SaveSub1(\&fred) ; |
1053 | CallSavedSub1() ; |
a0d0e21e |
1054 | |
d1b91892 |
1055 | SaveSub1( sub { print "Hello there\n" } ) ; |
1056 | CallSavedSub1() ; |
a0d0e21e |
1057 | |
d1b91892 |
1058 | By the time each of the C<SaveSub1> statements above have been executed, |
1059 | the SV*'s which corresponded to the parameters will no longer exist. |
1060 | Expect an error message from Perl of the form |
a0d0e21e |
1061 | |
d1b91892 |
1062 | Can't use an undefined value as a subroutine reference at ... |
a0d0e21e |
1063 | |
d1b91892 |
1064 | for each of the C<CallSavedSub1> lines. |
a0d0e21e |
1065 | |
d1b91892 |
1066 | Similarly, with this code |
a0d0e21e |
1067 | |
d1b91892 |
1068 | $ref = \&fred ; |
1069 | SaveSub1($ref) ; |
1070 | $ref = 47 ; |
1071 | CallSavedSub1() ; |
a0d0e21e |
1072 | |
d1b91892 |
1073 | you can expect one of these messages (which you actually get is dependant on |
1074 | the version of Perl you are using) |
a0d0e21e |
1075 | |
d1b91892 |
1076 | Not a CODE reference at ... |
1077 | Undefined subroutine &main::47 called ... |
a0d0e21e |
1078 | |
d1b91892 |
1079 | The variable C<$ref> may have referred to the subroutine C<fred> |
1080 | whenever the call to C<SaveSub1> was made but by the time |
1081 | C<CallSavedSub1> gets called it now holds the number C<47>. Since we |
1082 | saved only a pointer to the original SV in C<SaveSub1>, any changes to |
1083 | C<$ref> will be tracked by the pointer C<rememberSub>. This means that |
1084 | whenever C<CallSavedSub1> gets called, it will attempt to execute the |
1085 | code which is referenced by the SV* C<rememberSub>. In this case |
1086 | though, it now refers to the integer C<47>, so expect Perl to complain |
1087 | loudly. |
a0d0e21e |
1088 | |
d1b91892 |
1089 | A similar but more subtle problem is illustrated with this code |
a0d0e21e |
1090 | |
d1b91892 |
1091 | $ref = \&fred ; |
1092 | SaveSub1($ref) ; |
1093 | $ref = \&joe ; |
1094 | CallSavedSub1() ; |
a0d0e21e |
1095 | |
d1b91892 |
1096 | This time whenever C<CallSavedSub1> get called it will execute the Perl |
1097 | subroutine C<joe> (assuming it exists) rather than C<fred> as was |
1098 | originally requested in the call to C<SaveSub1>. |
a0d0e21e |
1099 | |
d1b91892 |
1100 | To get around these problems it is necessary to take a full copy of the |
1101 | SV. The code below shows C<SaveSub2> modified to do that |
a0d0e21e |
1102 | |
d1b91892 |
1103 | static SV * keepSub = (SV*)NULL ; |
1104 | |
1105 | void |
1106 | SaveSub2(name) |
1107 | SV * name |
1108 | CODE: |
1109 | /* Take a copy of the callback */ |
1110 | if (keepSub == (SV*)NULL) |
1111 | /* First time, so create a new SV */ |
1112 | keepSub = newSVsv(name) ; |
1113 | else |
1114 | /* Been here before, so overwrite */ |
1115 | SvSetSV(keepSub, name) ; |
1116 | |
1117 | void |
1118 | CallSavedSub2() |
1119 | CODE: |
1120 | PUSHMARK(sp) ; |
1121 | perl_call_sv(keepSub, G_DISCARD|G_NOARGS) ; |
1122 | |
1123 | In order to avoid creating a new SV every time C<SaveSub2> is called, |
1124 | the function first checks to see if it has been called before. If not, |
1125 | then space for a new SV is allocated and the reference to the Perl |
1126 | subroutine, C<name> is copied to the variable C<keepSub> in one |
1127 | operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called |
1128 | the existing SV, C<keepSub>, is overwritten with the new value using |
1129 | C<SvSetSV>. |
1130 | |
1131 | =head2 Using perl_call_argv |
1132 | |
1133 | Here is a Perl subroutine which prints whatever parameters are passed |
1134 | to it. |
1135 | |
1136 | sub PrintList |
1137 | { |
1138 | my(@list) = @_ ; |
1139 | |
1140 | foreach (@list) { print "$_\n" } |
1141 | } |
1142 | |
1143 | and here is an example of I<perl_call_argv> which will call |
1144 | I<PrintList>. |
1145 | |
1146 | static char * words[] = {"alpha", "beta", "gamma", "delta", NULL} ; |
1147 | |
1148 | static void |
1149 | call_PrintList() |
1150 | { |
1151 | dSP ; |
1152 | |
1153 | perl_call_argv("PrintList", G_DISCARD, words) ; |
1154 | } |
1155 | |
1156 | Note that it is not necessary to call C<PUSHMARK> in this instance. |
1157 | This is because I<perl_call_argv> will do it for you. |
1158 | |
1159 | =head2 Using perl_call_method |
a0d0e21e |
1160 | |
1161 | Consider the following Perl code |
1162 | |
d1b91892 |
1163 | { |
1164 | package Mine ; |
1165 | |
1166 | sub new |
1167 | { |
1168 | my($type) = shift ; |
1169 | bless [@_] |
1170 | } |
1171 | |
1172 | sub Display |
1173 | { |
1174 | my ($self, $index) = @_ ; |
1175 | print "$index: $$self[$index]\n" ; |
1176 | } |
1177 | |
1178 | sub PrintID |
1179 | { |
1180 | my($class) = @_ ; |
1181 | print "This is Class $class version 1.0\n" ; |
1182 | } |
1183 | } |
1184 | |
1185 | It just implements a very simple class to manage an array. Apart from |
1186 | the constructor, C<new>, it declares methods, one static and one |
1187 | virtual. The static method, C<PrintID>, simply prints out the class |
1188 | name and a version number. The virtual method, C<Display>, prints out a |
1189 | single element of the array. Here is an all Perl example of using it. |
1190 | |
1191 | $a = new Mine ('red', 'green', 'blue') ; |
1192 | $a->Display(1) ; |
1193 | PrintID Mine; |
a0d0e21e |
1194 | |
d1b91892 |
1195 | will print |
a0d0e21e |
1196 | |
d1b91892 |
1197 | 1: green |
1198 | This is Class Mine version 1.0 |
a0d0e21e |
1199 | |
d1b91892 |
1200 | Calling a Perl method from C is fairly straightforward. The following |
1201 | things are required |
a0d0e21e |
1202 | |
d1b91892 |
1203 | =over 5 |
1204 | |
1205 | =item * |
1206 | |
1207 | a reference to the object for a virtual method or the name of the class |
1208 | for a static method. |
1209 | |
1210 | =item * |
1211 | |
1212 | the name of the method. |
1213 | |
1214 | =item * |
1215 | |
1216 | any other parameters specific to the method. |
1217 | |
1218 | =back |
1219 | |
1220 | Here is a simple XSUB which illustrates the mechanics of calling both |
1221 | the C<PrintID> and C<Display> methods from C. |
1222 | |
1223 | void |
1224 | call_Method(ref, method, index) |
1225 | SV * ref |
1226 | char * method |
1227 | int index |
1228 | CODE: |
1229 | PUSHMARK(sp); |
1230 | XPUSHs(ref); |
1231 | XPUSHs(sv_2mortal(newSViv(index))) ; |
1232 | PUTBACK; |
1233 | |
1234 | perl_call_method(method, G_DISCARD) ; |
1235 | |
1236 | void |
1237 | call_PrintID(class, method) |
1238 | char * class |
1239 | char * method |
1240 | CODE: |
1241 | PUSHMARK(sp); |
1242 | XPUSHs(sv_2mortal(newSVpv(class, 0))) ; |
1243 | PUTBACK; |
1244 | |
1245 | perl_call_method(method, G_DISCARD) ; |
1246 | |
1247 | |
1248 | So the methods C<PrintID> and C<Display> can be invoked like this |
1249 | |
1250 | $a = new Mine ('red', 'green', 'blue') ; |
1251 | call_Method($a, 'Display', 1) ; |
1252 | call_PrintID('Mine', 'PrintID') ; |
1253 | |
1254 | The only thing to note is that in both the static and virtual methods, |
1255 | the method name is not passed via the stack - it is used as the first |
1256 | parameter to I<perl_call_method>. |
1257 | |
1258 | =head2 Using GIMME |
1259 | |
1260 | Here is a trivial XSUB which prints the context in which it is |
1261 | currently executing. |
1262 | |
1263 | void |
1264 | PrintContext() |
1265 | CODE: |
1266 | if (GIMME == G_SCALAR) |
1267 | printf ("Context is Scalar\n") ; |
1268 | else |
1269 | printf ("Context is Array\n") ; |
1270 | |
1271 | and here is some Perl to test it |
1272 | |
1273 | $a = PrintContext ; |
1274 | @a = PrintContext ; |
1275 | |
1276 | The output from that will be |
1277 | |
1278 | Context is Scalar |
1279 | Context is Array |
1280 | |
1281 | =head2 Using Perl to dispose of temporaries |
1282 | |
1283 | In the examples given to date, any temporaries created in the callback |
1284 | (i.e. parameters passed on the stack to the I<perl_call_*> function or |
1285 | values returned via the stack) have been freed by one of these methods |
1286 | |
1287 | =over 5 |
1288 | |
1289 | =item * |
1290 | |
1291 | specifying the G_DISCARD flag with I<perl_call_*>. |
1292 | |
1293 | =item * |
1294 | |
1295 | explicitly disposed of using the C<ENTER>/C<SAVETMPS> - |
1296 | C<FREETMPS>/C<LEAVE> pairing. |
1297 | |
1298 | =back |
1299 | |
1300 | There is another method which can be used, namely letting Perl do it |
1301 | for you automatically whenever it regains control after the callback |
1302 | has terminated. This is done by simply not using the |
1303 | |
1304 | ENTER ; |
1305 | SAVETMPS ; |
1306 | ... |
1307 | FREETMPS ; |
1308 | LEAVE ; |
1309 | |
1310 | sequence in the callback (and not, of course, specifying the G_DISCARD |
1311 | flag). |
1312 | |
1313 | If you are going to use this method you have to be aware of a possible |
1314 | memory leak which can arise under very specific circumstances. To |
1315 | explain these circumstances you need to know a bit about the flow of |
1316 | control between Perl and the callback routine. |
1317 | |
1318 | The examples given at the start of the document (an error handler and |
1319 | an event driven program) are typical of the two main sorts of flow |
1320 | control that you are likely to encounter with callbacks. There is a |
1321 | very important distinction between them, so pay attention. |
1322 | |
1323 | In the first example, an error handler, the flow of control could be as |
1324 | follows. You have created an interface to an external library. |
1325 | Control can reach the external library like this |
1326 | |
1327 | perl --> XSUB --> external library |
1328 | |
1329 | Whilst control is in the library, an error condition occurs. You have |
1330 | previously set up a Perl callback to handle this situation, so it will |
1331 | get executed. Once the callback has finished, control will drop back to |
1332 | Perl again. Here is what the flow of control will be like in that |
1333 | situation |
1334 | |
1335 | perl --> XSUB --> external library |
1336 | ... |
1337 | error occurs |
1338 | ... |
1339 | external library --> perl_call --> perl |
1340 | | |
1341 | perl <-- XSUB <-- external library <-- perl_call <----+ |
1342 | |
1343 | After processing of the error using I<perl_call_*> is completed, |
1344 | control reverts back to Perl more or less immediately. |
1345 | |
1346 | In the diagram, the further right you go the more deeply nested the |
1347 | scope is. It is only when control is back with perl on the extreme |
1348 | left of the diagram that you will have dropped back to the enclosing |
1349 | scope and any temporaries you have left hanging around will be freed. |
1350 | |
1351 | In the second example, an event driven program, the flow of control |
1352 | will be more like this |
1353 | |
1354 | perl --> XSUB --> event handler |
1355 | ... |
1356 | event handler --> perl_call --> perl |
1357 | | |
1358 | event handler <-- perl_call --<--+ |
1359 | ... |
1360 | event handler --> perl_call --> perl |
1361 | | |
1362 | event handler <-- perl_call --<--+ |
1363 | ... |
1364 | event handler --> perl_call --> perl |
1365 | | |
1366 | event handler <-- perl_call --<--+ |
1367 | |
1368 | In this case the flow of control can consist of only the repeated |
1369 | sequence |
1370 | |
1371 | event handler --> perl_call --> perl |
1372 | |
1373 | for the practically the complete duration of the program. This means |
1374 | that control may I<never> drop back to the surrounding scope in Perl at |
1375 | the extreme left. |
1376 | |
1377 | So what is the big problem? Well, if you are expecting Perl to tidy up |
1378 | those temporaries for you, you might be in for a long wait. For Perl |
1379 | to actually dispose of your temporaries, control must drop back to the |
1380 | enclosing scope at some stage. In the event driven scenario that may |
1381 | never happen. This means that as time goes on, your program will |
1382 | create more and more temporaries, none of which will ever be freed. As |
1383 | each of these temporaries consumes some memory your program will |
1384 | eventually consume all the available memory in your system - kapow! |
1385 | |
1386 | So here is the bottom line - if you are sure that control will revert |
1387 | back to the enclosing Perl scope fairly quickly after the end of your |
1388 | callback, then it isn't absolutely necessary to explicitly dispose of |
1389 | any temporaries you may have created. Mind you, if you are at all |
1390 | uncertain about what to do, it doesn't do any harm to tidy up anyway. |
1391 | |
1392 | |
1393 | =head2 Strategies for storing Callback Context Information |
1394 | |
1395 | |
1396 | Potentially one of the trickiest problems to overcome when designing a |
1397 | callback interface can be figuring out how to store the mapping between |
1398 | the C callback function and the Perl equivalent. |
1399 | |
1400 | To help understand why this can be a real problem first consider how a |
1401 | callback is set up in an all C environment. Typically a C API will |
1402 | provide a function to register a callback. This will expect a pointer |
1403 | to a function as one of its parameters. Below is a call to a |
1404 | hypothetical function C<register_fatal> which registers the C function |
1405 | to get called when a fatal error occurs. |
1406 | |
1407 | register_fatal(cb1) ; |
1408 | |
1409 | The single parameter C<cb1> is a pointer to a function, so you must |
1410 | have defined C<cb1> in your code, say something like this |
1411 | |
1412 | static void |
1413 | cb1() |
1414 | { |
1415 | printf ("Fatal Error\n") ; |
1416 | exit(1) ; |
1417 | } |
1418 | |
1419 | Now change that to call a Perl subroutine instead |
1420 | |
1421 | static SV * callback = (SV*)NULL; |
1422 | |
1423 | static void |
1424 | cb1() |
1425 | { |
1426 | dSP ; |
1427 | |
1428 | PUSHMARK(sp) ; |
1429 | |
1430 | /* Call the Perl sub to process the callback */ |
1431 | perl_call_sv(callback, G_DISCARD) ; |
1432 | } |
1433 | |
1434 | |
1435 | void |
1436 | register_fatal(fn) |
1437 | SV * fn |
1438 | CODE: |
1439 | /* Remember the Perl sub */ |
1440 | if (callback == (SV*)NULL) |
1441 | callback = newSVsv(fn) ; |
1442 | else |
1443 | SvSetSV(callback, fn) ; |
1444 | |
1445 | /* register the callback with the external library */ |
1446 | register_fatal(cb1) ; |
1447 | |
1448 | where the Perl equivalent of C<register_fatal> and the callback it |
1449 | registers, C<pcb1>, might look like this |
1450 | |
1451 | # Register the sub pcb1 |
1452 | register_fatal(\&pcb1) ; |
1453 | |
1454 | sub pcb1 |
1455 | { |
1456 | die "I'm dying...\n" ; |
1457 | } |
1458 | |
1459 | The mapping between the C callback and the Perl equivalent is stored in |
1460 | the global variable C<callback>. |
1461 | |
1462 | This will be adequate if you ever need to have only 1 callback |
1463 | registered at any time. An example could be an error handler like the |
1464 | code sketched out above. Remember though, repeated calls to |
1465 | C<register_fatal> will replace the previously registered callback |
1466 | function with the new one. |
1467 | |
1468 | Say for example you want to interface to a library which allows asynchronous |
1469 | file i/o. In this case you may be able to register a callback whenever |
1470 | a read operation has completed. To be of any use we want to be able to |
1471 | call separate Perl subroutines for each file that is opened. As it |
1472 | stands, the error handler example above would not be adequate as it |
1473 | allows only a single callback to be defined at any time. What we |
1474 | require is a means of storing the mapping between the opened file and |
1475 | the Perl subroutine we want to be called for that file. |
1476 | |
1477 | Say the i/o library has a function C<asynch_read> which associates a C |
1478 | function C<ProcessRead> with a file handle C<fh> - this assumes that it |
1479 | has also provided some routine to open the file and so obtain the file |
1480 | handle. |
1481 | |
1482 | asynch_read(fh, ProcessRead) |
1483 | |
1484 | This may expect the C I<ProcessRead> function of this form |
1485 | |
1486 | void |
1487 | ProcessRead(fh, buffer) |
1488 | int fh ; |
1489 | char * buffer ; |
1490 | { |
1491 | ... |
1492 | } |
1493 | |
1494 | To provide a Perl interface to this library we need to be able to map |
1495 | between the C<fh> parameter and the Perl subroutine we want called. A |
1496 | hash is a convenient mechanism for storing this mapping. The code |
1497 | below shows a possible implementation |
1498 | |
1499 | static HV * Mapping = (HV*)NULL ; |
a0d0e21e |
1500 | |
d1b91892 |
1501 | void |
1502 | asynch_read(fh, callback) |
1503 | int fh |
1504 | SV * callback |
1505 | CODE: |
1506 | /* If the hash doesn't already exist, create it */ |
1507 | if (Mapping == (HV*)NULL) |
1508 | Mapping = newHV() ; |
1509 | |
1510 | /* Save the fh -> callback mapping */ |
1511 | hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0) ; |
1512 | |
1513 | /* Register with the C Library */ |
1514 | asynch_read(fh, asynch_read_if) ; |
1515 | |
1516 | and C<asynch_read_if> could look like this |
1517 | |
1518 | static void |
1519 | asynch_read_if(fh, buffer) |
1520 | int fh ; |
1521 | char * buffer ; |
1522 | { |
1523 | dSP ; |
1524 | SV ** sv ; |
1525 | |
1526 | /* Get the callback associated with fh */ |
1527 | sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE) ; |
1528 | if (sv == (SV**)NULL) |
1529 | croak("Internal error...\n") ; |
1530 | |
1531 | PUSHMARK(sp) ; |
1532 | XPUSHs(sv_2mortal(newSViv(fh))) ; |
1533 | XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ; |
1534 | PUTBACK ; |
1535 | |
1536 | /* Call the Perl sub */ |
1537 | perl_call_sv(*sv, G_DISCARD) ; |
1538 | } |
1539 | |
1540 | For completeness, here is C<asynch_close>. This shows how to remove |
1541 | the entry from the hash C<Mapping>. |
1542 | |
1543 | void |
1544 | asynch_close(fh) |
1545 | int fh |
1546 | CODE: |
1547 | /* Remove the entry from the hash */ |
1548 | (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD) ; |
a0d0e21e |
1549 | |
d1b91892 |
1550 | /* Now call the real asynch_close */ |
1551 | asynch_close(fh) ; |
a0d0e21e |
1552 | |
d1b91892 |
1553 | So the Perl interface would look like this |
1554 | |
1555 | sub callback1 |
1556 | { |
1557 | my($handle, $buffer) = @_ ; |
1558 | } |
a0d0e21e |
1559 | |
d1b91892 |
1560 | # Register the Perl callback |
1561 | asynch_read($fh, \&callback1) ; |
a0d0e21e |
1562 | |
d1b91892 |
1563 | asynch_close($fh) ; |
1564 | |
1565 | The mapping between the C callback and Perl is stored in the global |
1566 | hash C<Mapping> this time. Using a hash has the distinct advantage that |
1567 | it allows an unlimited number of callbacks to be registered. |
1568 | |
1569 | What if the interface provided by the C callback doesn't contain a |
1570 | parameter which allows the file handle to Perl subroutine mapping? Say |
1571 | in the asynchronous i/o package, the callback function gets passed only |
1572 | the C<buffer> parameter like this |
1573 | |
1574 | void |
1575 | ProcessRead(buffer) |
1576 | char * buffer ; |
1577 | { |
1578 | ... |
1579 | } |
a0d0e21e |
1580 | |
d1b91892 |
1581 | Without the file handle there is no straightforward way to map from the |
1582 | C callback to the Perl subroutine. |
a0d0e21e |
1583 | |
d1b91892 |
1584 | In this case a possible way around this problem is to pre-define a |
1585 | series of C functions to act as the interface to Perl, thus |
1586 | |
1587 | #define MAX_CB 3 |
1588 | #define NULL_HANDLE -1 |
1589 | typedef void (*FnMap)() ; |
1590 | |
1591 | struct MapStruct { |
1592 | FnMap Function ; |
1593 | SV * PerlSub ; |
1594 | int Handle ; |
1595 | } ; |
1596 | |
1597 | static void fn1() ; |
1598 | static void fn2() ; |
1599 | static void fn3() ; |
1600 | |
1601 | static struct MapStruct Map [MAX_CB] = |
1602 | { |
1603 | { fn1, NULL, NULL_HANDLE }, |
1604 | { fn2, NULL, NULL_HANDLE }, |
1605 | { fn3, NULL, NULL_HANDLE } |
1606 | } ; |
1607 | |
1608 | static void |
1609 | Pcb(index, buffer) |
1610 | int index ; |
1611 | char * buffer ; |
1612 | { |
1613 | dSP ; |
1614 | |
1615 | PUSHMARK(sp) ; |
1616 | XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ; |
1617 | PUTBACK ; |
1618 | |
1619 | /* Call the Perl sub */ |
1620 | perl_call_sv(Map[index].PerlSub, G_DISCARD) ; |
1621 | } |
1622 | |
1623 | static void |
1624 | fn1(buffer) |
1625 | char * buffer ; |
1626 | { |
1627 | Pcb(0, buffer) ; |
1628 | } |
1629 | |
1630 | static void |
1631 | fn2(buffer) |
1632 | char * buffer ; |
1633 | { |
1634 | Pcb(1, buffer) ; |
1635 | } |
1636 | |
1637 | static void |
1638 | fn3(buffer) |
1639 | char * buffer ; |
1640 | { |
1641 | Pcb(2, buffer) ; |
1642 | } |
1643 | |
1644 | void |
1645 | array_asynch_read(fh, callback) |
1646 | int fh |
1647 | SV * callback |
1648 | CODE: |
1649 | int index ; |
1650 | int null_index = MAX_CB ; |
1651 | |
1652 | /* Find the same handle or an empty entry */ |
1653 | for (index = 0 ; index < MAX_CB ; ++index) |
1654 | { |
1655 | if (Map[index].Handle == fh) |
1656 | break ; |
1657 | |
1658 | if (Map[index].Handle == NULL_HANDLE) |
1659 | null_index = index ; |
1660 | } |
1661 | |
1662 | if (index == MAX_CB && null_index == MAX_CB) |
1663 | croak ("Too many callback functions registered\n") ; |
1664 | |
1665 | if (index == MAX_CB) |
1666 | index = null_index ; |
1667 | |
1668 | /* Save the file handle */ |
1669 | Map[index].Handle = fh ; |
1670 | |
1671 | /* Remember the Perl sub */ |
1672 | if (Map[index].PerlSub == (SV*)NULL) |
1673 | Map[index].PerlSub = newSVsv(callback) ; |
1674 | else |
1675 | SvSetSV(Map[index].PerlSub, callback) ; |
1676 | |
1677 | asynch_read(fh, Map[index].Function) ; |
1678 | |
1679 | void |
1680 | array_asynch_close(fh) |
1681 | int fh |
1682 | CODE: |
1683 | int index ; |
1684 | |
1685 | /* Find the file handle */ |
1686 | for (index = 0; index < MAX_CB ; ++ index) |
1687 | if (Map[index].Handle == fh) |
1688 | break ; |
1689 | |
1690 | if (index == MAX_CB) |
1691 | croak ("could not close fh %d\n", fh) ; |
1692 | |
1693 | Map[index].Handle = NULL_HANDLE ; |
1694 | SvREFCNT_dec(Map[index].PerlSub) ; |
1695 | Map[index].PerlSub = (SV*)NULL ; |
1696 | |
1697 | asynch_close(fh) ; |
1698 | |
1699 | In this case the functions C<fn1>, C<fn2> and C<fn3> are used to |
1700 | remember the Perl subroutine to be called. Each of the functions holds |
1701 | a separate hard-wired index which is used in the function C<Pcb> to |
1702 | access the C<Map> array and actually call the Perl subroutine. |
1703 | |
1704 | There are some obvious disadvantages with this technique. |
1705 | |
1706 | Firstly, the code is considerably more complex than with the previous |
1707 | example. |
1708 | |
1709 | Secondly, there is a hard-wired limit (in this case 3) to the number of |
1710 | callbacks that can exist simultaneously. The only way to increase the |
1711 | limit is by modifying the code to add more functions and then |
1712 | re-compiling. None the less, as long as the number of functions is |
1713 | chosen with some care, it is still a workable solution and in some |
1714 | cases is the only one available. |
1715 | |
1716 | To summarize, here are a number of possible methods for you to consider |
1717 | for storing the mapping between C and the Perl callback |
1718 | |
1719 | =over 5 |
1720 | |
1721 | =item 1. Ignore the problem - Allow only 1 callback |
1722 | |
1723 | For a lot of situations, like interfacing to an error handler, this may |
1724 | be a perfectly adequate solution. |
1725 | |
1726 | =item 2. Create a sequence of callbacks - hard wired limit |
1727 | |
1728 | If it is impossible to tell from the parameters passed back from the C |
1729 | callback what the context is, then you may need to create a sequence of C |
1730 | callback interface functions, and store pointers to each in an array. |
1731 | |
1732 | =item 3. Use a parameter to map to the Perl callback |
1733 | |
1734 | A hash is an ideal mechanism to store the mapping between C and Perl. |
1735 | |
1736 | =back |
a0d0e21e |
1737 | |
a0d0e21e |
1738 | |
1739 | =head2 Alternate Stack Manipulation |
1740 | |
a0d0e21e |
1741 | |
d1b91892 |
1742 | Although I have made use of only the C<POP*> macros to access values |
1743 | returned from Perl subroutines, it is also possible to bypass these |
8e07c86e |
1744 | macros and read the stack using the C<ST> macro (See L<perlxs> for a |
d1b91892 |
1745 | full description of the C<ST> macro). |
1746 | |
1747 | Most of the time the C<POP*> macros should be adequate, the main |
1748 | problem with them is that they force you to process the returned values |
1749 | in sequence. This may not be the most suitable way to process the |
1750 | values in some cases. What we want is to be able to access the stack in |
1751 | a random order. The C<ST> macro as used when coding an XSUB is ideal |
1752 | for this purpose. |
1753 | |
1754 | The code below is the example given in the section I<Returning a list |
1755 | of values> recoded to use C<ST> instead of C<POP*>. |
1756 | |
1757 | static void |
1758 | call_AddSubtract2(a, b) |
1759 | int a ; |
1760 | int b ; |
1761 | { |
1762 | dSP ; |
1763 | I32 ax ; |
1764 | int count ; |
1765 | |
1766 | ENTER ; |
1767 | SAVETMPS; |
1768 | |
1769 | PUSHMARK(sp) ; |
1770 | XPUSHs(sv_2mortal(newSViv(a))); |
1771 | XPUSHs(sv_2mortal(newSViv(b))); |
1772 | PUTBACK ; |
1773 | |
1774 | count = perl_call_pv("AddSubtract", G_ARRAY); |
1775 | |
1776 | SPAGAIN ; |
1777 | sp -= count ; |
1778 | ax = (sp - stack_base) + 1 ; |
1779 | |
1780 | if (count != 2) |
1781 | croak("Big trouble\n") ; |
a0d0e21e |
1782 | |
d1b91892 |
1783 | printf ("%d + %d = %d\n", a, b, SvIV(ST(0))) ; |
1784 | printf ("%d - %d = %d\n", a, b, SvIV(ST(1))) ; |
1785 | |
1786 | PUTBACK ; |
1787 | FREETMPS ; |
1788 | LEAVE ; |
1789 | } |
1790 | |
1791 | Notes |
1792 | |
1793 | =over 5 |
1794 | |
1795 | =item 1. |
1796 | |
1797 | Notice that it was necessary to define the variable C<ax>. This is |
1798 | because the C<ST> macro expects it to exist. If we were in an XSUB it |
1799 | would not be necessary to define C<ax> as it is already defined for |
1800 | you. |
1801 | |
1802 | =item 2. |
1803 | |
1804 | The code |
1805 | |
1806 | SPAGAIN ; |
1807 | sp -= count ; |
1808 | ax = (sp - stack_base) + 1 ; |
1809 | |
1810 | sets the stack up so that we can use the C<ST> macro. |
1811 | |
1812 | =item 3. |
1813 | |
1814 | Unlike the original coding of this example, the returned |
1815 | values are not accessed in reverse order. So C<ST(0)> refers to the |
1816 | first value returned by the Perl subroutine and C<ST(count-1)> |
1817 | refers to the last. |
1818 | |
1819 | =back |
a0d0e21e |
1820 | |
1821 | =head1 SEE ALSO |
1822 | |
8e07c86e |
1823 | L<perlxs>, L<perlguts>, L<perlembed> |
a0d0e21e |
1824 | |
1825 | =head1 AUTHOR |
1826 | |
1827 | Paul Marquess <pmarquess@bfsec.bt.co.uk> |
1828 | |
d1b91892 |
1829 | Special thanks to the following people who assisted in the creation of |
1830 | the document. |
a0d0e21e |
1831 | |
d1b91892 |
1832 | Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem and Larry Wall. |
a0d0e21e |
1833 | |
1834 | =head1 DATE |
1835 | |
d1b91892 |
1836 | Version 1.1, 17th May 1995 |