Amiga Tools 5

home *** CD-ROM | disk | FTP | other *** search

/ Amiga Tools 5 / Amiga Tools 5.iso / tools / developer-tools / andere sprachen / perl5 / man / catp / perlcall.0 < prev next >

Wrap

Text File | 1996-03-02 | 132.8 KB | 2,311 lines

PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) NNNNAAAAMMMMEEEE perlcall - Perl calling conventions from C DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN The purpose of this document is to show you how to call Perl subroutines directly from C, i.e. how to write _c_a_l_l_b_a_c_k_s. Apart from discussing the C interface provided by Perl for writing callbacks the document uses a series of examples to show how the interface actually works in practice. In addition some techniques for coding callbacks are covered. Examples where callbacks are necessary include +o An Error Handler You have created an XSUB interface to an application's C API. A fairly common feature in applications is to allow you to define a C function that will be called whenever something nasty occurs. What we would like is to be able to specify a Perl subroutine that will be called instead. +o An Event Driven Program The classic example of where callbacks are used is when writing an event driven program like for an X windows application. In this case your register functions to be called whenever specific events occur, e.g. a mouse button is pressed, the cursor moves into a window or a menu item is selected. Although the techniques described here are applicable when embedding Perl in a C program, this is not the primary goal of this document. There are other details that must be considered and are specific to embedding Perl. For details on embedding Perl in C refer to the _p_e_r_l_e_m_b_e_d manpage. Before you launch yourself head first into the rest of this document, it would be a good idea to have read the following two documents - the _p_e_r_l_x_s manpage and the _p_e_r_l_g_u_t_s manpage. TTTTHHHHEEEE PPPPEEEERRRRLLLL____CCCCAAAALLLLLLLL FFFFUUUUNNNNCCCCTTTTIIIIOOOONNNNSSSS Although this stuff is easier to explain using examples, you first need be aware of a few important definitions. Perl has a number of C functions that allow you to call Perl subroutines. They are 29/Jan/96 perl 5.002 with 1 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) IIII33332222 ppppeeeerrrrllll____ccccaaaallllllll____ssssvvvv((((SSSSVVVV**** ssssvvvv,,,, IIII33332222 ffffllllaaaaggggssss)))) ;;;; IIII33332222 ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((cccchhhhaaaarrrr ****ssssuuuubbbbnnnnaaaammmmeeee,,,, IIII33332222 ffffllllaaaaggggssss)))) ;;;; IIII33332222 ppppeeeerrrrllll____ccccaaaallllllll____mmmmeeeetttthhhhoooodddd((((cccchhhhaaaarrrr ****mmmmeeeetttthhhhnnnnaaaammmmeeee,,,, IIII33332222 ffffllllaaaaggggssss)))) ;;;; IIII33332222 ppppeeeerrrrllll____ccccaaaallllllll____aaaarrrrggggvvvv((((cccchhhhaaaarrrr ****ssssuuuubbbbnnnnaaaammmmeeee,,,, IIII33332222 ffffllllaaaaggggssss,,,, rrrreeeeggggiiiisssstttteeeerrrr cccchhhhaaaarrrr ********aaaarrrrggggvvvv)))) ;;;; The key function is _p_e_r_l___c_a_l_l___s_v. All the other functions are fairly simple wrappers which make it easier to call Perl subroutines in special cases. At the end of the day they will all call _p_e_r_l___c_a_l_l___s_v to actually invoke the Perl subroutine. All the _p_e_r_l___c_a_l_l___* functions have a ffffllllaaaaggggssss parameter which is used to pass a bit mask of options to Perl. This bit mask operates identically for each of the functions. The settings available in the bit mask are discussed in the section on _F_L_A_G _V_A_L_U_E_S. Each of the functions will now be discussed in turn. ppppeeeerrrrllll____ccccaaaallllllll____ssssvvvv _p_e_r_l___c_a_l_l___s_v takes two parameters, the first, ssssvvvv, is an SV*. This allows you to specify the Perl subroutine to be called either as a C string (which has first been converted to an SV) or a reference to a subroutine. The section, _U_s_i_n_g _p_e_r_l___c_a_l_l___s_v, shows how you can make use of _p_e_r_l___c_a_l_l___s_v. ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv The function, _p_e_r_l___c_a_l_l___p_v, is similar to _p_e_r_l___c_a_l_l___s_v except it expects its first parameter to be a C char* which identifies the Perl subroutine you want to call, e.g. ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((""""ffffrrrreeeedddd"""",,,, 0000)))). If the subroutine you want to call is in another package, just include the package name in the string, e.g. """"ppppkkkkgggg::::::::ffffrrrreeeedddd"""". ppppeeeerrrrllll____ccccaaaallllllll____mmmmeeeetttthhhhoooodddd The function _p_e_r_l___c_a_l_l___m_e_t_h_o_d is used to call a method from a Perl class. The parameter mmmmeeeetttthhhhnnnnaaaammmmeeee corresponds to the name of the method to be called. Note that the class that the method belongs to is passed on the Perl stack rather than in the parameter list. This class can be either the name of the class (for a static method) or a reference to an object (for a virtual method). See the _p_e_r_l_o_b_j manpage for more information on static and virtual methods and the section on _U_s_i_n_g _p_e_r_l___c_a_l_l___m_e_t_h_o_d for an example of using _p_e_r_l___c_a_l_l___m_e_t_h_o_d. ppppeeeerrrrllll____ccccaaaallllllll____aaaarrrrggggvvvv _p_e_r_l___c_a_l_l___a_r_g_v calls the Perl subroutine specified by the C string stored in the ssssuuuubbbbnnnnaaaammmmeeee parameter. It also takes the usual ffffllllaaaaggggssss parameter. The final parameter, aaaarrrrggggvvvv, consists of a NULL terminated list 29/Jan/96 perl 5.002 with 2 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) of C strings to be passed as parameters to the Perl subroutine. See _U_s_i_n_g _p_e_r_l___c_a_l_l___a_r_g_v. All the functions return an integer. This is a count of the number of items returned by the Perl subroutine. The actual items returned by the subroutine are stored on the Perl stack. As a general rule you should _a_l_w_a_y_s check the return value from these functions. Even if you are expecting only a particular number of values to be returned from the Perl subroutine, there is nothing to stop someone from doing something unexpected - don't say you haven't been warned. FFFFLLLLAAAAGGGG VVVVAAAALLLLUUUUEEEESSSS The ffffllllaaaaggggssss parameter in all the _p_e_r_l___c_a_l_l___* functions is a bit mask which can consist of any combination of the symbols defined below, OR'ed together. GGGG____SSSSCCCCAAAALLLLAAAARRRR Calls the Perl subroutine in a scalar context. This is the default context flag setting for all the _p_e_r_l___c_a_l_l___* functions. This flag has 2 effects 1. it indicates to the subroutine being called that it is executing in a scalar context (if it executes _w_a_n_t_a_r_r_a_y the result will be false). 2. it ensures that only a scalar is actually returned from the subroutine. The subroutine can, of course, ignore the _w_a_n_t_a_r_r_a_y and return a list anyway. If so, then only the last element of the list will be returned. The value returned by the _p_e_r_l___c_a_l_l___* function indicates how may items have been returned by the Perl subroutine - in this case it will be either 0 or 1. If 0, then you have specified the G_DISCARD flag. If 1, then the item actually returned by the Perl subroutine will be stored on the Perl stack - the section _R_e_t_u_r_n_i_n_g _a _S_c_a_l_a_r shows how to access this value on the stack. Remember that regardless of how many items the Perl subroutine returns, only the last one will be accessible from the stack - think of the case where only one value is returned as being a list with only one element. Any other items that were returned will not exist by the time control returns from the _p_e_r_l___c_a_l_l___* function. The section _R_e_t_u_r_n_i_n_g _a _l_i_s_t _i_n _a _s_c_a_l_a_r _c_o_n_t_e_x_t shows an example of this behaviour. 29/Jan/96 perl 5.002 with 3 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) GGGG____AAAARRRRRRRRAAAAYYYY Calls the Perl subroutine in a list context. As with G_SCALAR, this flag has 2 effects 1. it indicates to the subroutine being called that it is executing in an array context (if it executes _w_a_n_t_a_r_r_a_y the result will be true). 2. it ensures that all items returned from the subroutine will be accessible when control returns from the _p_e_r_l___c_a_l_l___* function. The value returned by the _p_e_r_l___c_a_l_l___* function indicates how may items have been returned by the Perl subroutine. If 0, the you have specified the G_DISCARD flag. If not 0, then it will be a count of the number of items returned by the subroutine. These items will be stored on the Perl stack. The section _R_e_t_u_r_n_i_n_g _a _l_i_s_t _o_f _v_a_l_u_e_s gives an example of using the G_ARRAY flag and the mechanics of accessing the returned items from the Perl stack. GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD By default, the _p_e_r_l___c_a_l_l___* functions place the items returned from by the Perl subroutine on the stack. If you are not interested in these items, then setting this flag will make Perl get rid of them automatically for you. Note that it is still possible to indicate a context to the Perl subroutine by using either G_SCALAR or G_ARRAY. If you do not set this flag then it is _v_e_r_y important that you make sure that any temporaries (i.e. parameters passed to the Perl subroutine and values returned from the subroutine) are disposed of yourself. The section _R_e_t_u_r_n_i_n_g _a _S_c_a_l_a_r gives details of how to explicitly dispose of these temporaries and the section _U_s_i_n_g _P_e_r_l _t_o _d_i_s_p_o_s_e _o_f _t_e_m_p_o_r_a_r_i_e_s discusses the specific circumstances where you can ignore the problem and let Perl deal with it for you. GGGG____NNNNOOOOAAAARRRRGGGGSSSS Whenever a Perl subroutine is called using one of the _p_e_r_l___c_a_l_l___* functions, it is assumed by default that parameters are to be passed to the subroutine. If you are not passing any parameters to the Perl subroutine, you can save a bit of time by setting this flag. It has the effect of not creating the @@@@____ array for the Perl subroutine. 29/Jan/96 perl 5.002 with 4 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) Although the functionality provided by this flag may seem straightforward, it should be used only if there is a good reason to do so. The reason for being cautious is that even if you have specified the G_NOARGS flag, it is still possible for the Perl subroutine that has been called to think that you have passed it parameters. In fact, what can happen is that the Perl subroutine you have called can access the @@@@____ array from a previous Perl subroutine. This will occur when the code that is executing the _p_e_r_l___c_a_l_l___* function has itself been called from another Perl subroutine. The code below illustrates this ssssuuuubbbb ffffrrrreeeedddd {{{{ pppprrrriiiinnnntttt """"@@@@____\\\\nnnn"""" }}}} ssssuuuubbbb jjjjooooeeee {{{{ &&&&ffffrrrreeeedddd }}}} &&&&jjjjooooeeee((((1111,,,,2222,,,,3333)))) ;;;; This will print 1111 2222 3333 What has happened is that ffffrrrreeeedddd accesses the @@@@____ array which belongs to jjjjooooeeee. GGGG____EEEEVVVVAAAALLLL It is possible for the Perl subroutine you are calling to terminate abnormally, e.g. by calling _d_i_e explicitly or by not actually existing. By default, when either of these of events occurs, the process will terminate immediately. If though, you want to trap this type of event, specify the G_EVAL flag. It will put an _e_v_a_l _{ _} around the subroutine call. Whenever control returns from the _p_e_r_l___c_a_l_l___* function you need to check the $$$$@@@@ variable as you would in a normal Perl script. The value returned from the _p_e_r_l___c_a_l_l___* function is dependent on what other flags have been specified and whether an error has occurred. Here are all the different cases that can occur +o If the _p_e_r_l___c_a_l_l___* function returns normally, then the value returned is as specified in the previous sections. +o If G_DISCARD is specified, the return value will always be 0. 29/Jan/96 perl 5.002 with 5 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) +o If G_ARRAY is specified _a_n_d an error has occurred, the return value will always be 0. +o If G_SCALAR is specified _a_n_d an error has occurred, the return value will be 1 and the value on the top of the stack will be _u_n_d_e_f. This means that if you have already detected the error by checking $$$$@@@@ and you want the program to continue, you must remember to pop the _u_n_d_e_f from the stack. See _U_s_i_n_g _G___E_V_A_L for details of using G_EVAL. GGGG____KKKKEEEEEEEEPPPPEEEERRRRRRRR You may have noticed that using the G_EVAL flag described above will aaaallllwwwwaaaayyyyssss clear the $$$$@@@@ variable and set it to a string describing the error iff there was an error in the called code. This unqualified resetting of $$$$@@@@ can be problematic in the reliable identification of errors using the eeeevvvvaaaallll {{{{}}}} mechanism, because the possibility exists that perl will call other code (end of block processing code, for example) between the time the error causes $$$$@@@@ to be set within eeeevvvvaaaallll {{{{}}}}, and the subsequent statement which checks for the value of $$$$@@@@ gets executed in the user's script. This scenario will mostly be applicable to code that is meant to be called from within destructors, asynchronous callbacks, signal handlers, ________DDDDIIIIEEEE________ or ________WWWWAAAARRRRNNNN________ hooks, and ttttiiiieeee functions. In such situations, you will not want to clear $$$$@@@@ at all, but simply to append any new errors to any existing value of $$$$@@@@. The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in _p_e_r_l___c_a_l_l___* functions that are used to implement such code. This flag has no effect when G_EVAL is not used. When G_KEEPERR is used, any errors in the called code will be prefixed with the string "\t(in cleanup)", and appended to the current value of $$$$@@@@. The G_KEEPERR flag was introduced in Perl version 5.002. See _U_s_i_n_g _G___K_E_E_P_E_R_R for an example of a situation that warrants the use of this flag. DDDDeeeetttteeeerrrrmmmmiiiinnnniiiinnnngggg tttthhhheeee CCCCoooonnnntttteeeexxxxtttt As mentioned above, you can determine the context of the currently executing subroutine in Perl with _w_a_n_t_a_r_r_a_y. The equivalent test can be made in C by using the GGGGIIIIMMMMMMMMEEEE macro. This will return GGGG____SSSSCCCCAAAALLLLAAAARRRR if you have been called in a scalar context and GGGG____AAAARRRRRRRRAAAAYYYY if in an array context. An 29/Jan/96 perl 5.002 with 6 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) example of using the GGGGIIIIMMMMMMMMEEEE macro is shown in section _U_s_i_n_g _G_I_M_M_E. KKKKNNNNOOOOWWWWNNNN PPPPRRRROOOOBBBBLLLLEEEEMMMMSSSS This section outlines all known problems that exist in the _p_e_r_l___c_a_l_l___* functions. 1. If you are intending to make use of both the G_EVAL and G_SCALAR flags in your code, use a version of Perl greater than 5.000. There is a bug in version 5.000 of Perl which means that the combination of these two flags will not work as described in the section _F_L_A_G _V_A_L_U_E_S. Specifically, if the two flags are used when calling a subroutine and that subroutine does not call _d_i_e, the value returned by _p_e_r_l___c_a_l_l___* will be wrong. 2. In Perl 5.000 and 5.001 there is a problem with using _p_e_r_l___c_a_l_l___* if the Perl sub you are calling attempts to trap a _d_i_e. The symptom of this problem is that the called Perl sub will continue to completion, but whenever it attempts to pass control back to the XSUB, the program will immediately terminate. For example, say you want to call this Perl sub ssssuuuubbbb ffffrrrreeeedddd {{{{ eeeevvvvaaaallll {{{{ ddddiiiieeee """"FFFFaaaattttaaaallll EEEErrrrrrrroooorrrr"""" ;;;; }}}} pppprrrriiiinnnntttt """"TTTTrrrraaaappppppppeeeedddd eeeerrrrrrrroooorrrr:::: $$$$@@@@\\\\nnnn"""" iiiiffff $$$$@@@@ ;;;; }}}} via this XSUB vvvvooooiiiidddd CCCCaaaallllllll____ffffrrrreeeedddd(((()))) CCCCOOOODDDDEEEE:::: PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((""""ffffrrrreeeedddd"""",,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD||||GGGG____NNNNOOOOAAAARRRRGGGGSSSS)))) ;;;; ffffpppprrrriiiinnnnttttffff((((ssssttttddddeeeerrrrrrrr,,,, """"bbbbaaaacccckkkk iiiinnnn CCCCaaaallllllll____ffffrrrreeeedddd\\\\nnnn"""")))) ;;;; When CCCCaaaallllllll____ffffrrrreeeedddd is executed it will print TTTTrrrraaaappppppppeeeedddd eeeerrrrrrrroooorrrr:::: FFFFaaaattttaaaallll EEEErrrrrrrroooorrrr As control never returns to CCCCaaaallllllll____ffffrrrreeeedddd, the """"bbbbaaaacccckkkk iiiinnnn CCCCaaaallllllll____ffffrrrreeeedddd"""" string will not get printed. To work around this problem, you can either upgrade to Perl 5.002 (or later), or use the G_EVAL flag with 29/Jan/96 perl 5.002 with 7 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) _p_e_r_l___c_a_l_l___* as shown below vvvvooooiiiidddd CCCCaaaallllllll____ffffrrrreeeedddd(((()))) CCCCOOOODDDDEEEE:::: PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((""""ffffrrrreeeedddd"""",,,, GGGG____EEEEVVVVAAAALLLL||||GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD||||GGGG____NNNNOOOOAAAARRRRGGGGSSSS)))) ;;;; ffffpppprrrriiiinnnnttttffff((((ssssttttddddeeeerrrrrrrr,,,, """"bbbbaaaacccckkkk iiiinnnn CCCCaaaallllllll____ffffrrrreeeedddd\\\\nnnn"""")))) ;;;; EEEEXXXXAAAAMMMMPPPPLLLLEEEESSSS Enough of the definition talk, let's have a few examples. Perl provides many macros to assist in accessing the Perl stack. Wherever possible, these macros should always be used when interfacing to Perl internals. Hopefully this should make the code less vulnerable to any changes made to Perl in the future. Another point worth noting is that in the first series of examples I have made use of only the _p_e_r_l___c_a_l_l___p_v function. This has been done to keep the code simpler and ease you into the topic. Wherever possible, if the choice is between using _p_e_r_l___c_a_l_l___p_v and _p_e_r_l___c_a_l_l___s_v, you should always try to use _p_e_r_l___c_a_l_l___s_v. See _U_s_i_n_g _p_e_r_l___c_a_l_l___s_v for details. NNNNoooo PPPPaaaarrrraaaammmmeeeetttteeeerrrrssss,,,, NNNNooootttthhhhiiiinnnngggg rrrreeeettttuuuurrrrnnnneeeedddd This first trivial example will call a Perl subroutine, _P_r_i_n_t_U_I_D, to print out the UID of the process. ssssuuuubbbb PPPPrrrriiiinnnnttttUUUUIIIIDDDD {{{{ pppprrrriiiinnnntttt """"UUUUIIIIDDDD iiiissss $$$$<<<<\\\\nnnn"""" ;;;; }}}} and here is a C function to call it ssssttttaaaattttiiiicccc vvvvooooiiiidddd ccccaaaallllllll____PPPPrrrriiiinnnnttttUUUUIIIIDDDD(((()))) {{{{ ddddSSSSPPPP ;;;; PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((""""PPPPrrrriiiinnnnttttUUUUIIIIDDDD"""",,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD||||GGGG____NNNNOOOOAAAARRRRGGGGSSSS)))) ;;;; }}}} Simple, eh. A few points to note about this example. 1. Ignore ddddSSSSPPPP and PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) for now. They will be discussed in the next example. 29/Jan/96 perl 5.002 with 8 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) 2. We aren't passing any parameters to _P_r_i_n_t_U_I_D so G_NOARGS can be specified. 3. We aren't interested in anything returned from _P_r_i_n_t_U_I_D, so G_DISCARD is specified. Even if _P_r_i_n_t_U_I_D was changed to actually return some _v_a_l_u_e(s), having specified G_DISCARD will mean that they will be wiped by the time control returns from _p_e_r_l___c_a_l_l___p_v. 4. As _p_e_r_l___c_a_l_l___p_v is being used, the Perl subroutine is specified as a C string. In this case the subroutine name has been 'hard-wired' into the code. 5. Because we specified G_DISCARD, it is not necessary to check the value returned from _p_e_r_l___c_a_l_l___p_v. It will always be 0. PPPPaaaassssssssiiiinnnngggg PPPPaaaarrrraaaammmmeeeetttteeeerrrrssss Now let's make a slightly more complex example. This time we want to call a Perl subroutine, LLLLeeeeffffttttSSSSttttrrrriiiinnnngggg, which will take 2 parameters - a string ($$$$ssss) and an integer ($$$$nnnn). The subroutine will simply print the first $$$$nnnn characters of the string. So the Perl subroutine would look like this ssssuuuubbbb LLLLeeeeffffttttSSSSttttrrrriiiinnnngggg {{{{ mmmmyyyy(((($$$$ssss,,,, $$$$nnnn)))) ==== @@@@____ ;;;; pppprrrriiiinnnntttt ssssuuuubbbbssssttttrrrr(((($$$$ssss,,,, 0000,,,, $$$$nnnn)))),,,, """"\\\\nnnn"""" ;;;; }}}} The C function required to call _L_e_f_t_S_t_r_i_n_g would look like this. ssssttttaaaattttiiiicccc vvvvooooiiiidddd ccccaaaallllllll____LLLLeeeeffffttttSSSSttttrrrriiiinnnngggg((((aaaa,,,, bbbb)))) cccchhhhaaaarrrr **** aaaa ;;;; iiiinnnntttt bbbb ;;;; {{{{ ddddSSSSPPPP ;;;; PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVVppppvvvv((((aaaa,,,, 0000))))))))))));;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((bbbb))))))))))));;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((""""LLLLeeeeffffttttSSSSttttrrrriiiinnnngggg"""",,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD))));;;; }}}} Here are a few notes on the C function _c_a_l_l___L_e_f_t_S_t_r_i_n_g. 29/Jan/96 perl 5.002 with 9 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) 1. Parameters are passed to the Perl subroutine using the Perl stack. This is the purpose of the code beginning with the line ddddSSSSPPPP and ending with the line PPPPUUUUTTTTBBBBAAAACCCCKKKK. 2. If you are going to put something onto the Perl stack, you need to know where to put it. This is the purpose of the macro ddddSSSSPPPP - it declares and initializes a _l_o_c_a_l copy of the Perl stack pointer. All the other macros which will be used in this example require you to have used this macro. The exception to this rule is if you are calling a Perl subroutine directly from an XSUB function. In this case it is not necessary to explicitly use the ddddSSSSPPPP macro - it will be declared for you automatically. 3. Any parameters to be pushed onto the stack should be bracketed by the PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK and PPPPUUUUTTTTBBBBAAAACCCCKKKK macros. The purpose of these two macros, in this context, is to automatically count the number of parameters you are pushing. Then whenever Perl is creating the @@@@____ array for the subroutine, it knows how big to make it. The PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK macro tells Perl to make a mental note of the current stack pointer. Even if you aren't passing any parameters (like the example shown in the section _N_o _P_a_r_a_m_e_t_e_r_s_, _N_o_t_h_i_n_g _r_e_t_u_r_n_e_d) you must still call the PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK macro before you can call any of the _p_e_r_l___c_a_l_l___* functions - Perl still needs to know that there are no parameters. The PPPPUUUUTTTTBBBBAAAACCCCKKKK macro sets the global copy of the stack pointer to be the same as our local copy. If we didn't do this _p_e_r_l___c_a_l_l___p_v wouldn't know where the two parameters we pushed were - remember that up to now all the stack pointer manipulation we have done is with our local copy, _n_o_t the global copy. 4. The only flag specified this time is G_DISCARD. Since we are passing 2 parameters to the Perl subroutine this time, we have not specified G_NOARGS. 5. Next, we come to XPUSHs. This is where the parameters actually get pushed onto the stack. In this case we are pushing a string and an integer. See the section the section on _X_S_U_B_'_S _a_n_d _t_h_e _A_r_g_u_m_e_n_t _S_t_a_c_k in the _p_e_r_l_g_u_t_s manpage for details on how the XPUSH macros work. 29/Jan/96 perl 5.002 with 10 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) 6. Finally, _L_e_f_t_S_t_r_i_n_g can now be called via the _p_e_r_l___c_a_l_l___p_v function. RRRReeeettttuuuurrrrnnnniiiinnnngggg aaaa SSSSccccaaaallllaaaarrrr Now for an example of dealing with the items returned from a Perl subroutine. Here is a Perl subroutine, _A_d_d_e_r, which takes 2 integer parameters and simply returns their sum. ssssuuuubbbb AAAAddddddddeeeerrrr {{{{ mmmmyyyy(((($$$$aaaa,,,, $$$$bbbb)))) ==== @@@@____ ;;;; $$$$aaaa ++++ $$$$bbbb ;;;; }}}} Since we are now concerned with the return value from _A_d_d_e_r, the C function required to call it is now a bit more complex. ssssttttaaaattttiiiicccc vvvvooooiiiidddd ccccaaaallllllll____AAAAddddddddeeeerrrr((((aaaa,,,, bbbb)))) iiiinnnntttt aaaa ;;;; iiiinnnntttt bbbb ;;;; {{{{ ddddSSSSPPPP ;;;; iiiinnnntttt ccccoooouuuunnnntttt ;;;; EEEENNNNTTTTEEEERRRR ;;;; SSSSAAAAVVVVEEEETTTTMMMMPPPPSSSS;;;; PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((aaaa))))))))))));;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((bbbb))))))))))));;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; ccccoooouuuunnnntttt ==== ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((""""AAAAddddddddeeeerrrr"""",,,, GGGG____SSSSCCCCAAAALLLLAAAARRRR))));;;; SSSSPPPPAAAAGGGGAAAAIIIINNNN ;;;; iiiiffff ((((ccccoooouuuunnnntttt !!!!==== 1111)))) ccccrrrrooooaaaakkkk((((""""BBBBiiiigggg ttttrrrroooouuuubbbblllleeee\\\\nnnn"""")))) ;;;; pppprrrriiiinnnnttttffff ((((""""TTTThhhheeee ssssuuuummmm ooooffff %%%%dddd aaaannnndddd %%%%dddd iiiissss %%%%dddd\\\\nnnn"""",,,, aaaa,,,, bbbb,,,, PPPPOOOOPPPPiiii)))) ;;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; FFFFRRRREEEEEEEETTTTMMMMPPPPSSSS ;;;; LLLLEEEEAAAAVVVVEEEE ;;;; }}}} Points to note this time are 29/Jan/96 perl 5.002 with 11 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) 1. The only flag specified this time was G_SCALAR. That means the @@@@____ array will be created and that the value returned by _A_d_d_e_r will still exist after the call to _p_e_r_l___c_a_l_l___p_v. 2. Because we are interested in what is returned from _A_d_d_e_r we cannot specify G_DISCARD. This means that we will have to tidy up the Perl stack and dispose of any temporary values ourselves. This is the purpose of EEEENNNNTTTTEEEERRRR ;;;; SSSSAAAAVVVVEEEETTTTMMMMPPPPSSSS ;;;; at the start of the function, and FFFFRRRREEEEEEEETTTTMMMMPPPPSSSS ;;;; LLLLEEEEAAAAVVVVEEEE ;;;; at the end. The EEEENNNNTTTTEEEERRRR/SSSSAAAAVVVVEEEETTTTMMMMPPPPSSSS pair creates a boundary for any temporaries we create. This means that the temporaries we get rid of will be limited to those which were created after these calls. The FFFFRRRREEEEEEEETTTTMMMMPPPPSSSS/LLLLEEEEAAAAVVVVEEEE pair will get rid of any values returned by the Perl subroutine, plus it will also dump the mortal SV's we have created. Having EEEENNNNTTTTEEEERRRR/SSSSAAAAVVVVEEEETTTTMMMMPPPPSSSS at the beginning of the code makes sure that no other mortals are destroyed. Think of these macros as working a bit like using {{{{ and }}}} in Perl to limit the scope of local variables. See the section _U_s_i_n_g _P_e_r_l _t_o _d_i_s_p_o_s_e _o_f _t_e_m_p_o_r_a_r_i_e_s for details of an alternative to using these macros. 3. The purpose of the macro SSSSPPPPAAAAGGGGAAAAIIIINNNN is to refresh the local copy of the stack pointer. This is necessary because it is possible that the memory allocated to the Perl stack has been re-allocated whilst in the _p_e_r_l___c_a_l_l___p_v call. If you are making use of the Perl stack pointer in your code you must always refresh the your local copy using SPAGAIN whenever you make use of the _p_e_r_l___c_a_l_l___* functions or any other Perl internal function. 4. Although only a single value was expected to be returned from _A_d_d_e_r, it is still good practice to check the return code from _p_e_r_l___c_a_l_l___p_v anyway. Expecting a single value is not quite the same as knowing that there will be one. If someone modified 29/Jan/96 perl 5.002 with 12 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) _A_d_d_e_r to return a list and we didn't check for that possibility and take appropriate action the Perl stack would end up in an inconsistent state. That is something you _r_e_a_l_l_y don't want to ever happen. 5. The PPPPOOOOPPPPiiii macro is used here to pop the return value from the stack. In this case we wanted an integer, so PPPPOOOOPPPPiiii was used. Here is the complete list of POP macros available, along with the types they return. PPPPOOOOPPPPssss SSSSVVVV PPPPOOOOPPPPpppp ppppooooiiiinnnntttteeeerrrr PPPPOOOOPPPPnnnn ddddoooouuuubbbblllleeee PPPPOOOOPPPPiiii iiiinnnntttteeeeggggeeeerrrr PPPPOOOOPPPPllll lllloooonnnngggg 6. The final PPPPUUUUTTTTBBBBAAAACCCCKKKK is used to leave the Perl stack in a consistent state before exiting the function. This is necessary because when we popped the return value from the stack with PPPPOOOOPPPPiiii it updated only our local copy of the stack pointer. Remember, PPPPUUUUTTTTBBBBAAAACCCCKKKK sets the global stack pointer to be the same as our local copy. RRRReeeettttuuuurrrrnnnniiiinnnngggg aaaa lllliiiisssstttt ooooffff vvvvaaaalllluuuueeeessss Now, let's extend the previous example to return both the sum of the parameters and the difference. Here is the Perl subroutine ssssuuuubbbb AAAAddddddddSSSSuuuubbbbttttrrrraaaacccctttt {{{{ mmmmyyyy(((($$$$aaaa,,,, $$$$bbbb)))) ==== @@@@____ ;;;; (((($$$$aaaa++++$$$$bbbb,,,, $$$$aaaa----$$$$bbbb)))) ;;;; }}}} and this is the C function ssssttttaaaattttiiiicccc vvvvooooiiiidddd ccccaaaallllllll____AAAAddddddddSSSSuuuubbbbttttrrrraaaacccctttt((((aaaa,,,, bbbb)))) iiiinnnntttt aaaa ;;;; iiiinnnntttt bbbb ;;;; {{{{ ddddSSSSPPPP ;;;; iiiinnnntttt ccccoooouuuunnnntttt ;;;; EEEENNNNTTTTEEEERRRR ;;;; SSSSAAAAVVVVEEEETTTTMMMMPPPPSSSS;;;; 29/Jan/96 perl 5.002 with 13 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((aaaa))))))))))));;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((bbbb))))))))))));;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; ccccoooouuuunnnntttt ==== ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((""""AAAAddddddddSSSSuuuubbbbttttrrrraaaacccctttt"""",,,, GGGG____AAAARRRRRRRRAAAAYYYY))));;;; SSSSPPPPAAAAGGGGAAAAIIIINNNN ;;;; iiiiffff ((((ccccoooouuuunnnntttt !!!!==== 2222)))) ccccrrrrooooaaaakkkk((((""""BBBBiiiigggg ttttrrrroooouuuubbbblllleeee\\\\nnnn"""")))) ;;;; pppprrrriiiinnnnttttffff ((((""""%%%%dddd ---- %%%%dddd ==== %%%%dddd\\\\nnnn"""",,,, aaaa,,,, bbbb,,,, PPPPOOOOPPPPiiii)))) ;;;; pppprrrriiiinnnnttttffff ((((""""%%%%dddd ++++ %%%%dddd ==== %%%%dddd\\\\nnnn"""",,,, aaaa,,,, bbbb,,,, PPPPOOOOPPPPiiii)))) ;;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; FFFFRRRREEEEEEEETTTTMMMMPPPPSSSS ;;;; LLLLEEEEAAAAVVVVEEEE ;;;; }}}} If _c_a_l_l___A_d_d_S_u_b_t_r_a_c_t is called like this ccccaaaallllllll____AAAAddddddddSSSSuuuubbbbttttrrrraaaacccctttt((((7777,,,, 4444)))) ;;;; then here is the output 7777 ---- 4444 ==== 3333 7777 ++++ 4444 ==== 11111111 Notes 1. We wanted array context, so G_ARRAY was used. 2. Not surprisingly PPPPOOOOPPPPiiii is used twice this time because we were retrieving 2 values from the stack. The important thing to note is that when using the PPPPOOOOPPPP**** macros they come off the stack in _r_e_v_e_r_s_e order. RRRReeeettttuuuurrrrnnnniiiinnnngggg aaaa lllliiiisssstttt iiiinnnn aaaa ssssccccaaaallllaaaarrrr ccccoooonnnntttteeeexxxxtttt Say the Perl subroutine in the previous section was called in a scalar context, like this ssssttttaaaattttiiiicccc vvvvooooiiiidddd ccccaaaallllllll____AAAAddddddddSSSSuuuubbbbSSSSccccaaaallllaaaarrrr((((aaaa,,,, bbbb)))) iiiinnnntttt aaaa ;;;; iiiinnnntttt bbbb ;;;; {{{{ ddddSSSSPPPP ;;;; iiiinnnntttt ccccoooouuuunnnntttt ;;;; iiiinnnntttt iiii ;;;; EEEENNNNTTTTEEEERRRR ;;;; SSSSAAAAVVVVEEEETTTTMMMMPPPPSSSS;;;; 29/Jan/96 perl 5.002 with 14 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((aaaa))))))))))));;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((bbbb))))))))))));;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; ccccoooouuuunnnntttt ==== ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((""""AAAAddddddddSSSSuuuubbbbttttrrrraaaacccctttt"""",,,, GGGG____SSSSCCCCAAAALLLLAAAARRRR))));;;; SSSSPPPPAAAAGGGGAAAAIIIINNNN ;;;; pppprrrriiiinnnnttttffff ((((""""IIIItttteeeemmmmssss RRRReeeettttuuuurrrrnnnneeeedddd ==== %%%%dddd\\\\nnnn"""",,,, ccccoooouuuunnnntttt)))) ;;;; ffffoooorrrr ((((iiii ==== 1111 ;;;; iiii <<<<==== ccccoooouuuunnnntttt ;;;; ++++++++iiii)))) pppprrrriiiinnnnttttffff ((((""""VVVVaaaalllluuuueeee %%%%dddd ==== %%%%dddd\\\\nnnn"""",,,, iiii,,,, PPPPOOOOPPPPiiii)))) ;;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; FFFFRRRREEEEEEEETTTTMMMMPPPPSSSS ;;;; LLLLEEEEAAAAVVVVEEEE ;;;; }}}} The other modification made is that _c_a_l_l___A_d_d_S_u_b_S_c_a_l_a_r will print the number of items returned from the Perl subroutine and their value (for simplicity it assumes that they are integer). So if _c_a_l_l___A_d_d_S_u_b_S_c_a_l_a_r is called ccccaaaallllllll____AAAAddddddddSSSSuuuubbbbSSSSccccaaaallllaaaarrrr((((7777,,,, 4444)))) ;;;; then the output will be IIIItttteeeemmmmssss RRRReeeettttuuuurrrrnnnneeeedddd ==== 1111 VVVVaaaalllluuuueeee 1111 ==== 3333 In this case the main point to note is that only the last item in the list returned from the subroutine, _A_d_d_e_r actually made it back to _c_a_l_l___A_d_d_S_u_b_S_c_a_l_a_r. RRRReeeettttuuuurrrrnnnniiiinnnngggg DDDDaaaattttaaaa ffffrrrroooommmm PPPPeeeerrrrllll vvvviiiiaaaa tttthhhheeee ppppaaaarrrraaaammmmeeeetttteeeerrrr lllliiiisssstttt It is also possible to return values directly via the parameter list - whether it is actually desirable to do it is another matter entirely. The Perl subroutine, _I_n_c, below takes 2 parameters and increments each directly. ssssuuuubbbb IIIInnnncccc {{{{ ++++++++ $$$$____[[[[0000]]]] ;;;; ++++++++ $$$$____[[[[1111]]]] ;;;; }}}} and here is a C function to call it. 29/Jan/96 perl 5.002 with 15 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) ssssttttaaaattttiiiicccc vvvvooooiiiidddd ccccaaaallllllll____IIIInnnncccc((((aaaa,,,, bbbb)))) iiiinnnntttt aaaa ;;;; iiiinnnntttt bbbb ;;;; {{{{ ddddSSSSPPPP ;;;; iiiinnnntttt ccccoooouuuunnnntttt ;;;; SSSSVVVV **** ssssvvvvaaaa ;;;; SSSSVVVV **** ssssvvvvbbbb ;;;; EEEENNNNTTTTEEEERRRR ;;;; SSSSAAAAVVVVEEEETTTTMMMMPPPPSSSS;;;; ssssvvvvaaaa ==== ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((aaaa)))))))) ;;;; ssssvvvvbbbb ==== ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((bbbb)))))))) ;;;; PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvvaaaa))));;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvvbbbb))));;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; ccccoooouuuunnnntttt ==== ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((""""IIIInnnncccc"""",,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD))));;;; iiiiffff ((((ccccoooouuuunnnntttt !!!!==== 0000)))) ccccrrrrooooaaaakkkk ((((""""ccccaaaallllllll____IIIInnnncccc:::: eeeexxxxppppeeeecccctttteeeedddd 0000 vvvvaaaalllluuuueeeessss ffffrrrroooommmm ''''IIIInnnncccc'''',,,, ggggooootttt %%%%dddd\\\\nnnn"""",,,, ccccoooouuuunnnntttt)))) ;;;; pppprrrriiiinnnnttttffff ((((""""%%%%dddd ++++ 1111 ==== %%%%dddd\\\\nnnn"""",,,, aaaa,,,, SSSSvvvvIIIIVVVV((((ssssvvvvaaaa)))))))) ;;;; pppprrrriiiinnnnttttffff ((((""""%%%%dddd ++++ 1111 ==== %%%%dddd\\\\nnnn"""",,,, bbbb,,,, SSSSvvvvIIIIVVVV((((ssssvvvvbbbb)))))))) ;;;; FFFFRRRREEEEEEEETTTTMMMMPPPPSSSS ;;;; LLLLEEEEAAAAVVVVEEEE ;;;; }}}} To be able to access the two parameters that were pushed onto the stack after they return from _p_e_r_l___c_a_l_l___p_v it is necessary to make a note of their addresses - thus the two variables ssssvvvvaaaa and ssssvvvvbbbb. The reason this is necessary is that the area of the Perl stack which held them will very likely have been overwritten by something else by the time control returns from _p_e_r_l___c_a_l_l___p_v. UUUUssssiiiinnnngggg GGGG____EEEEVVVVAAAALLLL Now an example using G_EVAL. Below is a Perl subroutine which computes the difference of its 2 parameters. If this would result in a negative result, the subroutine calls _d_i_e. ssssuuuubbbb SSSSuuuubbbbttttrrrraaaacccctttt {{{{ mmmmyyyy (((($$$$aaaa,,,, $$$$bbbb)))) ==== @@@@____ ;;;; 29/Jan/96 perl 5.002 with 16 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) ddddiiiieeee """"ddddeeeeaaaatttthhhh ccccaaaannnn bbbbeeee ffffaaaattttaaaallll\\\\nnnn"""" iiiiffff $$$$aaaa <<<< $$$$bbbb ;;;; $$$$aaaa ---- $$$$bbbb ;;;; }}}} and some C to call it ssssttttaaaattttiiiicccc vvvvooooiiiidddd ccccaaaallllllll____SSSSuuuubbbbttttrrrraaaacccctttt((((aaaa,,,, bbbb)))) iiiinnnntttt aaaa ;;;; iiiinnnntttt bbbb ;;;; {{{{ ddddSSSSPPPP ;;;; iiiinnnntttt ccccoooouuuunnnntttt ;;;; EEEENNNNTTTTEEEERRRR ;;;; SSSSAAAAVVVVEEEETTTTMMMMPPPPSSSS;;;; PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((aaaa))))))))))));;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((bbbb))))))))))));;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; ccccoooouuuunnnntttt ==== ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((""""SSSSuuuubbbbttttrrrraaaacccctttt"""",,,, GGGG____EEEEVVVVAAAALLLL||||GGGG____SSSSCCCCAAAALLLLAAAARRRR))));;;; SSSSPPPPAAAAGGGGAAAAIIIINNNN ;;;; ////**** CCCChhhheeeecccckkkk tttthhhheeee eeeevvvvaaaallll ffffiiiirrrrsssstttt ****//// iiiiffff ((((SSSSvvvvTTTTRRRRUUUUEEEE((((GGGGvvvvSSSSVVVV((((eeeerrrrrrrrggggvvvv)))))))))))) {{{{ pppprrrriiiinnnnttttffff ((((""""UUUUhhhh oooohhhh ---- %%%%ssss\\\\nnnn"""",,,, SSSSvvvvPPPPVVVV((((GGGGvvvvSSSSVVVV((((eeeerrrrrrrrggggvvvv)))),,,, nnnnaaaa)))))))) ;;;; PPPPOOOOPPPPssss ;;;; }}}} eeeellllsssseeee {{{{ iiiiffff ((((ccccoooouuuunnnntttt !!!!==== 1111)))) ccccrrrrooooaaaakkkk((((""""ccccaaaallllllll____SSSSuuuubbbbttttrrrraaaacccctttt:::: wwwwaaaannnntttteeeedddd 1111 vvvvaaaalllluuuueeee ffffrrrroooommmm ''''SSSSuuuubbbbttttrrrraaaacccctttt'''',,,, ggggooootttt %%%%dddd\\\\nnnn"""",,,, ccccoooouuuunnnntttt)))) ;;;; pppprrrriiiinnnnttttffff ((((""""%%%%dddd ---- %%%%dddd ==== %%%%dddd\\\\nnnn"""",,,, aaaa,,,, bbbb,,,, PPPPOOOOPPPPiiii)))) ;;;; }}}} PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; FFFFRRRREEEEEEEETTTTMMMMPPPPSSSS ;;;; LLLLEEEEAAAAVVVVEEEE ;;;; }}}} If _c_a_l_l___S_u_b_t_r_a_c_t is called thus ccccaaaallllllll____SSSSuuuubbbbttttrrrraaaacccctttt((((4444,,,, 5555)))) the following will be printed UUUUhhhh oooohhhh ---- ddddeeeeaaaatttthhhh ccccaaaannnn bbbbeeee ffffaaaattttaaaallll 29/Jan/96 perl 5.002 with 17 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) Notes 1. We want to be able to catch the _d_i_e so we have used the G_EVAL flag. Not specifying this flag would mean that the program would terminate immediately at the _d_i_e statement in the subroutine _S_u_b_t_r_a_c_t. 2. The code iiiiffff ((((SSSSvvvvTTTTRRRRUUUUEEEE((((GGGGvvvvSSSSVVVV((((eeeerrrrrrrrggggvvvv)))))))))))) {{{{ pppprrrriiiinnnnttttffff ((((""""UUUUhhhh oooohhhh ---- %%%%ssss\\\\nnnn"""",,,, SSSSvvvvPPPPVVVV((((GGGGvvvvSSSSVVVV((((eeeerrrrrrrrggggvvvv)))),,,, nnnnaaaa)))))))) ;;;; PPPPOOOOPPPPssss ;;;; }}}} is the direct equivalent of this bit of Perl pppprrrriiiinnnntttt """"UUUUhhhh oooohhhh ---- $$$$@@@@\\\\nnnn"""" iiiiffff $$$$@@@@ ;;;; eeeerrrrrrrrggggvvvv is a perl global of type GGGGVVVV **** that points to the symbol table entry containing the error. GGGGvvvvSSSSVVVV((((eeeerrrrrrrrggggvvvv)))) therefore refers to the C equivalent of $$$$@@@@. 3. Note that the stack is popped using PPPPOOOOPPPPssss in the block where SSSSvvvvTTTTRRRRUUUUEEEE((((GGGGvvvvSSSSVVVV((((eeeerrrrrrrrggggvvvv)))))))) is true. This is necessary because whenever a _p_e_r_l___c_a_l_l___* function invoked with G_EVAL|G_SCALAR returns an error, the top of the stack holds the value _u_n_d_e_f. Since we want the program to continue after detecting this error, it is essential that the stack is tidied up by removing the _u_n_d_e_f. UUUUssssiiiinnnngggg GGGG____KKKKEEEEEEEEPPPPEEEERRRRRRRR Consider this rather facetious example, where we have used an XS version of the call_Subtract example above inside a destructor: ppppaaaacccckkkkaaaaggggeeee FFFFoooooooo;;;; ssssuuuubbbb nnnneeeewwww {{{{ bbbblllleeeessssssss {{{{}}}},,,, $$$$____[[[[0000]]]] }}}} ssssuuuubbbb SSSSuuuubbbbttttrrrraaaacccctttt {{{{ mmmmyyyy(((($$$$aaaa,,,,$$$$bbbb)))) ==== @@@@____;;;; ddddiiiieeee """"ddddeeeeaaaatttthhhh ccccaaaannnn bbbbeeee ffffaaaattttaaaallll"""" iiiiffff $$$$aaaa <<<< $$$$bbbb ;;;; $$$$aaaa ---- $$$$bbbb;;;; }}}} ssssuuuubbbb DDDDEEEESSSSTTTTRRRROOOOYYYY {{{{ ccccaaaallllllll____SSSSuuuubbbbttttrrrraaaacccctttt((((5555,,,, 4444))));;;; }}}} ssssuuuubbbb ffffoooooooo {{{{ ddddiiiieeee """"ffffoooooooo ddddiiiieeeessss"""";;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; eeeevvvvaaaallll {{{{ FFFFoooooooo---->>>>nnnneeeewwww---->>>>ffffoooooooo }}}};;;; pppprrrriiiinnnntttt """"SSSSaaaawwww:::: $$$$@@@@"""" iiiiffff $$$$@@@@;;;; #### sssshhhhoooouuuulllldddd bbbbeeee,,,, bbbbuuuutttt iiiissssnnnn''''tttt This example will fail to recognize that an error occurred 29/Jan/96 perl 5.002 with 18 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) inside the eeeevvvvaaaallll {{{{}}}}. Here's why: the call_Subtract code got executed while perl was cleaning up temporaries when exiting the eval block, and since call_Subtract is implemented with _p_e_r_l___c_a_l_l___p_v using the G_EVAL flag, it promptly reset $$$$@@@@. This results in the failure of the outermost test for $$$$@@@@, and thereby the failure of the error trap. Appending the G_KEEPERR flag, so that the _p_e_r_l___c_a_l_l___p_v call in call_Subtract reads: ccccoooouuuunnnntttt ==== ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((""""SSSSuuuubbbbttttrrrraaaacccctttt"""",,,, GGGG____EEEEVVVVAAAALLLL||||GGGG____SSSSCCCCAAAALLLLAAAARRRR||||GGGG____KKKKEEEEEEEEPPPPEEEERRRRRRRR))));;;; will preserve the error and restore reliable error handling. UUUUssssiiiinnnngggg ppppeeeerrrrllll____ccccaaaallllllll____ssssvvvv In all the previous examples I have 'hard-wired' the name of the Perl subroutine to be called from C. Most of the time though, it is more convenient to be able to specify the name of the Perl subroutine from within the Perl script. Consider the Perl code below ssssuuuubbbb ffffrrrreeeedddd {{{{ pppprrrriiiinnnntttt """"HHHHeeeelllllllloooo tttthhhheeeerrrreeee\\\\nnnn"""" ;;;; }}}} CCCCaaaallllllllSSSSuuuubbbbPPPPVVVV((((""""ffffrrrreeeedddd"""")))) ;;;; Here is a snippet of XSUB which defines _C_a_l_l_S_u_b_P_V. vvvvooooiiiidddd CCCCaaaallllllllSSSSuuuubbbbPPPPVVVV((((nnnnaaaammmmeeee)))) cccchhhhaaaarrrr **** nnnnaaaammmmeeee CCCCOOOODDDDEEEE:::: PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((nnnnaaaammmmeeee,,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD||||GGGG____NNNNOOOOAAAARRRRGGGGSSSS)))) ;;;; That is fine as far as it goes. The thing is, the Perl subroutine can be specified only as a string. For Perl 4 this was adequate, but Perl 5 allows references to subroutines and anonymous subroutines. This is where _p_e_r_l___c_a_l_l___s_v is useful. The code below for _C_a_l_l_S_u_b_S_V is identical to _C_a_l_l_S_u_b_P_V except that the nnnnaaaammmmeeee parameter is now defined as an SV* and we use _p_e_r_l___c_a_l_l___s_v instead of _p_e_r_l___c_a_l_l___p_v. 29/Jan/96 perl 5.002 with 19 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) vvvvooooiiiidddd CCCCaaaallllllllSSSSuuuubbbbSSSSVVVV((((nnnnaaaammmmeeee)))) SSSSVVVV **** nnnnaaaammmmeeee CCCCOOOODDDDEEEE:::: PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; ppppeeeerrrrllll____ccccaaaallllllll____ssssvvvv((((nnnnaaaammmmeeee,,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD||||GGGG____NNNNOOOOAAAARRRRGGGGSSSS)))) ;;;; Since we are using an SV to call _f_r_e_d the following can all be used CCCCaaaallllllllSSSSuuuubbbbSSSSVVVV((((""""ffffrrrreeeedddd"""")))) ;;;; CCCCaaaallllllllSSSSuuuubbbbSSSSVVVV((((\\\\&&&&ffffrrrreeeedddd)))) ;;;; $$$$rrrreeeeffff ==== \\\\&&&&ffffrrrreeeedddd ;;;; CCCCaaaallllllllSSSSuuuubbbbSSSSVVVV(((($$$$rrrreeeeffff)))) ;;;; CCCCaaaallllllllSSSSuuuubbbbSSSSVVVV(((( ssssuuuubbbb {{{{ pppprrrriiiinnnntttt """"HHHHeeeelllllllloooo tttthhhheeeerrrreeee\\\\nnnn"""" }}}} )))) ;;;; As you can see, _p_e_r_l___c_a_l_l___s_v gives you much greater flexibility in how you can specify the Perl subroutine. You should note that if it is necessary to store the SV (nnnnaaaammmmeeee in the example above) which corresponds to the Perl subroutine so that it can be used later in the program, it not enough to just store a copy of the pointer to the SV. Say the code above had been like this ssssttttaaaattttiiiicccc SSSSVVVV **** rrrreeeemmmmeeeemmmmbbbbeeeerrrrSSSSuuuubbbb ;;;; vvvvooooiiiidddd SSSSaaaavvvveeeeSSSSuuuubbbb1111((((nnnnaaaammmmeeee)))) SSSSVVVV **** nnnnaaaammmmeeee CCCCOOOODDDDEEEE:::: rrrreeeemmmmeeeemmmmbbbbeeeerrrrSSSSuuuubbbb ==== nnnnaaaammmmeeee ;;;; vvvvooooiiiidddd CCCCaaaallllllllSSSSaaaavvvveeeeddddSSSSuuuubbbb1111(((()))) CCCCOOOODDDDEEEE:::: PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; ppppeeeerrrrllll____ccccaaaallllllll____ssssvvvv((((rrrreeeemmmmeeeemmmmbbbbeeeerrrrSSSSuuuubbbb,,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD||||GGGG____NNNNOOOOAAAARRRRGGGGSSSS)))) ;;;; The reason this is wrong is that by the time you come to use the pointer rrrreeeemmmmeeeemmmmbbbbeeeerrrrSSSSuuuubbbb in CCCCaaaallllllllSSSSaaaavvvveeeeddddSSSSuuuubbbb1111, it may or may not still refer to the Perl subroutine that was recorded in SSSSaaaavvvveeeeSSSSuuuubbbb1111. This is particularly true for these cases SSSSaaaavvvveeeeSSSSuuuubbbb1111((((\\\\&&&&ffffrrrreeeedddd)))) ;;;; CCCCaaaallllllllSSSSaaaavvvveeeeddddSSSSuuuubbbb1111(((()))) ;;;; SSSSaaaavvvveeeeSSSSuuuubbbb1111(((( ssssuuuubbbb {{{{ pppprrrriiiinnnntttt """"HHHHeeeelllllllloooo tttthhhheeeerrrreeee\\\\nnnn"""" }}}} )))) ;;;; CCCCaaaallllllllSSSSaaaavvvveeeeddddSSSSuuuubbbb1111(((()))) ;;;; By the time each of the SSSSaaaavvvveeeeSSSSuuuubbbb1111 statements above have been executed, the SV*'s which corresponded to the parameters will no longer exist. Expect an error message 29/Jan/96 perl 5.002 with 20 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) from Perl of the form CCCCaaaannnn''''tttt uuuusssseeee aaaannnn uuuunnnnddddeeeeffffiiiinnnneeeedddd vvvvaaaalllluuuueeee aaaassss aaaa ssssuuuubbbbrrrroooouuuuttttiiiinnnneeee rrrreeeeffffeeeerrrreeeennnncccceeee aaaatttt ............ for each of the CCCCaaaallllllllSSSSaaaavvvveeeeddddSSSSuuuubbbb1111 lines. Similarly, with this code $$$$rrrreeeeffff ==== \\\\&&&&ffffrrrreeeedddd ;;;; SSSSaaaavvvveeeeSSSSuuuubbbb1111(((($$$$rrrreeeeffff)))) ;;;; $$$$rrrreeeeffff ==== 44447777 ;;;; CCCCaaaallllllllSSSSaaaavvvveeeeddddSSSSuuuubbbb1111(((()))) ;;;; you can expect one of these messages (which you actually get is dependant on the version of Perl you are using) NNNNooootttt aaaa CCCCOOOODDDDEEEE rrrreeeeffffeeeerrrreeeennnncccceeee aaaatttt ............ UUUUnnnnddddeeeeffffiiiinnnneeeedddd ssssuuuubbbbrrrroooouuuuttttiiiinnnneeee &&&&mmmmaaaaiiiinnnn::::::::44447777 ccccaaaalllllllleeeedddd ............ The variable $$$$rrrreeeeffff may have referred to the subroutine ffffrrrreeeedddd whenever the call to SSSSaaaavvvveeeeSSSSuuuubbbb1111 was made but by the time CCCCaaaallllllllSSSSaaaavvvveeeeddddSSSSuuuubbbb1111 gets called it now holds the number 44447777. Since we saved only a pointer to the original SV in SSSSaaaavvvveeeeSSSSuuuubbbb1111, any changes to $$$$rrrreeeeffff will be tracked by the pointer rrrreeeemmmmeeeemmmmbbbbeeeerrrrSSSSuuuubbbb. This means that whenever CCCCaaaallllllllSSSSaaaavvvveeeeddddSSSSuuuubbbb1111 gets called, it will attempt to execute the code which is referenced by the SV* rrrreeeemmmmeeeemmmmbbbbeeeerrrrSSSSuuuubbbb. In this case though, it now refers to the integer 44447777, so expect Perl to complain loudly. A similar but more subtle problem is illustrated with this code $$$$rrrreeeeffff ==== \\\\&&&&ffffrrrreeeedddd ;;;; SSSSaaaavvvveeeeSSSSuuuubbbb1111(((($$$$rrrreeeeffff)))) ;;;; $$$$rrrreeeeffff ==== \\\\&&&&jjjjooooeeee ;;;; CCCCaaaallllllllSSSSaaaavvvveeeeddddSSSSuuuubbbb1111(((()))) ;;;; This time whenever CCCCaaaallllllllSSSSaaaavvvveeeeddddSSSSuuuubbbb1111 get called it will execute the Perl subroutine jjjjooooeeee (assuming it exists) rather than ffffrrrreeeedddd as was originally requested in the call to SSSSaaaavvvveeeeSSSSuuuubbbb1111. To get around these problems it is necessary to take a full copy of the SV. The code below shows SSSSaaaavvvveeeeSSSSuuuubbbb2222 modified to do that ssssttttaaaattttiiiicccc SSSSVVVV **** kkkkeeeeeeeeppppSSSSuuuubbbb ==== ((((SSSSVVVV****))))NNNNUUUULLLLLLLL ;;;; 29/Jan/96 perl 5.002 with 21 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) vvvvooooiiiidddd SSSSaaaavvvveeeeSSSSuuuubbbb2222((((nnnnaaaammmmeeee)))) SSSSVVVV **** nnnnaaaammmmeeee CCCCOOOODDDDEEEE:::: ////**** TTTTaaaakkkkeeee aaaa ccccooooppppyyyy ooooffff tttthhhheeee ccccaaaallllllllbbbbaaaacccckkkk ****//// iiiiffff ((((kkkkeeeeeeeeppppSSSSuuuubbbb ======== ((((SSSSVVVV****))))NNNNUUUULLLLLLLL)))) ////**** FFFFiiiirrrrsssstttt ttttiiiimmmmeeee,,,, ssssoooo ccccrrrreeeeaaaatttteeee aaaa nnnneeeewwww SSSSVVVV ****//// kkkkeeeeeeeeppppSSSSuuuubbbb ==== nnnneeeewwwwSSSSVVVVssssvvvv((((nnnnaaaammmmeeee)))) ;;;; eeeellllsssseeee ////**** BBBBeeeeeeeennnn hhhheeeerrrreeee bbbbeeeeffffoooorrrreeee,,,, ssssoooo oooovvvveeeerrrrwwwwrrrriiiitttteeee ****//// SSSSvvvvSSSSeeeettttSSSSVVVV((((kkkkeeeeeeeeppppSSSSuuuubbbb,,,, nnnnaaaammmmeeee)))) ;;;; vvvvooooiiiidddd CCCCaaaallllllllSSSSaaaavvvveeeeddddSSSSuuuubbbb2222(((()))) CCCCOOOODDDDEEEE:::: PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; ppppeeeerrrrllll____ccccaaaallllllll____ssssvvvv((((kkkkeeeeeeeeppppSSSSuuuubbbb,,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD||||GGGG____NNNNOOOOAAAARRRRGGGGSSSS)))) ;;;; In order to avoid creating a new SV every time SSSSaaaavvvveeeeSSSSuuuubbbb2222 is called, the function first checks to see if it has been called before. If not, then space for a new SV is allocated and the reference to the Perl subroutine, nnnnaaaammmmeeee is copied to the variable kkkkeeeeeeeeppppSSSSuuuubbbb in one operation using nnnneeeewwwwSSSSVVVVssssvvvv. Thereafter, whenever SSSSaaaavvvveeeeSSSSuuuubbbb2222 is called the existing SV, kkkkeeeeeeeeppppSSSSuuuubbbb, is overwritten with the new value using SSSSvvvvSSSSeeeettttSSSSVVVV. UUUUssssiiiinnnngggg ppppeeeerrrrllll____ccccaaaallllllll____aaaarrrrggggvvvv Here is a Perl subroutine which prints whatever parameters are passed to it. ssssuuuubbbb PPPPrrrriiiinnnnttttLLLLiiiisssstttt {{{{ mmmmyyyy((((@@@@lllliiiisssstttt)))) ==== @@@@____ ;;;; ffffoooorrrreeeeaaaacccchhhh ((((@@@@lllliiiisssstttt)))) {{{{ pppprrrriiiinnnntttt """"$$$$____\\\\nnnn"""" }}}} }}}} and here is an example of _p_e_r_l___c_a_l_l___a_r_g_v which will call _P_r_i_n_t_L_i_s_t. ssssttttaaaattttiiiicccc cccchhhhaaaarrrr **** wwwwoooorrrrddddssss[[[[]]]] ==== {{{{""""aaaallllpppphhhhaaaa"""",,,, """"bbbbeeeettttaaaa"""",,,, """"ggggaaaammmmmmmmaaaa"""",,,, """"ddddeeeellllttttaaaa"""",,,, NNNNUUUULLLLLLLL}}}} ;;;; ssssttttaaaattttiiiicccc vvvvooooiiiidddd ccccaaaallllllll____PPPPrrrriiiinnnnttttLLLLiiiisssstttt(((()))) {{{{ ddddSSSSPPPP ;;;; ppppeeeerrrrllll____ccccaaaallllllll____aaaarrrrggggvvvv((((""""PPPPrrrriiiinnnnttttLLLLiiiisssstttt"""",,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD,,,, wwwwoooorrrrddddssss)))) ;;;; }}}} Note that it is not necessary to call PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK in this instance. This is because _p_e_r_l___c_a_l_l___a_r_g_v will do it for 29/Jan/96 perl 5.002 with 22 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) you. UUUUssssiiiinnnngggg ppppeeeerrrrllll____ccccaaaallllllll____mmmmeeeetttthhhhoooodddd Consider the following Perl code {{{{ ppppaaaacccckkkkaaaaggggeeee MMMMiiiinnnneeee ;;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy(((($$$$ttttyyyyppppeeee)))) ==== sssshhhhiiiifffftttt ;;;; bbbblllleeeessssssss [[[[@@@@____]]]] }}}} ssssuuuubbbb DDDDiiiissssppppllllaaaayyyy {{{{ mmmmyyyy (((($$$$sssseeeellllffff,,,, $$$$iiiinnnnddddeeeexxxx)))) ==== @@@@____ ;;;; pppprrrriiiinnnntttt """"$$$$iiiinnnnddddeeeexxxx:::: $$$$$$$$sssseeeellllffff[[[[$$$$iiiinnnnddddeeeexxxx]]]]\\\\nnnn"""" ;;;; }}}} ssssuuuubbbb PPPPrrrriiiinnnnttttIIIIDDDD {{{{ mmmmyyyy(((($$$$ccccllllaaaassssssss)))) ==== @@@@____ ;;;; pppprrrriiiinnnntttt """"TTTThhhhiiiissss iiiissss CCCCllllaaaassssssss $$$$ccccllllaaaassssssss vvvveeeerrrrssssiiiioooonnnn 1111....0000\\\\nnnn"""" ;;;; }}}} }}}} It just implements a very simple class to manage an array. Apart from the constructor, nnnneeeewwww, it declares methods, one static and one virtual. The static method, PPPPrrrriiiinnnnttttIIIIDDDD, simply prints out the class name and a version number. The virtual method, DDDDiiiissssppppllllaaaayyyy, prints out a single element of the array. Here is an all Perl example of using it. $$$$aaaa ==== nnnneeeewwww MMMMiiiinnnneeee ((((''''rrrreeeedddd'''',,,, ''''ggggrrrreeeeeeeennnn'''',,,, ''''bbbblllluuuueeee'''')))) ;;;; $$$$aaaa---->>>>DDDDiiiissssppppllllaaaayyyy((((1111)))) ;;;; PPPPrrrriiiinnnnttttIIIIDDDD MMMMiiiinnnneeee;;;; will print 1111:::: ggggrrrreeeeeeeennnn TTTThhhhiiiissss iiiissss CCCCllllaaaassssssss MMMMiiiinnnneeee vvvveeeerrrrssssiiiioooonnnn 1111....0000 Calling a Perl method from C is fairly straightforward. The following things are required +o a reference to the object for a virtual method or the name of the class for a static method. +o the name of the method. +o any other parameters specific to the method. 29/Jan/96 perl 5.002 with 23 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) Here is a simple XSUB which illustrates the mechanics of calling both the PPPPrrrriiiinnnnttttIIIIDDDD and DDDDiiiissssppppllllaaaayyyy methods from C. vvvvooooiiiidddd ccccaaaallllllll____MMMMeeeetttthhhhoooodddd((((rrrreeeeffff,,,, mmmmeeeetttthhhhoooodddd,,,, iiiinnnnddddeeeexxxx)))) SSSSVVVV **** rrrreeeeffff cccchhhhaaaarrrr **** mmmmeeeetttthhhhoooodddd iiiinnnntttt iiiinnnnddddeeeexxxx CCCCOOOODDDDEEEE:::: PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp))));;;; XXXXPPPPUUUUSSSSHHHHssss((((rrrreeeeffff))));;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((iiiinnnnddddeeeexxxx)))))))))))) ;;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK;;;; ppppeeeerrrrllll____ccccaaaallllllll____mmmmeeeetttthhhhoooodddd((((mmmmeeeetttthhhhoooodddd,,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD)))) ;;;; vvvvooooiiiidddd ccccaaaallllllll____PPPPrrrriiiinnnnttttIIIIDDDD((((ccccllllaaaassssssss,,,, mmmmeeeetttthhhhoooodddd)))) cccchhhhaaaarrrr **** ccccllllaaaassssssss cccchhhhaaaarrrr **** mmmmeeeetttthhhhoooodddd CCCCOOOODDDDEEEE:::: PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp))));;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVVppppvvvv((((ccccllllaaaassssssss,,,, 0000)))))))))))) ;;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK;;;; ppppeeeerrrrllll____ccccaaaallllllll____mmmmeeeetttthhhhoooodddd((((mmmmeeeetttthhhhoooodddd,,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD)))) ;;;; So the methods PPPPrrrriiiinnnnttttIIIIDDDD and DDDDiiiissssppppllllaaaayyyy can be invoked like this $$$$aaaa ==== nnnneeeewwww MMMMiiiinnnneeee ((((''''rrrreeeedddd'''',,,, ''''ggggrrrreeeeeeeennnn'''',,,, ''''bbbblllluuuueeee'''')))) ;;;; ccccaaaallllllll____MMMMeeeetttthhhhoooodddd(((($$$$aaaa,,,, ''''DDDDiiiissssppppllllaaaayyyy'''',,,, 1111)))) ;;;; ccccaaaallllllll____PPPPrrrriiiinnnnttttIIIIDDDD((((''''MMMMiiiinnnneeee'''',,,, ''''PPPPrrrriiiinnnnttttIIIIDDDD'''')))) ;;;; The only thing to note is that in both the static and virtual methods, the method name is not passed via the stack - it is used as the first parameter to _p_e_r_l___c_a_l_l___m_e_t_h_o_d. UUUUssssiiiinnnngggg GGGGIIIIMMMMMMMMEEEE Here is a trivial XSUB which prints the context in which it is currently executing. vvvvooooiiiidddd PPPPrrrriiiinnnnttttCCCCoooonnnntttteeeexxxxtttt(((()))) CCCCOOOODDDDEEEE:::: iiiiffff ((((GGGGIIIIMMMMMMMMEEEE ======== GGGG____SSSSCCCCAAAALLLLAAAARRRR)))) pppprrrriiiinnnnttttffff ((((""""CCCCoooonnnntttteeeexxxxtttt iiiissss SSSSccccaaaallllaaaarrrr\\\\nnnn"""")))) ;;;; eeeellllsssseeee pppprrrriiiinnnnttttffff ((((""""CCCCoooonnnntttteeeexxxxtttt iiiissss AAAArrrrrrrraaaayyyy\\\\nnnn"""")))) ;;;; and here is some Perl to test it 29/Jan/96 perl 5.002 with 24 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) $$$$aaaa ==== PPPPrrrriiiinnnnttttCCCCoooonnnntttteeeexxxxtttt ;;;; @@@@aaaa ==== PPPPrrrriiiinnnnttttCCCCoooonnnntttteeeexxxxtttt ;;;; The output from that will be CCCCoooonnnntttteeeexxxxtttt iiiissss SSSSccccaaaallllaaaarrrr CCCCoooonnnntttteeeexxxxtttt iiiissss AAAArrrrrrrraaaayyyy UUUUssssiiiinnnngggg PPPPeeeerrrrllll ttttoooo ddddiiiissssppppoooosssseeee ooooffff tttteeeemmmmppppoooorrrraaaarrrriiiieeeessss In the examples given to date, any temporaries created in the callback (i.e. parameters passed on the stack to the _p_e_r_l___c_a_l_l___* function or values returned via the stack) have been freed by one of these methods +o specifying the G_DISCARD flag with _p_e_r_l___c_a_l_l___*. +o explicitly disposed of using the EEEENNNNTTTTEEEERRRR/SSSSAAAAVVVVEEEETTTTMMMMPPPPSSSS - FFFFRRRREEEEEEEETTTTMMMMPPPPSSSS/LLLLEEEEAAAAVVVVEEEE pairing. There is another method which can be used, namely letting Perl do it for you automatically whenever it regains control after the callback has terminated. This is done by simply not using the EEEENNNNTTTTEEEERRRR ;;;; SSSSAAAAVVVVEEEETTTTMMMMPPPPSSSS ;;;; ............ FFFFRRRREEEEEEEETTTTMMMMPPPPSSSS ;;;; LLLLEEEEAAAAVVVVEEEE ;;;; sequence in the callback (and not, of course, specifying the G_DISCARD flag). If you are going to use this method you have to be aware of a possible memory leak which can arise under very specific circumstances. To explain these circumstances you need to know a bit about the flow of control between Perl and the callback routine. The examples given at the start of the document (an error handler and an event driven program) are typical of the two main sorts of flow control that you are likely to encounter with callbacks. There is a very important distinction between them, so pay attention. In the first example, an error handler, the flow of control could be as follows. You have created an interface to an external library. Control can reach the external library like this ppppeeeerrrrllll -------->>>> XXXXSSSSUUUUBBBB -------->>>> eeeexxxxtttteeeerrrrnnnnaaaallll lllliiiibbbbrrrraaaarrrryyyy 29/Jan/96 perl 5.002 with 25 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) Whilst control is in the library, an error condition occurs. You have previously set up a Perl callback to handle this situation, so it will get executed. Once the callback has finished, control will drop back to Perl again. Here is what the flow of control will be like in that situation ppppeeeerrrrllll -------->>>> XXXXSSSSUUUUBBBB -------->>>> eeeexxxxtttteeeerrrrnnnnaaaallll lllliiiibbbbrrrraaaarrrryyyy ............ eeeerrrrrrrroooorrrr ooooccccccccuuuurrrrssss ............ eeeexxxxtttteeeerrrrnnnnaaaallll lllliiiibbbbrrrraaaarrrryyyy -------->>>> ppppeeeerrrrllll____ccccaaaallllllll -------->>>> ppppeeeerrrrllll |||| ppppeeeerrrrllll <<<<-------- XXXXSSSSUUUUBBBB <<<<-------- eeeexxxxtttteeeerrrrnnnnaaaallll lllliiiibbbbrrrraaaarrrryyyy <<<<-------- ppppeeeerrrrllll____ccccaaaallllllll <<<<----------------++++ After processing of the error using _p_e_r_l___c_a_l_l___* is completed, control reverts back to Perl more or less immediately. In the diagram, the further right you go the more deeply nested the scope is. It is only when control is back with perl on the extreme left of the diagram that you will have dropped back to the enclosing scope and any temporaries you have left hanging around will be freed. In the second example, an event driven program, the flow of control will be more like this ppppeeeerrrrllll -------->>>> XXXXSSSSUUUUBBBB -------->>>> eeeevvvveeeennnntttt hhhhaaaannnnddddlllleeeerrrr ............ eeeevvvveeeennnntttt hhhhaaaannnnddddlllleeeerrrr -------->>>> ppppeeeerrrrllll____ccccaaaallllllll -------->>>> ppppeeeerrrrllll |||| eeeevvvveeeennnntttt hhhhaaaannnnddddlllleeeerrrr <<<<-------- ppppeeeerrrrllll____ccccaaaallllllll --------<<<<--------++++ ............ eeeevvvveeeennnntttt hhhhaaaannnnddddlllleeeerrrr -------->>>> ppppeeeerrrrllll____ccccaaaallllllll -------->>>> ppppeeeerrrrllll |||| eeeevvvveeeennnntttt hhhhaaaannnnddddlllleeeerrrr <<<<-------- ppppeeeerrrrllll____ccccaaaallllllll --------<<<<--------++++ ............ eeeevvvveeeennnntttt hhhhaaaannnnddddlllleeeerrrr -------->>>> ppppeeeerrrrllll____ccccaaaallllllll -------->>>> ppppeeeerrrrllll |||| eeeevvvveeeennnntttt hhhhaaaannnnddddlllleeeerrrr <<<<-------- ppppeeeerrrrllll____ccccaaaallllllll --------<<<<--------++++ In this case the flow of control can consist of only the repeated sequence eeeevvvveeeennnntttt hhhhaaaannnnddddlllleeeerrrr -------->>>> ppppeeeerrrrllll____ccccaaaallllllll -------->>>> ppppeeeerrrrllll for the practically the complete duration of the program. This means that control may _n_e_v_e_r drop back to the surrounding scope in Perl at the extreme left. So what is the big problem? Well, if you are expecting Perl to tidy up those temporaries for you, you might be in for a long wait. For Perl to actually dispose of your 29/Jan/96 perl 5.002 with 26 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) temporaries, control must drop back to the enclosing scope at some stage. In the event driven scenario that may never happen. This means that as time goes on, your program will create more and more temporaries, none of which will ever be freed. As each of these temporaries consumes some memory your program will eventually consume all the available memory in your system - kapow! So here is the bottom line - if you are sure that control will revert back to the enclosing Perl scope fairly quickly after the end of your callback, then it isn't absolutely necessary to explicitly dispose of any temporaries you may have created. Mind you, if you are at all uncertain about what to do, it doesn't do any harm to tidy up anyway. SSSSttttrrrraaaatttteeeeggggiiiieeeessss ffffoooorrrr ssssttttoooorrrriiiinnnngggg CCCCaaaallllllllbbbbaaaacccckkkk CCCCoooonnnntttteeeexxxxtttt IIIInnnnffffoooorrrrmmmmaaaattttiiiioooonnnn Potentially one of the trickiest problems to overcome when designing a callback interface can be figuring out how to store the mapping between the C callback function and the Perl equivalent. To help understand why this can be a real problem first consider how a callback is set up in an all C environment. Typically a C API will provide a function to register a callback. This will expect a pointer to a function as one of its parameters. Below is a call to a hypothetical function rrrreeeeggggiiiisssstttteeeerrrr____ffffaaaattttaaaallll which registers the C function to get called when a fatal error occurs. rrrreeeeggggiiiisssstttteeeerrrr____ffffaaaattttaaaallll((((ccccbbbb1111)))) ;;;; The single parameter ccccbbbb1111 is a pointer to a function, so you must have defined ccccbbbb1111 in your code, say something like this ssssttttaaaattttiiiicccc vvvvooooiiiidddd ccccbbbb1111(((()))) {{{{ pppprrrriiiinnnnttttffff ((((""""FFFFaaaattttaaaallll EEEErrrrrrrroooorrrr\\\\nnnn"""")))) ;;;; eeeexxxxiiiitttt((((1111)))) ;;;; }}}} Now change that to call a Perl subroutine instead ssssttttaaaattttiiiicccc SSSSVVVV **** ccccaaaallllllllbbbbaaaacccckkkk ==== ((((SSSSVVVV****))))NNNNUUUULLLLLLLL;;;; ssssttttaaaattttiiiicccc vvvvooooiiiidddd ccccbbbb1111(((()))) {{{{ ddddSSSSPPPP ;;;; PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; 29/Jan/96 perl 5.002 with 27 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) ////**** CCCCaaaallllllll tttthhhheeee PPPPeeeerrrrllll ssssuuuubbbb ttttoooo pppprrrroooocccceeeessssssss tttthhhheeee ccccaaaallllllllbbbbaaaacccckkkk ****//// ppppeeeerrrrllll____ccccaaaallllllll____ssssvvvv((((ccccaaaallllllllbbbbaaaacccckkkk,,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD)))) ;;;; }}}} vvvvooooiiiidddd rrrreeeeggggiiiisssstttteeeerrrr____ffffaaaattttaaaallll((((ffffnnnn)))) SSSSVVVV **** ffffnnnn CCCCOOOODDDDEEEE:::: ////**** RRRReeeemmmmeeeemmmmbbbbeeeerrrr tttthhhheeee PPPPeeeerrrrllll ssssuuuubbbb ****//// iiiiffff ((((ccccaaaallllllllbbbbaaaacccckkkk ======== ((((SSSSVVVV****))))NNNNUUUULLLLLLLL)))) ccccaaaallllllllbbbbaaaacccckkkk ==== nnnneeeewwwwSSSSVVVVssssvvvv((((ffffnnnn)))) ;;;; eeeellllsssseeee SSSSvvvvSSSSeeeettttSSSSVVVV((((ccccaaaallllllllbbbbaaaacccckkkk,,,, ffffnnnn)))) ;;;; ////**** rrrreeeeggggiiiisssstttteeeerrrr tttthhhheeee ccccaaaallllllllbbbbaaaacccckkkk wwwwiiiitttthhhh tttthhhheeee eeeexxxxtttteeeerrrrnnnnaaaallll lllliiiibbbbrrrraaaarrrryyyy ****//// rrrreeeeggggiiiisssstttteeeerrrr____ffffaaaattttaaaallll((((ccccbbbb1111)))) ;;;; where the Perl equivalent of rrrreeeeggggiiiisssstttteeeerrrr____ffffaaaattttaaaallll and the callback it registers, ppppccccbbbb1111, might look like this #### RRRReeeeggggiiiisssstttteeeerrrr tttthhhheeee ssssuuuubbbb ppppccccbbbb1111 rrrreeeeggggiiiisssstttteeeerrrr____ffffaaaattttaaaallll((((\\\\&&&&ppppccccbbbb1111)))) ;;;; ssssuuuubbbb ppppccccbbbb1111 {{{{ ddddiiiieeee """"IIII''''mmmm ddddyyyyiiiinnnngggg............\\\\nnnn"""" ;;;; }}}} The mapping between the C callback and the Perl equivalent is stored in the global variable ccccaaaallllllllbbbbaaaacccckkkk. This will be adequate if you ever need to have only 1 callback registered at any time. An example could be an error handler like the code sketched out above. Remember though, repeated calls to rrrreeeeggggiiiisssstttteeeerrrr____ffffaaaattttaaaallll will replace the previously registered callback function with the new one. Say for example you want to interface to a library which allows asynchronous file i/o. In this case you may be able to register a callback whenever a read operation has completed. To be of any use we want to be able to call separate Perl subroutines for each file that is opened. As it stands, the error handler example above would not be adequate as it allows only a single callback to be defined at any time. What we require is a means of storing the mapping between the opened file and the Perl subroutine we want to be called for that file. Say the i/o library has a function aaaassssyyyynnnncccchhhh____rrrreeeeaaaadddd which associates a C function PPPPrrrroooocccceeeessssssssRRRReeeeaaaadddd with a file handle ffffhhhh - this assumes that it has also provided some routine to open the file and so obtain the file handle. aaaassssyyyynnnncccchhhh____rrrreeeeaaaadddd((((ffffhhhh,,,, PPPPrrrroooocccceeeessssssssRRRReeeeaaaadddd)))) 29/Jan/96 perl 5.002 with 28 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) This may expect the C _P_r_o_c_e_s_s_R_e_a_d function of this form vvvvooooiiiidddd PPPPrrrroooocccceeeessssssssRRRReeeeaaaadddd((((ffffhhhh,,,, bbbbuuuuffffffffeeeerrrr)))) iiiinnnntttt ffffhhhh ;;;; cccchhhhaaaarrrr **** bbbbuuuuffffffffeeeerrrr ;;;; {{{{ ............ }}}} To provide a Perl interface to this library we need to be able to map between the ffffhhhh parameter and the Perl subroutine we want called. A hash is a convenient mechanism for storing this mapping. The code below shows a possible implementation ssssttttaaaattttiiiicccc HHHHVVVV **** MMMMaaaappppppppiiiinnnngggg ==== ((((HHHHVVVV****))))NNNNUUUULLLLLLLL ;;;; vvvvooooiiiidddd aaaassssyyyynnnncccchhhh____rrrreeeeaaaadddd((((ffffhhhh,,,, ccccaaaallllllllbbbbaaaacccckkkk)))) iiiinnnntttt ffffhhhh SSSSVVVV **** ccccaaaallllllllbbbbaaaacccckkkk CCCCOOOODDDDEEEE:::: ////**** IIIIffff tttthhhheeee hhhhaaaasssshhhh ddddooooeeeessssnnnn''''tttt aaaallllrrrreeeeaaaaddddyyyy eeeexxxxiiiisssstttt,,,, ccccrrrreeeeaaaatttteeee iiiitttt ****//// iiiiffff ((((MMMMaaaappppppppiiiinnnngggg ======== ((((HHHHVVVV****))))NNNNUUUULLLLLLLL)))) MMMMaaaappppppppiiiinnnngggg ==== nnnneeeewwwwHHHHVVVV(((()))) ;;;; ////**** SSSSaaaavvvveeee tttthhhheeee ffffhhhh ---->>>> ccccaaaallllllllbbbbaaaacccckkkk mmmmaaaappppppppiiiinnnngggg ****//// hhhhvvvv____ssssttttoooorrrreeee((((MMMMaaaappppppppiiiinnnngggg,,,, ((((cccchhhhaaaarrrr****))))&&&&ffffhhhh,,,, ssssiiiizzzzeeeeooooffff((((ffffhhhh)))),,,, nnnneeeewwwwSSSSVVVVssssvvvv((((ccccaaaallllllllbbbbaaaacccckkkk)))),,,, 0000)))) ;;;; ////**** RRRReeeeggggiiiisssstttteeeerrrr wwwwiiiitttthhhh tttthhhheeee CCCC LLLLiiiibbbbrrrraaaarrrryyyy ****//// aaaassssyyyynnnncccchhhh____rrrreeeeaaaadddd((((ffffhhhh,,,, aaaassssyyyynnnncccchhhh____rrrreeeeaaaadddd____iiiiffff)))) ;;;; and aaaassssyyyynnnncccchhhh____rrrreeeeaaaadddd____iiiiffff could look like this ssssttttaaaattttiiiicccc vvvvooooiiiidddd aaaassssyyyynnnncccchhhh____rrrreeeeaaaadddd____iiiiffff((((ffffhhhh,,,, bbbbuuuuffffffffeeeerrrr)))) iiiinnnntttt ffffhhhh ;;;; cccchhhhaaaarrrr **** bbbbuuuuffffffffeeeerrrr ;;;; {{{{ ddddSSSSPPPP ;;;; SSSSVVVV ******** ssssvvvv ;;;; ////**** GGGGeeeetttt tttthhhheeee ccccaaaallllllllbbbbaaaacccckkkk aaaassssssssoooocccciiiiaaaatttteeeedddd wwwwiiiitttthhhh ffffhhhh ****//// ssssvvvv ==== hhhhvvvv____ffffeeeettttcccchhhh((((MMMMaaaappppppppiiiinnnngggg,,,, ((((cccchhhhaaaarrrr****))))&&&&ffffhhhh ,,,, ssssiiiizzzzeeeeooooffff((((ffffhhhh)))),,,, FFFFAAAALLLLSSSSEEEE)))) ;;;; iiiiffff ((((ssssvvvv ======== ((((SSSSVVVV********))))NNNNUUUULLLLLLLL)))) ccccrrrrooooaaaakkkk((((""""IIIInnnntttteeeerrrrnnnnaaaallll eeeerrrrrrrroooorrrr............\\\\nnnn"""")))) ;;;; PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((ffffhhhh)))))))))))) ;;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVVppppvvvv((((bbbbuuuuffffffffeeeerrrr,,,, 0000)))))))))))) ;;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; 29/Jan/96 perl 5.002 with 29 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) ////**** CCCCaaaallllllll tttthhhheeee PPPPeeeerrrrllll ssssuuuubbbb ****//// ppppeeeerrrrllll____ccccaaaallllllll____ssssvvvv((((****ssssvvvv,,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD)))) ;;;; }}}} For completeness, here is aaaassssyyyynnnncccchhhh____cccclllloooosssseeee. This shows how to remove the entry from the hash MMMMaaaappppppppiiiinnnngggg. vvvvooooiiiidddd aaaassssyyyynnnncccchhhh____cccclllloooosssseeee((((ffffhhhh)))) iiiinnnntttt ffffhhhh CCCCOOOODDDDEEEE:::: ////**** RRRReeeemmmmoooovvvveeee tttthhhheeee eeeennnnttttrrrryyyy ffffrrrroooommmm tttthhhheeee hhhhaaaasssshhhh ****//// ((((vvvvooooiiiidddd)))) hhhhvvvv____ddddeeeelllleeeetttteeee((((MMMMaaaappppppppiiiinnnngggg,,,, ((((cccchhhhaaaarrrr****))))&&&&ffffhhhh,,,, ssssiiiizzzzeeeeooooffff((((ffffhhhh)))),,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD)))) ;;;; ////**** NNNNoooowwww ccccaaaallllllll tttthhhheeee rrrreeeeaaaallll aaaassssyyyynnnncccchhhh____cccclllloooosssseeee ****//// aaaassssyyyynnnncccchhhh____cccclllloooosssseeee((((ffffhhhh)))) ;;;; So the Perl interface would look like this ssssuuuubbbb ccccaaaallllllllbbbbaaaacccckkkk1111 {{{{ mmmmyyyy(((($$$$hhhhaaaannnnddddlllleeee,,,, $$$$bbbbuuuuffffffffeeeerrrr)))) ==== @@@@____ ;;;; }}}} #### RRRReeeeggggiiiisssstttteeeerrrr tttthhhheeee PPPPeeeerrrrllll ccccaaaallllllllbbbbaaaacccckkkk aaaassssyyyynnnncccchhhh____rrrreeeeaaaadddd(((($$$$ffffhhhh,,,, \\\\&&&&ccccaaaallllllllbbbbaaaacccckkkk1111)))) ;;;; aaaassssyyyynnnncccchhhh____cccclllloooosssseeee(((($$$$ffffhhhh)))) ;;;; The mapping between the C callback and Perl is stored in the global hash MMMMaaaappppppppiiiinnnngggg this time. Using a hash has the distinct advantage that it allows an unlimited number of callbacks to be registered. What if the interface provided by the C callback doesn't contain a parameter which allows the file handle to Perl subroutine mapping? Say in the asynchronous i/o package, the callback function gets passed only the bbbbuuuuffffffffeeeerrrr parameter like this vvvvooooiiiidddd PPPPrrrroooocccceeeessssssssRRRReeeeaaaadddd((((bbbbuuuuffffffffeeeerrrr)))) cccchhhhaaaarrrr **** bbbbuuuuffffffffeeeerrrr ;;;; {{{{ ............ }}}} Without the file handle there is no straightforward way to map from the C callback to the Perl subroutine. In this case a possible way around this problem is to pre- define a series of C functions to act as the interface to Perl, thus 29/Jan/96 perl 5.002 with 30 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) ####ddddeeeeffffiiiinnnneeee MMMMAAAAXXXX____CCCCBBBB 3333 ####ddddeeeeffffiiiinnnneeee NNNNUUUULLLLLLLL____HHHHAAAANNNNDDDDLLLLEEEE ----1111 ttttyyyyppppeeeeddddeeeeffff vvvvooooiiiidddd ((((****FFFFnnnnMMMMaaaapppp))))(((()))) ;;;; ssssttttrrrruuuucccctttt MMMMaaaappppSSSSttttrrrruuuucccctttt {{{{ FFFFnnnnMMMMaaaapppp FFFFuuuunnnnccccttttiiiioooonnnn ;;;; SSSSVVVV **** PPPPeeeerrrrllllSSSSuuuubbbb ;;;; iiiinnnntttt HHHHaaaannnnddddlllleeee ;;;; }}}} ;;;; ssssttttaaaattttiiiicccc vvvvooooiiiidddd ffffnnnn1111(((()))) ;;;; ssssttttaaaattttiiiicccc vvvvooooiiiidddd ffffnnnn2222(((()))) ;;;; ssssttttaaaattttiiiicccc vvvvooooiiiidddd ffffnnnn3333(((()))) ;;;; ssssttttaaaattttiiiicccc ssssttttrrrruuuucccctttt MMMMaaaappppSSSSttttrrrruuuucccctttt MMMMaaaapppp [[[[MMMMAAAAXXXX____CCCCBBBB]]]] ==== {{{{ {{{{ ffffnnnn1111,,,, NNNNUUUULLLLLLLL,,,, NNNNUUUULLLLLLLL____HHHHAAAANNNNDDDDLLLLEEEE }}}},,,, {{{{ ffffnnnn2222,,,, NNNNUUUULLLLLLLL,,,, NNNNUUUULLLLLLLL____HHHHAAAANNNNDDDDLLLLEEEE }}}},,,, {{{{ ffffnnnn3333,,,, NNNNUUUULLLLLLLL,,,, NNNNUUUULLLLLLLL____HHHHAAAANNNNDDDDLLLLEEEE }}}} }}}} ;;;; ssssttttaaaattttiiiicccc vvvvooooiiiidddd PPPPccccbbbb((((iiiinnnnddddeeeexxxx,,,, bbbbuuuuffffffffeeeerrrr)))) iiiinnnntttt iiiinnnnddddeeeexxxx ;;;; cccchhhhaaaarrrr **** bbbbuuuuffffffffeeeerrrr ;;;; {{{{ ddddSSSSPPPP ;;;; PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVVppppvvvv((((bbbbuuuuffffffffeeeerrrr,,,, 0000)))))))))))) ;;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; ////**** CCCCaaaallllllll tttthhhheeee PPPPeeeerrrrllll ssssuuuubbbb ****//// ppppeeeerrrrllll____ccccaaaallllllll____ssssvvvv((((MMMMaaaapppp[[[[iiiinnnnddddeeeexxxx]]]]....PPPPeeeerrrrllllSSSSuuuubbbb,,,, GGGG____DDDDIIIISSSSCCCCAAAARRRRDDDD)))) ;;;; }}}} ssssttttaaaattttiiiicccc vvvvooooiiiidddd ffffnnnn1111((((bbbbuuuuffffffffeeeerrrr)))) cccchhhhaaaarrrr **** bbbbuuuuffffffffeeeerrrr ;;;; {{{{ PPPPccccbbbb((((0000,,,, bbbbuuuuffffffffeeeerrrr)))) ;;;; }}}} ssssttttaaaattttiiiicccc vvvvooooiiiidddd ffffnnnn2222((((bbbbuuuuffffffffeeeerrrr)))) cccchhhhaaaarrrr **** bbbbuuuuffffffffeeeerrrr ;;;; {{{{ PPPPccccbbbb((((1111,,,, bbbbuuuuffffffffeeeerrrr)))) ;;;; }}}} 29/Jan/96 perl 5.002 with 31 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) ssssttttaaaattttiiiicccc vvvvooooiiiidddd ffffnnnn3333((((bbbbuuuuffffffffeeeerrrr)))) cccchhhhaaaarrrr **** bbbbuuuuffffffffeeeerrrr ;;;; {{{{ PPPPccccbbbb((((2222,,,, bbbbuuuuffffffffeeeerrrr)))) ;;;; }}}} vvvvooooiiiidddd aaaarrrrrrrraaaayyyy____aaaassssyyyynnnncccchhhh____rrrreeeeaaaadddd((((ffffhhhh,,,, ccccaaaallllllllbbbbaaaacccckkkk)))) iiiinnnntttt ffffhhhh SSSSVVVV **** ccccaaaallllllllbbbbaaaacccckkkk CCCCOOOODDDDEEEE:::: iiiinnnntttt iiiinnnnddddeeeexxxx ;;;; iiiinnnntttt nnnnuuuullllllll____iiiinnnnddddeeeexxxx ==== MMMMAAAAXXXX____CCCCBBBB ;;;; ////**** FFFFiiiinnnndddd tttthhhheeee ssssaaaammmmeeee hhhhaaaannnnddddlllleeee oooorrrr aaaannnn eeeemmmmppppttttyyyy eeeennnnttttrrrryyyy ****//// ffffoooorrrr ((((iiiinnnnddddeeeexxxx ==== 0000 ;;;; iiiinnnnddddeeeexxxx <<<< MMMMAAAAXXXX____CCCCBBBB ;;;; ++++++++iiiinnnnddddeeeexxxx)))) {{{{ iiiiffff ((((MMMMaaaapppp[[[[iiiinnnnddddeeeexxxx]]]]....HHHHaaaannnnddddlllleeee ======== ffffhhhh)))) bbbbrrrreeeeaaaakkkk ;;;; iiiiffff ((((MMMMaaaapppp[[[[iiiinnnnddddeeeexxxx]]]]....HHHHaaaannnnddddlllleeee ======== NNNNUUUULLLLLLLL____HHHHAAAANNNNDDDDLLLLEEEE)))) nnnnuuuullllllll____iiiinnnnddddeeeexxxx ==== iiiinnnnddddeeeexxxx ;;;; }}}} iiiiffff ((((iiiinnnnddddeeeexxxx ======== MMMMAAAAXXXX____CCCCBBBB &&&&&&&& nnnnuuuullllllll____iiiinnnnddddeeeexxxx ======== MMMMAAAAXXXX____CCCCBBBB)))) ccccrrrrooooaaaakkkk ((((""""TTTToooooooo mmmmaaaannnnyyyy ccccaaaallllllllbbbbaaaacccckkkk ffffuuuunnnnccccttttiiiioooonnnnssss rrrreeeeggggiiiisssstttteeeerrrreeeedddd\\\\nnnn"""")))) ;;;; iiiiffff ((((iiiinnnnddddeeeexxxx ======== MMMMAAAAXXXX____CCCCBBBB)))) iiiinnnnddddeeeexxxx ==== nnnnuuuullllllll____iiiinnnnddddeeeexxxx ;;;; ////**** SSSSaaaavvvveeee tttthhhheeee ffffiiiilllleeee hhhhaaaannnnddddlllleeee ****//// MMMMaaaapppp[[[[iiiinnnnddddeeeexxxx]]]]....HHHHaaaannnnddddlllleeee ==== ffffhhhh ;;;; ////**** RRRReeeemmmmeeeemmmmbbbbeeeerrrr tttthhhheeee PPPPeeeerrrrllll ssssuuuubbbb ****//// iiiiffff ((((MMMMaaaapppp[[[[iiiinnnnddddeeeexxxx]]]]....PPPPeeeerrrrllllSSSSuuuubbbb ======== ((((SSSSVVVV****))))NNNNUUUULLLLLLLL)))) MMMMaaaapppp[[[[iiiinnnnddddeeeexxxx]]]]....PPPPeeeerrrrllllSSSSuuuubbbb ==== nnnneeeewwwwSSSSVVVVssssvvvv((((ccccaaaallllllllbbbbaaaacccckkkk)))) ;;;; eeeellllsssseeee SSSSvvvvSSSSeeeettttSSSSVVVV((((MMMMaaaapppp[[[[iiiinnnnddddeeeexxxx]]]]....PPPPeeeerrrrllllSSSSuuuubbbb,,,, ccccaaaallllllllbbbbaaaacccckkkk)))) ;;;; aaaassssyyyynnnncccchhhh____rrrreeeeaaaadddd((((ffffhhhh,,,, MMMMaaaapppp[[[[iiiinnnnddddeeeexxxx]]]]....FFFFuuuunnnnccccttttiiiioooonnnn)))) ;;;; vvvvooooiiiidddd aaaarrrrrrrraaaayyyy____aaaassssyyyynnnncccchhhh____cccclllloooosssseeee((((ffffhhhh)))) iiiinnnntttt ffffhhhh CCCCOOOODDDDEEEE:::: iiiinnnntttt iiiinnnnddddeeeexxxx ;;;; ////**** FFFFiiiinnnndddd tttthhhheeee ffffiiiilllleeee hhhhaaaannnnddddlllleeee ****//// ffffoooorrrr ((((iiiinnnnddddeeeexxxx ==== 0000;;;; iiiinnnnddddeeeexxxx <<<< MMMMAAAAXXXX____CCCCBBBB ;;;; ++++++++ iiiinnnnddddeeeexxxx)))) iiiiffff ((((MMMMaaaapppp[[[[iiiinnnnddddeeeexxxx]]]]....HHHHaaaannnnddddlllleeee ======== ffffhhhh)))) bbbbrrrreeeeaaaakkkk ;;;; 29/Jan/96 perl 5.002 with 32 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) iiiiffff ((((iiiinnnnddddeeeexxxx ======== MMMMAAAAXXXX____CCCCBBBB)))) ccccrrrrooooaaaakkkk ((((""""ccccoooouuuulllldddd nnnnooootttt cccclllloooosssseeee ffffhhhh %%%%dddd\\\\nnnn"""",,,, ffffhhhh)))) ;;;; MMMMaaaapppp[[[[iiiinnnnddddeeeexxxx]]]]....HHHHaaaannnnddddlllleeee ==== NNNNUUUULLLLLLLL____HHHHAAAANNNNDDDDLLLLEEEE ;;;; SSSSvvvvRRRREEEEFFFFCCCCNNNNTTTT____ddddeeeecccc((((MMMMaaaapppp[[[[iiiinnnnddddeeeexxxx]]]]....PPPPeeeerrrrllllSSSSuuuubbbb)))) ;;;; MMMMaaaapppp[[[[iiiinnnnddddeeeexxxx]]]]....PPPPeeeerrrrllllSSSSuuuubbbb ==== ((((SSSSVVVV****))))NNNNUUUULLLLLLLL ;;;; aaaassssyyyynnnncccchhhh____cccclllloooosssseeee((((ffffhhhh)))) ;;;; In this case the functions ffffnnnn1111, ffffnnnn2222 and ffffnnnn3333 are used to remember the Perl subroutine to be called. Each of the functions holds a separate hard-wired index which is used in the function PPPPccccbbbb to access the MMMMaaaapppp array and actually call the Perl subroutine. There are some obvious disadvantages with this technique. Firstly, the code is considerably more complex than with the previous example. Secondly, there is a hard-wired limit (in this case 3) to the number of callbacks that can exist simultaneously. The only way to increase the limit is by modifying the code to add more functions and then re-compiling. None the less, as long as the number of functions is chosen with some care, it is still a workable solution and in some cases is the only one available. To summarize, here are a number of possible methods for you to consider for storing the mapping between C and the Perl callback 1. Ignore the problem - Allow only 1 callback For a lot of situations, like interfacing to an error handler, this may be a perfectly adequate solution. 2. Create a sequence of callbacks - hard wired limit If it is impossible to tell from the parameters passed back from the C callback what the context is, then you may need to create a sequence of C callback interface functions, and store pointers to each in an array. 3. Use a parameter to map to the Perl callback A hash is an ideal mechanism to store the mapping between C and Perl. AAAAlllltttteeeerrrrnnnnaaaatttteeee SSSSttttaaaacccckkkk MMMMaaaannnniiiippppuuuullllaaaattttiiiioooonnnn Although I have made use of only the PPPPOOOOPPPP**** macros to access values returned from Perl subroutines, it is also possible to bypass these macros and read the stack using the SSSSTTTT macro (See the _p_e_r_l_x_s manpage for a full description of the SSSSTTTT macro). 29/Jan/96 perl 5.002 with 33 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) Most of the time the PPPPOOOOPPPP**** macros should be adequate, the main problem with them is that they force you to process the returned values in sequence. This may not be the most suitable way to process the values in some cases. What we want is to be able to access the stack in a random order. The SSSSTTTT macro as used when coding an XSUB is ideal for this purpose. The code below is the example given in the section _R_e_t_u_r_n_i_n_g _a _l_i_s_t _o_f _v_a_l_u_e_s recoded to use SSSSTTTT instead of PPPPOOOOPPPP****. ssssttttaaaattttiiiicccc vvvvooooiiiidddd ccccaaaallllllll____AAAAddddddddSSSSuuuubbbbttttrrrraaaacccctttt2222((((aaaa,,,, bbbb)))) iiiinnnntttt aaaa ;;;; iiiinnnntttt bbbb ;;;; {{{{ ddddSSSSPPPP ;;;; IIII33332222 aaaaxxxx ;;;; iiiinnnntttt ccccoooouuuunnnntttt ;;;; EEEENNNNTTTTEEEERRRR ;;;; SSSSAAAAVVVVEEEETTTTMMMMPPPPSSSS;;;; PPPPUUUUSSSSHHHHMMMMAAAARRRRKKKK((((sssspppp)))) ;;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((aaaa))))))))))));;;; XXXXPPPPUUUUSSSSHHHHssss((((ssssvvvv____2222mmmmoooorrrrttttaaaallll((((nnnneeeewwwwSSSSVVVViiiivvvv((((bbbb))))))))))));;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; ccccoooouuuunnnntttt ==== ppppeeeerrrrllll____ccccaaaallllllll____ppppvvvv((((""""AAAAddddddddSSSSuuuubbbbttttrrrraaaacccctttt"""",,,, GGGG____AAAARRRRRRRRAAAAYYYY))));;;; SSSSPPPPAAAAGGGGAAAAIIIINNNN ;;;; sssspppp ----==== ccccoooouuuunnnntttt ;;;; aaaaxxxx ==== ((((sssspppp ---- ssssttttaaaacccckkkk____bbbbaaaasssseeee)))) ++++ 1111 ;;;; iiiiffff ((((ccccoooouuuunnnntttt !!!!==== 2222)))) ccccrrrrooooaaaakkkk((((""""BBBBiiiigggg ttttrrrroooouuuubbbblllleeee\\\\nnnn"""")))) ;;;; pppprrrriiiinnnnttttffff ((((""""%%%%dddd ++++ %%%%dddd ==== %%%%dddd\\\\nnnn"""",,,, aaaa,,,, bbbb,,,, SSSSvvvvIIIIVVVV((((SSSSTTTT((((0000)))))))))))) ;;;; pppprrrriiiinnnnttttffff ((((""""%%%%dddd ---- %%%%dddd ==== %%%%dddd\\\\nnnn"""",,,, aaaa,,,, bbbb,,,, SSSSvvvvIIIIVVVV((((SSSSTTTT((((1111)))))))))))) ;;;; PPPPUUUUTTTTBBBBAAAACCCCKKKK ;;;; FFFFRRRREEEEEEEETTTTMMMMPPPPSSSS ;;;; LLLLEEEEAAAAVVVVEEEE ;;;; }}}} Notes 1. Notice that it was necessary to define the variable aaaaxxxx. This is because the SSSSTTTT macro expects it to exist. If we were in an XSUB it would not be necessary to define aaaaxxxx as it is already defined for you. 29/Jan/96 perl 5.002 with 34 PERLCALL(1) User Contributed Perl Documentation PERLCALL(1) 2. The code SSSSPPPPAAAAGGGGAAAAIIIINNNN ;;;; sssspppp ----==== ccccoooouuuunnnntttt ;;;; aaaaxxxx ==== ((((sssspppp ---- ssssttttaaaacccckkkk____bbbbaaaasssseeee)))) ++++ 1111 ;;;; sets the stack up so that we can use the SSSSTTTT macro. 3. Unlike the original coding of this example, the returned values are not accessed in reverse order. So SSSSTTTT((((0000)))) refers to the first value returned by the Perl subroutine and SSSSTTTT((((ccccoooouuuunnnntttt----1111)))) refers to the last. SSSSEEEEEEEE AAAALLLLSSSSOOOO the _p_e_r_l_x_s manpage, the _p_e_r_l_g_u_t_s manpage, the _p_e_r_l_e_m_b_e_d manpage AAAAUUUUTTTTHHHHOOOORRRR Paul Marquess <pmarquess@bfsec.bt.co.uk> Special thanks to the following people who assisted in the creation of the document. Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy and Larry Wall. DDDDAAAATTTTEEEE Version 1.2, 16th Jan 1996 29/Jan/96 perl 5.002 with 35