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
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.
The classic example of where callbacks are used is when writing an event driven program, such as for an X11 application. In this case you register functions to be called whenever specific events occur, e.g., a mouse button is pressed, the cursor moves into a window or a menu item is selected.
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 perlembed.
Before you launch yourself head first into the rest of this document, it would be a good idea to have read the following two documents---perlxs and perlguts.
Perl has a number of C functions that allow you to call Perl subroutines. They are
I32 call_sv(SV* sv, I32 flags);
I32 call_pv(char *subname, I32 flags);
I32 call_method(char *methname, I32 flags);
I32 call_argv(char *subname, I32 flags, char **argv);
The key function is 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 call_sv to invoke the Perl subroutine.
All the 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.
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.
This flag has 2 effects:
The value returned by the call_* function indicates how many items have been returned by the Perl subroutine---in this case it will be 0.
This flag has 2 effects:
The value returned by the call_* function indicates how many items have been returned by the Perl subroutine - in this case it will be either 0 or 1.
If 0, then you have specified the G_DISCARD flag.
If 1, then the item actually returned by the Perl subroutine will be stored on the Perl stack - the section ``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 call_* function. The section ``Returning a List in Scalar Context'' shows an example of this behavior.
As with G_SCALAR, this flag has 2 effects:
The value returned by the call_* function indicates how many items have been returned by the Perl subroutine.
If 0, then you have specified the G_DISCARD flag.
If not 0, then it will be a count of the number of items returned by the subroutine. These items will be stored on the Perl stack. The section ``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.
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 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 call_* function you need to check the $@ variable as you would in a normal Perl script.
The value returned from the 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 on using G_EVAL.
This scenario will mostly be applicable to code that is meant to be called from within destructors, asynchronous callbacks, and signal handlers. In such situations, where the code being called has little relation to the surrounding dynamic context, the main program needs to be insulated from errors in the called code, even if they can't be handled intelligently. It may also be useful to do this with code for "__DIE__" or "__WARN__" hooks, and "tie" functions.
The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in call_* functions that are used to implement such code, or with "eval_sv". This flag has no effect on the "call_*" functions when G_EVAL is not used.
When G_KEEPERR is used, any error in the called code will terminate the call as usual, and the error will not propagate beyond the call (as usual for G_EVAL), but it will not go into $@. Instead the error will be converted into a warning, prefixed with the string ``\t(in cleanup)''. This can be disabled using "no warnings 'misc'". If there is no error, $@ will not be cleared.
Note that the G_KEEPERR flag does not propagate into inner evals; these may still set $@.
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.
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 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 call_pv and call_sv, you should always try to use call_sv. See ``Using call_sv'' for details.
sub PrintUID
{
print "UID is $<\n";
}
and here is a C function to call it
static void
call_PrintUID()
{
dSP;
PUSHMARK(SP);
call_pv("PrintUID", G_DISCARD|G_NOARGS);
}
Simple, eh?
A few points to note about this example:
So the Perl subroutine would look like this:
sub LeftString
{
my($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;
ENTER;
SAVETMPS;
PUSHMARK(SP);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSVpv(a, 0)));
PUSHs(sv_2mortal(newSViv(b)));
PUTBACK;
call_pv("LeftString", G_DISCARD);
FREETMPS;
LEAVE;
}
Here are a few notes on the C function 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.
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 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, 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.
Alternatively you can use the XPUSHs() macro, which combines a "EXTEND(SP, 1)" and "PUSHs()". This is less efficient if you're pushing multiple values.
See ``XSUBs and the Argument Stack'' in perlguts for details on how the PUSH macros work.
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 (see next example), plus it will also dump the mortal SVs 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 "{" 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.
Here is a Perl subroutine, Adder, that takes 2 integer parameters and simply returns their sum.
sub Adder
{
my($a, $b) = @_;
$a + $b;
}
Because we are now concerned with the return value from Adder, the C function required to call it is now a bit more complex.
static void
call_Adder(a, b)
int a;
int b;
{
dSP;
int count;
ENTER;
SAVETMPS;
PUSHMARK(SP);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(a)));
PUSHs(sv_2mortal(newSViv(b)));
PUTBACK;
count = call_pv("Adder", G_SCALAR);
SPAGAIN;
if (count != 1)
croak("Big trouble\n");
printf ("The sum of %d and %d is %d\n", a, b, POPi);
PUTBACK;
FREETMPS;
LEAVE;
}
Points to note this time are
If you are making use of the Perl stack pointer in your code you must always refresh the local copy using SPAGAIN whenever you make use of the call_* functions or any other Perl internal function.
Expecting a single value is not quite the same as knowing that there will be one. If someone modified Adder to return a list and we didn't check for that possibility and take appropriate action the Perl stack would end up in an inconsistent state. That is something you really don't want to happen ever.
Here is the complete list of POP macros available, along with the types they return.
POPs SV
POPp pointer (PV)
POPpbytex pointer to bytes (PV)
POPn double (NV)
POPi integer (IV)
POPu unsigned integer (UV)
POPl long
POPul unsigned long
Since these macros have side-effects don't use them as arguments to macros that may evaluate their argument several times, for example:
/* Bad idea, don't do this */ STRLEN len; const char *s = SvPV(POPs, len);
Instead, use a temporary:
STRLEN len; SV *sv = POPs; const char *s = SvPV(sv, len);
or a macro that guarantees it will evaluate its arguments only once:
STRLEN len; const char *s = SvPVx(POPs, len);
Here is the Perl subroutine
sub AddSubtract
{
my($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);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(a)));
PUSHs(sv_2mortal(newSViv(b)));
PUTBACK;
count = 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
static void
call_AddSubScalar(a, b)
int a;
int b;
{
dSP;
int count;
int i;
ENTER;
SAVETMPS;
PUSHMARK(SP);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(a)));
PUSHs(sv_2mortal(newSViv(b)));
PUTBACK;
count = 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 is returned from the subroutine. AddSubtract actually made it back to call_AddSubScalar.
The Perl subroutine, Inc, below takes 2 parameters and increments each directly.
sub Inc
{
++ $_[0];
++ $_[1];
}
and here is a C function to call it.
static void
call_Inc(a, b)
int a;
int b;
{
dSP;
int count;
SV * sva;
SV * svb;
ENTER;
SAVETMPS;
sva = sv_2mortal(newSViv(a));
svb = sv_2mortal(newSViv(b));
PUSHMARK(SP);
EXTEND(SP, 2);
PUSHs(sva);
PUSHs(svb);
PUTBACK;
count = call_pv("Inc", G_DISCARD);
if (count != 0)
croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
count);
printf ("%d + 1 = %d\n", a, SvIV(sva));
printf ("%d + 1 = %d\n", b, SvIV(svb));
FREETMPS;
LEAVE;
}
To be able to access the two parameters that were pushed onto the stack after they return from call_pv it is necessary to make a note of their addresses---thus the two variables "sva" and "svb".
The reason this is necessary is that the area of the Perl stack which held them will very likely have been overwritten by something else by the time control returns from call_pv.
sub Subtract
{
my ($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;
SV *err_tmp;
ENTER;
SAVETMPS;
PUSHMARK(SP);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(a)));
PUSHs(sv_2mortal(newSViv(b)));
PUTBACK;
count = call_pv("Subtract", G_EVAL|G_SCALAR);
SPAGAIN;
/* Check the eval first */
err_tmp = ERRSV;
if (SvTRUE(err_tmp))
{
printf ("Uh oh - %s\n", SvPV_nolen(err_tmp));
POPs;
}
else
{
if (count != 1)
croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",
count);
printf ("%d - %d = %d\n", a, b, POPi);
}
PUTBACK;
FREETMPS;
LEAVE;
}
If call_Subtract is called thus
call_Subtract(4, 5)
the following will be printed
Uh oh - death can be fatal
Notes
err_tmp = ERRSV;
if (SvTRUE(err_tmp))
{
printf ("Uh oh - %s\n", SvPV_nolen(err_tmp));
POPs;
}
is the direct equivalent of this bit of Perl
print "Uh oh - $@\n" if $@;
"PL_errgv" is a perl global of type "GV *" that points to the symbol table entry containing the error. "ERRSV" therefore refers to the C equivalent of $@. We use a local temporary, "err_tmp", since "ERRSV" is a macro that calls a function, and "SvTRUE(ERRSV)" would end up calling that function multiple times.
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;
{
my $foo = Foo->new;
eval { $foo->foo };
}
print "Saw: $@" if $@; # should be, but isn't
This example will fail to recognize that an error occurred inside the "eval {}". Here's why: the call_Subtract code got executed while perl was cleaning up temporaries when exiting the outer braced block, and because call_Subtract is implemented with 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 call_pv call in call_Subtract reads:
count = 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);
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, however, Perl allows references to subroutines and anonymous subroutines. This is where 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 call_sv instead of call_pv.
void
CallSubSV(name)
SV * name
CODE:
PUSHMARK(SP);
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, 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);
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 has 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);
$ref = \&joe;
CallSavedSub1();
This time whenever "CallSavedSub1" gets called it will execute the Perl subroutine "joe" (assuming it exists) rather than "fred" as was originally requested in the call to "SaveSub1".
To get around these problems it is necessary to take a full copy of the SV. The code below shows "SaveSub2" modified to do that.
/* this isn't thread-safe */
static SV * keepSub = (SV*)NULL;
void
SaveSub2(name)
SV * name
CODE:
/* Take a copy of the callback */
if (keepSub == (SV*)NULL)
/* First time, so create a new SV */
keepSub = newSVsv(name);
else
/* Been here before, so overwrite */
SvSetSV(keepSub, name);
void
CallSavedSub2()
CODE:
PUSHMARK(SP);
call_sv(keepSub, G_DISCARD|G_NOARGS);
To avoid creating a new SV every time "SaveSub2" is called, the function first checks to see if it has been called before. If not, then space for a new SV is allocated and the reference to the Perl subroutine "name" is copied to the variable "keepSub" in one operation using "newSVsv". Thereafter, whenever "SaveSub2" is called, the existing SV, "keepSub", is overwritten with the new value using "SvSetSV".
Note: using a static or global variable to store the SV isn't thread-safe. You can either use the "MY_CXT" mechanism documented in ``Safely Storing Static Data in XS'' in perlxs which is fast, or store the values in perl global variables, using get_sv(), which is much slower.
sub PrintList
{
my(@list) = @_;
foreach (@list) { print "$_\n" }
}
And here is an example of call_argv which will call PrintList.
static char * words[] = {"alpha", "beta", "gamma", "delta", NULL};
static void
call_PrintList()
{
call_argv("PrintList", G_DISCARD, words);
}
Note that it is not necessary to call "PUSHMARK" in this instance. This is because 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 = Mine->new('red', 'green', 'blue');
$a->Display(1);
Mine->PrintID;
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 "PrintID" and "Display" methods from C.
void
call_Method(ref, method, index)
SV * ref
char * method
int index
CODE:
PUSHMARK(SP);
EXTEND(SP, 2);
PUSHs(ref);
PUSHs(sv_2mortal(newSViv(index)));
PUTBACK;
call_method(method, G_DISCARD);
void
call_PrintID(class, method)
char * class
char * method
CODE:
PUSHMARK(SP);
XPUSHs(sv_2mortal(newSVpv(class, 0)));
PUTBACK;
call_method(method, G_DISCARD);
So the methods "PrintID" and "Display" can be invoked like this:
$a = Mine->new('red', 'green', 'blue');
call_Method($a, 'Display', 1);
call_PrintID('Mine', 'PrintID');
The only thing to note is that, in both the static and virtual methods, the method name is not passed via the stack---it is used as the first parameter to call_method.
void
PrintContext()
CODE:
U8 gimme = GIMME_V;
if (gimme == G_VOID)
printf ("Context is Void\n");
else if (gimme == G_SCALAR)
printf ("Context is Scalar\n");
else
printf ("Context is Array\n");
And here is some Perl to test it.
PrintContext;
$a = PrintContext;
@a = PrintContext;
The output from that will be
Context is Void
Context is Scalar
Context is Array
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 --> call_* --> perl
|
perl <-- XSUB <-- external library <-- call_* <----+
After processing of the error using 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 --> call_* --> perl
|
event handler <-- call_* <----+
...
event handler --> call_* --> perl
|
event handler <-- call_* <----+
...
event handler --> call_* --> perl
|
event handler <-- call_* <----+
In this case the flow of control can consist of only the repeated sequence
event handler --> call_* --> perl
for 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 */
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);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(fh)));
PUSHs(sv_2mortal(newSVpv(buffer, 0)));
PUTBACK;
/* Call the Perl sub */
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 predefine 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 */
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 recompiling. None the less, as long as the number of functions is chosen with some care, it is still a workable solution and in some cases is the only one available.
To summarize, here are a number of possible methods for you to consider for storing the mapping between C and the Perl callback
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);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(a)));
PUSHs(sv_2mortal(newSViv(b)));
PUTBACK;
count = call_pv("AddSubtract", G_ARRAY);
SPAGAIN;
SP -= count;
ax = (SP - PL_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 - PL_stack_base) + 1;
sets the stack up so that we can use the "ST" macro.
...
SV *cvrv
= eval_pv("sub {
print 'You will not find me cluttering any namespace!'
}", TRUE);
...
call_sv(cvrv, G_VOID|G_NOARGS);
"eval_pv" is used to compile the anonymous subroutine, which will be the return value as well (read more about "eval_pv" in ``eval_pv'' in perlapi). Once this code reference is in hand, it can be mixed in with all the previous examples we've shown.
In this case it is possible to speed up the routine (often quite substantially) by using the lightweight callback API. The idea is that the calling context only needs to be created and destroyed once, and the sub can be called arbitrarily many times in between.
It is usual to pass parameters using global variables (typically $_ for one parameter, or $a and $b for two parameters) rather than via @_. (It is possible to use the @_ mechanism if you know what you're doing, though there is as yet no supported API for it. It's also inherently slower.)
The pattern of macro calls is like this:
dMULTICALL; /* Declare local variables */
U8 gimme = G_SCALAR; /* context of the call: G_SCALAR,
* G_ARRAY, or G_VOID */
PUSH_MULTICALL(cv); /* Set up the context for calling cv,
and set local vars appropriately */
/* loop */ {
/* set the value(s) af your parameter variables */
MULTICALL; /* Make the actual call */
} /* end of loop */
POP_MULTICALL; /* Tear down the calling context */
For some concrete examples, see the implementation of the first() and reduce() functions of List::Util 1.18. There you will also find a header file that emulates the multicall API on older versions of perl.
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.