NAMEperlcall - Perl calling conventions from C
DESCRIPTIONThe purpose of this document is to show you how to call Perl subroutines directly from C, i.e., how to write callbacks. 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
Although the techniques described here are applicable when embedding Perl in a C program, this is not the primary goal of this document. There are other details that must be considered and are specific to embedding Perl. For details on embedding Perl in C refer to the 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 CALL_ FUNCTIONSAlthough this stuff is easier to explain using examples, you first need be aware of a few important definitions. Perl has a number of C functions that allow you to call Perl subroutines. They are I32 call_sv(SV* sv, I32 flags) ; I32 call_pv(char *subname, I32 flags) ; I32 call_method(char *methname, I32 flags) ; I32 call_argv(char *subname, I32 flags, register char **argv) ; The key function is call_sv. All the other functions are fairly simple wrappers which make it easier to call Perl subroutines in special cases. At the end of the day they will all call call_sv to invoke the Perl subroutine. All the call_* functions have a Each of the functions will now be discussed in turn.
All the functions return an integer. This is a count of the number of items returned by the Perl subroutine. The actual items returned by the subroutine are stored on the Perl stack. As a general rule you should always check the return value from these functions. Even if you are expecting only a particular number of values to be returned from the Perl subroutine, there is nothing to stop someone from doing something unexpected--don't say you haven't been warned.
FLAG VALUESThe
G_VOIDCalls the Perl subroutine in a void context. This flag has 2 effects:
The value returned by the call_* function indicates how many items have been returned by the Perl subroutine - in this case it will be 0.
G_SCALARCalls the Perl subroutine in a scalar context. This is the default context flag setting for all the call_* functions. This flag has 2 effects:
The value returned by the call_* function indicates how many items have been returned by the Perl subroutine - in this case it will be either 0 or 1. If 0, then you have specified the G_DISCARD flag. If 1, then the item actually returned by the Perl subroutine will be stored on the Perl stack - the section Returning a Scalar shows how to access this value on the stack. Remember that regardless of how many items the Perl subroutine returns, only the last one will be accessible from the stack - think of the case where only one value is returned as being a list with only one element. Any other items that were returned will not exist by the time control returns from the call_* function. The section Returning a list in a scalar context shows an example of this behavior.
G_ARRAYCalls the Perl subroutine in a list context. As with G_SCALAR, this flag has 2 effects:
The value returned by the call_* function indicates how many items have been returned by the Perl subroutine. If 0, then you have specified the G_DISCARD flag. If not 0, then it will be a count of the number of items returned by the subroutine. These items will be stored on the Perl stack. The section Returning a list of values gives an example of using the G_ARRAY flag and the mechanics of accessing the returned items from the Perl stack.
G_DISCARDBy default, the 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. If you do not set this flag then it is very important that you make sure that any temporaries (i.e., parameters passed to the Perl subroutine and values returned from the subroutine) are disposed of yourself. The section Returning a Scalar gives details of how to dispose of these temporaries explicitly and the section Using Perl to dispose of temporaries discusses the specific circumstances where you can ignore the problem and let Perl deal with it for you.
G_NOARGSWhenever a Perl subroutine is called using one of the 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 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 sub fred { print "@_\n" } sub joe { &fred } &joe(1,2,3) ; This will print 1 2 3 What has happened is that
G_EVALIt 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 events occurs, the process will terminate immediately. If you want to trap this type of event, specify the G_EVAL flag. It will put an eval { } around the subroutine call. Whenever control returns from the call_* function you need to
check the The value returned from the call_* function is dependent on what other flags have been specified and whether an error has occurred. Here are all the different cases that can occur:
See Using G_EVAL for details on using G_EVAL.
G_KEEPERRYou may have noticed that using the G_EVAL flag described above will
always clear the 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 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.
Determining the ContextAs 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
KNOWN PROBLEMSThis section outlines all known problems that exist in the call_* functions.
EXAMPLESEnough 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 interfacing to Perl internals. We hope this should make the code less vulnerable to any changes made to Perl in the future. Another point worth noting is that in the first series of examples I have made use of only the call_pv function. This has been done to keep the code simpler and ease you into the topic. Wherever possible, if the choice is between using call_pv and call_sv, you should always try to use call_sv. See Using call_sv for details.
No Parameters, Nothing returnedThis first trivial example will call a Perl subroutine, PrintUID, to print out the UID of the process. sub PrintUID { print "UID is $<\n" ; } and here is a C function to call it static void call_PrintUID() { dSP ; PUSHMARK(SP) ; call_pv("PrintUID", G_DISCARD|G_NOARGS) ; } Simple, eh. A few points to note about this example.
Passing ParametersNow let's make a slightly more complex example. This time we want to
call a Perl subroutine, So the Perl subroutine would look like this sub LeftString { my($s, $n) = @_ ; print substr($s, 0, $n), "\n" ; } The C function required to call LeftString would look like this. static void call_LeftString(a, b) char * a ; int b ; { dSP ; ENTER ; SAVETMPS ; PUSHMARK(SP) ; XPUSHs(sv_2mortal(newSVpv(a, 0))); XPUSHs(sv_2mortal(newSViv(b))); PUTBACK ; call_pv("LeftString", G_DISCARD); FREETMPS ; LEAVE ; } Here are a few notes on the C function call_LeftString.
Returning a ScalarNow for an example of dealing with the items returned from a Perl subroutine. Here is a Perl subroutine, Adder, that takes 2 integer parameters and simply returns their sum. sub Adder { my($a, $b) = @_ ; $a + $b ; } Because we are now concerned with the return value from Adder, the C function required to call it is now a bit more complex. static void call_Adder(a, b) int a ; int b ; { dSP ; int count ; ENTER ; SAVETMPS; PUSHMARK(SP) ; XPUSHs(sv_2mortal(newSViv(a))); XPUSHs(sv_2mortal(newSViv(b))); PUTBACK ; count = call_pv("Adder", G_SCALAR); SPAGAIN ; if (count != 1) croak("Big trouble\n") ; printf ("The sum of %d and %d is %d\n", a, b, POPi) ; PUTBACK ; FREETMPS ; LEAVE ; } Points to note this time are
Returning a list of valuesNow, let's extend the previous example to return both the sum of the parameters and the difference. Here is the Perl subroutine sub AddSubtract { my($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 = 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
Returning a list in a scalar contextSay the Perl subroutine in the previous section was called in a scalar context, 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 = call_pv("AddSubtract", G_SCALAR); SPAGAIN ; printf ("Items Returned = %d\n", count) ; for (i = 1 ; i <= count ; ++i) printf ("Value %d = %d\n", i, POPi) ; PUTBACK ; FREETMPS ; LEAVE ; } The other modification made is that call_AddSubScalar will print the number of items returned from the Perl subroutine and their value (for simplicity it assumes that they are integer). So if call_AddSubScalar is called call_AddSubScalar(7, 4) ; then the output will be Items Returned = 1 Value 1 = 3 In this case the main point to note is that only the last item in the list is returned from the subroutine, AddSubtract actually made it back to call_AddSubScalar.
Returning Data from Perl via the parameter listIt 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 increments each directly. sub Inc { ++ $_[0] ; ++ $_[1] ; } and here is a C function to call it. static void call_Inc(a, b) int a ; int b ; { dSP ; int count ; SV * sva ; SV * svb ; ENTER ; SAVETMPS; sva = sv_2mortal(newSViv(a)) ; svb = sv_2mortal(newSViv(b)) ; PUSHMARK(SP) ; XPUSHs(sva); XPUSHs(svb); PUTBACK ; count = call_pv("Inc", G_DISCARD); if (count != 0) croak ("call_Inc: expected 0 values from 'Inc', got %d\n", count) ; printf ("%d + 1 = %d\n", a, SvIV(sva)) ; printf ("%d + 1 = %d\n", b, SvIV(svb)) ; FREETMPS ; LEAVE ; } To be able to access the two parameters that were pushed onto the stack
after they return from call_pv it is necessary to make a note
of their addresses--thus the two variables The reason this is necessary is that the area of the Perl stack which held them will very likely have been overwritten by something else by the time control returns from call_pv.
Using G_EVALNow an example using G_EVAL. Below is a Perl subroutine which computes the difference of its 2 parameters. If this would result in a negative result, the subroutine calls die. sub Subtract { my ($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 = call_pv("Subtract", G_EVAL|G_SCALAR); SPAGAIN ; /* Check the eval first */ if (SvTRUE(ERRSV)) { STRLEN n_a; printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ; 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
Using G_KEEPERRConsider this rather facetious example, where we have used an XS version 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 the
Appending the G_KEEPERR flag, so that the call_pv call in call_Subtract reads: count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR); will preserve the error and restore reliable error handling.
Using call_svIn 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. Consider the Perl code below sub fred { print "Hello there\n" ; } CallSubPV("fred") ; Here is a snippet of XSUB which defines CallSubPV. void CallSubPV(name) char * name CODE: PUSHMARK(SP) ; call_pv(name, G_DISCARD|G_NOARGS) ; That is fine as far as it goes. The thing is, the Perl subroutine can be specified as only a string. For Perl 4 this was adequate, but Perl 5 allows references to subroutines and anonymous subroutines. This is where call_sv is useful. The code below for CallSubSV is identical to CallSubPV except
that the void CallSubSV(name) SV * name CODE: PUSHMARK(SP) ; call_sv(name, G_DISCARD|G_NOARGS) ; Because we are using an SV to call fred the following can all be used CallSubSV("fred") ; CallSubSV(\&fred) ; $ref = \&fred ; CallSubSV($ref) ; CallSubSV( sub { print "Hello there\n" } ) ; As you can see, call_sv gives you much greater flexibility in how you can specify the Perl subroutine. You should note that if it is necessary to store the SV ( static SV * rememberSub ; void SaveSub1(name) SV * name CODE: rememberSub = name ; void CallSavedSub1() CODE: PUSHMARK(SP) ; call_sv(rememberSub, G_DISCARD|G_NOARGS) ; The reason this is wrong is that by the time you come to use the
pointer SaveSub1(\&fred) ; CallSavedSub1() ; SaveSub1( sub { print "Hello there\n" } ) ; CallSavedSub1() ; By the time each of the Can't use an undefined value as a subroutine reference at ... for each of the Similarly, with this code $ref = \&fred ; SaveSub1($ref) ; $ref = 47 ; CallSavedSub1() ; you can expect one of these messages (which you actually get is dependent on the version of Perl you are using) Not a CODE reference at ... Undefined subroutine &main::47 called ... The variable $ref may have referred to the subroutine A similar but more subtle problem is illustrated with this code $ref = \&fred ; SaveSub1($ref) ; $ref = \&joe ; CallSavedSub1() ; This time whenever To get around these problems it is necessary to take a full copy of the
SV. The code below shows static SV * keepSub = (SV*)NULL ; void SaveSub2(name) SV * name CODE: /* Take a copy of the callback */ if (keepSub == (SV*)NULL) /* First time, so create a new SV */ keepSub = newSVsv(name) ; else /* Been here before, so overwrite */ SvSetSV(keepSub, name) ; void CallSavedSub2() CODE: PUSHMARK(SP) ; call_sv(keepSub, G_DISCARD|G_NOARGS) ; To avoid creating a new SV every time
Using call_argvHere is a Perl subroutine which prints whatever parameters are passed to it. sub PrintList { my(@list) = @_ ; foreach (@list) { print "$_\n" } } and here is an example of call_argv which will call PrintList. static char * words[] = {"alpha", "beta", "gamma", "delta", NULL} ; static void call_PrintList() { dSP ; call_argv("PrintList", G_DISCARD, words) ; } Note that it is not necessary to call
Using call_methodConsider 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 from
the constructor, $a = new Mine ('red', 'green', 'blue') ; $a->Display(1) ; PrintID Mine; will print 1: green This is Class Mine version 1.0 Calling a Perl method from C is fairly straightforward. The following things are required
Here is a simple XSUB which illustrates the mechanics of calling both
the void call_Method(ref, method, index) SV * ref char * method int index CODE: PUSHMARK(SP); XPUSHs(ref); XPUSHs(sv_2mortal(newSViv(index))) ; PUTBACK; call_method(method, G_DISCARD) ; void call_PrintID(class, method) char * class char * method CODE: PUSHMARK(SP); XPUSHs(sv_2mortal(newSVpv(class, 0))) ; PUTBACK; call_method(method, G_DISCARD) ; So the methods $a = new Mine ('red', 'green', 'blue') ; call_Method($a, 'Display', 1) ; call_PrintID('Mine', 'PrintID') ; The only thing to note is that in both the static and virtual methods, the method name is not passed via the stack--it is used as the first parameter to call_method.
Using GIMME_VHere is a trivial XSUB which prints the context in which it is currently 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 temporariesIn the examples given to date, any temporaries created in the callback (i.e., parameters passed on the stack to the 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 ; sequence in the callback (and not, of course, specifying the G_DISCARD flag). If you are going to use this method you have to be aware of a possible memory leak which can arise under very specific circumstances. To explain these circumstances you need to know a bit about the flow of control between Perl and the callback routine. The examples given at the start of the document (an error handler and an event driven program) are typical of the two main sorts of flow control that you are likely to encounter with callbacks. There is a very important distinction between them, so pay attention. In the first example, an error handler, the flow of control could be as follows. You have created an interface to an external library. Control can reach the external library like this perl --> XSUB --> external library Whilst control is in the library, an error condition occurs. You have previously set up a Perl callback to handle this situation, so it will get executed. Once the callback has finished, control will drop back to Perl again. Here is what the flow of control will be like in that situation perl --> XSUB --> external library ... error occurs ... external library --> call_* --> perl | perl <-- XSUB <-- external library <-- call_* <----+ After processing of the error using call_* is completed, control reverts back to Perl more or less immediately. In the diagram, the further right you go the more deeply nested the scope is. It is only when control is back with perl on the extreme left of the diagram that you will have dropped back to the enclosing scope and any temporaries you have left hanging around will be freed. In the second example, an event driven program, the flow of control will be more like this perl --> XSUB --> event handler ... event handler --> call_* --> perl | event handler <-- call_* <----+ ... event handler --> call_* --> perl | event handler <-- call_* <----+ ... event handler --> call_* --> perl | event handler <-- call_* <----+ In this case the flow of control can consist of only the repeated sequence event handler --> call_* --> perl for practically the complete duration of the program. This means that control may never drop back to the surrounding scope in Perl at the extreme left. So what is the big problem? Well, if you are expecting Perl to tidy up those temporaries for you, you might be in for a long wait. For Perl to dispose of your temporaries, control must drop back to the enclosing scope at some stage. In the event driven scenario that may never happen. This means that as time goes on, your program will create more and more temporaries, none of which will ever be freed. As each of these temporaries consumes some memory your program will eventually consume all the available memory in your system--kapow! So here is the bottom line--if you are sure that control will revert back to the enclosing Perl scope fairly quickly after the end of your callback, then it isn't absolutely necessary to dispose explicitly of any temporaries you may have created. Mind you, if you are at all uncertain about what to do, it doesn't do any harm to tidy up anyway.
Strategies for storing Callback Context InformationPotentially 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(cb1) ; The single parameter static void cb1() { printf ("Fatal Error\n") ; exit(1) ; } Now change that to call a Perl subroutine instead static SV * callback = (SV*)NULL; static void cb1() { dSP ; PUSHMARK(SP) ; /* Call the Perl sub to process the callback */ call_sv(callback, G_DISCARD) ; } void register_fatal(fn) SV * fn CODE: /* Remember the Perl sub */ if (callback == (SV*)NULL) callback = newSVsv(fn) ; else SvSetSV(callback, fn) ; /* register the callback with the external library */ register_fatal(cb1) ; where the Perl equivalent of # Register the sub pcb1 register_fatal(\&pcb1) ; sub pcb1 { die "I'm dying...\n" ; } The mapping between the C callback and the Perl equivalent is stored in
the global variable This will be adequate if you ever need to have only one callback
registered at any time. An example could be an error handler like the
code sketched out above. Remember though, repeated calls to
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(fh, ProcessRead) This may expect the C ProcessRead function of this form void ProcessRead(fh, buffer) int fh ; char * buffer ; { ... } To provide a Perl interface to this library we need to be able to map
between the 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 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 */ call_sv(*sv, G_DISCARD) ; } For completeness, here is void asynch_close(fh) int fh CODE: /* Remove the entry from the hash */ (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD) ; /* Now call the real asynch_close */ asynch_close(fh) ; So the Perl interface would look like this sub callback1 { my($handle, $buffer) = @_ ; } # Register the Perl callback asynch_read($fh, \&callback1) ; asynch_close($fh) ; The mapping between the C callback and Perl is stored in the global
hash 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 void ProcessRead(buffer) char * buffer ; { ... } Without the file handle there is no straightforward way to map from the C callback to the Perl subroutine. In this case a possible way around this problem is to predefine a series of C functions to act as the interface to Perl, thus #define MAX_CB 3 #define NULL_HANDLE -1 typedef void (*FnMap)() ; struct MapStruct { FnMap Function ; SV * PerlSub ; int Handle ; } ; static void fn1() ; static void fn2() ; static void fn3() ; static struct MapStruct Map [MAX_CB] = { { fn1, NULL, NULL_HANDLE }, { fn2, NULL, NULL_HANDLE }, { fn3, NULL, NULL_HANDLE } } ; static void Pcb(index, buffer) int index ; char * buffer ; { dSP ; PUSHMARK(SP) ; XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ; PUTBACK ; /* Call the Perl sub */ call_sv(Map[index].PerlSub, G_DISCARD) ; } static void fn1(buffer) char * buffer ; { Pcb(0, buffer) ; } static void fn2(buffer) char * buffer ; { Pcb(1, buffer) ; } static void fn3(buffer) char * buffer ; { Pcb(2, buffer) ; } void array_asynch_read(fh, callback) int fh SV * callback CODE: int index ; int null_index = MAX_CB ; /* Find the same handle or an empty entry */ for (index = 0 ; index < MAX_CB ; ++index) { if (Map[index].Handle == fh) break ; if (Map[index].Handle == NULL_HANDLE) null_index = index ; } if (index == MAX_CB && null_index == MAX_CB) croak ("Too many callback functions registered\n") ; if (index == MAX_CB) index = null_index ; /* Save the file handle */ Map[index].Handle = fh ; /* Remember the Perl sub */ if (Map[index].PerlSub == (SV*)NULL) Map[index].PerlSub = newSVsv(callback) ; else SvSetSV(Map[index].PerlSub, callback) ; asynch_read(fh, Map[index].Function) ; void array_asynch_close(fh) int fh CODE: int index ; /* Find the file handle */ for (index = 0; index < MAX_CB ; ++ index) if (Map[index].Handle == fh) break ; if (index == MAX_CB) croak ("could not close fh %d\n", fh) ; Map[index].Handle = NULL_HANDLE ; SvREFCNT_dec(Map[index].PerlSub) ; Map[index].PerlSub = (SV*)NULL ; asynch_close(fh) ; In this case the functions There are some obvious disadvantages with this technique. Firstly, the code is considerably more complex than with the previous example. Secondly, there is a hard-wired limit (in this case 3) to the number of callbacks that can exist simultaneously. The only way to increase the limit is by modifying the code to add more functions and then recompiling. None the less, as long as the number of functions is chosen with some care, it is still a workable solution and in some cases is the only one available. To summarize, here are a number of possible methods for you to consider for storing the mapping between C and the Perl callback
Alternate Stack ManipulationAlthough I have made use of only the Most of the time the The code below is the example given in the section Returning a list
of values recoded to use 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 = call_pv("AddSubtract", G_ARRAY); SPAGAIN ; SP -= count ; ax = (SP - PL_stack_base) + 1 ; if (count != 2) croak("Big trouble\n") ; printf ("%d + %d = %d\n", a, b, SvIV(ST(0))) ; printf ("%d - %d = %d\n", a, b, SvIV(ST(1))) ; PUTBACK ; FREETMPS ; LEAVE ; } Notes
Creating and calling an anonymous subroutine in CAs we've already shown, ... SV *cvrv = eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE); ... call_sv(cvrv, G_VOID|G_NOARGS);
SEE ALSOthe perlxs manpage, the perlguts manpage, the perlembed manpage
AUTHORPaul Marquess 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.
DATEVersion 1.3, 14th Apr 1997
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