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NAME
perlcall - Perl calling conventions from C
---------------------------------------------------------------------------
DESCRIPTION
The purpose of this document is to show you how to call Perl subroutines
directly from C, i.e. how to write callbacks.
Apart from discussing the C interface provided by Perl for writing
callbacks the document uses a series of examples to show how the interface
actually works in practice. In addition some techniques for coding
callbacks are covered.
Examples where callbacks are necessary include
* An Error Handler
You have created an XSUB interface to an application's C API.
A fairly common feature in applications is to allow you to define a C
function that will be called whenever something nasty occurs. What we
would like is to be able to specify a Perl subroutine that will be
called instead.
* 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
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
menu item is selected.
Although the techniques described here are applicable when embedding.Perl
in a C program, this is not the primary goal of this document. There are
other details that must be considered and are specific to embedding Perl.
For details on embedding Perl in C refer to the perlembed manpage .
Before you launch yourself head first into the rest of this document, it
would be a good idea to have read the following two documents - the perlxs
manpage and the perlguts manpage .
---------------------------------------------------------------------------
THE PERL_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) ;
The key function is 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 perl_call_sv to actually
invoke the Perl subroutine.
All the perl_call_* functions have a flags parameter which is used to pass
a bit mask of options to Perl. This bit mask operates identically for each
of the functions. The settings available in the bit mask are discussed in
FLAG VALUES .
Each of the functions will now be discussed in turn.
perl_call_sv
perl_call_sv takes two parameters, the first, sv, is an SV*. This
allows you to specify the Perl subroutine to be called either as a C
string (which has first been converted to an SV) or a reference to a
subroutine. The section, Using perl_call_sv , shows how you can make
use of perl_call_sv .
perl_call_pv
The function, perl_call_pv , is similar to 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. 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. ``pkg::fred''.
perl_call_method
The function perl_call_method is used to call a method from a Perl
class. The parameter methname corresponds to the name of the method to
be called. Note that the class that the method belongs to is passed on
the Perl stack rather than in the parameter list. This class can be
either the name of the class (for a static method) or a reference to
an object (for a virtual method). See the perlobj manpage for more
information on static and virtual methods and Using perl_call_method
for an example of using perl_call_method .
perl_call_argv
perl_call_argv calls the Perl subroutine specified by the C string
stored in the subname parameter. It also takes the usual flags
parameter. The final parameter, argv, consists of a NULL terminated
list of C strings to be passed as parameters to the Perl subroutine.
See Using perl_call_argv .
All the functions return an integer. This is a count of the number of.items
returned by the Perl subroutine. The actual items returned by the
subroutine are stored on the Perl stack.
As a general rule you should always check the return value from these
functions. Even if you are expecting only a particular number of values to
be returned from the Perl subroutine, there is nothing to stop someone from
doing something unexpected - don't say you haven't been warned.
---------------------------------------------------------------------------
FLAG VALUES
The flags parameter in all the perl_call_* functions is a bit mask which
can consist of any combination of the symbols defined below, OR'ed
together.
---------------------------------------------------------------------------
G_SCALAR
Calls the Perl subroutine in a scalar context. This is the default context
flag setting for all the perl_call_* functions.
This flag has 2 effects
1. it indicates to the subroutine being called that it is executing in a
scalar context (if it executes wantarray the result will be false).
2. it ensures that only a scalar is actually returned from the
subroutine. The subroutine can, of course, ignore the wantarray and
return a list anyway. If so, then only the last element of the list
will be returned.
The value returned by the perl_call_* function indicates how may.items have
been returned by the Perl subroutine - in this case it will be either 0 or
1.
If 0, then you have specified the G_DISCARD flag.
If 1, then the item actually returned by the Perl subroutine will be stored
on the Perl stack - the section Returning a Scalar shows how to access this
value on the stack. Remember that regardless of how many items the Perl
subroutine returns, only the last one will be accessible from the stack -
think of the case where only one value is returned as being a list with
only one element. Any other items that were returned will not exist by the
time control returns from the perl_call_* function. The section I
---------------------------------------------------------------------------
G_ARRAY
Calls the Perl subroutine in a list context.
As with G_SCALAR, this flag has 2 effects
1. it indicates to the subroutine being called that it is executing in an
array context (if it executes wantarray the result will be true).
2. it ensures that all items returned from the subroutine will be
accessible when control returns from the perl_call_* function.
The value returned by the perl_call_* function indicates how may.items have
been returned by the Perl subroutine.
If 0, the you have specified the G_DISCARD flag.
If not 0, then it will be a count of the number of items returned by the
subroutine. These items will be stored on the Perl stack. The section
Returning a list of values gives an example of using the G_ARRAY flag and
the mechanics of accessing the returned items from the Perl stack.
---------------------------------------------------------------------------
G_DISCARD
By default, the perl_call_* functions place the items returned from by the
Perl subroutine on the stack. If you are not interested in these items,
then setting this flag will make Perl get rid of them automatically for
you. Note that it is still possible to indicate a context to the Perl
subroutine by using either G_SCALAR or G_ARRAY.
If you do not set this flag then it is very important that you make sure
that any temporaries (i.e. parameters passed to the Perl subroutine and
values returned from the subroutine) are disposed of yourself. The section
Returning a Scalar gives details of how to explicitly dispose of these
temporaries and the section I
---------------------------------------------------------------------------
G_NOARGS
Whenever a Perl subroutine is called using one of the perl_call_*
functions, it is assumed by default that parameters are to be passed to the
subroutine. If you are not passing any parameters to the Perl subroutine,
you can save a bit of time by setting this flag. It has the effect of not
creating the @_ array for the Perl subroutine.
Although the functionality provided by this flag may seem straightforward,
it should be used only if there is a good reason to do so. The reason for
being cautious is that even if you have specified the G_NOARGS flag, it is
still possible for the Perl subroutine that has been called to think that
you have passed it parameters.
In fact, what can happen is that the Perl subroutine you have called can
access the @_ array from a previous Perl subroutine. This will occur when
the code that is executing the perl_call_* function has itself been called
from another Perl subroutine. The code below illustrates this
sub fred
{ print "@_\n" }
sub joe
{ &fred }
&joe(1,2,3) ;
This will print
1 2 3
What has happened is that fred accesses the @_ array which belongs to joe.
---------------------------------------------------------------------------
G_EVAL
It is possible for the Perl subroutine you are calling to terminate
abnormally, e.g. by calling die explicitly or by not actually existing. By
default, when either of these of events occurs, the process will terminate
immediately. If though, you want to trap this type of event, specify the
G_EVAL flag. It will put an eval { } around the subroutine call.
Whenever control returns from the perl_call_* function you need to check
the $@ variable as you would in a normal Perl script.
The value returned from the 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
* If the perl_call_* function returns normally, then the value returned
is as specified in the previous sections.
* If G_DISCARD is specified, the return value will always be 0.
* If G_ARRAY is specified and an error has occurred, the return value
will always be 0.
* If G_SCALAR is specified and an error has occurred, the return value
will be 1 and the value on the top of the stack will be undef . This
means that if you have already detected the error by checking $@ and
you want the program to continue, you must remember to pop the undef
from the stack.
See Using G_EVAL for details of using G_EVAL..
---------------------------------------------------------------------------
Determining the Context
As mentioned above, you can determine the context of the currently
executing subroutine in Perl with wantarray . The equivalent test can be
made in C by using the GIMME macro. This will return G_SCALAR if you have
been called in a scalar context and G_ARRAY if in an array context. An
example of using the GIMME macro is shown in section Using GIMME .
---------------------------------------------------------------------------
KNOWN PROBLEMS
This section outlines all known problems that exist in the perl_call_*
functions.
1. If you are intending to make use of both the G_EVAL and G_SCALAR flags
in your code, use a version of Perl greater than 5.000. There is a bug
in version 5.000 of Perl which means that the combination of these two
flags will not work as described in the section FLAG VALUES .
Specifically, if the two flags are used when calling a subroutine and
that subroutine does not call die , the value returned by perl_call_*
will be wrong.
2. In Perl 5.000 and 5.001 there is a problem with using perl_call_* if
the Perl sub you are calling attempts to trap a die .
The symptom of this problem is that the called Perl sub will continue
to completion, but whenever it attempts to pass control back to the
XSUB, the program will immediately terminate.
For example, say you want to call this Perl sub
sub fred
{
eval { die "Fatal Error" ; }
print "Trapped error: $@\n"
if $@ ;
}
via this XSUB
void
Call_fred()
CODE:
PUSHMARK(sp) ;
perl_call_pv("fred", G_DISCARD|G_NOARGS) ;
fprintf(stderr, "back in Call_fred\n") ;
When Call_fred is executed it will print
Trapped error: Fatal Error
As control never returns to Call_fred, the ``back in Call_fred''
string will not get printed.
To work around this problem, you can either upgrade to Perl 5.002 (or
later), or use the G_EVAL flag with 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") ;
..
---------------------------------------------------------------------------
EXAMPLES
Enough of the definition talk, let's have a few examples.
Perl provides many macros to assist in accessing the Perl stack. Wherever
possible, these macros should always be used when interfacing to Perl
internals. Hopefully this should make the code less vulnerable to any
changes made to Perl in the future.
Another point worth noting is that in the first series of examples I have
made use of only the perl_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 perl_call_pv and perl_call_sv , you should always try to
use perl_call_sv . See Using perl_call_sv for details.
---------------------------------------------------------------------------
No Parameters, Nothing returned
This first trivial example will call a Perl subroutine, PrintUID, to print
out the UID of the process.
sub PrintUID
{
print "UID is $<\n" ;
}
and here is a C function to call it
static void
call_PrintUID()
{
dSP ;
PUSHMARK(sp) ;
perl_call_pv("PrintUID", G_DISCARD|G_NOARGS) ;
}
Simple, eh.
A few points to note about this example.
1. Ignore dSP and PUSHMARK(sp) for now. They will be discussed in the
next example.
2. We aren't passing any parameters to PrintUID so G_NOARGS can be
specified.
3. We aren't interested in anything returned from PrintUID, so G_DISCARD
is specified. Even if PrintUID was changed to actually return some
value(s), having specified G_DISCARD will mean that they will be wiped
by the time control returns from perl_call_pv .
4. As perl_call_pv is being used, the Perl subroutine is specified as a C
string. In this case the subroutine name has been 'hard-wired' into
the code.
5. Because we specified G_DISCARD, it is not necessary to check the value
returned from perl_call_pv . It will always be 0.
..
---------------------------------------------------------------------------
Passing Parameters
Now let's make a slightly more complex example. This time we want to call a
Perl subroutine, LeftString, which will take 2 parameters - a string ($s)
and an integer ($n). The subroutine will simply print the first $n
characters of the string.
So the Perl subroutine would look like this
sub LeftString
{
my($s, $n) = @_ ;
print substr($s, 0, $n), "\n" ;
}
The C function required to call LeftString would look like this.
static void
call_LeftString(a, b)
char * a ;
int b ;
{
dSP ;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSVpv(a, 0)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
perl_call_pv("LeftString", G_DISCARD);
}
Here are a few notes on the C function call_LeftString.
1. Parameters are passed to the Perl subroutine using the Perl stack.
This is the purpose of the code beginning with the line dSP and ending
with the line PUTBACK.
2. If you are going to put something onto the Perl stack, you need to
know where to put it. This is the purpose of the macro dSP - it
declares and initializes a local copy of the Perl stack pointer.
All the other macros which will be used in this example require you to
have used this macro.
The exception to this rule is if you are calling a Perl subroutine
directly from an XSUB function. In this case it is not necessary to
explicitly use the dSP macro - it will be declared for you
automatically.
3. Any parameters to be pushed onto the stack should be bracketed by the
PUSHMARK and PUTBACK macros. The purpose of these two macros, in this
context, is to automatically count the number of parameters you are
pushing. Then whenever Perl is creating the @_ array for the
subroutine, it knows how big to make it.
The PUSHMARK macro tells Perl to make a mental note of the current
stack pointer. Even if you aren't passing any parameters (like the
example shown in the section No Parameters, Nothing returned ) you
must still call the PUSHMARK macro before you can call any of the
perl_call_* functions - Perl still needs to know that there are no
parameters.
The PUTBACK macro sets the global copy of the stack pointer to be the
same as our local copy. If we didn't do this perl_call_pv wouldn't
know where the two parameters we pushed were - remember that up to now
all the stack pointer manipulation we have done is with our local
copy, not the global copy.
4. The only flag specified this time is G_DISCARD. Since we are passing 2
parameters to the Perl subroutine this time, we have not specified
G_NOARGS.
5. Next, we come to XPUSHs. This is where the parameters actually get
pushed onto the stack. In this case we are pushing a string and an
integer.
See the section ``XSUB'S and the Argument Stack'' for details on how
the XPUSH macros work.
6. Finally, LeftString can now be called via the perl_call_pv function.
..
---------------------------------------------------------------------------
Returning a Scalar
Now for an example of dealing with the items returned from a Perl
subroutine.
Here is a Perl subroutine, Adder, which takes 2 integer parameters and
simply returns their sum.
sub Adder
{
my($a, $b) = @_ ;
$a + $b ;
}
Since we are now concerned with the return value from Adder, the C function
required to call it is now a bit more complex.
static void
call_Adder(a, b)
int a ;
int b ;
{
dSP ;
int count ;
ENTER ;
SAVETMPS;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = perl_call_pv("Adder", G_SCALAR);
SPAGAIN ;
if (count != 1)
croak("Big trouble\n") ;
printf ("The sum of %d and %d is %d\n", a, b, POPi) ;
PUTBACK ;
FREETMPS ;
LEAVE ;
}
Points to note this time are
1. The only flag specified this time was G_SCALAR. That means the @_
array will be created and that the value returned by Adder will still
exist after the call to perl_call_pv .
2. Because we are interested in what is returned from Adder we cannot
specify G_DISCARD. This means that we will have to tidy up the Perl
stack and dispose of any temporary values ourselves. This is the
purpose of
ENTER ;
SAVETMPS ;
at the start of the function, and
FREETMPS ;
LEAVE ;
at the end. The ENTER/SAVETMPS pair creates a boundary for any
temporaries we create. This means that the temporaries we get rid of
will be limited to those which were created after these calls.
The FREETMPS/LEAVE pair will get rid of any values returned by the
Perl subroutine, plus it will also dump the mortal SV's we have
created. Having ENTER/SAVETMPS at the beginning of the code makes sure
that no other mortals are destroyed.
Think of these macros as working a bit like using { and } in Perl to
limit the scope of local variables.
See the section Using Perl to dispose of temporaries for details of an
alternative to using these macros.
3. The purpose of the macro SPAGAIN is to refresh the local copy of the
stack pointer. This is necessary because it is possible that the
memory allocated to the Perl stack has been re-allocated whilst in the
perl_call_pv call.
If you are making use of the Perl stack pointer in your code you must
always refresh the your local copy using SPAGAIN whenever you make use
of the perl_call_* functions or any other Perl internal function.
4. Although only a single value was expected to be returned from Adder,
it is still good practice to check the return code from perl_call_pv
anyway.
Expecting a single value is not quite the same as knowing that there
will be one. If someone modified Adder to return a list and we didn't
check for that possibility and take appropriate action the Perl stack
would end up in an inconsistent state. That is something you really
don't want to ever happen.
5. The POPi macro is used here to pop the return value from the stack. In
this case we wanted an integer, so POPi was used.
Here is the complete list of POP macros available, along with the
types they return.
o POPs SV
o POPp pointer
o POPn double
o POPi integer
o POPl long
6. The final PUTBACK is used to leave the Perl stack in a consistent
state before exiting the function. This is necessary because when we
popped the return value from the stack with POPi it updated only our
local copy of the stack pointer. Remember, PUTBACK sets the global
stack pointer to be the same as our local copy.
..
---------------------------------------------------------------------------
Returning a list of values
Now, let's extend the previous example to return both the sum of the
parameters and the difference.
Here is the Perl subroutine
sub AddSubtract
{
my($a, $b) = @_ ;
($a+$b, $a-$b) ;
}
and this is the C function
static void
call_AddSubtract(a, b)
int a ;
int b ;
{
dSP ;
int count ;
ENTER ;
SAVETMPS;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = perl_call_pv("AddSubtract", G_ARRAY);
SPAGAIN ;
if (count != 2)
croak("Big trouble\n") ;
printf ("%d - %d = %d\n", a, b, POPi) ;
printf ("%d + %d = %d\n", a, b, POPi) ;
PUTBACK ;
FREETMPS ;
LEAVE ;
}
If call_AddSubtract is called like this
call_AddSubtract(7, 4) ;
then here is the output
7 - 4 = 3
7 + 4 = 11
Notes
1. We wanted array context, so G_ARRAY was used.
2. Not surprisingly POPi is used twice this time because we were
retrieving 2 values from the stack. The important thing to note is
that when using the POP* macros they come off the stack in reverse
order.
..
---------------------------------------------------------------------------
Returning a list in a scalar context
Say the Perl subroutine in the previous section was called in a scalar
context, like this
static void
call_AddSubScalar(a, b)
int a ;
int b ;
{
dSP ;
int count ;
int i ;
ENTER ;
SAVETMPS;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = perl_call_pv("AddSubtract", G_SCALAR);
SPAGAIN ;
printf ("Items Returned = %d\n", count) ;
for (i = 1 ; i <= count ; ++i)
printf ("Value %d = %d\n", i, POPi) ;
PUTBACK ;
FREETMPS ;
LEAVE ;
}
The other modification made is that call_AddSubScalar will print the number
of items returned from the Perl subroutine and their value (for simplicity
it assumes that they are integer). So if call_AddSubScalar is called
call_AddSubScalar(7, 4) ;
then the output will be
Items Returned = 1
Value 1 = 3
In this case the main point to note is that only the last item in the list
returned from the subroutine, Adder actually made it back to
call_AddSubScalar.
---------------------------------------------------------------------------
Returning Data from Perl via the parameter list
It is also possible to return values directly via the parameter list -
whether it is actually desirable to do it is another matter entirely.
The Perl subroutine, Inc, below takes 2 parameters and increments each
directly.
sub Inc
{
++ $_[0] ;
++ $_[1] ;
}
and here is a C function to call it.
static void
call_Inc(a, b)
int a ;
int b ;
{
dSP ;
int count ;
SV * sva ;
SV * svb ;
ENTER ;
SAVETMPS;
sva = sv_2mortal(newSViv(a)) ;
svb = sv_2mortal(newSViv(b)) ;
PUSHMARK(sp) ;
XPUSHs(sva);
XPUSHs(svb);
PUTBACK ;
count = perl_call_pv("Inc", G_DISCARD);
if (count != 0)
croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
count) ;
printf ("%d + 1 = %d\n", a, SvIV(sva)) ;
printf ("%d + 1 = %d\n", b, SvIV(svb)) ;
FREETMPS ;
LEAVE ;
}
To be able to access the two parameters that were pushed onto the stack
after they return from perl_call_pv it is necessary to make a note of their
addresses - thus the two variables sva and svb.
The reason this is necessary is that the area of the Perl stack which held
them will very likely have been overwritten by something else by the time
control returns from perl_call_pv .
---------------------------------------------------------------------------
Using G_EVAL
Now an example using G_EVAL. Below is a Perl subroutine which computes the
difference of its 2 parameters. If this would result in a negative result,
the subroutine calls die .
sub Subtract
{
my ($a, $b) = @_ ;
die "death can be fatal\n" if $a < $b ;
$a - $b ;
}
and some C to call it
static void
call_Subtract(a, b)
int a ;
int b ;
{
dSP ;
int count ;
SV * sv ;
ENTER ;
SAVETMPS;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = perl_call_pv("Subtract", G_EVAL|G_SCALAR);
SPAGAIN ;
/* Check the eval first */
sv = GvSV(gv_fetchpv("@", TRUE, SVt_PV));
if (SvTRUE(sv))
{
printf ("Uh oh - %s\n", SvPV(sv, 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 ;
}
If call_Subtract is called thus
call_Subtract(4, 5)
the following will be printed
Uh oh - death can be fatal
Notes
1. We want to be able to catch the die so we have used the G_EVAL flag.
Not specifying this flag would mean that the program would terminate
immediately at the die statement in the subroutine Subtract.
2. The code
sv = GvSV(gv_fetchpv("@", TRUE, SVt_PV));
if (SvTRUE(sv))
{
printf ("Uh oh - %s\n", SvPVx(sv, na)) ;
POPs ;
}
is the direct equivalent of this bit of Perl
print "Uh oh - $@\n" if $@ ;
3. Note that the stack is popped using POPs in the block where SvTRUE(sv)
is true. This is necessary because whenever a perl_call_* function
invoked with G_EVAL|G_SCALAR returns an error, the top of the stack
holds the value undef . Since we want the program to continue after
detecting this error, it is essential that the stack is tidied up by
removing the undef .
..
---------------------------------------------------------------------------
Using perl_call_sv
In all the previous examples I have 'hard-wired' the name of the Perl
subroutine to be called from C. Most of the time though, it is more
convenient to be able to specify the name of the Perl subroutine from
within the Perl script.
Consider the Perl code below
sub fred
{
print "Hello there\n" ;
}
CallSubPV("fred") ;
Here is a snippet of XSUB which defines CallSubPV.
void
CallSubPV(name)
char * name
CODE:
PUSHMARK(sp) ;
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, but Perl 5 allows
references to subroutines and anonymous subroutines. This is where
perl_call_sv is useful.
The code below for CallSubSV is identical to CallSubPV except that the name
parameter is now defined as an SV* and we use perl_call_sv instead of
perl_call_pv .
void
CallSubSV(name)
SV * name
CODE:
PUSHMARK(sp) ;
perl_call_sv(name, G_DISCARD|G_NOARGS) ;
Since we are using an SV to call fred the following can all be used
CallSubSV("fred") ;
CallSubSV(\&fred) ;
$ref = \&fred ;
CallSubSV($ref) ;
CallSubSV( sub { print "Hello there\n" } ) ;
As you can see, perl_call_sv gives you much greater flexibility in how you
can specify the Perl subroutine.
You should note that if it is necessary to store the SV (name in the
example above) which corresponds to the Perl subroutine so that it can be
used later in the program, it not enough to just store a copy of the
pointer to the SV. Say the code above had been like this
static SV * rememberSub ;
void
SaveSub1(name)
SV * name
CODE:
rememberSub = name ;
void
CallSavedSub1()
CODE:
PUSHMARK(sp) ;
perl_call_sv(rememberSub, G_DISCARD|G_NOARGS) ;
The reason this is wrong is that by the time you come to use the pointer
rememberSub in CallSavedSub1, it may or may not still refer to the Perl
subroutine that was recorded in SaveSub1. This is particularly true for
these cases
SaveSub1(\&fred) ;
CallSavedSub1() ;
SaveSub1( sub { print "Hello there\n" } ) ;
CallSavedSub1() ;
By the time each of the SaveSub1 statements above have been executed, the
SV*'s which corresponded to the parameters will no longer exist. Expect an
error message from Perl of the form
Can't use an undefined value as a subroutine reference at ...
for each of the CallSavedSub1 lines.
Similarly, with this code
$ref = \&fred ;
SaveSub1($ref) ;
$ref = 47 ;
CallSavedSub1() ;
you can expect one of these messages (which you actually get is dependant
on the version of Perl you are using)
Not a CODE reference at ...
Undefined subroutine &main::47 called ...
The variable $ref may have referred to the subroutine fred whenever the
call to SaveSub1 was made but by the time CallSavedSub1 gets called it now
holds the number 47. Since we saved only a pointer to the original SV in
SaveSub1, any changes to $ref will be tracked by the pointer rememberSub.
This means that whenever CallSavedSub1 gets called, it will attempt to
execute the code which is referenced by the SV* rememberSub. In this case
though, it now refers to the integer 47, so expect Perl to complain loudly.
A similar but more subtle problem is illustrated with this code
$ref = \&fred ;
SaveSub1($ref) ;
$ref = \&joe ;
CallSavedSub1() ;
This time whenever CallSavedSub1 get called it will execute the Perl
subroutine joe (assuming it exists) rather than fred as was originally
requested in the call to SaveSub1.
To get around these problems it is necessary to take a full copy of the SV.
The code below shows SaveSub2 modified to do that
static SV * keepSub = (SV*)NULL ;
void
SaveSub2(name)
SV * name
CODE:
/* Take a copy of the callback */
if (keepSub == (SV*)NULL)
/* First time, so create a new SV */
keepSub = newSVsv(name) ;
else
/* Been here before, so overwrite */
SvSetSV(keepSub, name) ;
void
CallSavedSub2()
CODE:
PUSHMARK(sp) ;
perl_call_sv(keepSub, G_DISCARD|G_NOARGS) ;
In order to avoid creating a new SV every time SaveSub2 is called, the
function first checks to see if it has been called before. If not, then
space for a new SV is allocated and the reference to the Perl subroutine,
name is copied to the variable keepSub in one operation using newSVsv.
Thereafter, whenever SaveSub2 is called the existing SV, keepSub, is
overwritten with the new value using SvSetSV.
---------------------------------------------------------------------------
Using perl_call_argv
Here is a Perl subroutine which prints whatever parameters are passed to
it.
sub PrintList
{
my(@list) = @_ ;
foreach (@list) { print "$_\n" }
}
and here is an example of perl_call_argv which will call PrintList.
static char * words[] = {"alpha", "beta", "gamma", "delta", NULL} ;
static void
call_PrintList()
{
dSP ;
perl_call_argv("PrintList", G_DISCARD, words) ;
}
Note that it is not necessary to call PUSHMARK in this instance. This is
because perl_call_argv will do it for you.
---------------------------------------------------------------------------
Using perl_call_method
Consider the following Perl code
{
package Mine ;
sub new
{
my($type) = shift ;
bless [@_]
}
sub Display
{
my ($self, $index) = @_ ;
print "$index: $$self[$index]\n" ;
}
sub PrintID
{
my($class) = @_ ;
print "This is Class $class version 1.0\n" ;
}
}
It just implements a very simple class to manage an array. Apart from the
constructor, new, it declares methods, one static and one virtual. The
static method, PrintID, simply prints out the class name and a version
number. The virtual method, Display, prints out a single element of the
array. Here is an all Perl example of using it.
$a = new Mine ('red', 'green', 'blue') ;
$a->Display(1) ;
PrintID Mine;
will print
1: green
This is Class Mine version 1.0
Calling a Perl method from C is fairly straightforward. The following
things are required
* a reference to the object for a virtual method or the name of the
class for a static method.
* the name of the method.
* any other parameters specific to the method.
Here is a simple XSUB which illustrates the mechanics of calling both.the
PrintID and Display methods from C.
void
call_Method(ref, method, index)
SV * ref
char * method
int index
CODE:
PUSHMARK(sp);
XPUSHs(ref);
XPUSHs(sv_2mortal(newSViv(index))) ;
PUTBACK;
perl_call_method(method, G_DISCARD) ;
void
call_PrintID(class, method)
char * class
char * method
CODE:
PUSHMARK(sp);
XPUSHs(sv_2mortal(newSVpv(class, 0))) ;
PUTBACK;
perl_call_method(method, G_DISCARD) ;
So the methods PrintID and Display can be invoked like this
$a = new Mine ('red', 'green', 'blue') ;
call_Method($a, 'Display', 1) ;
call_PrintID('Mine', 'PrintID') ;
The only thing to note is that in both the static and virtual methods, the
method name is not passed via the stack - it is used as the first parameter
to perl_call_method .
---------------------------------------------------------------------------
Using GIMME
Here is a trivial XSUB which prints the context in which it is currently
executing.
void
PrintContext()
CODE:
if (GIMME == G_SCALAR)
printf ("Context is Scalar\n") ;
else
printf ("Context is Array\n") ;
and here is some Perl to test it
$a = PrintContext ;
@a = PrintContext ;
The output from that will be
Context is Scalar
Context is Array
---------------------------------------------------------------------------
Using Perl to dispose of temporaries
In the examples given to date, any temporaries created in the callback
(i.e. parameters passed on the stack to the perl_call_* function or values
returned via the stack) have been freed by one of these methods
* specifying the G_DISCARD flag with perl_call_*.
* explicitly disposed of using the ENTER/SAVETMPS - FREETMPS/LEAVE
pairing.
There is another method which can be used, namely letting Perl do it.for
you automatically whenever it regains control after the callback has
terminated. This is done by simply not using the
ENTER ;
SAVETMPS ;
...
FREETMPS ;
LEAVE ;
sequence in the callback (and not, of course, specifying the G_DISCARD
flag).
If you are going to use this method you have to be aware of a possible
memory leak which can arise under very specific circumstances. To explain
these circumstances you need to know a bit about the flow of control
between Perl and the callback routine.
The examples given at the start of the document (an error handler and an
event driven program) are typical of the two main sorts of flow control
that you are likely to encounter with callbacks. There is a very important
distinction between them, so pay attention.
In the first example, an error handler, the flow of control could be as
follows. You have created an interface to an external library. Control can
reach the external library like this
perl --> XSUB --> external library
Whilst control is in the library, an error condition occurs. You have
previously set up a Perl callback to handle this situation, so it will get
executed. Once the callback has finished, control will drop back to Perl
again. Here is what the flow of control will be like in that situation
perl --> XSUB --> external library
...
error occurs
...
external library --> perl_call --> perl
|
perl <-- XSUB <-- external library <-- perl_call <----+
After processing of the error using perl_call_* is completed, control
reverts back to Perl more or less immediately.
In the diagram, the further right you go the more deeply nested the scope
is. It is only when control is back with perl on the extreme left of the
diagram that you will have dropped back to the enclosing scope and any
temporaries you have left hanging around will be freed.
In the second example, an event driven program, the flow of control will be
more like this
perl --> XSUB --> event handler
...
event handler --> perl_call --> perl
|
event handler <-- perl_call --<--+
...
event handler --> perl_call --> perl
|
event handler <-- perl_call --<--+
...
event handler --> perl_call --> perl
|
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 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
enclosing scope at some stage. In the event driven scenario that may never
happen. This means that as time goes on, your program will create more and
more temporaries, none of which will ever be freed. As each of these
temporaries consumes some memory your program will eventually consume all
the available memory in your system - kapow!
So here is the bottom line - if you are sure that control will revert back
to the enclosing Perl scope fairly quickly after the end of your callback,
then it isn't absolutely necessary to explicitly dispose of any temporaries
you may have created. Mind you, if you are at all uncertain about what to
do, it doesn't do any harm to tidy up anyway.
---------------------------------------------------------------------------
Strategies for storing Callback Context Information
Potentially one of the trickiest problems to overcome when designing a
callback interface can be figuring out how to store the mapping between the
C callback function and the Perl equivalent.
To help understand why this can be a real problem first consider how a
callback is set up in an all C environment. Typically a C API will provide
a function to register a callback. This will expect a pointer to a function
as one of its parameters. Below is a call to a hypothetical function
register_fatal which registers the C function to get called when a fatal
error occurs.
register_fatal(cb1) ;
The single parameter cb1 is a pointer to a function, so you must have
defined cb1 in your code, say something like this
static void
cb1()
{
printf ("Fatal Error\n") ;
exit(1) ;
}
Now change that to call a Perl subroutine instead
static SV * callback = (SV*)NULL;
static void
cb1()
{
dSP ;
PUSHMARK(sp) ;
/* Call the Perl sub to process the callback */
perl_call_sv(callback, G_DISCARD) ;
}
void
register_fatal(fn)
SV * fn
CODE:
/* Remember the Perl sub */
if (callback == (SV*)NULL)
callback = newSVsv(fn) ;
else
SvSetSV(callback, fn) ;
/* register the callback with the external library */
register_fatal(cb1) ;
where the Perl equivalent of register_fatal and the callback it registers,
pcb1, might look like this
# Register the sub pcb1
register_fatal(\&pcb1) ;
sub pcb1
{
die "I'm dying...\n" ;
}
The mapping between the C callback and the Perl equivalent is stored in the
global variable callback.
This will be adequate if you ever need to have only 1 callback registered
at any time. An example could be an error handler like the code sketched
out above. Remember though, repeated calls to register_fatal will replace
the previously registered callback function with the new one.
Say for example you want to interface to a library which allows
asynchronous file i/o. In this case you may be able to register a callback
whenever a read operation has completed. To be of any use we want to be
able to call separate Perl subroutines for each file that is opened. As it
stands, the error handler example above would not be adequate as it allows
only a single callback to be defined at any time. What we require is a
means of storing the mapping between the opened file and the Perl
subroutine we want to be called for that file.
Say the i/o library has a function asynch_read which associates a C
function ProcessRead with a file handle fh - this assumes that it has also
provided some routine to open the file and so obtain the file handle.
asynch_read(fh, ProcessRead)
This may expect the C ProcessRead function of this form
void
ProcessRead(fh, buffer)
int fh ;
char * buffer ;
{
...
}
To provide a Perl interface to this library we need to be able to map
between the fh parameter and the Perl subroutine we want called. A hash is
a convenient mechanism for storing this mapping. The code below shows a
possible implementation
static HV * Mapping = (HV*)NULL ;
void
asynch_read(fh, callback)
int fh
SV * callback
CODE:
/* If the hash doesn't already exist, create it */
if (Mapping == (HV*)NULL)
Mapping = newHV() ;
/* Save the fh -> callback mapping */
hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0) ;
/* Register with the C Library */
asynch_read(fh, asynch_read_if) ;
and asynch_read_if could look like this
static void
asynch_read_if(fh, buffer)
int fh ;
char * buffer ;
{
dSP ;
SV ** sv ;
/* Get the callback associated with fh */
sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE) ;
if (sv == (SV**)NULL)
croak("Internal error...\n") ;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSViv(fh))) ;
XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
PUTBACK ;
/* Call the Perl sub */
perl_call_sv(*sv, G_DISCARD) ;
}
For completeness, here is asynch_close. This shows how to remove the entry
from the hash Mapping.
void
asynch_close(fh)
int fh
CODE:
/* Remove the entry from the hash */
(void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD) ;
/* Now call the real asynch_close */
asynch_close(fh) ;
So the Perl interface would look like this
sub callback1
{
my($handle, $buffer) = @_ ;
}
# Register the Perl callback
asynch_read($fh, \&callback1) ;
asynch_close($fh) ;
The mapping between the C callback and Perl is stored in the global hash
Mapping this time. Using a hash has the distinct advantage that it allows
an unlimited number of callbacks to be registered.
What if the interface provided by the C callback doesn't contain a
parameter which allows the file handle to Perl subroutine mapping? Say in
the asynchronous i/o package, the callback function gets passed only the
buffer parameter like this
void
ProcessRead(buffer)
char * buffer ;
{
...
}
Without the file handle there is no straightforward way to map from the C
callback to the Perl subroutine.
In this case a possible way around this problem is to pre-define a series
of C functions to act as the interface to Perl, thus
#define MAX_CB 3
#define NULL_HANDLE -1
typedef void (*FnMap)() ;
struct MapStruct {
FnMap Function ;
SV * PerlSub ;
int Handle ;
} ;
static void fn1() ;
static void fn2() ;
static void fn3() ;
static struct MapStruct Map [MAX_CB] =
{
{ fn1, NULL, NULL_HANDLE },
{ fn2, NULL, NULL_HANDLE },
{ fn3, NULL, NULL_HANDLE }
} ;
static void
Pcb(index, buffer)
int index ;
char * buffer ;
{
dSP ;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
PUTBACK ;
/* Call the Perl sub */
perl_call_sv(Map[index].PerlSub, G_DISCARD) ;
}
static void
fn1(buffer)
char * buffer ;
{
Pcb(0, buffer) ;
}
static void
fn2(buffer)
char * buffer ;
{
Pcb(1, buffer) ;
}
static void
fn3(buffer)
char * buffer ;
{
Pcb(2, buffer) ;
}
void
array_asynch_read(fh, callback)
int fh
SV * callback
CODE:
int index ;
int null_index = MAX_CB ;
/* Find the same handle or an empty entry */
for (index = 0 ; index < MAX_CB ; ++index)
{
if (Map[index].Handle == fh)
break ;
if (Map[index].Handle == NULL_HANDLE)
null_index = index ;
}
if (index == MAX_CB && null_index == MAX_CB)
croak ("Too many callback functions registered\n") ;
if (index == MAX_CB)
index = null_index ;
/* Save the file handle */
Map[index].Handle = fh ;
/* Remember the Perl sub */
if (Map[index].PerlSub == (SV*)NULL)
Map[index].PerlSub = newSVsv(callback) ;
else
SvSetSV(Map[index].PerlSub, callback) ;
asynch_read(fh, Map[index].Function) ;
void
array_asynch_close(fh)
int fh
CODE:
int index ;
/* Find the file handle */
for (index = 0; index < MAX_CB ; ++ index)
if (Map[index].Handle == fh)
break ;
if (index == MAX_CB)
croak ("could not close fh %d\n", fh) ;
Map[index].Handle = NULL_HANDLE ;
SvREFCNT_dec(Map[index].PerlSub) ;
Map[index].PerlSub = (SV*)NULL ;
asynch_close(fh) ;
In this case the functions fn1, fn2 and fn3 are used to remember the Perl
subroutine to be called. Each of the functions holds a separate hard-wired
index which is used in the function Pcb to access the Map array and
actually call the Perl subroutine.
There are some obvious disadvantages with this technique.
Firstly, the code is considerably more complex than with the previous
example.
Secondly, there is a hard-wired limit (in this case 3) to the number of
callbacks that can exist simultaneously. The only way to increase the limit
is by modifying the code to add more functions and then re-compiling. None
the less, as long as the number of functions is chosen with some care, it
is still a workable solution and in some cases is the only one available.
To summarize, here are a number of possible methods for you to consider for
storing the mapping between C and the Perl callback
1. For a lot of situations, like interfacing to an error handler, this
may be a perfectly adequate solution.
2. If it is impossible to tell from the parameters passed back from the C
callback what the context is, then you may need to create a sequence
of C callback interface functions, and store pointers to each in an
array.
3. A hash is an ideal mechanism to store the mapping between C and Perl.
..
---------------------------------------------------------------------------
Alternate Stack Manipulation
Although I have made use of only the POP* macros to access values returned
from Perl subroutines, it is also possible to bypass these macros and read
the stack using the ST macro (See the perlxs manpage for a full description
of the ST macro).
Most of the time the POP* macros should be adequate, the main problem with
them is that they force you to process the returned values in sequence.
This may not be the most suitable way to process the values in some cases.
What we want is to be able to access the stack in a random order. The ST
macro as used when coding an XSUB is ideal for this purpose.
The code below is the example given in the section IST instead of POP*.
static void
call_AddSubtract2(a, b)
int a ;
int b ;
{
dSP ;
I32 ax ;
int count ;
ENTER ;
SAVETMPS;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = perl_call_pv("AddSubtract", G_ARRAY);
SPAGAIN ;
sp -= count ;
ax = (sp - stack_base) + 1 ;
if (count != 2)
croak("Big trouble\n") ;
printf ("%d + %d = %d\n", a, b, SvIV(ST(0))) ;
printf ("%d - %d = %d\n", a, b, SvIV(ST(1))) ;
PUTBACK ;
FREETMPS ;
LEAVE ;
}
Notes
1. Notice that it was necessary to define the variable ax. This is
because the ST macro expects it to exist. If we were in an XSUB it
would not be necessary to define ax as it is already defined for you.
2. The code
SPAGAIN ;
sp -= count ;
ax = (sp - stack_base) + 1 ;
sets the stack up so that we can use the ST macro.
3. Unlike the original coding of this example, the returned values are
not accessed in reverse order. So ST(0) refers to the first value
returned by the Perl subroutine and ST(count-1) refers to the last.
..
---------------------------------------------------------------------------
SEE ALSO
the perlxs manpage , the perlguts manpage , the perlembed manpage
---------------------------------------------------------------------------
AUTHOR
Paul Marquess <pmarquess@bfsec.bt.co.uk>
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.
---------------------------------------------------------------------------
DATE
Version 1.1, 17th May 1995
--------------36A73D714DFA--
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