X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlcall.pod;h=e691e759a1d22b0971b27bf1a5e475283b8f3677;hb=a3dfe201291c96fc01babd3d4782d52ba945f2a3;hp=9a4a886a595d6172a39d2241ed5c90dd2b7d5643;hpb=72451f4af0d31f24ef5b12bc5d034e3e8b35d43d;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlcall.pod b/pod/perlcall.pod index 9a4a886..e691e75 100644 --- a/pod/perlcall.pod +++ b/pod/perlcall.pod @@ -72,7 +72,7 @@ Each of the functions will now be discussed in turn. =over 5 -=item B +=item perl_call_sv I takes two parameters, the first, C, is an SV*. This allows you to specify the Perl subroutine to be called either as a @@ -80,7 +80,7 @@ C string (which has first been converted to an SV) or a reference to a subroutine. The section, I, shows how you can make use of I. -=item B +=item perl_call_pv The function, I, is similar to I except it expects its first parameter to be a C char* which identifies the Perl @@ -88,7 +88,7 @@ subroutine you want to call, e.g., C. If the subroutine you want to call is in another package, just include the package name in the string, e.g., C<"pkg::fred">. -=item B +=item perl_call_method The function I is used to call a method from a Perl class. The parameter C corresponds to the name of the method @@ -99,7 +99,7 @@ object (for a virtual method). See L for more information on static and virtual methods and L for an example of using I. -=item B +=item perl_call_argv I calls the Perl subroutine specified by the C string stored in the C parameter. It also takes the usual C @@ -116,7 +116,7 @@ subroutine are stored on the Perl stack. As a general rule you should I 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 +stop someone from doing something unexpected--don't say you haven't been warned. =head1 FLAG VALUES @@ -126,6 +126,31 @@ which can consist of any combination of the symbols defined below, OR'ed together. +=head2 G_VOID + +Calls the Perl subroutine in a void context. + +This flag has 2 effects: + +=over 5 + +=item 1. + +It indicates to the subroutine being called that it is executing in +a void context (if it executes I the result will be the +undefined value). + +=item 2. + +It ensures that nothing is actually returned from the subroutine. + +=back + +The value returned by the I function indicates how many +items have been returned by the Perl subroutine - in this case it will +be 0. + + =head2 G_SCALAR Calls the Perl subroutine in a scalar context. This is the default @@ -140,7 +165,6 @@ This flag has 2 effects: It indicates to the subroutine being called that it is executing in a scalar context (if it executes I the result will be false). - =item 2. It ensures that only a scalar is actually returned from the subroutine. @@ -164,7 +188,7 @@ 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 I function. The section I shows an example of this behaviour. +context> shows an example of this behavior. =head2 G_ARRAY @@ -251,12 +275,12 @@ What has happened is that C accesses the C<@_> array which belongs to C. -=head2 G_EVAL +=head2 G_EVAL It is possible for the Perl subroutine you are calling to terminate abnormally, e.g., by calling I explicitly or by not actually -existing. By default, when either of these of events occurs, the -process will terminate immediately. If though, you want to trap this +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 I around the subroutine call. @@ -293,7 +317,7 @@ from the stack. =back -See I for details of using G_EVAL. +See I for details on using G_EVAL. =head2 G_KEEPERR @@ -326,14 +350,17 @@ The G_KEEPERR flag was introduced in Perl version 5.002. See I for an example of a situation that warrants the use of this flag. -=head2 Determining the Context +=head2 Determining the Context As mentioned above, you can determine the context of the currently -executing subroutine in Perl with I. The equivalent test can -be made in C by using the C macro. This will return C -if you have been called in a scalar context and C if in an -array context. An example of using the C macro is shown in -section I. +executing subroutine in Perl with I. The equivalent test +can be made in C by using the C macro, which returns +C if you have been called in an array context, C if +in a scalar context, or C if in a void context (i.e. the +return value will not be used). An older version of this macro is +called C; in a void context it returns C instead of +C. An example of using the C macro is shown in +section I. =head1 KNOWN PROBLEMS @@ -368,7 +395,7 @@ For example, say you want to call this Perl sub sub fred { eval { die "Fatal Error" ; } - print "Trapped error: $@\n" + print "Trapped error: $@\n" if $@ ; } @@ -377,7 +404,7 @@ via this XSUB void Call_fred() CODE: - PUSHMARK(sp) ; + PUSHMARK(SP) ; perl_call_pv("fred", G_DISCARD|G_NOARGS) ; fprintf(stderr, "back in Call_fred\n") ; @@ -388,13 +415,13 @@ When C is executed it will print As control never returns to C, the C<"back in Call_fred"> string will not get printed. -To work around this problem, you can either upgrade to Perl 5.002 (or -later), or use the G_EVAL flag with I as shown below +To work around this problem, you can either upgrade to Perl 5.002 or +higher, or use the G_EVAL flag with I as shown below void Call_fred() CODE: - PUSHMARK(sp) ; + PUSHMARK(SP) ; perl_call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ; fprintf(stderr, "back in Call_fred\n") ; @@ -435,7 +462,7 @@ and here is a C function to call it { dSP ; - PUSHMARK(sp) ; + PUSHMARK(SP) ; perl_call_pv("PrintUID", G_DISCARD|G_NOARGS) ; } @@ -447,7 +474,7 @@ A few points to note about this example. =item 1. -Ignore C and C for now. They will be discussed in +Ignore C and C for now. They will be discussed in the next example. =item 2. @@ -478,9 +505,9 @@ returned from I. It will always be 0. =head2 Passing Parameters Now let's make a slightly more complex example. This time we want to -call a Perl subroutine, C, which will take 2 parameters - a -string (C<$s>) and an integer (C<$n>). The subroutine will simply -print the first C<$n> characters of the string. +call a Perl subroutine, C, which will take 2 parameters--a +string ($s) and an integer ($n). The subroutine will simply +print the first $n characters of the string. So the Perl subroutine would look like this @@ -499,12 +526,18 @@ The C function required to call I would look like this. { dSP ; - PUSHMARK(sp) ; + ENTER ; + SAVETMPS ; + + PUSHMARK(SP) ; XPUSHs(sv_2mortal(newSVpv(a, 0))); XPUSHs(sv_2mortal(newSViv(b))); PUTBACK ; perl_call_pv("LeftString", G_DISCARD); + + FREETMPS ; + LEAVE ; } Here are a few notes on the C function I. @@ -515,13 +548,14 @@ Here are a few notes on the C function I. Parameters are passed to the Perl subroutine using the Perl stack. This is the purpose of the code beginning with the line C and -ending with the line C. - +ending with the line C. The C declares a local copy +of the stack pointer. This local copy should B be accessed +as C. =item 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 C - it declares +where to put it. This is the purpose of the macro C--it declares and initializes a I copy of the Perl stack pointer. All the other macros which will be used in this example require you to @@ -529,7 +563,7 @@ have used this macro. The exception to this rule is if you are calling a Perl subroutine directly from an XSUB function. In this case it is not necessary to -use the C macro explicitly - it will be declared for you +use the C macro explicitly--it will be declared for you automatically. =item 3. @@ -544,12 +578,12 @@ The C 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 I) you must still call the C macro before you can call any of the -I functions - Perl still needs to know that there are no +I functions--Perl still needs to know that there are no parameters. The C macro sets the global copy of the stack pointer to be the same as our local copy. If we didn't do this I -wouldn't know where the two parameters we pushed were - remember that +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, I the global copy. @@ -565,11 +599,41 @@ 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 L for details +See L for details on how the XPUSH macros work. =item 6. +Because we created temporary values (by means of sv_2mortal() calls) +we will have to tidy up the Perl stack and dispose of mortal SVs. + +This is the purpose of + + ENTER ; + SAVETMPS ; + +at the start of the function, and + + FREETMPS ; + LEAVE ; + +at the end. The C/C 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 C/C pair will get rid of any values returned by +the Perl subroutine (see next example), plus it will also dump the +mortal SVs we have created. Having C/C at the +beginning of the code makes sure that no other mortals are destroyed. + +Think of these macros as working a bit like using C<{> and C<}> in Perl +to limit the scope of local variables. + +See the section I for details of +an alternative to using these macros. + +=item 7. + Finally, I can now be called via the I function. @@ -603,7 +667,7 @@ function required to call it is now a bit more complex. ENTER ; SAVETMPS; - PUSHMARK(sp) ; + PUSHMARK(SP) ; XPUSHs(sv_2mortal(newSViv(a))); XPUSHs(sv_2mortal(newSViv(b))); PUTBACK ; @@ -626,56 +690,24 @@ Points to note this time are =over 5 -=item 1. +=item 1. The only flag specified this time was G_SCALAR. That means the C<@_> array will be created and that the value returned by I will still exist after the call to I. - - =item 2. -Because we are interested in what is returned from I 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 C/C 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 C/C 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 C/C at the beginning of the code -makes sure that no other mortals are destroyed. - -Think of these macros as working a bit like using C<{> and C<}> in Perl -to limit the scope of local variables. - -See the section I for details of -an alternative to using these macros. - -=item 3. - The purpose of the macro C 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 +allocated to the Perl stack has been reallocated whilst in the I 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 +always refresh the local copy using SPAGAIN whenever you make use of the I functions or any other Perl internal function. -=item 4. +=item 3. Although only a single value was expected to be returned from I, it is still good practice to check the return code from I @@ -687,7 +719,7 @@ didn't check for that possibility and take appropriate action the Perl stack would end up in an inconsistent state. That is something you I don't want to happen ever. -=item 5. +=item 4. The C macro is used here to pop the return value from the stack. In this case we wanted an integer, so C was used. @@ -702,7 +734,7 @@ they return. POPi integer POPl long -=item 6. +=item 5. The final C is used to leave the Perl stack in a consistent state before exiting the function. This is necessary because when we @@ -739,7 +771,7 @@ and this is the C function ENTER ; SAVETMPS; - PUSHMARK(sp) ; + PUSHMARK(SP) ; XPUSHs(sv_2mortal(newSViv(a))); XPUSHs(sv_2mortal(newSViv(b))); PUTBACK ; @@ -802,7 +834,7 @@ context, like this ENTER ; SAVETMPS; - PUSHMARK(sp) ; + PUSHMARK(SP) ; XPUSHs(sv_2mortal(newSViv(a))); XPUSHs(sv_2mortal(newSViv(b))); PUTBACK ; @@ -834,7 +866,7 @@ then the output will be Value 1 = 3 In this case the main point to note is that only the last item in the -list returned from the subroutine, I actually made it back to +list is returned from the subroutine, I actually made it back to I. @@ -870,7 +902,7 @@ and here is a C function to call it. sva = sv_2mortal(newSViv(a)) ; svb = sv_2mortal(newSViv(b)) ; - PUSHMARK(sp) ; + PUSHMARK(SP) ; XPUSHs(sva); XPUSHs(svb); PUTBACK ; @@ -890,7 +922,7 @@ 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 I it is necessary to make a note -of their addresses - thus the two variables C and C. +of their addresses--thus the two variables C and C. 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 @@ -927,7 +959,7 @@ and some C to call it ENTER ; SAVETMPS; - PUSHMARK(sp) ; + PUSHMARK(SP) ; XPUSHs(sv_2mortal(newSViv(a))); XPUSHs(sv_2mortal(newSViv(b))); PUTBACK ; @@ -937,9 +969,10 @@ and some C to call it SPAGAIN ; /* Check the eval first */ - if (SvTRUE(GvSV(errgv))) + if (SvTRUE(ERRSV)) { - printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ; + STRLEN n_a; + printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ; POPs ; } else @@ -977,11 +1010,12 @@ I. =item 2. -The code +The code - if (SvTRUE(GvSV(errgv))) + if (SvTRUE(ERRSV)) { - printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ; + STRLEN n_a; + printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ; POPs ; } @@ -989,14 +1023,14 @@ is the direct equivalent of this bit of Perl print "Uh oh - $@\n" if $@ ; -C is a perl global of type C that points to the -symbol table entry containing the error. C therefore +C is a perl global of type C that points to the +symbol table entry containing the error. C therefore refers to the C equivalent of C<$@>. =item 3. Note that the stack is popped using C in the block where -C is true. This is necessary because whenever a +C is true. This is necessary because whenever a I function invoked with G_EVAL|G_SCALAR returns an error, the top of the stack holds the value I. Because we want the program to continue after detecting this error, it is essential that @@ -1012,7 +1046,7 @@ version of the call_Subtract example above inside a destructor: package Foo; sub new { bless {}, $_[0] } - sub Subtract { + sub Subtract { my($a,$b) = @_; die "death can be fatal" if $a < $b ; $a - $b; @@ -1060,10 +1094,10 @@ Here is a snippet of XSUB which defines I. CallSubPV(name) char * name CODE: - PUSHMARK(sp) ; + PUSHMARK(SP) ; perl_call_pv(name, G_DISCARD|G_NOARGS) ; -That is fine as far as it goes. The thing is, the Perl subroutine +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 I is useful. @@ -1076,7 +1110,7 @@ I instead of I. CallSubSV(name) SV * name CODE: - PUSHMARK(sp) ; + PUSHMARK(SP) ; perl_call_sv(name, G_DISCARD|G_NOARGS) ; Because we are using an SV to call I the following can all be used @@ -1106,7 +1140,7 @@ pointer to the SV. Say the code above had been like this void CallSavedSub1() CODE: - PUSHMARK(sp) ; + PUSHMARK(SP) ; perl_call_sv(rememberSub, G_DISCARD|G_NOARGS) ; The reason this is wrong is that by the time you come to use the @@ -1121,31 +1155,31 @@ particularly true for these cases CallSavedSub1() ; By the time each of the C statements above have been executed, -the SV*'s which corresponded to the parameters will no longer exist. +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 C lines. -Similarly, with this code +Similarly, with this code $ref = \&fred ; SaveSub1($ref) ; $ref = 47 ; CallSavedSub1() ; -you can expect one of these messages (which you actually get is dependent on -the version of Perl you are using) +you can expect one of these messages (which you actually get is dependent on +the version of Perl you are using) Not a CODE reference at ... Undefined subroutine &main::47 called ... -The variable C<$ref> may have referred to the subroutine C +The variable $ref may have referred to the subroutine C whenever the call to C was made but by the time C gets called it now holds the number C<47>. Because we saved only a pointer to the original SV in C, any changes to -C<$ref> will be tracked by the pointer C. This means that +$ref will be tracked by the pointer C. This means that whenever C gets called, it will attempt to execute the code which is referenced by the SV* C. In this case though, it now refers to the integer C<47>, so expect Perl to complain @@ -1159,7 +1193,7 @@ A similar but more subtle problem is illustrated with this code CallSavedSub1() ; This time whenever C get called it will execute the Perl -subroutine C (assuming it exists) rather than C as was +subroutine C (assuming it exists) rather than C as was originally requested in the call to C. To get around these problems it is necessary to take a full copy of the @@ -1182,7 +1216,7 @@ SV. The code below shows C modified to do that void CallSavedSub2() CODE: - PUSHMARK(sp) ; + PUSHMARK(SP) ; perl_call_sv(keepSub, G_DISCARD|G_NOARGS) ; To avoid creating a new SV every time C is called, @@ -1260,7 +1294,7 @@ single element of the array. Here is an all Perl example of using it. will print 1: green - This is Class Mine version 1.0 + This is Class Mine version 1.0 Calling a Perl method from C is fairly straightforward. The following things are required @@ -1291,7 +1325,7 @@ the C and C methods from C. char * method int index CODE: - PUSHMARK(sp); + PUSHMARK(SP); XPUSHs(ref); XPUSHs(sv_2mortal(newSViv(index))) ; PUTBACK; @@ -1303,7 +1337,7 @@ the C and C methods from C. char * class char * method CODE: - PUSHMARK(sp); + PUSHMARK(SP); XPUSHs(sv_2mortal(newSVpv(class, 0))) ; PUTBACK; @@ -1317,29 +1351,34 @@ So the methods C and C can be invoked like this 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 +the method name is not passed via the stack--it is used as the first parameter to I. -=head2 Using GIMME +=head2 Using GIMME_V -Here is a trivial XSUB which prints the context in which it is +Here is a trivial XSUB which prints the context in which it is currently executing. void PrintContext() CODE: - if (GIMME == G_SCALAR) + 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 @@ -1418,26 +1457,26 @@ will be more like this perl --> XSUB --> event handler ... - event handler --> perl_call --> perl + event handler --> perl_call --> perl | - event handler <-- perl_call --<--+ + event handler <-- perl_call <----+ ... - event handler --> perl_call --> perl + event handler --> perl_call --> perl | - event handler <-- perl_call --<--+ + event handler <-- perl_call <----+ ... - event handler --> perl_call --> perl + event handler --> perl_call --> perl | - event handler <-- perl_call --<--+ + event handler <-- perl_call <----+ In this case the flow of control can consist of only the repeated sequence event handler --> perl_call --> perl -for the practically the complete duration of the program. This means -that control may I drop back to the surrounding scope in Perl at -the extreme left. +for practically the complete duration of the program. This means that +control may I 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 @@ -1446,9 +1485,9 @@ 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! +eventually consume all the available memory in your system--kapow! -So here is the bottom line - if you are sure that control will revert +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 @@ -1490,7 +1529,7 @@ Now change that to call a Perl subroutine instead { dSP ; - PUSHMARK(sp) ; + PUSHMARK(SP) ; /* Call the Perl sub to process the callback */ perl_call_sv(callback, G_DISCARD) ; @@ -1540,7 +1579,7 @@ 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 C which associates a C -function C with a file handle C - this assumes that it +function C with a file handle C--this assumes that it has also provided some routine to open the file and so obtain the file handle. @@ -1553,7 +1592,7 @@ This may expect the C I function of this form int fh ; char * buffer ; { - ... + ... } To provide a Perl interface to this library we need to be able to map @@ -1593,7 +1632,7 @@ and C could look like this if (sv == (SV**)NULL) croak("Internal error...\n") ; - PUSHMARK(sp) ; + PUSHMARK(SP) ; XPUSHs(sv_2mortal(newSViv(fh))) ; XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ; PUTBACK ; @@ -1646,7 +1685,7 @@ the C 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 +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 @@ -1677,7 +1716,7 @@ series of C functions to act as the interface to Perl, thus { dSP ; - PUSHMARK(sp) ; + PUSHMARK(SP) ; XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ; PUTBACK ; @@ -1774,7 +1813,7 @@ 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 +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. @@ -1831,7 +1870,7 @@ of values> recoded to use C instead of C. ENTER ; SAVETMPS; - PUSHMARK(sp) ; + PUSHMARK(SP) ; XPUSHs(sv_2mortal(newSViv(a))); XPUSHs(sv_2mortal(newSViv(b))); PUTBACK ; @@ -1839,8 +1878,8 @@ of values> recoded to use C instead of C. count = perl_call_pv("AddSubtract", G_ARRAY); SPAGAIN ; - sp -= count ; - ax = (sp - stack_base) + 1 ; + SP -= count ; + ax = (SP - PL_stack_base) + 1 ; if (count != 2) croak("Big trouble\n") ; @@ -1869,8 +1908,8 @@ you. The code SPAGAIN ; - sp -= count ; - ax = (sp - stack_base) + 1 ; + SP -= count ; + ax = (SP - PL_stack_base) + 1 ; sets the stack up so that we can use the C macro. @@ -1878,18 +1917,38 @@ sets the stack up so that we can use the C macro. Unlike the original coding of this example, the returned values are not accessed in reverse order. So C refers to the -first value returned by the Perl subroutine and C +first value returned by the Perl subroutine and C refers to the last. =back +=head2 Creating and calling an anonymous subroutine in C + +As we've already shown, C can be used to invoke an +anonymous subroutine. However, our example showed a Perl script +invoking an XSUB to perform this operation. Let's see how it can be +done 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); + +C is used to compile the anonymous subroutine, which +will be the return value as well (read more about C in +L). Once this code reference is in hand, it +can be mixed in with all the previous examples we've shown. + =head1 SEE ALSO L, L, L =head1 AUTHOR -Paul Marquess > +Paul Marquess Special thanks to the following people who assisted in the creation of the document. @@ -1899,4 +1958,4 @@ and Larry Wall. =head1 DATE -Version 1.2, 16th Jan 1996 +Version 1.3, 14th Apr 1997