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Inline::C - Write Perl Subroutines in C


Inline::C is a module that allows you to write Perl subroutines in C. Since version 0.30 the Inline module supports multiple programming languages and each language has its own support module. This document describes how to use Inline with the C programming language. It also goes a bit into Perl C internals.

If you want to start working with programming examples right away, check out Inline::C-Cookbook. For more information on Inline in general, see Inline.


You never actually use Inline::C directly. It is just a support module for using with C. So the usage is always:

    use Inline C => ...;


    bind Inline C => ...;

Function Definitions

The Inline grammar for C recognizes certain function definitions (or signatures) in your C code. If a signature is recognized by Inline, then it will be available in Perl-space. That is, Inline will generate the "glue" necessary to call that function as if it were a Perl subroutine. If the signature is not recognized, Inline will simply ignore it, with no complaints. It will not be available from Perl-space, although it will be available from C-space.

Inline looks for ANSI/prototype style function definitions. They must be of the form:

    return-type function-name ( type-name-pairs ) { ... }

The most common types are: int, long, double, char*, and SV*. But you can use any type for which Inline can find a typemap. Inline uses the typemap file distributed with Perl as the default. You can specify more typemaps with the TYPEMAPS configuration option.

A return type of void may also be used. The following are examples of valid function definitions.

    int Foo(double num, char* str) {
    void Foo(double num, char* str) {
    SV* Foo() {
    void Foo(SV*, ...) {
    long Foo(int i, int j, ...) {

The following definitions would not be recognized:

    Foo(int i) {               # no return type
    int Foo(float f) {         # no (default) typemap for float
    int Foo(num, str) double num; char* str; {
    void Foo(void) {           # void only valid for return type

Notice that Inline only looks for function definitions, not function prototypes. Definitions are the syntax directly preceeding a function body. Also Inline does not scan external files, like headers. Only the code passed to Inline is used to create bindings; although other libraries can linked in, and called from C-space.

C Configuration Options

For information on how to specify Inline configuration options, see Inline. This section describes each of the configuration options available for C. Most of the options correspond either to MakeMaker or XS options of the same name. See ExtUtils::MakeMaker and perlxs.


Specifies extra statements to automatically included. They will be added onto the defaults. A newline char will be automatically added.

    use Inline C => Config => AUTO_INCLUDE => '#include "yourheader.h"';


If you 'ENABLE => AUTOWRAP', Inline::C will parse function declarations (prototype statements) in your C code. For each declaration it can bind to, it will create a dummy wrapper that will call the real function which may be in an external library. This is a nice convenience for functions that would otherwise just require an empty wrapper function.

This is similar to the base functionality you get from h2xs. It can be very useful for binding to external libraries.


Specifies C code to be executed in the XS BOOT section. Corresponds to the XS parameter.


Specify which compiler to use.


Specify extra compiler flags.


Allows you to specify a list of source code filters. If more than one is requested, be sure to group them with an array ref. The filters can either be subroutine references or names of filters provided by the supplementary Inline::Filters module.

Your source code will be filtered just before it is parsed by Inline. The MD5 fingerprint is generated before filtering. Source code filters can be used to do things like stripping out POD documentation, pre-expanding #include statements or whatever else you please. For example:

    use Inline C => DATA =>
               FILTERS => [Strip_POD => \&MyFilter => Preprocess ];

Filters are invoked in the order specified. See Inline::Filters for more information.


Specifies an include path to use. Corresponds to the MakeMaker parameter. Expects a fully qualified path.

    use Inline C => Config => INC => '-I/inc/path';


Specify which linker to use.


Specify which linker flags to use.

NOTE: These flags will completely override the existing flags, instead of just adding to them. So if you need to use those too, you must respecify them here.


Specifies external libraries that should be linked into your code. Corresponds to the MakeMaker parameter. Provide a fully qualified path with the -L switch if the library is in a location where it won't be found automatically.

    use Inline C => Config => LIBS => '-lyourlib';


    use Inline C => Config => LIBS => '-L/your/path -lyourlib';


Specify the name of the 'make' utility to use.


Specifies a user compiled object that should be linked in. Corresponds to the MakeMaker parameter. Expects a fully qualified path.

    use Inline C => Config => MYEXTLIB => '/your/path/';


This controls the MakeMaker OPTIMIZE setting. By setting this value to '-g', you can turn on debugging support for your Inline extensions. This will allow you to be able to set breakpoints in your C code using a debugger like gdb.


Specifies a prefix that will be automatically stripped from C functions when they are bound to Perl. Useful for creating wrappers for shared library API-s, and binding to the original names in Perl. Also useful when names conflict with Perl internals. Corresponds to the XS parameter.

    use Inline C => Config => PREFIX => 'ZLIB_';


Specifies extra typemap files to use. These types will modify the behaviour of the C parsing. Corresponds to the MakeMaker parameter. Expects a fully qualified path.

    use Inline C => Config => TYPEMAPS => '/your/path/typemap';

C-Perl Bindings

This section describes how the Perl variables get mapped to C variables and back again.

First, you need to know how Perl passes arguments back and forth to subroutines. Basically it uses a stack (also known as the Stack). When a sub is called, all of the parenthesized arguments get expanded into a list of scalars and pushed onto the Stack. The subroutine then pops all of its parameters off of the Stack. When the sub is done, it pushes all of its return values back onto the Stack.

The Stack is an array of scalars known internally as SV's. The Stack is actually an array of pointers to SV or SV*; therefore every element of the Stack is natively a SV*. For FMTYEWTK about this, read perldoc perlguts.

So back to variable mapping. XS uses a thing known as "typemaps" to turn each SV* into a C type and back again. This is done through various XS macro calls, casts and the Perl API. See perldoc perlapi. XS allows you to define your own typemaps as well for fancier non-standard types such as typedef-ed structs.

Inline uses the default Perl typemap file for its default types. This file is called /usr/local/lib/perl5/5.6.1/ExtUtils/typemap, or something similar, depending on your Perl installation. It has definitions for over 40 types, which are automatically used by Inline. (You should probably browse this file at least once, just to get an idea of the possibilities.)

Inline parses your code for these types and generates the XS code to map them. The most commonly used types are:

 - int
 - long
 - double
 - char*
 - void
 - SV*

If you need to deal with a type that is not in the defaults, just use the generic SV* type in the function definition. Then inside your code, do the mapping yourself. Alternatively, you can create your own typemap files and specify them using the TYPEMAPS configuration option.

A return type of void has a special meaning to Inline. It means that you plan to push the values back onto the Stack yourself. This is what you need to do to return a list of values. If you really don't want to return anything (the traditional meaning of void) then simply don't push anything back.

If ellipsis or ... is used at the end of an argument list, it means that any number of SV*s may follow. Again you will need to pop the values off of the Stack yourself.

See "Examples" below.

The Inline Stack Macros

When you write Inline C, the following lines are automatically prepended to your code (by default):

    #include "EXTERN.h"
    #include "perl.h"
    #include "XSUB.h"
    #include "INLINE.h"

The file INLINE.h defines a set of macros that are useful for handling the Perl Stack from your C functions.


You'll need to use this one, if you want to use the others. It sets up a few local variables: sp, items, ax and mark, for use by the other macros. It's not important to know what they do, but I mention them to avoid possible name conflicts.

NOTE: Since this macro declares variables, you'll need to put it with your other variable declarations at the top of your function. It must come before any executable statements and before any other Inline_Stack macros.


Returns the number of arguments passed in on the Stack.


Refers to a particular SV* in the Stack, where i is an index number starting from zero. Can be used to get or set the value.


Use this before pushing anything back onto the Stack. It resets the internal Stack pointer to the beginning of the Stack.


Push a return value back onto the Stack. The value must be of type SV*.


After you have pushed all of your return values, you must call this macro.


Return n items on the Stack.


A special macro to indicate that you really don't want to return anything. Same as:


Please note that this macro actually returns from your function.

Each of these macros is available in 3 different styles to suit your coding tastes. The following macros are equivalent.


All of this functionality is available through XS macro calls as well. So why duplicate the functionality? There are a few reasons why I decided to offer this set of macros. First, as a convenient way to access the Stack. Second, for consistent, self documenting, non-cryptic coding. Third, for future compatibility. It occured to me that if a lot of people started using XS macros for their C code, the interface might break under Perl6. By using this set, hopefully I will be able to insure future compatibility of argument handling.

Of course, if you use the rest of the Perl API, your code will most likely break under Perl6. So this is not a 100% guarantee. But since argument handling is the most common interface you're likely to use, it seemed like a wise thing to do.

Writing C Subroutines

The definitions of your C functions will fall into one of the following four categories. For each category there are special considerations.

  1.     int Foo(int arg1, char* arg2, SV* arg3) {

    This is the simplest case. You have a non void return type and a fixed length argument list. You don't need to worry about much. All the conversions will happen automatically.

  2.     void Foo(int arg1, char* arg2, SV* arg3) {

    In this category you have a void return type. This means that either you want to return nothing, or that you want to return a list. In the latter case you'll need to push values onto the Stack yourself. There are a few Inline macros that make this easy. Code something like this:

        int i, max; SV* my_sv[10];
        for (i = 0; i < max; i++)

    After resetting the Stack pointer, this code pushes a series of return values. At the end it uses Inline_Stack_Done to mark the end of the return stack.

    If you really want to return nothing, then don't use the Inline_Stack_ macros. If you must use them, then set use Inline_Stack_Void at the end of your function.

  3.     char* Foo(SV* arg1, ...) {

    In this category you have an unfixed number of arguments. This means that you'll have to pop values off the Stack yourself. Do it like this:

        int i;
        for (i = 0; i < Inline_Stack_Items; i++)

    The return type of Inline_Stack_Item(i) is SV*.

  4.     void* Foo(SV* arg1, ...) {

    In this category you have both a void return type and an unfixed number of arguments. Just combine the techniques from Categories 3 and 4.


Here are a few examples. Each one is a complete program that you can try running yourself. For many more examples see Inline::C-Cookbook.

Example #1 - Greetings

This example will take one string argument (a name) and print a greeting. The function is called with a string and with a number. In the second case the number is forced to a string.

Notice that you do not need to #include <stdio.h>. The perl.h header file which gets included by default, automatically loads the standard C header files for you.

    use Inline C;
    void greet(char* name) {
      printf("Hello %s!\n", name);

Example #2 - and Salutations

This is similar to the last example except that the name is passed in as a SV* (pointer to Scalar Value) rather than a string (char*). That means we need to convert the SV to a string ourselves. This is accomplished using the SvPVX function which is part of the Perl internal API. See perldoc perlapi for more info.

One problem is that SvPVX doesn't automatically convert strings to numbers, so we get a little surprise when we try to greet 42. The program segfaults, a common occurence when delving into the guts of Perl.

    use Inline C;
    void greet(SV* sv_name) {
      printf("Hello %s!\n", SvPVX(sv_name));

Example #3 - Fixing the problem

We can fix the problem in Example #2 by using the SvPV function instead. This function will stringify the SV if it does not contain a string. SvPV returns the length of the string as it's second parameter. Since we don't care about the length, we can just put PL_na there, which is a special variable designed for that purpose.

    use Inline C;
    void greet(SV* sv_name) {
      printf("Hello %s!\n", SvPV(sv_name, PL_na));


For general information about Inline see Inline.

For sample programs using Inline with C see Inline::C-Cookbook.

For information on supported languages and platforms see Inline-Support.

For information on writing your own Inline Language Support Module, see Inline-API.

Inline's mailing list is

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  1. If you use C function names that happen to be used internally by Perl, you will get a load error at run time. There is currently no functionality to prevent this or to warn you. For now, a list of Perl's internal symbols is packaged in the Inline module distribution under the filename 'symbols.perl'. Avoid using these in your code.


Brian Ingerson <>

Sisyphus <> fixed some bugs and is current co-maintainer.


Copyright (c) 2000-2002. Brian Ingerson.

Copyright (c) 2008, 2010, 2011. Sisyphus.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.