{
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
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.
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.
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 CWflags parameter which is used to pass a bit mask of options to Perl. This bit mask operates identically for each of the functions. The settings available in the bit mask are discussed in the section on FLAG VALUES.
Each of the functions will now be discussed in turn.
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.
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
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 Returning a list in a scalar context shows an example of this behaviour.
G_ARRAY
Calls the Perl subroutine in a list context.
As with G_SCALAR, this flag has 2 effects
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 Using Perl to dispose of temporaries discusses the specific circumstances where you can ignore the problem and let Perl deal with it for you.
G_NOARGS
Whenever a Perl subroutine is called using one of the 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 CW@_ 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 CW@_ 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 CWfred accesses the CW@_ array which belongs to CWjoe.
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 CW$@ variable as you would in a normal Perl script.
The value returned from the perl_call_* function is dependent on what other flags have been specified and whether an error has occurred. Here are all the different cases that can occur
See Using G_EVAL for details of using G_EVAL.
G_KEEPERR
You may have noticed that using the G_EVAL flag described above will always clear the CW$@ variable and set it to a string describing the error iff there was an error in the called code. This unqualified resetting of CW$@ can be problematic in the reliable identification of errors using the CWeval {} mechanism, because the possibility exists that perl will call other code (end of block processing code, for example) between the time the error causes CW$@ to be set within CWeval {}, and the subsequent statement which checks for the value of CW$@ gets executed in the user's script.
This scenario will mostly be applicable to code that is meant to be called from within destructors, asynchronous callbacks, signal handlers, CW__DIE__ or CW__WARN__ hooks, and CWtie functions. In such situations, you will not want to clear CW$@ at all, but simply to append any new errors to any existing value of CW$@.
The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in perl_call_* functions that are used to implement such code. This flag has no effect when G_EVAL is not used.
When G_KEEPERR is used, any errors in the called code will be prefixed with the string ``\t(in cleanup)'', and appended to the current value of CW$@.
The G_KEEPERR flag was introduced in Perl version 5.002.
See Using G_KEEPERR for an example of a situation that warrants the use of this flag.
Determining the Context
As mentioned above, you can determine the context of the currently executing subroutine in Perl with wantarray. The equivalent test can be made in C by using the CWGIMME macro. This will return CWG_SCALAR if you have been called in a scalar context and CWG_ARRAY if in an array context. An example of using the CWGIMME macro is shown in section Using GIMME.
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.
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 CWCall_fred is executed it will print
Trapped error: Fatal Error
As control never returns to CWCall_fred, the CW"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") ;
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 $<\n" ;
}
and here is a C function to call it
static void
call_PrintUID()
{
dSP ;
PUSHMARK(sp) ;
perl_call_pv("PrintUID", G_DISCARD|G_NOARGS) ;
}
Simple, eh.
A few points to note about this example.
Passing Parameters
Now let's make a slightly more complex example. This time we want to call a Perl subroutine, CWLeftString, which will take 2 parameters - a string (CW$s) and an integer (CW$n). The subroutine will simply print the first CW$n characters of the string.
So the Perl subroutine would look like this
sub LeftString
{
my($s, $n) = @_ ;
print substr($s, 0, $n), "\n" ;
}
The C function required to call LeftString would look like this.
static void
call_LeftString(a, b)
char * a ;
int b ;
{
dSP ;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSVpv(a, 0)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
perl_call_pv("LeftString", G_DISCARD);
}
Here are a few notes on the C function call_LeftString.
All the other macros which will be used in this example require you to have used this macro.
The exception to this rule is if you are calling a Perl subroutine directly from an XSUB function. In this case it is not necessary to explicitly use the CWdSP macro - it will be declared for you automatically.
The CWPUSHMARK macro tells Perl to make a mental note of the current stack pointer. Even if you aren't passing any parameters (like the example shown in the section No Parameters, Nothing returned) you must still call the CWPUSHMARK macro before you can call any of the perl_call_* functions - Perl still needs to know that there are no parameters.
The CWPUTBACK macro sets the global copy of the stack pointer to be the same as our local copy. If we didn't do this perl_call_pv wouldn't know where the two parameters we pushed were - remember that up to now all the stack pointer manipulation we have done is with our local copy, not the global copy.
See the section the section on XSUB'S and the Argument Stack in the perlguts manpage for details on how the XPUSH macros work.
Returning a Scalar
Now for an example of dealing with the items returned from a Perl subroutine.
Here is a Perl subroutine, Adder, which takes 2 integer parameters and simply returns their sum.
sub Adder
{
my($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
ENTER ;
SAVETMPS ;
at the start of the function, and
FREETMPS ;
LEAVE ;
at the end. The CWENTER/CWSAVETMPS 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 CWFREETMPS/CWLEAVE 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 CWENTER/CWSAVETMPS at the beginning of the code makes sure that no other mortals are destroyed.
Think of these macros as working a bit like using CW{ and CW} 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.
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.
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.
Here is the complete list of POP macros available, along with the types they return.
POPs SV
POPp pointer
POPn double
POPi integer
POPl long
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
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] ;
++ $_[1] ;
}
and here is a C function to call it.
static void
call_Inc(a, b)
int a ;
int b ;
{
dSP ;
int count ;
SV * sva ;
SV * svb ;
ENTER ;
SAVETMPS;
sva = sv_2mortal(newSViv(a)) ;
svb = sv_2mortal(newSViv(b)) ;
PUSHMARK(sp) ;
XPUSHs(sva);
XPUSHs(svb);
PUTBACK ;
count = perl_call_pv("Inc", G_DISCARD);
if (count != 0)
croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
count) ;
printf ("%d + 1 = %d\n", a, SvIV(sva)) ;
printf ("%d + 1 = %d\n", b, SvIV(svb)) ;
FREETMPS ;
LEAVE ;
}
To be able to access the two parameters that were pushed onto the stack after they return from perl_call_pv it is necessary to make a note of their addresses - thus the two variables CWsva and CWsvb.
The reason this is necessary is that the area of the Perl stack which held them will very likely have been overwritten by something else by the time control returns from perl_call_pv.
Using G_EVAL
Now an example using G_EVAL. Below is a Perl subroutine which computes the difference of its 2 parameters. If this would result in a negative result, the subroutine calls die.
sub Subtract
{
my ($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 ;
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 ;
}
If call_Subtract is called thus
call_Subtract(4, 5)
the following will be printed
Uh oh - death can be fatal
Notes
if (SvTRUE(GvSV(errgv)))
{
printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ;
POPs ;
}
is the direct equivalent of this bit of Perl
print "Uh oh - $@\n" if $@ ;
CWerrgv is a perl global of type CWGV * that points to the symbol table entry containing the error. CWGvSV(errgv) therefore refers to the C equivalent of CW$@.
Using G_KEEPERR
Consider this rather facetious example, where we have used an XS version of the call_Subtract example above inside a destructor:
package Foo;
sub new { bless {}, $_[0] }
sub Subtract {
my($a,$b) = @_;
die "death can be fatal" if $a < $b ;
$a - $b;
}
sub DESTROY { call_Subtract(5, 4); }
sub foo { die "foo dies"; }
package main;
eval { Foo->new->foo };
print "Saw: $@" if $@; # should be, but isn't
This example will fail to recognize that an error occurred inside the CWeval {}. Here's why: the call_Subtract code got executed while perl was cleaning up temporaries when exiting the eval block, and since call_Subtract is implemented with perl_call_pv using the G_EVAL flag, it promptly reset CW$@. This results in the failure of the outermost test for CW$@, and thereby the failure of the error trap.
Appending the G_KEEPERR flag, so that the perl_call_pv call in call_Subtract reads:
count = perl_call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);
will preserve the error and restore reliable error handling.
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 CWname 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 (CWname 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 CWrememberSub in CWCallSavedSub1, it may or may not still refer to the Perl subroutine that was recorded in CWSaveSub1. This is particularly true for these cases
SaveSub1(\&fred) ;
CallSavedSub1() ;
SaveSub1( sub { print "Hello there\n" } ) ;
CallSavedSub1() ;
By the time each of the CWSaveSub1 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 CWCallSavedSub1 lines.
Similarly, with this code
$ref = \&fred ;
SaveSub1($ref) ;
$ref = 47 ;
CallSavedSub1() ;
you can expect one of these messages (which you actually get is dependant on the version of Perl you are using)
Not a CODE reference at ...
Undefined subroutine &main::47 called ...
The variable CW$ref may have referred to the subroutine CWfred whenever the call to CWSaveSub1 was made but by the time CWCallSavedSub1 gets called it now holds the number CW47. Since we saved only a pointer to the original SV in CWSaveSub1, any changes to CW$ref will be tracked by the pointer CWrememberSub. This means that whenever CWCallSavedSub1 gets called, it will attempt to execute the code which is referenced by the SV* CWrememberSub. In this case though, it now refers to the integer CW47, so expect Perl to complain loudly.
A similar but more subtle problem is illustrated with this code
$ref = \&fred ;
SaveSub1($ref) ;
$ref = \&joe ;
CallSavedSub1() ;
This time whenever CWCallSavedSub1 get called it will execute the Perl subroutine CWjoe (assuming it exists) rather than CWfred as was originally requested in the call to CWSaveSub1.
To get around these problems it is necessary to take a full copy of the SV. The code below shows CWSaveSub2 modified to do that
static SV * keepSub = (SV*)NULL ;
void
SaveSub2(name)
SV * name
CODE:
/* Take a copy of the callback */
if (keepSub == (SV*)NULL)
/* First time, so create a new SV */
keepSub = newSVsv(name) ;
else
/* Been here before, so overwrite */
SvSetSV(keepSub, name) ;
void
CallSavedSub2()
CODE:
PUSHMARK(sp) ;
perl_call_sv(keepSub, G_DISCARD|G_NOARGS) ;
In order to avoid creating a new SV every time CWSaveSub2 is called, the function first checks to see if it has been called before. If not, then space for a new SV is allocated and the reference to the Perl subroutine, CWname is copied to the variable CWkeepSub in one operation using CWnewSVsv. Thereafter, whenever CWSaveSub2 is called the existing SV, CWkeepSub, is overwritten with the new value using CWSvSetSV.
Using perl_call_argv
Here is a Perl subroutine which prints whatever parameters are passed to it.
sub PrintList
{
my(@list) = @_ ;
foreach (@list) { print "$_\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 CWPUSHMARK 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, CWnew, it declares methods, one static and one virtual. The static method, CWPrintID, simply prints out the class name and a version number. The virtual method, CWDisplay, 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
Here is a simple XSUB which illustrates the mechanics of calling both the CWPrintID and CWDisplay 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 CWPrintID and CWDisplay 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
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 CWregister_fatal which registers the C function to get called when a fatal error occurs.
register_fatal(cb1) ;
The single parameter CWcb1 is a pointer to a function, so you must have defined CWcb1 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 CWregister_fatal and the callback it registers, CWpcb1, 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 CWcallback.
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 CWregister_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 CWasynch_read which associates a C function CWProcessRead with a file handle CWfh - 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 CWfh 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 CWasynch_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 CWasynch_close. This shows how to remove the entry from the hash CWMapping.
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 CWMapping 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 CWbuffer 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 CWfn1, CWfn2 and CWfn3 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 CWPcb to access the CWMap 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
Alternate Stack Manipulation
Although I have made use of only the CWPOP* macros to access values returned from Perl subroutines, it is also possible to bypass these macros and read the stack using the CWST macro (See the perlxs manpage for a full description of the CWST macro).
Most of the time the CWPOP* 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 CWST macro as used when coding an XSUB is ideal for this purpose.
The code below is the example given in the section Returning a list of values recoded to use CWST instead of CWPOP*.
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
SPAGAIN ;
sp -= count ;
ax = (sp - stack_base) + 1 ;
sets the stack up so that we can use the CWST macro.
Special thanks to the following people who assisted in the creation of the document.
Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy and Larry Wall.