NAME

DESCRIPTION

Apart from discussing the C interface provided by Perl for writing callbacks the document uses a series of examples to show how the interface actually works in practice. In addition some techniques for coding callbacks are covered.

Examples where callbacks are necessary include

* An Error Handler

You have created an XSUB interface to an application's C API.

A fairly common feature in applications is to allow you to define a C function that will be called whenever something nasty occurs. What we would like is to be able to specify a Perl subroutine that will be called instead.

* An Event Driven Program

The classic example of where callbacks are used is when writing an event driven program like for an X windows application. In this case your register functions to be called whenever specific events occur, e.g. a mouse button is pressed, the cursor moves into a window or a menu item is selected.

Before you launch yourself head first into the rest of this document, it would be a good idea to have read the following two documents - the perlxs manpage and the perlguts manpage .

THE PERL_CALL FUNCTIONS

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.

perl_call_sv

perl_call_sv takes two parameters, the first, sv, is an SV*. This allows you to specify the Perl subroutine to be called either as a C string (which has first been converted to an SV) or a reference to a subroutine. The section, Using perl_call_sv , shows how you can make use of perl_call_sv .

perl_call_pv

The function, perl_call_pv , is similar to perl_call_sv except it expects its first parameter to be a C char* which identifies the Perl subroutine you want to call, e.g. perl_call_pv(``fred'', 0) . If the subroutine you want to call is in another package, just include the package name in the string, e.g. ``pkg::fred''.

perl_call_method

The function perl_call_method is used to call a method from a Perl class. The parameter methname corresponds to the name of the method to be called. Note that the class that the method belongs to is passed on the Perl stack rather than in the parameter list. This class can be either the name of the class (for a static method) or a reference to an object (for a virtual method). See the perlobj manpage for more information on static and virtual methods and Using perl_call_method for an example of using perl_call_method .

perl_call_argv

perl_call_argv calls the Perl subroutine specified by the C string stored in the subname parameter. It also takes the usual flags parameter. The final parameter, argv, consists of a NULL terminated list of C strings to be passed as parameters to the Perl subroutine. See Using perl_call_argv .

As a general rule you should always check the return value from these functions. Even if you are expecting only a particular number of values to be returned from the Perl subroutine, there is nothing to stop someone from doing something unexpected - don't say you haven't been warned.

FLAG VALUES

G_SCALAR

This flag has 2 effects

1. it indicates to the subroutine being called that it is executing in a scalar context (if it executes wantarray the result will be false).

2. it ensures that only a scalar is actually returned from the subroutine. The subroutine can, of course, ignore the wantarray and return a list anyway. If so, then only the last element of the list will be returned.

If 0, then you have specified the G_DISCARD flag.

If 1, then the item actually returned by the Perl subroutine will be stored on the Perl stack - the section Returning a Scalar shows how to access this value on the stack. Remember that regardless of how many items the Perl subroutine returns, only the last one will be accessible from the stack - think of the case where only one value is returned as being a list with only one element. Any other items that were returned will not exist by the time control returns from the perl_call_* function. The section I

G_ARRAY

As with G_SCALAR, this flag has 2 effects

1. it indicates to the subroutine being called that it is executing in an array context (if it executes wantarray the result will be true).

2. it ensures that all items returned from the subroutine will be accessible when control returns from the perl_call_* function.

If 0, the you have specified the G_DISCARD flag.

If not 0, then it will be a count of the number of items returned by the subroutine. These items will be stored on the Perl stack. The section Returning a list of values gives an example of using the G_ARRAY flag and the mechanics of accessing the returned items from the Perl stack.

G_DISCARD

If you do not set this flag then it is very important that you make sure that any temporaries (i.e. parameters passed to the Perl subroutine and values returned from the subroutine) are disposed of yourself. The section Returning a Scalar gives details of how to explicitly dispose of these temporaries and the section I

G_NOARGS

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

This will print

What has happened is that fred accesses the @_ array which belongs to joe.

G_EVAL

Whenever control returns from the perl_call_* function you need to check the $@ variable as you would in a normal Perl script.

The value returned from the perl_call_* function is dependent on what other flags have been specified and whether an error has occurred. Here are all the different cases that can occur

• If the perl_call_* function returns normally, then the value returned is as specified in the previous sections.

• If G_DISCARD is specified, the return value will always be 0.

• If G_ARRAY is specified and an error has occurred, the return value will always be 0.

• If G_SCALAR is specified and an error has occurred, the return value will be 1 and the value on the top of the stack will be undef . This means that if you have already detected the error by checking $@ and you want the program to continue, you must remember to pop the undef from the stack.

Determining the Context

KNOWN PROBLEMS

1. If you are intending to make use of both the G_EVAL and G_SCALAR flags in your code, use a version of Perl greater than 5.000. There is a bug in version 5.000 of Perl which means that the combination of these two flags will not work as described in the section FLAG VALUES .

   Specifically, if the two flags are used when calling a subroutine and that subroutine does not call die , the value returned by perl_call_* will be wrong.

2. In Perl 5.000 and 5.001 there is a problem with using perl_call_* if the Perl sub you are calling attempts to trap a die .

   The symptom of this problem is that the called Perl sub will continue to completion, but whenever it attempts to pass control back to the XSUB, the program will immediately terminate.

   For example, say you want to call this Perl sub

   sub fred { eval { die "Fatal Error" ; } print "Trapped error: $@\n" if $@ ; }

   via this XSUB

   void Call_fred() CODE: PUSHMARK(sp) ; perl_call_pv("fred", G_DISCARD|G_NOARGS) ; fprintf(stderr, "back in Call_fred\n") ;

   When Call_fred is executed it will print

   Trapped error: Fatal Error

   As control never returns to Call_fred, the ``back in Call_fred'' string will not get printed.

   To work around this problem, you can either upgrade to Perl 5.002 (or later), or use the G_EVAL flag with perl_call_* as shown below

   void Call_fred() CODE: PUSHMARK(sp) ; perl_call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ; fprintf(stderr, "back in Call_fred\n") ;

EXAMPLES

Perl provides many macros to assist in accessing the Perl stack. Wherever possible, these macros should always be used when interfacing to Perl internals. Hopefully this should make the code less vulnerable to any changes made to Perl in the future.

Another point worth noting is that in the first series of examples I have made use of only the perl_call_pv function. This has been done to keep the code simpler and ease you into the topic. Wherever possible, if the choice is between using perl_call_pv and perl_call_sv , you should always try to use perl_call_sv . See Using perl_call_sv for details.

No Parameters, Nothing returned

and here is a C function to call it

Simple, eh.

A few points to note about this example.

1. Ignore dSP and PUSHMARK(sp) for now. They will be discussed in the next example.

2. We aren't passing any parameters to PrintUID so G_NOARGS can be specified.

3. We aren't interested in anything returned from PrintUID, so G_DISCARD is specified. Even if PrintUID was changed to actually return some value(s), having specified G_DISCARD will mean that they will be wiped by the time control returns from perl_call_pv .

4. As perl_call_pv is being used, the Perl subroutine is specified as a C string. In this case the subroutine name has been 'hard-wired' into the code.

5. Because we specified G_DISCARD, it is not necessary to check the value returned from perl_call_pv . It will always be 0.

Passing Parameters

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.

1. Parameters are passed to the Perl subroutine using the Perl stack. This is the purpose of the code beginning with the line dSP and ending with the line PUTBACK.

2. If you are going to put something onto the Perl stack, you need to know where to put it. This is the purpose of the macro dSP - it declares and initializes a local copy of the Perl stack pointer.

   All the other macros which will be used in this example require you to have used this macro.

   The exception to this rule is if you are calling a Perl subroutine directly from an XSUB function. In this case it is not necessary to explicitly use the dSP macro - it will be declared for you automatically.

3. Any parameters to be pushed onto the stack should be bracketed by the PUSHMARK and PUTBACK macros. The purpose of these two macros, in this context, is to automatically count the number of parameters you are pushing. Then whenever Perl is creating the @_ array for the subroutine, it knows how big to make it.

   The PUSHMARK macro tells Perl to make a mental note of the current stack pointer. Even if you aren't passing any parameters (like the example shown in the section No Parameters, Nothing returned ) you must still call the PUSHMARK macro before you can call any of the perl_call_* functions - Perl still needs to know that there are no parameters.

   The PUTBACK macro sets the global copy of the stack pointer to be the same as our local copy. If we didn't do this perl_call_pv wouldn't know where the two parameters we pushed were - remember that up to now all the stack pointer manipulation we have done is with our local copy, not the global copy.

4. The only flag specified this time is G_DISCARD. Since we are passing 2 parameters to the Perl subroutine this time, we have not specified G_NOARGS.

5. Next, we come to XPUSHs. This is where the parameters actually get pushed onto the stack. In this case we are pushing a string and an integer.

   See the section ``XSUB'S and the Argument Stack'' for details on how the XPUSH macros work.

6. Finally, LeftString can now be called via the perl_call_pv function.

Returning a Scalar

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

1. The only flag specified this time was G_SCALAR. That means the @_ array will be created and that the value returned by Adder will still exist after the call to perl_call_pv .

2. Because we are interested in what is returned from Adder we cannot specify G_DISCARD. This means that we will have to tidy up the Perl stack and dispose of any temporary values ourselves. This is the purpose of
   ENTER ; SAVETMPS ;

   at the start of the function, and

   FREETMPS ; LEAVE ;

   at the end. The ENTER/SAVETMPS pair creates a boundary for any temporaries we create. This means that the temporaries we get rid of will be limited to those which were created after these calls.

   The FREETMPS/LEAVE pair will get rid of any values returned by the Perl subroutine, plus it will also dump the mortal SV's we have created. Having ENTER/SAVETMPS at the beginning of the code makes sure that no other mortals are destroyed.

   Think of these macros as working a bit like using { and } in Perl to limit the scope of local variables.

   See the section Using Perl to dispose of temporaries for details of an alternative to using these macros.

3. The purpose of the macro SPAGAIN is to refresh the local copy of the stack pointer. This is necessary because it is possible that the memory allocated to the Perl stack has been re-allocated whilst in the perl_call_pv call.

   If you are making use of the Perl stack pointer in your code you must always refresh the your local copy using SPAGAIN whenever you make use of the perl_call_* functions or any other Perl internal function.

4. Although only a single value was expected to be returned from Adder, it is still good practice to check the return code from perl_call_pv anyway.

   Expecting a single value is not quite the same as knowing that there will be one. If someone modified Adder to return a list and we didn't check for that possibility and take appropriate action the Perl stack would end up in an inconsistent state. That is something you really don't want to ever happen.

5. The POPi macro is used here to pop the return value from the stack. In this case we wanted an integer, so POPi was used.

   Here is the complete list of POP macros available, along with the types they return.

   • POPs SV
   • POPp pointer
   • POPn double
   • POPi integer
   • POPl long

6. The final PUTBACK is used to leave the Perl stack in a consistent state before exiting the function. This is necessary because when we popped the return value from the stack with POPi it updated only our local copy of the stack pointer. Remember, PUTBACK sets the global stack pointer to be the same as our local copy.

Returning a list of values

Here is the Perl subroutine

and this is the C function

If call_AddSubtract is called like this

then here is the output

Notes

1. We wanted array context, so G_ARRAY was used.

2. Not surprisingly POPi is used twice this time because we were retrieving 2 values from the stack. The important thing to note is that when using the POP* macros they come off the stack in reverse order.

Returning a list in a scalar context

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.

Returning Data from Perl via the parameter list

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 sva and svb.

The reason this is necessary is that the area of the Perl stack which held them will very likely have been overwritten by something else by the time control returns from perl_call_pv .

Using G_EVAL

and some C to call it

If call_Subtract is called thus

the following will be printed

Notes

1. We want to be able to catch the die so we have used the G_EVAL flag. Not specifying this flag would mean that the program would terminate immediately at the die statement in the subroutine Subtract.

2. The code
   sv = GvSV(gv_fetchpv("@", TRUE, SVt_PV)); if (SvTRUE(sv)) { printf ("Uh oh - %s\n", SvPVx(sv, na)) ; POPs ; }

   is the direct equivalent of this bit of Perl

   print "Uh oh - $@\n" if $@ ;

3. Note that the stack is popped using POPs in the block where SvTRUE(sv) is true. This is necessary because whenever a perl_call_* function invoked with G_EVAL|G_SCALAR returns an error, the top of the stack holds the value undef . Since we want the program to continue after detecting this error, it is essential that the stack is tidied up by removing the undef .

Using perl_call_sv

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 name parameter is now defined as an SV* and we use perl_call_sv instead of perl_call_pv .

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 (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

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

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

for each of the CallSavedSub1 lines.

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 $ref may have referred to the subroutine fred whenever the call to SaveSub1 was made but by the time CallSavedSub1 gets called it now holds the number 47. Since we saved only a pointer to the original SV in SaveSub1, any changes to $ref will be tracked by the pointer rememberSub. This means that whenever CallSavedSub1 gets called, it will attempt to execute the code which is referenced by the SV* rememberSub. In this case though, it now refers to the integer 47, so expect Perl to complain loudly.

A similar but more subtle problem is illustrated with this code

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

In order to avoid creating a new SV every time SaveSub2 is called, the function first checks to see if it has been called before. If not, then space for a new SV is allocated and the reference to the Perl subroutine, name is copied to the variable keepSub in one operation using newSVsv. Thereafter, whenever SaveSub2 is called the existing SV, keepSub, is overwritten with the new value using SvSetSV.

Using perl_call_argv

and here is an example of perl_call_argv which will call PrintList.

Note that it is not necessary to call PUSHMARK in this instance. This is because perl_call_argv will do it for you.

Using perl_call_method

It just implements a very simple class to manage an array. Apart from the constructor, new, it declares methods, one static and one virtual. The static method, PrintID, simply prints out the class name and a version number. The virtual method, Display, prints out a single element of the array. Here is an all Perl example of using it.

will print

Calling a Perl method from C is fairly straightforward. The following things are required

• a reference to the object for a virtual method or the name of the class for a static method.

• the name of the method.

• any other parameters specific to the method.

So the methods PrintID and Display can be invoked like this

The only thing to note is that in both the static and virtual methods, the method name is not passed via the stack - it is used as the first parameter to perl_call_method .

Using GIMME

and here is some Perl to test it

The output from that will be

Using Perl to dispose of temporaries

• specifying the G_DISCARD flag with perl_call_*.

• explicitly disposed of using the ENTER/SAVETMPS - FREETMPS/LEAVE pairing.

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.

Strategies for storing Callback Context Information

Potentially one of the trickiest problems to overcome when designing a callback interface can be figuring out how to store the mapping between the C callback function and the Perl equivalent.

To help understand why this can be a real problem first consider how a callback is set up in an all C environment. Typically a C API will provide a function to register a callback. This will expect a pointer to a function as one of its parameters. Below is a call to a hypothetical function register_fatal which registers the C function to get called when a fatal error occurs.

The single parameter cb1 is a pointer to a function, so you must have defined cb1 in your code, say something like this

Now change that to call a Perl subroutine instead

where the Perl equivalent of register_fatal and the callback it registers, pcb1, might look like this

The mapping between the C callback and the Perl equivalent is stored in the global variable callback.

This will be adequate if you ever need to have only 1 callback registered at any time. An example could be an error handler like the code sketched out above. Remember though, repeated calls to register_fatal will replace the previously registered callback function with the new one.

Say for example you want to interface to a library which allows asynchronous file i/o. In this case you may be able to register a callback whenever a read operation has completed. To be of any use we want to be able to call separate Perl subroutines for each file that is opened. As it stands, the error handler example above would not be adequate as it allows only a single callback to be defined at any time. What we require is a means of storing the mapping between the opened file and the Perl subroutine we want to be called for that file.

Say the i/o library has a function asynch_read which associates a C function ProcessRead with a file handle fh - this assumes that it has also provided some routine to open the file and so obtain the file handle.

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 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

and asynch_read_if could look like this

For completeness, here is asynch_close. This shows how to remove the entry from the hash Mapping.

So the Perl interface would look like this

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

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 fn1, fn2 and fn3 are used to remember the Perl subroutine to be called. Each of the functions holds a separate hard-wired index which is used in the function Pcb to access the Map array and actually call the Perl subroutine.

There are some obvious disadvantages with this technique.

Firstly, the code is considerably more complex than with the previous example.

Secondly, there is a hard-wired limit (in this case 3) to the number of callbacks that can exist simultaneously. The only way to increase the limit is by modifying the code to add more functions and then re-compiling. None the less, as long as the number of functions is chosen with some care, it is still a workable solution and in some cases is the only one available.

To summarize, here are a number of possible methods for you to consider for storing the mapping between C and the Perl callback

1. For a lot of situations, like interfacing to an error handler, this may be a perfectly adequate solution.

2. If it is impossible to tell from the parameters passed back from the C callback what the context is, then you may need to create a sequence of C callback interface functions, and store pointers to each in an array.

3. A hash is an ideal mechanism to store the mapping between C and Perl.

Alternate Stack Manipulation

Although I have made use of only the POP* macros to access values returned from Perl subroutines, it is also possible to bypass these macros and read the stack using the ST macro (See the perlxs manpage for a full description of the ST macro).

Most of the time the POP* macros should be adequate, the main problem with them is that they force you to process the returned values in sequence. This may not be the most suitable way to process the values in some cases. What we want is to be able to access the stack in a random order. The ST macro as used when coding an XSUB is ideal for this purpose.

The code below is the example given in the section IST instead of POP*.

Notes

1. Notice that it was necessary to define the variable ax. This is because the ST macro expects it to exist. If we were in an XSUB it would not be necessary to define ax as it is already defined for you.

2. The code
   SPAGAIN ; sp -= count ; ax = (sp - stack_base) + 1 ;

   sets the stack up so that we can use the ST macro.

3. Unlike the original coding of this example, the returned values are not accessed in reverse order. So ST(0) refers to the first value returned by the Perl subroutine and ST(count-1) refers to the last.

SEE ALSO

AUTHOR

Special thanks to the following people who assisted in the creation of the document.

Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem and Larry Wall.

DATE