path: root/doc/api.texi

\input texinfo   @c -*-texinfo-*-
@c %**start of header (This is for running Texinfo on a region.)
@setfilename api.info
@settitle Writing Extensions For Gawk
@c %**end of header (This is for running Texinfo on a region.)

@dircategory Text creation and manipulation
@direntry
* Gawk: (gawk).                 A text scanning and processing language.
@end direntry
@dircategory Individual utilities
@direntry
* awk: (gawk)Invoking gawk.                     Text scanning and processing.
@end direntry

@set xref-automatic-section-title

@c The following information should be updated here only!
@c This sets the edition of the document, the version of gawk it
@c applies to and all the info about who's publishing this edition

@c These apply across the board.
@set UPDATE-MONTH October, 2012
@set VERSION 4.1
@set PATCHLEVEL 0

@set FSF

@set TITLE Writing Extensions for Gawk
@set SUBTITLE A Temporary Manual
@set EDITION 1

@iftex
@set DOCUMENT book
@set CHAPTER chapter
@set APPENDIX appendix
@set SECTION section
@set SUBSECTION subsection
@set DARKCORNER @inmargin{@image{lflashlight,1cm}, @image{rflashlight,1cm}}
@set COMMONEXT (c.e.)
@end iftex
@ifinfo
@set DOCUMENT Info file
@set CHAPTER major node
@set APPENDIX major node
@set SECTION minor node
@set SUBSECTION node
@set DARKCORNER (d.c.)
@set COMMONEXT (c.e.)
@end ifinfo
@ifhtml
@set DOCUMENT Web page
@set CHAPTER chapter
@set APPENDIX appendix
@set SECTION section
@set SUBSECTION subsection
@set DARKCORNER (d.c.)
@set COMMONEXT (c.e.)
@end ifhtml
@ifdocbook
@set DOCUMENT book
@set CHAPTER chapter
@set APPENDIX appendix
@set SECTION section
@set SUBSECTION subsection
@set DARKCORNER (d.c.)
@set COMMONEXT (c.e.)
@end ifdocbook
@ifplaintext
@set DOCUMENT book
@set CHAPTER chapter
@set APPENDIX appendix
@set SECTION section
@set SUBSECTION subsection
@set DARKCORNER (d.c.)
@set COMMONEXT (c.e.)
@end ifplaintext

@c some special symbols
@iftex
@set LEQ @math{@leq}
@set PI @math{@pi}
@end iftex
@ifnottex
@set LEQ <=
@set PI @i{pi}
@end ifnottex

@ifnottex
@macro ii{text}
@i{\text\}
@end macro
@end ifnottex

@c For HTML, spell out email addresses, to avoid problems with
@c address harvesters for spammers.
@ifhtml
@macro EMAIL{real,spelled}
``\spelled\''
@end macro
@end ifhtml
@ifnothtml
@macro EMAIL{real,spelled}
@email{\real\}
@end macro
@end ifnothtml

@set FN file name
@set FFN File Name
@set DF data file
@set DDF Data File
@set PVERSION version
@set CTL Ctrl

@ignore
Some comments on the layout for TeX.
1. Use at least texinfo.tex 2000-09-06.09
2. I have done A LOT of work to make this look good. There are  `@page' commands
   and use of `@group ... @end group' in a number of places. If you muck
   with anything, it's your responsibility not to break the layout.
@end ignore

@c merge the function and variable indexes into the concept index
@ifinfo
@synindex fn cp
@synindex vr cp
@end ifinfo
@iftex
@syncodeindex fn cp
@syncodeindex vr cp
@end iftex
@ifxml
@syncodeindex fn cp
@syncodeindex vr cp
@end ifxml

@c If "finalout" is commented out, the printed output will show
@c black boxes that mark lines that are too long.  Thus, it is
@c unwise to comment it out when running a master in case there are
@c overfulls which are deemed okay.

@iftex
@finalout
@end iftex

@copying
Copyright @copyright{} 2012
Free Software Foundation, Inc.
@sp 2

This is Edition @value{EDITION} of @cite{@value{TITLE}: @value{SUBTITLE}},
for the @value{VERSION}.@value{PATCHLEVEL} (or later) version of the GNU
implementation of AWK.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'', the Front-Cover
texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below).  A copy of the license is included in the section entitled
``GNU Free Documentation License''.

@enumerate a
@item
``A GNU Manual''

@item
``You have the freedom to
copy and modify this GNU manual.  Buying copies from the FSF
supports it in developing GNU and promoting software freedom.''
@end enumerate
@end copying

@c Comment out the "smallbook" for technical review.  Saves
@c considerable paper.  Remember to turn it back on *before*
@c starting the page-breaking work.

@c 4/2002: Karl Berry recommends commenting out this and the
@c `@setchapternewpage odd', and letting users use `texi2dvi -t'
@c if they want to waste paper.
@c @smallbook


@c Uncomment this for the release.  Leaving it off saves paper
@c during editing and review.
@setchapternewpage odd

@titlepage
@title @value{TITLE}
@subtitle @value{SUBTITLE}
@subtitle Edition @value{EDITION}
@subtitle @value{UPDATE-MONTH}
@author Arnold D. Robbins

@c Include the Distribution inside the titlepage environment so
@c that headings are turned off.  Headings on and off do not work.

@page
@vskip 0pt plus 1filll
``To boldly go where no man has gone before'' is a
Registered Trademark of Paramount Pictures Corporation. @*
@c sorry, i couldn't resist
@sp 3
Published by:
@sp 1

Free Software Foundation @*
51 Franklin Street, Fifth Floor @*
Boston, MA  02110-1301 USA @*
Phone: +1-617-542-5942 @*
Fax: +1-617-542-2652 @*
Email: @email{gnu@@gnu.org} @*
URL: @uref{http://www.gnu.org/} @*

@c This one is correct for gawk 3.1.0 from the FSF
ISBN 1-882114-28-0 @*
@sp 2
@insertcopying
@end titlepage

@node Extension API
@chapter Writing Extensions for @command{gawk}

This @value{CHAPTER} describes how to extend @command{gawk} using
code written in C or C++.  If you don't know anything about C
programming, you can safely skip this @value{CHAPTER}, although you
may wish to review the documentation on the extensions that come
with @command{gawk} (@pxref{Extension Samples}).

@menu
@end menu

@node Extension Intro
@section Introduction

An @dfn{extension} (sometimes called a @dfn{plug-in}) is a piece of
external compiled code that @command{gawk} can load at runtime to
provide additional functionality, over and above the built-in capabilities
described in the rest of this @value{DOCUMENT}.

Extensions are useful because they allow you (of course) to extend
@command{gawk}'s functionality. For example, they can provide access to
system calls (such as @code{chdir()} to change directory) and to other
C library routines that could be of use.  As with most software,
``the sky is the limit;'' if you can imagine something that you might
want to do and can write in C or C++, you can write an extension to do it!

Extensions are written in C or C++, using the @dfn{Application Programming
Interface} (API) defined for this purpose by the @command{gawk}
developers.  The rest of this @value{CHAPTER} explains the design
decisions behind the API, the facilities it provides and how to use
them, and presents a small sample extension.  In addition, it documents
the sample extensions included in the @command{gawk} distribution.

@node Extension Design
@section Extension API Design

The first version of extensions for @command{gawk} was developed in
the mid-1990s and released with @command{gawk} 3.1 in the late 1990s.
The basic mechanisms and design remained unchanged for close to 15 years,
until 2012.

The old extension mechanism used data types and functions from
@command{gawk} itself, with a ``clever hack'' to install extension
functions.

@command{gawk} included some sample extensions, of which a few were
really useful.  However, it was clear from the outset that the extension
mechanism was bolted onto the side and was not really thought out.

@menu
@end menu

@node Old Extension Problems
@subsection Problems With The Old Mechanism

The old extension mechanism had several problems:

@itemize @bullet
@item
It depended heavily upon @command{gawk} internals.  Any time the
@code{NODE} structure@footnote{A critical central data structure
inside @command{gawk}.} changed, an extension would have to be
recompiled. Furthermore, to really write extensions required understanding
something about @command{gawk}'s internal functions.  There was some
documentation in this @value{DOCUMENT}, but it was quite minimal.

@item
Being able to call into @command{gawk} from an extension required linker
facilities that are common on Unix-derived systems but that did
not work on Windows systems; users wanting extensions on Windows
had to statically link them into @command{gawk}, even though Windows supports
dynamic loading of shared objects.

@item
The API would change occasionally as @command{gawk} changed; no compatibility
between versions was ever offered or planned for.
@end itemize

Despite the drawbacks, the @command{xgawk} project developers forked
@command{gawk} and developed several significant extensions. They also
enhanced @command{gawk}'s facilities relating to file inclusion and
shared object access.

A new API was desired for a long time, but only in 2012 did the
@command{gawk} maintainer and the @command{xgawk} developers finally
start working on it together.  More information about the @command{xgawk}
project is provided in @ref{gawkextlib}.

@node Extension New Mechansim Goals
@subsection Goals For A New Mechansim

Some goals for the new API were:

@itemize @bullet
@item
The API should be independent of @command{gawk} internals.  Changes in
@command{gawk} internals should not be visible to the writer of an
extension function.

@item
The API should provide @emph{binary} compatibility across @command{gawk}
releases as long as the API itself does not change.

@item
The API should enable extensions written in C to have roughly the
same ``appearance'' to @command{awk}-level code as @command{awk}
functions do. This means that extensions should have:

@itemize @minus
@item
The ability to access function parameters.

@item
The ability to turn an undefined parameter into an array (call by reference).

@item
The ability to create, access and update global variables.

@item
Easy access to all the elements of an array at once (``array flattening'')
in order to loop over all the element in an easy fashion for C code.
@end itemize

@item
The ability to create arrays (including @command{gawk}'s true
multi-dimensional arrays).
@end itemize

Some additional important goals were:

@itemize @bullet
@item
The API should use only features in ISO C 90, so that extensions
can be written using the widest range of C and C++ compilers. The header
should include the appropriate @samp{#ifdef __cplusplus} and @samp{extern "C"}
magic so that a C++ compiler could be used.  (If using the C++, the runtime
system has to be smart enough to call any constructors and destructors,
as @command{gawk} is a C program. As of this writing, this has not been
tested.)

@item
The API mechanism should not require access to @command{gawk}'s
symbols@footnote{The @dfn{symbols} are the variables and functions
defined inside @command{gawk}.  Access to these symbols by code
external to @command{gawk} loaded dynamically at runtime is
problematic on Windows.} by the compile-time or dynamic linker,
in order to enable creation of extensions that will also work on Windows.
@end itemize

During development, it became clear that there were other features
that should be available to extensions, which were also subsequently
provided:

@itemize @bullet
@item
Extensions should have the ability to hook into @command{gawk}'s
I/O redirection mechanism.  In particular, the @command{xgawk}
developers provided a so-called ``open hook'' to take over reading
records.  During the development, this was generalized to allow
extensions to hook into input processing, output processing, and
two-way I/O.

@item
An extension should be able to provide a ``call back'' function
to perform clean up actions when @command{gawk} exits.

@item
An extension should be able to provide a version string so that
@command{gawk}'s @option{--version} option can provide information
about extensions as well.
@end itemize

@node Extension Other Design Decisions
@subsection Other Design Decisions

As an ``arbitrary'' design decision, extensions can read the values of
built-in variables and arrays (such as @code{ARGV} and @code{FS}), but cannot
change them, with the exception of @code{PROCINFO}.

The reason for this is to prevent an extension function from affecting
the flow of an @command{awk} program outside its control.  While a real
@command{awk} function can do what it likes, that is at the discretion
of the programmer.  An extension function should provide a service or
make a C API available for use within @command{awk}, and not mess with
@code{FS} or @code{ARGC} and @code{ARGV}.

In addition, it becomes easy to start down a slippery slope. How
much access to @command{gawk} facilities do extensions need?
Do they need @code{getline}?  What about calling @code{gsub()} or
compiling regular expressions?  What about calling into @command{awk}
functions? (@emph{That} would be messy.)

In order to avoid these issues, the @command{gawk} developers chose
to start with the simplest, most basic features that are still truly useful.

Another decision is that although @command{gawk} provides nice things like
MPFR, and arrays indexed internally by integers, these features are not
being brought out to the API in order to keep things simple and close to
traditional @command{awk} semantics.  (In fact, arrays indexed internally
by integers are so transparent that they aren't even documented!)

With time, the API will undoubtedly evolve; the @command{gawk} developers
expect this to be driven by user needs. For now, the current API seems
to provide a minimal yet powerful set of features for extension creation.

@node Extension Mechanism Outline
@subsection At A High Level How It Works

The requirement to avoid access to @command{gawk}'s symbols is, at first
glance, a difficult one to meet.

One design, apparently used by Perl and Ruby and maybe others, would
be to make the mainline @command{gawk} code into a library, with the
@command{gawk} program a small C @code{main()} function linked against
the library.

This seemed like the tail wagging the dog, complicating build and
installation and making a simple copy of the @command{gawk} executable
from one system to another (or one place to another on the same
system!) into a chancy operation.

Pat Rankin suggested the solution that was adopted. Communication between
@command{gawk} and an extension is two-way.  First, when an extension
is loaded, it is passed a pointer to a @code{struct} whose fields are
function pointers.

FIXME: Figure 1

The extension can call functions inside @command{gawk} through these
function pointers, at runtime, without needing (link-time) access
to @command{gawk}'s symbols.  One of these function pointers is to a
function for ``registering'' new built-in functions.

FIXME: Figure 2

In the other direction, the extension registers its new functions
with @command{gawk} by passing function pointers to the functions that
provide the new feature (@code{do_chdir()}, for example).  @command{gawk}
associates the function pointer with a name and can then call it, using a
defined calling convention. The @code{do_@var{xxx}()} function, in turn,
then uses the function pointers in the API @code{struct} to do its work,
such as updating variables or arrays, printing messages, setting @code{ERRNO},
and so on.

FIXME: Figure 3

Convenience macros in the @file{gawkapi.h} header file make calling
through the function pointers look like regular function calls so that
extension code is quite readable and understandable.

Although all of this sounds medium complicated, the result is that
extension code is quite clean and straightforward. This can be seen in
the sample extensions @file{filefuncs.c} and also the @file{testext.c}
code for testing the APIs.

Some other bits and pieces:

@itemize @bullet
@item
The API provides access to @command{gawk}'s @code{do_@var{xxx}} values,
reflecting command line options, like @code{do_lint}, @code{do_profiling}
and so on (@pxref{Extension API Variables}).
These are informational: an extension cannot affect these
inside @command{gawk}.  In addtion, attempting to assign to them
produces a compile-time error.

@item
The API also provides major and minor version numbers, so that an
extension can check if the @command{gawk} it is loaded with supports the
facilties it was compiled with.  (Version mismatches ``shouldn't''
happen, but we all know how @emph{that} goes.)
@xref{Extension Versioning}, for details.

@item
An extension may register a version string with @command{gawk}; this
allows @command{gawk} to dump extension version information when
invoked with the @option{--version} option.
@end itemize

@node Extension Future Grouth
@subsection Room For Future Growth

The API provides room for future growth, in two ways.

An ``extension id'' is passed into the extension when its loaded. This
extension id is then passed back to @command{gawk} with each function
call.  This allows @command{gawk} to identify the extension calling it,
should it need to know.

A ``name space'' is passed into @command{gawk} when an extension function
is registered.  This provides for a future mechanism for grouping
extension functions and possibly avoiding name conflicts.

Of course, as of this writing, no decisions have been made with respect
to any of the above.

@node Extension API Description
@section API Description

This (rather large) @value{SECTION} describes the API in detail.

@menu
@end menu

@node Extension API Functions Introduction
@subsection Introduction

Access to facilities within @command{gawk} are made available
by calling through function pointers passed into your extension.

API function pointers are provided for the following kinds of operations:

@itemize @bullet
@item
Registrations functions. You may register
@itemize @bullet
@item
extension functions,
@item
input parsers,
@item
output wrappers,
@item
two-way processors,
@item
exit callbacks,
@item
and a version string.
@end itemize
All of these are discussed in detail, later in this @value{CHAPTER}.

@item
Printing fatal, warning, and lint warning messages.

@item
Updating @code{ERRNO}, or unsetting it.

@item
Accessing parameters, including converting an undefined paramater into
an array.

@item
Symbol table access: retreiving a global variable, creating one,
or changing one.  This also includes the ability to create a scalar
variable that will be @emph{constant} within @command{awk} code.

@item
Creating and releasing cached values; this provides an
efficient way to use values for multiple variables and
can be a big performance win.

@item
Manipulating arrays:
@itemize @minus
@item
Retrieving, adding, deleting, and modifying elements
@item
Getting the count of elements in an array
@item
Creating a new array
@item
Clearing an array
@item
Flattening an array for easy C style looping over an array
@end itemize
@end itemize

Some points about using the API:

@itemize @bullet
@item
You must include @code{<sys/types.h>} and @code{<sys/stat.h>} before including
the @file{gawkapi.h} header file. In addition, you must include either
@code{<stddef.h>} or @code{<stdlib.h>} to get the definition of @code{size_t}.
Finally, if you wish to use the boilerplate @code{dl_load_func} macro, you will
need to include @code{<stdio.h>} as well.

@item
Although the API only uses ISO C 90 features, there is an exception; the
``constructor'' functions use the @code{inline} keyword. If your compiler
does not support this keyword, you should either place
@samp{-Dinline=''} on your command line, or use the autotools and include a
@file{config.h} file in your extensions.

@item
All pointers filled in by @command{gawk} are to memory
managed by @command{gawk} and should be treated by the extension as
read-only.  Memory for @emph{all} strings passed into @command{gawk}
from the extension @emph{must} come from @code{malloc()} and is managed
by @command{gawk} from then on.

@item
The API defines several simple structs that map values as seen
from @command{awk}.  A value can be a @code{double}, a string, or an
array (as in multidimensional arrays, or when creating a new array).
Strings maintain both pointer and length since embedded @code{NUL}
characters are allowed.

By intent, strings are maintained using the current multibyte encoding (as
defined by @env{LC_@var{xxx}} environment variables) and not using wide
characters.  This matches how @command{gawk} stores strings internally
and also how characters are likely to be input and output from files.

@item
When retrieving a value (such as a parameter or that of a global variable
or array element), the extension requests a specific type (number, string,
scalars, value cookie, array, or ``undefined'').  When the request is
``undefined,'' the returned value will have the real underlying type.

However, if the request and actual type don't match, the access function
returns ``false'' and fills in the type of the actual value that is there,
so that the extension can, e.g., print an error message
(``scalar passed where array expected'').

@c This is documented in the header file and needs some expanding upon.
@c The table there should be presented here
@end itemize

While you may call the API functions by using the function pointers
directly, the interface is not so pretty. To make extension code look
more like regular code, the @file{gawkapi.h} header file defines a number
of macros which you should use in your code.  This @value{SECTION} presents
the macros as if they were functions.

@node General Data Types
@subsection General Purpose Data Types

@quotation
@i{I have a true love/hate relationship with unions.}@*
Arnold Robbins

@i{That's the thing about unions: the compiler will arrange things so they
can accommodate both love and hate.}@*
Chet Ramey
@end quotation

The extension API defines a number of simple types and structures for general
purpose use. Additional, more specialized, data structures, are introduced
in subsequent @value{SECTION}s, together with the functions that use them.

@table @code
@item typedef void *awk_ext_id_t;
A value of this type is received from @command{gawk} when an extension is loaded.
That value must then be passed back to @command{gawk} as the first parameter of
each API function.

@item #define awk_const @dots{}
This macro expands to @code{const} when compiling an extension,
and to nothing when compiling @command{gawk} itself.  This enables making
certain fields in the API data structures unwritable from extension code,
while allowing @command{gawk} to use them as it needs to.

@item typedef int awk_bool_t;
A simple boolean type. As of this moment, the API does not define special
``true'' and ``false'' values, although perhaps it should.

@item typedef struct @{
@itemx @ @ @ @ char *str;@ @ @ @ @ @ /* data */
@itemx @ @ @ @ size_t len;@ @ @ @ @ /* length thereof, in chars */
@itemx @} awk_string_t;
This represents a mutable string. @command{gawk}
owns the memory pointed to if it supplied
the value. Otherwise, it takes ownership of the memory pointed to.
@strong{Such memory must come from @code{malloc()}!}

As mentioned earlier, strings are maintained using the current
multibyte encoding.

@item typedef enum @{
@itemx @ @ @ @ AWK_UNDEFINED,
@itemx @ @ @ @ AWK_NUMBER,
@itemx @ @ @ @ AWK_STRING,
@itemx @ @ @ @ AWK_ARRAY,
@itemx @ @ @ @ AWK_SCALAR,@ @ @ @ @ @ @ @ @ /* opaque access to a variable */
@itemx @ @ @ @ AWK_VALUE_COOKIE,@ @ @ /* for updating a previously created value */
@itemx @} awk_valtype_t;
This @code{enum} indicates the type of a value.
It is used in the following @code{struct}.

@item typedef struct @{
@itemx @ @ @ @ awk_valtype_t   val_type;
@itemx @ @ @ @ union @{
@itemx @ @ @ @ @ @ @ @ awk_string_t@ @ @ @ @ @ @ s;
@itemx @ @ @ @ @ @ @ @ double@ @ @ @ @ @ @ @ @ @ @ @ @ d;
@itemx @ @ @ @ @ @ @ @ awk_array_t@ @ @ @ @ @ @ @ a;
@itemx @ @ @ @ @ @ @ @ awk_scalar_t@ @ @ @ @ @ @ scl;
@itemx @ @ @ @ @ @ @ @ awk_value_cookie_t vc;
@itemx @ @ @ @ @} u;
@itemx @} awk_value_t;
An ``@command{awk} value.''  
The @code{val_type} member indicates what kind of value the
@code{union} holds, and each member is of the appropriate type.

@item #define str_value@ @ @ @ @ @ u.s
@itemx #define num_value@ @ @ @ @ @ u.d
@itemx #define array_cookie@ @ @ u.a
@itemx #define scalar_cookie@ @ u.scl
@itemx #define value_cookie@ @ @ u.vc
These macros make accessing the fields of the @code{awk_value_t} more
readable.

@item typedef void *awk_scalar_t;
Scalars can be represented as an opaque type. These values are obtained from
@command{gawk} and then passed back into it. This is discussed below.

@item typedef void *awk_value_cookie_t;
A ``value cookie'' is an opaque type representing a cached value.
This is also discussed below.
@end table

Scalar values in @command{awk} are either numbers or strings. The
@code{awk_value_t} struct represents values.  The @code{val_type} member
indicates what is in the @code{union}.

Representing numbers is easy---the API uses a C @code{double}.  Strings
require more work. Since @command{gawk} allows embedded @code{NUL} bytes
in string values, a string must be represented as a pair containing a
data-pointer and length. This is the @code{awk_string_t} type.

Identifiers (i.e., the names of global variables) can be associated
with either scalar values or with arrays.  In addition, @command{gawk}
provides true arrays of arrays, where any given array element can
itself be an array.  Discussion of arrays is delayed until
FIXME: ref.

The various macros listed earlier make it easier to use the elements
of the @code{union} as if they were fields in a @code{struct}; this
is a common coding practice in C.  Such code is easier to write and to
read, however it remains @emph{your} responsibility to make sure that
the @code{val_type} member correctly reflects the type of the value in
the @code{awk_value_t}.

Conceptually, the first three members of the @code{union} (number, string,
and array) are all that is needed for working with @command{awk} values.
However, since the API provides routines for accessing and changing
the value of global scalar variables only by using the variable's name,
there is a performance penalty: @command{gawk} must find the variable
each time it is accessed and changed.  This turns out to be a real issue,
not just a theoretical one.

Thus, if you know that your extension will spend considerable time
reading and/or changing the value of one or more scalar variables, you
can obtain a @dfn{scalar cookie}@footnote{See
@uref{http://catb.org/jargon/html/C/cookie.html, the ``cookie'' entry in the Jargon file} for a
definition of @dfn{cookie}, and @uref{http://catb.org/jargon/html/M/magic-cookie.html,
the ``magic cookie'' entry in the Jargon file} for a nice example. See
also the entry in the FIXME ref to glossary.}
object for that variable, and then use
the cookie for getting the variable's value for changing the variable's
value.
This is the @code{awk_scalar_t} type and @code{scalar_cookie} macro.
Given a scalar cookie, @command{gawk} can directly retrieve or
modify the value, as required, without having to first find it.

The @code{awk_value_cookie_t} type and @code{value_cookie} macro are similar.
If you know that you wish to
use the same numeric or string @emph{value} for one or more variables,
you can create the value once, retaining a @dfn{value cookie} for it,
and then pass in that value cookie whenever you wish to set the value of a
variable.  This saves both storage space within the running @command{gawk}
process as well as the time needed to create the value.

@node Requesting Values
@subsection Requesting Values

All of the functions that return values from @command{gawk}
work in the same way. You pass in an @code{awk_valtype_t} value
to indicate what kind of value you want.  If the actual value
matches what you requested, the function returns true and fills
in the @code{awk_value_t} result.
Otherwise, the function returns false, and the @code{val_type}
member indicates the type of the actual value.  You may then
print an error message, or reissue the request for the actual
value type, as appropriate.  This behavior is summarised in
@ref{table-value-types-returned}.

@ifnotplaintext
@float Table,table-value-types-returned
@caption{Value Types Returned}
@multitable @columnfractions .50 .50
@headitem @tab Type of Actual Value:
@end multitable
@multitable @columnfractions .166 .166 .198 .15 .15 .166
@headitem @tab @tab String @tab Number @tab Array @tab Undefined
@item @tab @b{String} @tab String @tab String @tab false @tab false
@item @tab @b{Number} @tab Number if can be converted, else false @tab Number @tab false @tab false
@item @b{Type} @tab @b{Array} @tab false @tab false @tab Array @tab false
@item @b{Requested:} @tab @b{Scalar} @tab Scalar @tab Scalar @tab false @tab false
@item @tab @b{Undefined} @tab String @tab Number @tab Array @tab Undefined
@item @tab @b{Value Cookie} @tab false @tab false @tab false @tab false
@end multitable
@end float
@end ifnotplaintext
@ifplaintext
@float Table,table-value-types-returned
@caption{Value Types Returned}
@example
                        +-------------------------------------------------+
                        |                Type of Actual Value:            |
                        +------------+------------+-----------+-----------+
                        |   String   |   Number   | Array     | Undefined |
+-----------+-----------+------------+------------+-----------+-----------+
|           | String    |   String   |   String   | false     | false     |
|           |-----------+------------+------------+-----------+-----------+
|           | Number    | Number if  |   Number   | false     | false     |
|           |           | can be     |            |           |           |
|           |           | converted, |            |           |           |
|           |           | else false |            |           |           |
|           |-----------+------------+------------+-----------+-----------+
|   Type    | Array     |   false    |   false    | Array     | false     |
| Requested |-----------+------------+------------+-----------+-----------+
|           | Scalar    |   Scalar   |   Scalar   | false     | false     |
|           |-----------+------------+------------+-----------+-----------+
|           | Undefined |  String    |   Number   | Array     | Undefined |
|           |-----------+------------+------------+-----------+-----------+
|           | Value     |   false    |   false    | false     | false     |
|           | Cookie    |            |            |           |           |
+-----------+-----------+------------+------------+-----------+-----------+
@end example
@end float
@end ifplaintext

@node Constructor Functions
@subsection Constructor Functions and Convenience Macros

The API provides a number of @dfn{constructor} functions for creating
string and numeric values, as well as a number of convenience macros.
This @value{SUBSECTION} presents them all as function prototypes, in
the way that extension code would use them.

@table @code
@item static inline awk_value_t *
@itemx make_const_string(const char *string, size_t length, awk_value_t *result)
This function creates a string value in the @code{awk_value_t} variable
pointed to by @code{result}. It expects @code{string} to be a C string constant
(or other string data), and automatically creates a @emph{copy} of the data
for storage in @code{result}.

@item static inline awk_value_t *
@itemx make_malloced_string(const char *string, size_t length, awk_value_t *result)
This function creates a string value in the @code{awk_value_t} variable
pointed to by @code{result}. It expects @code{string} to be a @samp{char *}
value pointing to data previously obtained from @code{malloc()}. The idea here
is that the data will be passed directly to @command{gawk}, which will assume
responsibility for it.

@item static inline awk_value_t *
@itemx make_null_string(awk_value_t *result)
This specialized function creates a null string (the ``undefined'' value)
in the @code{awk_value_t} variable pointed to by @code{result}.

@item static inline awk_value_t *
@itemx make_number(double num, awk_value_t *result)
This function simply creates a numeric value in the @code{awk_value_t} variable
pointed to by @code{result}.
@end table

Two convenience macros may be used for allocating storage from @code{malloc()}
and @code{realloc()}. If the allocation fails, they cause @command{gawk} to
exit with a fatal error message.  They should be used as if they were
procedure calls that do not return a value.

@table @code
@item emalloc(pointer, type, size, message)
The arguments to this macro are as follows:
@c nested table
@table @code
@item pointer
The pointer variable to point at the allocated storage.

@item type
The type of the pointer variable, used to create a cast for the call to @code{malloc()}.

@item size
The total number of bytes to be allocated.

@item message
A message to be prefixed to the fatal error message. Typically this is the name
of the function using the macro.
@end table

@noindent
For example, you might allocate a string value like so:

@example
awk_value_t result;
char *message;
const char greet[] = "Don't Panic!";

emalloc(message, char *, sizeof(greet), "myfunc");
strcpy(message, greet);
make_malloced_string(message, strlen(message), & result);
@end example

@item erealloc(pointer, type, size, message)
The arguments are the same as for the @code{emalloc()} macro.
@end table

@node Registration Functions
@subsection Registration Functions

This @value{SECTION} describes the API functions which let you
register parts of your extension with @command{gawk}.

@menu
@end menu

@node Extension Functions
@subsubsection Registering An Extension Function

Extension functions are described by the following record:

@example
typedef struct @{
@ @ @ @ const char *name;
@ @ @ @ awk_value_t *(*function)(int num_actual_args, awk_value_t *result);
@ @ @ @ size_t num_expected_args;
@} awk_ext_func_t;
@end example

The fields are:

@table @code
@item const char *name;
The name of the new function.
@command{awk} level code will call the function by this name.

@item awk_value_t *(*function)(int num_actual_args, awk_value_t *result);
This is a pointer to the C function that provides the desired
functionality.
The function must fill in the result with either a number
or a string. @command{awk takes ownership of any string memory}.
As mentioned earlier, string memory @strong{must} come from @code{malloc()}.

The function must return the value of @code{result}.
This is for the convenience of the calling code inside @command{gawk}.

@item size_t num_expected_args;
This is the number of arguments the function expects to receive.
Each extension function may decide what to do if the number of
arguments isn't what it expected.  Following @command{awk} functions, it
is likely OK to ignore extra arguments.
@end table

Once you have a record representing your extension function, you register
it with @command{gawk} using this API function:

@table @code
@item awk_bool_t add_ext_func(const char *namespace, const awk_ext_func_t *func);
This function returns true upon success, false otherwise.
The @code{namespace} parameter is currently not used; you should pass in an
empty string (@code{""}).  The @code{func} pointer is the address of a
@code{struct} describing your function, as just described.
@end table

@node Input Parsers
@subsubsection Customized Input Parsers

By default, @command{gawk} reads text files as its input. It uses the value
of @code{RS} to find the end of the record, and then uses @code{FS}
(or @code{FIELDWIDTHS}) to split it into fields.  Additionally, it sets
the value of @code{RT}.  (FIXME: pxrefs as needed.)

If you want, you can provide your own, custom, input parser.  An input
parser's job is to return a record to the @command{gawk} record processing
code, along with indicators for the value and length of the data to be
used for @code{RT}, if any.

To provide an input parser, you must first provide two functions
(where @var{XXX} is a prefix name for your extension):

@table @code
@item awk_bool_t @var{XXX}_can_take_file(const awk_input_buf_t *iobuf)
This function examines the information available in @code{iobuf}
(which we discuss shortly).  Based on the information there, it
decides if the input parser should be used for this file.
If so, it should return true (non-zero). Otherwise, it should
return false (zero).

@item awk_bool_t @var{XXX}_take_control_of(awk_input_buf_t *iobuf)
When @command{gawk} decides to hand control of the file over to the
input parser, it calls this function.  This function in turn must fill
in certain fields in the @code{awk_input_buf_t} structure, and ensure
that certain conditions are true.  It should then return true. If an
error of some kind occurs, it should not fill in any fields, and should
return false; then @command{gawk} will not use the input parser.
The details are presented shortly.
@end table

Your extension should package these functions inside an
@code{awk_input_parser_t}, which looks like this:

@example
typedef struct input_parser @{
    const char *name;   /* name of parser */
    awk_bool_t (*can_take_file)(const awk_input_buf_t *iobuf);
    awk_bool_t (*take_control_of)(awk_input_buf_t *iobuf);
    awk_const struct input_parser *awk_const next;    /* for use by gawk */
@} awk_input_parser_t;
@end example

The steps are as follows:

@enumerate
@item
Create a @code{static awk_input_parser_t} variable and initialize it
appropriately.

@item
When your extension is loaded, register your input parser with
@command{gawk} using the @code{register_input_parser()} API function
(described below).
@end enumerate

An @code{awk_input_buf_t} looks like this:

@example
typedef struct awk_input @{
    const char *name;       /* filename */
    int fd;                 /* file descriptor */
#define INVALID_HANDLE (-1)
    void *opaque;           /* private data for input parsers */
    int (*get_record)(char **out, struct awk_input *, int *errcode,
            char **rt_start, size_t *rt_len);
    void (*close_func)(struct awk_input *);
    struct stat sbuf;       /* stat buf */
@} awk_input_buf_t;
@end example

The fields can be divided into two categories: those for use (initially,
at least) by @code{@var{XXX}_can_take_file()}, and those for use by
@code{@var{XXX}_take_control_of()}.  The first group of fields and their uses
are as follows:

@table @code
@item const char *name;
The name of the file.

@item int fd;
A file descriptor for the file.  If @command{gawk} was able to
open the file, then it will @emph{not} be equal to 
@code{INVALID_HANDLE}. Otherwise, it will.

@item struct stat sbuf;
If file descriptor is valid, then @command{gawk} will have filled
in this structure with a call to the @code{fstat()} system call.
@end table

The @code{@var{XXX}_can_take_file()} function should examine these
fields and decide if the input parser should be used for the file.
The decision can be made based upon @command{gawk} state (the value
of a variable defined previously by the extension and set by
@command{awk} code), the name of the
file, whether or not the file descriptor is valid, the information
in the @code{struct stat}, or any combination of the above.

Once @code{@var{XXX}_can_take_file()} has returned true, and
@command{gawk} has decided to use your input parser, it will call
@code{@var{XXX}_take_control_of()}.  That function then fills in at
least the @code{get_record} field of the @code{awk_input_buf_t}.  It must
also ensure that @code{fd} is not set to @code{INVALID_HANDLE}.  All of
the fields that may be filled by @code{@var{XXX}_take_control_of()}
are as follows:

@table @code
@item void *opaque;
This is used to hold any state information needed by the input parser
for this file.  It is ``opaque'' to @command{gawk}.  The input parser
is not required to use this pointer.

@item int (*get_record)(char **out, struct awk_input *, int *errcode,
@itemx                   char **rt_start, size_t *rt_len);
This is a function pointer that should be set to point to the
function that creates the input records.
Said function is the core of the input parser.  Its behavior is
described below.

@item void (*close_func)(struct awk_input *);
This is a function pointer that should be set to point to the
function that does the ``tear down.'' It should release any resources
allocated by @code{@var{XXX}_take_control_of()}.  It may also close
the file. If it does so, it shold set the @code{fd} field to
@code{INVALID_HANDLE}.

Having a ``tear down'' function is optional. If your input parser does
not need it, do not set this field.  In that case, @command{gawk}
will close the regular @code{close()} system call on the
file descriptor, so it should be valid.
@end table

The @code{@var{XXX}_get_record()} function does the work of creating
input records.  The parameters are as follows:

@table @code
@item char **out
This is a pointer to a @code{char *} variable which is set to point
to the record.  @command{gawk} will make its own copy of the data, so
the extension must manage this storage.

@item struct awk_input *iobuf
This is the @code{awk_input_buf_t} for the file.  The fields should be
used for reading data (@code{fd}) and for managing private state
(@code{opaque}), if any.

@item int *errcode
If an error occurs, @code{*errcode} should be set to an appropriate
code from @code{<errno.h>}.

@item char **rt_start
@itemx size_t *rt_len
If the concept of a ``record terminator'' makes sense, then
@code{*rt_start} should be set to point to the data to be used for
@code{RT}, and @code{*rt_len} should be set to the length of the
data. Otherwise, @code{*rt_len} should be set to zero.
@code{gawk} makes its own copy of this data, so the
extension must manage the storage.
@end table

The return value is the length of the buffer pointed to by
@code{*out}, or @code{EOF} if end-of-file was reached or an
error occurred.

It is guaranteed that @code{errcode} is a valid pointer, so there is no
need to test for a @code{NULL} value.  @command{gawk} sets @code{*errcode}
to zero, so there is no need to set it unless an error occurs.

If an error does occur, the function should return @code{EOF} and set
@code{*errcode} to a non-zero value.  In that case, if @code{*errcode}
does not equal @minus{}1, @command{gawk|} will automatically update
the @code{ERRNO} variable based on the value of @code{*errcode} (e.g.,
setting @samp{*errcode = errno} should do the right thing).

@command{gawk} ships with a sample extension (@pxref{Extension Sample
Readdir}) that reads directories, returning records for each entry in
the directory.  You may wish to use that code as a guide for writing
your own input parser.

When writing an input parser, you should think about (and document)
how it is expected to interact with @command{awk} code.  You may want
it to always be called, and take effect as appropriate (as the
@code{readdir} extension does).  Or you may want it to take effect
based upon the value of an @code{awk} variable, as the XML extension
from the @code{gawkextlib} project does (@pxref{gawkextlib}).
In the latter case, code in a @code{BEGINFILE} section (FIXME: pxref)
can look at @code{FILENAME} and @code{ERRNO} to decide whether or
not to activate an input parser.

You register your input parser with the following function:

@table @code
@item void register_input_parser(awk_input_parser_t *input_parser);
Register the input parser pointed to by @code{input_parser} with
@command{gawk}.
@end table

@node Output Wrappers
@subsubsection Customized Output Wrappers

An @dfn{output wrapper} is the mirror image of an input parser.
It allows an extension to take over the output to a file (opened
with the @samp{>} or @samp{>>} operators, FIXME pxref).

The output wrapper is very similar to the input parser structure:

@example
typedef struct output_wrapper @{
    const char *name;   /* name of the wrapper */
    awk_bool_t (*can_take_file)(const awk_output_buf_t *outbuf);
    awk_bool_t (*take_control_of)(awk_output_buf_t *outbuf);
    awk_const struct output_wrapper *awk_const next;  /* for use by gawk */
@} awk_output_wrapper_t;
@end example

The members are as follows:

@table @code
@item const char *name;
This is the name of the output wrapper.

@item awk_bool_t (*can_take_file)(const awk_output_buf_t *outbuf);
This points to a function that examines the information in
the @code{awk_output_buf_t} structure pointed to by @code{outbuf}.
It should return true if the output wrapper wants to take over the
file, and false otherwise.  It should not change any state (variable
values, etc.) within @command{gawk}.

@item awk_bool_t (*take_control_of)(awk_output_buf_t *outbuf);
The function pointed to by this field is called when @command{gawk}
decides to let the output wrapper take control of the file. It should
fill in appropriate members of the @code{awk_output_buf_t} structure,
as described below, and return true if successful, false otherwise.

@item awk_const struct output_wrapper *awk_const next;
This is for use by @command{gawk}.
@end table

The @code{awk_output_buf_t} structure looks like this:

@example
typedef struct @{
    const char *name;   /* name of output file */
    const char *mode;   /* mode argument to fopen */
    FILE *fp;           /* stdio file pointer */
    awk_bool_t redirected;  /* true if a wrapper is active */
    void *opaque;       /* for use by output wrapper */
    size_t (*gawk_fwrite)(const void *buf, size_t size, size_t count,
                FILE *fp, void *opaque);
    int (*gawk_fflush)(FILE *fp, void *opaque);
    int (*gawk_ferror)(FILE *fp, void *opaque);
    int (*gawk_fclose)(FILE *fp, void *opaque);
@} awk_output_buf_t;
@end example

Here too, your extension will define @code{@var{XXX}_can_take_file()}
and @code{@var{XXX}_take_control_of()} functions that examine and update
data members in the @code{awk_output_buf_t}.
The data members are as follows:

@table @code
@item const char *name;
The name of the output file.

@item const char *mode;
The mode string (as would be used in the second argument to @code{fopen()}
with which the file was opened.

@item FILE *fp;
The @code{FILE} pointer from @code{<stdio.h>}. @command{gawk} opens the file
before attempting to find an output wrapper.

@item awk_bool_t redirected;
The field should be set to true in the @code{@var{XXX}_take_control_of()} function.

@item void *opaque;
This pointer is opaque to @command{gawk}. The extension should use it to store
a pointer to any private data associated with the file.

@item size_t (*gawk_fwrite)(const void *buf, size_t size, size_t count,
@itemx @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ FILE *fp, void *opaque);
@itemx int (*gawk_fflush)(FILE *fp, void *opaque);
@itemx int (*gawk_ferror)(FILE *fp, void *opaque);
@itemx int (*gawk_fclose)(FILE *fp, void *opaque);
These pointers should be set to point to functions that perform
the equivalent function as the @code{<stdio.h>} functions do, if appropriate.
@command{gawk} uses these function pointers for all output.
@command{gawk} initializes the pointers to point to internal, ``pass through''
functions that just call the regular @code{<stdio.h>} functions, so an
extension only needs to redefine those functions that are appropriate for
what it does.
@end table

The @code{@var{XXX}_can_take_file()} function should make a decision based
upon the @code{name} and @code{mode} fields, and any additional state
(such as @command{awk} variable values) that is appropriate.

When @command{gawk} calls @code{@var{XXX}_take_control_of()}, it should fill
in the other fields, as appropriate, except for @code{fp}, which it should just
use normally.

You register your output wrapper with the following function:

@table @code
@item void register_output_wrapper(awk_output_wrapper_t *output_wrapper);
Register the output wrapper pointed to by @code{output_wrapper} with
@command{gawk}.
@end table

@node Two-way processors
@subsubsection Customized Two-way Processors

A @dfn{two-way processor} combines an input parser and an output wrapper for
two-way I/O with the @samp{|&} operator (FIXME: pxref).  It makes identical
use of the @code{awk_input_parser_t} and @code{awk_output_buf_t} structures,
as described earlier.

A two-way processor is represented by the following structure:

@example
typedef struct two_way_processor @{
    const char *name;   /* name of the two-way processor */
    awk_bool_t (*can_take_two_way)(const char *name);
    awk_bool_t (*take_control_of)(const char *name, awk_input_buf_t *inbuf, awk_output_buf_t *outbuf);
    awk_const struct two_way_processor *awk_const next;  /* for use by gawk */
@} awk_two_way_processor_t;
@end example

The fields are as follows:

@table @code
@item const char *name;
The name of the two-way processor.

@item awk_bool_t (*can_take_two_way)(const char *name);
This function returns true if it wants to take over the two-way I/O for this filename.

@item awk_bool_t (*take_control_of)(const char *name, awk_input_buf_t *inbuf, awk_output_buf_t *outbuf);
This function should fill in the @code{awk_input_buf_t} and
@code{awk_outut_buf_t} structures pointed to by @code{inbuf} and
@code{outbuf}, respectively.  These structures were described earlier.

@item awk_const struct two_way_processor *awk_const next;
This is for use by @command{gawk}.
@end table

As with the input parser and output processor, you provide
``yes I can take this'' and ``take over for this'' functions,
@code{@var{XXX}_can_take_two_way()} and @code{@var{XXX}_take_control_of()}.

You register your two-way processor with the following function:

@table @code
@item void register_two_way_processor(awk_two_way_processor_t *two_way_processor);
Register the two-way processor pointed to by @code{two_way_processor} with
@command{gawk}.
@end table

@node Exit Callback Functions
@subsubsection Registering An Exit Callback Function

An @dfn{exit callback} function is a function that
@command{gawk} calls before it exits.
Such functions are useful if you have general ``clean up'' tasks
that should be performed in your extension (such as closing data
base connections or other resource deallocations).
You can register such
a function with @command{gawk} using the following function.

@table @code
@item void awk_atexit(void (*funcp)(void *data, int exit_status),
@itemx @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ void *arg0);
The parameters are:
@c nested table
@table @code
@item funcp
Points to the function to be called before @command{gawk} exits. The @code{data}
parameter will be the original value of @code{arg0}.
The @code{exit_status} parameter is
the exit status value that @command{gawk} will pass to the @code{exit()} system call.

@item arg0
A pointer to private data which @command{gawk} saves in order to pass to
the function pointed to by @code{funcp}.
@end table
@end table

Exit callback functions are called in Last-In-First-Out (LIFO) order---that is, in
the reverse order in which they are registered with @command{gawk}.

@node Extension Version String
@subsubsection Registering An Extension Version String

You can register a version string which indicates the name and
version of your extension, with @command{gawk}, as follows:

@table @code
@item void register_ext_version(const char *version);
Register the string pointed to by @code{version} with @command{gawk}.
@end table

@command{gawk} prints all registered extension version strings when it
is invoked with the @option{--version} option.

@node Printing Messages
@subsection Printing Messages

You can print different kinds of warning messages from your
extension, as described below.  Note that for these functions,
you must pass in the extension id received from @command{gawk}
when the extension was loaded.@footnote{Because the API uses only ISO C 90
features, it cannot make use of the ISO C 99 variadic macro feature to hide
that parameter. More's the pity.}

@table @code
@item void fatal(awk_ext_id_t id, const char *format, ...);
Print a message and then cause @command{gawk} to exit immediately.

@item void warning(awk_ext_id_t id, const char *format, ...);
Print a warning message.

@item void lintwarn(awk_ext_id_t id, const char *format, ...);
Print a ``lint warning.''  Normally this is the same as printing a
warning message, but if @command{gawk} was invoked with @samp{--lint=fatal},
then they become fatal error messages.
@end table

All of these functions are otherwise like the C @code{printf()}
family of functions, where the @code{format} parameter is a string
with literal characters and formatting codes intermixed.

@node Updating @code{ERRNO}
@subsection Updating @code{ERRNO}

The following functions allow you to update the @code{ERRNO}
variable.

@table @code
@item void update_ERRNO_int(int errno_val);
Set @code{ERRNO} to the string equivalent of the error code
in @code{errno_val}. The value should be one of the defined
error codes in @code{<errno.h>}, and @command{gawk} will turn it
into a (possibly translated) string using the C @code{strerror()} function.

@item void update_ERRNO_string(const char *string);
Set @code{ERRNO} directly to the string value of @code{ERRNO}.
@command{gawk} will make a copy of the value of @code{string}.

@item void unset_ERRNO();
Unset @code{ERRNO}.
@end table

@node Accessing Parameters
@subsection Accessing and Updating Parameters

@node Symbol Table Access
@subsection Symbol Table Access

@c @menu
@c @end menu
@c subsubsection - regular routines
@c subsubsection - cookie routines
@c subsubsection - value routines

@node Array Manipulation
@subsection Array Manipulation

@c @item
typedef void *awk_array_t;
Arrays are represented as an opaque type. These values are obtained from
@command{gawk} and then passed back into it.

In order to make working with arrays manageable,
the @code{awk_array_t} type represents an array to @command{gawk}.

If you request the value of an array variable, you get back an
@code{awk_array_t} value. This value is opaque@footnote{It is also
a ``cookie,'' but the gawk developers did not wish to overuse this
term.} to the extension; it uniquely identifies the array but can
only be used by passing it into API functions or receiving it from API
functions. This is very similar to way @samp{FILE *} values are used
with the @code{<stdio.h>} library routines. FIXME: XREF, for how to use
the value.

 * 2. Due to gawk internals, after using sym_update() to install an array
 * into gawk, you have to retrieve the array cookie from the value
 * passed in to sym_update().  Like so:
 *
 *	new_array = create_array();
 *	val.val_type = AWK_ARRAY;
 *	val.array_cookie = new_array;
 *	sym_update("array", & val);	// install array in the symbol table
 *
 *	new_array = val.array_cookie;	// MUST DO THIS
 *
 *	// fill in new array with lots of subscripts and values
 *
 * Similarly, if installing a new array as a subarray of an existing
 * array, you must add the new array to its parent before adding any
 * elements to it.
 *
 * You must also retrieve the value of the array_cookie after the call
 * to set_element().
 *
 * Thus, the correct way to build an array is to work "top down".
 * Create the array, and immediately install it in gawk's symbol table
 * using sym_update(), or install it as an element in a previously
 * existing array using set_element().
 *
 * Thus the new array must ultimately be rooted in a global symbol. This is
 * necessary before installing any subarrays in it, due to gawk's
 * internal implementation.  Strictly speaking, this is required only
 * for arrays that will have subarrays as elements; however it is
 * a good idea to always do this.  This restriction may be relaxed
 * in a subsequent revision of the API.


@node Extension API Variables
@subsection Variables

The API provides two sets of variables.  The first provides information
about the version of the API (both with which the extension was compiled,
and with which @command{gawk} was compiled).  The second provides
information about how @command{gawk} was invoked.

@menu
@end menu

@node Extension Versioning
@subsubsection API Version Constants and Variables

The API provides both a ``major'' and a ``minor'' version number.
The API versions are available at compile time as constants:

@table @code
@item GAWK_API_MAJOR_VERSION
The major version of the API.

@item GAWK_API_MINOR_VERSION
The minor version of the API.
@end table

The minor version increases when new functions are added to the API. Such
new functions are always added to the end of the API @code{struct}.

The major version increases (and the minor version is reset to zero) if any
of the data types change size or member order, or if any of the existing
functions change signature.

It could happen that an extension may be compiled against one version
of the API but loaded by a version of @command{gawk} using a different
version. For this reason, the major and minor API versions of the
running @command{gawk} are included in the API @code{struct} as read-only
constant integers:

@table @code
@item api->major_version
The major version of the running @command{gawk}.

@item api->minor_version
The minor version of the running @command{gawk}.
@end table

It is up to the extension to decide if there are API incompatibilities.
Typically a check like this is enough:

@example
if (api->major_version != GAWK_API_MAJOR_VERSION
    || api->minor_version < GAWK_API_MINOR_VERSION) @{
        fprintf(stderr, "foo_extension: version mismatch with gawk!\n");
        fprintf(stderr, "\tmy version (%d, %d), gawk version (%d, %d)\n",
                GAWK_API_MAJOR_VERSION, GAWK_API_MINOR_VERSION,
                api->major_version, api->minor_version);
        exit(1);
@}
@end example

Such code is included in the boilerplate @code{dl_load_func} macro
provided in @file{gawkapi.h} (discussed later, in PXREF).


@node Extension API Informational Variables
@subsubsection Informational Variables

The API provides access to several variables that describe
whether the corresponding command-line options were enabled when
@command{gawk} was invoked.  The variables are:

@table @code
@item do_lint
This variable will be true if the @option{--lint} option was passed
(FIXME: pxref).

@item do_traditional
This variable will be true if the @option{--traditional} option was passed.

@item do_profile
This variable will be true if the @option{--profile} option was passed.

@item do_sandbox
This variable will be true if the @option{--sandbox} option was passed.

@item do_debug
This variable will be true if the @option{--debug} option was passed.

@item do_mpfr
This variable will be true if the @option{--bignum} option was passed.
@end table

The value of @code{do_lint} can change if @command{awk} code
modifies the @code{LINT} built-in variable (FIXME: pxref).
The others should not change during execution.

@node Extension API Boilerplate
@subsection Boilerplate Code

@node Extension Example
@section Example: Some File Functions

@c It's enough to show chdir and stat, no need for fts

@node Extension Samples
@section Sample Extensions

@menu
@end menu

@node Extension Sample File Functions
@subsection File Related Functions

@c can pull doc from man pages in extension directory

@node Extension Sample Fnmatch
@subsection Interface To @code{fnmatch()}

@node Extension Sample Fork
@subsection Interface to @code{fork()}, @code{wait()} and @code{waitpid()}

@node Extension Sample Ord
@subsection Character and Numeric values: @code{ord()} and @code{chr()}

@node Extension Sample Readdir
@subsection Reading Directories

@node Extension Sample Revout
@subsection Reversing Output

@node Extension Sample Rev2way
@subsection Two-Way I/O Example

@node Extension Sample Read write array
@subsection Dumping and Restoring An Array

@node Extension Sample Readfile
@subsection Reading An Entire File

@node Extension Sample API Tests
@subsection API Tests

@node Extension Sample Time
@subsection Time Functions

@cindex time
@cindex sleep

These functions  can be used by either invoking @command{gawk}
with a command-line argument of @option{-l time} or by
inserting @code{@@load "time"} in your script.

@table @code

@cindex @code{gettimeofday} time extension function
@item gettimeofday()
This function returns the time that has elapsed since 1970-01-01 UTC
as a floating point value.  It should have sub-second precision, but
the actual precision will vary based on the platform.  If the time
is unavailable on this platform, it returns @minus{}1 and sets @code{ERRNO}.
If the standard C @code{gettimeofday()} system call is available on this platform,
then it simply returns the value.  Otherwise, if on Windows,
it tries to use @code{GetSystemTimeAsFileTime()}.

@cindex @code{sleep} time extension function
@item sleep(@var{seconds})
This function attempts to sleep for @var{seconds} seconds. 
Note that @var{seconds} may be a floating-point (non-integral) value.
If @var{seconds} is negative, or the attempt to sleep fails,
then it returns @minus{}1 and sets @code{ERRNO}.
Otherwise, the function should return 0 after sleeping
for the indicated amount of time.  Implementation details: depending
on platform availability, it tries to use @code{nanosleep()} or @code{select()}
to implement the delay.

@end table

@node gawkextlib
@section The @code{gawkextlib} Project

The @uref{http://sourceforge.net/projects/gawkextlib/, @code{gawkextlib}}
project provides a number of @command{gawk} extensions, including one for
processing XML files.  This is the evolution of the original @command{xgawk}
(XML @command{gawk}) project.

As of this writing, there are four extensions:

@itemize @bullet
@item
XML parser extension, using the @uref{http://expat.sourceforge.net, Expat}
XML parsing library

@item
Postgres SQL extension

@item
GD graphics library extension

@item
MPFR library extension.
This provides access to a number of MPFR functions which @command{gawk}'s
native MPFR support does not.
@end itemize

The @code{time} extension described earlier
(@pxref{Extension Sample Time})
was originally from this project but has been moved in to the
main @command{gawk} distribution.

You can check out the code for the @code{gawkextlib} project
using the @uref{http://git-scm.com, GIT} distributed source
code control system.  The command is as follows:

@example
git clone git://git.code.sf.net/p/gawkextlib/code gawkextlib-code
@end example

You will need to have the @uref{http://expat.sourceforge.net, Expat}
XML parser library installed in order to build and use the XML extension.

In addition, you should have the GNU Autotools installed (Autoconf,
Automake, Libtool and Gettext). FIXME: Need URLs.

The simple recipe for building and testing @code{gawkextlib} is as follows.
First, build and install @command{gawk}:

@example
cd .../path/to/gawk/code
./configure --prefix=/tmp/newgawk     @i{Install in /tmp/newgawk for now}
make && make check                    @i{Build and check that all is OK}
make install                          @i{Install gawk}
@end example

Next, build @code{gawkextlib} and test it:

@example
cd .../path/to/gawkextlib-code
./update-autotools                    @i{Generate configure, etc. May have to run twice}
./configure --with-gawk=/tmp/newgawk  @i{Configure, point at ``installed'' gawk}
make && make check                    @i{Build and check that all is OK}
@end example

@bye

From: Doug McIlroy <doug@cs.dartmouth.edu>
Date: Sat, 13 Oct 2012 19:55:25 -0400
To: arnold@skeeve.com
Subject: Re: origin of the term "cookie"?

I believe the term "cookie", for a more or less inscrutable
saying or crumb of information, was injected into Unix
jargon by Bob Morris, who used the word quite frequently.
It had no fixed meaning as it now does in browsers.

The word had been around long before it was recognized in
the 8th edition glossary (earlier editions had no glossary):

cookie   a peculiar goodie, token, saying or remembrance
returned by or presented to a program. [I would say that
"returned by" would better read "produced by", and assume
responsibility for the inexactitude.]

Doug McIlroy