perlxs - XS language reference manual

- NAME
- DESCRIPTION
  - Introduction
  - On The Road
  - The Anatomy of an XSUB
  - The Argument Stack
  - The RETVAL Variable
  - The MODULE Keyword
  - The PACKAGE Keyword
  - The PREFIX Keyword
  - The OUTPUT: Keyword
  - The CODE: Keyword
  - The INIT: Keyword
  - The NO_INIT Keyword
  - Initializing Function Parameters
  - Default Parameter Values
  - The PREINIT: Keyword
  - The SCOPE: Keyword
  - The INPUT: Keyword
  - Variable-length Parameter Lists
  - The PPCODE: Keyword
  - Returning Undef And Empty Lists
  - The REQUIRE: Keyword
  - The CLEANUP: Keyword
  - The BOOT: Keyword
  - The VERSIONCHECK: Keyword
  - The PROTOTYPES: Keyword
  - The PROTOTYPE: Keyword
  - The ALIAS: Keyword
  - The INCLUDE: Keyword
  - The CASE: Keyword
  - The & Unary Operator
  - Inserting Comments and C Preprocessor Directives
  - Using XS With C++
  - Interface Strategy
  - Perl Objects And C Structures
  - The Typemap
- EXAMPLES
- XS VERSION
- AUTHOR

# NAME

perlxs - XS language reference manual

# DESCRIPTION

## Introduction

XS is a language used to create an extension interface between Perl and some C library which one wishes to use with Perl. The XS interface is combined with the library to create a new library which can be linked to Perl. An XSUB is a function in the XS language and is the core component of the Perl application interface.

The XS compiler is called xsubpp. This compiler will embed the constructs necessary to let an XSUB, which is really a C function in disguise, manipulate Perl values and creates the glue necessary to let Perl access the XSUB. The compiler uses typemaps to determine how to map C function parameters and variables to Perl values. The default typemap handles many common C types. A supplement typemap must be created to handle special structures and types for the library being linked.

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

## 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 `EXTERN.h`, `perl.h`, and `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 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.

## The Anatomy of an XSUB

The following XSUB allows a Perl program to access a C library function called `sin().` The XSUB will imitate the C function which takes a single argument and returns a single value.

         double
         sin(x)
           double x

When using C pointers the indirection operator `*` should be considered part of the type and the address operator `&` should be considered part of the variable, as is demonstrated in the `rpcb_gettime()` function above. See the section on typemaps for more about handling qualifiers and unary operators in C types.

The function name and the return type must be placed on separate lines.

      INCORRECT                        CORRECT

      double sin(x)                    double
        double x                       sin(x)
                                         double x

The function body may be indented or left-adjusted. The following example shows a function with its body left-adjusted. Most examples in this document will indent the body.

      CORRECT

      double
      sin(x)
      double x

## The Argument Stack

The argument stack is used to store the values which are sent as parameters to the XSUB and to store the XSUB's return value. In reality all Perl functions keep their values on this stack at the same time, each limited to its own range of positions on the stack. In this document the first position on that stack which belongs to the active function will be referred to as position 0 for that function.

XSUBs refer to their stack arguments with the macro ST(x), where 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 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.

## The RETVAL Variable

The RETVAL variable is a magic variable which always matches the return type of the C library function. The xsubpp compiler will supply this variable in each XSUB and by default will use it to hold the return value of the C library function being called. In simple cases the value of RETVAL will be placed in `ST(0)` of the argument stack where it can be received by Perl as the return value of the XSUB.

If the XSUB has a return type of void then the compiler will not supply a RETVAL variable for that function. When using the PPCODE: directive the RETVAL variable is not needed, unless used explicitly.

If PPCODE: directive is not used, void return value should be used only 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 return value in such cases. It was discovered that this could lead to segfaults in cases when XSUB was truely void. This practice is now deprecated, and may be not supported at some future version. Use the return value SV * in such cases. (Currently xsubpp contains some heuristic code which tries to disambiguate between ``truely-void'' and ``old-practice-declared-as-void'' functions. Hence your code is at mercy of this heuristics unless you use SV * as return value.)

## The MODULE Keyword

The MODULE keyword is used to start the XS code and to specify the package of the functions which are being defined. All text preceding the first MODULE keyword is considered C code and is passed through to the output untouched. Every XS module will have a bootstrap function which is used to hook the XSUBs into Perl. The package name of this bootstrap function will match the value of the last MODULE statement in the XS source files. The value of MODULE should always remain constant within the same XS file, though this is not required.

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

         MODULE = RPC

## The PACKAGE Keyword

When functions within an XS source file must be separated into packages the PACKAGE keyword should be used. This keyword is used with the MODULE keyword and must follow immediately after it when used.

         MODULE = RPC  PACKAGE = RPC

         [ XS code in package RPC ]

         MODULE = RPC  PACKAGE = RPCB

         [ XS code in package RPCB ]

         MODULE = RPC  PACKAGE = RPC

         [ XS code in package RPC ]

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

## The PREFIX Keyword

The PREFIX keyword designates prefixes which should be removed from the Perl function names. If the C function is `rpcb_gettime()` and the PREFIX value is `rpcb_` then Perl will see this function as `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_

## The OUTPUT: Keyword

The OUTPUT: keyword indicates that certain function parameters should be updated (new values made visible to Perl) when the XSUB terminates or that certain values should be returned to the calling Perl function. For simple functions, such as the `sin()` function above, the RETVAL variable is automatically designated as an output value. In more complex functions the 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);

## The CODE: Keyword

This keyword is used in more complicated XSUBs which require special handling for the C function. The RETVAL variable is available but will not be returned unless it is specified under the OUTPUT: keyword.

The following XSUB is for a C function which requires special handling of its parameters. The Perl usage is given first.

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

The XSUB follows.

         bool_t
         rpcb_gettime(host,timep)
              char *host
              time_t timep
              CODE:
                   RETVAL = rpcb_gettime( host, &timep );
              OUTPUT:
              timep
              RETVAL

## 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

## 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 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

## Initializing Function Parameters

Function parameters are normally initialized with their values from the argument stack. The typemaps contain the code segments which are used to transfer the Perl values to the C parameters. The programmer, however, is allowed to override the typemaps and supply alternate initialization code.

The following code demonstrates how to supply initialization code for function parameters. The initialization code is eval'd by the compiler before it is added to the output so anything which should be interpreted literally, such as double quotes, must be protected with backslashes.

         bool_t
         rpcb_gettime(host,timep)
              char *host = (char *)SvPV(ST(0),na);
              time_t &timep = 0;
              OUTPUT:
              timep

This should not be used to supply default values for parameters. One would normally use this when a function parameter must be processed by another library function before it can be used. Default parameters are covered in the next section.

## Default Parameter Values

Default values can be specified for function parameters by placing an assignment statement in the parameter list. The default value may be a number or a string. Defaults should always be used on the right-most parameters only.

To allow the XSUB for `rpcb_gettime()` to have a default host value the parameters to the XSUB could be rearranged. The XSUB will then call the real `rpcb_gettime()` function with the parameters in the correct order. Perl will call this XSUB with either of the following statements.

         $status = rpcb_gettime( $timep, $host );

         $status = rpcb_gettime( $timep );

The XSUB will look like the code which follows. A CODE: block is used to call the real `rpcb_gettime()` function with the parameters in the correct order for that function.

         bool_t
         rpcb_gettime(timep,host="localhost")
              char *host
              time_t timep = NO_INIT
              CODE:
                   RETVAL = rpcb_gettime( host, &timep );
              OUTPUT:
              timep
              RETVAL

## The PREINIT: Keyword

The PREINIT: keyword allows extra variables to be declared before the typemaps are expanded. If a variable is declared in a CODE: block then that variable will follow any typemap code. This may result in a C syntax error. To force the variable to be declared before the typemap code, place it into a PREINIT: block. The PREINIT: keyword may be used one or more times within an XSUB.

The following examples are equivalent, but if the code is using complex typemaps then the first example is safer.

         bool_t
         rpcb_gettime(timep)
              time_t timep = NO_INIT
              PREINIT:
              char *host = "localhost";
              CODE:
              RETVAL = rpcb_gettime( host, &timep );
              OUTPUT:
              timep
              RETVAL

A correct, but error-prone example.

         bool_t
         rpcb_gettime(timep)
              time_t timep = NO_INIT
              CODE:
              char *host = "localhost";
              RETVAL = rpcb_gettime( host, &timep );
              OUTPUT:
              timep
              RETVAL

## The SCOPE: Keyword

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

To support potentially complex type mappings, if a typemap entry used by this XSUB contains a comment like `/*scope*/` then scoping will automatically be enabled for that XSUB.

To enable scoping:

        SCOPE: ENABLE

To disable scoping:

        SCOPE: DISABLE

## 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 `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

## 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 that found in ANSI C. The programmer is able to determine the number of arguments passed to the XSUB by examining the items variable which the 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 host parameter for the `rpcb_gettime()` XSUB can be optional so the ellipsis can be used to indicate that the XSUB will take a variable number of parameters. Perl should be able to call this XSUB with either of the following statements.

         $status = rpcb_gettime( $timep, $host );

         $status = rpcb_gettime( $timep );

The XS code, with ellipsis, follows.

         bool_t
         rpcb_gettime(timep, ...)
              time_t timep = NO_INIT
              PREINIT:
              char *host = "localhost";
              CODE:
                      if( items > 1 )
                           host = (char *)SvPV(ST(1), na);
                      RETVAL = rpcb_gettime( host, &timep );
              OUTPUT:
              timep
              RETVAL

## The PPCODE: Keyword

The PPCODE: keyword is an alternate form of the CODE: keyword and is used to tell the xsubpp compiler that the programmer is supplying the code to control the argument stack for the XSUBs return values. Occasionally one will want an XSUB to return a list of values rather than a single value. In these cases one must use PPCODE: and then explicitly push the list of values on the stack. The PPCODE: and CODE: keywords are not used together within the same XSUB.

The following XSUB will call the C `rpcb_gettime()` function and will return its two output values, timep and status, to Perl as a single list.

         void
         rpcb_gettime(host)
              char *host
              PREINIT:
              time_t  timep;
              bool_t  status;
              PPCODE:
              status = rpcb_gettime( host, &timep );
              EXTEND(sp, 2);
              PUSHs(sv_2mortal(newSViv(status)));
              PUSHs(sv_2mortal(newSViv(timep)));

Notice that the programmer must supply the C code necessary to have the real `rpcb_gettime()` function called and to have the return values properly placed on the argument stack.

The void return type for this function tells the 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 xsubpp compiler to create a stack pointer called `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");

## Returning Undef And Empty Lists

Occasionally the programmer will want to return simply 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 SV * return type as a mneumonic only, and uses a CODE: block to indicate to the compiler that the programmer has supplied all the necessary code. The `sv_newmortal()` call will initialize the return value to undef, making that the default return value.

         SV *
         rpcb_gettime(host)
              char *  host
              PREINIT:
              time_t  timep;
              bool_t x;
              CODE:
              ST(0) = sv_newmortal();
              if( rpcb_gettime( host, &timep ) )
                   sv_setnv( ST(0), (double)timep);

The next example demonstrates how one would place an explicit undef in the return value, should the need arise.

         SV *
         rpcb_gettime(host)
              char *  host
              PREINIT:
              time_t  timep;
              bool_t x;
              CODE:
              ST(0) = sv_newmortal();
              if( rpcb_gettime( host, &timep ) ){
                   sv_setnv( ST(0), (double)timep);
              }
              else{
                   ST(0) = &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 return in the above XSUB, rather than letting control fall through to the end. In those situations XSRETURN_EMPTY should be used, instead. This will ensure that the XSUB stack is properly adjusted. Consult API LISTING for other XSRETURN macros.

## The REQUIRE: Keyword

The REQUIRE: keyword is used to indicate the minimum version of the xsubpp compiler needed to compile the XS module. An XS module which contains the following statement will compile with only xsubpp version 1.922 or greater:

            REQUIRE: 1.922

## 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.

## The BOOT: Keyword

The BOOT: keyword is used to add code to the extension's bootstrap function. The bootstrap function is generated by the 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");

## The VERSIONCHECK: Keyword

The VERSIONCHECK: keyword corresponds to xsubpp's `-versioncheck` and `-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

## The PROTOTYPES: Keyword

The PROTOTYPES: keyword corresponds to xsubpp's `-prototypes` and `-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

## The PROTOTYPE: Keyword

This keyword is similar to the PROTOTYPES: keyword above but can be used to force 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 Prototypes for information about Perl prototypes.

        bool_t
        rpcb_gettime(timep, ...)
              time_t timep = NO_INIT
              PROTOTYPE: $;$
              PREINIT:
              char *host = "localhost";
              CODE:
                      if( items > 1 )
                           host = (char *)SvPV(ST(1), na);
                      RETVAL = rpcb_gettime( host, &timep );
              OUTPUT:
              timep
              RETVAL

## The ALIAS: Keyword

The ALIAS: keyword allows an XSUB to have two 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 ix which contain the index of the alias which was used. When the XSUB is called with its declared 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 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 Rpcb1.xsh contains our `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 (`|`) then the compiler will interpret the parameters as a command.

        INCLUDE: cat Rpcb1.xsh |

## 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 ix ALIAS: variable (see The ALIAS: Keyword), or maybe via the items variable (see Variable-length Parameter Lists). The last CASE: becomes the default case if it is not associated with a conditional. The following example shows CASE switched via ix with a function `rpcb_gettime()` having an alias `x_gettime()`. When the function is called as `rpcb_gettime()` its parameters are the usual `(char *host, time_t *timep)`, but when the function is called as `x_gettime()` its parameters are reversed, `(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 );

## The & Unary Operator

The & unary operator is used to tell the compiler that it should dereference the object when it calls the C function. This is used when a CODE: block is not used and the object is a not a pointer type (the object is an int or `long` but not a int* or `long*`).

The following XSUB will generate incorrect C code. The xsubpp compiler will turn this into code which calls `rpcb_gettime()` with parameters `(char *host, time_t timep)`, but the real `rpcb_gettime()` wants the `timep` parameter to be of type `time_t*` rather than `time_t`.

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

That problem is corrected by using the `&` operator. The xsubpp compiler will now turn this into code which calls `rpcb_gettime()` correctly with parameters `(char *host, time_t *timep)`. It does this by carrying the `&` through, so the function call looks like `rpcb_gettime(host, &timep)`.

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

## Inserting Comments and C Preprocessor Directives

C preprocessor directives are allowed within BOOT:, PREINIT: INIT:, CODE:, PPCODE:, and CLEANUP: blocks, as well as outside the functions. Comments are allowed anywhere after the MODULE keyword. The compiler will pass the preprocessor directives through untouched and will remove the commented lines.

Comments can be added to XSUBs by placing a `#` 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 `#`.

If you use preprocessor directives to choose one of two versions of a function, use

        #if ... version1
        #else /* ... version2  */
        #endif

and not

        #if ... version1
        #endif
        #if ... version2
        #endif

because otherwise xsubpp will believe that you made a duplicate definition of the function. Also, put a blank line before the #else/#endif so it will not be seen as part of the function body.

## Using XS With C++

If a function is defined as a C++ method then it will assume its first argument is an object pointer. The object pointer will be stored in a variable called THIS. The object should have been created by C++ with the `new()` function and should be blessed by Perl with the `sv_setref_pv()` macro. The blessing of the object by Perl can be handled by a typemap. An example typemap is shown at the end of this section.

If the method is defined as static it will call the C++ function using the class::method() syntax. If the method is not static the function will be called using the THIS- >`method()` syntax.

The next examples will use the following C++ class.

         class color {
              public:
              color();
              ~color();
              int blue();
              void set_blue( int );

              private:
              int c_blue;
         };

The XSUBs for the `blue()` and `set_blue()` methods are defined with the class name but the parameter for the object (THIS, or ``self'') is implicit and is not listed.

         int
         color::blue()

         void
         color::set_blue( val )
              int val

Both functions will expect an object as the first parameter. The xsubpp compiler will call that object THIS and will use it to call the specified method. So in the C++ code the `blue()` and `set_blue()` methods will be called in the following manner.

         RETVAL = THIS->blue();

         THIS->set_blue( val );

If the function's name is DESTROY then the C++ delete function will be called and THIS will be given as its parameter.

         void
         color::DESTROY()

The C++ code will call delete.

         delete THIS;

If the function's name is new then the 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 CLASS, to be given as the first argument.

         color *
         color::new()

The C++ code will call `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;
            }

## Interface Strategy

When designing an interface between Perl and a C library a straight translation from C to XS is often sufficient. The interface will often be very C-like and occasionally nonintuitive, especially when the C function modifies one of its parameters. In cases where the programmer wishes to create a more Perl-like interface the following strategy may help to identify the more critical parts of the interface.

Identify the C functions which modify their parameters. The XSUBs for these functions may be able to return lists to Perl, or may be candidates to return undef or an empty list in case of failure.

Identify which values are used by only the C and XSUB functions themselves. If Perl does not need to access the contents of the value then it may not be necessary to provide a translation for that value from C to Perl.

Identify the pointers in the C function parameter lists and return values. Some pointers can be handled in XS with the & unary operator on the variable name while others will require the use of the * operator on the type name. In general it is easier to work with the & operator.

Identify the structures used by the C functions. In many cases it may be helpful to use the T_PTROBJ typemap for these structures so they can be manipulated by Perl as blessed objects.

## Perl Objects And C Structures

When dealing with C structures one should select either T_PTROBJ or 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 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 `typedef` will be created for `struct netconfig`. The Perl object will be blessed in a class matching the name of the C type, with the tag `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.

## The Typemap

The typemap is a collection of code fragments which are used by the xsubpp compiler to map C function parameters and values to Perl values. The typemap file may consist of three sections labeled `TYPEMAP`, INPUT, and `OUTPUT`. The INPUT section tells the compiler how to translate Perl values into variables of certain C types. The OUTPUT section tells the compiler how to translate the values from certain C types into values Perl can understand. The TYPEMAP section tells the compiler which of the INPUT and OUTPUT code fragments should be used to map a given C type to a Perl value. Each of the sections of the typemap must be preceded by one of the TYPEMAP, INPUT, or OUTPUT keywords.

The default typemap in the `ext` directory of the Perl source contains many useful types which can be used by Perl extensions. Some extensions define additional typemaps which they keep in their own directory. These additional typemaps may reference INPUT and OUTPUT maps in the main typemap. The 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 `*` is considered to be a part of the C type name.

         TYPEMAP
         Netconfig *<tab>T_PTROBJ

# EXAMPLES

File `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 `typemap`: Custom typemap for RPC.xs.

         TYPEMAP
         Netconfig *  T_PTROBJ

File `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 `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";

# XS VERSION

This document covers features supported by xsubpp 1.935.

# AUTHOR

Dean Roehrich <roehrich@cray.com> Jul 8, 1996