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.
From C this function will be called with the following statements.
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.
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.
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.
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.
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.
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.
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 may not be needed.
The following example will start the XS code and will place all functions in a package named 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.
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.
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.
The OUTPUT: keyword will also allow an output parameter to be mapped to a matching piece of code rather than to a typemap.
The following XSUB is for a C function which requires special handling of its parameters. The Perl usage is given first.
The XSUB follows.
In many of the examples shown here the CODE: block (and
other blocks) will often be contained within braces ( { and
} ). This protects the CODE: block from complex INPUT
typemaps and ensures the resulting C code is legal.
The following example shows a variation of the rpcb_gettime() function. This function uses the timep variable as only an output variable and does not care about its initial contents.
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.
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.
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.
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.
(...) 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.
The XS code, with ellipsis, follows.
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.
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.
The following XSUB uses the void return type to disable the generation of
the RETVAL variable 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.
The next example demonstrates how one would place an explicit undef in the return value, should the need arise.
To return an empty list one must use a PPCODE: block and then not push return values on the stack.
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.
# at the beginning of the line. Care
should be taken to avoid making the comment look like a C
preprocessor directive, lest it be interpreted as such.
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.
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.
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.
If the function's name is DESTROY then the C++
delete
function will be
called and THIS will be given as its parameter.
The C++ code will call delete .
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.
The C++ code will call new.
The following is an example of a typemap that could be used for this C++ example.
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.
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.
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.
This example requires the following typemap entry. Consult the typemap section for more information about adding new typemaps for an extension.
This example will be used with the following Perl statements.
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.
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.
RPC.xs: Interface to some ONC+ RPC bind library functions.
File typemap: Custom typemap for RPC.xs.
File RPC.pm: Perl module for the RPC extension.
File rpctest.pl: Perl test program for the RPC extension.