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Section: Perl Programmers Reference Guide (1)
Updated: 2018-03-01


perlxs - XS language reference manual 




XS is an interface description file format used to create an extensioninterface between Perl and C code (or a C library) which one wishesto use with Perl. The XS interface is combined with the library tocreate a new library which can then be either dynamically loadedor statically linked into perl. The XS interface description iswritten in the XS language and is the core component of the Perlextension interface.

Before writing XS, read the ``CAVEATS'' section below.

An XSUB forms the basic unit of the XS interface. After compilationby the xsubpp compiler, each XSUB amounts to a C function definitionwhich will provide the glue between Perl calling conventions and Ccalling conventions.

The glue code pulls the arguments from the Perl stack, converts thesePerl 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 Cfunction arguments that may serve as output parameters. These returnvalues may be passed back to Perl either by putting them on thePerl stack, or by modifying the arguments supplied from the Perl side.

The above is a somewhat simplified view of what really happens. SincePerl allows more flexible calling conventions than C, XSUBs may do muchmore in practice, such as checking input parameters for validity,throwing exceptions (or returning undef/empty list) if the return valuefrom the C function indicates failure, calling different C functionsbased on numbers and types of the arguments, providing an object-orientedinterface, etc.

Of course, one could write such glue code directly in C. However, thiswould be a tedious task, especially if one needs to write glue formultiple C functions, and/or one is not familiar enough with the Perlstack discipline and other such arcana. XS comes to the rescue here:instead of writing this glue C code in long-hand, one can writea more concise short-hand description of what should be done bythe glue, and let the XS compiler xsubpp handle the rest.

The XS language allows one to describe the mapping between how the Croutine is used, and how the corresponding Perl routine is used. Italso allows creation of Perl routines which are directly translated toC code and which are not related to a pre-existing C function. In caseswhen the C interface coincides with the Perl interface, the XSUBdeclaration is almost identical to a declaration of a C function (in K&Rstyle). In such circumstances, there is another tool called "h2xs"that is able to translate an entire C header file into a correspondingXS file that will provide glue to the functions/macros described inthe header file.

The XS compiler is called xsubpp. This compiler createsthe constructs necessary to let an XSUB manipulate Perl values, andcreates the glue necessary to let Perl call the XSUB. The compileruses typemaps to determine how to map C function parametersand output values to Perl values and back. The default typemap(which comes with Perl) handles many common C types. A supplementarytypemap may also be needed to handle any special structures and typesfor the library being linked. For more information on typemaps,see perlxstypemap.

A file in XS format starts with a C language section which goes until thefirst "MODULE =" directive. Other XS directives and XSUB definitionsmay follow this line. The ``language'' used in this part of the fileis usually referred to as the XS language. xsubpp recognizes andskips POD (see perlpod) in both the C and XS language sections, whichallows the XS file to contain embedded documentation.

See perlxstut for a tutorial on the whole extension creation process.

Note: For some extensions, Dave Beazley's SWIG system may provide asignificantly more convenient mechanism for creating the extensionglue code. See <> for more information. 

On The Road

Many of the examples which follow will concentrate on creating an interfacebetween Perl and the ONC+ RPC bind library functions. The rpcb_gettime()function is used to demonstrate many features of the XS language. Thisfunction has two parameters; the first is an input parameter and the secondis 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 followingstatements.

     #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 functionand 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, whichdemonstrates one possible interface to the rpcb_gettime()function. This XSUB represents a direct translation betweenC and Perl and so preserves the interface even from Perl.This XSUB will be invoked from Perl with the usage shownabove. Note that the first three #include statements, for"EXTERN.h", "perl.h", and "XSUB.h", will always be present at thebeginning of an XS file. This approach and others will beexpanded later in this document. A #define for "PERL_NO_GET_CONTEXT"should be present to fetch the interpreter context more efficiently,see perlguts for details.

     #define PERL_NO_GET_CONTEXT     #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 whichpulls the extension into Perl. This module will export theextension's functions and variables to the Perl program andwill cause the extension's XSUBs to be linked into Perl.The following module will be used for most of the examplesin this document and should be used from Perl with the "use"command as shown earlier. Perl modules are explained inmore 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 differentorders or will take different numbers of parameters. In each case theXSUB 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 willallow the programmer to create a more Perl-like interface to the Cfunction. 

The Anatomy of an XSUB

The simplest XSUBs consist of 3 parts: a description of the returnvalue, 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 functioncalled sin(). The XSUB will imitate the C function which takes a singleargument and returns a single value.

     double     sin(x)       double x

Optionally, one can merge the description of types and the list ofargument names, rewriting this as

     double     sin(double x)

This makes this XSUB look similar to an ANSI C declaration. An optionalsemicolon is allowed after the argument list, as in

     double     sin(double x);

Parameters with C pointer types can have different semantic: C functionswith 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 functionscould be described xsubpp like this:

     char *  s     char    &c

Both these XS declarations correspond to the "char*" C type, but they havedifferent semantics, see ``The & Unary Operator''.

It is convenient to think that the indirection operator"*" should be considered as a part of the type and the address operator "&"should be considered part of the variable. See perlxstypemapfor more info about handling qualifiers and unary operators in C types.

The function name and the return type must be placed onseparate 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. Thefollowing example shows a function with its body left-adjusted. Mostexamples 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 ofan 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 itsparameters. Whatever immediately follows these is considered to bean INPUT: section unless explicitly marked with another keyword.(See ``The INPUT: Keyword''.)

An XSUB section continues until another section-start keyword is found. 

The Argument Stack

The Perl argument stack is used to store the values which aresent as parameters to the XSUB and to store the XSUB'sreturn value(s). In reality all Perl functions (including non-XSUBones) keep their values on this stack all the same time, each limitedto its own range of positions on the stack. In this document thefirst position on that stack which belongs to the activefunction will be referred to as position 0 for that function.

XSUBs refer to their stack arguments with the macro ST(x), where xrefers to a position in this XSUB's part of the stack. Position 0 for thatfunction would be known to the XSUB as ST(0). The XSUB's incomingparameters and outgoing return values always begin at ST(0). For manysimple cases the xsubpp compiler will generate the code necessary tohandle the argument stack by embedding code fragments found in thetypemaps. In more complex cases the programmer must supply the code. 

The RETVAL Variable

The RETVAL variable is a special C variable that is declared automaticallyfor you. The C type of RETVAL matches the return type of the C libraryfunction. The xsubpp compiler will declare this variable in each XSUBwith non-"void" return type. By default the generated C functionwill use RETVAL to hold the return value of the C library function beingcalled. In simple cases the value of RETVAL will be placed in ST(0) ofthe argument stack where it can be received by Perl as the return valueof the XSUB.

If the XSUB has a return type of "void" then the compiler willnot declare a RETVAL variable for that function. When usinga PPCODE: section no manipulation of the RETVAL variable is required, thesection may use direct stack manipulation to place output values on the stack.

If PPCODE: directive is not used, "void" return value should be usedonly for subroutines which do not return a value, even if CODE:directive is used which sets ST(0) explicitly.

Older versions of this document recommended to use "void" returnvalue in such cases. It was discovered that this could lead tosegfaults in cases when XSUB was truly "void". This practice isnow deprecated, and may be not supported at some future version. Usethe return value "SV *" in such cases. (Currently "xsubpp" containssome heuristic code which tries to disambiguate between ``truly-void''and ``old-practice-declared-as-void'' functions. Hence your code is atmercy of this heuristics unless you use "SV *" as return value.) 

Returning SVs, AVs and HVs through RETVAL

When you're using RETVAL to return an "SV *", there's some magicgoing on behind the scenes that should be mentioned. When you'remanipulating the argument stack using the ST(x) macro, for example,you usually have to pay special attention to reference counts. (Formore about reference counts, see perlguts.) To make your lifeeasier, the typemap file automatically makes "RETVAL" mortal whenyou're returning an "SV *". Thus, the following two XSUBs are moreor 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. Whilethis works fine for an "SV *", it's unfortunately not as easyto have "AV *" or "HV *" as a return value. You should beable 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 existingCPAN modules) in the typemap file, the reference count of the "AV *"is not properly decremented. Thus, the above XSUB would leak memorywhenever it is being called. The same problem exists for "HV *","CV *", and "SVREF" (which indicates a scalar reference, nota general "SV *").In XS code on perls starting with perl 5.16, you can override thetypemaps for any of these types with a version that has properhandling of refcounts. In your "TYPEMAP" section, do


to get the repaired variant. For backward compatibility with olderversions of perl, you can instead decrement the reference countmanually when you're returning one of the aforementionedtypes using "sv_2mortal":

  AV *  array()      CODE:          RETVAL = newAV();          sv_2mortal((SV*)RETVAL);          /* do something with RETVAL */      OUTPUT:          RETVAL

Remember that you don't have to do this for an "SV *". The referencedocumentation for all core typemaps can be found in perlxstypemap. 

The MODULE Keyword

The MODULE keyword is used to start the XS code and to specify the packageof the functions which are being defined. All text preceding the firstMODULE keyword is considered C code and is passed through to the output withPOD stripped, but otherwise untouched. Every XS module will have abootstrap function which is used to hook the XSUBs into Perl. The packagename of this bootstrap function will match the value of the last MODULEstatement in the XS source files. The value of MODULE should always remainconstant within the same XS file, though this is not required.

The following example will start the XS code and will placeall functions in a package named RPC.


The PACKAGE Keyword

When functions within an XS source file must be separated into packagesthe PACKAGE keyword should be used. This keyword is used with the MODULEkeyword 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 fornon-contiguous code. This is useful if you have a stronger orderingprinciple than package names.

Although this keyword is optional and in some cases provides redundantinformation it should always be used. This keyword will ensure that theXSUBs appear in the desired package. 

The PREFIX Keyword

The PREFIX keyword designates prefixes which should beremoved from the Perl function names. If the C function is"rpcb_gettime()" and the PREFIX value is "rpcb_" then Perl willsee this function as "gettime()".

This keyword should follow the PACKAGE keyword when used.If PACKAGE is not used then PREFIX should follow the MODULEkeyword.


The OUTPUT: Keyword

The OUTPUT: keyword indicates that certain function parameters should beupdated (new values made visible to Perl) when the XSUB terminates or thatcertain values should be returned to the calling Perl function. Forsimple functions which have no CODE: or PPCODE: section,such as the sin() function above, the RETVAL variable isautomatically designated as an output value. For more complex functionsthe xsubpp compiler will need help to determine which variables are outputvariables.

This keyword will normally be used to complement the CODE: keyword.The RETVAL variable is not recognized as an output variable when theCODE: keyword is present. The OUTPUT: keyword is used in thissituation to tell the compiler that RETVAL really is an outputvariable.

The OUTPUT: keyword can also be used to indicate that function parametersare output variables. This may be necessary when a parameter has beenmodified within the function and the programmer would like the update tobe 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 tobe mapped to a matching piece of code rather than to atypemap.

     bool_t     rpcb_gettime(host,timep)          char *host          time_t &timep        OUTPUT:          timep sv_setnv(ST(1), (double)timep);

xsubpp emits an automatic "SvSETMAGIC()" for all parameters in theOUTPUT section of the XSUB, except RETVAL. This is the usually desiredbehavior, as it takes care of properly invoking 'set' magic on outputparameters (needed for hash or array element parameters that must becreated if they didn't exist). If for some reason, this behavior isnot desired, the OUTPUT section may contain a "SETMAGIC: DISABLE" lineto disable it for the remainder of the parameters in the OUTPUT section.Likewise, "SETMAGIC: ENABLE" can be used to reenable it for theremainder of the OUTPUT section. See perlguts for more detailsabout 'set' magic. 

The NO_OUTPUT Keyword

The NO_OUTPUT can be placed as the first token of the XSUB. This keywordindicates that while the C subroutine we provide an interface to hasa non-"void" return type, the return value of this C subroutine should notbe returned from the generated Perl subroutine.

With this keyword present ``The RETVAL Variable'' is created, and in thegenerated call to the subroutine this variable is assigned to, but the valueof this variable is not going to be used in the auto-generated code.

This keyword makes sense only if "RETVAL" is going to be accessed by theuser-supplied code. It is especially useful to make a function interfacemore Perl-like, especially when the C return value is just an error conditionindicator. 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. 

The CODE: Keyword

This keyword is used in more complicated XSUBs which requirespecial handling for the C function. The RETVAL variable isstill declared, but it will not be returned unless it is specifiedin the OUTPUT: section.

The following XSUB is for a C function which requires special handling ofits 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

The INIT: Keyword

The INIT: keyword allows initialization to be inserted into the XSUB beforethe compiler generates the call to the C function. Unlike the CODE: keywordabove, 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 beforemaking 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");

The NO_INIT Keyword

The NO_INIT keyword is used to indicate that a functionparameter is being used only as an output value. The xsubppcompiler will normally generate code to read the values ofall function parameters from the argument stack and assignthem to C variables upon entry to the function. NO_INITwill tell the compiler that some parameters will be used foroutput rather than for input and that they will be handledbefore 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 doesnot care about its initial contents.

     bool_t     rpcb_gettime(host,timep)          char *host          time_t &timep = NO_INIT        OUTPUT:          timep

The TYPEMAP: Keyword

Starting with Perl 5.16, you can embed typemaps into your XS codeinstead of or in addition to typemaps in a separate file. Multiplesuch embedded typemaps will be processed in order of appearance inthe XS code and like local typemap files take precedence over thedefault typemap, the embedded typemaps may overwrite previousdefinitions of TYPEMAP, INPUT, and OUTPUT stanzas. The syntax forembedded typemaps is

      TYPEMAP: <<HERE      ... your typemap code here ...      HERE

where the "TYPEMAP" keyword must appear in the first column of anew line.

Refer to perlxstypemap for details on writing typemaps. 

Initializing Function Parameters

C function parameters are normally initialized with their values fromthe argument stack (which in turn contains the parameters that werepassed to the XSUB from Perl). The typemaps contain thecode segments which are used to translate the Perl values tothe C parameters. The programmer, however, is allowed tooverride the typemaps and supply alternate (or additional)initialization code. Initialization code starts with the first"=", ";" or "+" on a line in the INPUT: section. The onlyexception happens if this ";" terminates the line, then this ";"is quietly ignored.

The following code demonstrates how to supply initialization code forfunction parameters. The initialization code is eval'ed within doublequotes by the compiler before it is added to the output so anythingwhich should be interpreted literally [mainly "$", "@", or "\\"]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. Onewould normally use this when a function parameter must be processed byanother library function before it can be used. Default parameters arecovered in the next section.

If the initialization begins with "=", then it is output inthe declaration for the input variable, replacing the initializationsupplied by the typemap. If the initializationbegins with ";" or "+", then it is performed afterall of the input variables have been declared. In the ";"case the initialization normally supplied by the typemap is not performed.For the "+" case, the declaration for the variable will include theinitialization from the typemap. A globalvariable, %v, is available for the truly rare case whereinformation from one initialization is needed in anotherinitialization.

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 "\$v{timep}=@{[$v{timep}=$arg]}" used in the aboveexample has a two-fold purpose: first, when this line is processed byxsubpp, the Perl snippet "$v{timep}=$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 "char *host" line,$arg will evaluate to ST(0), and $v{timep} will evaluate toST(1). 

Default Parameter Values

Default values for XSUB arguments can be specified by placing anassignment statement in the parameter list. The default value maybe a number, a string or the special string "NO_INIT". Defaults shouldalways be used on the right-most parameters only.

To allow the XSUB for rpcb_gettime() to have a default hostvalue the parameters to the XSUB could be rearranged. TheXSUB will then call the real rpcb_gettime() function withthe parameters in the correct order. This XSUB can be calledfrom 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 withthe 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

The PREINIT: Keyword

The PREINIT: keyword allows extra variables to be declared immediatelybefore or after the declarations of the parameters from the INPUT: sectionare emitted.

If a variable is declared inside a CODE: section it will follow any typemapcode that is emitted for the input parameters. This may result in thedeclaration ending up after C code, which is C syntax error. Similarerrors may happen with an explicit ";"-type or "+"-type initialization ofparameters is used (see ``Initializing Function Parameters''). Declaringthese variables in an INIT: section will not help.

In such cases, to force an additional variable to be declared togetherwith declarations of other variables, place the declaration into aPREINIT: section. The PREINIT: keyword may be used one or more timeswithin an XSUB.

The following examples are equivalent, but if the code is using complextypemaps 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 thesame 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 "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 areprocessed is very handy in the cases when typemap conversions manipulatesome 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 "MyObject" in the INPUT: section and fromMyObject when processing RETVAL will modify a global variable "global_state".After these conversions are performed, we restore the old value of"global_state" (to avoid memory leaks, for example).

There is another way to trade clarity for compactness: INPUT sections allowdeclaration of C variables which do not appear in the parameter list ofa 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

The SCOPE: Keyword

The SCOPE: keyword allows scoping to be enabled for a particular XSUB. Ifenabled, the XSUB will invoke ENTER and LEAVE automatically.

To support potentially complex type mappings, if a typemap entry usedby an XSUB contains a comment like "/*scope*/" then scoping willbe automatically enabled for that XSUB.

To enable scoping:


To disable scoping:


The INPUT: Keyword

The XSUB's parameters are usually evaluated immediately after entering theXSUB. The INPUT: keyword can be used to force those parameters to beevaluated a little later. The INPUT: keyword can be used multiple timeswithin an XSUB and can be used to list one or more input variables. Thiskeyword is used with the PREINIT: keyword.

The following example shows how the input parameter "timep" can beevaluated 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 appearin 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 "char *" is a ``simple'' one,thus "host" is initialized on the declaration line, and our assignment"h = host" is not performed too early. Otherwise one would need to have theassignment "h = host" in a CODE: or INIT: section.) 


In the list of parameters for an XSUB, one can precede parameter namesby the "IN"/"OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT" keywords."IN" keyword is the default, the other keywords indicate how the Perlinterface should differ from the C interface.

Parameters preceded by "OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT"keywords are considered to be used by the C subroutine viapointers. "OUTLIST"/"OUT" keywords indicate that the C subroutinedoes not inspect the memory pointed by this parameter, but will writethrough this pointer to provide additional return values.

Parameters preceded by "OUTLIST" keyword do not appear in the usagesignature of the generated Perl function.

Parameters preceded by "IN_OUTLIST"/"IN_OUT"/"OUT" do appear asparameters to the Perl function. With the exception of"OUT"-parameters, these parameters are converted to the correspondingC type, then pointers to these data are given as arguments to the Cfunction. It is expected that the C function will write through thesepointers.

The return list of the generated Perl function consists of the C return valuefrom the function (unless the XSUB is of "void" return type or"The NO_OUTPUT Keyword" was used) followed by all the "OUTLIST"and "IN_OUTLIST" parameters (in the order of appearance). On thereturn from the XSUB the "IN_OUT"/"OUT" Perl parameter will bemodified 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 "IN"/"OUTLIST"/"IN_OUTLIST"/"IN_OUT"/"OUT" keywords can bemixed with ANSI-style declarations, as in

  void  day_month(OUTLIST int day, int unix_time, OUTLIST int month)

(here the optional "IN" keyword is omitted).

The "IN_OUT" parameters are identical with parameters introduced with``The & Unary Operator'' and put into the "OUTPUT:" section (see``The OUTPUT: Keyword''). The "IN_OUTLIST" parameters are very similar,the only difference being that the value C function writes through thepointer would not modify the Perl parameter, but is put in the outputlist.

The "OUTLIST"/"OUT" parameter differ from "IN_OUTLIST"/"IN_OUT"parameters only by the initial value of the Perl parameter notbeing read (and not being given to the C function - which gets somegarbage instead). For example, the same C function as above can beinterfaced with as

  void day_month(OUT int day, int unix_time, OUT int month);


  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);

The length(NAME) Keyword

If one of the input arguments to the C function is the length of a stringargument "NAME", one can substitute the name of the length-argument by"length(NAME)" in the XSUB declaration. This argument must be omitted whenthe 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 "dump_chars($string)".

This directive is supported with ANSI-type function declarations only. 

Variable-length Parameter Lists

XSUBs can have variable-length parameter lists by specifying an ellipsis"(...)" in the parameter list. This use of the ellipsis is similar to thatfound in ANSI C. The programmer is able to determine the number ofarguments passed to the XSUB by examining the "items" variable which thexsubpp compiler supplies for all XSUBs. By using this mechanism one cancreate an XSUB which accepts a list of parameters of unknown length.

The host parameter for the rpcb_gettime() XSUB can beoptional so the ellipsis can be used to indicate that theXSUB will take a variable number of parameters. Perl shouldbe 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

The C_ARGS: Keyword

The C_ARGS: keyword allows creating of XSUBS which have differentcalling sequence from Perl than from C, without a need to writeCODE: or PPCODE: section. The contents of the C_ARGS: paragraph isput 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"default_flags". Suppose that you want to create an interface whichis 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

The PPCODE: Keyword

The PPCODE: keyword is an alternate form of the CODE: keyword and is usedto tell the xsubpp compiler that the programmer is supplying the code tocontrol the argument stack for the XSUBs return values. Occasionally onewill 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 ofvalues on the stack. The PPCODE: and CODE: keywords should not be usedtogether within the same XSUB.

The actual difference between PPCODE: and CODE: sections is in theinitialization of "SP" macro (which stands for the current Perlstack pointer), and in the handling of data on the stack when returningfrom an XSUB. In CODE: sections SP preserves the value which was onentry to the XSUB: SP is on the function pointer (which follows thelast parameter). In PPCODE: sections SP is moved backward to thebeginning of the parameter list, which allows "PUSH*()" macrosto place output values in the place Perl expects them to be whenthe XSUB returns back to Perl.

The generated trailer for a CODE: section ensures that the number of returnvalues Perl will see is either 0 or 1 (depending on the "void"ness of thereturn value of the C function, and heuristics mentioned in``The RETVAL Variable''). The trailer generated for a PPCODE: sectionis based on the number of return values and on the number of times"SP" was updated by "[X]PUSH*()" macros.

Note that macros ST(i), "XST_m*()" and "XSRETURN*()" work equallywell in CODE: sections and PPCODE: sections.

The following XSUB will call the C rpcb_gettime() functionand will return its two output values, timep and status, toPerl 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 necessaryto have the real rpcb_gettime() function called and to havethe return values properly placed on the argument stack.

The "void" return type for this function tells the xsubpp compiler thatthe 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 argumentstack for 2 return values. The PPCODE: directive causes thexsubpp compiler to create a stack pointer available as "SP", and itis 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 withthe following statement.

     ($status, $timep) = rpcb_gettime("localhost");

When handling output parameters with a PPCODE section, be sure to handle'set' magic properly. See perlguts for details about 'set' magic. 

Returning Undef And Empty Lists

Occasionally the programmer will want to return simply"undef" or an empty list if a function fails rather than aseparate status value. The rpcb_gettime() function offersjust this situation. If the function succeeds we would liketo have it return the time and if it fails we would like tohave undef returned. In the following Perl code the valueof $timep will either be undef or it will be a valid time.

     $timep = rpcb_gettime( "localhost" );

The following XSUB uses the "SV *" return type as a mnemonic only,and uses a CODE: block to indicate to the compilerthat the programmer has supplied all the necessary code. Thesv_newmortal() call will initialize the return value to undef, making thatthe 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 thereturn 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 andthen 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 "return" in the aboveXSUB, rather than letting control fall through to the end. In thosesituations "XSRETURN_EMPTY" should be used, instead. This will ensure thatthe XSUB stack is properly adjusted. Consult perlapi for other"XSRETURN" macros.

Since "XSRETURN_*" macros can be used with CODE blocks as well, one canrewrite 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. Togetherwith PREINIT: simplifications, this leads to:

     int     rpcb_gettime(host)          char *host          time_t  timep;        POSTCALL:          if (RETVAL == 0)                XSRETURN_UNDEF;

The REQUIRE: Keyword

The REQUIRE: keyword is used to indicate the minimum version of thexsubpp compiler needed to compile the XS module. An XS module whichcontains the following statement will compile with only xsubpp version1.922 or greater:

        REQUIRE: 1.922

The CLEANUP: Keyword

This keyword can be used when an XSUB requires special cleanup proceduresbefore it terminates. When the CLEANUP: keyword is used it must followany CODE:, or OUTPUT: blocks which are present in the XSUB. The codespecified for the cleanup block will be added as the last statements inthe XSUB. 

The POSTCALL: Keyword

This keyword can be used when an XSUB requires special proceduresexecuted after the C subroutine call is performed. When the POSTCALL:keyword is used it must precede OUTPUT: and CLEANUP: blocks which arepresent in the XSUB.

See examples in ``The NO_OUTPUT Keyword'' and ``Returning Undef And Empty Lists''.

The POSTCALL: block does not make a lot of sense when the C subroutinecall is supplied by user by providing either CODE: or PPCODE: section. 

The BOOT: Keyword

The BOOT: keyword is used to add code to the extension's bootstrapfunction. The bootstrap function is generated by the xsubpp compiler andnormally holds the statements necessary to register any XSUBs with Perl.With the BOOT: keyword the programmer can tell the compiler to add extrastatements to the bootstrap function.

This keyword may be used any time after the first MODULE keyword and shouldappear on a line by itself. The first blank line after the keyword willterminate the code block.

     BOOT:     # The following message will be printed when the     # bootstrap function executes.     printf("Hello from the bootstrap!\n");


The VERSIONCHECK: keyword corresponds to xsubpp's "-versioncheck" and"-noversioncheck" options. This keyword overrides the command lineoptions. Version checking is enabled by default. When version checking isenabled the XS module will attempt to verify that its version matches theversion of the PM module.

To enable version checking:


To disable version checking:


Note that if the version of the PM module is an NV (a floating pointnumber), it will be stringified with a possible loss of precision(currently chopping to nine decimal places) so that it may not matchthe version of the XS module anymore. Quoting the $VERSION declarationto make it a string is recommended if long version numbers are used. 


The PROTOTYPES: keyword corresponds to xsubpp's "-prototypes" and"-noprototypes" options. This keyword overrides the command line options.Prototypes are disabled by default. When prototypes are enabled, XSUBs willbe given Perl prototypes. This keyword may be used multiple times in an XSmodule to enable and disable prototypes for different parts of the module.Note that xsubpp will nag you if you don't explicitly enable or disableprototypes, with:

    Please specify prototyping behavior for Foo.xs (see perlxs manual)

To enable prototypes:


To disable prototypes:


The PROTOTYPE: Keyword

This keyword is similar to the PROTOTYPES: keyword above but can be used toforce xsubpp to use a specific prototype for the XSUB. This keywordoverrides all other prototype options and keywords but affects only thecurrent XSUB. Consult ``Prototypes'' in perlsub for information about Perlprototypes.

    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 givenXSUB as in the following example:

    void    rpcb_gettime_noproto()        PROTOTYPE: DISABLE    ...

The ALIAS: Keyword

The ALIAS: keyword allows an XSUB to have two or more unique Perl namesand to know which of those names was used when it was invoked. The Perlnames may be fully-qualified with package names. Each alias is given anindex. The compiler will setup a variable called "ix" which contain theindex of the alias which was used. When the XSUB is called with itsdeclared name "ix" will be 0.

The following example will create aliases "FOO::gettime()" and"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

The OVERLOAD: Keyword

Instead of writing an overloaded interface using pure Perl, youcan also use the OVERLOAD keyword to define additional Perl namesfor your functions (like the ALIAS: keyword above). However, theoverloaded functions must be defined with three parameters (exceptfor the nomethod() function which needs four parameters). If anyfunction has the OVERLOAD: keyword, several additional lineswill be defined in the c file generated by xsubpp in order toregister with the overload magic.

Since blessed objects are actually stored as RV's, it is usefulto use the typemap features to preprocess parameters and extractthe actual SV stored within the blessed RV. See the sample forT_PTROBJ_SPECIAL below.

To use the OVERLOAD: keyword, create an XS function which takesthree input parameters ( or use the c style '...' definition) likethis:

    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 waycomparison operators. For all overload operations using non-alphacharacters, you must type the parameter without quoting, separatingmultiple overloads with whitespace. Note that "`` (the stringifyoverload) should be entered as \''\" (i.e. escaped). 

The FALLBACK: Keyword

In addition to the OVERLOAD keyword, if you need to control howPerl autogenerates missing overloaded operators, you can set theFALLBACK keyword in the module header section, like this:


where FALLBACK can take any of the three values TRUE, FALSE, orUNDEF. If you do not set any FALLBACK value when using OVERLOAD,it defaults to UNDEF. FALLBACK is not used except when one ormore functions using OVERLOAD have been defined. Please see``fallback'' in overload for more details. 

The INTERFACE: Keyword

This keyword declares the current XSUB as a keeper of the givencalling signature. If some text follows this keyword, it isconsidered as a list of functions which have this signature, andshould 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 th