The OpenNET Project / Index page

[ новости /+++ | форум | теги | ]

Интерактивная система просмотра системных руководств (man-ов)

 ТемаНаборКатегория 
 
 [Cписок руководств | Печать]

perlcall (1)
  • >> perlcall (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perlcall (1) ( Разные man: Команды и прикладные программы пользовательского уровня )
  • 
    
    

    NAME

         perlcall - Perl calling conventions from C
    
    
    

    DESCRIPTION

         The purpose of this document is to show you how to call Perl
         subroutines directly from C, i.e., how to write callbacks.
    
         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 you 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 perlembed 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 perlxs manpage and the perlguts manpage.
    
    
    

    THE CALL_ FUNCTIONS

         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
    
    
    
             I32 call_sv(SV* sv, I32 flags) ;
             I32 call_pv(char *subname, I32 flags) ;
             I32 call_method(char *methname, I32 flags) ;
             I32 call_argv(char *subname, I32 flags, register char **argv) ;
    
         The key function is call_sv.  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 call_sv to invoke the Perl subroutine.
    
         All the call_* functions have a `flags' 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 FLAG
         VALUES entry elsewhere in this document.
    
         Each of the functions will now be discussed in turn.
    
         call_sv
              call_sv takes two parameters, the first, `sv', 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, Using call_sv, shows how you can make use of
              call_sv.
    
         call_pv
              The function, call_pv, is similar to call_sv except it
              expects its first parameter to be a C char* which
              identifies the Perl subroutine you want to call, e.g.,
              `call_pv("fred", 0)'.  If the subroutine you want to
              call is in another package, just include the package
              name in the string, e.g., `"pkg::fred"'.
    
         call_method
              The function call_method is used to call a method from
              a Perl class.  The parameter `methname' 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 perlobj manpage for more information on static
              and virtual methods and the Using call_method entry
              elsewhere in this document for an example of using
              call_method.
    
         call_argv
              call_argv calls the Perl subroutine specified by the C
              string stored in the `subname' parameter. It also takes
              the usual `flags' parameter.  The final parameter,
              `argv', consists of a NULL terminated list of C strings
              to be passed as parameters to the Perl subroutine.  See
              Using call_argv.
    
         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 always 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.
    
    
    

    FLAG VALUES

         The `flags' parameter in all the call_* functions is a bit
         mask which can consist of any combination of the symbols
         defined below, OR'ed together.
    
         G_VOID
    
         Calls the Perl subroutine in a void context.
    
         This flag has 2 effects:
    
         1.   It indicates to the subroutine being called that it is
              executing in a void context (if it executes wantarray
              the result will be the undefined value).
    
         2.   It ensures that nothing is actually returned from the
              subroutine.
    
         The value returned by the call_* function indicates how many
         items have been returned by the Perl subroutine - in this
         case it will be 0.
    
         G_SCALAR
    
         Calls the Perl subroutine in a scalar context.  This is the
         default context flag setting for all the call_* 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 wantarray
              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 wantarray and return a list anyway. If so, then
              only the last element of the list will be returned.
    
         The value returned by the call_* function indicates how many
         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 Returning a
         Scalar 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 call_* function.  The section Returning a
         list in a scalar context shows an example of this behavior.
    
         G_ARRAY
    
         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 wantarray
              the result will be true).
    
         2.   It ensures that all items returned from the subroutine
              will be accessible when control returns from the call_*
              function.
    
         The value returned by the call_* function indicates how many
         items have been returned by the Perl subroutine.
    
         If 0, then 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 Returning a list of values
         gives an example of using the G_ARRAY flag and the mechanics
         of accessing the returned items from the Perl stack.
    
         G_DISCARD
    
         By default, the call_* 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 very 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 Returning
         a Scalar gives details of how to dispose of these
         temporaries explicitly and the section Using Perl to dispose
         of temporaries discusses the specific circumstances where
         you can ignore the problem and let Perl deal with it for
         you.
    
         G_NOARGS
    
         Whenever a Perl subroutine is called using one of the call_*
         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.
    
         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 call_* function has itself been called from another Perl
         subroutine. The code below illustrates this
    
             sub fred
               { print "@_\n"  }
    
             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
    
         It is possible for the Perl subroutine you are calling to
         terminate abnormally, e.g., by calling die explicitly or by
         not actually existing.  By default, when either of these
         events occurs, the process will terminate immediately.  If
         you want to trap this type of event, specify the G_EVAL
         flag.  It will put an eval { } around the subroutine call.
    
         Whenever control returns from the call_* function you need
         to check the `$@' variable as you would in a normal Perl
         script.
    
         The value returned from the call_* 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 call_* 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.
    
         o    If G_ARRAY is specified and an error has occurred, the
              return value will always be 0.
    
         o    If G_SCALAR is specified and an error has occurred, the
              return value will be 1 and the value on the top of the
              stack will be undef. 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 undef from the stack.
    
         See Using G_EVAL for details on using G_EVAL.
    
         G_KEEPERR
    
         You may have noticed that using the G_EVAL flag described
         above will always 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 `eval {}' mechanism, because the possibility exists that
         perl will call other code (end of block processing code, for
         example) between the time the error causes `$@' to be set
         within `eval {}', and the subsequent statement which checks
         for the value of `$@' 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, `__DIE__' or `__WARN__' hooks,
         and `tie' 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 call_* 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 Using G_KEEPERR for an example of a situation that
         warrants the use of this flag.
    
         Determining the Context
    
         As mentioned above, you can determine the context of the
         currently executing subroutine in Perl with wantarray.  The
         equivalent test can be made in C by using the `GIMME_V'
         macro, which returns `G_ARRAY' if you have been called in an
         array context, `G_SCALAR' if in a scalar context, or
         `G_VOID' if in a void context (i.e. the return value will
         not be used).  An older version of this macro is called
         `GIMME'; in a void context it returns `G_SCALAR' instead of
         `G_VOID'.  An example of using the `GIMME_V' macro is shown
         in section Using GIMME_V.
    
    
    

    KNOWN PROBLEMS

         This section outlines all known problems that exist in the
         call_* 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 FLAG
              VALUES.
    
              Specifically, if the two flags are used when calling a
              subroutine and that subroutine does not call die, the
              value returned by call_* will be wrong.
    
         2.   In Perl 5.000 and 5.001 there is a problem with using
              call_* if the Perl sub you are calling attempts to trap
              a die.
    
              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
    
                  sub fred
                  {
                      eval { die "Fatal Error" ; }
                      print "Trapped error: $@\n"
                          if $@ ;
                  }
    
              via this XSUB
    
                  void
                  Call_fred()
                      CODE:
                      PUSHMARK(SP) ;
                      call_pv("fred", G_DISCARD|G_NOARGS) ;
                      fprintf(stderr, "back in Call_fred\n") ;
    
              When `Call_fred' is executed it will print
    
                  Trapped error: Fatal Error
    
              As control never returns to `Call_fred', the `"back in
              Call_fred"' string will not get printed.
    
              To work around this problem, you can either upgrade to
              Perl 5.002 or higher, or use the G_EVAL flag with
              call_* as shown below
    
                  void
                  Call_fred()
                      CODE:
                      PUSHMARK(SP) ;
                      call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ;
                      fprintf(stderr, "back in Call_fred\n") ;
    
    
    
    

    EXAMPLES

         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.  We hope 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 call_pv function.  This
         has been done to keep the code simpler and ease you into the
         topic.  Wherever possible, if the choice is between using
         call_pv and call_sv, you should always try to use call_sv.
         See Using call_sv for details.
    
    
         No Parameters, Nothing returned
    
         This first trivial example will call a Perl subroutine,
         PrintUID, to print out the UID of the process.
    
             sub PrintUID
             {
                 print "UID is $<\n" ;
             }
    
         and here is a C function to call it
    
             static void
             call_PrintUID()
             {
                 dSP ;
    
                 PUSHMARK(SP) ;
                 call_pv("PrintUID", G_DISCARD|G_NOARGS) ;
             }
    
         Simple, eh.
    
         A few points to note about this example.
    
         1.   Ignore `dSP' and `PUSHMARK(SP)' for now. They will be
              discussed in the next example.
    
         2.   We aren't passing any parameters to PrintUID so
              G_NOARGS can be specified.
    
         3.   We aren't interested in anything returned from
              PrintUID, so G_DISCARD is specified. Even if PrintUID
              was changed to return some value(s), having specified
              G_DISCARD will mean that they will be wiped by the time
              control returns from call_pv.
    
         4.   As call_pv 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 call_pv. It will always
              be 0.
    
         Passing Parameters
    
         Now let's make a slightly more complex example. This time we
         want to call a Perl subroutine, `LeftString', which will
         take 2 parameters--a string ($s) and an integer ($n).  The
         subroutine will simply print the first $n characters of the
         string.
         So the Perl subroutine would look like this
    
             sub LeftString
             {
                 my($s, $n) = @_ ;
                 print substr($s, 0, $n), "\n" ;
             }
    
         The C function required to call LeftString would look like
         this.
    
             static void
             call_LeftString(a, b)
             char * a ;
             int b ;
             {
                 dSP ;
    
                 ENTER ;
                 SAVETMPS ;
    
                 PUSHMARK(SP) ;
                 XPUSHs(sv_2mortal(newSVpv(a, 0)));
                 XPUSHs(sv_2mortal(newSViv(b)));
                 PUTBACK ;
    
                 call_pv("LeftString", G_DISCARD);
    
                 FREETMPS ;
                 LEAVE ;
             }
    
         Here are a few notes on the C function call_LeftString.
    
         1.   Parameters are passed to the Perl subroutine using the
              Perl stack.  This is the purpose of the code beginning
              with the line `dSP' and ending with the line `PUTBACK'.
              The `dSP' declares a local copy of the stack pointer.
              This local copy should always be accessed as `SP'.
    
         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 `dSP'--it declares and initializes 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.
    
              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 use the `dSP' macro
              explicitly--it will be declared for you automatically.
    
         3.   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
              count the number of parameters you are pushing
              automatically.  Then whenever Perl is creating the `@_'
              array for the subroutine, 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 the example shown in the
              section No Parameters, Nothing returned) you must still
              call the `PUSHMARK' macro before you can call any of
              the 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 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.
    
         4.   The only flag specified this time is G_DISCARD. Because
              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 XSUBs and the Argument Stack entry in the
              perlguts manpage for details on how the XPUSH macros
              work.
    
         6.   Because we created temporary values (by means of
              sv_2mortal() calls) we will have to tidy up the Perl
              stack and dispose of mortal SVs.
    
              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 subroutine (see next example),
              plus it will also dump the mortal SVs we have created.
              Having `ENTER'/`SAVETMPS' 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 Using Perl to dispose of temporaries
              for details of an alternative to using these macros.
    
         7.   Finally, LeftString can now be called via the call_pv
              function.
    
         Returning a Scalar
    
         Now for an example of dealing with the items returned from a
         Perl subroutine.
    
         Here is a Perl subroutine, Adder, that takes 2 integer
         parameters and simply returns their sum.
    
             sub Adder
             {
                 my($a, $b) = @_ ;
                 $a + $b ;
             }
    
         Because we are now concerned with the return value from
         Adder, the C function required to call it 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 = 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) ;
    
                 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
              call_pv.
    
         2.   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 reallocated whilst in the call_pv
              call.
    
              If you are making use of the Perl stack pointer in your
              code you must always refresh the local copy using
              SPAGAIN whenever you make use of the call_* functions
              or any other Perl internal function.
    
         3.   Although only a single value was expected to be
              returned from Adder, it is still good practice to check
              the return code from 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 inconsistent state. That is
              something you really don't want to happen ever.
    
         4.   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
    
         5.   The final `PUTBACK' 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 `POPi' it updated only our local
              copy of the stack pointer.  Remember, `PUTBACK' sets
              the global stack pointer to be the same as our local
              copy.
    
         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 subroutine
    
             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 = 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 ;
             }
    
         If call_AddSubtract is called like this
    
             call_AddSubtract(7, 4) ;
    
         then here is the output
    
             7 - 4 = 3
             7 + 4 = 11
    
         Notes
    
         1.   We wanted array context, so G_ARRAY was used.
    
         2.   Not surprisingly `POPi' is used twice this time because
              we were retrieving 2 values from the stack. The
              important thing to note is that when using the `POP*'
              macros they come off the stack in reverse order.
    
         Returning a list in a scalar context
    
         Say the Perl subroutine in the previous section was called
         in a scalar context, like this
    
             static void
             call_AddSubScalar(a, b)
             int a ;
             int b ;
             {
                 dSP ;
                 int count ;
                 int i ;
    
                 ENTER ;
                 SAVETMPS;
    
                 PUSHMARK(SP) ;
                 XPUSHs(sv_2mortal(newSViv(a)));
                 XPUSHs(sv_2mortal(newSViv(b)));
                 PUTBACK ;
    
                 count = call_pv("AddSubtract", G_SCALAR);
    
                 SPAGAIN ;
    
                 printf ("Items Returned = %d\n", count) ;
    
    
                 for (i = 1 ; i <= count ; ++i)
                     printf ("Value %d = %d\n", i, POPi) ;
    
                 PUTBACK ;
                 FREETMPS ;
                 LEAVE ;
             }
    
         The other modification made is that call_AddSubScalar will
         print the number of items returned from the Perl subroutine
         and their value (for simplicity it assumes that they are
         integer).  So if call_AddSubScalar is called
    
             call_AddSubScalar(7, 4) ;
    
         then the output will be
    
             Items Returned = 1
             Value 1 = 3
    
         In this case the main point to note is that only the last
         item in the list is returned from the subroutine,
         AddSubtract actually made it back to call_AddSubScalar.
    
         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 subroutine, Inc, below takes 2 parameters and
         increments each directly.
    
             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 = call_pv("Inc", G_DISCARD);
    
                 if (count != 0)
                     croak ("call_Inc: expected 0 values 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 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 call_pv.
    
         Using G_EVAL
    
         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 die.
    
             sub Subtract
             {
                 my ($a, $b) = @_ ;
    
                 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 ;
    
                 ENTER ;
                 SAVETMPS;
    
                 PUSHMARK(SP) ;
                 XPUSHs(sv_2mortal(newSViv(a)));
                 XPUSHs(sv_2mortal(newSViv(b)));
                 PUTBACK ;
    
                 count = call_pv("Subtract", G_EVAL|G_SCALAR);
    
                 SPAGAIN ;
    
                 /* Check the eval first */
                 if (SvTRUE(ERRSV))
                 {
                     STRLEN n_a;
                     printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ;
                     POPs ;
                 }
                 else
                 {
                     if (count != 1)
                        croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",
                                 count) ;
    
                     printf ("%d - %d = %d\n", 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 immediately at the die
              statement in the subroutine Subtract.
    
         2.   The code
    
                  if (SvTRUE(ERRSV))
                  {
                      STRLEN n_a;
                      printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ;
                      POPs ;
                  }
    
              is the direct equivalent of this bit of Perl
    
                  print "Uh oh - $@\n" if $@ ;
    
              `PL_errgv' is a perl global of type `GV *' that points
              to the symbol table entry containing the error.
              `ERRSV' therefore refers to the C equivalent of `$@'.
    
         3.   Note that the stack is popped using `POPs' in the block
              where `SvTRUE(ERRSV)' is true.  This is necessary
              because whenever a call_* function invoked with
              G_EVAL|G_SCALAR returns an error, the top of the stack
              holds the value undef. Because we want the program to
              continue after detecting this error, it is essential
              that the stack is tidied up by removing the undef.
    
         Using G_KEEPERR
    
         Consider this rather facetious example, where we have used
         an XS version of the call_Subtract example above inside a
         destructor:
    
             package Foo;
             sub new { bless {}, $_[0] }
             sub Subtract {
                 my($a,$b) = @_;
                 die "death can be fatal" if $a < $b ;
                 $a - $b;
             }
             sub DESTROY { call_Subtract(5, 4); }
             sub foo { die "foo dies"; }
    
             package main;
             eval { Foo->new->foo };
             print "Saw: $@" if $@;             # should be, but isn't
    
         This example will fail to recognize that an error occurred
         inside the `eval {}'.  Here's why: the call_Subtract code
         got executed while perl was cleaning up temporaries when
         exiting the eval block, and because call_Subtract is
         implemented with call_pv 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 call_pv call in
         call_Subtract reads:
    
                 count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);
    
         will preserve the error and restore reliable error handling.
    
         Using call_sv
    
         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
    
             sub fred
             {
                 print "Hello there\n" ;
             }
    
             CallSubPV("fred") ;
    
         Here is a snippet of XSUB which defines CallSubPV.
    
             void
             CallSubPV(name)
                 char *  name
                 CODE:
                 PUSHMARK(SP) ;
                 call_pv(name, G_DISCARD|G_NOARGS) ;
    
         That is fine as far as it goes. The thing is, the Perl
         subroutine can be specified as only a string.  For Perl 4
         this was adequate, but Perl 5 allows references to
         subroutines and anonymous subroutines.  This is where
         call_sv is useful.
    
         The code below for CallSubSV is identical to CallSubPV
         except that the `name' parameter is now defined as an SV*
         and we use call_sv instead of call_pv.
    
    
    
             void
             CallSubSV(name)
                 SV *    name
                 CODE:
                 PUSHMARK(SP) ;
                 call_sv(name, G_DISCARD|G_NOARGS) ;
    
         Because we are using an SV to call fred the following can
         all be used
    
             CallSubSV("fred") ;
             CallSubSV(\&fred) ;
             $ref = \&fred ;
             CallSubSV($ref) ;
             CallSubSV( sub { print "Hello there\n" } ) ;
    
         As you can see, call_sv 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
         (`name' in the example above) which corresponds to the Perl
         subroutine so that it can be used later in the program, it
         not enough just to store a copy of the pointer to the SV.
         Say the code above had been like this
    
             static SV * rememberSub ;
    
             void
             SaveSub1(name)
                 SV *    name
                 CODE:
                 rememberSub = name ;
    
             void
             CallSavedSub1()
                 CODE:
                 PUSHMARK(SP) ;
                 call_sv(rememberSub, G_DISCARD|G_NOARGS) ;
    
         The reason this is wrong is that by the time you come to use
         the pointer `rememberSub' in `CallSavedSub1', it may or may
         not still refer to the Perl subroutine that was recorded in
         `SaveSub1'.  This is particularly true for these cases
    
             SaveSub1(\&fred) ;
             CallSavedSub1() ;
    
             SaveSub1( sub { print "Hello there\n" } ) ;
             CallSavedSub1() ;
    
         By the time each of the `SaveSub1' statements above have
         been executed, the SV*s which corresponded to the parameters
         will no longer exist.  Expect an error message from Perl of
         the form
    
             Can't use an undefined value as a subroutine reference at ...
    
         for each of the `CallSavedSub1' lines.
    
         Similarly, with this code
    
             $ref = \&fred ;
             SaveSub1($ref) ;
             $ref = 47 ;
             CallSavedSub1() ;
    
         you can expect one of these messages (which you actually get
         is dependent on the version of Perl you are using)
    
             Not a CODE reference at ...
             Undefined subroutine &main::47 called ...
    
         The variable $ref may have referred to the subroutine `fred'
         whenever the call to `SaveSub1' was made but by the time
         `CallSavedSub1' gets called it now holds the number `47'.
         Because we saved only a pointer to the original SV in
         `SaveSub1', any changes to $ref will be tracked by the
         pointer `rememberSub'. This means that whenever
         `CallSavedSub1' gets called, it will attempt to execute the
         code which is referenced by the SV* `rememberSub'.  In this
         case though, it now refers to the integer `47', so expect
         Perl to complain loudly.
    
         A similar but more subtle problem is illustrated with this
         code
    
             $ref = \&fred ;
             SaveSub1($ref) ;
             $ref = \&joe ;
             CallSavedSub1() ;
    
         This time whenever `CallSavedSub1' get called it will
         execute the Perl subroutine `joe' (assuming it exists)
         rather than `fred' as was originally requested in the call
         to `SaveSub1'.
    
         To get around these problems it is necessary to take a full
         copy of the SV.  The code below shows `SaveSub2' modified to
         do that
    
             static SV * keepSub = (SV*)NULL ;
    
    
    
             void
             SaveSub2(name)
                 SV *    name
                 CODE:
                 /* Take a copy of the callback */
                 if (keepSub == (SV*)NULL)
                     /* First time, so create a new SV */
                     keepSub = newSVsv(name) ;
                 else
                     /* Been here before, so overwrite */
                     SvSetSV(keepSub, name) ;
    
             void
             CallSavedSub2()
                 CODE:
                 PUSHMARK(SP) ;
                 call_sv(keepSub, G_DISCARD|G_NOARGS) ;
    
         To avoid creating a new SV every time `SaveSub2' 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, `name' is copied to
         the variable `keepSub' in one operation using `newSVsv'.
         Thereafter, whenever `SaveSub2' is called the existing SV,
         `keepSub', is overwritten with the new value using
         `SvSetSV'.
    
         Using call_argv
    
         Here is a Perl subroutine which prints whatever parameters
         are passed to it.
    
             sub PrintList
             {
                 my(@list) = @_ ;
    
                 foreach (@list) { print "$_\n" }
             }
    
         and here is an example of call_argv which will call
         PrintList.
    
             static char * words[] = {"alpha", "beta", "gamma", "delta", NULL} ;
    
             static void
             call_PrintList()
             {
                 dSP ;
    
                 call_argv("PrintList", G_DISCARD, words) ;
             }
    
         Note that it is not necessary to call `PUSHMARK' in this
         instance.  This is because call_argv will do it for you.
    
         Using call_method
    
         Consider the following Perl code
    
             {
                 package Mine ;
    
                 sub new
                 {
                     my($type) = shift ;
                     bless [@_]
                 }
    
                 sub Display
                 {
                     my ($self, $index) = @_ ;
                     print "$index: $$self[$index]\n" ;
                 }
    
                 sub PrintID
                 {
                     my($class) = @_ ;
                     print "This is Class $class version 1.0\n" ;
                 }
             }
    
         It implements just a very simple class to manage an array.
         Apart from the constructor, `new', it declares methods, one
         static and one virtual. The static method, `PrintID', prints
         out simply the class name and a version number. The virtual
         method, `Display', prints out a single element of the array.
         Here is an all Perl example of using it.
    
             $a = new Mine ('red', 'green', 'blue') ;
             $a->Display(1) ;
             PrintID Mine;
    
         will print
    
             1: green
             This is Class Mine version 1.0
    
         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.
    
         Here is a simple XSUB which illustrates the mechanics of
         calling both the `PrintID' and `Display' methods from C.
    
             void
             call_Method(ref, method, index)
                 SV *    ref
                 char *  method
                 int             index
                 CODE:
                 PUSHMARK(SP);
                 XPUSHs(ref);
                 XPUSHs(sv_2mortal(newSViv(index))) ;
                 PUTBACK;
    
                 call_method(method, G_DISCARD) ;
    
             void
             call_PrintID(class, method)
                 char *  class
                 char *  method
                 CODE:
                 PUSHMARK(SP);
                 XPUSHs(sv_2mortal(newSVpv(class, 0))) ;
                 PUTBACK;
    
                 call_method(method, G_DISCARD) ;
    
         So the methods `PrintID' and `Display' can be invoked like
         this
    
             $a = new Mine ('red', 'green', 'blue') ;
             call_Method($a, 'Display', 1) ;
             call_PrintID('Mine', 'PrintID') ;
    
         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 call_method.
    
         Using GIMME_V
    
         Here is a trivial XSUB which prints the context in which it
         is currently executing.
    
    
    
             void
             PrintContext()
                 CODE:
                 I32 gimme = GIMME_V;
                 if (gimme == G_VOID)
                     printf ("Context is Void\n") ;
                 else if (gimme == G_SCALAR)
                     printf ("Context is Scalar\n") ;
                 else
                     printf ("Context is Array\n") ;
    
         and here is some Perl to test it
    
             PrintContext ;
             $a = PrintContext ;
             @a = PrintContext ;
    
         The output from that will be
    
             Context is Void
             Context is Scalar
             Context is Array
    
    
         Using Perl to dispose of temporaries
    
         In the examples given to date, any temporaries created in
         the callback (i.e., parameters passed on the stack to the
         call_* function or values returned via the stack) have been
         freed by one of these methods
    
         o    specifying the G_DISCARD flag with call_*.
    
         o    explicitly disposed of using the `ENTER'/`SAVETMPS' -
              `FREETMPS'/`LEAVE' 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
    
             ENTER ;
             SAVETMPS ;
             ...
             FREETMPS ;
             LEAVE ;
    
         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
    
             perl --> XSUB --> external library
    
         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
    
             perl --> XSUB --> external library
                               ...
                               error occurs
                               ...
                               external library --> call_* --> perl
                                                                   |
             perl <-- XSUB <-- external library <-- call_* <----+
    
         After processing of the error using call_* 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
    
    
    
             perl --> XSUB --> event handler
                               ...
                               event handler --> call_* --> perl
                                                                |
                               event handler <-- call_* <----+
                               ...
                               event handler --> call_* --> perl
                                                                |
                               event handler <-- call_* <----+
                               ...
                               event handler --> call_* --> perl
                                                                |
                               event handler <-- call_* <----+
    
         In this case the flow of control can consist of only the
         repeated sequence
    
             event handler --> call_* --> perl
    
         for practically the complete duration of the program.  This
         means that control may never 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 dispose of your 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 dispose explicitly 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.
    
         Strategies for storing Callback Context Information
    
         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 `register_fatal' which registers the C function to
         get called when a fatal error occurs.
    
             register_fatal(cb1) ;
    
         The single parameter `cb1' is a pointer to a function, so
         you must have defined `cb1' in your code, say something like
         this
    
             static void
             cb1()
             {
                 printf ("Fatal Error\n") ;
                 exit(1) ;
             }
    
         Now change that to call a Perl subroutine instead
    
             static SV * callback = (SV*)NULL;
    
             static void
             cb1()
             {
                 dSP ;
    
                 PUSHMARK(SP) ;
    
                 /* Call the Perl sub to process the callback */
                 call_sv(callback, G_DISCARD) ;
             }
    
             void
             register_fatal(fn)
                 SV *    fn
                 CODE:
                 /* Remember the Perl sub */
                 if (callback == (SV*)NULL)
                     callback = newSVsv(fn) ;
                 else
                     SvSetSV(callback, fn) ;
    
                 /* register the callback with the external library */
                 register_fatal(cb1) ;
    
         where the Perl equivalent of `register_fatal' and the
         callback it registers, `pcb1', might look like this
    
             # Register the sub pcb1
             register_fatal(\&pcb1) ;
    
    
             sub pcb1
             {
                 die "I'm dying...\n" ;
             }
    
         The mapping between the C callback and the Perl equivalent
         is stored in the global variable `callback'.
    
         This will be adequate if you ever need to have only one
         callback registered at any time. An example could be an
         error handler like the code sketched out above. Remember
         though, repeated calls to `register_fatal' 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 `asynch_read' which
         associates a C function `ProcessRead' with a file handle
         `fh'--this assumes that it has also provided some routine to
         open the file and so obtain the file handle.
    
             asynch_read(fh, ProcessRead)
    
         This may expect the C ProcessRead function of this form
    
             void
             ProcessRead(fh, buffer)
             int fh ;
             char *      buffer ;
             {
                  ...
             }
    
         To provide a Perl interface to this library we need to be
         able to map between the `fh' 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
    
             static HV * Mapping = (HV*)NULL ;
    
    
    
             void
             asynch_read(fh, callback)
                 int     fh
                 SV *    callback
                 CODE:
                 /* If the hash doesn't already exist, create it */
                 if (Mapping == (HV*)NULL)
                     Mapping = newHV() ;
    
                 /* Save the fh -> callback mapping */
                 hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0) ;
    
                 /* Register with the C Library */
                 asynch_read(fh, asynch_read_if) ;
    
         and `asynch_read_if' could look like this
    
             static void
             asynch_read_if(fh, buffer)
             int fh ;
             char *      buffer ;
             {
                 dSP ;
                 SV ** sv ;
    
                 /* Get the callback associated with fh */
                 sv =  hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE) ;
                 if (sv == (SV**)NULL)
                     croak("Internal error...\n") ;
    
                 PUSHMARK(SP) ;
                 XPUSHs(sv_2mortal(newSViv(fh))) ;
                 XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
                 PUTBACK ;
    
                 /* Call the Perl sub */
                 call_sv(*sv, G_DISCARD) ;
             }
    
         For completeness, here is `asynch_close'.  This shows how to
         remove the entry from the hash `Mapping'.
    
             void
             asynch_close(fh)
                 int     fh
                 CODE:
                 /* Remove the entry from the hash */
                 (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD) ;
    
                 /* Now call the real asynch_close */
                 asynch_close(fh) ;
    
         So the Perl interface would look like this
    
             sub callback1
             {
                 my($handle, $buffer) = @_ ;
             }
    
             # Register the Perl callback
             asynch_read($fh, \&callback1) ;
    
             asynch_close($fh) ;
    
         The mapping between the C callback and Perl is stored in the
         global hash `Mapping' 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 `buffer'
         parameter like this
    
             void
             ProcessRead(buffer)
             char *      buffer ;
             {
                 ...
             }
    
         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
         predefine a series of C functions to act as the interface to
         Perl, thus
    
             #define MAX_CB              3
             #define NULL_HANDLE -1
             typedef void (*FnMap)() ;
    
             struct MapStruct {
                 FnMap    Function ;
                 SV *     PerlSub ;
                 int      Handle ;
               } ;
    
             static void  fn1() ;
             static void  fn2() ;
             static void  fn3() ;
    
    
             static struct MapStruct Map [MAX_CB] =
                 {
                     { fn1, NULL, NULL_HANDLE },
                     { fn2, NULL, NULL_HANDLE },
                     { fn3, NULL, NULL_HANDLE }
                 } ;
    
             static void
             Pcb(index, buffer)
             int index ;
             char * buffer ;
             {
                 dSP ;
    
                 PUSHMARK(SP) ;
                 XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
                 PUTBACK ;
    
                 /* Call the Perl sub */
                 call_sv(Map[index].PerlSub, G_DISCARD) ;
             }
    
             static void
             fn1(buffer)
             char * buffer ;
             {
                 Pcb(0, buffer) ;
             }
    
             static void
             fn2(buffer)
             char * buffer ;
             {
                 Pcb(1, buffer) ;
             }
    
             static void
             fn3(buffer)
             char * buffer ;
             {
                 Pcb(2, buffer) ;
             }
    
             void
             array_asynch_read(fh, callback)
                 int             fh
                 SV *    callback
                 CODE:
                 int index ;
                 int null_index = MAX_CB ;
    
    
                 /* Find the same handle or an empty entry */
                 for (index = 0 ; index < MAX_CB ; ++index)
                 {
                     if (Map[index].Handle == fh)
                         break ;
    
                     if (Map[index].Handle == NULL_HANDLE)
                         null_index = index ;
                 }
    
                 if (index == MAX_CB && null_index == MAX_CB)
                     croak ("Too many callback functions registered\n") ;
    
                 if (index == MAX_CB)
                     index = null_index ;
    
                 /* Save the file handle */
                 Map[index].Handle = fh ;
    
                 /* Remember the Perl sub */
                 if (Map[index].PerlSub == (SV*)NULL)
                     Map[index].PerlSub = newSVsv(callback) ;
                 else
                     SvSetSV(Map[index].PerlSub, callback) ;
    
                 asynch_read(fh, Map[index].Function) ;
    
             void
             array_asynch_close(fh)
                 int     fh
                 CODE:
                 int index ;
    
                 /* Find the file handle */
                 for (index = 0; index < MAX_CB ; ++ index)
                     if (Map[index].Handle == fh)
                         break ;
    
                 if (index == MAX_CB)
                     croak ("could not close fh %d\n", fh) ;
    
                 Map[index].Handle = NULL_HANDLE ;
                 SvREFCNT_dec(Map[index].PerlSub) ;
                 Map[index].PerlSub = (SV*)NULL ;
    
                 asynch_close(fh) ;
    
         In this case the functions `fn1', `fn2', and `fn3' 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 `Pcb' to access the `Map' 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 recompiling.  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.
    
         Alternate Stack Manipulation
    
         Although I have made use of only the `POP*' macros to access
         values returned from Perl subroutines, it is also possible
         to bypass these macros and read the stack using the `ST'
         macro (See the perlxs manpage for a full description of the
         `ST' macro).
    
         Most of the time the `POP*' 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 `ST' macro as used when coding an XSUB is ideal for this
         purpose.
    
         The code below is the example given in the section Returning
         a list of values recoded to use `ST' instead of `POP*'.
    
    
    
             static void
             call_AddSubtract2(a, b)
             int a ;
             int b ;
             {
                 dSP ;
                 I32 ax ;
                 int count ;
    
                 ENTER ;
                 SAVETMPS;
    
                 PUSHMARK(SP) ;
                 XPUSHs(sv_2mortal(newSViv(a)));
                 XPUSHs(sv_2mortal(newSViv(b)));
                 PUTBACK ;
    
                 count = call_pv("AddSubtract", G_ARRAY);
    
                 SPAGAIN ;
                 SP -= count ;
                 ax = (SP - PL_stack_base) + 1 ;
    
                 if (count != 2)
                     croak("Big trouble\n") ;
    
                 printf ("%d + %d = %d\n", a, b, SvIV(ST(0))) ;
                 printf ("%d - %d = %d\n", a, b, SvIV(ST(1))) ;
    
                 PUTBACK ;
                 FREETMPS ;
                 LEAVE ;
             }
    
         Notes
    
         1.   Notice that it was necessary to define the variable
              `ax'.  This is because the `ST' macro expects it to
              exist.  If we were in an XSUB it would not be necessary
              to define `ax' as it is already defined for you.
    
         2.   The code
    
                      SPAGAIN ;
                      SP -= count ;
                      ax = (SP - PL_stack_base) + 1 ;
    
              sets the stack up so that we can use the `ST' macro.
    
         3.   Unlike the original coding of this example, the
              returned values are not accessed in reverse order.  So
              `ST(0)' refers to the first value returned by the Perl
              subroutine and `ST(count-1)' refers to the last.
    
         Creating and calling an anonymous subroutine in C
    
         As we've already shown, `call_sv' can be used to invoke an
         anonymous subroutine.  However, our example showed a Perl
         script invoking an XSUB to perform this operation.  Let's
         see how it can be done inside our C code:
    
          ...
    
          SV *cvrv = eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE);
    
          ...
    
          call_sv(cvrv, G_VOID|G_NOARGS);
    
         `eval_pv' is used to compile the anonymous subroutine, which
         will be the return value as well (read more about `eval_pv'
         in the eval_pv entry in the perlapi manpage).  Once this
         code reference is in hand, it can be mixed in with all the
         previous examples we've shown.
    
    
    

    SEE ALSO

         the perlxs 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, Nick Gianniotis, Steve Kelem,
         Gurusamy Sarathy and Larry Wall.
    
    
    

    DATE

         Version 1.3, 14th Apr 1997
    
    
    
    


    Поиск по тексту MAN-ов: 




    Партнёры:
    PostgresPro
    Inferno Solutions
    Hosting by Hoster.ru
    Хостинг:

    Закладки на сайте
    Проследить за страницей
    Created 1996-2024 by Maxim Chirkov
    Добавить, Поддержать, Вебмастеру