=head1 DESCRIPTION
The purpose of this document is to show you how to call Perl subroutines
-directly from C, i.e. how to write I<callbacks>.
+directly from C, i.e., how to write I<callbacks>.
Apart from discussing the C interface provided by Perl for writing
callbacks the document uses a series of examples to show how the
The classic example of where callbacks are used is when writing an
event driven program like for an X windows application. In this case
-your register functions to be called whenever specific events occur,
-e.g. a mouse button is pressed, the cursor moves into a window or a
+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.
=back
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 -
-L<perlapi> and L<perlguts>.
+L<perlxs> and L<perlguts>.
=head1 THE PERL_CALL FUNCTIONS
The key function is I<perl_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 I<perl_call_sv>
-to actually invoke the Perl subroutine.
+to invoke the Perl subroutine.
All the I<perl_call_*> functions have a C<flags> parameter which is
used to pass a bit mask of options to Perl. This bit mask operates
=over 5
-=item B<perl_call_sv>
+=item perl_call_sv
I<perl_call_sv> takes two parameters, the first, C<sv>, is an SV*.
This allows you to specify the Perl subroutine to be called either as a
subroutine. The section, I<Using perl_call_sv>, shows how you can make
use of I<perl_call_sv>.
-=item B<perl_call_pv>
+=item perl_call_pv
The function, I<perl_call_pv>, is similar to I<perl_call_sv> except it
expects its first parameter to be a C char* which identifies the Perl
-subroutine you want to call, e.g. C<perl_call_pv("fred", 0)>. If the
+subroutine you want to call, e.g., C<perl_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. C<"pkg::fred">.
+package name in the string, e.g., C<"pkg::fred">.
-=item B<perl_call_method>
+=item perl_call_method
The function I<perl_call_method> is used to call a method from a Perl
class. The parameter C<methname> corresponds to the name of the method
static and virtual methods and L<Using perl_call_method> for an example
of using I<perl_call_method>.
-=item B<perl_call_argv>
+=item perl_call_argv
I<perl_call_argv> calls the Perl subroutine specified by the C string
stored in the C<subname> parameter. It also takes the usual C<flags>
As a general rule you should I<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
+stop someone from doing something unexpected--don't say you haven't
been warned.
=head1 FLAG VALUES
OR'ed together.
+=head2 G_VOID
+
+Calls the Perl subroutine in a void context.
+
+This flag has 2 effects:
+
+=over 5
+
+=item 1.
+
+It indicates to the subroutine being called that it is executing in
+a void context (if it executes I<wantarray> the result will be the
+undefined value).
+
+=item 2.
+
+It ensures that nothing is actually returned from the subroutine.
+
+=back
+
+The value returned by the I<perl_call_*> function indicates how many
+items have been returned by the Perl subroutine - in this case it will
+be 0.
+
+
=head2 G_SCALAR
Calls the Perl subroutine in a scalar context. This is the default
context flag setting for all the I<perl_call_*> functions.
-This flag has 2 effects
+This flag has 2 effects:
=over 5
=item 1.
-it indicates to the subroutine being called that it is executing in a
+It indicates to the subroutine being called that it is executing in a
scalar context (if it executes I<wantarray> the result will be false).
-
=item 2.
-it ensures that only a scalar is actually returned from the subroutine.
+It ensures that only a scalar is actually returned from the subroutine.
The subroutine can, of course, ignore the I<wantarray> and return a
list anyway. If so, then only the last element of the list will be
returned.
=back
-The value returned by the I<perl_call_*> function indicates how may
+The value returned by the I<perl_call_*> function indicates how many
items have been returned by the Perl subroutine - in this case it will
be either 0 or 1.
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
I<perl_call_*> function. The section I<Returning a list in a scalar
-context> shows an example of this behaviour.
+context> shows an example of this behavior.
=head2 G_ARRAY
Calls the Perl subroutine in a list context.
-As with G_SCALAR, this flag has 2 effects
+As with G_SCALAR, this flag has 2 effects:
=over 5
=item 1.
-it indicates to the subroutine being called that it is executing in an
+It indicates to the subroutine being called that it is executing in an
array context (if it executes I<wantarray> the result will be true).
=item 2.
-it ensures that all items returned from the subroutine will be
+It ensures that all items returned from the subroutine will be
accessible when control returns from the I<perl_call_*> function.
=back
-The value returned by the I<perl_call_*> function indicates how may
+The value returned by the I<perl_call_*> function indicates how many
items have been returned by the Perl subroutine.
-If 0, the you have specified the G_DISCARD flag.
+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
context to the Perl subroutine by using either G_SCALAR or G_ARRAY.
If you do not set this flag then it is I<very> important that you make
-sure that any temporaries (i.e. parameters passed to the Perl
+sure that any temporaries (i.e., parameters passed to the Perl
subroutine and values returned from the subroutine) are disposed of
yourself. The section I<Returning a Scalar> gives details of how to
-explicitly dispose of these temporaries and the section I<Using Perl to
+dispose of these temporaries explicitly and the section I<Using Perl to
dispose of temporaries> discusses the specific circumstances where you
can ignore the problem and let Perl deal with it for you.
belongs to C<joe>.
-=head2 G_EVAL
+=head2 G_EVAL
It is possible for the Perl subroutine you are calling to terminate
-abnormally, e.g. by calling I<die> explicitly or by not actually
-existing. By default, when either of these of events occurs, the
-process will terminate immediately. If though, you want to trap this
+abnormally, e.g., by calling I<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 I<eval { }>
around the subroutine call.
The value returned from the I<perl_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
+occurred. Here are all the different cases that can occur:
=over 5
=back
-See I<Using G_EVAL> for details of using G_EVAL.
+See I<Using G_EVAL> for details on using G_EVAL.
+
+=head2 G_KEEPERR
+
+You may have noticed that using the G_EVAL flag described above will
+B<always> clear the C<$@> variable and set it to a string describing
+the error iff there was an error in the called code. This unqualified
+resetting of C<$@> can be problematic in the reliable identification of
+errors using the C<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 C<$@> to be set within
+C<eval {}>, and the subsequent statement which checks for the value of
+C<$@> gets executed in the user's script.
-=head2 Determining the Context
+This scenario will mostly be applicable to code that is meant to be
+called from within destructors, asynchronous callbacks, signal
+handlers, C<__DIE__> or C<__WARN__> hooks, and C<tie> functions. In
+such situations, you will not want to clear C<$@> at all, but simply to
+append any new errors to any existing value of C<$@>.
+
+The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in
+I<perl_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 C<$@>.
+
+The G_KEEPERR flag was introduced in Perl version 5.002.
+
+See I<Using G_KEEPERR> for an example of a situation that warrants the
+use of this flag.
+
+=head2 Determining the Context
As mentioned above, you can determine the context of the currently
-executing subroutine in Perl with I<wantarray>. The equivalent test can
-be made in C by using the C<GIMME> macro. This will return C<G_SCALAR>
-if you have been called in a scalar context and C<G_ARRAY> if in an
-array context. An example of using the C<GIMME> macro is shown in
-section I<Using GIMME>.
+executing subroutine in Perl with I<wantarray>. The equivalent test
+can be made in C by using the C<GIMME_V> macro, which returns
+C<G_ARRAY> if you have been called in an array context, C<G_SCALAR> if
+in a scalar context, or C<G_VOID> if in a void context (i.e. the
+return value will not be used). An older version of this macro is
+called C<GIMME>; in a void context it returns C<G_SCALAR> instead of
+C<G_VOID>. An example of using the C<GIMME_V> macro is shown in
+section I<Using GIMME_V>.
=head1 KNOWN PROBLEMS
sub fred
{
eval { die "Fatal Error" ; }
- print "Trapped error: $@\n"
+ print "Trapped error: $@\n"
if $@ ;
}
void
Call_fred()
CODE:
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
perl_call_pv("fred", G_DISCARD|G_NOARGS) ;
fprintf(stderr, "back in Call_fred\n") ;
As control never returns to C<Call_fred>, the C<"back in Call_fred">
string will not get printed.
-To work around this problem, you can either upgrade to Perl 5.002 (or
-later), or use the G_EVAL flag with I<perl_call_*> as shown below
+To work around this problem, you can either upgrade to Perl 5.002 or
+higher, or use the G_EVAL flag with I<perl_call_*> as shown below
void
Call_fred()
CODE:
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
perl_call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ;
fprintf(stderr, "back in Call_fred\n") ;
Perl provides many macros to assist in accessing the Perl stack.
Wherever possible, these macros should always be used when interfacing
-to Perl internals. Hopefully this should make the code less vulnerable
+to Perl internals. We hope this should make the code less vulnerable
to any changes made to Perl in the future.
Another point worth noting is that in the first series of examples I
{
dSP ;
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
perl_call_pv("PrintUID", G_DISCARD|G_NOARGS) ;
}
=item 1.
-Ignore C<dSP> and C<PUSHMARK(sp)> for now. They will be discussed in
+Ignore C<dSP> and C<PUSHMARK(SP)> for now. They will be discussed in
the next example.
=item 2.
=item 3.
We aren't interested in anything returned from I<PrintUID>, so
-G_DISCARD is specified. Even if I<PrintUID> was changed to actually
+G_DISCARD is specified. Even if I<PrintUID> was changed to
return some value(s), having specified G_DISCARD will mean that they
will be wiped by the time control returns from I<perl_call_pv>.
=head2 Passing Parameters
Now let's make a slightly more complex example. This time we want to
-call a Perl subroutine, C<LeftString>, which will take 2 parameters - a
-string (C<$s>) and an integer (C<$n>). The subroutine will simply
-print the first C<$n> characters of the string.
+call a Perl subroutine, C<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
{
dSP ;
- PUSHMARK(sp) ;
+ ENTER ;
+ SAVETMPS ;
+
+ PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSVpv(a, 0)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
perl_call_pv("LeftString", G_DISCARD);
+
+ FREETMPS ;
+ LEAVE ;
}
Here are a few notes on the C function I<call_LeftString>.
Parameters are passed to the Perl subroutine using the Perl stack.
This is the purpose of the code beginning with the line C<dSP> and
-ending with the line C<PUTBACK>.
-
+ending with the line C<PUTBACK>. The C<dSP> declares a local copy
+of the stack pointer. This local copy should B<always> be accessed
+as C<SP>.
=item 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 C<dSP> - it declares
+where to put it. This is the purpose of the macro C<dSP>--it declares
and initializes a I<local> copy of the Perl stack pointer.
All the other macros which will be used in this example require you to
The exception to this rule is if you are calling a Perl subroutine
directly from an XSUB function. In this case it is not necessary to
-explicitly use the C<dSP> macro - it will be declared for you
+use the C<dSP> macro explicitly--it will be declared for you
automatically.
=item 3.
Any parameters to be pushed onto the stack should be bracketed by the
C<PUSHMARK> and C<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 C<@_> array for the
+this context, is to count the number of parameters you are
+pushing automatically. Then whenever Perl is creating the C<@_> array for the
subroutine, it knows how big to make it.
The C<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 I<No Parameters, Nothing returned>) you
must still call the C<PUSHMARK> macro before you can call any of the
-I<perl_call_*> functions - Perl still needs to know that there are no
+I<perl_call_*> functions--Perl still needs to know that there are no
parameters.
The C<PUTBACK> macro sets the global copy of the stack pointer to be
the same as our local copy. If we didn't do this I<perl_call_pv>
-wouldn't know where the two parameters we pushed were - remember that
+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, I<not> the global copy.
=item 4.
-The only flag specified this time is G_DISCARD. Since we are passing 2
+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.
pushed onto the stack. In this case we are pushing a string and an
integer.
-See the section L<perlguts/"XSUB'S and the Argument Stack"> for details
+See L<perlguts/"XSUBs and the Argument Stack"> for details
on how the XPUSH macros work.
=item 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 C<ENTER>/C<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 C<FREETMPS>/C<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 C<ENTER>/C<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 C<{> and C<}> in Perl
+to limit the scope of local variables.
+
+See the section I<Using Perl to dispose of temporaries> for details of
+an alternative to using these macros.
+
+=item 7.
+
Finally, I<LeftString> can now be called via the I<perl_call_pv>
function.
Now for an example of dealing with the items returned from a Perl
subroutine.
-Here is a Perl subroutine, I<Adder>, which takes 2 integer parameters
+Here is a Perl subroutine, I<Adder>, that takes 2 integer parameters
and simply returns their sum.
sub Adder
$a + $b ;
}
-Since we are now concerned with the return value from I<Adder>, the C
+Because we are now concerned with the return value from I<Adder>, the C
function required to call it is now a bit more complex.
static void
ENTER ;
SAVETMPS;
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
=over 5
-=item 1.
+=item 1.
The only flag specified this time was G_SCALAR. That means the C<@_>
array will be created and that the value returned by I<Adder> will
still exist after the call to I<perl_call_pv>.
-
-
=item 2.
-Because we are interested in what is returned from I<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 C<ENTER>/C<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 C<FREETMPS>/C<LEAVE> pair will get rid of any values returned by
-the Perl subroutine, plus it will also dump the mortal SV's we have
-created. Having C<ENTER>/C<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 C<{> and C<}> in Perl
-to limit the scope of local variables.
-
-See the section I<Using Perl to dispose of temporaries> for details of
-an alternative to using these macros.
-
-=item 3.
-
The purpose of the macro C<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
+allocated to the Perl stack has been reallocated whilst in the
I<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
+always refresh the local copy using SPAGAIN whenever you make use
of the I<perl_call_*> functions or any other Perl internal function.
-=item 4.
+=item 3.
Although only a single value was expected to be returned from I<Adder>,
it is still good practice to check the return code from I<perl_call_pv>
will be one. If someone modified I<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
-I<really> don't want to ever happen.
+I<really> don't want to happen ever.
-=item 5.
+=item 4.
The C<POPi> macro is used here to pop the return value from the stack.
In this case we wanted an integer, so C<POPi> was used.
POPi integer
POPl long
-=item 6.
+=item 5.
The final C<PUTBACK> is used to leave the Perl stack in a consistent
state before exiting the function. This is necessary because when we
ENTER ;
SAVETMPS;
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
ENTER ;
SAVETMPS;
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
Value 1 = 3
In this case the main point to note is that only the last item in the
-list returned from the subroutine, I<Adder> actually made it back to
+list is returned from the subroutine, I<AddSubtract> actually made it back to
I<call_AddSubScalar>.
sva = sv_2mortal(newSViv(a)) ;
svb = sv_2mortal(newSViv(b)) ;
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
XPUSHs(sva);
XPUSHs(svb);
PUTBACK ;
To be able to access the two parameters that were pushed onto the stack
after they return from I<perl_call_pv> it is necessary to make a note
-of their addresses - thus the two variables C<sva> and C<svb>.
+of their addresses--thus the two variables C<sva> and C<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
{
dSP ;
int count ;
- SV * sv ;
ENTER ;
SAVETMPS;
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
SPAGAIN ;
/* Check the eval first */
- sv = GvSV(gv_fetchpv("@", TRUE, SVt_PV));
- if (SvTRUE(sv))
+ if (SvTRUE(ERRSV))
{
- printf ("Uh oh - %s\n", SvPV(sv, na)) ;
+ STRLEN n_a;
+ printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ;
POPs ;
}
else
=item 2.
-The code
+The code
- sv = GvSV(gv_fetchpv("@", TRUE, SVt_PV));
- if (SvTRUE(sv))
+ if (SvTRUE(ERRSV))
{
- printf ("Uh oh - %s\n", SvPVx(sv, na)) ;
+ STRLEN n_a;
+ printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ;
POPs ;
}
print "Uh oh - $@\n" if $@ ;
+C<PL_errgv> is a perl global of type C<GV *> that points to the
+symbol table entry containing the error. C<ERRSV> therefore
+refers to the C equivalent of C<$@>.
+
=item 3.
Note that the stack is popped using C<POPs> in the block where
-C<SvTRUE(sv)> is true. This is necessary because whenever a
+C<SvTRUE(ERRSV)> is true. This is necessary because whenever a
I<perl_call_*> function invoked with G_EVAL|G_SCALAR returns an error,
-the top of the stack holds the value I<undef>. Since we want the
+the top of the stack holds the value I<undef>. Because we want the
program to continue after detecting this error, it is essential that
the stack is tidied up by removing the I<undef>.
=back
+=head2 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 $@; # should be, but isn't
+
+This example will fail to recognize that an error occurred inside the
+C<eval {}>. Here's why: the call_Subtract code got executed while perl
+was cleaning up temporaries when exiting the eval block, and because
+call_Subtract is implemented with I<perl_call_pv> using the G_EVAL
+flag, it promptly reset C<$@>. This results in the failure of the
+outermost test for C<$@>, and thereby the failure of the error trap.
+
+Appending the G_KEEPERR flag, so that the I<perl_call_pv> call in
+call_Subtract reads:
+
+ count = perl_call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);
+
+will preserve the error and restore reliable error handling.
+
=head2 Using perl_call_sv
In all the previous examples I have 'hard-wired' the name of the Perl
CallSubPV(name)
char * name
CODE:
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
perl_call_pv(name, G_DISCARD|G_NOARGS) ;
-That is fine as far as it goes. The thing is, the Perl subroutine
-can be specified only as a string. For Perl 4 this was adequate,
+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 I<perl_call_sv> is useful.
CallSubSV(name)
SV * name
CODE:
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
perl_call_sv(name, G_DISCARD|G_NOARGS) ;
-Since we are using an SV to call I<fred> the following can all be used
+Because we are using an SV to call I<fred> the following can all be used
CallSubSV("fred") ;
CallSubSV(\&fred) ;
You should note that if it is necessary to store the SV (C<name> in the
example above) which corresponds to the Perl subroutine so that it can
-be used later in the program, it not enough to just store a copy of the
+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
CallSavedSub1()
CODE:
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
perl_call_sv(rememberSub, G_DISCARD|G_NOARGS) ;
The reason this is wrong is that by the time you come to use the
CallSavedSub1() ;
By the time each of the C<SaveSub1> statements above have been executed,
-the SV*'s which corresponded to the parameters will no longer exist.
+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 C<CallSavedSub1> lines.
-Similarly, with this code
+Similarly, with this code
$ref = \&fred ;
SaveSub1($ref) ;
$ref = 47 ;
CallSavedSub1() ;
-you can expect one of these messages (which you actually get is dependant on
-the version of Perl you are using)
+you can expect one of these messages (which you actually get is dependent on
+the version of Perl you are using)
Not a CODE reference at ...
Undefined subroutine &main::47 called ...
-The variable C<$ref> may have referred to the subroutine C<fred>
+The variable $ref may have referred to the subroutine C<fred>
whenever the call to C<SaveSub1> was made but by the time
-C<CallSavedSub1> gets called it now holds the number C<47>. Since we
+C<CallSavedSub1> gets called it now holds the number C<47>. Because we
saved only a pointer to the original SV in C<SaveSub1>, any changes to
-C<$ref> will be tracked by the pointer C<rememberSub>. This means that
+$ref will be tracked by the pointer C<rememberSub>. This means that
whenever C<CallSavedSub1> gets called, it will attempt to execute the
code which is referenced by the SV* C<rememberSub>. In this case
though, it now refers to the integer C<47>, so expect Perl to complain
CallSavedSub1() ;
This time whenever C<CallSavedSub1> get called it will execute the Perl
-subroutine C<joe> (assuming it exists) rather than C<fred> as was
+subroutine C<joe> (assuming it exists) rather than C<fred> as was
originally requested in the call to C<SaveSub1>.
To get around these problems it is necessary to take a full copy of the
void
CallSavedSub2()
CODE:
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
perl_call_sv(keepSub, G_DISCARD|G_NOARGS) ;
-In order to avoid creating a new SV every time C<SaveSub2> is called,
+To avoid creating a new SV every time C<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, C<name> is copied to the variable C<keepSub> in one
}
}
-It just implements a very simple class to manage an array. Apart from
+It implements just a very simple class to manage an array. Apart from
the constructor, C<new>, it declares methods, one static and one
-virtual. The static method, C<PrintID>, simply prints out the class
+virtual. The static method, C<PrintID>, prints out simply the class
name and a version number. The virtual method, C<Display>, prints out a
single element of the array. Here is an all Perl example of using it.
will print
1: green
- This is Class Mine version 1.0
+ This is Class Mine version 1.0
Calling a Perl method from C is fairly straightforward. The following
things are required
char * method
int index
CODE:
- PUSHMARK(sp);
+ PUSHMARK(SP);
XPUSHs(ref);
XPUSHs(sv_2mortal(newSViv(index))) ;
PUTBACK;
char * class
char * method
CODE:
- PUSHMARK(sp);
+ PUSHMARK(SP);
XPUSHs(sv_2mortal(newSVpv(class, 0))) ;
PUTBACK;
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
+the method name is not passed via the stack--it is used as the first
parameter to I<perl_call_method>.
-=head2 Using GIMME
+=head2 Using GIMME_V
-Here is a trivial XSUB which prints the context in which it is
+Here is a trivial XSUB which prints the context in which it is
currently executing.
void
PrintContext()
CODE:
- if (GIMME == G_SCALAR)
+ 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
=head2 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 I<perl_call_*> function or
+(i.e., parameters passed on the stack to the I<perl_call_*> function or
values returned via the stack) have been freed by one of these methods
=over 5
perl --> XSUB --> event handler
...
- event handler --> perl_call --> perl
+ event handler --> perl_call --> perl
|
- event handler <-- perl_call --<--+
+ event handler <-- perl_call <----+
...
- event handler --> perl_call --> perl
+ event handler --> perl_call --> perl
|
- event handler <-- perl_call --<--+
+ event handler <-- perl_call <----+
...
- event handler --> perl_call --> perl
+ event handler --> perl_call --> perl
|
- event handler <-- perl_call --<--+
+ event handler <-- perl_call <----+
In this case the flow of control can consist of only the repeated
sequence
event handler --> perl_call --> perl
-for the practically the complete duration of the program. This means
-that control may I<never> drop back to the surrounding scope in Perl at
-the extreme left.
+for practically the complete duration of the program. This means that
+control may I<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 actually dispose of your temporaries, control must drop back to the
+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!
+eventually consume all the available memory in your system--kapow!
-So here is the bottom line - if you are sure that control will revert
+So here is the bottom line--if you are sure that control will revert
back to the enclosing Perl scope fairly quickly after the end of your
-callback, then it isn't absolutely necessary to explicitly dispose of
+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.
{
dSP ;
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
/* Call the Perl sub to process the callback */
perl_call_sv(callback, G_DISCARD) ;
The mapping between the C callback and the Perl equivalent is stored in
the global variable C<callback>.
-This will be adequate if you ever need to have only 1 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
C<register_fatal> will replace the previously registered callback
the Perl subroutine we want to be called for that file.
Say the i/o library has a function C<asynch_read> which associates a C
-function C<ProcessRead> with a file handle C<fh> - this assumes that it
+function C<ProcessRead> with a file handle C<fh>--this assumes that it
has also provided some routine to open the file and so obtain the file
handle.
int fh ;
char * buffer ;
{
- ...
+ ...
}
To provide a Perl interface to this library we need to be able to map
if (sv == (SV**)NULL)
croak("Internal error...\n") ;
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSViv(fh))) ;
XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
PUTBACK ;
Without the file handle there is no straightforward way to map from the
C callback to the Perl subroutine.
-In this case a possible way around this problem is to pre-define a
+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
{
dSP ;
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
PUTBACK ;
asynch_close(fh) ;
-In this case the functions C<fn1>, C<fn2> and C<fn3> are used to
+In this case the functions C<fn1>, C<fn2>, and C<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 C<Pcb> to
access the C<Map> array and actually call the Perl subroutine.
Secondly, there is a hard-wired limit (in this case 3) to the number of
callbacks that can exist simultaneously. The only way to increase the
limit is by modifying the code to add more functions and then
-re-compiling. None the less, as long as the number of functions is
+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.
Although I have made use of only the C<POP*> macros to access values
returned from Perl subroutines, it is also possible to bypass these
-macros and read the stack using the C<ST> macro (See L<perlapi> for a
+macros and read the stack using the C<ST> macro (See L<perlxs> for a
full description of the C<ST> macro).
Most of the time the C<POP*> macros should be adequate, the main
ENTER ;
SAVETMPS;
- PUSHMARK(sp) ;
+ PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = perl_call_pv("AddSubtract", G_ARRAY);
SPAGAIN ;
- sp -= count ;
- ax = (sp - stack_base) + 1 ;
+ SP -= count ;
+ ax = (SP - PL_stack_base) + 1 ;
if (count != 2)
croak("Big trouble\n") ;
The code
SPAGAIN ;
- sp -= count ;
- ax = (sp - stack_base) + 1 ;
+ SP -= count ;
+ ax = (SP - PL_stack_base) + 1 ;
sets the stack up so that we can use the C<ST> macro.
Unlike the original coding of this example, the returned
values are not accessed in reverse order. So C<ST(0)> refers to the
-first value returned by the Perl subroutine and C<ST(count-1)>
+first value returned by the Perl subroutine and C<ST(count-1)>
refers to the last.
=back
+=head2 Creating and calling an anonymous subroutine in C
+
+As we've already shown, C<perl_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 = perl_eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE);
+
+ ...
+
+ perl_call_sv(cvrv, G_VOID|G_NOARGS);
+
+C<perl_eval_pv> is used to compile the anonymous subroutine, which
+will be the return value as well (read more about C<perl_eval_pv> in
+L<perlguts/perl_eval_pv>). Once this code reference is in hand, it
+can be mixed in with all the previous examples we've shown.
+
=head1 SEE ALSO
-L<perlapi>, L<perlguts>, L<perlembed>
+L<perlxs>, L<perlguts>, L<perlembed>
=head1 AUTHOR
-Paul Marquess <pmarquess@bfsec.bt.co.uk>
+Paul Marquess
Special thanks to the following people who assisted in the creation of
the document.
-Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem and Larry Wall.
+Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy
+and Larry Wall.
=head1 DATE
-Version 1.1, 17th May 1995
+Version 1.3, 14th Apr 1997
projects / p5sagit/p5-mst-13.2.git / blobdiff