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.
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 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.
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.
"pkg::fred".
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.
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.
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.
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.
This flag has 2 effects:
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.
As with G_SCALAR, this flag has 2 effects:
If 0, then you have specified the G_DISCARD flag.
If not 0, then it will be a count of the number of items returned by the subroutine. These items will be stored on the Perl stack. The section Returning a list of values gives an example of using the G_ARRAY flag and the mechanics of accessing the returned items from the Perl stack.
If you do not set this flag then it is very important that you make sure that any temporaries (i.e., parameters passed to the Perl subroutine and values returned from the subroutine) are disposed of yourself. The section Returning a Scalar gives details of how to dispose of these temporaries explicitly and the section Using Perl to dispose of temporaries discusses the specific circumstances where you can ignore the problem and let Perl deal with it for you.
@_ 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.
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:
$@ and you want the program to continue, you must remember to pop the undef
from the stack.
$@ variable and set it to a string describing the error iff there was an error
in the called code. This unqualified resetting of $@ can be problematic in the reliable identification of errors using the eval {} mechanism, because the possibility exists that perl will call other code
(end of block processing code, for example) between the time the error
causes $@ to be set within
eval {}, and the subsequent statement which checks for the value of
$@ gets executed in the user's script.
This scenario will mostly be applicable to code that is meant to be called
from within destructors, asynchronous callbacks, signal handlers, __DIE__ or __WARN__ hooks, and tie functions. In such situations, you will not want to clear $@ at all, but simply to append any new errors to any existing value of $@.
The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in 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 $@.
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.
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 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") ;
Perl provides many macros to assist in accessing the Perl stack. Wherever possible, these macros should always be used when interfacing to Perl internals. We hope this should make the code less vulnerable to any changes made to Perl in the future.
Another point worth noting is that in the first series of examples I have made use of only the 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.
sub PrintUID
{
print "UID is $<\n" ;
}
and here is a C function to call it
static void
call_PrintUID()
{
dSP ;
PUSHMARK(sp) ;
perl_call_pv("PrintUID", G_DISCARD|G_NOARGS) ;
}
Simple, eh.
A few points to note about this example.
value, having specified
G_DISCARD will mean that they will be wiped by the
time control returns from perl_call_pv.
LeftString, which will take 2 parameters - a string ($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, $n), "\n" ;
}
The C function required to call LeftString would look like this.
static void
call_LeftString(a, b)
char * a ;
int b ;
{
dSP ;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSVpv(a, 0)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
perl_call_pv("LeftString", G_DISCARD);
}
Here are a few notes on the C function call_LeftString.
All the other macros which will be used in this example require you to have used this macro.
The exception to this rule is if you are calling a Perl subroutine directly from an XSUB function. In this case it is not necessary to use the dSP macro explicitly - it will be declared for you automatically.
@_ array for the subroutine, it knows how big to make it.
The PUSHMARK macro tells Perl to make a mental note of the current stack pointer. Even if you aren't passing any parameters (like the example shown in the section No Parameters, Nothing returned) you must still call the PUSHMARK macro before you can call any of the perl_call_* functions - Perl still needs to know that there are no parameters.
The PUTBACK macro sets the global copy of the stack pointer to be the same as our local copy. If we didn't do this perl_call_pv wouldn't know where the two parameters we pushed were - remember that up to now all the stack pointer manipulation we have done is with our local copy, not the global copy.
See the XSUB's and the Argument Stack for details on how the XPUSH macros work.
Here is a Perl subroutine, Adder, that takes 2 integer parameters and simply returns their sum.
sub Adder
{
my($a, $b) = @_ ;
$a + $b ;
}
Because we are now concerned with the return value from Adder, the C function required to call it is now a bit more complex.
static void
call_Adder(a, b)
int a ;
int b ;
{
dSP ;
int count ;
ENTER ;
SAVETMPS;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = 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
@_
array will be created and that the value returned by Adder will still exist after the call to perl_call_pv.
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.
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 happen ever.
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
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
POP* macros they come off the stack in reverse
order.
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.
The Perl subroutine, Inc, below takes 2 parameters and increments each directly.
sub Inc
{
++ $_[0] ;
++ $_[1] ;
}
and here is a C function to call it.
static void
call_Inc(a, b)
int a ;
int b ;
{
dSP ;
int count ;
SV * sva ;
SV * svb ;
ENTER ;
SAVETMPS;
sva = sv_2mortal(newSViv(a)) ;
svb = sv_2mortal(newSViv(b)) ;
PUSHMARK(sp) ;
XPUSHs(sva);
XPUSHs(svb);
PUTBACK ;
count = perl_call_pv("Inc", G_DISCARD);
if (count != 0)
croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
count) ;
printf ("%d + 1 = %d\n", a, SvIV(sva)) ;
printf ("%d + 1 = %d\n", b, SvIV(svb)) ;
FREETMPS ;
LEAVE ;
}
To be able to access the two parameters that were pushed onto the stack
after they return from perl_call_pv it is necessary to make a note of their addresses - thus the two variables sva and svb.
The reason this is necessary is that the area of the Perl stack which held them will very likely have been overwritten by something else by the time control returns from perl_call_pv.
sub Subtract
{
my ($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 $@ ;
errgv is a perl global of type GV * that points to the symbol table entry containing the error. GvSV therefore refers to the
C equivalent of $@.
SvTRUE(GvSV) 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. Because we want the program to continue after detecting this error, it is
essential that the stack is tidied up by removing the undef.
package Foo;
sub new { bless {}, $_[0] }
sub Subtract {
my($a,$b) = @_;
die "death can be fatal" if $a < $b ;
$a - $b;
}
sub DESTROY { call_Subtract(5, 4); }
sub foo { die "foo dies"; }
package main;
eval { Foo->new->foo };
print "Saw: $@" if $@; # should be, but isn't
This example will fail to recognize that an error occurred inside the
eval {}. Here's why: the call_Subtract code got executed while perl was cleaning
up temporaries when exiting the eval block, and because call_Subtract is
implemented with perl_call_pv using the
G_EVAL flag, it promptly reset $@. This results in the failure of the outermost test for $@, and thereby the failure of the error trap.
Appending the G_KEEPERR flag, so that the 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.
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 as only 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) ;
Because we are using an SV to call fred the following can all be used
CallSubSV("fred") ;
CallSubSV(\&fred) ;
$ref = \&fred ;
CallSubSV($ref) ;
CallSubSV( sub { print "Hello there\n" } ) ;
As you can see, 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 just to store a copy of the pointer to the
SV. Say the code above had been like this
static SV * rememberSub ;
void
SaveSub1(name)
SV * name
CODE:
rememberSub = name ;
void
CallSavedSub1()
CODE:
PUSHMARK(sp) ;
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 dependent on the version of Perl you are using)
Not a CODE reference at ...
Undefined subroutine &main::47 called ...
The variable $ref may have referred to the subroutine fred
whenever the call to SaveSub1 was made but by the time
CallSavedSub1 gets called it now holds the number 47. Because we saved only a pointer to the original
SV in SaveSub1, any changes to
$ref will be tracked by the pointer rememberSub. This means that whenever CallSavedSub1 gets called, it will attempt to execute the code which is referenced by the
SV*
rememberSub. In this case though, it now refers to the integer 47, so expect Perl to complain loudly.
A similar but more subtle problem is illustrated with this code
$ref = \&fred ;
SaveSub1($ref) ;
$ref = \&joe ;
CallSavedSub1() ;
This time whenever CallSavedSub1 get called it will execute the Perl subroutine joe (assuming it exists) rather than fred as was originally requested in the call to SaveSub1.
To get around these problems it is necessary to take a full copy of the
SV. The code below shows SaveSub2 modified to do that
static SV * keepSub = (SV*)NULL ;
void
SaveSub2(name)
SV * name
CODE:
/* Take a copy of the callback */
if (keepSub == (SV*)NULL)
/* First time, so create a new SV */
keepSub = newSVsv(name) ;
else
/* Been here before, so overwrite */
SvSetSV(keepSub, name) ;
void
CallSavedSub2()
CODE:
PUSHMARK(sp) ;
perl_call_sv(keepSub, G_DISCARD|G_NOARGS) ;
To avoid creating a new
SV every time SaveSub2 is called, the function first checks to see if it has been called before. If not, then space for a new
SV is allocated and the reference to the Perl subroutine,
name is copied to the variable keepSub in one operation using newSVsv. Thereafter, whenever SaveSub2 is called the existing
SV, keepSub, is overwritten with the new value using
SvSetSV.
sub PrintList
{
my(@list) = @_ ;
foreach (@list) { print "$_\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.
{
package Mine ;
sub new
{
my($type) = shift ;
bless [@_]
}
sub Display
{
my ($self, $index) = @_ ;
print "$index: $$self[$index]\n" ;
}
sub PrintID
{
my($class) = @_ ;
print "This is Class $class version 1.0\n" ;
}
}
It implements just a very simple class to manage an array. Apart from the
constructor, new, it declares methods, one static and one virtual. The static method, PrintID, prints out simply the class name and a version number. The virtual
method, Display, prints out a single element of the array. Here is an all Perl example of
using it.
$a = new Mine ('red', 'green', 'blue') ;
$a->Display(1) ;
PrintID Mine;
will print
1: green
This is Class Mine version 1.0
Calling a Perl method from C is fairly straightforward. The following things are required
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.
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
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 dispose of your temporaries, control must drop back to the enclosing scope at some stage. In the event driven scenario that may never happen. This means that as time goes on, your program will create more and more temporaries, none of which will ever be freed. As each of these temporaries consumes some memory your program will eventually consume all the available memory in your system - kapow!
So here is the bottom line - if you are sure that control will revert back to the enclosing Perl scope fairly quickly after the end of your callback, then it isn't absolutely necessary to dispose explicitly of any temporaries you may have created. Mind you, if you are at all uncertain about what to do, it doesn't do any harm to tidy up anyway.
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 one callback registered
at any time. An example could be an error handler like the code sketched
out above. Remember though, repeated calls to
register_fatal will replace the previously registered callback function with the new one.
Say for example you want to interface to a library which allows asynchronous file i/o. In this case you may be able to register a callback whenever a read operation has completed. To be of any use we want to be able to call separate Perl subroutines for each file that is opened. As it stands, the error handler example above would not be adequate as it allows only a single callback to be defined at any time. What we require is a means of storing the mapping between the opened file and the Perl subroutine we want to be called for that file.
Say the i/o library has a function asynch_read which associates a
C function ProcessRead with a file handle fh - this assumes that it has also provided some routine to open the file and
so obtain the file handle.
asynch_read(fh, ProcessRead)
This may expect the C ProcessRead function of this form
void
ProcessRead(fh, buffer)
int fh ;
char * buffer ;
{
...
}
To provide a Perl interface to this library we need to be able to map
between the fh parameter and the Perl subroutine we want called.
A hash is a convenient mechanism for storing this
mapping. The code below shows a possible implementation
static HV * Mapping = (HV*)NULL ;
void
asynch_read(fh, callback)
int fh
SV * callback
CODE:
/* If the hash doesn't already exist, create it */
if (Mapping == (HV*)NULL)
Mapping = newHV() ;
/* Save the fh -> callback mapping */
hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0) ;
/* Register with the C Library */
asynch_read(fh, asynch_read_if) ;
and asynch_read_if could look like this
static void
asynch_read_if(fh, buffer)
int fh ;
char * buffer ;
{
dSP ;
SV ** sv ;
/* Get the callback associated with fh */
sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE) ;
if (sv == (SV**)NULL)
croak("Internal error...\n") ;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSViv(fh))) ;
XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
PUTBACK ;
/* Call the Perl sub */
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
POP* macros to access values returned from Perl subroutines, it is also possible
to bypass these macros and read the stack using the ST macro (See the perlxs manpage for a full description of the ST macro).
Most of the time the POP* macros should be adequate, the main problem with them is that they force
you to process the returned values in sequence. This may not be the most
suitable way to process the values in some cases. What we want is to be
able to access the stack in a random order. The ST macro as used when coding an
XSUB is ideal for this purpose.
The code below is the example given in the section Returning a list
of values recoded to use ST instead of POP*.
static void
call_AddSubtract2(a, b)
int a ;
int b ;
{
dSP ;
I32 ax ;
int count ;
ENTER ;
SAVETMPS;
PUSHMARK(sp) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = 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
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.
SPAGAIN ;
sp -= count ;
ax = (sp - stack_base) + 1 ;
sets the stack up so that we can use the ST 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.