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
The key function is perl_call_sv . All the other functions are fairly simple wrappers which make it easier to call Perl subroutines in special cases. At the end of the day they will all call perl_call_sv to actually invoke the Perl subroutine.
All the perl_call_* functions have a flags parameter which is
used to pass a bit mask of options to Perl. This bit mask operates
identically for each of the functions. The settings available in the
bit mask are discussed in
FLAG VALUES
.
Each of the functions will now be discussed in turn.
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 I As with G_SCALAR, this flag has 2 effects
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.
If you do not set this flag then it is very important that you make
sure that any temporaries (i.e. parameters passed to the Perl
subroutine and values returned from the subroutine) are disposed of
yourself. The section
Returning a Scalar
gives details of how to
explicitly dispose of these temporaries and the section I 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
This will print
What has happened is that
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
This scenario will mostly be applicable to code that is meant to be
called from within destructors, asynchronous callbacks, signal
handlers, 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
via this XSUB
When
As control never returns to 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
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.
and here is a C function to call it
Simple, eh.
A few points to note about this example.
So the Perl subroutine would look like this
The C function required to call LeftString would look like this.
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
The The
See the section ``XSUB'S and the Argument Stack'' for details
on how the XPUSH macros work.
Here is a Perl subroutine, Adder, which takes 2 integer parameters
and simply returns their sum.
Since we are now concerned with the return value from Adder, the C
function required to call it is now a bit more complex.
Points to note this time are
at the start of the function, and
at the end. The The Think of these macros as working a bit like using 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.
Here is the Perl subroutine
and this is the C function
If call_AddSubtract is called like this
then here is the output
Notes
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
then the output will be
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.
and here is a C function to call it.
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 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
.
and some C to call it
If call_Subtract is called thus
the following will be printed
Notes
is the direct equivalent of this bit of Perl
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 since
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:
will preserve the error and restore reliable error handling.
Consider the Perl code below
Here is a snippet of XSUB which defines CallSubPV.
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
Since we are using an SV to call fred the following can all be used
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 (
The reason this is wrong is that by the time you come to use the
pointer
By the time each of the
for each of the Similarly, with this code
you can expect one of these messages (which you actually get is dependant on
the version of Perl you are using)
The variable A similar but more subtle problem is illustrated with this code
This time whenever To get around these problems it is necessary to take a full copy of the
SV. The code below shows
In order to avoid creating a new SV every time
and here is an example of
perl_call_argv
which will call
PrintList.
Note that it is not necessary to call
It just implements a very simple class to manage an array. Apart from
the constructor,
will print
Calling a Perl method from C is fairly straightforward. The following
things are required
So the methods
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
.
and here is some Perl to test it
The output from that will be
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
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
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
In this case the flow of control can consist of only the repeated
sequence
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.
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
The single parameter
Now change that to call a Perl subroutine instead
where the Perl equivalent of
The mapping between the C callback and the Perl equivalent is stored in
the global variable 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
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
This may expect the C ProcessRead function of this form
To provide a Perl interface to this library we need to be able to map
between the
and
For completeness, here is
So the Perl interface would look like this
The mapping between the C callback and Perl is stored in the global
hash 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
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
In this case the functions 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
Although I have made use of only the Most of the time the The code below is the example given in the section I
Notes
sets the stack up so that we can use the
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.
G_ARRAY
Calls the Perl subroutine in a list context.
The value returned by the perl_call_* function indicates how may.items have been returned by the Perl subroutine.
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.
G_NOARGS
Whenever a Perl subroutine is called using one of the perl_call_*
functions, it is assumed by default that parameters are to be passed to
the subroutine. If you are not passing any parameters to the Perl
subroutine, you can save a bit of time by setting this flag. It has
the effect of not creating the @_ array for the Perl subroutine.
@_ 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
fred accesses the @_ array which
belongs to joe.
G_EVAL
It is possible for the Perl subroutine you are calling to terminate
abnormally, e.g. by calling
die
explicitly or by not actually
existing. By default, when either of these of events occurs, the
process will terminate immediately. If though, you want to trap this
type of event, specify the G_EVAL flag. It will put an
eval { }
around the subroutine call.
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
$@
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.
__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
$@
.
Determining the Context
As mentioned above, you can determine the context of the currently
executing subroutine in Perl with
wantarray
. The equivalent test can
be made in C by using the GIMME macro. This will return
G_SCALAR
if you have been called in a scalar context and
G_ARRAY
if in an
array context. An example of using the GIMME macro is shown in
section
Using GIMME
.
KNOWN PROBLEMS
This section outlines all known problems that exist in the
perl_call_* functions.
.
Call_fred is executed it will print
Call_fred, the ``back in Call_fred''
string will not get printed.
EXAMPLES
Enough of the definition talk, let's have a few examples.
No Parameters, Nothing returned
This first trivial example will call a Perl subroutine, PrintUID, to
print out the UID of the process.
.
dSP and PUSHMARK(sp) for now. They will be discussed in
the next example.
Passing Parameters
Now let's make a slightly more complex example. This time we want to
call a Perl subroutine, LeftString, which will take 2 parameters - a
string ($s) and an integer ($n). The subroutine will simply
print the first $n characters of the string.
.
dSP and
ending with the line PUTBACK.
dSP - it declares
and initializes a
local
copy of the Perl stack pointer.
dSP macro - it will be declared for you
automatically.
PUSHMARK and PUTBACK macros. The purpose of these two macros, in
this context, is to automatically count the number of parameters you
are pushing. Then whenever Perl is creating the @_ array for the
subroutine, it knows how big to make it.
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.
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.
Returning a Scalar
Now for an example of dealing with the items returned from a Perl
subroutine.
.
@_
array will be created and that the value returned by Adder will
still exist after the call to
perl_call_pv
.
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.
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.
{ and } in Perl
to limit the scope of local variables.
SPAGAIN is to refresh the local copy of the
stack pointer. This is necessary because it is possible that the memory
allocated to the Perl stack has been re-allocated whilst in the
perl_call_pv
call.
POPi macro is used here to pop the return value from the stack.
In this case we wanted an integer, so POPi was used.
PUTBACK is used to leave the Perl stack in a consistent
state before exiting the function. This is necessary because when we
popped the return value from the stack with POPi it updated only our
local copy of the stack pointer. Remember, PUTBACK sets the global
stack pointer to be the same as our local copy.
Returning a list of values
Now, let's extend the previous example to return both the sum of the
parameters and the difference.
.
POPi is used twice this time because we were
retrieving 2 values from the stack. The important thing to note is that
when using the POP* macros they come off the stack in
reverse
order.
Returning a list in a scalar context
Say the Perl subroutine in the previous section was called in a scalar
context, like this
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.
sva and svb.
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
.
.
errgv is a perl global of type GV * that points to the
symbol table entry containing the error. GvSV(errgv) therefore
refers to the C equivalent of
$@
.
POPs in the block where
SvTRUE(GvSV(errgv)) is true. This is necessary because whenever a
perl_call_* function invoked with G_EVAL|G_SCALAR returns an error,
the top of the stack holds the value
undef
. Since we want the
program to continue after detecting this error, it is essential that
the stack is tidied up by removing the
undef
.
Using G_KEEPERR
Consider this rather facetious example, where we have used an XS
version of the call_Subtract example above inside a destructor:
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.
name parameter is now defined as an SV* and we use
perl_call_sv
instead of
perl_call_pv
.
name in the
example above) which corresponds to the Perl subroutine so that it can
be used later in the program, it not enough to just store a copy of the
pointer to the SV. Say the code above had been like this
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 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
CallSavedSub1 lines.
$ref may have referred to the subroutine fred
whenever the call to SaveSub1 was made but by the time
CallSavedSub1 gets called it now holds the number 47. Since we
saved only a pointer to the original SV in SaveSub1, any changes to
$ref will be tracked by the pointer rememberSub. This means that
whenever CallSavedSub1 gets called, it will attempt to execute the
code which is referenced by the SV* rememberSub. In this case
though, it now refers to the integer 47, so expect Perl to complain
loudly.
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.
SaveSub2 modified to do that
SaveSub2 is called,
the function first checks to see if it has been called before. If not,
then space for a new SV is allocated and the reference to the Perl
subroutine, name is copied to the variable keepSub in one
operation using newSVsv. Thereafter, whenever SaveSub2 is called
the existing SV, keepSub, is overwritten with the new value using
SvSetSV.
Using perl_call_argv
Here is a Perl subroutine which prints whatever parameters are passed
to it.
PUSHMARK in this instance.
This is because
perl_call_argv
will do it for you.
Using perl_call_method
Consider the following Perl code
new, it declares methods, one static and one
virtual. The static method, PrintID, simply prints out the class
name and a version number. The virtual method, Display, prints out a
single element of the array. Here is an all Perl example of using it.
Here is a simple XSUB which illustrates the mechanics of calling both.the PrintID and Display methods from C.
PrintID and Display can be invoked like this
Using GIMME
Here is a trivial XSUB which prints the context in which it is
currently executing.
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 pairing.
Strategies for storing Callback Context Information
register_fatal which registers the C function
to get called when a fatal error occurs.
cb1 is a pointer to a function, so you must
have defined cb1 in your code, say something like this
register_fatal and the callback it
registers, pcb1, might look like this
callback.
register_fatal will replace the previously registered callback
function with the new one.
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.
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
asynch_read_if could look like this
asynch_close. This shows how to remove
the entry from the hash Mapping.
Mapping this time. Using a hash has the distinct advantage that
it allows an unlimited number of callbacks to be registered.
buffer parameter like this
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.
.
Alternate Stack Manipulation
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).
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.
POP*.
.
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.
ST macro.
ST(0) refers to the
first value returned by the Perl subroutine and ST(count-1)
refers to the last.
SEE ALSO
the perlxs manpage
,
the perlguts manpage
,
the perlembed manpage
AUTHOR
Paul Marquess <pmarquess@bfsec.bt.co.uk>
DATE
Version 1.2, 16th Jan 1996