Amiga Magazin: Amiga-CD 1996 July/AMIGA_1996_7.BIN/ausgabe_7_96/pd-programmierung/perl5_002bin.lha/man/catp/perlcall.0

Text File | 1996-03-02 | 136KB | 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






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           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



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            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.



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       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.



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       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.



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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



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       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



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            _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.



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       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.





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       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.





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       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





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       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



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            _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;;;;





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               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;;;;



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               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.






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           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)))) ==== @@@@____ ;;;;



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               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



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       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



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       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.






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           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



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       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 ;;;;









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           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



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       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.




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       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




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           $$$$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




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       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



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       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)))) ;;;;



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               ////**** 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))))



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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 ;;;;





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               ////**** 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




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           ####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)))) ;;;;
           }}}}








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           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 ;;;;





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               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).



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       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.




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       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 <[e-mail]>

       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





























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