would like is to be able to specify a Perl subroutine that will be
called instead.
-=item * An Event Driven Program
+=item * 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
+event driven program, such as for an X11 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.
L<perlembed>.
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<perlxs> and L<perlguts>.
+it would be a good idea to have read the following two documents--L<perlxs>
+and L<perlguts>.
-=head1 THE PERL_CALL FUNCTIONS
+=head1 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 perl_call_sv(SV* sv, I32 flags) ;
- I32 perl_call_pv(char *subname, I32 flags) ;
- I32 perl_call_method(char *methname, I32 flags) ;
- I32 perl_call_argv(char *subname, I32 flags, register char **argv) ;
+ 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, char **argv);
-The key function is I<perl_call_sv>. All the other functions are
+The key function is I<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>
+special cases. At the end of the day they will all call I<call_sv>
to invoke the Perl subroutine.
-All the I<perl_call_*> functions have a C<flags> parameter which is
+All the I<call_*> functions have a C<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 L<FLAG VALUES>.
+bit mask are discussed in L</FLAG VALUES>.
Each of the functions will now be discussed in turn.
=over 5
-=item B<perl_call_sv>
+=item call_sv
-I<perl_call_sv> takes two parameters, the first, C<sv>, is an SV*.
+I<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
C string (which has first been converted to an SV) or a reference to a
-subroutine. The section, I<Using perl_call_sv>, shows how you can make
-use of I<perl_call_sv>.
+subroutine. The section, L</Using call_sv>, shows how you can make
+use of I<call_sv>.
-=item B<perl_call_pv>
+=item call_pv
-The function, I<perl_call_pv>, is similar to I<perl_call_sv> except it
+The function, I<call_pv>, is similar to I<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<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">.
-=item B<perl_call_method>
+=item call_method
-The function I<perl_call_method> is used to call a method from a Perl
+The function I<call_method> is used to call a method from a Perl
class. The parameter C<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 L<perlobj> for more information on
-static and virtual methods and L<Using perl_call_method> for an example
-of using I<perl_call_method>.
+static and virtual methods and L</Using call_method> for an example
+of using I<call_method>.
-=item B<perl_call_argv>
+=item call_argv
-I<perl_call_argv> calls the Perl subroutine specified by the C string
+I<call_argv> calls the Perl subroutine specified by the C string
stored in the C<subname> parameter. It also takes the usual C<flags>
-parameter. The final parameter, C<argv>, consists of a NULL terminated
+parameter. The final parameter, C<argv>, consists of a NULL-terminated
list of C strings to be passed as parameters to the Perl subroutine.
-See I<Using perl_call_argv>.
+See L</Using call_argv>.
=back
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
-The C<flags> parameter in all the I<perl_call_*> functions is a bit mask
-which can consist of any combination of the symbols defined below,
-OR'ed together.
-
+The C<flags> parameter in all the I<call_*> functions is one of G_VOID,
+G_SCALAR, or G_ARRAY, which indicate the call context, OR'ed together
+with a bit mask of any combination of the other G_* symbols defined below.
=head2 G_VOID
=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
+The value returned by the I<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.
+context flag setting for all the I<call_*> functions.
This flag has 2 effects:
=back
-The value returned by the I<perl_call_*> function indicates how many
+The value returned by the I<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 I<Returning a Scalar> shows how
+stored on the Perl stack - the section L</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
-I<perl_call_*> function. The section I<Returning a list in a scalar
-context> shows an example of this behavior.
+I<call_*> function. The section L</Returning a List in Scalar
+Context> shows an example of this behavior.
=head2 G_ARRAY
=item 1.
-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).
-
+It indicates to the subroutine being called that it is executing in a
+list context (if it executes I<wantarray> the result will be true).
=item 2.
It ensures that all items returned from the subroutine will be
-accessible when control returns from the I<perl_call_*> function.
+accessible when control returns from the I<call_*> function.
=back
-The value returned by the I<perl_call_*> function indicates how many
+The value returned by the I<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 I<Returning a list of values> gives an example of using the
+section L</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.
=head2 G_DISCARD
-By default, the I<perl_call_*> functions place the items returned from
+By default, the I<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
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
subroutine and values returned from the subroutine) are disposed of
-yourself. The section I<Returning a Scalar> gives details of how to
-dispose of these temporaries explicitly and the section I<Using Perl to
-dispose of temporaries> discusses the specific circumstances where you
+yourself. The section L</Returning a Scalar> gives details of how to
+dispose of these temporaries explicitly and the section L</Using Perl to
+Dispose of Temporaries> discusses the specific circumstances where you
can ignore the problem and let Perl deal with it for you.
=head2 G_NOARGS
-Whenever a Perl subroutine is called using one of the I<perl_call_*>
+Whenever a Perl subroutine is called using one of the I<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
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
+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 C<@_> array from a previous Perl subroutine. This will
-occur when the code that is executing the I<perl_call_*> function has
+occur when the code that is executing the I<call_*> function has
itself been called from another Perl subroutine. The code below
illustrates this
sub joe
{ &fred }
- &joe(1,2,3) ;
+ &joe(1,2,3);
This will print
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
+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.
-Whenever control returns from the I<perl_call_*> function you need to
+Whenever control returns from the I<call_*> function you need to
check the C<$@> variable as you would in a normal Perl script.
-The value returned from the I<perl_call_*> function is dependent on
+The value returned from the I<call_*> function is dependent on
what other flags have been specified and whether an error has
occurred. Here are all the different cases that can occur:
=item *
-If the I<perl_call_*> function returns normally, then the value
+If the I<call_*> function returns normally, then the value
returned is as specified in the previous sections.
=item *
=back
-See I<Using G_EVAL> for details on using G_EVAL.
+See L</Using G_EVAL> for details on using G_EVAL.
=head2 G_KEEPERR
-You may have noticed that using the G_EVAL flag described above will
-B<always> clear the C<$@> 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<$@> gets executed in the user's script.
-
-This scenario will mostly be applicable to code that is meant to be
-called from within destructors, asynchronous callbacks, signal
-handlers, C<__DIE__> or C<__WARN__> hooks, and C<tie> functions. In
-such situations, you will not want to clear C<$@> at all, but simply to
-append any new errors to any existing value of C<$@>.
+Using the G_EVAL flag described above will always set C<$@>: clearing
+it if there was no error, and setting it to describe the error if there
+was an error in the called code. This is what you want if your intention
+is to handle possible errors, but sometimes you just want to trap errors
+and stop them interfering with the rest of the program.
+
+This scenario will mostly be applicable to code that is meant to be called
+from within destructors, asynchronous callbacks, and signal handlers.
+In such situations, where the code being called has little relation to the
+surrounding dynamic context, the main program needs to be insulated from
+errors in the called code, even if they can't be handled intelligently.
+It may also be useful to do this with code for C<__DIE__> or C<__WARN__>
+hooks, and C<tie> functions.
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.
+I<call_*> functions that are used to implement such code, or with
+C<eval_sv>. This flag has no effect on the C<call_*> functions 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<$@>.
+When G_KEEPERR is used, any error in the called code will terminate the
+call as usual, and the error will not propagate beyond the call (as usual
+for G_EVAL), but it will not go into C<$@>. Instead the error will be
+converted into a warning, prefixed with the string "\t(in cleanup)".
+This can be disabled using C<no warnings 'misc'>. If there is no error,
+C<$@> will not be cleared.
+
+Note that the G_KEEPERR flag does not propagate into inner evals; these
+may still set C<$@>.
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
+See L</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_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
+C<G_ARRAY> if you have been called in a list 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
-
-This section outlines all known problems that exist in the
-I<perl_call_*> functions.
-
-=over 5
-
-=item 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 I<FLAG VALUES>.
-
-Specifically, if the two flags are used when calling a subroutine and
-that subroutine does not call I<die>, the value returned by
-I<perl_call_*> will be wrong.
-
-
-=item 2.
-
-In Perl 5.000 and 5.001 there is a problem with using I<perl_call_*> if
-the Perl sub you are calling attempts to trap a I<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) ;
- perl_call_pv("fred", G_DISCARD|G_NOARGS) ;
- fprintf(stderr, "back in Call_fred\n") ;
-
-When C<Call_fred> is executed it will print
-
- Trapped error: Fatal Error
-
-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
-higher, or use the G_EVAL flag with I<perl_call_*> as shown below
-
- void
- Call_fred()
- CODE:
- PUSHMARK(SP) ;
- perl_call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ;
- fprintf(stderr, "back in Call_fred\n") ;
-
-=back
-
-
+section L</Using GIMME_V>.
=head1 EXAMPLES
-Enough of the definition talk, let's have a few 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 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 I<perl_call_pv> function. This has been done
+have made use of only the I<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 I<perl_call_pv> and
-I<perl_call_sv>, you should always try to use I<perl_call_sv>. See
-I<Using perl_call_sv> for details.
+possible, if the choice is between using I<call_pv> and
+I<call_sv>, you should always try to use I<call_sv>. See
+L</Using call_sv> for details.
-=head2 No Parameters, Nothing returned
+=head2 No Parameters, Nothing Returned
This first trivial example will call a Perl subroutine, I<PrintUID>, to
print out the UID of the process.
sub PrintUID
{
- print "UID is $<\n" ;
+ print "UID is $<\n";
}
and here is a C function to call it
static void
call_PrintUID()
{
- dSP ;
+ dSP;
- PUSHMARK(SP) ;
- perl_call_pv("PrintUID", G_DISCARD|G_NOARGS) ;
+ PUSHMARK(SP);
+ call_pv("PrintUID", G_DISCARD|G_NOARGS);
}
-Simple, eh.
+Simple, eh?
-A few points to note about this example.
+A few points to note about this example:
=over 5
We aren't interested in anything returned from I<PrintUID>, so
G_DISCARD is specified. Even if I<PrintUID> was changed to
return some value(s), having specified G_DISCARD will mean that they
-will be wiped by the time control returns from I<perl_call_pv>.
+will be wiped by the time control returns from I<call_pv>.
=item 4.
-As I<perl_call_pv> is being used, the Perl subroutine is specified as a
+As I<call_pv> is being used, the Perl subroutine is specified as a
C string. In this case the subroutine name has been 'hard-wired' into the
code.
=item 5.
Because we specified G_DISCARD, it is not necessary to check the value
-returned from I<perl_call_pv>. It will always be 0.
+returned from I<call_pv>. It will always be 0.
=back
=head2 Passing Parameters
Now let's make a slightly more complex example. This time we want to
-call a Perl subroutine, C<LeftString>, which will take 2 parameters - a
-string (C<$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 $n characters of the string.
-So the Perl subroutine would look like this
+So the Perl subroutine would look like this:
sub LeftString
{
- my($s, $n) = @_ ;
- print substr($s, 0, $n), "\n" ;
+ my($s, $n) = @_;
+ print substr($s, 0, $n), "\n";
}
-The C function required to call I<LeftString> would look like this.
+The C function required to call I<LeftString> would look like this:
static void
call_LeftString(a, b)
- char * a ;
- int b ;
+ char * a;
+ int b;
{
- dSP ;
+ dSP;
- PUSHMARK(SP) ;
- XPUSHs(sv_2mortal(newSVpv(a, 0)));
- XPUSHs(sv_2mortal(newSViv(b)));
- PUTBACK ;
+ ENTER;
+ SAVETMPS;
- perl_call_pv("LeftString", G_DISCARD);
+ PUSHMARK(SP);
+ EXTEND(SP, 2);
+ PUSHs(sv_2mortal(newSVpv(a, 0)));
+ PUSHs(sv_2mortal(newSViv(b)));
+ PUTBACK;
+
+ 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>. The <dSP> declares a local copy
+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
-use the C<dSP> macro explicitly - it will be declared for you
+use the C<dSP> macro explicitly--it will be declared for you
automatically.
=item 3.
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
+example shown in the section L</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<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
+the same as our local copy. If we didn't do this, I<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, I<not> the global copy.
=item 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.
+Next, we come to EXTEND and PUSHs. This is where the parameters
+actually get pushed onto the stack. In this case we are pushing a
+string and an integer.
+
+Alternatively you can use the XPUSHs() macro, which combines a
+C<EXTEND(SP, 1)> and C<PUSHs()>. This is less efficient if you're
+pushing multiple values.
+
+See L<perlguts/"XSUBs and the Argument Stack"> for details
+on how the PUSH macros work.
=item 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.
+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.
-See L<perlguts/"XSUBs and the Argument Stack"> for details
-on how the XPUSH macros work.
+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 C<{> and C<}> in Perl
+to limit the scope of local variables.
+
+See the section L</Using Perl to Dispose of Temporaries> for details of
+an alternative to using these macros.
=item 6.
-Finally, I<LeftString> can now be called via the I<perl_call_pv>
-function.
+Finally, I<LeftString> can now be called via the I<call_pv> function.
+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.
=back
sub Adder
{
- my($a, $b) = @_ ;
- $a + $b ;
+ my($a, $b) = @_;
+ $a + $b;
}
Because we are now concerned with the return value from I<Adder>, the C
static void
call_Adder(a, b)
- int a ;
- int b ;
+ int a;
+ int b;
{
- dSP ;
- int count ;
+ dSP;
+ int count;
- ENTER ;
+ ENTER;
SAVETMPS;
- PUSHMARK(SP) ;
- XPUSHs(sv_2mortal(newSViv(a)));
- XPUSHs(sv_2mortal(newSViv(b)));
- PUTBACK ;
+ PUSHMARK(SP);
+ EXTEND(SP, 2);
+ PUSHs(sv_2mortal(newSViv(a)));
+ PUSHs(sv_2mortal(newSViv(b)));
+ PUTBACK;
- count = perl_call_pv("Adder", G_SCALAR);
+ count = call_pv("Adder", G_SCALAR);
- SPAGAIN ;
+ SPAGAIN;
if (count != 1)
- croak("Big trouble\n") ;
+ croak("Big trouble\n");
- printf ("The sum of %d and %d is %d\n", a, b, POPi) ;
+ printf ("The sum of %d and %d is %d\n", a, b, POPi);
- PUTBACK ;
- FREETMPS ;
- LEAVE ;
+ PUTBACK;
+ FREETMPS;
+ LEAVE;
}
Points to note this time are
=item 1.
-The only flag specified this time was G_SCALAR. That means the C<@_>
+The only flag specified this time was G_SCALAR. That means that 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>.
-
-
+still exist after the call to I<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 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 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 reallocated whilst in the
-I<perl_call_pv> call.
+allocated to the Perl stack has been reallocated during the
+I<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 I<perl_call_*> functions or any other Perl internal function.
+of the I<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>
+it is still good practice to check the return code from I<call_pv>
anyway.
Expecting a single value is not quite the same as knowing that there
stack would end up in an inconsistent state. That is something you
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.
they return.
POPs SV
- POPp pointer
- POPn double
- POPi integer
+ POPp pointer (PV)
+ POPpbytex pointer to bytes (PV)
+ POPn double (NV)
+ POPi integer (IV)
+ POPu unsigned integer (UV)
POPl long
+ POPul unsigned long
-=item 6.
+Since these macros have side-effects don't use them as arguments to
+macros that may evaluate their argument several times, for example:
+
+ /* Bad idea, don't do this */
+ STRLEN len;
+ const char *s = SvPV(POPs, len);
+
+Instead, use a temporary:
+
+ STRLEN len;
+ SV *sv = POPs;
+ const char *s = SvPV(sv, len);
+
+or a macro that guarantees it will evaluate its arguments only once:
+
+ STRLEN len;
+ const char *s = SvPVx(POPs, len);
+
+=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
=back
-=head2 Returning a list of values
+=head2 Returning a List of Values
Now, let's extend the previous example to return both the sum of the
parameters and the difference.
sub AddSubtract
{
- my($a, $b) = @_ ;
- ($a+$b, $a-$b) ;
+ my($a, $b) = @_;
+ ($a+$b, $a-$b);
}
and this is the C function
static void
call_AddSubtract(a, b)
- int a ;
- int b ;
+ int a;
+ int b;
{
- dSP ;
- int count ;
+ dSP;
+ int count;
- ENTER ;
+ ENTER;
SAVETMPS;
- PUSHMARK(SP) ;
- XPUSHs(sv_2mortal(newSViv(a)));
- XPUSHs(sv_2mortal(newSViv(b)));
- PUTBACK ;
+ PUSHMARK(SP);
+ EXTEND(SP, 2);
+ PUSHs(sv_2mortal(newSViv(a)));
+ PUSHs(sv_2mortal(newSViv(b)));
+ PUTBACK;
- count = perl_call_pv("AddSubtract", G_ARRAY);
+ count = call_pv("AddSubtract", G_ARRAY);
- SPAGAIN ;
+ SPAGAIN;
if (count != 2)
- croak("Big trouble\n") ;
+ croak("Big trouble\n");
- printf ("%d - %d = %d\n", a, b, POPi) ;
- printf ("%d + %d = %d\n", a, b, POPi) ;
+ printf ("%d - %d = %d\n", a, b, POPi);
+ printf ("%d + %d = %d\n", a, b, POPi);
- PUTBACK ;
- FREETMPS ;
- LEAVE ;
+ PUTBACK;
+ FREETMPS;
+ LEAVE;
}
If I<call_AddSubtract> is called like this
- call_AddSubtract(7, 4) ;
+ call_AddSubtract(7, 4);
then here is the output
=item 1.
-We wanted array context, so G_ARRAY was used.
+We wanted list context, so G_ARRAY was used.
=item 2.
=back
-=head2 Returning a list in a scalar context
+=head2 Returning a List in 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 ;
+ int a;
+ int b;
{
- dSP ;
- int count ;
- int i ;
+ dSP;
+ int count;
+ int i;
- ENTER ;
+ ENTER;
SAVETMPS;
- PUSHMARK(SP) ;
- XPUSHs(sv_2mortal(newSViv(a)));
- XPUSHs(sv_2mortal(newSViv(b)));
- PUTBACK ;
+ PUSHMARK(SP);
+ EXTEND(SP, 2);
+ PUSHs(sv_2mortal(newSViv(a)));
+ PUSHs(sv_2mortal(newSViv(b)));
+ PUTBACK;
- count = perl_call_pv("AddSubtract", G_SCALAR);
+ count = call_pv("AddSubtract", G_SCALAR);
- SPAGAIN ;
+ SPAGAIN;
- printf ("Items Returned = %d\n", count) ;
+ printf ("Items Returned = %d\n", count);
- for (i = 1 ; i <= count ; ++i)
- printf ("Value %d = %d\n", i, POPi) ;
+ for (i = 1; i <= count; ++i)
+ printf ("Value %d = %d\n", i, POPi);
- PUTBACK ;
- FREETMPS ;
- LEAVE ;
+ PUTBACK;
+ FREETMPS;
+ LEAVE;
}
The other modification made is that I<call_AddSubScalar> will print the
simplicity it assumes that they are integer). So if
I<call_AddSubScalar> is called
- call_AddSubScalar(7, 4) ;
+ call_AddSubScalar(7, 4);
then the output will be
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
, I<AddSubtract> actually made it back to
+list is returned from the subroutine
. I<AddSubtract> actually made it back to
I<call_AddSubScalar>.
-=head2 Returning Data from Perl via the parameter list
+=head2 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.
+It is also possible to return values directly via the parameter
+list--whether it is actually desirable to do it is another matter entirely.
The Perl subroutine, I<Inc>, below takes 2 parameters and increments
each directly.
sub Inc
{
- ++ $_[0] ;
- ++ $_[1] ;
+ ++ $_[0];
+ ++ $_[1];
}
and here is a C function to call it.
static void
call_Inc(a, b)
- int a ;
- int b ;
+ int a;
+ int b;
{
- dSP ;
- int count ;
- SV * sva ;
- SV * svb ;
+ dSP;
+ int count;
+ SV * sva;
+ SV * svb;
- ENTER ;
+ ENTER;
SAVETMPS;
- sva = sv_2mortal(newSViv(a)) ;
- svb = sv_2mortal(newSViv(b)) ;
+ sva = sv_2mortal(newSViv(a));
+ svb = sv_2mortal(newSViv(b));
- PUSHMARK(SP) ;
- XPUSHs(sva);
- XPUSHs(svb);
- PUTBACK ;
+ PUSHMARK(SP);
+ EXTEND(SP, 2);
+ PUSHs(sva);
+ PUSHs(svb);
+ PUTBACK;
- count = perl_call_pv("Inc", G_DISCARD);
+ count = call_pv("Inc", G_DISCARD);
if (count != 0)
croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
- count) ;
+ count);
- printf ("%d + 1 = %d\n", a, SvIV(sva)) ;
- printf ("%d + 1 = %d\n", b, SvIV(svb)) ;
+ printf ("%d + 1 = %d\n", a, SvIV(sva));
+ printf ("%d + 1 = %d\n", b, SvIV(svb));
- FREETMPS ;
- LEAVE ;
+ FREETMPS;
+ LEAVE;
}
To be able to access the two parameters that were pushed onto the stack
-after they return from I<perl_call_pv> it is necessary to make a note
-of their addresses - thus the two variables C<sva> and C<svb>.
+after they return from I<call_pv> it is necessary to make a note
+of their addresses--thus the two variables C<sva> and C<svb>.
The reason this is necessary is that the area of the Perl stack which
held them will very likely have been overwritten by something else by
-the time control returns from I<perl_call_pv>.
+the time control returns from I<call_pv>.
sub Subtract
{
- my ($a, $b) = @_ ;
+ my ($a, $b) = @_;
- die "death can be fatal\n" if $a < $b ;
+ die "death can be fatal\n" if $a < $b;
- $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 = perl_call_pv("Subtract", G_EVAL|G_SCALAR);
-
- SPAGAIN ;
-
- /* Check the eval first */
- if (SvTRUE(GvSV(errgv)))
- {
- printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ;
- 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 ;
- }
+ static void
+ call_Subtract(a, b)
+ int a;
+ int b;
+ {
+ dSP;
+ int count;
+ SV *err_tmp;
+
+ ENTER;
+ SAVETMPS;
+
+ PUSHMARK(SP);
+ EXTEND(SP, 2);
+ PUSHs(sv_2mortal(newSViv(a)));
+ PUSHs(sv_2mortal(newSViv(b)));
+ PUTBACK;
+
+ count = call_pv("Subtract", G_EVAL|G_SCALAR);
+
+ SPAGAIN;
+
+ /* Check the eval first */
+ err_tmp = ERRSV;
+ if (SvTRUE(err_tmp))
+ {
+ printf ("Uh oh - %s\n", SvPV_nolen(err_tmp));
+ 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 I<call_Subtract> is called thus
The code
- if (SvTRUE(GvSV(errgv)))
+ err_tmp = ERRSV;
+ if (SvTRUE(err_tmp))
{
- printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ;
- POPs ;
+ printf ("Uh oh - %s\n", SvPV_nolen(err_tmp));
+ POPs;
}
is the direct equivalent of this bit of Perl
- print "Uh oh - $@\n" if $@ ;
+ print "Uh oh - $@\n" if $@;
-C<errgv> is a perl global of type C<GV *> that points to the
-symbol table entry containing the error. C<GvSV(errgv)> therefore
-refers to the C equivalent of C<$@>.
+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<$@>. We use a local temporary, C<err_tmp>, since
+C<ERRSV> is a macro that calls a function, and C<SvTRUE(ERRSV)> would
+end up calling that function multiple times.
=item 3.
Note that the stack is popped using C<POPs> in the block where
-C<SvTRUE(GvSV(errgv))> is true. This is necessary because whenever a
-I<perl_call_*> function invoked with G_EVAL|G_SCALAR returns an error,
+C<SvTRUE(err_tmp)> is true. This is necessary because whenever a
+I<call_*> function invoked with G_EVAL|G_SCALAR returns an error,
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>.
+the stack be tidied up by removing the I<undef>.
=back
sub new { bless {}, $_[0] }
sub Subtract {
my($a,$b) = @_;
- die "death can be fatal" if $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 };
+ {
+ my $foo = Foo->new;
+ eval { $foo->foo };
+ }
print "Saw: $@" if $@; # should be, but isn't
This example will fail to recognize that an error occurred inside the
C<eval {}>. Here's why: the call_Subtract code got executed while perl
-was cleaning up temporaries when exiting the eval block, and because
-call_Subtract is implemented with I<perl_call_pv> using the G_EVAL
+was cleaning up temporaries when exiting the outer braced block, and because
+call_Subtract is implemented with I<call_pv> using the G_EVAL
flag, it promptly reset C<$@>. 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
+Appending the G_KEEPERR flag, so that the I<call_pv> call in
call_Subtract reads:
- count = perl_call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);
+ count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);
will preserve the error and restore reliable error handling.
-=head2 Using perl_call_sv
+=head2 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.
+within the Perl script, and you'll want to use
+L<call_sv|perlapi/call_sv>.
Consider the Perl code below
sub fred
{
- print "Hello there\n" ;
+ print "Hello there\n";
}
- CallSubPV("fred") ;
+ CallSubPV("fred");
Here is a snippet of XSUB which defines I<CallSubPV>.
CallSubPV(name)
char * name
CODE:
- PUSHMARK(SP) ;
- perl_call_pv(name, G_DISCARD|G_NOARGS) ;
+ 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 I<perl_call_sv> is useful.
+can be specified as only a string, however, Perl allows references
+to subroutines and anonymous subroutines.
+This is where I<call_sv> is useful.
The code below for I<CallSubSV> is identical to I<CallSubPV> except
that the C<name> parameter is now defined as an SV* and we use
-I<perl_call_sv> instead of I<perl_call_pv>.
+I<call_sv> instead of I<call_pv>.
void
CallSubSV(name)
SV * name
CODE:
- PUSHMARK(SP) ;
- perl_call_sv(name, G_DISCARD|G_NOARGS) ;
+ PUSHMARK(SP);
+ call_sv(name, G_DISCARD|G_NOARGS);
-Because 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) ;
- $ref = \&fred ;
- CallSubSV($ref) ;
- CallSubSV( sub { print "Hello there\n" } ) ;
+ CallSubSV("fred");
+ CallSubSV(\&fred);
+ $ref = \&fred;
+ CallSubSV($ref);
+ CallSubSV( sub { print "Hello there\n" } );
-As you can see, I<perl_call_sv> gives you much greater flexibility in
+As you can see, I<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 (C<name> in the
+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 just to store a copy of the
-pointer to the SV. Say the code above had been like this
+pointer to the SV. Say the code above had been like this:
- static SV * rememberSub ;
+ static SV * rememberSub;
void
SaveSub1(name)
SV * name
CODE:
- rememberSub = name ;
+ rememberSub = name;
void
CallSavedSub1()
CODE:
- PUSHMARK(SP) ;
- perl_call_sv(rememberSub, G_DISCARD|G_NOARGS) ;
+ PUSHMARK(SP);
+ call_sv(rememberSub, G_DISCARD|G_NOARGS);
-The reason this is wrong is that by the time you come to use the
+The reason this is wrong is that, by the time you come to use the
pointer C<rememberSub> in C<CallSavedSub1>, it may or may not still refer
to the Perl subroutine that was recorded in C<SaveSub1>. This is
-particularly true for these cases
+particularly true for these cases:
- SaveSub1(\&fred) ;
- CallSavedSub1() ;
+ SaveSub1(\&fred);
+ CallSavedSub1();
- SaveSub1( sub { print "Hello there\n" } ) ;
- CallSavedSub1() ;
+ SaveSub1( sub { print "Hello there\n" } );
+ CallSavedSub1();
-By the time each of the C<SaveSub1> statements above have been executed,
+By the time each of the C<SaveSub1> statements above has been executed,
the SV*s which corresponded to the parameters will no longer exist.
Expect an error message from Perl of the form
Similarly, with this code
- $ref = \&fred ;
- SaveSub1($ref) ;
- $ref = 47 ;
- CallSavedSub1() ;
+ $ref = \&fred;
+ SaveSub1($ref);
+ $ref = 47;
+ CallSavedSub1();
you can expect one of these messages (which you actually get is dependent on
the version of Perl you are using)
Not a CODE reference at ...
Undefined subroutine &main::47 called ...
-The variable C<$ref> may have referred to the subroutine C<fred>
+The variable $ref may have referred to the subroutine C<fred>
whenever the call to C<SaveSub1> was made but by the time
C<CallSavedSub1> gets called it now holds the number C<47>. Because we
saved only a pointer to the original SV in C<SaveSub1>, any changes to
-C<$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
loudly.
-A similar but more subtle problem is illustrated with this code
+A similar but more subtle problem is illustrated with this code:
- $ref = \&fred ;
- SaveSub1($ref) ;
- $ref = \&joe ;
- CallSavedSub1() ;
+ $ref = \&fred;
+ SaveSub1($ref);
+ $ref = \&joe;
+ CallSavedSub1();
-This time whenever C<CallSavedSub1> get called it will execute the Perl
+This time whenever C<CallSavedSub1> gets called it will execute the Perl
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
-SV. The code below shows C<SaveSub2> modified to do that
+SV. The code below shows C<SaveSub2> modified to do that.
- static SV * keepSub = (SV*)NULL ;
+ /* this isn't thread-safe */
+ static SV * keepSub = (SV*)NULL;
void
SaveSub2(name)
/* Take a copy of the callback */
if (keepSub == (SV*)NULL)
/* First time, so create a new SV */
- keepSub = newSVsv(name) ;
+ keepSub = newSVsv(name);
else
/* Been here before, so overwrite */
- SvSetSV(keepSub, name) ;
+ SvSetSV(keepSub, name);
void
CallSavedSub2()
CODE:
- PUSHMARK(SP) ;
- perl_call_sv(keepSub, G_DISCARD|G_NOARGS) ;
+ PUSHMARK(SP);
+ call_sv(keepSub, G_DISCARD|G_NOARGS);
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
-operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called
+subroutine C<name> is copied to the variable C<keepSub> in one
+operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called,
the existing SV, C<keepSub>, is overwritten with the new value using
C<SvSetSV>.
-=head2 Using perl_call_argv
+Note: using a static or global variable to store the SV isn't
+thread-safe. You can either use the C<MY_CXT> mechanism documented in
+L<perlxs/Safely Storing Static Data in XS> which is fast, or store the
+values in perl global variables, using get_sv(), which is much slower.
+
+=head2 Using call_argv
Here is a Perl subroutine which prints whatever parameters are passed
to it.
sub PrintList
{
- my(@list) = @_ ;
+ my(@list) = @_;
foreach (@list) { print "$_\n" }
}
-and here is an example of I<perl_call_argv> which will call
+And here is an example of I<call_argv> which will call
I<PrintList>.
- static char * words[] = {"alpha", "beta", "gamma", "delta", NULL} ;
+ static char * words[] = {"alpha", "beta", "gamma", "delta", NULL};
static void
call_PrintList()
{
- dSP ;
-
- perl_call_argv("PrintList", G_DISCARD, words) ;
+ call_argv("PrintList", G_DISCARD, words);
}
Note that it is not necessary to call C<PUSHMARK> in this instance.
-This is because I<perl_call_argv> will do it for you.
+This is because I<call_argv> will do it for you.
-=head2 Using perl_call_method
+=head2 Using call_method
-Consider the following Perl code
+Consider the following Perl code:
{
- package Mine ;
+ package Mine;
sub new
{
- my($type) = shift ;
+ my($type) = shift;
bless [@_]
}
sub Display
{
- my ($self, $index) = @_ ;
- print "$index: $$self[$index]\n" ;
+ my ($self, $index) = @_;
+ print "$index: $$self[$index]\n";
}
sub PrintID
{
- my($class) = @_ ;
- print "This is Class $class version 1.0\n" ;
+ my($class) = @_;
+ print "This is Class $class version 1.0\n";
}
}
the constructor, C<new>, it declares methods, one static and one
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.
+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;
+ $a = Mine->new('red', 'green', 'blue');
+ $a->Display(1);
+ Mine->PrintID;
will print
This is Class Mine version 1.0
Calling a Perl method from C is fairly straightforward. The following
-things are required
+things are required:
=over 5
=item *
-a reference to the object for a virtual method or the name of the class
-for a static method.
+A reference to the object for a virtual method or the name of the class
+for a static method
=item *
-the name of the method.
+The name of the method
=item *
-any other parameters specific to the method.
+Any other parameters specific to the method
=back
int index
CODE:
PUSHMARK(SP);
- XPUSHs(ref);
- XPUSHs(sv_2mortal(newSViv(index))) ;
+ EXTEND(SP, 2);
+ PUSHs(ref);
+ PUSHs(sv_2mortal(newSViv(index)));
PUTBACK;
- perl_call_method(method, G_DISCARD) ;
+ call_method(method, G_DISCARD);
void
call_PrintID(class, method)
char * method
CODE:
PUSHMARK(SP);
- XPUSHs(sv_2mortal(newSVpv(class, 0))) ;
+ XPUSHs(sv_2mortal(newSVpv(class, 0)));
PUTBACK;
- perl_call_method(method, G_DISCARD) ;
+ call_method(method, G_DISCARD);
-So the methods C<PrintID> and C<Display> can be invoked like this
+So the methods C<PrintID> and C<Display> can be invoked like this:
- $a = new Mine ('red', 'green', 'blue') ;
- call_Method($a, 'Display', 1) ;
- call_PrintID('Mine', 'PrintID') ;
+ $a = Mine->new('red', 'green', 'blue');
+ call_Method($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 I<perl_call_method>.
+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 I<call_method>.
=head2 Using GIMME_V
void
PrintContext()
CODE:
- I32 gimme = GIMME_V;
+ U8 gimme = GIMME_V;
if (gimme == G_VOID)
- printf ("Context is Void\n") ;
+ printf ("Context is Void\n");
else if (gimme == G_SCALAR)
- printf ("Context is Scalar\n") ;
+ printf ("Context is Scalar\n");
else
- printf ("Context is Array\n") ;
+ printf ("Context is Array\n");
-and here is some Perl to test it
+And here is some Perl to test it.
- PrintContext ;
- $a = PrintContext ;
- @a = PrintContext ;
+ PrintContext;
+ $a = PrintContext;
+ @a = PrintContext;
The output from that will be
Context is Scalar
Context is Array
-=head2 Using Perl to dispose of temporaries
+=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
-values returned via the stack) have been freed by one of these methods
+(i.e., parameters passed on the stack to the I<call_*> function or
+values returned via the stack) have been freed by one of these methods:
=over 5
=item *
-specifying the G_DISCARD flag with I<perl_call_*>.
+Specifying the G_DISCARD flag with I<call_*>
=item *
-explicitly disposed of using the C<ENTER>/C<SAVETMPS> -
-C<FREETMPS>/C<LEAVE> pairing.
+Explicitly using the C<ENTER>/C<SAVETMPS>--C<FREETMPS>/C<LEAVE> pairing
=back
for you automatically whenever it regains control after the callback
has terminated. This is done by simply not using the
- ENTER ;
- SAVETMPS ;
+ ENTER;
+ SAVETMPS;
...
- FREETMPS ;
- LEAVE ;
+ FREETMPS;
+ LEAVE;
sequence in the callback (and not, of course, specifying the G_DISCARD
flag).
...
error occurs
...
- external library --> perl_call --> perl
+ external library --> call_* --> perl
|
- perl <-- XSUB <-- external library <-- perl_call <----+
+ perl <-- XSUB <-- external library <-- call_* <----+
-After processing of the error using I<perl_call_*> is completed,
+After processing of the error using I<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
perl --> XSUB --> event handler
...
- event handler --> perl_call --> perl
+ event handler --> call_* --> perl
|
- event handler <-- perl_call <----+
+ event handler <-- call_* <----+
...
- event handler --> perl_call --> perl
+ event handler --> call_* --> perl
|
- event handler <-- perl_call <----+
+ event handler <-- call_* <----+
...
- event handler --> perl_call --> perl
+ event handler --> call_* --> perl
|
- event handler <-- perl_call <----+
+ event handler <-- call_* <----+
In this case the flow of control can consist of only the repeated
sequence
- event handler --> perl_call --> perl
+ event handler --> call_* --> perl
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
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
+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 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.
-=head2 Strategies for storing Callback Context Information
+=head2 Strategies for Storing Callback Context Information
Potentially one of the trickiest problems to overcome when designing a
hypothetical function C<register_fatal> which registers the C function
to get called when a fatal error occurs.
- register_fatal(cb1) ;
+ register_fatal(cb1);
The single parameter C<cb1> is a pointer to a function, so you must
have defined C<cb1> in your code, say something like this
static void
cb1()
{
- printf ("Fatal Error\n") ;
- exit(1) ;
+ printf ("Fatal Error\n");
+ exit(1);
}
Now change that to call a Perl subroutine instead
static void
cb1()
{
- dSP ;
+ dSP;
- PUSHMARK(SP) ;
+ PUSHMARK(SP);
/* Call the Perl sub to process the callback */
- perl_call_sv(callback, G_DISCARD) ;
+ call_sv(callback, G_DISCARD);
}
CODE:
/* Remember the Perl sub */
if (callback == (SV*)NULL)
- callback = newSVsv(fn) ;
+ callback = newSVsv(fn);
else
- SvSetSV(callback, fn) ;
+ SvSetSV(callback, fn);
/* register the callback with the external library */
- register_fatal(cb1) ;
+ register_fatal(cb1);
where the Perl equivalent of C<register_fatal> and the callback it
registers, C<pcb1>, might look like this
# Register the sub pcb1
- register_fatal(\&pcb1) ;
+ register_fatal(\&pcb1);
sub pcb1
{
- die "I'm dying...\n" ;
+ die "I'm dying...\n";
}
The mapping between the C callback and the Perl equivalent is stored in
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.
void
ProcessRead(fh, buffer)
- int fh ;
- char * buffer ;
+ int fh;
+ char * buffer;
{
...
}
hash is a convenient mechanism for storing this mapping. The code
below shows a possible implementation
- static HV * Mapping = (HV*)NULL ;
+ static HV * Mapping = (HV*)NULL;
void
asynch_read(fh, callback)
CODE:
/* If the hash doesn't already exist, create it */
if (Mapping == (HV*)NULL)
- Mapping = newHV() ;
+ Mapping = newHV();
/* Save the fh -> callback mapping */
- hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0) ;
+ hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0);
/* Register with the C Library */
- asynch_read(fh, asynch_read_if) ;
+ asynch_read(fh, asynch_read_if);
and C<asynch_read_if> could look like this
static void
asynch_read_if(fh, buffer)
- int fh ;
- char * buffer ;
+ int fh;
+ char * buffer;
{
- dSP ;
- SV ** sv ;
+ dSP;
+ SV ** sv;
/* Get the callback associated with fh */
- sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE) ;
+ sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE);
if (sv == (SV**)NULL)
- croak("Internal error...\n") ;
+ croak("Internal error...\n");
- PUSHMARK(SP) ;
- XPUSHs(sv_2mortal(newSViv(fh))) ;
- XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
- PUTBACK ;
+ PUSHMARK(SP);
+ EXTEND(SP, 2);
+ PUSHs(sv_2mortal(newSViv(fh)));
+ PUSHs(sv_2mortal(newSVpv(buffer, 0)));
+ PUTBACK;
/* Call the Perl sub */
- perl_call_sv(*sv, G_DISCARD) ;
+ call_sv(*sv, G_DISCARD);
}
For completeness, here is C<asynch_close>. This shows how to remove
int fh
CODE:
/* Remove the entry from the hash */
- (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD) ;
+ (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD);
/* Now call the real asynch_close */
- asynch_close(fh) ;
+ asynch_close(fh);
So the Perl interface would look like this
sub callback1
{
- my($handle, $buffer) = @_ ;
+ my($handle, $buffer) = @_;
}
# Register the Perl callback
- asynch_read($fh, \&callback1) ;
+ asynch_read($fh, \&callback1);
- asynch_close($fh) ;
+ asynch_close($fh);
The mapping between the C callback and Perl is stored in the global
hash C<Mapping> this time. Using a hash has the distinct advantage that
void
ProcessRead(buffer)
- char * buffer ;
+ char * buffer;
{
...
}
#define MAX_CB 3
#define NULL_HANDLE -1
- typedef void (*FnMap)() ;
+ typedef void (*FnMap)();
struct MapStruct {
- FnMap Function ;
- SV * PerlSub ;
- int Handle ;
- } ;
+ FnMap Function;
+ SV * PerlSub;
+ int Handle;
+ };
- static void fn1() ;
- static void fn2() ;
- static void fn3() ;
+ 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 ;
+ int index;
+ char * buffer;
{
- dSP ;
+ dSP;
- PUSHMARK(SP) ;
- XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
- PUTBACK ;
+ PUSHMARK(SP);
+ XPUSHs(sv_2mortal(newSVpv(buffer, 0)));
+ PUTBACK;
/* Call the Perl sub */
- perl_call_sv(Map[index].PerlSub, G_DISCARD) ;
+ call_sv(Map[index].PerlSub, G_DISCARD);
}
static void
fn1(buffer)
- char * buffer ;
+ char * buffer;
{
- Pcb(0, buffer) ;
+ Pcb(0, buffer);
}
static void
fn2(buffer)
- char * buffer ;
+ char * buffer;
{
- Pcb(1, buffer) ;
+ Pcb(1, buffer);
}
static void
fn3(buffer)
- char * buffer ;
+ char * buffer;
{
- Pcb(2, buffer) ;
+ Pcb(2, buffer);
}
void
int fh
SV * callback
CODE:
- int index ;
- int null_index = MAX_CB ;
+ int index;
+ int null_index = MAX_CB;
/* Find the same handle or an empty entry */
- for (index = 0 ; index < MAX_CB ; ++index)
+ for (index = 0; index < MAX_CB; ++index)
{
if (Map[index].Handle == fh)
- break ;
+ break;
if (Map[index].Handle == NULL_HANDLE)
- null_index = index ;
+ null_index = index;
}
if (index == MAX_CB && null_index == MAX_CB)
- croak ("Too many callback functions registered\n") ;
+ croak ("Too many callback functions registered\n");
if (index == MAX_CB)
- index = null_index ;
+ index = null_index;
/* Save the file handle */
- Map[index].Handle = fh ;
+ Map[index].Handle = fh;
/* Remember the Perl sub */
if (Map[index].PerlSub == (SV*)NULL)
- Map[index].PerlSub = newSVsv(callback) ;
+ Map[index].PerlSub = newSVsv(callback);
else
- SvSetSV(Map[index].PerlSub, callback) ;
+ SvSetSV(Map[index].PerlSub, callback);
- asynch_read(fh, Map[index].Function) ;
+ asynch_read(fh, Map[index].Function);
void
array_asynch_close(fh)
int fh
CODE:
- int index ;
+ int index;
/* Find the file handle */
- for (index = 0; index < MAX_CB ; ++ index)
+ for (index = 0; index < MAX_CB; ++ index)
if (Map[index].Handle == fh)
- break ;
+ break;
if (index == MAX_CB)
- croak ("could not close fh %d\n", fh) ;
+ croak ("could not close fh %d\n", fh);
- Map[index].Handle = NULL_HANDLE ;
- SvREFCNT_dec(Map[index].PerlSub) ;
- Map[index].PerlSub = (SV*)NULL ;
+ Map[index].Handle = NULL_HANDLE;
+ SvREFCNT_dec(Map[index].PerlSub);
+ Map[index].PerlSub = (SV*)NULL;
- asynch_close(fh) ;
+ asynch_close(fh);
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
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
+Most of the time the C<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 C<ST> macro as used when coding an XSUB is ideal
for this purpose.
-The code below is the example given in the section I<Returning a list
-of
values> recoded to use C<ST> instead of C<POP*>.
+The code below is the example given in the section L</Returning a List
+of
Values> recoded to use C<ST> instead of C<POP*>.
static void
call_AddSubtract2(a, b)
- int a ;
- int b ;
+ int a;
+ int b;
{
- dSP ;
- I32 ax ;
- int count ;
+ dSP;
+ I32 ax;
+ int count;
- ENTER ;
+ ENTER;
SAVETMPS;
- PUSHMARK(SP) ;
- XPUSHs(sv_2mortal(newSViv(a)));
- XPUSHs(sv_2mortal(newSViv(b)));
- PUTBACK ;
+ PUSHMARK(SP);
+ EXTEND(SP, 2);
+ PUSHs(sv_2mortal(newSViv(a)));
+ PUSHs(sv_2mortal(newSViv(b)));
+ PUTBACK;
- count = perl_call_pv("AddSubtract", G_ARRAY);
+ count = call_pv("AddSubtract", G_ARRAY);
- SPAGAIN ;
- SP -= count ;
- ax = (SP - stack_base) + 1 ;
+ SPAGAIN;
+ SP -= count;
+ ax = (SP - PL_stack_base) + 1;
if (count != 2)
- croak("Big trouble\n") ;
+ croak("Big trouble\n");
- printf ("%d + %d = %d\n", a, b, SvIV(ST(0))) ;
- printf ("%d - %d = %d\n", a, b, SvIV(ST(1))) ;
+ printf ("%d + %d = %d\n", a, b, SvIV(ST(0)));
+ printf ("%d - %d = %d\n", a, b, SvIV(ST(1)));
- PUTBACK ;
- FREETMPS ;
- LEAVE ;
+ PUTBACK;
+ FREETMPS;
+ LEAVE;
}
Notes
Notice that it was necessary to define the variable C<ax>. This is
because the C<ST> macro expects it to exist. If we were in an XSUB it
would not be necessary to define C<ax> as it is already defined for
-you.
+us.
=item 2.
The code
- SPAGAIN ;
- SP -= count ;
- ax = (SP - stack_base) + 1 ;
+ SPAGAIN;
+ SP -= count;
+ ax = (SP - PL_stack_base) + 1;
sets the stack up so that we can use the C<ST> macro.
=back
-=head2 Creating and calling an anonymous subroutine in C
+=head2 Creating and Calling an Anonymous Subroutine in C
-As we've already shown, L<perl_call_sv> can be used to invoke an
-anonymous subroutine. However, our example showed how Perl script
-invoking an XSUB to preform this operation. Let's see how it can be
+As we've already shown, C<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);
+ SV *cvrv
+ = eval_pv("sub {
+ print 'You will not find me cluttering any namespace!'
+ }", TRUE);
...
- perl_call_sv(cvrv, G_VOID|G_NOARGS);
+ call_sv(cvrv, G_VOID|G_NOARGS);
-L<perlguts/perl_eval_pv> is used to compile the anonymous subroutine, which
-will be the return value as well. Once this code reference is in hand, it
+C<eval_pv> is used to compile the anonymous subroutine, which
+will be the return value as well (read more about C<eval_pv> in
+L<perlapi/eval_pv>). Once this code reference is in hand, it
can be mixed in with all the previous examples we've shown.
+=head1 LIGHTWEIGHT CALLBACKS
+
+Sometimes you need to invoke the same subroutine repeatedly.
+This usually happens with a function that acts on a list of
+values, such as Perl's built-in sort(). You can pass a
+comparison function to sort(), which will then be invoked
+for every pair of values that needs to be compared. The first()
+and reduce() functions from L<List::Util> follow a similar
+pattern.
+
+In this case it is possible to speed up the routine (often
+quite substantially) by using the lightweight callback API.
+The idea is that the calling context only needs to be
+created and destroyed once, and the sub can be called
+arbitrarily many times in between.
+
+It is usual to pass parameters using global variables (typically
+$_ for one parameter, or $a and $b for two parameters) rather
+than via @_. (It is possible to use the @_ mechanism if you know
+what you're doing, though there is as yet no supported API for
+it. It's also inherently slower.)
+
+The pattern of macro calls is like this:
+
+ dMULTICALL; /* Declare local variables */
+ U8 gimme = G_SCALAR; /* context of the call: G_SCALAR,
+ * G_ARRAY, or G_VOID */
+
+ PUSH_MULTICALL(cv); /* Set up the context for calling cv,
+ and set local vars appropriately */
+
+ /* loop */ {
+ /* set the value(s) af your parameter variables */
+ MULTICALL; /* Make the actual call */
+ } /* end of loop */
+
+ POP_MULTICALL; /* Tear down the calling context */
+
+For some concrete examples, see the implementation of the
+first() and reduce() functions of List::Util 1.18. There you
+will also find a header file that emulates the multicall API
+on older versions of perl.
+
=head1 SEE ALSO
L<perlxs>, L<perlguts>, L<perlembed>
=head1 AUTHOR
-Paul Marquess <F<pmarquess@bfsec.bt.co.uk>>
+Paul Marquess
Special thanks to the following people who assisted in the creation of
the document.
=head1 DATE
-Version 1.3, 14th Apr 1997
+Last updated for perl 5.23.1.
https://perl5.git.perl.org / perl5.git / blobdiff