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diff --git a/Master/tlpkg/tlperl0/lib/pods/perlxs.pod b/Master/tlpkg/tlperl0/lib/pods/perlxs.pod new file mode 100755 index 00000000000..5ad24d8aa3d --- /dev/null +++ b/Master/tlpkg/tlperl0/lib/pods/perlxs.pod @@ -0,0 +1,2126 @@ +=head1 NAME + +perlxs - XS language reference manual + +=head1 DESCRIPTION + +=head2 Introduction + +XS is an interface description file format used to create an extension +interface between Perl and C code (or a C library) which one wishes +to use with Perl. The XS interface is combined with the library to +create a new library which can then be either dynamically loaded +or statically linked into perl. The XS interface description is +written in the XS language and is the core component of the Perl +extension interface. + +An B<XSUB> forms the basic unit of the XS interface. After compilation +by the B<xsubpp> compiler, each XSUB amounts to a C function definition +which will provide the glue between Perl calling conventions and C +calling conventions. + +The glue code pulls the arguments from the Perl stack, converts these +Perl values to the formats expected by a C function, call this C function, +transfers the return values of the C function back to Perl. +Return values here may be a conventional C return value or any C +function arguments that may serve as output parameters. These return +values may be passed back to Perl either by putting them on the +Perl stack, or by modifying the arguments supplied from the Perl side. + +The above is a somewhat simplified view of what really happens. Since +Perl allows more flexible calling conventions than C, XSUBs may do much +more in practice, such as checking input parameters for validity, +throwing exceptions (or returning undef/empty list) if the return value +from the C function indicates failure, calling different C functions +based on numbers and types of the arguments, providing an object-oriented +interface, etc. + +Of course, one could write such glue code directly in C. However, this +would be a tedious task, especially if one needs to write glue for +multiple C functions, and/or one is not familiar enough with the Perl +stack discipline and other such arcana. XS comes to the rescue here: +instead of writing this glue C code in long-hand, one can write +a more concise short-hand I<description> of what should be done by +the glue, and let the XS compiler B<xsubpp> handle the rest. + +The XS language allows one to describe the mapping between how the C +routine is used, and how the corresponding Perl routine is used. It +also allows creation of Perl routines which are directly translated to +C code and which are not related to a pre-existing C function. In cases +when the C interface coincides with the Perl interface, the XSUB +declaration is almost identical to a declaration of a C function (in K&R +style). In such circumstances, there is another tool called C<h2xs> +that is able to translate an entire C header file into a corresponding +XS file that will provide glue to the functions/macros described in +the header file. + +The XS compiler is called B<xsubpp>. This compiler creates +the constructs necessary to let an XSUB manipulate Perl values, and +creates the glue necessary to let Perl call the XSUB. The compiler +uses B<typemaps> to determine how to map C function parameters +and output values to Perl values and back. The default typemap +(which comes with Perl) handles many common C types. A supplementary +typemap may also be needed to handle any special structures and types +for the library being linked. + +A file in XS format starts with a C language section which goes until the +first C<MODULE =Z<>> directive. Other XS directives and XSUB definitions +may follow this line. The "language" used in this part of the file +is usually referred to as the XS language. B<xsubpp> recognizes and +skips POD (see L<perlpod>) in both the C and XS language sections, which +allows the XS file to contain embedded documentation. + +See L<perlxstut> for a tutorial on the whole extension creation process. + +Note: For some extensions, Dave Beazley's SWIG system may provide a +significantly more convenient mechanism for creating the extension +glue code. See http://www.swig.org/ 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 simplest XSUBs consist of 3 parts: a description of the return +value, the name of the XSUB routine and the names of its arguments, +and a description of types or formats of the arguments. + +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 + +Optionally, one can merge the description of types and the list of +argument names, rewriting this as + + double + sin(double x) + +This makes this XSUB look similar to an ANSI C declaration. An optional +semicolon is allowed after the argument list, as in + + double + sin(double x); + +Parameters with C pointer types can have different semantic: C functions +with similar declarations + + bool string_looks_as_a_number(char *s); + bool make_char_uppercase(char *c); + +are used in absolutely incompatible manner. Parameters to these functions +could be described B<xsubpp> like this: + + char * s + char &c + +Both these XS declarations correspond to the C<char*> C type, but they have +different semantics, see L<"The & Unary Operator">. + +It is convenient to think that the indirection operator +C<*> should be considered as a part of the type and the address operator C<&> +should be considered part of the variable. See L<"The Typemap"> +for more info about handling qualifiers and unary operators in C types. + +The function name and the return type must be placed on +separate lines and should be flush left-adjusted. + + INCORRECT CORRECT + + double sin(x) double + double x sin(x) + double x + +The rest of the function description 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 for better readability. + + CORRECT + + double + sin(x) + double x + +More complicated XSUBs may contain many other sections. Each section of +an XSUB starts with the corresponding keyword, such as INIT: or CLEANUP:. +However, the first two lines of an XSUB always contain the same data: +descriptions of the return type and the names of the function and its +parameters. Whatever immediately follows these is considered to be +an INPUT: section unless explicitly marked with another keyword. +(See L<The INPUT: Keyword>.) + +An XSUB section continues until another section-start keyword is found. + +=head2 The Argument Stack + +The Perl argument stack is used to store the values which are +sent as parameters to the XSUB and to store the XSUB's +return value(s). In reality all Perl functions (including non-XSUB +ones) keep their values on this stack all 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 special C variable that is declared automatically +for you. The C type of RETVAL matches the return type of the C library +function. The B<xsubpp> compiler will declare this variable in each XSUB +with non-C<void> return type. By default the generated C function +will use RETVAL 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 declare a RETVAL variable for that function. When using +a PPCODE: section no manipulation of the RETVAL variable is required, the +section may use direct stack manipulation to place output values on the stack. + +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<truly> 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 "truly-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 Returning SVs, AVs and HVs through RETVAL + +When you're using RETVAL to return an C<SV *>, there's some magic +going on behind the scenes that should be mentioned. When you're +manipulating the argument stack using the ST(x) macro, for example, +you usually have to pay special attention to reference counts. (For +more about reference counts, see L<perlguts>.) To make your life +easier, the typemap file automatically makes C<RETVAL> mortal when +you're returning an C<SV *>. Thus, the following two XSUBs are more +or less equivalent: + + void + alpha() + PPCODE: + ST(0) = newSVpv("Hello World",0); + sv_2mortal(ST(0)); + XSRETURN(1); + + SV * + beta() + CODE: + RETVAL = newSVpv("Hello World",0); + OUTPUT: + RETVAL + +This is quite useful as it usually improves readability. While +this works fine for an C<SV *>, it's unfortunately not as easy +to have C<AV *> or C<HV *> as a return value. You I<should> be +able to write: + + AV * + array() + CODE: + RETVAL = newAV(); + /* do something with RETVAL */ + OUTPUT: + RETVAL + +But due to an unfixable bug (fixing it would break lots of existing +CPAN modules) in the typemap file, the reference count of the C<AV *> +is not properly decremented. Thus, the above XSUB would leak memory +whenever it is being called. The same problem exists for C<HV *>. + +When you're returning an C<AV *> or a C<HV *>, you have to make sure +their reference count is decremented by making the AV or HV mortal: + + AV * + array() + CODE: + RETVAL = newAV(); + sv_2mortal((SV*)RETVAL); + /* do something with RETVAL */ + OUTPUT: + RETVAL + +And also remember that you don't have to do this for an C<SV *>. + +=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 with +POD stripped, but otherwise 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 ] + +The same package name can be used more than once, allowing for +non-contiguous code. This is useful if you have a stronger ordering +principle than package names. + +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 which have no CODE: or PPCODE: section, +such as the sin() function above, the RETVAL variable is +automatically designated as an output value. For 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 NO_OUTPUT Keyword + +The NO_OUTPUT can be placed as the first token of the XSUB. This keyword +indicates that while the C subroutine we provide an interface to has +a non-C<void> return type, the return value of this C subroutine should not +be returned from the generated Perl subroutine. + +With this keyword present L<The RETVAL Variable> is created, and in the +generated call to the subroutine this variable is assigned to, but the value +of this variable is not going to be used in the auto-generated code. + +This keyword makes sense only if C<RETVAL> is going to be accessed by the +user-supplied code. It is especially useful to make a function interface +more Perl-like, especially when the C return value is just an error condition +indicator. For example, + + NO_OUTPUT int + delete_file(char *name) + POSTCALL: + if (RETVAL != 0) + croak("Error %d while deleting file '%s'", RETVAL, name); + +Here the generated XS function returns nothing on success, and will die() +with a meaningful error message on error. + +=head2 The CODE: Keyword + +This keyword is used in more complicated XSUBs which require +special handling for the C function. The RETVAL variable is +still declared, but it will not be returned unless it is specified +in the OUTPUT: section. + +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 + +Another use for the INIT: section is to check for preconditions before +making a call to the C function: + + long long + lldiv(a,b) + long long a + long long b + INIT: + if (a == 0 && b == 0) + XSRETURN_UNDEF; + if (b == 0) + croak("lldiv: cannot divide by 0"); + +=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 + +C function parameters are normally initialized with their values from +the argument stack (which in turn contains the parameters that were +passed to the XSUB from Perl). The typemaps contain the +code segments which are used to translate the Perl values to +the C parameters. The programmer, however, is allowed to +override the typemaps and supply alternate (or additional) +initialization code. Initialization code starts with the first +C<=>, C<;> or C<+> on a line in the INPUT: section. The only +exception happens if this C<;> terminates the line, then this C<;> +is quietly ignored. + +The following code demonstrates how to supply initialization code for +function parameters. The initialization code is eval'ed 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 $var, $arg, +and $type can be used as in typemaps. + + bool_t + rpcb_gettime(host,timep) + char *host = (char *)SvPV_nolen($arg); + 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 in +the declaration for the input variable, replacing the initialization +supplied by the typemap. If the initialization +begins with C<;> or C<+>, then it is performed after +all of the input variables have been declared. In the C<;> +case the initialization normally supplied by the typemap is not performed. +For the C<+> case, the declaration for the variable will include the +initialization from the typemap. A global +variable, C<%v>, is available for the truly rare case where +information from one initialization is needed in another +initialization. + +Here's a truly obscure example: + + bool_t + rpcb_gettime(host,timep) + time_t &timep; /* \$v{timep}=@{[$v{timep}=$arg]} */ + char *host + SvOK($v{timep}) ? SvPV_nolen($arg) : NULL; + OUTPUT: + timep + +The construct C<\$v{timep}=@{[$v{timep}=$arg]}> used in the above +example has a two-fold purpose: first, when this line is processed by +B<xsubpp>, the Perl snippet C<$v{timep}=$arg> is evaluated. Second, +the text of the evaluated snippet is output into the generated C file +(inside a C comment)! During the processing of C<char *host> line, +$arg will evaluate to C<ST(0)>, and C<$v{timep}> will evaluate to +C<ST(1)>. + +=head2 Default Parameter Values + +Default values for XSUB arguments can be specified by placing an +assignment statement in the parameter list. The default value may +be a number, a string or the special string C<NO_INIT>. 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. This XSUB can be called +from Perl 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 immediately +before or after the declarations of the parameters from the INPUT: section +are emitted. + +If a variable is declared inside a CODE: section it will follow any typemap +code that is emitted for the input parameters. This may result in the +declaration ending up after C code, which is C syntax error. Similar +errors may happen with an explicit C<;>-type or C<+>-type initialization of +parameters is used (see L<"Initializing Function Parameters">). Declaring +these variables in an INIT: section will not help. + +In such cases, to force an additional variable to be declared together +with declarations of other variables, place the declaration into a +PREINIT: section. 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 + +For this particular case an INIT: keyword would generate the +same C code as the PREINIT: keyword. Another 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 + +Another way to declare C<host> is to use a C block in the CODE: section: + + bool_t + rpcb_gettime(timep) + time_t timep = NO_INIT + CODE: + { + char *host = "localhost"; + RETVAL = rpcb_gettime( host, &timep ); + } + OUTPUT: + timep + RETVAL + +The ability to put additional declarations before the typemap entries are +processed is very handy in the cases when typemap conversions manipulate +some global state: + + MyObject + mutate(o) + PREINIT: + MyState st = global_state; + INPUT: + MyObject o; + CLEANUP: + reset_to(global_state, st); + +Here we suppose that conversion to C<MyObject> in the INPUT: section and from +MyObject when processing RETVAL will modify a global variable C<global_state>. +After these conversions are performed, we restore the old value of +C<global_state> (to avoid memory leaks, for example). + +There is another way to trade clarity for compactness: INPUT sections allow +declaration of C variables which do not appear in the parameter list of +a subroutine. Thus the above code for mutate() can be rewritten as + + MyObject + mutate(o) + MyState st = global_state; + MyObject o; + CLEANUP: + reset_to(global_state, st); + +and the code for rpcb_gettime() can be rewritten as + + bool_t + rpcb_gettime(timep) + time_t timep = NO_INIT + char *host = "localhost"; + C_ARGS: + 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 an XSUB contains a comment like C</*scope*/> then scoping will +be automatically 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 + +Since INPUT sections allow declaration of C variables which do not appear +in the parameter list of a subroutine, this may be shortened to: + + bool_t + rpcb_gettime(host,timep) + time_t tt; + char *host; + char *h = host; + time_t timep; + CODE: + RETVAL = rpcb_gettime( h, &tt ); + timep = tt; + OUTPUT: + timep + RETVAL + +(We used our knowledge that input conversion for C<char *> is a "simple" one, +thus C<host> is initialized on the declaration line, and our assignment +C<h = host> is not performed too early. Otherwise one would need to have the +assignment C<h = host> in a CODE: or INIT: section.) + +=head2 The IN/OUTLIST/IN_OUTLIST/OUT/IN_OUT Keywords + +In the list of parameters for an XSUB, one can precede parameter names +by the C<IN>/C<OUTLIST>/C<IN_OUTLIST>/C<OUT>/C<IN_OUT> keywords. +C<IN> keyword is the default, the other keywords indicate how the Perl +interface should differ from the C interface. + +Parameters preceded by C<OUTLIST>/C<IN_OUTLIST>/C<OUT>/C<IN_OUT> +keywords are considered to be used by the C subroutine I<via +pointers>. C<OUTLIST>/C<OUT> keywords indicate that the C subroutine +does not inspect the memory pointed by this parameter, but will write +through this pointer to provide additional return values. + +Parameters preceded by C<OUTLIST> keyword do not appear in the usage +signature of the generated Perl function. + +Parameters preceded by C<IN_OUTLIST>/C<IN_OUT>/C<OUT> I<do> appear as +parameters to the Perl function. With the exception of +C<OUT>-parameters, these parameters are converted to the corresponding +C type, then pointers to these data are given as arguments to the C +function. It is expected that the C function will write through these +pointers. + +The return list of the generated Perl function consists of the C return value +from the function (unless the XSUB is of C<void> return type or +C<The NO_OUTPUT Keyword> was used) followed by all the C<OUTLIST> +and C<IN_OUTLIST> parameters (in the order of appearance). On the +return from the XSUB the C<IN_OUT>/C<OUT> Perl parameter will be +modified to have the values written by the C function. + +For example, an XSUB + + void + day_month(OUTLIST day, IN unix_time, OUTLIST month) + int day + int unix_time + int month + +should be used from Perl as + + my ($day, $month) = day_month(time); + +The C signature of the corresponding function should be + + void day_month(int *day, int unix_time, int *month); + +The C<IN>/C<OUTLIST>/C<IN_OUTLIST>/C<IN_OUT>/C<OUT> keywords can be +mixed with ANSI-style declarations, as in + + void + day_month(OUTLIST int day, int unix_time, OUTLIST int month) + +(here the optional C<IN> keyword is omitted). + +The C<IN_OUT> parameters are identical with parameters introduced with +L<The & Unary Operator> and put into the C<OUTPUT:> section (see +L<The OUTPUT: Keyword>). The C<IN_OUTLIST> parameters are very similar, +the only difference being that the value C function writes through the +pointer would not modify the Perl parameter, but is put in the output +list. + +The C<OUTLIST>/C<OUT> parameter differ from C<IN_OUTLIST>/C<IN_OUT> +parameters only by the initial value of the Perl parameter not +being read (and not being given to the C function - which gets some +garbage instead). For example, the same C function as above can be +interfaced with as + + void day_month(OUT int day, int unix_time, OUT int month); + +or + + void + day_month(day, unix_time, month) + int &day = NO_INIT + int unix_time + int &month = NO_INIT + OUTPUT: + day + month + +However, the generated Perl function is called in very C-ish style: + + my ($day, $month); + day_month($day, time, $month); + +=head2 The C<length(NAME)> Keyword + +If one of the input arguments to the C function is the length of a string +argument C<NAME>, one can substitute the name of the length-argument by +C<length(NAME)> in the XSUB declaration. This argument must be omitted when +the generated Perl function is called. E.g., + + void + dump_chars(char *s, short l) + { + short n = 0; + while (n < l) { + printf("s[%d] = \"\\%#03o\"\n", n, (int)s[n]); + n++; + } + } + + MODULE = x PACKAGE = x + + void dump_chars(char *s, short length(s)) + +should be called as C<dump_chars($string)>. + +This directive is supported with ANSI-type function declarations only. + +=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_nolen(ST(1)); + 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 PPCODE: 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 a 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 should not be used +together within the same XSUB. + +The actual difference between PPCODE: and CODE: sections is in the +initialization of C<SP> macro (which stands for the I<current> Perl +stack pointer), and in the handling of data on the stack when returning +from an XSUB. In CODE: sections SP preserves the value which was on +entry to the XSUB: SP is on the function pointer (which follows the +last parameter). In PPCODE: sections SP is moved backward to the +beginning of the parameter list, which allows C<PUSH*()> macros +to place output values in the place Perl expects them to be when +the XSUB returns back to Perl. + +The generated trailer for a CODE: section ensures that the number of return +values Perl will see is either 0 or 1 (depending on the C<void>ness of the +return value of the C function, and heuristics mentioned in +L<"The RETVAL Variable">). The trailer generated for a PPCODE: section +is based on the number of return values and on the number of times +C<SP> was updated by C<[X]PUSH*()> macros. + +Note that macros C<ST(i)>, C<XST_m*()> and C<XSRETURN*()> work equally +well in CODE: sections and PPCODE: sections. + +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: + if( rpcb_gettime( host, &timep ) ){ + ST(0) = sv_newmortal(); + 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<perlapi> for other +C<XSRETURN> macros. + +Since C<XSRETURN_*> macros can be used with CODE blocks as well, one can +rewrite this example as: + + int + rpcb_gettime(host) + char *host + PREINIT: + time_t timep; + CODE: + RETVAL = rpcb_gettime( host, &timep ); + if (RETVAL == 0) + XSRETURN_UNDEF; + OUTPUT: + RETVAL + +In fact, one can put this check into a POSTCALL: section as well. Together +with PREINIT: simplifications, this leads to: + + int + rpcb_gettime(host) + char *host + time_t timep; + POSTCALL: + if (RETVAL == 0) + XSRETURN_UNDEF; + +=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 POSTCALL: Keyword + +This keyword can be used when an XSUB requires special procedures +executed after the C subroutine call is performed. When the POSTCALL: +keyword is used it must precede OUTPUT: and CLEANUP: blocks which are +present in the XSUB. + +See examples in L<"The NO_OUTPUT Keyword"> and L<"Returning Undef And Empty Lists">. + +The POSTCALL: block does not make a lot of sense when the C subroutine +call is supplied by user by providing either CODE: or PPCODE: section. + +=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 + +Note that if the version of the PM module is an NV (a floating point +number), it will be stringified with a possible loss of precision +(currently chopping to nine decimal places) so that it may not match +the version of the XS module anymore. Quoting the $VERSION declaration +to make it a string is recommended if long version numbers are used. + +=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_nolen(ST(1)); + RETVAL = rpcb_gettime( host, &timep ); + OUTPUT: + timep + RETVAL + +If the prototypes are enabled, you can disable it locally for a given +XSUB as in the following example: + + void + rpcb_gettime_noproto() + PROTOTYPE: DISABLE + ... + +=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 OVERLOAD: Keyword + +Instead of writing an overloaded interface using pure Perl, you +can also use the OVERLOAD keyword to define additional Perl names +for your functions (like the ALIAS: keyword above). However, the +overloaded functions must be defined with three parameters (except +for the nomethod() function which needs four parameters). If any +function has the OVERLOAD: keyword, several additional lines +will be defined in the c file generated by xsubpp in order to +register with the overload magic. + +Since blessed objects are actually stored as RV's, it is useful +to use the typemap features to preprocess parameters and extract +the actual SV stored within the blessed RV. See the sample for +T_PTROBJ_SPECIAL below. + +To use the OVERLOAD: keyword, create an XS function which takes +three input parameters ( or use the c style '...' definition) like +this: + + SV * + cmp (lobj, robj, swap) + My_Module_obj lobj + My_Module_obj robj + IV swap + OVERLOAD: cmp <=> + { /* function defined here */} + +In this case, the function will overload both of the three way +comparison operators. For all overload operations using non-alpha +characters, you must type the parameter without quoting, separating +multiple overloads with whitespace. Note that "" (the stringify +overload) should be entered as \"\" (i.e. escaped). + +=head2 The FALLBACK: Keyword + +In addition to the OVERLOAD keyword, if you need to control how +Perl autogenerates missing overloaded operators, you can set the +FALLBACK keyword in the module header section, like this: + + MODULE = RPC PACKAGE = RPC + + FALLBACK: TRUE + ... + +where FALLBACK can take any of the three values TRUE, FALSE, or +UNDEF. If you do not set any FALLBACK value when using OVERLOAD, +it defaults to UNDEF. FALLBACK is not used except when one or +more functions using OVERLOAD have been defined. Please see +L<overload/Fallback> for more details. + +=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 the current XSUB. + +For example, if you have 4 C functions multiply(), divide(), add(), +subtract() all having the signature: + + symbolic f(symbolic, symbolic); + +you can make them all to use the same XSUB using this: + + symbolic + interface_s_ss(arg1, arg2) + symbolic arg1 + symbolic arg2 + INTERFACE: + multiply divide + add subtract + +(This is the complete XSUB code for 4 Perl functions!) Four generated +Perl function share names with corresponding C functions. + +The advantage of this approach comparing to ALIAS: keyword is that there +is no need to code a switch statement, each Perl function (which shares +the same XSUB) knows which C function it should call. Additionally, 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); + +say, from another XSUB. (This example supposes that there was no +INTERFACE_MACRO: section, otherwise one needs to use something else instead of +C<XSINTERFACE_FUNC_SET>, see the next section.) + +=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_ANON(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 C<&> unary operator in the INPUT: section is used to tell B<xsubpp> +that it should convert a Perl value to/from C using the C type to the left +of C<&>, but provide a pointer to this value when the C function is called. + +This is useful to avoid a CODE: block for a C function which takes a parameter +by reference. Typically, the parameter should be not a pointer type (an +C<int> or C<long> but not an C<int*> or C<long*>). + +The following XSUB will generate incorrect C code. The B<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 B<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 POD, Comments and C Preprocessor Directives + +C preprocessor directives are allowed within BOOT:, PREINIT: INIT:, CODE:, +PPCODE:, POSTCALL:, 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. POD documentation is allowed at any point, both in the +C and XS language sections. POD must be terminated with a C<=cut> command; +C<xsubpp> will exit with an error if it does not. It is very unlikely that +human generated C code will be mistaken for POD, as most indenting styles +result in whitespace in front of any line starting with C<=>. Machine +generated XS files may fall into this trap unless care is taken to +ensure that a space breaks the sequence "\n=". + +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 B<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 an XSUB name contains C<::>, it is considered to be a C++ method. +The generated Perl function will assume that +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 return type of the XSUB includes C<static>, the method is considered +to be a static method. 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 Perl functions will expect an object as the first parameter. In the +generated C++ code the object is called C<THIS>, and the method call will +be performed on this object. So in the C++ code the blue() and set_blue() +methods will be called as this: + + RETVAL = THIS->blue(); + + THIS->set_blue( val ); + +You could also write a single get/set method using an optional argument: + + int + color::blue( val = NO_INIT ) + int val + PROTOTYPE $;$ + CODE: + if (items > 1) + THIS->set_blue( val ); + RETVAL = THIS->blue(); + OUTPUT: + RETVAL + +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. The generated C++ code for + + void + color::DESTROY() + +will look like this: + + color *THIS = ...; // Initialized as in typemap + + 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 generated 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 (such as created by C<h2xs -x>) is often sufficient. +However, sometimes the interface will look +very C-like and occasionally nonintuitive, especially when the C function +modifies one of its parameters, or returns failure inband (as in "negative +return values mean failure"). 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 with input/output or output parameters. The XSUBs for +these functions may be able to return lists to Perl. + +Identify the C functions which use some inband info as an indication +of failure. They may be +candidates to return undef or an empty list in case of failure. If the +failure may be detected without a call to the C function, you may want to use +an INIT: section to report the failure. For failures detectable after the C +function returns one may want to use a POSTCALL: section to process the +failure. In more complicated cases use CODE: or PPCODE: sections. + +If many functions use the same failure indication based on the return value, +you may want to create a special typedef to handle this situation. Put + + typedef int negative_is_failure; + +near the beginning of XS file, and create an OUTPUT typemap entry +for C<negative_is_failure> which converts negative values to C<undef>, or +maybe croak()s. After this the return value of type C<negative_is_failure> +will create more Perl-like interface. + +Identify which values are used by only the C and XSUB functions +themselves, say, when a parameter to a function should be a contents of a +global variable. 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 may be used to implement input/output or +output parameters, they can be handled in XS with the C<&> unary operator, +and, possibly, using the NO_INIT keyword. +Some others will require handling of types like C<int *>, and one needs +to decide what a useful Perl translation will do in such a case. When +the semantic is clear, it is advisable to put the translation into a typemap +file. + +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. (This is handled automatically by C<h2xs -x>.) + +If the same C type is used in several different contexts which require +different translations, C<typedef> several new types mapped to this C type, +and create separate F<typemap> entries for these new types. Use these +types in declarations of return type and parameters to XSUBs. + +=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 labelled C<TYPEMAP>, C<INPUT>, and +C<OUTPUT>. An unlabelled initial section is assumed to be a C<TYPEMAP> +section. 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. +The section labels C<TYPEMAP>, C<INPUT>, or C<OUTPUT> must begin +in the first column on a line by themselves, and must be in uppercase. + +The default typemap in the C<lib/ExtUtils> 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)); + $var = INT2PTR($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. + +The INT2PTR macro (defined in perl.h) casts an integer to a pointer, +of a given type, taking care of the possible different size of integers +and pointers. There are also PTR2IV, PTR2UV, PTR2NV macros, +to map the other way, which may be useful in OUTPUT sections. + +=head2 Safely Storing Static Data in XS + +Starting with Perl 5.8, a macro framework has been defined to allow +static data to be safely stored in XS modules that will be accessed from +a multi-threaded Perl. + +Although primarily designed for use with multi-threaded Perl, the macros +have been designed so that they will work with non-threaded Perl as well. + +It is therefore strongly recommended that these macros be used by all +XS modules that make use of static data. + +The easiest way to get a template set of macros to use is by specifying +the C<-g> (C<--global>) option with h2xs (see L<h2xs>). + +Below is an example module that makes use of the macros. + + #include "EXTERN.h" + #include "perl.h" + #include "XSUB.h" + + /* Global Data */ + + #define MY_CXT_KEY "BlindMice::_guts" XS_VERSION + + typedef struct { + int count; + char name[3][100]; + } my_cxt_t; + + START_MY_CXT + + MODULE = BlindMice PACKAGE = BlindMice + + BOOT: + { + MY_CXT_INIT; + MY_CXT.count = 0; + strcpy(MY_CXT.name[0], "None"); + strcpy(MY_CXT.name[1], "None"); + strcpy(MY_CXT.name[2], "None"); + } + + int + newMouse(char * name) + char * name; + PREINIT: + dMY_CXT; + CODE: + if (MY_CXT.count >= 3) { + warn("Already have 3 blind mice"); + RETVAL = 0; + } + else { + RETVAL = ++ MY_CXT.count; + strcpy(MY_CXT.name[MY_CXT.count - 1], name); + } + + char * + get_mouse_name(index) + int index + CODE: + dMY_CXT; + RETVAL = MY_CXT.lives ++; + if (index > MY_CXT.count) + croak("There are only 3 blind mice."); + else + RETVAL = newSVpv(MY_CXT.name[index - 1]); + + void + CLONE(...) + CODE: + MY_CXT_CLONE; + +B<REFERENCE> + +=over 5 + +=item MY_CXT_KEY + +This macro is used to define a unique key to refer to the static data +for an XS module. The suggested naming scheme, as used by h2xs, is to +use a string that consists of the module name, the string "::_guts" +and the module version number. + + #define MY_CXT_KEY "MyModule::_guts" XS_VERSION + +=item typedef my_cxt_t + +This struct typedef I<must> always be called C<my_cxt_t> -- the other +C<CXT*> macros assume the existence of the C<my_cxt_t> typedef name. + +Declare a typedef named C<my_cxt_t> that is a structure that contains +all the data that needs to be interpreter-local. + + typedef struct { + int some_value; + } my_cxt_t; + +=item START_MY_CXT + +Always place the START_MY_CXT macro directly after the declaration +of C<my_cxt_t>. + +=item MY_CXT_INIT + +The MY_CXT_INIT macro initialises storage for the C<my_cxt_t> struct. + +It I<must> be called exactly once -- typically in a BOOT: section. If you +are maintaining multiple interpreters, it should be called once in each +interpreter instance, except for interpreters cloned from existing ones. +(But see C<MY_CXT_CLONE> below.) + +=item dMY_CXT + +Use the dMY_CXT macro (a declaration) in all the functions that access +MY_CXT. + +=item MY_CXT + +Use the MY_CXT macro to access members of the C<my_cxt_t> struct. For +example, if C<my_cxt_t> is + + typedef struct { + int index; + } my_cxt_t; + +then use this to access the C<index> member + + dMY_CXT; + MY_CXT.index = 2; + +=item aMY_CXT/pMY_CXT + +C<dMY_CXT> may be quite expensive to calculate, and to avoid the overhead +of invoking it in each function it is possible to pass the declaration +onto other functions using the C<aMY_CXT>/C<pMY_CXT> macros, eg + + void sub1() { + dMY_CXT; + MY_CXT.index = 1; + sub2(aMY_CXT); + } + + void sub2(pMY_CXT) { + MY_CXT.index = 2; + } + +Analogously to C<pTHX>, there are equivalent forms for when the macro is the +first or last in multiple arguments, where an underscore represents a +comma, i.e. C<_aMY_CXT>, C<aMY_CXT_>, C<_pMY_CXT> and C<pMY_CXT_>. + +=item MY_CXT_CLONE + +By default, when a new interpreter is created as a copy of an existing one +(eg via C<< threads->create() >>), both interpreters share the same physical +my_cxt_t structure. Calling C<MY_CXT_CLONE> (typically via the package's +C<CLONE()> function), causes a byte-for-byte copy of the structure to be +taken, and any future dMY_CXT will cause the copy to be accessed instead. + +=item MY_CXT_INIT_INTERP(my_perl) + +=item dMY_CXT_INTERP(my_perl) + +These are versions of the macros which take an explicit interpreter as an +argument. + +=back + +Note that these macros will only work together within the I<same> source +file; that is, a dMY_CTX in one source file will access a different structure +than a dMY_CTX in another source file. + +=head2 Thread-aware system interfaces + +Starting from Perl 5.8, in C/C++ level Perl knows how to wrap +system/library interfaces that have thread-aware versions +(e.g. getpwent_r()) into frontend macros (e.g. getpwent()) that +correctly handle the multithreaded interaction with the Perl +interpreter. This will happen transparently, the only thing +you need to do is to instantiate a Perl interpreter. + +This wrapping happens always when compiling Perl core source +(PERL_CORE is defined) or the Perl core extensions (PERL_EXT is +defined). When compiling XS code outside of Perl core the wrapping +does not take place. Note, however, that intermixing the _r-forms +(as Perl compiled for multithreaded operation will do) and the _r-less +forms is neither well-defined (inconsistent results, data corruption, +or even crashes become more likely), nor is it very portable. + +=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 + +Originally written by Dean Roehrich <F<roehrich@cray.com>>. + +Maintained since 1996 by The Perl Porters <F<perlbug@perl.org>>. |