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perlxs - XS language reference manualDESCRIPTION
Introduction XS is an interface description file format used to create an extension interface between Perl and C code (or a C library) which one wishes to use with Perl. The XS interface is combined with the library to create a new library which can then be either dynamically loaded or statically linked into perl. The XS interface description is written in the XS language and is the core component of the Perl extension interface. An XSUB forms the basic unit of the XS interface. After compilation by the xsubpp compiler, each XSUB amounts to a C function definition which will provide the glue between Perl calling conventions and C calling conventions. The glue code pulls the arguments from the Perl stack, converts these Perl values to the formats expected by a C function, call this C function, transfers the return values of the C function back to Perl. Return values here may be a conventional C return value or any C function arguments that may serve as output parameters. These return values may be passed back to Perl either by putting them on the Perl stack, or by modifying the arguments supplied from the Perl side. The above is a somewhat simplified view of what really happens. Since Perl allows more flexible calling conventions than C, XSUBs may do much more in practice, such as checking input parameters for validity, throwing exceptions (or returning undef/empty list) if the return value from the C function indicates failure, calling different C functions based on numbers and types of the arguments, providing an object-oriented interface, etc. Of course, one could write such glue code directly in C. However, this would be a tedious task, especially if one needs to write glue for multiple C functions, and/or one is not familiar enough with the Perl stack discipline and other such arcana. XS comes to the rescue here: instead of writing this glue C code in long-hand, one can write a more concise short-hand description of what should be done by the glue, and let the XS compiler xsubpp handle the rest. The XS language allows one to describe the mapping between how the C routine is used, and how the corresponding Perl routine is used. It also allows creation of Perl routines which are directly translated to C code and which are not related to a pre-existing C function. In cases when the C interface coincides with the Perl interface, the XSUB declaration is almost identical to a declaration of a C function (in K&R style). In such circumstances, there is another tool called `h2xs' that is able to translate an entire C header file into a corresponding XS file that will provide glue to the functions/macros described in the header file. The XS compiler is called xsubpp. This compiler creates the constructs necessary to let an XSUB manipulate Perl values, and creates the glue necessary to let Perl call the XSUB. The compiler uses typemaps to determine how to map C function parameters and output values to Perl values and back. The default typemap (which comes with Perl) handles many common C types. A supplementary typemap may also be needed to handle any special structures and types for the library being linked. A file in XS format starts with a C language section which goes until the first `MODULE =' directive. Other XS directives and XSUB definitions may follow this line. The "language" used in this part of the file is usually referred to as the XS language. See the perlxstut manpage for a tutorial on the whole extension creation process. Note: For some extensions, Dave Beazley's SWIG system may provide a significantly more convenient mechanism for creating the extension glue code. See the section on "/www.swig.org" in the http: manpage for more information. 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 simplest XSUBs consist of 3 parts: a description of the return value, the name of the XSUB routine and the names of its arguments, and a description of types or formats of the arguments. The following XSUB allows a Perl program to access a C library function called sin(). The XSUB will imitate the C function which takes a single argument and returns a single value. double sin(x) double x When using parameters with C pointer types, as in double string_to_double(char *s); there may be two ways to describe this argument to xsubpp: char * s char &s Both these XS declarations correspond to the `char*' C type, but they have different semantics. 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 the section on "The Typemap" and the section on "The & Unary Operator" for more info about handling qualifiers and unary operators in C types. The function name and the return type must be placed on separate lines and should be flush left-adjusted. INCORRECT CORRECT double sin(x) double double x sin(x) double x The 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 for better readability. CORRECT double sin(x) double x More complicated XSUBs may contain many other sections. Each section of an XSUB starts with the corresponding keyword, such as INIT: or CLEANUP:. However, the first two lines of an XSUB always contain the same data: descriptions of the return type and the names of the function and its parameters. Whatever immediately follows these is considered to be an INPUT: section unless explicitly marked with another keyword. (See the The INPUT: Keyword entry elsewhere in this document.) 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 are sent as parameters to the XSUB and to store the XSUB's return value(s). In reality all Perl functions (including non-XSUB ones) keep their values on this stack all the same time, each limited to its own range of positions on the stack. In this document the first position on that stack which belongs to the active function will be referred to as position 0 for that function. XSUBs refer to their stack arguments with the macro SSTT(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 special C variable that is declared automatically for you. The C type of RETVAL matches the return type of the C library function. The xsubpp compiler will declare this variable in each XSUB with non-`void' return type. By default the generated C function will use RETVAL to hold the return value of the C library function being called. In simple cases the value of RETVAL will be placed in ST(0) of the argument stack where it can be received by Perl as the return value of the XSUB. If the XSUB has a return type of `void' then the compiler will not declare a RETVAL variable for that function. When using a PPCODE: section no manipulation of the RETVAL variable is required, the section may use direct stack manipulation to place output values on the stack. If PPCODE: directive is not used, `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 truly `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 "truly-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 which have no CODE: or PPCODE: section, such as the sin() function above, the RETVAL variable is automatically designated as an output value. For more complex functions the 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); xsubpp emits an automatic `SvSETMAGIC()' for all parameters in the OUTPUT section of the XSUB, except RETVAL. This is the usually desired behavior, as it takes care of properly invoking 'set' magic on output parameters (needed for hash or array element parameters that must be created if they didn't exist). If for some reason, this behavior is not desired, the OUTPUT section may contain a `SETMAGIC: DISABLE' line to disable it for the remainder of the parameters in the OUTPUT section. Likewise, `SETMAGIC: ENABLE' can be used to reenable it for the remainder of the OUTPUT section. See the perlguts manpage for more details about 'set' magic. The CODE: Keyword This keyword is used in more complicated XSUBs which require special handling for the C function. The RETVAL variable is still declared, but it will not be returned unless it is specified in the OUTPUT: section. The following XSUB is for a C function which requires special handling of its parameters. The Perl usage is given first. $status = rpcb_gettime( "localhost", $timep ); The XSUB follows. bool_t rpcb_gettime(host,timep) char *host time_t timep CODE: RETVAL = rpcb_gettime( host, &timep ); OUTPUT: timep RETVAL The INIT: Keyword The INIT: keyword allows initialization to be inserted into the XSUB before the compiler generates the call to the C function. Unlike the CODE: keyword above, this keyword does not affect the way the compiler handles RETVAL. bool_t rpcb_gettime(host,timep) char *host time_t &timep INIT: printf("# Host is %s\n", host ); OUTPUT: timep Another use for the INIT: section is to check for preconditions before making a call to the C function: long long lldiv(a,b) long long a long long b INIT: if (a == 0 && b == 0) XSRETURN_UNDEF; if (b == 0) croak("lldiv: cannot divide by 0"); 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 C function parameters are normally initialized with their values from the argument stack (which in turn contains the parameters that were passed to the XSUB from Perl). The typemaps contain the code segments which are used to translate the Perl values to the C parameters. The programmer, however, is allowed to override the typemaps and supply alternate (or additional) initialization code. Initialization code starts with the first `=', `;' or `+' on a line in the INPUT: section. The only exception happens if this `;' terminates the line, then this `;' is quietly ignored. The following code demonstrates how to supply initialization code for function parameters. The initialization code is eval'd within double quotes by the compiler before it is added to the output so anything which should be interpreted literally [mainly `$', `@', or `\\'] must be protected with backslashes. The variables $var, $arg, and $type can be used as in typemaps. bool_t rpcb_gettime(host,timep) char *host = (char *)SvPV($arg,PL_na); time_t &timep = 0; OUTPUT: timep This should not be used to supply default values for parameters. One would normally use this when a function parameter must be processed by another library function before it can be used. Default parameters are covered in the next section. If the initialization begins with `=', then it is output in the declaration for the input variable, replacing the initialization supplied by the typemap. If the initialization begins with `;' or `+', then it is performed after all 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 the initialization from the typemap. A global variable, `%v', is available for the truly rare case where information from one initialization is needed in another initialization. Here's a truly obscure example: bool_t rpcb_gettime(host,timep) time_t &timep ; /* \$v{timep}=@{[$v{timep}=$arg]} */ char *host + SvOK($v{timep}) ? SvPV($arg,PL_na) : NULL; OUTPUT: timep The construct `\$v{timep}=@{[$v{timep}=$arg]}' used in the above example has a two-fold purpose: first, when this line is processed by xsubpp, 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 to `ST(1)'. Default Parameter Values Default values for XSUB arguments can be specified by placing an assignment statement in the parameter list. The default value may be a number, a string or the special string `NO_INIT'. Defaults should always be used on the right-most parameters only. To allow the XSUB for rpcb_gettime() to have a default host value the parameters to the XSUB could be rearranged. The XSUB will then call the real rpcb_gettime() function with the parameters in the correct order. This XSUB can be called from Perl with either of the following statements: $status = rpcb_gettime( $timep, $host ); $status = rpcb_gettime( $timep ); The XSUB will look like the code which follows. A CODE: block is used to call the real rpcb_gettime() function with the parameters in the correct order for that function. bool_t rpcb_gettime(timep,host="localhost") char *host time_t timep = NO_INIT CODE: RETVAL = rpcb_gettime( host, &timep ); OUTPUT: timep RETVAL The PREINIT: Keyword The PREINIT: keyword allows extra variables to be declared immediately before or after the declartions of the parameters from the INPUT: section are emitted. If a variable is declared inside a CODE: section it will follow any typemap code that is emitted for the input parameters. This may result in the declaration ending up after C code, which is C syntax error. Similar errors may happen with an explicit `;'-type or `+'-type initialization of parameters is used (see the section on "Initializing Function Parameters"). Declaring these variables in an INIT: section will not help. In such cases, to force an additional variable to be declared together with declarations of other variables, place the declaration into a PREINIT: section. The PREINIT: keyword may be used one or more times within an XSUB. The following examples are equivalent, but if the code is using complex typemaps then the first example is safer. bool_t rpcb_gettime(timep) time_t timep = NO_INIT PREINIT: char *host = "localhost"; CODE: RETVAL = rpcb_gettime( host, &timep ); OUTPUT: timep RETVAL For this particular case an INIT: keyword would generate the same C code as the PREINIT: keyword. Another correct, but error-prone example: bool_t rpcb_gettime(timep) time_t timep = NO_INIT CODE: char *host = "localhost"; RETVAL = rpcb_gettime( host, &timep ); OUTPUT: timep RETVAL Another way to declare `host' is to use a C block in the CODE: section: bool_t rpcb_gettime(timep) time_t timep = NO_INIT CODE: { char *host = "localhost"; RETVAL = rpcb_gettime( host, &timep ); } OUTPUT: timep RETVAL The ability to put additional declarations before the typemap entries are processed is very handy in the cases when typemap conversions manipulate some global state: MyObject mutate(o) PREINIT: MyState st = global_state; INPUT: MyObject o; CLEANUP: reset_to(global_state, st); Here we suppose that conversion to `MyObject' in the INPUT: section and from MyObject 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 allow declaration of C variables which do not appear in the parameter list of a subroutine. Thus the above code for mutate() can be rewritten as MyObject mutate(o) MyState st = global_state; MyObject o; CLEANUP: reset_to(global_state, st); and the code for rpcb_gettime() can be rewritten as bool_t rpcb_gettime(timep) time_t timep = NO_INIT char *host = "localhost"; C_ARGS: host, &timep OUTPUT: timep RETVAL The SCOPE: Keyword The SCOPE: keyword allows scoping to be enabled for a particular XSUB. If enabled, the XSUB will invoke ENTER and LEAVE automatically. To support potentially complex type mappings, if a typemap entry used by an XSUB contains a comment like `/*scope*/' then scoping will be automatically 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 Since INPUT sections allow declaration of C variables which do not appear in the parameter list of a subroutine, this may be shortened to: bool_t rpcb_gettime(host,timep) time_t tt; char *host; char *h = host; time_t timep; CODE: RETVAL = rpcb_gettime( h, &tt ); timep = tt; OUTPUT: timep RETVAL (We used our knowledge that input conversion for `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 the assignment `h = host' in a CODE: or INIT: section.) 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"; STRLEN n_a; CODE: if( items > 1 ) host = (char *)SvPV(ST(1), n_a); RETVAL = rpcb_gettime( host, &timep ); OUTPUT: timep RETVAL The C_ARGS: Keyword The C_ARGS: keyword allows creating of XSUBS which have different calling sequence from Perl than from C, without a need to write CODE: or PPCODE: section. The contents of the C_ARGS: paragraph is put as the argument to the called C function without any change. For example, suppose that a C function is declared as symbolic nth_derivative(int n, symbolic function, int flags); and that the default flags are kept in a global C variable `default_flags'. Suppose that you want to create an interface which is called as $second_deriv = $function->nth_derivative(2); To do this, declare the XSUB as symbolic nth_derivative(function, n) symbolic function int n C_ARGS: n, function, default_flags 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 should not be used together within the same XSUB. The actual difference between PPCODE: and CODE: sections is in the initialization of `SP' macro (which stands for the current Perl stack pointer), and in the handling of data on the stack when returning from an XSUB. In CODE: sections SP preserves the value which was on entry to the XSUB: SP is on the function pointer (which follows the last parameter). In PPCODE: sections SP is moved backward to the beginning of the parameter list, which allows `PUSH*()' macros to place output values in the place Perl expects them to be when the XSUB returns back to Perl. The generated trailer for a CODE: section ensures that the number of return values Perl will see is either 0 or 1 (depending on the `void'ness of the return value of the C function, and heuristics mentioned in the section on "The RETVAL Variable"). The trailer generated for a PPCODE: section is 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 equally well in CODE: sections and PPCODE: sections. The following XSUB will call the C rpcb_gettime() function and will return its two output values, timep and status, to Perl as a single list. void rpcb_gettime(host) char *host PREINIT: time_t timep; bool_t status; PPCODE: status = rpcb_gettime( host, &timep ); EXTEND(SP, 2); PUSHs(sv_2mortal(newSViv(status))); PUSHs(sv_2mortal(newSViv(timep))); Notice that the programmer must supply the C code necessary to have the real rpcb_gettime() function called and to have the return values properly placed on the argument stack. The `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 available as `SP', and it is this pointer which is being used in the EXTEND() macro. The values are then pushed onto the stack with the PUSHs() macro. Now the rpcb_gettime() function can be used from Perl with the following statement. ($status, $timep) = rpcb_gettime("localhost"); When handling output parameters with a PPCODE section, be sure to handle 'set' magic properly. See the perlguts manpage 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 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 mnemonic only, and uses a CODE: block to indicate to the compiler that the programmer has supplied all the necessary code. The sv_newmortal() call will initialize the return value to undef, making that the default return value. SV * rpcb_gettime(host) char * host PREINIT: time_t timep; bool_t x; CODE: ST(0) = sv_newmortal(); if( rpcb_gettime( host, &timep ) ) sv_setnv( ST(0), (double)timep); The next example demonstrates how one would place an explicit undef in the return value, should the need arise. SV * rpcb_gettime(host) char * host PREINIT: time_t timep; bool_t x; CODE: ST(0) = sv_newmortal(); if( rpcb_gettime( host, &timep ) ){ sv_setnv( ST(0), (double)timep); } else{ ST(0) = &PL_sv_undef; } To return an empty list one must use a PPCODE: block and then not push return values on the stack. void rpcb_gettime(host) char *host PREINIT: time_t timep; PPCODE: if( rpcb_gettime( host, &timep ) ) PUSHs(sv_2mortal(newSViv(timep))); else{ /* Nothing pushed on stack, so an empty * list is implicitly returned. */ } Some people may be inclined to include an explicit `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 the API LISTING entry in the perlguts manpage for other `XSRETURN' macros. Since `XSRETURN_*' macros can be used with CODE blocks as well, one can rewrite this example as: int rpcb_gettime(host) char *host PREINIT: time_t timep; CODE: RETVAL = rpcb_gettime( host, &timep ); if (RETVAL == 0) XSRETURN_UNDEF; OUTPUT: RETVAL In fact, one can put this check into a CLEANUP: section as well. Together with PREINIT: simplifications, this leads to: int rpcb_gettime(host) char *host time_t timep; CLEANUP: if (RETVAL == 0) XSRETURN_UNDEF; 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 the Prototypes entry in the perlsub manpage for information about Perl prototypes. bool_t rpcb_gettime(timep, ...) time_t timep = NO_INIT PROTOTYPE: $;$ PREINIT: char *host = "localhost"; STRLEN n_a; CODE: if( items > 1 ) host = (char *)SvPV(ST(1), n_a); RETVAL = rpcb_gettime( host, &timep ); OUTPUT: timep RETVAL The ALIAS: Keyword The ALIAS: keyword allows an XSUB to have two or more unique Perl names and to know which of those names was used when it was invoked. The Perl names may be fully-qualified with package names. Each alias is given an index. The compiler will setup a variable called `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 INTERFACE: Keyword This keyword declares the current XSUB as a keeper of the given calling signature. If some text follows this keyword, it is considered as a list of functions which have this signature, and should be attached to the current XSUB. For example, if you have 4 C functions multiply(), divide(), add(), subtract() all having the signature: symbolic f(symbolic, symbolic); you can make them all to use the same XSUB using this: symbolic interface_s_ss(arg1, arg2) symbolic arg1 symbolic arg2 INTERFACE: multiply divide add subtract (This is the complete XSUB code for 4 Perl functions!) Four generated Perl function share names with corresponding C functions. The advantage of this approach comparing to ALIAS: keyword is that there is no need to code a switch statement, each Perl function (which shares the same XSUB) knows which C function it should call. Additionally, one can attach an extra function remainder() at runtime by using CV *mycv = newXSproto("Symbolic::remainder", XS_Symbolic_interface_s_ss, __FILE__, "$$"); XSINTERFACE_FUNC_SET(mycv, remainder); say, from another XSUB. (This example supposes that there was no INTERFACE_MACRO: section, otherwise one needs to use something else instead of `XSINTERFACE_FUNC_SET', see the next section.) The INTERFACE_MACRO: Keyword This keyword allows one to define an INTERFACE using a different way to extract a function pointer from an XSUB. The text which follows this keyword should give the name of macros which would extract/set a function pointer. The extractor macro is given return type, `CV*', and `XSANY.any_dptr' for this `CV*'. The setter macro is given cv, and the function pointer. The default value is `XSINTERFACE_FUNC' and `XSINTERFACE_FUNC_SET'. An INTERFACE keyword with an empty list of functions can be omitted if INTERFACE_MACRO keyword is used. Suppose that in the previous example functions pointers for multiply(), divide(), add(), subtract() are kept in a global C array `fp[]' with offsets being `multiply_off', `divide_off', `add_off', `subtract_off'. Then one can use #define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \ ((XSINTERFACE_CVT(ret,))fp[CvXSUBANY(cv).any_i32]) #define XSINTERFACE_FUNC_BYOFFSET_set(cv,f) \ CvXSUBANY(cv).any_i32 = CAT2( f, _off ) in C section, symbolic interface_s_ss(arg1, arg2) symbolic arg1 symbolic arg2 INTERFACE_MACRO: XSINTERFACE_FUNC_BYOFFSET XSINTERFACE_FUNC_BYOFFSET_set INTERFACE: multiply divide add subtract in XSUB section. 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 section on "The ALIAS: Keyword"), or maybe via the `items' variable (see the section on "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 in the INPUT: section is used to tell xsubpp that it should convert a Perl value to/from C using the C type to the left of `&', but provide a pointer to this value when the C function is called. This is useful to avoid a CODE: block for a C function which takes a parameter by reference. Typically, the parameter should be not a pointer type (an `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 an XSUB name contains `::', it is considered to be a C++ method. The generated Perl function will assume that its first argument is an object pointer. The object pointer will be stored in a variable called THIS. The object should have been created by C++ with the new() function and should be blessed by Perl with the sv_setref_pv() macro. The blessing of the object by Perl can be handled by a typemap. An example typemap is shown at the end of this section. If the return type of the XSUB includes `static', the method is considered to be a static method. It will call the C++ function using the class::method() syntax. If the method is not static the function will be called using the THIS- >method() syntax. The next examples will use the following C++ class. class color { public: color(); ~color(); int blue(); void set_blue( int ); private: int c_blue; }; The XSUBs for the blue() and set_blue() methods are defined with the class name but the parameter for the object (THIS, or "self") is implicit and is not listed. int color::blue() void color::set_blue( val ) int val Both Perl functions will expect an object as the first parameter. In the generated C++ code the object is called `THIS', and the method call will be performed on this object. So in the C++ code the blue() and set_blue() methods will be called as this: RETVAL = THIS->blue(); THIS->set_blue( val ); You could also write a single get/set method using an optional argument: int color::blue( val = NO_INIT ) int val PROTOTYPE $;$ CODE: if (items > 1) THIS->set_blue( val ); RETVAL = THIS->blue(); OUTPUT: RETVAL If the function's name is DESTROY then the C++ `delete' function will be called and `THIS' will be given as its parameter. The generated C++ code for void color::DESTROY() will look like this: color *THIS = ...; // Initialized as in typemap delete THIS; If the function's name is 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 generated 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 (such as created by `h2xs -x') is often sufficient. However, sometimes the interface will look very C-like and occasionally nonintuitive, especially when the C function modifies one of its parameters, or returns failure inband (as in "negative return values mean failure"). In cases where the programmer wishes to create a more Perl-like interface the following strategy may help to identify the more critical parts of the interface. Identify the C functions with input/output or output parameters. The XSUBs for these functions may be able to return lists to Perl. Identify the C functions which use some inband info as an indication of failure. They may be candidates to return undef or an empty list in case of failure. If the failure may be detected without a call to the C function, you may want to use an INIT: section to report the failure. For failures detectable after the C function returns one may want to use a CLEANUP: section to process the failure. In more complicated cases use CODE: or PPCODE: sections. If many functions use the same failure indication based on the return value, you may want to create a special typedef to handle this situation. Put typedef int negative_is_failure; near the beginning of XS file, and create an OUTPUT typemap entry for `negative_is_failure' which converts negative values to `undef', or maybe croak()s. After this the return value of type `negative_is_failure' will create more Perl- like interface. Identify which values are used by only the C and XSUB functions themselves, say, when a parameter to a function should be a contents of a global variable. If Perl does not need to access the contents of the value then it may not be necessary to provide a translation for that value from C to Perl. Identify the pointers in the C function parameter lists and return values. Some pointers may be used to implement input/output or output parameters, they can be handled in XS with the `&' unary operator, and, possibly, using the NO_INIT keyword. Some others will require handling of types like `int *', and one needs to decide what a useful Perl translation will do in such a case. When the semantic is clear, it is advisable to put the translation into a typemap file. Identify the structures used by the C functions. In many cases it may be helpful to use the T_PTROBJ typemap for these structures so they can be manipulated by Perl as blessed objects. (This is handled automatically by `h2xs -x'.) If the same C type is used in several different contexts which require different translations, `typedef' several new types mapped to this C type, and create separate typemap entries for these new types. Use these types in declarations of return type and parameters to XSUBs. 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'. An unlabelled initial section is assumed to be a `TYPEMAP' section. The INPUT section tells the compiler how to translate Perl values into variables of certain C types. The OUTPUT section tells the compiler how to translate the values from certain C types into values Perl can understand. The TYPEMAP section tells the compiler which of the INPUT and OUTPUT code fragments should be used to map a given C type to a Perl value. The section labels `TYPEMAP', `INPUT', or `OUTPUT' must begin in the first column on a line by themselves, and must be in uppercase. 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 Here's a more complicated example: suppose that you wanted `struct netconfig' to be blessed into the class `Net::Config'. One way to do this is to use underscores (_) to separate package names, as follows: typedef struct netconfig * Net_Config; And then provide a typemap entry `T_PTROBJ_SPECIAL' that maps underscores to double-colons (::), and declare `Net_Config' to be of that type: TYPEMAP Net_Config T_PTROBJ_SPECIAL INPUT T_PTROBJ_SPECIAL if (sv_derived_from($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")) { IV tmp = SvIV((SV*)SvRV($arg)); $var = ($type) tmp; } else croak(\"$var is not of type ${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\") OUTPUT T_PTROBJ_SPECIAL sv_setref_pv($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\", (void*)$var); The INPUT and OUTPUT sections substitute underscores for double-colons on the fly, giving the desired effect. This example demonstrates some of the power and versatility of the typemap facility.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
Originally written by Dean Roehrich <roehrich@cray.com>. Maintained since 1996 by The Perl Porters <perlbug@perl.com>.
