Skunkware 5/Skunkware 5.iso/man/cat.1/perlcall.1

Text File | 1995-07-25 | 40KB | 1,057 lines

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     NNNNAAAAMMMMEEEE
          perlcall - Perl calling conventions from C

     DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
          WWWWAAAARRRRNNNNIIIINNNNGGGG :::: TTTThhhhiiiissss ddddooooccccuuuummmmeeeennnntttt iiiissss ssssttttiiiillllllll uuuunnnnddddeeeerrrr ccccoooonnnnssssttttrrrruuuuccccttttiiiioooonnnn.... TTTThhhheeeerrrreeee
          aaaarrrreeee bbbboooouuuunnnndddd ttttoooo bbbbeeee aaaa nnnnuuuummmmbbbbeeeerrrr ooooffff iiiinnnnaaaaccccccccuuuurrrraaaacccciiiieeeessss,,,, ssssoooo ttttrrrreeeeaaaadddd vvvveeeerrrryyyy
          ccccaaaarrrreeeeffffuuuullllllllyyyy ffffoooorrrr nnnnoooowwww....

          The purpose of this document is to show you how to write
          _c_a_l_l_b_a_c_k_s, i.e. how to call Perl from C. The main focus is
          on how to interface back to Perl from a bit of C code that
          has itself been run by Perl, i.e. the 'main' program is a
          Perl script; you are using it to execute a section of code
          written in C; that bit of C code wants you to do something
          with a particular event, so you want a Perl sub to be
          executed whenever it happens.

          Examples where this is necessary include

          o+    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 get called whenever
               something nasty occurs.  What we would like is for a
               Perl sub to be called instead.

          o+    The classic example of where callbacks are used is in
               an event driven program like for X-windows.  In this
               case your register functions to be called whenever a
               specific events occur, e.g. a mouse button is pressed.

          Although the techniques described are applicable to
          embedding Perl in a C program, this is not the primary goal
          of this document. For details on embedding Perl in C refer
          to the _p_e_r_l_e_m_b_e_d manpage (currently unwritten).

          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_a_p_i manpage and the _p_e_r_l_g_u_t_s
          manpage.

          This stuff is easier to explain using examples. But first
          here are a few definitions anyway.

          DDDDeeeeffffiiiinnnniiiittttiiiioooonnnnssss

          Perl has a number of C functions which allow you to call
          Perl subs. They are






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              I32 perl_call_sv(SV* sv, I32 flags) ;
              I32 perl_call_pv(char *subname, I32 flags) ;
              I32 perl_call_method(char *methname, I32 flags) ;
              I32 perl_call_argv(char *subname, I32 flags, register char **argv) ;

          The key function is _p_e_r_l__c_a_l_l__s_v. All the other functions
          make use of _p_e_r_l__c_a_l_l__s_v to do what they do.

          _p_e_r_l__c_a_l_l__s_v takes two parameters, the first is an SV*. This
          allows you to specify the Perl sub to be called either as a
          C string (which has first been converted to an SV) or a
          reference to a sub. Example 7, shows you how you can make
          use of _p_e_r_l__c_a_l_l__s_v.  The second parameter, flags, is a
          general purpose option command. This parameter is common to
          all the _p_e_r_l__c_a_l_l_* functions. It is discussed in the next
          section.

          The function, _p_e_r_l__c_a_l_l__p_v, is similar as _p_e_r_l__c_a_l_l__s_v
          except it expects it's first parameter has to be a C char*
          which identifies the Perl sub you want to call, e.g.
          perl_call_pv("fred", 0).

          The function _p_e_r_l__c_a_l_l__m_e_t_h_o_d expects its first argument to
          contain a blessed reference to a class. Using that reference
          it looks up and calls methname from that class. See example
          9.

          _p_e_r_l__c_a_l_l__a_r_g_v calls the Perl sub specified by the subname
          parameter. It also takes the usual flags parameter. The
          final parameter, argv, consists of a list of C strings to be
          sent to the Perl sub. See example 8.

          All the functions return a number. This is a count of the
          number of items returned by the Perl sub on the stack.

          As a general rule you should _a_l_w_a_y_s check the return value
          from these functions.  Even if you are only expecting a
          particular number of values to be returned from the Perl
          sub, there is nothing to stop someone from doing something
          unexpected - don't say you havn't been warned.

          FFFFllllaaaagggg VVVVaaaalllluuuueeeessss

          The flags parameter in all the _p_e_r_l__c_a_l_l_* functions
          consists of any combination of the symbols defined below,
          OR'ed together.

          G_SCALAR
               Calls the Perl sub in a scalar context.

               Whatever the Perl sub actually returns, we only want a
               scalar. If the perl sub does return a scalar, the



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               return value from the _p_e_r_l__c_a_l_l_* function will be 1 or
               0. If 1, then the value actually returned by the Perl
               sub will be contained on the top of the stack. If 0,
               then the sub has probably called _d_i_e or you have used
               the G_DISCARD flag.

               If the Perl sub returns a list, the _p_e_r_l__c_a_l_l_*
               function will still only return 1 or 0. If 1, then the
               number of elements in the list will be stored on top of
               the stack.  The actual values of the list will not be
               accessable.

               G_SCALAR is the default flag setting for all the
               functions.

          G_ARRAY
               Calls the Perl sub in a list context.

               The return code from the _p_e_r_l__c_a_l_l_* functions will
               indicate how many elements of the stack are used to
               store the array.

          G_DISCARD
               If you are not interested in the values returned by the
               Perl sub then setting this flag will make Perl get rid
               of them automatically for you. This will take
               precedence to either G_SCALAR or G_ARRAY.

               If you do not set this flag then you may need to
               explicitly get rid of temporary values.  See example 3
               for details.

          G_NOARGS
               If you are not passing any parameters to the Perl sub,
               you can save a bit of time by setting this flag. It has
               the effect of of not creating the @_ array for the Perl
               sub.

               A point worth noting is that if this flag is specified
               the Perl sub called can still access an @_ array from a
               previous Perl sub. This functionality can be
               illustrated with the perl code below

                       sub fred
                         { print "@_\n"  }

                       sub joe
                         { &fred }

                       &joe(1,2,3) ;

               This will print



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                       1 2 3

               What has happened is that fred accesses the @_ array
               which belongs to joe.

          G_EVAL
               If the Perl sub you are calling has the ability to
               terminate abnormally, e.g. by calling _d_i_e or by not
               actually existing, and you want to catch this type of
               event, specify this flag setting. It will put an _e_v_a_l {
               } around the sub 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. See example 6 for details of how to
               do this.

     EEEEXXXXAAAAMMMMPPPPLLLLEEEESSSS
          Enough of the definition talk, let's have a few examples.

          Perl provides many macros to assist in accessing the Perl
          stack. These macros should always be used when interfacing
          to Perl internals.  Hopefully this should make the code less
          vulnerable to changes made to Perl in the future.

          Another point worth noting is that in the first series of
          examples I have only made use of the _p_e_r_l__c_a_l_l__p_v function.
          This has only been done to 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, I would always try to use _p_e_r_l__c_a_l_l__s_v.

          The code for these examples is stored in the file
          _p_e_r_l_c_a_l_l._t_a_r. (Once this document settles down, all the
          example code will be available in the file).

          EEEExxxxaaaammmmpppplllleeee1111:::: NNNNoooo PPPPaaaarrrraaaammmmeeeetttteeeerrrrssss,,,, NNNNooootttthhhhiiiinnnngggg rrrreeeettttuuuurrrrnnnneeeedddd

          This first trivial example will call a Perl sub, _P_r_i_n_t_U_I_D,
          to print out the UID of the process.

              sub PrintUID
              {
                  print "UID is $<\n" ;
              }

          and here is the C to call it

              void
              call_PrintUID()
              {
                  dSP ;




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                  PUSHMARK(sp) ;
                  perl_call_pv("PrintUID", G_DISCARD|G_NOARGS) ;
              }

          Simple, eh.

          A few points to note about this example.

          1.   We aren't passing any parameters to _P_r_i_n_t_U_I_D so
               G_NOARGS can be specified.

          2.   Ignore dSP and PUSHMARK(sp) for now. They will be
               discussed in the next example.

          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.   Because we specified G_DISCARD, it is not necessary to
               check the value returned from _p_e_r_l__c_a_l_l__s_v. It will
               always be 0.

          5.   As _p_e_r_l__c_a_l_l__p_v is being used, the Perl sub is
               specified as a C string.

          EEEExxxxaaaammmmpppplllleeee 2222:::: PPPPaaaassssssssiiiinnnngggg PPPPaaaarrrraaaammmmeeeetttteeeerrrrssss

          Now let's make a slightly more complex example. This time we
          want to call a Perl sub which will take 2 parameters - a
          string ($s) and an integer ($n). The sub will simply print
          the first $n characters of the string.

          So the Perl sub would look like this

              sub LeftString
              {
                  my($s, $n) = @_ ;
                  print substr($s, 0, $n), "\n" ;
              }

          The C function required to call _L_e_f_t_S_t_r_i_n_g would look like
          this.

              static void
              call_LeftString(a, b)
              char * a ;
              int b ;
              {
                  dSP ;




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                  PUSHMARK(sp) ;
                  XPUSHs(sv_2mortal(newSVpv(a, 0)));
                  XPUSHs(sv_2mortal(newSViv(b)));
                  PUTBACK ;

                  perl_call_pv("LeftString", G_DISCARD);
              }

          Here are a few notes on the C function _c_a_l_l__L_e_f_t_S_t_r_i_n_g.

          1.   The only flag specified this time is G_DISCARD. As we
               are passing 2 parameters to the Perl sub this time, we
               have not specified G_NOARGS.

          2.   Parameters are passed to the Perl sub using the Perl
               stack.  This is the purpose of the code beginning with
               the line dSP and ending with the line PUTBACK.

          3.   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 dSP - it declares and initialises a local
               copy of the Perl stack pointer.

               All the other macros which will be used in this example
               require you to have used this macro.

               If you are calling a Perl sub directly from an XSUB
               function, it is not necessary to explicitly use the dSP
               macro - it will be declared for you.

          4.   Any parameters to be pushed onto the stack should be
               bracketed by the PUSHMARK and PUTBACK 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 sub, it knows how big to make it.

               The PUSHMARK macro tells Perl to make a mental note of
               the current stack pointer. Even if you aren't passing
               any parameters (like in Example 1) you must still call
               the PUSHMARK 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 PUTBACK 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.





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          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 _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.

          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.

          EEEExxxxaaaammmmpppplllleeee 3333:::: RRRReeeettttuuuurrrrnnnniiiinnnngggg aaaa SSSSccccaaaallllaaaarrrr

          Now for an example of dealing with the values returned from
          a Perl sub.

          Here is a Perl sub, _A_d_d_e_r,  which takes 2 integer parameters
          and simply returns their sum.

              sub Adder
              {
                  my($a, $b) = @_ ;
                  $a + $b ;
              }

          As we are now concerned with the return value from _A_d_d_e_r,
          the C function is now a bit more complex.

              static void
              call_Adder(a, b)
              int a ;
              int b ;
              {
                  dSP ;
                  int count ;

                  ENTER ;
                  SAVETMPS;

                  PUSHMARK(sp) ;
                  XPUSHs(sv_2mortal(newSViv(a)));
                  XPUSHs(sv_2mortal(newSViv(b)));
                  PUTBACK ;

                  count = perl_call_pv("Adder", G_SCALAR);

                  SPAGAIN ;

                  if (count != 1)
                      croak("Big trouble\n") ;

                  printf ("The sum of %d and %d is %d\n", a, b, POPi) ;



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                  PUTBACK ;
                  FREETMPS ;
                  LEAVE ;
              }

          Points to note this time are

          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

                       ENTER ;
                       SAVETMPS ;

               at the start of the function, and

                       FREETMPS ;
                       LEAVE ;

               at the end. The ENTER/SAVETMPS 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 FREETMPS/LEAVE pair will get rid of any values
               returned by the Perl sub, plus it will also dump the
               mortal SV's we created. Having ENTER/SAVETMPS at the
               beginning of the code makes sure that no other mortals
               are destroyed.

          3.   The purpose of the macro SPAGAIN 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 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



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               knowing that there will be one. If someone modified
               _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 inconsistant state. That is
               something you _r_e_a_l_l_y don't want to ever happen.

          5.   The POPi macro is used here to pop the return value
               from the stack. In this case we wanted an integer, so
               POPi was used.

               Here is the complete list of POP macros available,
               along with the types they return.

                       POPs    SV
                       POPp    pointer
                       POPn    double
                       POPi    integer
                       POPl    long


          6.   The final PUTBACK is used to leave the Perl stack in a
               consistant state before exiting the function. This is
               necessary because when we popped the return value from
               the stack with POPi it only updated our local copy of
               the stack pointer. Remember, PUTBACK sets the global
               stack pointer to be the same as our local copy.

          EEEExxxxaaaammmmpppplllleeee 4444:::: 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 sub

              sub AddSubtract
              {
                 my($a, $b) = @_ ;
                 ($a+$b, $a-$b) ;
              }

          and this is the C function

              static void
              call_AddSubtract(a, b)
              int a ;
              int b ;
              {
                  dSP ;
                  int count ;

                  ENTER ;
                  SAVETMPS;



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                  PUSHMARK(sp) ;
                  XPUSHs(sv_2mortal(newSViv(a)));
                  XPUSHs(sv_2mortal(newSViv(b)));
                  PUTBACK ;

                  count = perl_call_pv("AddSubtract", G_ARRAY);

                  SPAGAIN ;

                  if (count != 2)
                      croak("Big trouble\n") ;

                  printf ("%d - %d = %d\n", a, b, POPi) ;
                  printf ("%d + %d = %d\n", a, b, POPi) ;

                  PUTBACK ;
                  FREETMPS ;
                  LEAVE ;
              }

          Notes

          1.   We wanted array context, so we used G_ARRAY.

          2.   Not surprisingly there are 2 POPi's this time  because
               we were retrieving 2 values from the stack. The main
               point to note is that they came off the stack in
               reverse order.

          EEEExxxxaaaammmmpppplllleeee 5555:::: 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 sub, _I_n_c, below takes 2 parameters and increments
          each.

              sub Inc
              {
                  ++ $_[0] ;
                  ++ $_[1] ;
              }

          and here is a C function to call it.










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              static void
              call_Inc(a, b)
              int a ;
              int b ;
              {
                  dSP ;
                  int count ;
                  SV * sva ;
                  SV * svb ;

                  ENTER ;
                  SAVETMPS;

                  sva = sv_2mortal(newSViv(a)) ;
                  svb = sv_2mortal(newSViv(b)) ;

                  PUSHMARK(sp) ;
                  XPUSHs(sva);
                  XPUSHs(svb);
                  PUTBACK ;

                  count = perl_call_pv("Inc", G_DISCARD);

                  if (count != 0)
                      croak ("call_Inc : expected 0 return value from 'Inc', got %d\n", count) ;

                  printf ("%d + 1 = %d\n", a, SvIV(sva)) ;
                  printf ("%d + 1 = %d\n", b, SvIV(svb)) ;

                  FREETMPS ;
                  LEAVE ;
              }

          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 sva and svb.

          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.

          EEEExxxxaaaammmmpppplllleeee 6666:::: UUUUssssiiiinnnngggg GGGG____EEEEVVVVAAAALLLL

          Now an example using G_EVAL. Below is a Perl sub which
          computes the difference of its 2 parameters. If this would
          result in a negative result, the sub calls _d_i_e.

              sub Subtract
              {
                  my ($a, $b) = @_ ;



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                  die "death can be fatal\n" if $a < $b ;

                  $a - $b ;
              }

          and some C to call it

              static void
              call_Subtract(a, b)
              int a ;
              int b ;
              {
                  dSP ;
                  int count ;
                  SV * sv ;

                  ENTER ;
                  SAVETMPS;

                  PUSHMARK(sp) ;
                  XPUSHs(sv_2mortal(newSViv(a)));
                  XPUSHs(sv_2mortal(newSViv(b)));
                  PUTBACK ;

                  count = perl_call_pv("Subtract", G_EVAL|G_SCALAR);

                  /* Check the eval first */
                  sv = GvSV(gv_fetchpv("@", TRUE, SVt_PV));
                  if (SvTRUE(sv))
                      printf ("Uh oh - %s\n", SvPV(sv, na)) ;

                  SPAGAIN ;

                  if (count != 1)
                      croak ("call_Subtract : expected 1 return value from 'Subtract', got %d\n", count) ;

                  printf ("%d - %d = %d\n", a, b, POPi) ;

                  PUTBACK ;
                  FREETMPS ;
                  LEAVE ;

              }

          If _c_a_l_l__S_u_b_t_r_a_c_t is called thus

                  call_Subtract(4, 5)

          the following will be printed

                  Uh oh - death can be fatal




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     PPPPEEEERRRRLLLLCCCCAAAALLLLLLLL((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLCCCCAAAALLLLLLLL((((1111))))



          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.

          2.   The code

                       sv = GvSV(gv_fetchpv("@", TRUE, SVt_PV));
                       if (SvTRUE(sv))
                           printf ("Uh oh - %s\n", SvPVx(sv, na)) ;

               is the equivalent of this bit of Perl

                       print "Uh oh - $@\n" if $@ ;


          EEEExxxxaaaammmmpppplllleeee 7777:::: UUUUssssiiiinnnngggg ppppeeeerrrrllll____ccccaaaallllllll____ssssvvvv

          In all the previous examples I have 'hard-wried' the name of
          the Perl sub to be called from C. Sometimes though, it is
          necessary to be able to specify the name of the Perl sub
          from within the Perl script.

          Consider the Perl code below

                  sub fred
                  {
                      print "Hello there\n" ;
                  }

                  CallSub("fred") ;

          here is a snippet of XSUB which defines _C_a_l_l_S_u_b.

                  void
                  CallSub(name)
                          char *  name
                          CODE:
                          PUSHMARK(sp) ;
                          perl_call_pv(name, G_DISCARD|G_NOARGS) ;

          That is fine as far as it goes. The thing is, it only allows
          the Perl sub to be specified as a string. For perl 4 this
          was adequate, but Perl 5 allows references to subs and
          anonymous subs. 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 is identical to the previous time
          except that the name 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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     PPPPEEEERRRRLLLLCCCCAAAALLLLLLLL((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLCCCCAAAALLLLLLLL((((1111))))



                  void
                  CallSub(name)
                          SV*     name
                          CODE:
                          PUSHMARK(sp) ;
                          perl_call_sv(name, G_DISCARD|G_NOARGS) ;

          As we are using an SV to call _f_r_e_d the following can all be
          used

                  CallSub("fred") ;
                  Callsub(\&fred) ;
                  $ref = \&fred ;
                  CallSub($ref) ;
                  CallSub( sub { print "Hello there\n" } ) ;

          As you can see, _p_e_r_l__c_a_l_l__s_v gives you greater flexibility
          in how you can specify the Perl sub.

          EEEExxxxaaaammmmpppplllleeee 8888:::: UUUUssssiiiinnnngggg ppppeeeerrrrllll____ccccaaaallllllll____aaaarrrrggggvvvv

          Here is a Perl sub which prints whatever parameters are
          passed to it.

                  sub PrintList
                  {
                      my(@list) = @_ ;

                      foreach (@list) { print "$_\n" }
                  }

          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.

                  call_PrintList
                  {
                      dSP ;
                      char * words[] = {"alpha", "beta", "gamma", "delta", NULL } ;

                      perl_call_argv("PrintList", words, G_DISCARD) ;
                  }

          Note that it is not necessary to call PUSHMARK in this
          instance. This is because _p_e_r_l__c_a_l_l__a_r_g_v will do it for you.

          EEEExxxxaaaammmmpppplllleeee 9999:::: UUUUssssiiiinnnngggg ppppeeeerrrrllll____ccccaaaallllllll____mmmmeeeetttthhhhoooodddd

          [This section is under construction]

          Consider the following Perl code





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     PPPPEEEERRRRLLLLCCCCAAAALLLLLLLL((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLCCCCAAAALLLLLLLL((((1111))))



                  {
                    package Mine ;

                    sub new     { bless [@_] }
                    sub Display { print $_[0][1], "\n" }
                  }

                  $a = new Mine ('red', 'green', 'blue') ;
                  call_Display($a, 'Display') ;

          The method Display just prints out the first element of the
          list.  Here is a XSUB implementation of _c_a_l_l__D_i_s_p_l_a_y.

                  void
                  call_Display(ref, method)
                      SV *    ref
                      char *  method
                      CODE:
                      PUSHMARK(sp);
                      XPUSHs(ref);
                      PUTBACK;

                      perl_call_method(method, G_DISCARD) ;


          SSSSttttrrrraaaatttteeeeggggiiiieeeessss ffffoooorrrr ssssttttoooorrrriiiinnnngggg CCCCoooonnnntttteeeexxxxtttt IIIInnnnffffoooorrrrmmmmaaaattttiiiioooonnnn

          [This section is under construction]

          One of the trickiest problems to overcome when designing a
          callback interface is figuring out how to store the mapping
          between the C callback functions and the Perl equivalent.

          Consider the following example.

          AAAAlllltttteeeerrrrnnnnaaaatttteeee SSSSttttaaaacccckkkk MMMMaaaannnniiiippppuuuullllaaaattttiiiioooonnnn

          [This section is under construction]

          Although I have only made use of the POP* macros to access
          values returned from Perl subs, it is also possible to
          bypass these macros and read the stack directly.

          The code below is example 4 recoded to

     SSSSEEEEEEEE AAAALLLLSSSSOOOO
          the _p_e_r_l_a_p_i 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]>




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     PPPPEEEERRRRLLLLCCCCAAAALLLLLLLL((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLCCCCAAAALLLLLLLL((((1111))))



          Special thanks to the following people who assisted in the
          creation of the document.

          Jeff Okamoto, Tim Bunce.

     DDDDAAAATTTTEEEE
          Version 0.4, 17th October 1994
















































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