MAN page from Old RedHat 5.X perl-5.004-4.i386.rpm

PERLCALL

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index

NAME

perlcall - Perl calling conventions from C

DESCRIPTION

The purpose of this document is to show you how to call Perl subroutinesdirectly from C, i.e., how to write callbacks.

Apart from discussing the C interface provided by Perl for writingcallbacks the document uses a series of examples to show how theinterface actually works in practice. In addition some techniques forcoding 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 Cfunction that will be called whenever something nasty occurs. What wewould like is to be able to specify a Perl subroutine that will becalled instead.
* An Event Driven Program: The classic example of where callbacks are used is when writing anevent driven program like for an X windows application. In this caseyou register functions to be called whenever specific events occur,e.g., a mouse button is pressed, the cursor moves into a window or amenu item is selected.

Although the techniques described here are applicable when embeddingPerl in a C program, this is not the primary goal of this document.There are other details that must be considered and are specific toembedding Perl. For details on embedding Perl in C refer tothe perlembed manpage.

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

THE PERL_CALL FUNCTIONS

Although this stuff is easier to explain using examples, you first needbe aware of a few important definitions.

Perl has a number of C functions that allow you to call Perlsubroutines. They are

    I32 perl_call_sv(SV* sv, I32 flags) ;    I32 perl_call_pv(char *subname, I32 flags) ;    I32 perl_call_method(char *methname, I32 flags) ;    I32 perl_call_argv(char *subname, I32 flags, register char **argv) ;

The key function is perl_call_sv. All the other functions arefairly simple wrappers which make it easier to call Perl subroutines inspecial cases. At the end of the day they will all call perl_call_svto invoke the Perl subroutine.

All the perl_call_* functions have a flags parameter which isused to pass a bit mask of options to Perl. This bit mask operatesidentically for each of the functions. The settings available in thebit mask are discussed in the section on 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 aC string (which has first been converted to an SV) or a reference to asubroutine. The section, Using perl_call_sv, shows how you can makeuse of perl_call_sv.
perl_call_pv: The function, perl_call_pv, is similar to perl_call_sv except itexpects its first parameter to be a C char* which identifies the Perlsubroutine you want to call, e.g., perl_call_pv("fred", 0). If thesubroutine you want to call is in another package, just include thepackage name in the string, e.g., "pkg::fred".
perl_call_method: The function perl_call_method is used to call a method from a Perlclass. The parameter methname corresponds to the name of the methodto be called. Note that the class that the method belongs to is passedon the Perl stack rather than in the parameter list. This class can beeither the name of the class (for a static method) or a reference to anobject (for a virtual method). See the perlobj manpage for more information onstatic and virtual methods and the section on Using perl_call_method for an exampleof using perl_call_method.
perl_call_argv: perl_call_argv calls the Perl subroutine specified by the C stringstored in the subname parameter. It also takes the usual flagsparameter. The final parameter, argv, consists of a NULL terminatedlist of C strings to be passed as parameters to the Perl subroutine.See Using perl_call_argv.

All the functions return an integer. This is a count of the number ofitems returned by the Perl subroutine. The actual items returned by thesubroutine are stored on the Perl stack.

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

FLAG VALUES

The flags parameter in all the perl_call_* functions is a bit maskwhich can consist of any combination of the symbols defined below,OR'ed together.

G_VOID

Calls the Perl subroutine in a void context.

This flag has 2 effects:

1.: It indicates to the subroutine being called that it is executing ina void context (if it executes wantarray the result will be theundefined value).
2.: It ensures that nothing is actually returned from the subroutine.

The value returned by the perl_call_* function indicates how manyitems have been returned by the Perl subroutine - in this case it willbe 0.

G_SCALAR

Calls the Perl subroutine in a scalar context. This is the defaultcontext flag setting for all the perl_call_* functions.

This flag has 2 effects:

1.: It indicates to the subroutine being called that it is executing in ascalar 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 alist anyway. If so, then only the last element of the list will bereturned.

The value returned by the perl_call_* function indicates how manyitems have been returned by the Perl subroutine - in this case it willbe 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 bestored on the Perl stack - the section Returning a Scalar shows howto access this value on the stack. Remember that regardless of howmany items the Perl subroutine returns, only the last one will beaccessible from the stack - think of the case where only one value isreturned as being a list with only one element. Any other items thatwere returned will not exist by the time control returns from theperl_call_* function. The section Returning a list in a scalarcontext shows an example of this behavior.

G_ARRAY

Calls the Perl subroutine in a list context.

As with G_SCALAR, this flag has 2 effects:

1.: It indicates to the subroutine being called that it is executing in anarray context (if it executes wantarray the result will be true).
2.: It ensures that all items returned from the subroutine will beaccessible when control returns from the perl_call_* function.

The value returned by the perl_call_* function indicates how manyitems 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 bythe subroutine. These items will be stored on the Perl stack. Thesection Returning a list of values gives an example of using theG_ARRAY flag and the mechanics of accessing the returned items from thePerl stack.

G_DISCARD

By default, the perl_call_* functions place the items returned fromby the Perl subroutine on the stack. If you are not interested inthese items, then setting this flag will make Perl get rid of themautomatically for you. Note that it is still possible to indicate acontext to the Perl subroutine by using either G_SCALAR or G_ARRAY.

If you do not set this flag then it is very important that you makesure that any temporaries (i.e., parameters passed to the Perlsubroutine and values returned from the subroutine) are disposed ofyourself. The section Returning a Scalar gives details of how todispose of these temporaries explicitly and the section Using Perl todispose of temporaries discusses the specific circumstances where youcan ignore the problem and let Perl deal with it for you.

G_NOARGS

Whenever a Perl subroutine is called using one of the perl_call_*functions, it is assumed by default that parameters are to be passed tothe subroutine. If you are not passing any parameters to the Perlsubroutine, you can save a bit of time by setting this flag. It hasthe effect of not creating the @_ array for the Perl subroutine.

Although the functionality provided by this flag may seemstraightforward, it should be used only if there is a good reason to doso. The reason for being cautious is that even if you have specifiedthe G_NOARGS flag, it is still possible for the Perl subroutine thathas been called to think that you have passed it parameters.

In fact, what can happen is that the Perl subroutine you have calledcan access the @_ array from a previous Perl subroutine. This willoccur when the code that is executing the perl_call_* function hasitself been called from another Perl subroutine. The code belowillustrates 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 whichbelongs to joe.

G_EVAL

It is possible for the Perl subroutine you are calling to terminateabnormally, e.g., by calling die explicitly or by not actuallyexisting. By default, when either of these of events occurs, theprocess will terminate immediately. If though, you want to trap thistype of event, specify the G_EVAL flag. It will put an eval { }around the subroutine call.

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

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

*: If the perl_call_* function returns normally, then the valuereturned 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 valuewill always be 0.
*: If G_SCALAR is specified and an error has occurred, the return valuewill be 1 and the value on the top of the stack will be undef. Thismeans that if you have already detected the error by checking $@ andyou want the program to continue, you must remember to pop the undeffrom the stack.

See Using G_EVAL for details on using G_EVAL.

G_KEEPERR

You may have noticed that using the G_EVAL flag described above willalways clear the $@ variable and set it to a string describingthe error iff there was an error in the called code. This unqualifiedresetting of $@ can be problematic in the reliable identification oferrors using the eval {} mechanism, because the possibility existsthat perl will call other code (end of block processing code, forexample) between the time the error causes $@ to be set withineval {}, and the subsequent statement which checks for the value of$@ gets executed in the user's script.

This scenario will mostly be applicable to code that is meant to becalled from within destructors, asynchronous callbacks, signalhandlers, __DIE__ or __WARN__ hooks, and tie functions. Insuch situations, you will not want to clear $@ at all, but simply toappend any new errors to any existing value of $@.

The G_KEEPERR flag is meant to be used in conjunction with G_EVAL inperl_call_* functions that are used to implement such code. This flaghas no effect when G_EVAL is not used.

When G_KEEPERR is used, any errors in the called code will be prefixedwith the string ``\t(in cleanup)'', and appended to the current valueof $@.

The G_KEEPERR flag was introduced in Perl version 5.002.

See Using G_KEEPERR for an example of a situation that warrants theuse of this flag.

Determining the Context

As mentioned above, you can determine the context of the currentlyexecuting subroutine in Perl with wantarray. The equivalent testcan be made in C by using the GIMME_V macro, which returnsG_ARRAY if you have been called in an array context, G_SCALAR ifin a scalar context, or G_VOID if in a void context (i.e. thereturn value will not be used). An older version of this macro iscalled GIMME; in a void context it returns G_SCALAR instead ofG_VOID. An example of using the GIMME_V macro is shown insection Using GIMME_V.

KNOWN PROBLEMS

This section outlines all known problems that exist in theperl_call_* functions.

1.

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

Specifically, if the two flags are used when calling a subroutine andthat subroutine does not call die, the value returned byperl_call_* will be wrong.

2.

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

The symptom of this problem is that the called Perl sub will continueto completion, but whenever it attempts to pass control back to theXSUB, 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 orhigher, 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

Enough of the definition talk, let's have a few examples.

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

Another point worth noting is that in the first series of examples Ihave made use of only the perl_call_pv function. This has been doneto keep the code simpler and ease you into the topic. Whereverpossible, if the choice is between using perl_call_pv andperl_call_sv, you should always try to use perl_call_sv. SeeUsing perl_call_sv for details.

Parameters, Nothing returned

This first trivial example will call a Perl subroutine, PrintUID, toprint out the UID of the process.

    sub PrintUID    {        print "UID is $<\n" ;    }

and here is a C function to call it

    static void    call_PrintUID()    {        dSP ;

        PUSHMARK(sp) ;        perl_call_pv("PrintUID", G_DISCARD|G_NOARGS) ;    }

Simple, eh.

A few points to note about this example.

1.: Ignore dSP and PUSHMARK(sp) for now. They will be discussed inthe next example.
2.: We aren't passing any parameters to PrintUID so G_NOARGS can bespecified.
3.: We aren't interested in anything returned from PrintUID, soG_DISCARD is specified. Even if PrintUID was changed toreturn some value(s), having specified G_DISCARD will mean that theywill 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 aC string. In this case the subroutine name has been `hard-wired' into thecode.
5.: Because we specified G_DISCARD, it is not necessary to check the valuereturned from perl_call_pv. It will always be 0.

Passing Parameters

Now let's make a slightly more complex example. This time we want tocall a Perl subroutine, LeftString, which will take 2 parameters - astring ($s) and an integer ($n). The subroutine will simplyprint the first $n characters of the string.

So the Perl subroutine would look like this

    sub LeftString    {        my($s, $n) = @_ ;        print substr($s, 0, $n), "\n" ;    }

The C function required to call LeftString would look like this.

    static void    call_LeftString(a, b)    char * a ;    int b ;    {        dSP ;

        PUSHMARK(sp) ;        XPUSHs(sv_2mortal(newSVpv(a, 0)));        XPUSHs(sv_2mortal(newSViv(b)));        PUTBACK ;

        perl_call_pv("LeftString", G_DISCARD);    }

Here are a few notes on the C function call_LeftString.

1.

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

2.

If you are going to put something onto the Perl stack, you need to knowwhere to put it. This is the purpose of the macro dSP - it declaresand initializes a local copy of the Perl stack pointer.

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

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

3.

Any parameters to be pushed onto the stack should be bracketed by thePUSHMARK and PUTBACK macros. The purpose of these two macros, inthis context, is to count the number of parameters you arepushing automatically. Then whenever Perl is creating the @_ array for thesubroutine, it knows how big to make it.

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

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

4.

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

5.

Next, we come to XPUSHs. This is where the parameters actually getpushed onto the stack. In this case we are pushing a string and aninteger.

See the section on XSUBs and the Argument Stack in the perlguts manpage for detailson how the XPUSH macros work.

6.

Finally, LeftString can now be called via the perl_call_pvfunction.

Returning a Scalar

Now for an example of dealing with the items returned from a Perlsubroutine.

Here is a Perl subroutine, Adder, that takes 2 integer parametersand simply returns their sum.

    sub Adder    {        my($a, $b) = @_ ;        $a + $b ;    }

Because we are now concerned with the return value from Adder, the Cfunction required to call it is now a bit more complex.

    static void    call_Adder(a, b)    int a ;    int b ;    {        dSP ;        int count ;

        ENTER ;        SAVETMPS;

        PUSHMARK(sp) ;        XPUSHs(sv_2mortal(newSViv(a)));        XPUSHs(sv_2mortal(newSViv(b)));        PUTBACK ;

        count = perl_call_pv("Adder", G_SCALAR);

        SPAGAIN ;

        if (count != 1)            croak("Big trouble\n") ;

        printf ("The sum of %d and %d is %d\n", a, b, POPi) ;

        PUTBACK ;        FREETMPS ;        LEAVE ;    }

Points to note this time are

1.

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

2.

Because we are interested in what is returned from Adder we cannotspecify G_DISCARD. This means that we will have to tidy up the Perlstack and dispose of any temporary values ourselves. This is thepurpose of

    ENTER ;    SAVETMPS ;

at the start of the function, and

    FREETMPS ;    LEAVE ;

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

The FREETMPS/LEAVE pair will get rid of any values returned bythe Perl subroutine, plus it will also dump the mortal SVs we havecreated. Having ENTER/SAVETMPS at the beginning of the codemakes sure that no other mortals are destroyed.

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

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

3.

The purpose of the macro SPAGAIN is to refresh the local copy of thestack pointer. This is necessary because it is possible that the memoryallocated to the Perl stack has been reallocated whilst in theperl_call_pv call.

If you are making use of the Perl stack pointer in your code you mustalways refresh the local copy using SPAGAIN whenever you make useof 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_pvanyway.

Expecting a single value is not quite the same as knowing that therewill be one. If someone modified Adder to return a list and wedidn't check for that possibility and take appropriate action the Perlstack would end up in an inconsistent state. That is something youreally don't want to happen ever.

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 typesthey return.

    POPs        SV    POPp        pointer    POPn        double    POPi        integer    POPl        long

6.

The final PUTBACK is used to leave the Perl stack in a consistentstate before exiting the function. This is necessary because when wepopped the return value from the stack with POPi it updated only ourlocal copy of the stack pointer. Remember, PUTBACK sets the globalstack 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 theparameters and the difference.

Here is the Perl subroutine

    sub AddSubtract    {       my($a, $b) = @_ ;       ($a+$b, $a-$b) ;    }

and this is the C function

    static void    call_AddSubtract(a, b)    int a ;    int b ;    {        dSP ;        int count ;

        ENTER ;        SAVETMPS;

        PUSHMARK(sp) ;        XPUSHs(sv_2mortal(newSViv(a)));        XPUSHs(sv_2mortal(newSViv(b)));        PUTBACK ;

        count = perl_call_pv("AddSubtract", G_ARRAY);

        SPAGAIN ;

        if (count != 2)            croak("Big trouble\n") ;

        printf ("%d - %d = %d\n", a, b, POPi) ;        printf ("%d + %d = %d\n", a, b, POPi) ;

        PUTBACK ;        FREETMPS ;        LEAVE ;    }

If call_AddSubtract is called like this

    call_AddSubtract(7, 4) ;

then here is the output

    7 - 4 = 3    7 + 4 = 11

Notes

1.: We wanted array context, so G_ARRAY was used.
2.: Not surprisingly POPi is used twice this time because we wereretrieving 2 values from the stack. The important thing to note is thatwhen using the POP* macros they come off the stack in reverseorder.

Returning a list in a scalar context

Say the Perl subroutine in the previous section was called in a scalarcontext, like this

    static void    call_AddSubScalar(a, b)    int a ;    int b ;    {        dSP ;        int count ;        int i ;

        ENTER ;        SAVETMPS;

        PUSHMARK(sp) ;        XPUSHs(sv_2mortal(newSViv(a)));        XPUSHs(sv_2mortal(newSViv(b)));        PUTBACK ;

        count = perl_call_pv("AddSubtract", G_SCALAR);

        SPAGAIN ;

        printf ("Items Returned = %d\n", count) ;

        for (i = 1 ; i <= count ; ++i)            printf ("Value %d = %d\n", i, POPi) ;

        PUTBACK ;        FREETMPS ;        LEAVE ;    }

The other modification made is that call_AddSubScalar will print thenumber of items returned from the Perl subroutine and their value (forsimplicity it assumes that they are integer). So ifcall_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 thelist is returned from the subroutine, AddSubtract actually made it back tocall_AddSubScalar.

Returning Data from Perl via the parameter list

It is also possible to return values directly via the parameter list -whether it is actually desirable to do it is another matter entirely.

The Perl subroutine, Inc, below takes 2 parameters and incrementseach directly.

    sub Inc    {        ++ $_[0] ;        ++ $_[1] ;    }

and here is a C function to call it.

    static void    call_Inc(a, b)    int a ;    int b ;    {        dSP ;        int count ;        SV * sva ;        SV * svb ;

        ENTER ;        SAVETMPS;

        sva = sv_2mortal(newSViv(a)) ;        svb = sv_2mortal(newSViv(b)) ;

        PUSHMARK(sp) ;        XPUSHs(sva);        XPUSHs(svb);        PUTBACK ;

        count = perl_call_pv("Inc", G_DISCARD);

        if (count != 0)            croak ("call_Inc: expected 0 values from 'Inc', got %d\n",                   count) ;

        printf ("%d + 1 = %d\n", a, SvIV(sva)) ;        printf ("%d + 1 = %d\n", b, SvIV(svb)) ;

        FREETMPS ;        LEAVE ;    }

To be able to access the two parameters that were pushed onto the stackafter they return from perl_call_pv it is necessary to make a noteof their addresses - thus the two variables sva and svb.

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

Using G_EVAL

Now an example using G_EVAL. Below is a Perl subroutine which computesthe difference of its 2 parameters. If this would result in a negativeresult, the subroutine calls die.

    sub Subtract    {        my ($a, $b) = @_ ;

        die "death can be fatal\n" if $a < $b ;

        $a - $b ;    }

and some C to call it

    static void    call_Subtract(a, b)    int a ;    int b ;    {        dSP ;        int count ;

        ENTER ;        SAVETMPS;

        PUSHMARK(sp) ;        XPUSHs(sv_2mortal(newSViv(a)));        XPUSHs(sv_2mortal(newSViv(b)));        PUTBACK ;

        count = perl_call_pv("Subtract", G_EVAL|G_SCALAR);

        SPAGAIN ;

        /* Check the eval first */        if (SvTRUE(GvSV(errgv)))        {            printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ;            POPs ;        }        else        {            if (count != 1)               croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",                        count) ;

            printf ("%d - %d = %d\n", a, b, POPi) ;        }

        PUTBACK ;        FREETMPS ;        LEAVE ;    }

If call_Subtract is called thus

    call_Subtract(4, 5)

the following will be printed

    Uh oh - death can be fatal

Notes

1.

We want to be able to catch the die so we have used the G_EVALflag. Not specifying this flag would mean that the program wouldterminate immediately at the die statement in the subroutineSubtract.

2.

The code

    if (SvTRUE(GvSV(errgv)))    {        printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ;        POPs ;    }

is the direct equivalent of this bit of Perl

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

errgv is a perl global of type GV * that points to thesymbol table entry containing the error. GvSV(errgv) thereforerefers to the C equivalent of $@.

3.

Note that the stack is popped using POPs in the block whereSvTRUE(GvSV(errgv)) is true. This is necessary because whenever aperl_call_* function invoked with G_EVAL|G_SCALAR returns an error,the top of the stack holds the value undef. Because we want theprogram to continue after detecting this error, it is essential thatthe stack is tidied up by removing the undef.

Using G_KEEPERR

Consider this rather facetious example, where we have used an XSversion of the call_Subtract example above inside a destructor:

    package Foo;    sub new { bless {}, $_[0] }    sub Subtract {        my($a,$b) = @_;        die "death can be fatal" if $a < $b ;        $a - $b;    }    sub DESTROY { call_Subtract(5, 4); }    sub foo { die "foo dies"; }

    package main;    eval { Foo->new->foo };    print "Saw: $@" if $@;             # should be, but isn't

This example will fail to recognize that an error occurred inside theeval {}. Here's why: the call_Subtract code got executed while perlwas cleaning up temporaries when exiting the eval block, and becausecall_Subtract is implemented with perl_call_pv using the G_EVALflag, it promptly reset $@. This results in the failure of theoutermost test for $@, and thereby the failure of the error trap.

Appending the G_KEEPERR flag, so that the perl_call_pv call incall_Subtract reads:

        count = perl_call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);

will preserve the error and restore reliable error handling.

Using perl_call_sv

In all the previous examples I have `hard-wired' the name of the Perlsubroutine to be called from C. Most of the time though, it is moreconvenient to be able to specify the name of the Perl subroutine fromwithin the Perl script.

Consider the Perl code below

    sub fred    {        print "Hello there\n" ;    }

    CallSubPV("fred") ;

Here is a snippet of XSUB which defines CallSubPV.

    void    CallSubPV(name)        char *  name        CODE:        PUSHMARK(sp) ;        perl_call_pv(name, G_DISCARD|G_NOARGS) ;

That is fine as far as it goes. The thing is, the Perl subroutinecan be specified as only a string. For Perl 4 this was adequate,but Perl 5 allows references to subroutines and anonymous subroutines.This is where perl_call_sv is useful.

The code below for CallSubSV is identical to CallSubPV exceptthat the name parameter is now defined as an SV* and we useperl_call_sv instead of perl_call_pv.

    void    CallSubSV(name)        SV *    name        CODE:        PUSHMARK(sp) ;        perl_call_sv(name, G_DISCARD|G_NOARGS) ;

Because we are using an SV to call fred the following can all be used

    CallSubSV("fred") ;    CallSubSV(\&fred) ;    $ref = \&fred ;    CallSubSV($ref) ;    CallSubSV( sub { print "Hello there\n" } ) ;

As you can see, perl_call_sv gives you much greater flexibility inhow you can specify the Perl subroutine.

You should note that if it is necessary to store the SV (name in theexample above) which corresponds to the Perl subroutine so that it canbe used later in the program, it not enough just to store a copy of thepointer to the SV. Say the code above had been like this

    static SV * rememberSub ;

    void    SaveSub1(name)        SV *    name        CODE:        rememberSub = name ;

    void    CallSavedSub1()        CODE:        PUSHMARK(sp) ;        perl_call_sv(rememberSub, G_DISCARD|G_NOARGS) ;

The reason this is wrong is that by the time you come to use thepointer rememberSub in CallSavedSub1, it may or may not still referto the Perl subroutine that was recorded in SaveSub1. This isparticularly true for these cases

    SaveSub1(\&fred) ;    CallSavedSub1() ;

    SaveSub1( sub { print "Hello there\n" } ) ;    CallSavedSub1() ;

By the time each of the SaveSub1 statements above have been executed,the SV*s which corresponded to the parameters will no longer exist.Expect an error message from Perl of the form

    Can't use an undefined value as a subroutine reference at ...

for each of the CallSavedSub1 lines.

Similarly, with this code

    $ref = \&fred ;    SaveSub1($ref) ;    $ref = 47 ;    CallSavedSub1() ;

you can expect one of these messages (which you actually get is dependent onthe version of Perl you are using)

    Not a CODE reference at ...    Undefined subroutine &main::47 called ...

The variable $ref may have referred to the subroutine fredwhenever the call to SaveSub1 was made but by the timeCallSavedSub1 gets called it now holds the number 47. Because wesaved only a pointer to the original SV in SaveSub1, any changes to$ref will be tracked by the pointer rememberSub. This means thatwhenever CallSavedSub1 gets called, it will attempt to execute thecode which is referenced by the SV* rememberSub. In this casethough, it now refers to the integer 47, so expect Perl to complainloudly.

A similar but more subtle problem is illustrated with this code

    $ref = \&fred ;    SaveSub1($ref) ;    $ref = \&joe ;    CallSavedSub1() ;

This time whenever CallSavedSub1 get called it will execute the Perlsubroutine joe (assuming it exists) rather than fred as wasoriginally requested in the call to SaveSub1.

To get around these problems it is necessary to take a full copy of theSV. The code below shows SaveSub2 modified to do that

    static SV * keepSub = (SV*)NULL ;

    void    SaveSub2(name)        SV *    name        CODE:        /* Take a copy of the callback */        if (keepSub == (SV*)NULL)            /* First time, so create a new SV */            keepSub = newSVsv(name) ;        else            /* Been here before, so overwrite */            SvSetSV(keepSub, name) ;

    void    CallSavedSub2()        CODE:        PUSHMARK(sp) ;        perl_call_sv(keepSub, G_DISCARD|G_NOARGS) ;

To avoid creating a new SV every time SaveSub2 is called,the function first checks to see if it has been called before. If not,then space for a new SV is allocated and the reference to the Perlsubroutine, name is copied to the variable keepSub in oneoperation using newSVsv. Thereafter, whenever SaveSub2 is calledthe existing SV, keepSub, is overwritten with the new value usingSvSetSV.

Using perl_call_argv

Here is a Perl subroutine which prints whatever parameters are passedto it.

    sub PrintList    {        my(@list) = @_ ;

        foreach (@list) { print "$_\n" }    }

and here is an example of perl_call_argv which will callPrintList.

    static char * words[] = {"alpha", "beta", "gamma", "delta", NULL} ;

    static void    call_PrintList()    {        dSP ;

        perl_call_argv("PrintList", G_DISCARD, words) ;    }

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

Using perl_call_method

Consider the following Perl code

    {        package Mine ;

        sub new        {            my($type) = shift ;            bless [@_]        }

        sub Display        {            my ($self, $index) = @_ ;            print "$index: $$self[$index]\n" ;        }

        sub PrintID        {            my($class) = @_ ;            print "This is Class $class version 1.0\n" ;        }    }

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

    $a = new Mine ('red', 'green', 'blue') ;    $a->Display(1) ;    PrintID Mine;

will print

    1: green    This is Class Mine version 1.0

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

*: a reference to the object for a virtual method or the name of the classfor a static method.
*: the name of the method.
*: any other parameters specific to the method.

Here is a simple XSUB which illustrates the mechanics of calling boththe PrintID and Display methods from C.

    void    call_Method(ref, method, index)        SV *    ref        char *  method        int             index        CODE:        PUSHMARK(sp);        XPUSHs(ref);        XPUSHs(sv_2mortal(newSViv(index))) ;        PUTBACK;

        perl_call_method(method, G_DISCARD) ;

    void    call_PrintID(class, method)        char *  class        char *  method        CODE:        PUSHMARK(sp);        XPUSHs(sv_2mortal(newSVpv(class, 0))) ;        PUTBACK;

        perl_call_method(method, G_DISCARD) ;

So the methods PrintID and Display can be invoked like this

    $a = new Mine ('red', 'green', 'blue') ;    call_Method($a, 'Display', 1) ;    call_PrintID('Mine', 'PrintID') ;

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

Using GIMME_V

Here is a trivial XSUB which prints the context in which it iscurrently executing.

    void    PrintContext()        CODE:        I32 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

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 orvalues returned via the stack) have been freed by one of these methods

*: specifying the G_DISCARD flag with perl_call_*.
*: explicitly disposed of using the ENTER/SAVETMPS -FREETMPS/LEAVE pairing.

There is another method which can be used, namely letting Perl do itfor you automatically whenever it regains control after the callbackhas terminated. This is done by simply not using the

    ENTER ;    SAVETMPS ;    ...    FREETMPS ;    LEAVE ;

sequence in the callback (and not, of course, specifying the G_DISCARDflag).

If you are going to use this method you have to be aware of a possiblememory leak which can arise under very specific circumstances. Toexplain these circumstances you need to know a bit about the flow ofcontrol between Perl and the callback routine.

The examples given at the start of the document (an error handler andan event driven program) are typical of the two main sorts of flowcontrol that you are likely to encounter with callbacks. There is avery important distinction between them, so pay attention.

In the first example, an error handler, the flow of control could be asfollows. 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 havepreviously set up a Perl callback to handle this situation, so it willget executed. Once the callback has finished, control will drop back toPerl again. Here is what the flow of control will be like in thatsituation

    perl --> XSUB --> external library                      ...                      error occurs                      ...                      external library --> perl_call --> perl                                                          |    perl <-- XSUB <-- external library <-- perl_call <----+

After processing of the error using perl_call_* is completed,control reverts back to Perl more or less immediately.

In the diagram, the further right you go the more deeply nested thescope is. It is only when control is back with perl on the extremeleft of the diagram that you will have dropped back to the enclosingscope and any temporaries you have left hanging around will be freed.

In the second example, an event driven program, the flow of controlwill be more like this

    perl --> XSUB --> event handler                      ...                      event handler --> perl_call --> perl                                                       |                      event handler <-- perl_call <----+                      ...                      event handler --> perl_call --> perl                                                       |                      event handler <-- perl_call <----+                      ...                      event handler --> perl_call --> perl                                                       |                      event handler <-- perl_call <----+

In this case the flow of control can consist of only the repeatedsequence

    event handler --> perl_call --> perl

for practically the complete duration of the program. This means thatcontrol may never drop back to the surrounding scope in Perl at theextreme left.

So what is the big problem? Well, if you are expecting Perl to tidy upthose temporaries for you, you might be in for a long wait. For Perlto dispose of your temporaries, control must drop back to theenclosing scope at some stage. In the event driven scenario that maynever happen. This means that as time goes on, your program willcreate more and more temporaries, none of which will ever be freed. Aseach of these temporaries consumes some memory your program willeventually consume all the available memory in your system - kapow!

So here is the bottom line - if you are sure that control will revertback to the enclosing Perl scope fairly quickly after the end of yourcallback, then it isn't absolutely necessary to dispose explicitly ofany temporaries you may have created. Mind you, if you are at alluncertain 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 acallback interface can be figuring out how to store the mapping betweenthe C callback function and the Perl equivalent.

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

    register_fatal(cb1) ;

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

    static void    cb1()    {        printf ("Fatal Error\n") ;        exit(1) ;    }

Now change that to call a Perl subroutine instead

    static SV * callback = (SV*)NULL;

    static void    cb1()    {        dSP ;

        PUSHMARK(sp) ;

        /* Call the Perl sub to process the callback */        perl_call_sv(callback, G_DISCARD) ;    }

    void    register_fatal(fn)        SV *    fn        CODE:        /* Remember the Perl sub */        if (callback == (SV*)NULL)            callback = newSVsv(fn) ;        else            SvSetSV(callback, fn) ;

        /* register the callback with the external library */        register_fatal(cb1) ;

where the Perl equivalent of register_fatal and the callback itregisters, 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 inthe global variable callback.

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

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

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

    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 mapbetween the fh parameter and the Perl subroutine we want called. Ahash is a convenient mechanism for storing this mapping. The codebelow shows a possible implementation

    static HV * Mapping = (HV*)NULL ;

    void    asynch_read(fh, callback)        int     fh        SV *    callback        CODE:        /* If the hash doesn't already exist, create it */        if (Mapping == (HV*)NULL)            Mapping = newHV() ;

        /* Save the fh -> callback mapping */        hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0) ;

        /* Register with the C Library */        asynch_read(fh, asynch_read_if) ;

and asynch_read_if could look like this

    static void    asynch_read_if(fh, buffer)    int fh ;    char *      buffer ;    {        dSP ;        SV ** sv ;

        /* Get the callback associated with fh */        sv =  hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE) ;        if (sv == (SV**)NULL)            croak("Internal error...\n") ;

        PUSHMARK(sp) ;        XPUSHs(sv_2mortal(newSViv(fh))) ;        XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;        PUTBACK ;

        /* Call the Perl sub */        perl_call_sv(*sv, G_DISCARD) ;    }

For completeness, here is asynch_close. This shows how to removethe 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 globalhash Mapping this time. Using a hash has the distinct advantage thatit allows an unlimited number of callbacks to be registered.

What if the interface provided by the C callback doesn't contain aparameter which allows the file handle to Perl subroutine mapping? Sayin the asynchronous i/o package, the callback function gets passed onlythe buffer parameter like this

    void    ProcessRead(buffer)    char *      buffer ;    {        ...    }

Without the file handle there is no straightforward way to map from theC callback to the Perl subroutine.

In this case a possible way around this problem is to predefine aseries of C functions to act as the interface to Perl, thus

    #define MAX_CB              3    #define NULL_HANDLE -1    typedef void (*FnMap)() ;

    struct MapStruct {        FnMap    Function ;        SV *     PerlSub ;        int      Handle ;      } ;

    static void  fn1() ;    static void  fn2() ;    static void  fn3() ;

    static struct MapStruct Map [MAX_CB] =        {            { fn1, NULL, NULL_HANDLE },            { fn2, NULL, NULL_HANDLE },            { fn3, NULL, NULL_HANDLE }        } ;

    static void    Pcb(index, buffer)    int index ;    char * buffer ;    {        dSP ;

        PUSHMARK(sp) ;        XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;        PUTBACK ;

        /* Call the Perl sub */        perl_call_sv(Map[index].PerlSub, G_DISCARD) ;    }

    static void    fn1(buffer)    char * buffer ;    {        Pcb(0, buffer) ;    }

    static void    fn2(buffer)    char * buffer ;    {        Pcb(1, buffer) ;    }

    static void    fn3(buffer)    char * buffer ;    {        Pcb(2, buffer) ;    }

    void    array_asynch_read(fh, callback)        int             fh        SV *    callback        CODE:        int index ;        int null_index = MAX_CB ;

        /* Find the same handle or an empty entry */        for (index = 0 ; index < MAX_CB ; ++index)        {            if (Map[index].Handle == fh)                break ;

            if (Map[index].Handle == NULL_HANDLE)                null_index = index ;        }

        if (index == MAX_CB && null_index == MAX_CB)            croak ("Too many callback functions registered\n") ;

        if (index == MAX_CB)            index = null_index ;

        /* Save the file handle */        Map[index].Handle = fh ;

        /* Remember the Perl sub */        if (Map[index].PerlSub == (SV*)NULL)            Map[index].PerlSub = newSVsv(callback) ;        else            SvSetSV(Map[index].PerlSub, callback) ;

        asynch_read(fh, Map[index].Function) ;

    void    array_asynch_close(fh)        int     fh        CODE:        int index ;

        /* Find the file handle */        for (index = 0; index < MAX_CB ; ++ index)            if (Map[index].Handle == fh)                break ;

        if (index == MAX_CB)            croak ("could not close fh %d\n", fh) ;

        Map[index].Handle = NULL_HANDLE ;        SvREFCNT_dec(Map[index].PerlSub) ;        Map[index].PerlSub = (SV*)NULL ;

        asynch_close(fh) ;

In this case the functions fn1, fn2, and fn3 are used toremember the Perl subroutine to be called. Each of the functions holdsa separate hard-wired index which is used in the function Pcb toaccess 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 previousexample.

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

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

1. Ignore the problem - Allow only 1 callback: For a lot of situations, like interfacing to an error handler, this maybe a perfectly adequate solution.
2. Create a sequence of callbacks - hard wired limit: If it is impossible to tell from the parameters passed back from the Ccallback what the context is, then you may need to create a sequence of Ccallback interface functions, and store pointers to each in an array.
3. Use a parameter to map to the Perl callback: 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 valuesreturned from Perl subroutines, it is also possible to bypass thesemacros and read the stack using the ST macro (See the perlxs manpage for afull description of the ST macro).

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

The code below is the example given in the section Returning a listof values recoded to use ST instead of POP*.

    static void    call_AddSubtract2(a, b)    int a ;    int b ;    {        dSP ;        I32 ax ;        int count ;

        ENTER ;        SAVETMPS;

        PUSHMARK(sp) ;        XPUSHs(sv_2mortal(newSViv(a)));        XPUSHs(sv_2mortal(newSViv(b)));        PUTBACK ;

        count = perl_call_pv("AddSubtract", G_ARRAY);

        SPAGAIN ;        sp -= count ;        ax = (sp - stack_base) + 1 ;

        if (count != 2)            croak("Big trouble\n") ;

        printf ("%d + %d = %d\n", a, b, SvIV(ST(0))) ;        printf ("%d - %d = %d\n", a, b, SvIV(ST(1))) ;

        PUTBACK ;        FREETMPS ;        LEAVE ;    }

Notes

1.

Notice that it was necessary to define the variable ax. This isbecause the ST macro expects it to exist. If we were in an XSUB itwould not be necessary to define ax as it is already defined foryou.

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 returnedvalues are not accessed in reverse order. So ST(0) refers to thefirst value returned by the Perl subroutine and ST(count-1)refers to the last.

Creating and calling an anonymous subroutine in C

As we've already shown, the perl_call_sv manpage can be used to invoke ananonymous subroutine. However, our example showed how Perl scriptinvoking an XSUB to preform this operation. Let's see how it can bedone inside our C code:

...

 SV *cvrv = perl_eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE);

...

 perl_call_sv(cvrv, G_VOID|G_NOARGS);

the perl_eval_pv entry in the perlguts manpage is used to compile the anonymous subroutine, whichwill be the return value as well. Once this code reference is in hand, itcan be mixed in with all the previous examples we've shown.

AUTHOR

Paul Marquess <pmarquessAATTbfsec.bt.co.uk>

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

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

DATE

Version 1.3, 14th Apr 1997

PERLCALL

NAME

DESCRIPTION

THE PERL_CALL FUNCTIONS

FLAG VALUES

G_VOID

G_SCALAR

G_ARRAY

G_DISCARD

G_NOARGS

G_EVAL

G_KEEPERR

Determining the Context

KNOWN PROBLEMS

EXAMPLES

Parameters, Nothing returned

Passing Parameters

Returning a Scalar

Returning a list of values

Returning a list in a scalar context

Returning Data from Perl via the parameter list

Using G_EVAL

Using G_KEEPERR

Using perl_call_sv

Using perl_call_argv

Using perl_call_method

Using GIMME_V

Using Perl to dispose of temporaries

Strategies for storing Callback Context Information

Alternate Stack Manipulation

Creating and calling an anonymous subroutine in C

SEE ALSO

AUTHOR

DATE