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Man Page for PERLCALL




NAME
     perlcall - Perl calling conventions from C

DESCRIPTION
     WARNING : This document is still under construction. There
     are bound to be a number of inaccuracies, so tread very
     carefully for now.

     The purpose of this document is to show you how to write
     callbacks, 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

     +    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.

     +    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 perlembed 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 perlapi manpage and the perlguts
     manpage.

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

     Definitions

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





         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 perl_call_sv. All the other functions
     make use of perl_call_sv to do what they do.

     perl_call_sv 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 perl_call_sv.  The second parameter, flags, is a
     general purpose option command. This parameter is common to
     all the perl_call_* functions. It is discussed in the next
     section.

     The function, perl_call_pv, is similar as perl_call_sv
     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 perl_call_method 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.

     perl_call_argv 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 always 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.

     Flag Values

     The flags parameter in all the perl_call_* 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


          return value from the perl_call_* 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 die or you have used
          the G_DISCARD flag.

          If the Perl sub returns a list, the perl_call_*
          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 perl_call_* 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 "@_0  }

                  sub joe
                    { &fred }

                  &joe(1,2,3) ;

          This will print


                  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 die or by not
          actually existing, and you want to catch this type of
          event, specify this flag setting. It will put an eval {
          } around the sub call.

          Whenever control returns from the perl_call_* 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.

EXAMPLES
     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 perl_call_pv  function.
     This has only been done to ease you into the topic. Wherever
     possible, if the choice is between using perl_call_pv and
     perl_call_sv, I would always try to use perl_call_sv.

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

     Example1: No Parameters, Nothing returned

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

         sub PrintUID
         {
             print "UID is $<0 ;
         }

     and here is the C to call it

         void
         call_PrintUID()
         {
             dSP ;



             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 PrintUID 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
          PrintUID, so G_DISCARD is specified. Even if PrintUID
          was changed to actually return some value(s), having
          specified G_DISCARD will mean that they will be wiped
          by the time control returns from perl_call_pv.

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

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

     Example 2: Passing Parameters

     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), "0 ;
         }

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

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



             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 call_LeftString.

     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
          perl_call_* 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 perl_call_pv 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, not the global copy.




     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 XSUB's AND  THE ARGUMENT STACK in the
          perlguts manpage for details on how the XPUSH macros
          work.

     6.   Finally, LeftString can now be called via the
          perl_call_pv function.

     Example 3: Returning a Scalar

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

     Here is a Perl sub, Adder,  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 Adder,
     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 trouble0) ;

             printf ("The sum of %d and %d is %d0, a, b, POPi) ;


             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 Adder will still exist after the call to
          perl_call_pv.

     2.   Because we are interested in what is returned from
          Adder 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
          perl_call_pv 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 perl_call_*
          functions or any other Perl internal function.

     4.   Although only a single value was expected to be
          returned from Adder, it is still good practice to check
          the return code from perl_call_pv anyway.

          Expecting a single value is not quite the same as


          knowing that there will be one. If someone modified
          Adder 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 really 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.

     Example 4: Returning a list of values

     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;


             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 trouble0) ;

             printf ("%d - %d = %d0, a, b, POPi) ;
             printf ("%d + %d = %d0, 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.

     Example 5: Returning Data from Perl via the parameter list

     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, Inc, below takes 2 parameters and increments
     each.

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

     and here is a C function to call it.









         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 %d0, count) ;

             printf ("%d + 1 = %d0, a, SvIV(sva)) ;
             printf ("%d + 1 = %d0, b, SvIV(svb)) ;

             FREETMPS ;
             LEAVE ;
         }

     To be able to access the two parameters that were pushed
     onto the stack after they return from perl_call_pv 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
     perl_call_pv.

     Example 6: Using G_EVAL

     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 die.

         sub Subtract
         {

	     my ($a, $b) = @_ ;

             die "death can be fatal0 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 - %s0, SvPV(sv, na)) ;

             SPAGAIN ;

             if (count != 1)
                 croak ("call_Subtract : expected 1 return  value
from 'Subtract', got %d0, count) ;

             printf ("%d - %d = %d0, a, b, POPi) ;

             PUTBACK ;
             FREETMPS ;
             LEAVE ;

         }

     If call_Subtract is called thus

             call_Subtract(4, 5)

     the following will be printed

             Uh oh - death can be fatal


     Notes

     1.   We want to be able to catch the die 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 - %s0, SvPVx(sv, na)) ;

          is the equivalent of this bit of Perl

                  print "Uh oh - $@0 if $@ ;


     Example 7: Using perl_call_sv

     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 there0 ;
             }

             CallSub("fred") ;

     here is a snippet of XSUB which defines CallSub.

             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 perl_call_sv is useful.

     The code below for CallSub is identical to the previous time
     except that the name parameter is now defined as an SV* and
     we use perl_call_sv instead of perl_call_pv.

PERLCALL(1)              USER COMMANDS  Release 5.0 Patchlevel 00



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

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

             CallSub("fred") ;
             Callsub(fred) ;
             $ref = fred ;
             CallSub($ref) ;
             CallSub( sub { print "Hello there0 } ) ;

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

     Example 8: Using perl_call_argv

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

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

                 foreach (@list) { print "$_0 }
             }

     and here is an example of perl_call_argv which will call
     PrintList.

             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 perl_call_argv will do it for you.

     Example 9: Using perl_call_method

     [This section is under construction]

     Consider the following Perl code


             {
               package Mine ;

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

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

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

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

                 perl_call_method(method, G_DISCARD) ;


     Strategies for storing Context Information

     [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.

     Alternate Stack Manipulation

     [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

SEE ALSO
     the perlapi manpage, the perlguts manpage, the perlembed
     manpage

AUTHOR
     Paul Marquess <>

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

     Jeff Okamoto, Tim Bunce.

DATE
     Version 0.4, 17th October 1994


 

 

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