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diff --git a/contrib/perl5/pod/perlxs.pod b/contrib/perl5/pod/perlxs.pod new file mode 100644 index 0000000..c578a2e --- /dev/null +++ b/contrib/perl5/pod/perlxs.pod @@ -0,0 +1,1348 @@ +=head1 NAME + +perlxs - XS language reference manual + +=head1 DESCRIPTION + +=head2 Introduction + +XS is a language used to create an extension interface +between Perl and some C library which one wishes to use with +Perl. The XS interface is combined with the library to +create a new library which can be linked to Perl. An B<XSUB> +is a function in the XS language and is the core component +of the Perl application interface. + +The XS compiler is called B<xsubpp>. This compiler will embed +the constructs necessary to let an XSUB, which is really a C +function in disguise, manipulate Perl values and creates the +glue necessary to let Perl access the XSUB. The compiler +uses B<typemaps> to determine how to map C function parameters +and variables to Perl values. The default typemap handles +many common C types. A supplement typemap must be created +to handle special structures and types for the library being +linked. + +See L<perlxstut> for a tutorial on the whole extension creation process. + +Note: For many extensions, Dave Beazley's SWIG system provides a +significantly more convenient mechanism for creating the XS glue +code. See L<http://www.cs.utah.edu/~beazley/SWIG> for more +information. + +=head2 On The Road + +Many of the examples which follow will concentrate on creating an interface +between Perl and the ONC+ RPC bind library functions. The rpcb_gettime() +function is used to demonstrate many features of the XS language. This +function has two parameters; the first is an input parameter and the second +is an output parameter. The function also returns a status value. + + bool_t rpcb_gettime(const char *host, time_t *timep); + +From C this function will be called with the following +statements. + + #include <rpc/rpc.h> + bool_t status; + time_t timep; + status = rpcb_gettime( "localhost", &timep ); + +If an XSUB is created to offer a direct translation between this function +and Perl, then this XSUB will be used from Perl with the following code. +The $status and $timep variables will contain the output of the function. + + use RPC; + $status = rpcb_gettime( "localhost", $timep ); + +The following XS file shows an XS subroutine, or XSUB, which +demonstrates one possible interface to the rpcb_gettime() +function. This XSUB represents a direct translation between +C and Perl and so preserves the interface even from Perl. +This XSUB will be invoked from Perl with the usage shown +above. Note that the first three #include statements, for +C<EXTERN.h>, C<perl.h>, and C<XSUB.h>, will always be present at the +beginning of an XS file. This approach and others will be +expanded later in this document. + + #include "EXTERN.h" + #include "perl.h" + #include "XSUB.h" + #include <rpc/rpc.h> + + MODULE = RPC PACKAGE = RPC + + bool_t + rpcb_gettime(host,timep) + char *host + time_t &timep + OUTPUT: + timep + +Any extension to Perl, including those containing XSUBs, +should have a Perl module to serve as the bootstrap which +pulls the extension into Perl. This module will export the +extension's functions and variables to the Perl program and +will cause the extension's XSUBs to be linked into Perl. +The following module will be used for most of the examples +in this document and should be used from Perl with the C<use> +command as shown earlier. Perl modules are explained in +more detail later in this document. + + package RPC; + + require Exporter; + require DynaLoader; + @ISA = qw(Exporter DynaLoader); + @EXPORT = qw( rpcb_gettime ); + + bootstrap RPC; + 1; + +Throughout this document a variety of interfaces to the rpcb_gettime() +XSUB will be explored. The XSUBs will take their parameters in different +orders or will take different numbers of parameters. In each case the +XSUB is an abstraction between Perl and the real C rpcb_gettime() +function, and the XSUB must always ensure that the real rpcb_gettime() +function is called with the correct parameters. This abstraction will +allow the programmer to create a more Perl-like interface to the C +function. + +=head2 The Anatomy of an XSUB + +The following XSUB allows a Perl program to access a C library function +called sin(). The XSUB will imitate the C function which takes a single +argument and returns a single value. + + double + sin(x) + double x + +When using C pointers the indirection operator C<*> should be considered +part of the type and the address operator C<&> should be considered part of +the variable, as is demonstrated in the rpcb_gettime() function above. See +the section on typemaps for more about handling qualifiers and unary +operators in C types. + +The function name and the return type must be placed on +separate lines. + + INCORRECT CORRECT + + double sin(x) double + double x sin(x) + double x + +The function body may be indented or left-adjusted. The following example +shows a function with its body left-adjusted. Most examples in this +document will indent the body. + + CORRECT + + double + sin(x) + double x + +=head2 The Argument Stack + +The argument stack is used to store the values which are +sent as parameters to the XSUB and to store the XSUB's +return value. In reality all Perl functions keep their +values on this stack at the same time, each limited to its +own range of positions on the stack. In this document the +first position on that stack which belongs to the active +function will be referred to as position 0 for that function. + +XSUBs refer to their stack arguments with the macro B<ST(x)>, where I<x> +refers to a position in this XSUB's part of the stack. Position 0 for that +function would be known to the XSUB as ST(0). The XSUB's incoming +parameters and outgoing return values always begin at ST(0). For many +simple cases the B<xsubpp> compiler will generate the code necessary to +handle the argument stack by embedding code fragments found in the +typemaps. In more complex cases the programmer must supply the code. + +=head2 The RETVAL Variable + +The RETVAL variable is a magic variable which always matches +the return type of the C library function. The B<xsubpp> compiler will +supply this variable in each XSUB and by default will use it to hold the +return value of the C library function being called. In simple cases the +value of RETVAL will be placed in ST(0) of the argument stack where it can +be received by Perl as the return value of the XSUB. + +If the XSUB has a return type of C<void> then the compiler will +not supply a RETVAL variable for that function. When using +the PPCODE: directive the RETVAL variable is not needed, unless used +explicitly. + +If PPCODE: directive is not used, C<void> return value should be used +only for subroutines which do not return a value, I<even if> CODE: +directive is used which sets ST(0) explicitly. + +Older versions of this document recommended to use C<void> return +value in such cases. It was discovered that this could lead to +segfaults in cases when XSUB was I<truely> C<void>. This practice is +now deprecated, and may be not supported at some future version. Use +the return value C<SV *> in such cases. (Currently C<xsubpp> contains +some heuristic code which tries to disambiguate between "truely-void" +and "old-practice-declared-as-void" functions. Hence your code is at +mercy of this heuristics unless you use C<SV *> as return value.) + +=head2 The MODULE Keyword + +The MODULE keyword is used to start the XS code and to +specify the package of the functions which are being +defined. All text preceding the first MODULE keyword is +considered C code and is passed through to the output +untouched. Every XS module will have a bootstrap function +which is used to hook the XSUBs into Perl. The package name +of this bootstrap function will match the value of the last +MODULE statement in the XS source files. The value of +MODULE should always remain constant within the same XS +file, though this is not required. + +The following example will start the XS code and will place +all functions in a package named RPC. + + MODULE = RPC + +=head2 The PACKAGE Keyword + +When functions within an XS source file must be separated into packages +the PACKAGE keyword should be used. This keyword is used with the MODULE +keyword and must follow immediately after it when used. + + MODULE = RPC PACKAGE = RPC + + [ XS code in package RPC ] + + MODULE = RPC PACKAGE = RPCB + + [ XS code in package RPCB ] + + MODULE = RPC PACKAGE = RPC + + [ XS code in package RPC ] + +Although this keyword is optional and in some cases provides redundant +information it should always be used. This keyword will ensure that the +XSUBs appear in the desired package. + +=head2 The PREFIX Keyword + +The PREFIX keyword designates prefixes which should be +removed from the Perl function names. If the C function is +C<rpcb_gettime()> and the PREFIX value is C<rpcb_> then Perl will +see this function as C<gettime()>. + +This keyword should follow the PACKAGE keyword when used. +If PACKAGE is not used then PREFIX should follow the MODULE +keyword. + + MODULE = RPC PREFIX = rpc_ + + MODULE = RPC PACKAGE = RPCB PREFIX = rpcb_ + +=head2 The OUTPUT: Keyword + +The OUTPUT: keyword indicates that certain function parameters should be +updated (new values made visible to Perl) when the XSUB terminates or that +certain values should be returned to the calling Perl function. For +simple functions, such as the sin() function above, the RETVAL variable is +automatically designated as an output value. In more complex functions +the B<xsubpp> compiler will need help to determine which variables are output +variables. + +This keyword will normally be used to complement the CODE: keyword. +The RETVAL variable is not recognized as an output variable when the +CODE: keyword is present. The OUTPUT: keyword is used in this +situation to tell the compiler that RETVAL really is an output +variable. + +The OUTPUT: keyword can also be used to indicate that function parameters +are output variables. This may be necessary when a parameter has been +modified within the function and the programmer would like the update to +be seen by Perl. + + bool_t + rpcb_gettime(host,timep) + char *host + time_t &timep + OUTPUT: + timep + +The OUTPUT: keyword will also allow an output parameter to +be mapped to a matching piece of code rather than to a +typemap. + + bool_t + rpcb_gettime(host,timep) + char *host + time_t &timep + OUTPUT: + timep sv_setnv(ST(1), (double)timep); + +B<xsubpp> emits an automatic C<SvSETMAGIC()> for all parameters in the +OUTPUT section of the XSUB, except RETVAL. This is the usually desired +behavior, as it takes care of properly invoking 'set' magic on output +parameters (needed for hash or array element parameters that must be +created if they didn't exist). If for some reason, this behavior is +not desired, the OUTPUT section may contain a C<SETMAGIC: DISABLE> line +to disable it for the remainder of the parameters in the OUTPUT section. +Likewise, C<SETMAGIC: ENABLE> can be used to reenable it for the +remainder of the OUTPUT section. See L<perlguts> for more details +about 'set' magic. + +=head2 The CODE: Keyword + +This keyword is used in more complicated XSUBs which require +special handling for the C function. The RETVAL variable is +available but will not be returned unless it is specified +under the OUTPUT: keyword. + +The following XSUB is for a C function which requires special handling of +its parameters. The Perl usage is given first. + + $status = rpcb_gettime( "localhost", $timep ); + +The XSUB follows. + + bool_t + rpcb_gettime(host,timep) + char *host + time_t timep + CODE: + RETVAL = rpcb_gettime( host, &timep ); + OUTPUT: + timep + RETVAL + +=head2 The INIT: Keyword + +The INIT: keyword allows initialization to be inserted into the XSUB before +the compiler generates the call to the C function. Unlike the CODE: keyword +above, this keyword does not affect the way the compiler handles RETVAL. + + bool_t + rpcb_gettime(host,timep) + char *host + time_t &timep + INIT: + printf("# Host is %s\n", host ); + OUTPUT: + timep + +=head2 The NO_INIT Keyword + +The NO_INIT keyword is used to indicate that a function +parameter is being used only as an output value. The B<xsubpp> +compiler will normally generate code to read the values of +all function parameters from the argument stack and assign +them to C variables upon entry to the function. NO_INIT +will tell the compiler that some parameters will be used for +output rather than for input and that they will be handled +before the function terminates. + +The following example shows a variation of the rpcb_gettime() function. +This function uses the timep variable only as an output variable and does +not care about its initial contents. + + bool_t + rpcb_gettime(host,timep) + char *host + time_t &timep = NO_INIT + OUTPUT: + timep + +=head2 Initializing Function Parameters + +Function parameters are normally initialized with their +values from the argument stack. The typemaps contain the +code segments which are used to transfer the Perl values to +the C parameters. The programmer, however, is allowed to +override the typemaps and supply alternate (or additional) +initialization code. + +The following code demonstrates how to supply initialization code for +function parameters. The initialization code is eval'd within double +quotes by the compiler before it is added to the output so anything +which should be interpreted literally [mainly C<$>, C<@>, or C<\\>] +must be protected with backslashes. The variables C<$var>, C<$arg>, +and C<$type> can be used as in typemaps. + + bool_t + rpcb_gettime(host,timep) + char *host = (char *)SvPV($arg,PL_na); + time_t &timep = 0; + OUTPUT: + timep + +This should not be used to supply default values for parameters. One +would normally use this when a function parameter must be processed by +another library function before it can be used. Default parameters are +covered in the next section. + +If the initialization begins with C<=>, then it is output on +the same line where the input variable is declared. If the +initialization begins with C<;> or C<+>, then it is output after +all of the input variables have been declared. The C<=> and C<;> +cases replace the initialization normally supplied from the typemap. +For the C<+> case, the initialization from the typemap will preceed +the initialization code included after the C<+>. A global +variable, C<%v>, is available for the truely rare case where +information from one initialization is needed in another +initialization. + + bool_t + rpcb_gettime(host,timep) + time_t &timep ; /*\$v{time}=@{[$v{time}=$arg]}*/ + char *host + SvOK($v{time}) ? SvPV($arg,PL_na) : NULL; + OUTPUT: + timep + +=head2 Default Parameter Values + +Default values can be specified for function parameters by +placing an assignment statement in the parameter list. The +default value may be a number or a string. Defaults should +always be used on the right-most parameters only. + +To allow the XSUB for rpcb_gettime() to have a default host +value the parameters to the XSUB could be rearranged. The +XSUB will then call the real rpcb_gettime() function with +the parameters in the correct order. Perl will call this +XSUB with either of the following statements. + + $status = rpcb_gettime( $timep, $host ); + + $status = rpcb_gettime( $timep ); + +The XSUB will look like the code which follows. A CODE: +block is used to call the real rpcb_gettime() function with +the parameters in the correct order for that function. + + bool_t + rpcb_gettime(timep,host="localhost") + char *host + time_t timep = NO_INIT + CODE: + RETVAL = rpcb_gettime( host, &timep ); + OUTPUT: + timep + RETVAL + +=head2 The PREINIT: Keyword + +The PREINIT: keyword allows extra variables to be declared before the +typemaps are expanded. If a variable is declared in a CODE: block then that +variable will follow any typemap code. This may result in a C syntax +error. To force the variable to be declared before the typemap code, place +it into a PREINIT: block. The PREINIT: keyword may be used one or more +times within an XSUB. + +The following examples are equivalent, but if the code is using complex +typemaps then the first example is safer. + + bool_t + rpcb_gettime(timep) + time_t timep = NO_INIT + PREINIT: + char *host = "localhost"; + CODE: + RETVAL = rpcb_gettime( host, &timep ); + OUTPUT: + timep + RETVAL + +A correct, but error-prone example. + + bool_t + rpcb_gettime(timep) + time_t timep = NO_INIT + CODE: + char *host = "localhost"; + RETVAL = rpcb_gettime( host, &timep ); + OUTPUT: + timep + RETVAL + +=head2 The SCOPE: Keyword + +The SCOPE: keyword allows scoping to be enabled for a particular XSUB. If +enabled, the XSUB will invoke ENTER and LEAVE automatically. + +To support potentially complex type mappings, if a typemap entry used +by this XSUB contains a comment like C</*scope*/> then scoping will +automatically be enabled for that XSUB. + +To enable scoping: + + SCOPE: ENABLE + +To disable scoping: + + SCOPE: DISABLE + +=head2 The INPUT: Keyword + +The XSUB's parameters are usually evaluated immediately after entering the +XSUB. The INPUT: keyword can be used to force those parameters to be +evaluated a little later. The INPUT: keyword can be used multiple times +within an XSUB and can be used to list one or more input variables. This +keyword is used with the PREINIT: keyword. + +The following example shows how the input parameter C<timep> can be +evaluated late, after a PREINIT. + + bool_t + rpcb_gettime(host,timep) + char *host + PREINIT: + time_t tt; + INPUT: + time_t timep + CODE: + RETVAL = rpcb_gettime( host, &tt ); + timep = tt; + OUTPUT: + timep + RETVAL + +The next example shows each input parameter evaluated late. + + bool_t + rpcb_gettime(host,timep) + PREINIT: + time_t tt; + INPUT: + char *host + PREINIT: + char *h; + INPUT: + time_t timep + CODE: + h = host; + RETVAL = rpcb_gettime( h, &tt ); + timep = tt; + OUTPUT: + timep + RETVAL + +=head2 Variable-length Parameter Lists + +XSUBs can have variable-length parameter lists by specifying an ellipsis +C<(...)> in the parameter list. This use of the ellipsis is similar to that +found in ANSI C. The programmer is able to determine the number of +arguments passed to the XSUB by examining the C<items> variable which the +B<xsubpp> compiler supplies for all XSUBs. By using this mechanism one can +create an XSUB which accepts a list of parameters of unknown length. + +The I<host> parameter for the rpcb_gettime() XSUB can be +optional so the ellipsis can be used to indicate that the +XSUB will take a variable number of parameters. Perl should +be able to call this XSUB with either of the following statements. + + $status = rpcb_gettime( $timep, $host ); + + $status = rpcb_gettime( $timep ); + +The XS code, with ellipsis, follows. + + bool_t + rpcb_gettime(timep, ...) + time_t timep = NO_INIT + PREINIT: + char *host = "localhost"; + CODE: + if( items > 1 ) + host = (char *)SvPV(ST(1), PL_na); + RETVAL = rpcb_gettime( host, &timep ); + OUTPUT: + timep + RETVAL + +=head2 The C_ARGS: Keyword + +The C_ARGS: keyword allows creating of XSUBS which have different +calling sequence from Perl than from C, without a need to write +CODE: or CPPCODE: section. The contents of the C_ARGS: paragraph is +put as the argument to the called C function without any change. + +For example, suppose that C function is declared as + + symbolic nth_derivative(int n, symbolic function, int flags); + +and that the default flags are kept in a global C variable +C<default_flags>. Suppose that you want to create an interface which +is called as + + $second_deriv = $function->nth_derivative(2); + +To do this, declare the XSUB as + + symbolic + nth_derivative(function, n) + symbolic function + int n + C_ARGS: + n, function, default_flags + +=head2 The PPCODE: Keyword + +The PPCODE: keyword is an alternate form of the CODE: keyword and is used +to tell the B<xsubpp> compiler that the programmer is supplying the code to +control the argument stack for the XSUBs return values. Occasionally one +will want an XSUB to return a list of values rather than a single value. +In these cases one must use PPCODE: and then explicitly push the list of +values on the stack. The PPCODE: and CODE: keywords are not used +together within the same XSUB. + +The following XSUB will call the C rpcb_gettime() function +and will return its two output values, timep and status, to +Perl as a single list. + + void + rpcb_gettime(host) + char *host + PREINIT: + time_t timep; + bool_t status; + PPCODE: + status = rpcb_gettime( host, &timep ); + EXTEND(SP, 2); + PUSHs(sv_2mortal(newSViv(status))); + PUSHs(sv_2mortal(newSViv(timep))); + +Notice that the programmer must supply the C code necessary +to have the real rpcb_gettime() function called and to have +the return values properly placed on the argument stack. + +The C<void> return type for this function tells the B<xsubpp> compiler that +the RETVAL variable is not needed or used and that it should not be created. +In most scenarios the void return type should be used with the PPCODE: +directive. + +The EXTEND() macro is used to make room on the argument +stack for 2 return values. The PPCODE: directive causes the +B<xsubpp> compiler to create a stack pointer available as C<SP>, and it +is this pointer which is being used in the EXTEND() macro. +The values are then pushed onto the stack with the PUSHs() +macro. + +Now the rpcb_gettime() function can be used from Perl with +the following statement. + + ($status, $timep) = rpcb_gettime("localhost"); + +When handling output parameters with a PPCODE section, be sure to handle +'set' magic properly. See L<perlguts> for details about 'set' magic. + +=head2 Returning Undef And Empty Lists + +Occasionally the programmer will want to return simply +C<undef> or an empty list if a function fails rather than a +separate status value. The rpcb_gettime() function offers +just this situation. If the function succeeds we would like +to have it return the time and if it fails we would like to +have undef returned. In the following Perl code the value +of $timep will either be undef or it will be a valid time. + + $timep = rpcb_gettime( "localhost" ); + +The following XSUB uses the C<SV *> return type as a mnemonic only, +and uses a CODE: block to indicate to the compiler +that the programmer has supplied all the necessary code. The +sv_newmortal() call will initialize the return value to undef, making that +the default return value. + + SV * + rpcb_gettime(host) + char * host + PREINIT: + time_t timep; + bool_t x; + CODE: + ST(0) = sv_newmortal(); + if( rpcb_gettime( host, &timep ) ) + sv_setnv( ST(0), (double)timep); + +The next example demonstrates how one would place an explicit undef in the +return value, should the need arise. + + SV * + rpcb_gettime(host) + char * host + PREINIT: + time_t timep; + bool_t x; + CODE: + ST(0) = sv_newmortal(); + if( rpcb_gettime( host, &timep ) ){ + sv_setnv( ST(0), (double)timep); + } + else{ + ST(0) = &PL_sv_undef; + } + +To return an empty list one must use a PPCODE: block and +then not push return values on the stack. + + void + rpcb_gettime(host) + char *host + PREINIT: + time_t timep; + PPCODE: + if( rpcb_gettime( host, &timep ) ) + PUSHs(sv_2mortal(newSViv(timep))); + else{ + /* Nothing pushed on stack, so an empty */ + /* list is implicitly returned. */ + } + +Some people may be inclined to include an explicit C<return> in the above +XSUB, rather than letting control fall through to the end. In those +situations C<XSRETURN_EMPTY> should be used, instead. This will ensure that +the XSUB stack is properly adjusted. Consult L<perlguts/"API LISTING"> for +other C<XSRETURN> macros. + +=head2 The REQUIRE: Keyword + +The REQUIRE: keyword is used to indicate the minimum version of the +B<xsubpp> compiler needed to compile the XS module. An XS module which +contains the following statement will compile with only B<xsubpp> version +1.922 or greater: + + REQUIRE: 1.922 + +=head2 The CLEANUP: Keyword + +This keyword can be used when an XSUB requires special cleanup procedures +before it terminates. When the CLEANUP: keyword is used it must follow +any CODE:, PPCODE:, or OUTPUT: blocks which are present in the XSUB. The +code specified for the cleanup block will be added as the last statements +in the XSUB. + +=head2 The BOOT: Keyword + +The BOOT: keyword is used to add code to the extension's bootstrap +function. The bootstrap function is generated by the B<xsubpp> compiler and +normally holds the statements necessary to register any XSUBs with Perl. +With the BOOT: keyword the programmer can tell the compiler to add extra +statements to the bootstrap function. + +This keyword may be used any time after the first MODULE keyword and should +appear on a line by itself. The first blank line after the keyword will +terminate the code block. + + BOOT: + # The following message will be printed when the + # bootstrap function executes. + printf("Hello from the bootstrap!\n"); + +=head2 The VERSIONCHECK: Keyword + +The VERSIONCHECK: keyword corresponds to B<xsubpp>'s C<-versioncheck> and +C<-noversioncheck> options. This keyword overrides the command line +options. Version checking is enabled by default. When version checking is +enabled the XS module will attempt to verify that its version matches the +version of the PM module. + +To enable version checking: + + VERSIONCHECK: ENABLE + +To disable version checking: + + VERSIONCHECK: DISABLE + +=head2 The PROTOTYPES: Keyword + +The PROTOTYPES: keyword corresponds to B<xsubpp>'s C<-prototypes> and +C<-noprototypes> options. This keyword overrides the command line options. +Prototypes are enabled by default. When prototypes are enabled XSUBs will +be given Perl prototypes. This keyword may be used multiple times in an XS +module to enable and disable prototypes for different parts of the module. + +To enable prototypes: + + PROTOTYPES: ENABLE + +To disable prototypes: + + PROTOTYPES: DISABLE + +=head2 The PROTOTYPE: Keyword + +This keyword is similar to the PROTOTYPES: keyword above but can be used to +force B<xsubpp> to use a specific prototype for the XSUB. This keyword +overrides all other prototype options and keywords but affects only the +current XSUB. Consult L<perlsub/Prototypes> for information about Perl +prototypes. + + bool_t + rpcb_gettime(timep, ...) + time_t timep = NO_INIT + PROTOTYPE: $;$ + PREINIT: + char *host = "localhost"; + CODE: + if( items > 1 ) + host = (char *)SvPV(ST(1), PL_na); + RETVAL = rpcb_gettime( host, &timep ); + OUTPUT: + timep + RETVAL + +=head2 The ALIAS: Keyword + +The ALIAS: keyword allows an XSUB to have two or more unique Perl names +and to know which of those names was used when it was invoked. The Perl +names may be fully-qualified with package names. Each alias is given an +index. The compiler will setup a variable called C<ix> which contain the +index of the alias which was used. When the XSUB is called with its +declared name C<ix> will be 0. + +The following example will create aliases C<FOO::gettime()> and +C<BAR::getit()> for this function. + + bool_t + rpcb_gettime(host,timep) + char *host + time_t &timep + ALIAS: + FOO::gettime = 1 + BAR::getit = 2 + INIT: + printf("# ix = %d\n", ix ); + OUTPUT: + timep + +=head2 The INTERFACE: Keyword + +This keyword declares the current XSUB as a keeper of the given +calling signature. If some text follows this keyword, it is +considered as a list of functions which have this signature, and +should be attached to XSUBs. + +Say, if you have 4 functions multiply(), divide(), add(), subtract() all +having the signature + + symbolic f(symbolic, symbolic); + +you code them all by using XSUB + + symbolic + interface_s_ss(arg1, arg2) + symbolic arg1 + symbolic arg2 + INTERFACE: + multiply divide + add subtract + +The advantage of this approach comparing to ALIAS: keyword is that one +can attach an extra function remainder() at runtime by using + + CV *mycv = newXSproto("Symbolic::remainder", + XS_Symbolic_interface_s_ss, __FILE__, "$$"); + XSINTERFACE_FUNC_SET(mycv, remainder); + +(This example supposes that there was no INTERFACE_MACRO: section, +otherwise one needs to use something else instead of +C<XSINTERFACE_FUNC_SET>.) + +=head2 The INTERFACE_MACRO: Keyword + +This keyword allows one to define an INTERFACE using a different way +to extract a function pointer from an XSUB. The text which follows +this keyword should give the name of macros which would extract/set a +function pointer. The extractor macro is given return type, C<CV*>, +and C<XSANY.any_dptr> for this C<CV*>. The setter macro is given cv, +and the function pointer. + +The default value is C<XSINTERFACE_FUNC> and C<XSINTERFACE_FUNC_SET>. +An INTERFACE keyword with an empty list of functions can be omitted if +INTERFACE_MACRO keyword is used. + +Suppose that in the previous example functions pointers for +multiply(), divide(), add(), subtract() are kept in a global C array +C<fp[]> with offsets being C<multiply_off>, C<divide_off>, C<add_off>, +C<subtract_off>. Then one can use + + #define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \ + ((XSINTERFACE_CVT(ret,))fp[CvXSUBANY(cv).any_i32]) + #define XSINTERFACE_FUNC_BYOFFSET_set(cv,f) \ + CvXSUBANY(cv).any_i32 = CAT2( f, _off ) + +in C section, + + symbolic + interface_s_ss(arg1, arg2) + symbolic arg1 + symbolic arg2 + INTERFACE_MACRO: + XSINTERFACE_FUNC_BYOFFSET + XSINTERFACE_FUNC_BYOFFSET_set + INTERFACE: + multiply divide + add subtract + +in XSUB section. + +=head2 The INCLUDE: Keyword + +This keyword can be used to pull other files into the XS module. The other +files may have XS code. INCLUDE: can also be used to run a command to +generate the XS code to be pulled into the module. + +The file F<Rpcb1.xsh> contains our C<rpcb_gettime()> function: + + bool_t + rpcb_gettime(host,timep) + char *host + time_t &timep + OUTPUT: + timep + +The XS module can use INCLUDE: to pull that file into it. + + INCLUDE: Rpcb1.xsh + +If the parameters to the INCLUDE: keyword are followed by a pipe (C<|>) then +the compiler will interpret the parameters as a command. + + INCLUDE: cat Rpcb1.xsh | + +=head2 The CASE: Keyword + +The CASE: keyword allows an XSUB to have multiple distinct parts with each +part acting as a virtual XSUB. CASE: is greedy and if it is used then all +other XS keywords must be contained within a CASE:. This means nothing may +precede the first CASE: in the XSUB and anything following the last CASE: is +included in that case. + +A CASE: might switch via a parameter of the XSUB, via the C<ix> ALIAS: +variable (see L<"The ALIAS: Keyword">), or maybe via the C<items> variable +(see L<"Variable-length Parameter Lists">). The last CASE: becomes the +B<default> case if it is not associated with a conditional. The following +example shows CASE switched via C<ix> with a function C<rpcb_gettime()> +having an alias C<x_gettime()>. When the function is called as +C<rpcb_gettime()> its parameters are the usual C<(char *host, time_t *timep)>, +but when the function is called as C<x_gettime()> its parameters are +reversed, C<(time_t *timep, char *host)>. + + long + rpcb_gettime(a,b) + CASE: ix == 1 + ALIAS: + x_gettime = 1 + INPUT: + # 'a' is timep, 'b' is host + char *b + time_t a = NO_INIT + CODE: + RETVAL = rpcb_gettime( b, &a ); + OUTPUT: + a + RETVAL + CASE: + # 'a' is host, 'b' is timep + char *a + time_t &b = NO_INIT + OUTPUT: + b + RETVAL + +That function can be called with either of the following statements. Note +the different argument lists. + + $status = rpcb_gettime( $host, $timep ); + + $status = x_gettime( $timep, $host ); + +=head2 The & Unary Operator + +The & unary operator is used to tell the compiler that it should dereference +the object when it calls the C function. This is used when a CODE: block is +not used and the object is a not a pointer type (the object is an C<int> or +C<long> but not a C<int*> or C<long*>). + +The following XSUB will generate incorrect C code. The xsubpp compiler will +turn this into code which calls C<rpcb_gettime()> with parameters C<(char +*host, time_t timep)>, but the real C<rpcb_gettime()> wants the C<timep> +parameter to be of type C<time_t*> rather than C<time_t>. + + bool_t + rpcb_gettime(host,timep) + char *host + time_t timep + OUTPUT: + timep + +That problem is corrected by using the C<&> operator. The xsubpp compiler +will now turn this into code which calls C<rpcb_gettime()> correctly with +parameters C<(char *host, time_t *timep)>. It does this by carrying the +C<&> through, so the function call looks like C<rpcb_gettime(host, &timep)>. + + bool_t + rpcb_gettime(host,timep) + char *host + time_t &timep + OUTPUT: + timep + +=head2 Inserting Comments and C Preprocessor Directives + +C preprocessor directives are allowed within BOOT:, PREINIT: INIT:, +CODE:, PPCODE:, and CLEANUP: blocks, as well as outside the functions. +Comments are allowed anywhere after the MODULE keyword. The compiler +will pass the preprocessor directives through untouched and will remove +the commented lines. + +Comments can be added to XSUBs by placing a C<#> as the first +non-whitespace of a line. Care should be taken to avoid making the +comment look like a C preprocessor directive, lest it be interpreted as +such. The simplest way to prevent this is to put whitespace in front of +the C<#>. + +If you use preprocessor directives to choose one of two +versions of a function, use + + #if ... version1 + #else /* ... version2 */ + #endif + +and not + + #if ... version1 + #endif + #if ... version2 + #endif + +because otherwise xsubpp will believe that you made a duplicate +definition of the function. Also, put a blank line before the +#else/#endif so it will not be seen as part of the function body. + +=head2 Using XS With C++ + +If a function is defined as a C++ method then it will assume +its first argument is an object pointer. The object pointer +will be stored in a variable called THIS. The object should +have been created by C++ with the new() function and should +be blessed by Perl with the sv_setref_pv() macro. The +blessing of the object by Perl can be handled by a typemap. An example +typemap is shown at the end of this section. + +If the method is defined as static it will call the C++ +function using the class::method() syntax. If the method is not static +the function will be called using the THIS-E<gt>method() syntax. + +The next examples will use the following C++ class. + + class color { + public: + color(); + ~color(); + int blue(); + void set_blue( int ); + + private: + int c_blue; + }; + +The XSUBs for the blue() and set_blue() methods are defined with the class +name but the parameter for the object (THIS, or "self") is implicit and is +not listed. + + int + color::blue() + + void + color::set_blue( val ) + int val + +Both functions will expect an object as the first parameter. The xsubpp +compiler will call that object C<THIS> and will use it to call the specified +method. So in the C++ code the blue() and set_blue() methods will be called +in the following manner. + + RETVAL = THIS->blue(); + + THIS->set_blue( val ); + +If the function's name is B<DESTROY> then the C++ C<delete> function will be +called and C<THIS> will be given as its parameter. + + void + color::DESTROY() + +The C++ code will call C<delete>. + + delete THIS; + +If the function's name is B<new> then the C++ C<new> function will be called +to create a dynamic C++ object. The XSUB will expect the class name, which +will be kept in a variable called C<CLASS>, to be given as the first +argument. + + color * + color::new() + +The C++ code will call C<new>. + + RETVAL = new color(); + +The following is an example of a typemap that could be used for this C++ +example. + + TYPEMAP + color * O_OBJECT + + OUTPUT + # The Perl object is blessed into 'CLASS', which should be a + # char* having the name of the package for the blessing. + O_OBJECT + sv_setref_pv( $arg, CLASS, (void*)$var ); + + INPUT + O_OBJECT + if( sv_isobject($arg) && (SvTYPE(SvRV($arg)) == SVt_PVMG) ) + $var = ($type)SvIV((SV*)SvRV( $arg )); + else{ + warn( \"${Package}::$func_name() -- $var is not a blessed SV reference\" ); + XSRETURN_UNDEF; + } + +=head2 Interface Strategy + +When designing an interface between Perl and a C library a straight +translation from C to XS is often sufficient. The interface will often be +very C-like and occasionally nonintuitive, especially when the C function +modifies one of its parameters. In cases where the programmer wishes to +create a more Perl-like interface the following strategy may help to +identify the more critical parts of the interface. + +Identify the C functions which modify their parameters. The XSUBs for +these functions may be able to return lists to Perl, or may be +candidates to return undef or an empty list in case of failure. + +Identify which values are used by only the C and XSUB functions +themselves. If Perl does not need to access the contents of the value +then it may not be necessary to provide a translation for that value +from C to Perl. + +Identify the pointers in the C function parameter lists and return +values. Some pointers can be handled in XS with the & unary operator on +the variable name while others will require the use of the * operator on +the type name. In general it is easier to work with the & operator. + +Identify the structures used by the C functions. In many +cases it may be helpful to use the T_PTROBJ typemap for +these structures so they can be manipulated by Perl as +blessed objects. + +=head2 Perl Objects And C Structures + +When dealing with C structures one should select either +B<T_PTROBJ> or B<T_PTRREF> for the XS type. Both types are +designed to handle pointers to complex objects. The +T_PTRREF type will allow the Perl object to be unblessed +while the T_PTROBJ type requires that the object be blessed. +By using T_PTROBJ one can achieve a form of type-checking +because the XSUB will attempt to verify that the Perl object +is of the expected type. + +The following XS code shows the getnetconfigent() function which is used +with ONC+ TIRPC. The getnetconfigent() function will return a pointer to a +C structure and has the C prototype shown below. The example will +demonstrate how the C pointer will become a Perl reference. Perl will +consider this reference to be a pointer to a blessed object and will +attempt to call a destructor for the object. A destructor will be +provided in the XS source to free the memory used by getnetconfigent(). +Destructors in XS can be created by specifying an XSUB function whose name +ends with the word B<DESTROY>. XS destructors can be used to free memory +which may have been malloc'd by another XSUB. + + struct netconfig *getnetconfigent(const char *netid); + +A C<typedef> will be created for C<struct netconfig>. The Perl +object will be blessed in a class matching the name of the C +type, with the tag C<Ptr> appended, and the name should not +have embedded spaces if it will be a Perl package name. The +destructor will be placed in a class corresponding to the +class of the object and the PREFIX keyword will be used to +trim the name to the word DESTROY as Perl will expect. + + typedef struct netconfig Netconfig; + + MODULE = RPC PACKAGE = RPC + + Netconfig * + getnetconfigent(netid) + char *netid + + MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_ + + void + rpcb_DESTROY(netconf) + Netconfig *netconf + CODE: + printf("Now in NetconfigPtr::DESTROY\n"); + free( netconf ); + +This example requires the following typemap entry. Consult the typemap +section for more information about adding new typemaps for an extension. + + TYPEMAP + Netconfig * T_PTROBJ + +This example will be used with the following Perl statements. + + use RPC; + $netconf = getnetconfigent("udp"); + +When Perl destroys the object referenced by $netconf it will send the +object to the supplied XSUB DESTROY function. Perl cannot determine, and +does not care, that this object is a C struct and not a Perl object. In +this sense, there is no difference between the object created by the +getnetconfigent() XSUB and an object created by a normal Perl subroutine. + +=head2 The Typemap + +The typemap is a collection of code fragments which are used by the B<xsubpp> +compiler to map C function parameters and values to Perl values. The +typemap file may consist of three sections labeled C<TYPEMAP>, C<INPUT>, and +C<OUTPUT>. The INPUT section tells the compiler how to translate Perl values +into variables of certain C types. The OUTPUT section tells the compiler +how to translate the values from certain C types into values Perl can +understand. The TYPEMAP section tells the compiler which of the INPUT and +OUTPUT code fragments should be used to map a given C type to a Perl value. +Each of the sections of the typemap must be preceded by one of the TYPEMAP, +INPUT, or OUTPUT keywords. + +The default typemap in the C<ext> directory of the Perl source contains many +useful types which can be used by Perl extensions. Some extensions define +additional typemaps which they keep in their own directory. These +additional typemaps may reference INPUT and OUTPUT maps in the main +typemap. The B<xsubpp> compiler will allow the extension's own typemap to +override any mappings which are in the default typemap. + +Most extensions which require a custom typemap will need only the TYPEMAP +section of the typemap file. The custom typemap used in the +getnetconfigent() example shown earlier demonstrates what may be the typical +use of extension typemaps. That typemap is used to equate a C structure +with the T_PTROBJ typemap. The typemap used by getnetconfigent() is shown +here. Note that the C type is separated from the XS type with a tab and +that the C unary operator C<*> is considered to be a part of the C type name. + + TYPEMAP + Netconfig *<tab>T_PTROBJ + +Here's a more complicated example: suppose that you wanted C<struct +netconfig> to be blessed into the class C<Net::Config>. One way to do +this is to use underscores (_) to separate package names, as follows: + + typedef struct netconfig * Net_Config; + +And then provide a typemap entry C<T_PTROBJ_SPECIAL> that maps underscores to +double-colons (::), and declare C<Net_Config> to be of that type: + + + TYPEMAP + Net_Config T_PTROBJ_SPECIAL + + INPUT + T_PTROBJ_SPECIAL + if (sv_derived_from($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")) { + IV tmp = SvIV((SV*)SvRV($arg)); + $var = ($type) tmp; + } + else + croak(\"$var is not of type ${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\") + + OUTPUT + T_PTROBJ_SPECIAL + sv_setref_pv($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\", + (void*)$var); + +The INPUT and OUTPUT sections substitute underscores for double-colons +on the fly, giving the desired effect. This example demonstrates some +of the power and versatility of the typemap facility. + +=head1 EXAMPLES + +File C<RPC.xs>: Interface to some ONC+ RPC bind library functions. + + #include "EXTERN.h" + #include "perl.h" + #include "XSUB.h" + + #include <rpc/rpc.h> + + typedef struct netconfig Netconfig; + + MODULE = RPC PACKAGE = RPC + + SV * + rpcb_gettime(host="localhost") + char *host + PREINIT: + time_t timep; + CODE: + ST(0) = sv_newmortal(); + if( rpcb_gettime( host, &timep ) ) + sv_setnv( ST(0), (double)timep ); + + Netconfig * + getnetconfigent(netid="udp") + char *netid + + MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_ + + void + rpcb_DESTROY(netconf) + Netconfig *netconf + CODE: + printf("NetconfigPtr::DESTROY\n"); + free( netconf ); + +File C<typemap>: Custom typemap for RPC.xs. + + TYPEMAP + Netconfig * T_PTROBJ + +File C<RPC.pm>: Perl module for the RPC extension. + + package RPC; + + require Exporter; + require DynaLoader; + @ISA = qw(Exporter DynaLoader); + @EXPORT = qw(rpcb_gettime getnetconfigent); + + bootstrap RPC; + 1; + +File C<rpctest.pl>: Perl test program for the RPC extension. + + use RPC; + + $netconf = getnetconfigent(); + $a = rpcb_gettime(); + print "time = $a\n"; + print "netconf = $netconf\n"; + + $netconf = getnetconfigent("tcp"); + $a = rpcb_gettime("poplar"); + print "time = $a\n"; + print "netconf = $netconf\n"; + + +=head1 XS VERSION + +This document covers features supported by C<xsubpp> 1.935. + +=head1 AUTHOR + +Dean Roehrich <F<roehrich@cray.com>> +Jul 8, 1996 |
