path: root/contrib/libxo/doc/libxo.txt

#
# Copyright (c) 2014, Juniper Networks, Inc.
# All rights reserved.
# This SOFTWARE is licensed under the LICENSE provided in the
# ../Copyright file. By downloading, installing, copying, or 
# using the SOFTWARE, you agree to be bound by the terms of that
# LICENSE.
# Phil Shafer, July 2014
#

* Overview

libxo - A Library for Generating Text, XML, JSON, and HTML Output

You want to prepare for the future, but you need to live in the
present.  You'd love a flying car, but need to get to work today.  You
want to support features like XML, JSON, and HTML rendering to allow
integration with NETCONF, REST, and web browsers, but you need to make
text output for command line users.  And you don't want multiple code
paths that can't help but get out of sync.  None of this "if (xml)
{... } else {...}"  logic.  And ifdefs are right out.  But you'd
really, really like all the fancy features that modern encoding
formats can provide.  libxo can help.

The libxo library allows an application to generate text, XML, JSON,
and HTML output using a common set of function calls.  The application
decides at run time which output style should be produced.  The
application calls a function "xo_emit" to product output that is
described in a format string.  A "field descriptor" tells libxo what
the field is and what it means.  Each field descriptor is placed in
braces with a printf-like format string (^format-strings^):

    xo_emit(" {:lines/%7ju} {:words/%7ju} "
            "{:characters/%7ju} {d:filename/%s}\n",
            linect, wordct, charct, file);

Each field can have a role, with the 'value' role being the default,
and the role tells libxo how and when to render that field.  Output
can then be generated in various style, using the "--libxo" option:

    % wc /etc/motd
          25     165    1140 /etc/motd
    % wc --libxo xml,pretty,warn /etc/motd
    <wc>
      <file>
        <lines>25</lines>
        <words>165</words>
        <characters>1140</characters>
        <filename>/etc/motd</filename>
      </file>
    </wc>
    % wc --libxo json,pretty,warn /etc/motd
    {
      "wc": {
        "file": [
          {
            "lines": 25,
            "words": 165,
            "characters": 1140,
            "filename": "/etc/motd"
          }
        ]
      }
    }
    % wc --libxo html,pretty,warn /etc/motd
    <div class="line">
      <div class="text"> </div>
      <div class="data" data-tag="lines">     25</div>
      <div class="text"> </div>
      <div class="data" data-tag="words">    165</div>
      <div class="text"> </div>
      <div class="data" data-tag="characters">   1140</div>
      <div class="text"> </div>
      <div class="data" data-tag="filename">/etc/motd</div>
    </div>

** Getting libxo

libxo lives on github as:

  https://github.com/Juniper/libxo

The latest release of libxo is available at:

  https://github.com/Juniper/libxo/releases

We are following the branching scheme from
^http://nvie.com/posts/a-successful-git-branching-model/^ which means
we will do development under the "develop" branch, and release from
the "master" branch.  To clone a developer tree, run the following
command:

  git clone https://github.com/Juniper/libxo.git -b develop

We're using semantic release numbering, as defined in
^http://semver.org/spec/v2.0.0.html^.

libxo is open source, distributed under the BSD license.  It shipped
as part of the FreeBSD operating system starting with release 11.0.

Issues, problems, and bugs should be directly to the issues page on
our github site.

*** Downloading libxo Source Code

You can retrieve the source for libxo in two ways:

A) Use a "distfile" for a specific release.  We use
github to maintain our releases.  Visit
github release page (^https://github.com/Juniper/libxo/releases^)
to see the list of releases.  To download the latest, look for the
release with the green "Latest release" button and the green
"libxo-RELEASE.tar.gz" button under that section.

After downloading that release's distfile, untar it as follows:

    tar -zxf libxo-RELEASE.tar.gz
    cd libxo-RELEASE

[Note: for Solaris users, your "tar" command lacks the "-z" flag,
so you'll need to substitute "gzip -dc "file" | tar xf -" instead of
"tar -zxf "file"".]

B) Use the current build from github.  This gives you the most recent
source code, which might be less stable than a specific release.  To
build libxo from the git repo:

    git clone https://github.com/Juniper/libxo.git
    cd libxo

_BE AWARE_: The github repository does _not_ contain the files
generated by "autoreconf", with the notable exception of the "m4"
directory.  Since these files (depcomp, configure, missing,
install-sh, etc) are generated files, we keep them out of the source
code repository.

This means that if you download the a release distfile, these files
will be ready and you'll just need to run "configure", but if you
download the source code from svn, then you'll need to run
"autoreconf" by hand.  This step is done for you by the "setup.sh"
script, described in the next section.

*** Building libxo

To build libxo, you'll need to set up the build, run the "configure"
script, run the "make" command, and run the regression tests.

The following is a summary of the commands needed.  These commands are
explained in detail in the rest of this section.

    sh bin/setup.sh
    cd build
    ../configure
    make
    make test
    sudo make install

The following sections will walk thru each of these steps with
additional details and options, but the above directions should be all
that's needed.

**** Setting up the build

[If you downloaded a distfile, you can skip this step.]

Run the "setup.sh" script to set up the build.  This script runs the
"autoreconf" command to generate the "configure" script and other
generated files.

    sh bin/setup.sh

Note: We're are currently using autoreconf version 2.69.

**** Running the "configure" Script

Configure (and autoconf in general) provides a means of building
software in diverse environments.  Our configure script supports
a set of options that can be used to adjust to your operating
environment. Use "configure --help" to view these options.

We use the "build" directory to keep object files and generated files
away from the source tree.

To run the configure script, change into the "build" directory, and
run the "configure" script.  Add any required options to the
"../configure" command line.

    cd build
    ../configure

Expect to see the "configure" script generate the following error:

    /usr/bin/rm: cannot remove `libtoolT': No such file or directory

This error is harmless and can be safely ignored.

By default, libxo installs architecture-independent files, including
extension library files, in the /usr/local directories. To specify an
installation prefix other than /usr/local for all installation files,
include the --prefix=prefix option and specify an alternate
location. To install just the extension library files in a different,
user-defined location, include the --with-extensions-dir=dir option
and specify the location where the extension libraries will live.

    cd build
    ../configure [OPTION]... [VAR=VALUE]...

**** Running the "make" command

Once the "configure" script is run, build the images using the "make"
command:

    make

**** Running the Regression Tests

libxo includes a set of regression tests that can be run to ensure
the software is working properly.  These test are optional, but will
help determine if there are any issues running libxo on your
machine.  To run the regression tests:

    make test

**** Installing libxo

Once the software is built, you'll need to install libxo using the
"make install" command.  If you are the root user, or the owner of the
installation directory, simply issue the command:

    make install

If you are not the "root" user and are using the "sudo" package, use:

    sudo make install

Verify the installation by viewing the output of "xo --version":

    % xo --version
    libxo version 0.3.5-git-develop
    xo version 0.3.5-git-develop

* Formatting with libxo

Most unix commands emit text output aimed at humans.  It is designed
to be parsed and understood by a user.  Humans are gifted at
extracting details and pattern matching in such output.  Often
programmers need to extract information from this human-oriented
output.  Programmers use tools like grep, awk, and regular expressions
to ferret out the pieces of information they need.  Such solutions are
fragile and require maintenance when output contents change or evolve,
along with testing and validation.

Modern tool developers favor encoding schemes like XML and JSON,
which allow trivial parsing and extraction of data.  Such formats are
simple, well understood, hierarchical, easily parsed, and often
integrate easier with common tools and environments.  Changes to
content can be done in ways that do not break existing users of the
data, which can reduce maintenance costs and increase feature velocity.

In addition, modern reality means that more output ends up in web
browsers than in terminals, making HTML output valuable.

libxo allows a single set of function calls in source code to generate
traditional text output, as well as XML and JSON formatted data.  HTML
can also be generated; "<div>" elements surround the traditional text
output, with attributes that detail how to render the data.

A single libxo function call in source code is all that's required:

    xo_emit("Connecting to {:host}.{:domain}...\n", host, domain);

    TEXT:
      Connecting to my-box.example.com...
    XML:
      <host>my-box</host>
      <domain>example.com</domain>
    JSON:
      "host": "my-box",
      "domain": "example.com"
    HTML:
       <div class="line">
         <div class="text">Connecting to </div>
         <div class="data" data-tag="host" 
              data-xpath="/top/host">my-box</div>
         <div class="text">.</div>
         <div class="data" data-tag="domain"
              data-xpath="/top/domain">example.com</div>
         <div class="text">...</div>
       </div>

** Encoding Styles

There are four encoding styles supported by libxo:

- TEXT output can be display on a terminal session, allowing
compatibility with traditional command line usage.
- XML output is suitable for tools like XPath and protocols like
NETCONF.
- JSON output can be used for RESTful APIs and integration with
languages like Javascript and Python.
- HTML can be matched with a small CSS file to permit rendering in any
HTML5 browser.

In general, XML and JSON are suitable for encoding data, while TEXT is
suited for terminal output and HTML is suited for display in a web
browser (see ^xohtml^).

*** Text Output

Most traditional programs generate text output on standard output,
with contents like:

    36      ./src
    40      ./bin
    90      .

In this example (taken from du source code), the code to generate this
data might look like:

    printf("%d\t%s\n", num_blocks, path);

Simple, direct, obvious.  But it's only making text output.  Imagine
using a single code path to make TEXT, XML, JSON or HTML, deciding at
run time which to generate.

libxo expands on the idea of printf format strings to make a single
format containing instructions for creating multiple output styles:

    xo_emit("{:blocks/%d}\t{:path/%s}\n", num_blocks, path);

This line will generate the same text output as the earlier printf
call, but also has enough information to generate XML, JSON, and HTML.

The following sections introduce the other formats.

*** XML Output

XML output consists of a hierarchical set of elements, each encoded
with a start tag and an end tag.  The element should be named for data
value that it is encoding:

    <item>
      <blocks>36</blocks>
      <path>./src</path>
    </item>
    <item>
      <blocks>40</blocks>
      <path>./bin</path>
    </item>
    <item>
      <blocks>90</blocks>
      <path>.</path>
    </item>

XML is a W3C standard for encoding data.  See w3c.org/TR/xml for
additional information.

*** JSON Output

JSON output consists of a hierarchical set of objects and lists, each
encoded with a quoted name, a colon, and a value.  If the value is a
string, it must be quoted, but numbers are not quoted.  Objects are
encoded using braces; lists are encoded using square brackets.
Data inside objects and lists is separated using commas:

    items: [
        { "blocks": 36, "path" : "./src" },
        { "blocks": 40, "path" : "./bin" },
        { "blocks": 90, "path" : "./" }
    ]

*** HTML Output

HTML output is designed to allow the output to be rendered in a web
browser with minimal effort.  Each piece of output data is rendered
inside a <div> element, with a class name related to the role of the
data.  By using a small set of class attribute values, a CSS
stylesheet can render the HTML into rich text that mirrors the
traditional text content.

Additional attributes can be enabled to provide more details about the
data, including data type, description, and an XPath location.

    <div class="line">
      <div class="data" data-tag="blocks">36</div>
      <div class="padding">      </div>
      <div class="data" data-tag="path">./src</div>
    </div>
    <div class="line">
      <div class="data" data-tag="blocks">40</div>
      <div class="padding">      </div>
      <div class="data" data-tag="path">./bin</div>
    </div>
    <div class="line">
      <div class="data" data-tag="blocks">90</div>
      <div class="padding">      </div>
      <div class="data" data-tag="path">./</div>
    </div>

** Format Strings @format-strings@

libxo uses format strings to control the rendering of data into the
various output styles.  Each format string contains a set of zero or
more field descriptions, which describe independent data fields.  Each
field description contains a set of modifiers, a content string, and
zero, one, or two format descriptors.  The modifiers tell libxo what
the field is and how to treat it, while the format descriptors are
formatting instructions using printf-style format strings, telling
libxo how to format the field.  The field description is placed inside
a set of braces, with a colon (":") after the modifiers and a slash
("/") before each format descriptors.  Text may be intermixed with
field descriptions within the format string.

The field description is given as follows:

    '{' [ role | modifier ]* [',' long-names ]* ':' [ content ]
            [ '/' field-format [ '/' encoding-format ]] '}'

The role describes the function of the field, while the modifiers
enable optional behaviors.  The contents, field-format, and
encoding-format are used in varying ways, based on the role.  These
are described in the following sections.

In the following example, three field descriptors appear.  The first
is a padding field containing three spaces of padding, the second is a
label ("In stock"), and the third is a value field ("in-stock").  The
in-stock field has a "%u" format that will parse the next argument
passed to the xo_emit function as an unsigned integer.

    xo_emit("{P:   }{Lwc:In stock}{:in-stock/%u}\n", 65);

This single line of code can generate text (" In stock: 65\n"), XML
("<in-stock>65</in-stock>"), JSON ('"in-stock": 6'), or HTML (too
lengthy to be listed here).

While roles and modifiers typically use single character for brevity,
there are alternative names for each which allow more verbose
formatting strings.  These names must be preceded by a comma, and may
follow any single-character values:

    xo_emit("{L,white,colon:In stock}{,key:in-stock/%u}\n", 65);

*** Field Roles

Field roles are optional, and indicate the role and formatting of the
content.  The roles are listed below; only one role is permitted:

|---+--------------+-------------------------------------------------|
| R | Name         | Description                                     |
|---+--------------+-------------------------------------------------|
| C | color        | Field has color and effect controls             |
| D | decoration   | Field is non-text (e.g., colon, comma)          |
| E | error        | Field is an error message                       |
| G | gettext      | Call gettext(3) on the format string            |
| L | label        | Field is text that prefixes a value             |
| N | note         | Field is text that follows a value              |
| P | padding      | Field is spaces needed for vertical alignment   |
| T | title        | Field is a title value for headings             |
| U | units        | Field is the units for the previous value field |
| V | value        | Field is the name of field (the default)        |
| W | warning      | Field is a warning message                      |
| [ | start-anchor | Begin a section of anchored variable-width text |
| ] | stop-anchor  | End a section of anchored variable-width text   |
|---+--------------+-------------------------------------------------|

    EXAMPLE:
        xo_emit("{L:Free}{D::}{P:   }{:free/%u} {U:Blocks}\n",
                free_blocks);

When a role is not provided, the "value" role is used as the default.

Roles and modifiers can also use more verbose names, when preceeded by
a comma:

    EXAMPLE:
        xo_emit("{,label:Free}{,decoration::}{,padding:   }"
                "{,value:free/%u} {,units:Blocks}\n",
                free_blocks);

**** The Color Role ({C:}) @color-role@

Colors and effects control how text values are displayed; they are
used for display styles (TEXT and HTML).

    xo_emit("{C:bold}{:value}{C:no-bold}\n", value);

Colors and effects remain in effect until modified by other "C"-role
fields.

    xo_emit("{C:bold}{C:inverse}both{C:no-bold}only inverse\n");

If the content is empty, the "reset" action is performed.

    xo_emit("{C:both,underline}{:value}{C:}\n", value);

The content should be a comma-separated list of zero or more colors or
display effects.

    xo_emit("{C:bold,inverse}Ugly{C:no-bold,no-inverse}\n");

The color content can be either static, when placed directly within
the field descriptor, or a printf-style format descriptor can be used,
if preceded by a slash ("/"):

   xo_emit("{C:/%s%s}{:value}{C:}", need_bold ? "bold" : "",
           need_underline ? "underline" : "", value);

Color names are prefixed with either "fg-" or "bg-" to change the
foreground and background colors, respectively.

    xo_emit("{C:/fg-%s,bg-%s}{Lwc:Cost}{:cost/%u}{C:reset}\n",
            fg_color, bg_color, cost);

The following table lists the supported effects:

|---------------+-------------------------------------------------|
|  Name         | Description                                     |
|---------------+-------------------------------------------------|
|  bg-XXXXX     | Change background color                         |
|  bold         | Start bold text effect                          |
|  fg-XXXXX     | Change foreground color                         |
|  inverse      | Start inverse (aka reverse) text effect         |
|  no-bold      | Stop bold text effect                           |
|  no-inverse   | Stop inverse (aka reverse) text effect          |
|  no-underline | Stop underline text effect                      |
|  normal       | Reset effects (only)                            |
|  reset        | Reset colors and effects (restore defaults)     |
|  underline    | Start underline text effect                     |
|---------------+-------------------------------------------------|

The following color names are supported:

|---------+--------------------------------------------|
| Name    | Description                                |
|---------+--------------------------------------------|
| black   |                                            |
| blue    |                                            |
| cyan    |                                            |
| default | Default color for foreground or background |
| green   |                                            |
| magenta |                                            |
| red     |                                            |
| white   |                                            |
| yellow  |                                            |
|---------+--------------------------------------------|

**** The Decoration Role ({D:})

Decorations are typically punctuation marks such as colons,
semi-colons, and commas used to decorate the text and make it simpler
for human readers.  By marking these distinctly, HTML usage scenarios
can use CSS to direct their display parameters.

    xo_emit("{D:((}{:name}{D:))}\n", name);

**** The Gettext Role ({G:}) @gettext-role@

libxo supports internationalization (i18n) through its use of
gettext(3).  Use the "{G:}" role to request that the remaining part of
the format string, following the "{G:}" field, be handled using
gettext().

Since gettext() uses the string as the key into the message catalog,
libxo uses a simplified version of the format string that removes
unimportant field formatting and modifiers, stopping minor formatting
changes from impacting the expensive translation process.  A developer
change such as changing "/%06d" to "/%08d" should not force hand
inspection of all .po files.

The simplified version can be generated for a single message using the
"xopo -s <text>" command, or an entire .pot can be translated using
the "xopo -f <input> -o <output>" command.

   xo_emit("{G:}Invalid token\n");

The {G:} role allows a domain name to be set.  gettext calls will
continue to use that domain name until the current format string
processing is complete, enabling a library function to emit strings
using it's own catalog.  The domain name can be either static as the
content of the field, or a format can be used to get the domain name
from the arguments.

   xo_emit("{G:libc}Service unavailable in restricted mode\n");

See ^howto-i18n^ for additional details.

**** The Label Role ({L:})

Labels are text that appears before a value.

    xo_emit("{Lwc:Cost}{:cost/%u}\n", cost);

**** The Note Role ({N:})

Notes are text that appears after a value.

    xo_emit("{:cost/%u} {N:per year}\n", cost);

**** The Padding Role ({P:}) @padding-role@

Padding represents whitespace used before and between fields.

The padding content can be either static, when placed directly within
the field descriptor, or a printf-style format descriptor can be used,
if preceded by a slash ("/"):

    xo_emit("{P:        }{Lwc:Cost}{:cost/%u}\n", cost);
    xo_emit("{P:/%30s}{Lwc:Cost}{:cost/%u}\n", "", cost);

**** The Title Role ({T:})

Title are heading or column headers that are meant to be displayed to
the user.  The title can be either static, when placed directly within
the field descriptor, or a printf-style format descriptor can be used,
if preceded by a slash ("/"):

    xo_emit("{T:Interface Statistics}\n");
    xo_emit("{T:/%20.20s}{T:/%6.6s}\n", "Item Name", "Cost");

Title fields have an extra convenience feature; if both content and
format are specified, instead of looking to the argument list for a
value, the content is used, allowing a mixture of format and content
within the field descriptor:

    xo_emit("{T:Name/%20s}{T:Count/%6s}\n");

Since the incoming argument is a string, the format must be "%s" or
something suitable.

**** The Units Role ({U:})

Units are the dimension by which values are measured, such as degrees,
miles, bytes, and decibels.  The units field carries this information
for the previous value field.

    xo_emit("{Lwc:Distance}{:distance/%u}{Uw:miles}\n", miles);

Note that the sense of the 'w' modifier is reversed for units;
a blank is added before the contents, rather than after it.

When the XOF_UNITS flag is set, units are rendered in XML as the
"units" attribute:

    <distance units="miles">50</distance>

Units can also be rendered in HTML as the "data-units" attribute:

    <div class="data" data-tag="distance" data-units="miles"
         data-xpath="/top/data/distance">50</div>

**** The Value Role ({V:} and {:})

The value role is used to represent the a data value that is
interesting for the non-display output styles (XML and JSON).  Value
is the default role; if no other role designation is given, the field
is a value.  The field name must appear within the field descriptor,
followed by one or two format descriptors.  The first format
descriptor is used for display styles (TEXT and HTML), while the
second one is used for encoding styles (XML and JSON).  If no second
format is given, the encoding format defaults to the first format,
with any minimum width removed.  If no first format is given, both
format descriptors default to "%s".

    xo_emit("{:length/%02u}x{:width/%02u}x{:height/%02u}\n",
            length, width, height);
    xo_emit("{:author} wrote \"{:poem}\" in {:year/%4d}\n,
            author, poem, year);

**** The Anchor Roles ({[:} and {]:}) @anchor-role@

The anchor roles allow a set of strings by be padded as a group,
but still be visible to xo_emit as distinct fields.  Either the start
or stop anchor can give a field width and it can be either directly in
the descriptor or passed as an argument.  Any fields between the start
and stop anchor are padded to meet the minimum width given.

To give a width directly, encode it as the content of the anchor tag:

    xo_emit("({[:10}{:min/%d}/{:max/%d}{]:})\n", min, max);

To pass a width as an argument, use "%d" as the format, which must
appear after the "/".  Note that only "%d" is supported for widths.
Using any other value could ruin your day.

    xo_emit("({[:/%d}{:min/%d}/{:max/%d}{]:})\n", width, min, max);

If the width is negative, padding will be added on the right, suitable
for left justification.  Otherwise the padding will be added to the
left of the fields between the start and stop anchors, suitable for
right justification.  If the width is zero, nothing happens.  If the
number of columns of output between the start and stop anchors is less
than the absolute value of the given width, nothing happens.

Widths over 8k are considered probable errors and not supported.  If
XOF_WARN is set, a warning will be generated.

*** Field Modifiers

Field modifiers are flags which modify the way content emitted for
particular output styles:

|---+---------------+-------------------------------------------------|
| M | Name          | Description                                     |
|---+---------------+-------------------------------------------------|
| c | colon         | A colon (":") is appended after the label       |
| d | display       | Only emit field for display styles (text/HTML)  |
| e | encoding      | Only emit for encoding styles (XML/JSON)        |
| g | gettext       | Call gettext on field's render content          |
| h | humanize (hn) | Format large numbers in human-readable style    |
|   | hn-space      | Humanize: Place space between numeric and unit  |
|   | hn-decimal    | Humanize: Add a decimal digit, if number < 10   |
|   | hn-1000       | Humanize: Use 1000 as divisor instead of 1024   |
| k | key           | Field is a key, suitable for XPath predicates   |
| l | leaf-list     | Field is a leaf-list                            |
| n | no-quotes     | Do not quote the field when using JSON style    |
| p | plural        | Gettext: Use comma-separated plural form        |
| q | quotes        | Quote the field when using JSON style           |
| t | trim          | Trim leading and trailing whitespace            |
| w | white         | A blank (" ") is appended after the label       |
|---+---------------+-------------------------------------------------|

Roles and modifiers can also use more verbose names, when preceeded by
a comma.  For example, the modifier string "Lwc" (or "L,white,colon")
means the field has a label role (text that describes the next field)
and should be followed by a colon ('c') and a space ('w').  The
modifier string "Vkq" (or ":key,quote") means the field has a value
role (the default role), that it is a key for the current instance,
and that the value should be quoted when encoded for JSON.

**** The Colon Modifier ({c:})

The colon modifier appends a single colon to the data value:

    EXAMPLE:
      xo_emit("{Lc:Name}{:name}\n", "phil");
    TEXT:
      Name:phil

The colon modifier is only used for the TEXT and HTML output
styles. It is commonly combined with the space modifier ('{w:}').
It is purely a convenience feature.

**** The Display Modifier ({d:})

The display modifier indicated the field should only be generated for
the display output styles, TEXT and HTML.

    EXAMPLE:
      xo_emit("{Lcw:Name}{d:name} {:id/%d}\n", "phil", 1);
    TEXT:
      Name: phil 1
    XML:
      <id>1</id>

The display modifier is the opposite of the encoding modifier, and
they are often used to give to distinct views of the underlying data.

**** The Encoding Modifier ({e:}) @e-modifier@

The display modifier indicated the field should only be generated for
the display output styles, TEXT and HTML.

    EXAMPLE:
      xo_emit("{Lcw:Name}{:name} {e:id/%d}\n", "phil", 1);
    TEXT:
      Name: phil
    XML:
      <name>phil</name><id>1</id>

The encoding modifier is the opposite of the display modifier, and
they are often used to give to distinct views of the underlying data.

**** The Gettext Modifier ({g:}) @gettext-modifier@

The gettext modifier is used to translate individual fields using the
gettext domain (typically set using the "{G:}" role) and current
language settings.  Once libxo renders the field value, it is passed
to gettext(3), where it is used as a key to find the native language
translation.

In the following example, the strings "State" and "full" are passed
to gettext() to find locale-based translated strings.

    xo_emit("{Lgwc:State}{g:state}\n", "full");

See ^gettext-role^, ^plural-modifier^, and ^howto-i18n^ for additional
details.

**** The Humanize Modifier ({h:})

The humanize modifier is used to render large numbers as in a
human-readable format.  While numbers like "44470272" are completely
readable to computers and savants, humans will generally find "44M"
more meaningful.

"hn" can be used as an alias for "humanize".

The humanize modifier only affects display styles (TEXT and HMTL).
The "no-humanize" option (See ^LIBXO_OPTIONS^) will block the function of
the humanize modifier.

There are a number of modifiers that affect details of humanization.
These are only available in as full names, not single characters.  The
"hn-space" modifier places a space between the number and any
multiplier symbol, such as "M" or "K" (ex: "44 K").  The "hn-decimal"
modifier will add a decimal point and a single tenths digit when the number is
less than 10 (ex: "4.4K").  The "hn-1000" modifier will use 1000 as divisor
instead of 1024, following the JEDEC-standard instead of the more
natural binary powers-of-two tradition.

    EXAMPLE:
        xo_emit("{h:input/%u}, {h,hn-space:output/%u}, "
	    "{h,hn-decimal:errors/%u}, {h,hn-1000:capacity/%u}, "
	    "{h,hn-decimal:remaining/%u}\n",
            input, output, errors, capacity, remaining);
    TEXT:
        21, 57 K, 96M, 44M, 1.2G

In the HTML style, the original numeric value is rendered in the
"data-number" attribute on the <div> element:

    <div class="data" data-tag="errors"
         data-number="100663296">96M</div>

**** The Key Modifier ({k:})

The key modifier is used to indicate that a particular field helps
uniquely identify an instance of list data.

    EXAMPLE:
        xo_open_list("user");
        for (i = 0; i < num_users; i++) {
	    xo_open_instance("user");
            xo_emit("User {k:name} has {:count} tickets\n",
               user[i].u_name, user[i].u_tickets);
            xo_close_instance("user");
        }
        xo_close_list("user");

Currently the key modifier is only used when generating XPath value
for the HTML output style when XOF_XPATH is set, but other uses are
likely in the near future.

**** The Leaf-List Modifier ({l:})

The leaf-list modifier is used to distinguish lists where each
instance consists of only a single value.  In XML, these are
rendered as single elements, where JSON renders them as arrays.

    EXAMPLE:
        for (i = 0; i < num_users; i++) {
            xo_emit("Member {l:user}\n", user[i].u_name);
        }
    XML:
        <user>phil</user>
        <user>pallavi</user>
    JSON:
        "user": [ "phil", "pallavi" ]

The name of the field must match the name of the leaf list.

**** The No-Quotes Modifier ({n:})

The no-quotes modifier (and its twin, the 'quotes' modifier) affect
the quoting of values in the JSON output style.  JSON uses quotes for
string value, but no quotes for numeric, boolean, and null data.
xo_emit applies a simple heuristic to determine whether quotes are
needed, but often this needs to be controlled by the caller.

    EXAMPLE:
      const char *bool = is_true ? "true" : "false";
      xo_emit("{n:fancy/%s}", bool);
    JSON:
      "fancy": true

**** The Plural Modifier ({p:}) @plural-modifier@

The plural modifier selects the appropriate plural form of an
expression based on the most recent number emitted and the current
language settings.  The contents of the field should be the singular
and plural English values, separated by a comma:

    xo_emit("{:bytes} {Ngp:byte,bytes}\n", bytes);

The plural modifier is meant to work with the gettext modifier ({g:})
but can work independently.  See ^gettext-modifier^.

When used without the gettext modifier or when the message does not
appear in the message catalog, the first token is chosen when the last
numeric value is equal to 1; otherwise the second value is used,
mimicking the simple pluralization rules of English.

When used with the gettext modifier, the ngettext(3) function is
called to handle the heavy lifting, using the message catalog to
convert the singular and plural forms into the native language.

**** The Quotes Modifier ({q:})

The quotes modifier (and its twin, the 'no-quotes' modifier) affect
the quoting of values in the JSON output style.  JSON uses quotes for
string value, but no quotes for numeric, boolean, and null data.
xo_emit applies a simple heuristic to determine whether quotes are
needed, but often this needs to be controlled by the caller.

    EXAMPLE:
      xo_emit("{q:time/%d}", 2014);
    JSON:
      "year": "2014"

**** The White Space Modifier ({w:})

The white space modifier appends a single space to the data value:

    EXAMPLE:
      xo_emit("{Lw:Name}{:name}\n", "phil");
    TEXT:
      Name phil

The white space modifier is only used for the TEXT and HTML output
styles. It is commonly combined with the colon modifier ('{c:}').
It is purely a convenience feature.

Note that the sense of the 'w' modifier is reversed for the units role
({Uw:}); a blank is added before the contents, rather than after it.

*** Field Formatting

The field format is similar to the format string for printf(3).  Its
use varies based on the role of the field, but generally is used to
format the field's contents.

If the format string is not provided for a value field, it defaults to
"%s".

Note a field definition can contain zero or more printf-style
'directives', which are sequences that start with a '%' and end with
one of following characters: "diouxXDOUeEfFgGaAcCsSp".  Each directive
is matched by one of more arguments to the xo_emit function.

The format string has the form:

  '%' format-modifier * format-character

The format- modifier can be:
- a '#' character, indicating the output value should be prefixed with
'0x', typically to indicate a base 16 (hex) value.
- a minus sign ('-'), indicating the output value should be padded on
the right instead of the left.
- a leading zero ('0') indicating the output value should be padded on the
left with zeroes instead of spaces (' ').
- one or more digits ('0' - '9') indicating the minimum width of the
argument.  If the width in columns of the output value is less that
the minumum width, the value will be padded to reach the minimum.
- a period followed by one or more digits indicating the maximum
number of bytes which will be examined for a string argument, or the maximum
width for a non-string argument.  When handling ASCII strings this
functions as the field width but for multi-byte characters, a single
character may be composed of multiple bytes.
xo_emit will never dereference memory beyond the given number of bytes.
- a second period followed by one or more digits indicating the maximum
width for a string argument.  This modifier cannot be given for non-string
arguments. 
- one or more 'h' characters, indicating shorter input data.
- one or more 'l' characters, indicating longer input data.
- a 'z' character, indicating a 'size_t' argument.
- a 't' character, indicating a 'ptrdiff_t' argument.
- a ' ' character, indicating a space should be emitted before
positive numbers.
- a '+' character, indicating sign should emitted before any number.

Note that 'q', 'D', 'O', and 'U' are considered deprecated and will be
removed eventually.

The format character is described in the following table:

|-----+-----------------+----------------------|
| Ltr | Argument Type   | Format               |
|-----+-----------------+----------------------|
| d   | int             | base 10 (decimal)    |
| i   | int             | base 10 (decimal)    |
| o   | int             | base 8 (octal)       |
| u   | unsigned        | base 10 (decimal)    |
| x   | unsigned        | base 16 (hex)        |
| X   | unsigned long   | base 16 (hex)        |
| D   | long            | base 10 (decimal)    |
| O   | unsigned long   | base 8 (octal)       |
| U   | unsigned long   | base 10 (decimal)    |
| e   | double          | [-]d.ddde+-dd        |
| E   | double          | [-]d.dddE+-dd        |
| f   | double          | [-]ddd.ddd           |
| F   | double          | [-]ddd.ddd           |
| g   | double          | as 'e' or 'f'        |
| G   | double          | as 'E' or 'F'        |
| a   | double          | [-]0xh.hhhp[+-]d     |
| A   | double          | [-]0Xh.hhhp[+-]d     |
| c   | unsigned char   | a character          |
| C   | wint_t          | a character          |
| s   | char *          | a UTF-8 string       |
| S   | wchar_t *       | a unicode/WCS string |
| p   | void *          | '%#lx'               |
|-----+-----------------+----------------------|

The 'h' and 'l' modifiers affect the size and treatment of the
argument:

|-----+-------------+--------------------|
| Mod | d, i        | o, u, x, X         |
|-----+-------------+--------------------|
| hh  | signed char | unsigned char      |
| h   | short       | unsigned short     |
| l   | long        | unsigned long      |
| ll  | long long   | unsigned long long |
| j   | intmax_t    | uintmax_t          |
| t   | ptrdiff_t   | ptrdiff_t          |
| z   | size_t      | size_t             |
| q   | quad_t      | u_quad_t           |
|-----+-------------+--------------------|

*** UTF-8 and Locale Strings

For strings, the 'h' and 'l' modifiers affect the interpretation of
the bytes pointed to argument.  The default '%s' string is a 'char *'
pointer to a string encoded as UTF-8.  Since UTF-8 is compatible with
ASCII data, a normal 7-bit ASCII string can be used.  '%ls' expects a
'wchar_t *' pointer to a wide-character string, encoded as a 32-bit
Unicode values.  '%hs' expects a 'char *' pointer to a multi-byte
string encoded with the current locale, as given by the LC_CTYPE,
LANG, or LC_ALL environment varibles.  The first of this list of
variables is used and if none of the variables are set, the locale
defaults to "UTF-8".

For example, a function is passed a locale-base name, a hat size,
and a time value.  The hat size is formatted in a UTF-8 (ASCII)
string, and the time value is formatted into a wchar_t string.

    void print_order (const char *name, int size,
                      struct tm *timep) {
        char buf[32];
        const char *size_val = "unknown";

	if (size > 0)
            snprintf(buf, sizeof(buf), "%d", size);
            size_val = buf;
        }

        wchar_t when[32];
        wcsftime(when, sizeof(when), L"%d%b%y", timep);

        xo_emit("The hat for {:name/%hs} is {:size/%s}.\n",
                name, size_val);
        xo_emit("It was ordered on {:order-time/%ls}.\n",
                when);
    }

It is important to note that xo_emit will perform the conversion
required to make appropriate output.  Text style output uses the
current locale (as described above), while XML, JSON, and HTML use
UTF-8.

UTF-8 and locale-encoded strings can use multiple bytes to encode one
column of data.  The traditional "precision'" (aka "max-width") value
for "%s" printf formatting becomes overloaded since it specifies both
the number of bytes that can be safely referenced and the maximum
number of columns to emit.  xo_emit uses the precision as the former,
and adds a third value for specifying the maximum number of columns.

In this example, the name field is printed with a minimum of 3 columns
and a maximum of 6.  Up to ten bytes of data at the location given by
'name' are in used in filling those columns.

    xo_emit("{:name/%3.10.6s}", name);

*** Characters Outside of Field Definitions

Characters in the format string that are not part of a field
definition are copied to the output for the TEXT style, and are
ignored for the JSON and XML styles.  For HTML, these characters are
placed in a <div> with class "text".

  EXAMPLE:
      xo_emit("The hat is {:size/%s}.\n", size_val);
  TEXT:
      The hat is extra small.
  XML:
      <size>extra small</size>
  JSON:
      "size": "extra small"
  HTML:
      <div class="text">The hat is </div>
      <div class="data" data-tag="size">extra small</div>
      <div class="text">.</div>

*** "%m" Is Supported

libxo supports the '%m' directive, which formats the error message
associated with the current value of "errno".  It is the equivalent
of "%s" with the argument strerror(errno).

    xo_emit("{:filename} cannot be opened: {:error/%m}", filename);
    xo_emit("{:filename} cannot be opened: {:error/%s}",
            filename, strerror(errno));

*** "%n" Is Not Supported

libxo does not support the '%n' directive.  It's a bad idea and we
just don't do it.

*** The Encoding Format (eformat)

The "eformat" string is the format string used when encoding the field
for JSON and XML.  If not provided, it defaults to the primary format
with any minimum width removed.  If the primary is not given, both
default to "%s".

*** Content Strings

For padding and labels, the content string is considered the content,
unless a format is given.

*** Argument Validation @printf-like@

Many compilers and tool chains support validation of printf-like
arguments.  When the format string fails to match the argument list, 
a warning is generated.  This is a valuable feature and while the
formatting strings for libxo differ considerably from printf, many of
these checks can still provide build-time protection against bugs.

libxo provide variants of functions that provide this ability, if the
"--enable-printflike" option is passed to the "configure" script.
These functions use the "_p" suffix, like "xo_emit_p()",
xo_emit_hp()", etc.

The following are features of libxo formatting strings that are
incompatible with printf-like testing:

- implicit formats, where "{:tag}" has an implicit "%s";
- the "max" parameter for strings, where "{:tag/%4.10.6s}" means up to
ten bytes of data can be inspected to fill a minimum of 4 columns and
a maximum of 6;
- percent signs in strings, where "{:filled}%" makes a single,
trailing percent sign;
- the "l" and "h" modifiers for strings, where "{:tag/%hs}" means
locale-based string and "{:tag/%ls}" means a wide character string;
- distinct encoding formats, where "{:tag/#%s/%s}" means the display
styles (text and HTML) will use "#%s" where other styles use "%s";

If none of these features are in use by your code, then using the "_p"
variants might be wise.

|------------------+------------------------|
| Function         | printf-like Equivalent |
|------------------+------------------------|
| xo_emit_hv       | xo_emit_hvp            |
| xo_emit_h        | xo_emit_hp             |
| xo_emit          | xo_emit_p              |
| xo_emit_warn_hcv | xo_emit_warn_hcvp      |
| xo_emit_warn_hc  | xo_emit_warn_hcp       |
| xo_emit_warn_c   | xo_emit_warn_cp        |
| xo_emit_warn     | xo_emit_warn_p         |
| xo_emit_warnx_   | xo_emit_warnx_p        |
| xo_emit_err      | xo_emit_err_p          |
| xo_emit_errx     | xo_emit_errx_p         |
| xo_emit_errc     | xo_emit_errc_p         |
|------------------+------------------------|

*** Example

In this example, the value for the number of items in stock is emitted:

        xo_emit("{P:   }{Lwc:In stock}{:in-stock/%u}\n",
                instock);

This call will generate the following output:

  TEXT: 
       In stock: 144
  XML:
      <in-stock>144</in-stock>
  JSON:
      "in-stock": 144,
  HTML:
      <div class="line">
        <div class="padding">   </div>
        <div class="label">In stock</div>
        <div class="decoration">:</div>
        <div class="padding"> </div>
        <div class="data" data-tag="in-stock">144</div>
      </div>

Clearly HTML wins the verbosity award, and this output does
not include XOF_XPATH or XOF_INFO data, which would expand the
penultimate line to:

       <div class="data" data-tag="in-stock"
          data-xpath="/top/data/item/in-stock"
          data-type="number"
          data-help="Number of items in stock">144</div>

** Command-line Arguments

libxo uses command line options to trigger rendering behavior.  The
following options are recognised:

- --libxo <options>
- --libxo=<options>
- --libxo:<brief-options>

Options is a comma-separated list of tokens that correspond to output
styles, flags, or features:

|-------------+-------------------------------------------------------|
| Token       | Action                                                |
|-------------+-------------------------------------------------------|
| color       | Enable colors/effects for display styles (TEXT, HTML) |
| dtrt        | Enable "Do The Right Thing" mode                      |
| html        | Emit HTML output                                      |
| indent=xx   | Set the indentation level                             |
| info        | Add info attributes (HTML)                            |
| json        | Emit JSON output                                      |
| keys        | Emit the key attribute for keys (XML)                 |
| log-gettext | Log (via stderr) each gettext(3) string lookup        |
| log-syslog  | Log (via stderr) each syslog message (via xo_syslog)  |
| no-humanize | Ignore the {h:} modifier (TEXT, HTML)                 |
| no-locale   | Do not initialize the locale setting                  |
| no-top      | Do not emit a top set of braces (JSON)                |
| not-first   | Pretend the 1st output item was not 1st (JSON)        |
| pretty      | Emit pretty-printed output                            |
| text        | Emit TEXT output                                      |
| underscores | Replace XML-friendly "-"s with JSON friendly "_"s e   |
| units       | Add the 'units' (XML) or 'data-units (HTML) attribute |
| warn        | Emit warnings when libxo detects bad calls            |
| warn-xml    | Emit warnings in XML                                  |
| xml         | Emit XML output                                       |
| xpath       | Add XPath expressions (HTML)                          |
|-------------+-------------------------------------------------------|

The brief options are detailed in ^LIBXO_OPTIONS^.

** Representing Hierarchy

For XML and JSON, individual fields appear inside hierarchies which
provide context and meaning to the fields.  Unfortunately, these
encoding have a basic disconnect between how lists is similar objects
are represented.

XML encodes lists as set of sequential elements:

    <user>phil</user>
    <user>pallavi</user>
    <user>sjg</user>

JSON encodes lists using a single name and square brackets:

    "user": [ "phil", "pallavi", "sjg" ]

This means libxo needs three distinct indications of hierarchy: one
for containers of hierarchy appear only once for any specific parent,
one for lists, and one for each item in a list.

*** Containers

A "container" is an element of a hierarchy that appears only once
under any specific parent.  The container has no value, but serves to
contain other nodes.

To open a container, call xo_open_container() or
xo_open_container_h().  The former uses the default handle and
the latter accepts a specific handle.

    int xo_open_container_h (xo_handle_t *xop, const char *name);
    int xo_open_container (const char *name);

To close a level, use the xo_close_container() or
xo_close_container_h() functions:

    int xo_close_container_h (xo_handle_t *xop, const char *name);
    int xo_close_container (const char *name);

Each open call must have a matching close call.  If the XOF_WARN flag
is set and the name given does not match the name of the currently open
container, a warning will be generated.

    Example:

        xo_open_container("top");
        xo_open_container("system");
        xo_emit("{:host-name/%s%s%s", hostname,
                domainname ? "." : "", domainname ?: "");
        xo_close_container("system");
        xo_close_container("top");

    Sample Output:
      Text:
        my-host.example.org
      XML:
        <top>
          <system>
              <host-name>my-host.example.org</host-name>
          </system>
        </top>
      JSON:
        "top" : {
          "system" : {
              "host-name": "my-host.example.org"
          }
        }
      HTML:
        <div class="data"
             data-tag="host-name">my-host.example.org</div>

*** Lists and Instances

A list is set of one or more instances that appear under the same
parent.  The instances contain details about a specific object.  One
can think of instances as objects or records.  A call is needed to
open and close the list, while a distinct call is needed to open and
close each instance of the list:

    xo_open_list("item");

    for (ip = list; ip->i_title; ip++) {
        xo_open_instance("item");
        xo_emit("{L:Item} '{:name/%s}':\n", ip->i_title);
        xo_close_instance("item");
    }

    xo_close_list("item");

Getting the list and instance calls correct is critical to the proper
generation of XML and JSON data.

*** DTRT Mode

Some users may find tracking the names of open containers, lists, and
instances inconvenient.  libxo offers a "Do The Right Thing" mode, where
libxo will track the names of open containers, lists, and instances so
the close function can be called without a name.  To enable DTRT mode,
turn on the XOF_DTRT flag prior to making any other libxo output.

    xo_set_flags(NULL, XOF_DTRT);

Each open and close function has a version with the suffix "_d", which
will close the open container, list, or instance:

    xo_open_container("top");
    ...
    xo_close_container_d();

This also works for lists and instances:

    xo_open_list("item");
    for (...) {
        xo_open_instance("item");
        xo_emit(...);
        xo_close_instance_d();
    }
    xo_close_list_d();

Note that the XOF_WARN flag will also cause libxo to track open
containers, lists, and instances.  A warning is generated when the
name given to the close function and the name recorded do not match.

*** Markers

Markers are used to protect and restore the state of open constructs.
While a marker is open, no other open constructs can be closed.  When
a marker is closed, all constructs open since the marker was opened
will be closed.

Markers use names which are not user-visible, allowing the caller to
choose appropriate internal names.

In this example, the code whiffles through a list of fish, calling a
function to emit details about each fish.  The marker "fish-guts" is
used to ensure that any constructs opened by the function are closed
properly.

    for (i = 0; fish[i]; i++) {
        xo_open_instance("fish");
        xo_open_marker("fish-guts");
        dump_fish_details(i);
        xo_close_marker("fish-guts");
    }

** Handles @handles@

libxo uses "handles" to control its rendering functionality.  The
handle contains state and buffered data, as well as callback functions
to process data.

A default handle is used when a NULL is passed to functions accepting
a handle.  This handle is initialized to write its data to stdout
using the default style of text (XO_STYLE_TEXT).

For the convenience of callers, the libxo library includes handle-less
functions that implicitly use the default handle.  Any function that
takes a handle will use the default handle is a value of NULL is
passed in place of a valid handle.

For example, the following are equivalent:

    xo_emit("test");
    xo_emit_h(NULL, "test");

Handles are created using xo_create() and destroy using xo_destroy().

** UTF-8

All strings for libxo must be UTF-8.  libxo will handle turning them
into locale-based strings for display to the user.

The only exception is argument formatted using the "%ls" format, which
require a wide character string (wchar_t *) as input.  libxo will
convert these arguments as needed to either UTF-8 (for XML, JSON, and
HTML styles) or locale-based strings for display in text style.

   xo_emit("Alll strings are utf-8 content {:tag/%ls}",
           L"except for wide strings");

"%S" is equivalent to "%ls".

* The libxo API

This section gives details about the functions in libxo, how to call
them, and the actions they perform.

** Handles

Handles give an abstraction for libxo that encapsulates the state of a
stream of output.  Handles have the data type "xo_handle_t" and are
opaque to the caller.

The library has a default handle that is automatically initialized.
By default, this handle will send text style output to standard output.
The xo_set_style and xo_set_flags functions can be used to change this
behavior.

Many libxo functions take a handle as their first parameter; most that
do not use the default handle.  Any function taking a handle can
be passed NULL to access the default handle.

For the typical command that is generating output on standard output,
there is no need to create an explicit handle, but they are available
when needed, e.g., for daemons that generate multiple streams of
output.

*** xo_create

A handle can be allocated using the xo_create() function:

    xo_handle_t *xo_create (unsigned style, unsigned flags);

  Example:
    xo_handle_t *xop = xo_create(XO_STYLE_JSON, XOF_WARN);
    ....
    xo_emit_h(xop, "testing\n");

See also ^styles^ and ^flags^.

*** xo_create_to_file

By default, libxo writes output to standard output.  A convenience
function is provided for situations when output should be written to
a different file:

    xo_handle_t *xo_create_to_file (FILE *fp, unsigned style,
                                    unsigned flags);

Use the XOF_CLOSE_FP flag to trigger a call to fclose() for
the FILE pointer when the handle is destroyed.

*** xo_set_writer

The xo_set_writer function allows custom 'write' functions
which can tailor how libxo writes data.  An opaque argument is
recorded and passed back to the write function, allowing the function
to acquire context information. The 'close' function can
release this opaque data and any other resources as needed.
The flush function can flush buffered data associated with the opaque
object.

    void xo_set_writer (xo_handle_t *xop, void *opaque,
                        xo_write_func_t write_func,
                        xo_close_func_t close_func);
                        xo_flush_func_t flush_func);

*** xo_set_style

To set the style, use the xo_set_style() function:

    void xo_set_style(xo_handle_t *xop, unsigned style);

To use the default handle, pass a NULL handle:

    xo_set_style(NULL, XO_STYLE_XML);

**** Output Styles (XO_STYLE_*) @styles@

The libxo functions accept a set of output styles:

|---------------+-------------------------|
| Flag          | Description             |
|---------------+-------------------------|
| XO_STYLE_TEXT | Traditional text output |
| XO_STYLE_XML  | XML encoded data        |
| XO_STYLE_JSON | JSON encoded data       |
| XO_STYLE_HTML | HTML encoded data       |
|---------------+-------------------------|

**** xo_set_style_name

The xo_set_style_name() can be used to set the style based on a name
encoded as a string:

    int xo_set_style_name (xo_handle_t *xop, const char *style);

The name can be any of the styles: "text", "xml", "json", or "html".

    EXAMPLE:
        xo_set_style_name(NULL, "html");

*** xo_set_flags

To set the flags, use the xo_set_flags() function:

    void xo_set_flags(xo_handle_t *xop, unsigned flags);

To use the default handle, pass a NULL handle:

    xo_set_style(NULL, XO_STYLE_XML);

**** Flags (XOF_*) @flags@

The set of valid flags include:

|-------------------+----------------------------------------|
| Flag              | Description                            |
|-------------------+----------------------------------------|
| XOF_CLOSE_FP      | Close file pointer on xo_destroy()     |
| XOF_COLOR         | Enable color and effects in output     |
| XOF_COLOR_ALLOWED | Allow color/effect for terminal output |
| XOF_DTRT          | Enable "do the right thing" mode       |
| XOF_INFO          | Display info data attributes (HTML)    |
| XOF_KEYS          | Emit the key attribute (XML)           |
| XOF_NO_ENV        | Do not use the LIBXO_OPTIONS env var   |
| XOF_NO_HUMANIZE   | Display humanization (TEXT, HTML)      |
| XOF_PRETTY        | Make 'pretty printed' output           |
| XOF_UNDERSCORES   | Replaces hyphens with underscores      |
| XOF_UNITS         | Display units (XML, HMTL)              |
| XOF_WARN          | Generate warnings for broken calls     |
| XOF_WARN_XML      | Generate warnings in XML on stdout     |
| XOF_XPATH         | Emit XPath expressions (HTML)          |
| XOF_COLUMNS       | Force xo_emit to return columns used   |
| XOF_FLUSH         | Flush output after each xo_emit call   |
|-------------------+----------------------------------------|

The XOF_CLOSE_FP flag will trigger the call of the close_func
(provided via xo_set_writer()) when the handle is destroyed.

The XOF_COLOR flag enables color and effects in output regardless of
output device, while the XOF_COLOR_ALLOWED flag allows color and
effects only if the output device is a terminal.

The XOF_PRETTY flag requests 'pretty printing', which will trigger the
addition of indentation and newlines to enhance the readability of
XML, JSON, and HTML output.  Text output is not affected.

The XOF_WARN flag requests that warnings will trigger diagnostic
output (on standard error) when the library notices errors during
operations, or with arguments to functions.  Without warnings enabled,
such conditions are ignored.

Warnings allow developers to debug their interaction with libxo.
The function "xo_failure" can used as a breakpoint for a debugger,
regardless of whether warnings are enabled.

If the style is XO_STYLE_HTML, the following additional flags can be
used:

|---------------+-----------------------------------------|
| Flag          | Description                             |
|---------------+-----------------------------------------|
| XOF_XPATH     | Emit "data-xpath" attributes            |
| XOF_INFO      | Emit additional info fields             |
|---------------+-----------------------------------------|

The XOF_XPATH flag enables the emission of XPath expressions detailing
the hierarchy of XML elements used to encode the data field, if the
XPATH style of output were requested.

The XOF_INFO flag encodes additional informational fields for HTML
output.  See ^info^ for details.

If the style is XO_STYLE_XML, the following additional flags can be
used:

|---------------+-----------------------------------------|
| Flag          | Description                             |
|---------------+-----------------------------------------|
| XOF_KEYS      | Flag 'key' fields for xml               |
|---------------+-----------------------------------------|

The XOF_KEYS flag adds 'key' attribute to the XML encoding for
field definitions that use the 'k' modifier.  The key attribute has
the value "key":

    xo_emit("{k:name}", item);

  XML:
      <name key="key">truck</name>

**** xo_clear_flags

The xo_clear_flags() function turns off the given flags in a specific
handle. 

    void xo_clear_flags (xo_handle_t *xop, xo_xof_flags_t flags);

**** xo_set_options

The xo_set_options() function accepts a comma-separated list of styles
and flags and enables them for a specific handle.

    int xo_set_options (xo_handle_t *xop, const char *input);

The options are identical to those listed in ^command-line-arguments^.

*** xo_destroy

The xo_destroy function releases a handle and any resources it is
using.  Calling xo_destroy with a NULL handle will release any
resources associated with the default handle.

    void xo_destroy(xo_handle_t *xop);

** Emitting Content (xo_emit)

The following functions are used to emit output:

    int xo_emit (const char *fmt, ...);
    int xo_emit_h (xo_handle_t *xop, const char *fmt, ...);
    int xo_emit_hv (xo_handle_t *xop, const char *fmt, va_list vap);

The "fmt" argument is a string containing field descriptors as
specified in ^format-strings^.  The use of a handle is optional and
NULL can be passed to access the internal 'default' handle.  See
^handles^.

The remaining arguments to xo_emit() and xo_emit_h() are a set of
arguments corresponding to the fields in the format string.  Care must
be taken to ensure the argument types match the fields in the format
string, since an inappropriate cast can ruin your day.  The vap
argument to xo_emit_hv() points to a variable argument list that can
be used to retrieve arguments via va_arg().

*** Attributes (xo_attr) @xo_attr@

The xo_attr() function emits attributes for the XML output style.


    int xo_attr (const char *name, const char *fmt, ...);
    int xo_attr_h (xo_handle_t *xop, const char *name, 
                   const char *fmt, ...);
    int xo_attr_hv (xo_handle_t *xop, const char *name, 
                   const char *fmt, va_list vap);

The name parameter give the name of the attribute to be encoded.  The
fmt parameter gives a printf-style format string used to format the
value of the attribute using any remaining arguments, or the vap
parameter passed to xo_attr_hv().

    EXAMPLE:
      xo_attr("seconds", "%ld", (unsigned long) login_time);
      struct tm *tmp = localtime(login_time);
      strftime(buf, sizeof(buf), "%R", tmp);
      xo_emit("Logged in at {:login-time}\n", buf);
    XML:
        <login-time seconds="1408336270">00:14</login-time>

xo_attr is placed on the next container, instance, leaf, or leaf list
that is emitted.

Since attributes are only emitted in XML, their use should be limited
to meta-data and additional or redundant representations of data
already emitted in other form.

*** Flushing Output (xo_flush)

libxo buffers data, both for performance and consistency, but also to
allow some advanced features to work properly.  At various times, the
caller may wish to flush any data buffered within the library.  The
xo_flush() call is used for this:

    void xo_flush (void);
    void xo_flush_h (xo_handle_t *xop);

Calling xo_flush also triggers the flush function associated with the
handle.  For the default handle, this is equivalent to
"fflush(stdio);". 

*** Finishing Output (xo_finish)

When the program is ready to exit or close a handle, a call to
xo_finish() is required.  This flushes any buffered data, closes
open libxo constructs, and completes any pending operations.

    int xo_finish (void);
    int xo_finish_h (xo_handle_t *xop);
    void xo_finish_atexit (void);

Calling this function is vital to the proper operation of libxo,
especially for the non-TEXT output styles.

xo_finish_atexit is suitable for use with atexit(3).

** Emitting Hierarchy

libxo represents to types of hierarchy: containers and lists.  A
container appears once under a given parent where a list contains
instances that can appear multiple times.  A container is used to hold
related fields and to give the data organization and scope.

To create a container, use the xo_open_container and
xo_close_container functions:

    int xo_open_container (const char *name);
    int xo_open_container_h (xo_handle_t *xop, const char *name);
    int xo_open_container_hd (xo_handle_t *xop, const char *name);
    int xo_open_container_d (const char *name);

    int xo_close_container (const char *name);
    int xo_close_container_h (xo_handle_t *xop, const char *name);
    int xo_close_container_hd (xo_handle_t *xop);
    int xo_close_container_d (void);

The name parameter gives the name of the container, encoded in UTF-8.
Since ASCII is a proper subset of UTF-8, traditional C strings can be
used directly.

The close functions with the "_d" suffix are used in "Do The Right
Thing" mode, where the name of the open containers, lists, and
instances are maintained internally by libxo to allow the caller to
avoid keeping track of the open container name.

Use the XOF_WARN flag to generate a warning if the name given on the
close does not match the current open container.

For TEXT and HTML output, containers are not rendered into output
text, though for HTML they are used when the XOF_XPATH flag is set.

    EXAMPLE:
       xo_open_container("system");
       xo_emit("The host name is {:host-name}\n", hn);
       xo_close_container("system");
    XML:
       <system><host-name>foo</host-name></system>

*** Lists and Instances

Lists are sequences of instances of homogeneous data objects.  Two
distinct levels of calls are needed to represent them in our output
styles.  Calls must be made to open and close a list, and for each
instance of data in that list, calls must be make to open and close
that instance.

The name given to all calls must be identical, and it is strongly
suggested that the name be singular, not plural, as a matter of
style and usage expectations.

    EXAMPLE:
        xo_open_list("user");
        for (i = 0; i < num_users; i++) {
            xo_open_instance("user");
            xo_emit("{k:name}:{:uid/%u}:{:gid/%u}:{:home}\n",
                    pw[i].pw_name, pw[i].pw_uid,
                    pw[i].pw_gid, pw[i].pw_dir);
            xo_close_instance("user");
        }
        xo_close_list("user");
    TEXT:
        phil:1001:1001:/home/phil
        pallavi:1002:1002:/home/pallavi
    XML:
        <user>
            <name>phil</name>
            <uid>1001</uid>
            <gid>1001</gid>
            <home>/home/phil</home>
        </user>
        <user>
            <name>pallavi</name>
            <uid>1002</uid>
            <gid>1002</gid>
            <home>/home/pallavi</home>
        </user>
    JSON:
        user: [
            {
                "name": "phil",
                "uid": 1001,
                "gid": 1001,
                "home": "/home/phil",
            },
            {
                "name": "pallavi",
                "uid": 1002,
                "gid": 1002,
                "home": "/home/pallavi",
            }
        ]

** Support Functions

*** Parsing Command-line Arguments (xo_parse_args) @xo_parse_args@

The xo_parse_args() function is used to process a program's
arguments.  libxo-specific options are processed and removed
from the argument list so the calling application does not
need to process them.  If successful, a new value for argc
is returned.  On failure, a message it emitted and -1 is returned.

    argc = xo_parse_args(argc, argv);
    if (argc < 0)
        exit(EXIT_FAILURE);

Following the call to xo_parse_args, the application can process the
remaining arguments in a normal manner.  See ^command-line-arguments^
for a description of valid arguments.

*** xo_set_program

The xo_set_program function sets name of the program as reported by
functions like xo_failure, xo_warn, xo_err, etc.  The program name is
initialized by xo_parse_args, but subsequent calls to xo_set_program
can override this value.

    xo_set_program(argv[0]);

Note that the value is not copied, so the memory passed to
xo_set_program (and xo_parse_args) must be maintained by the caller.

*** xo_set_version

The xo_set_version function records a version number to be emitted as
part of the data for encoding styles (XML and JSON).  This version
number is suitable for tracking changes in the content, allowing a
user of the data to discern which version of the data model is in use.

     void xo_set_version (const char *version);
     void xo_set_version_h (xo_handle_t *xop, const char *version);

*** Field Information (xo_info_t) @info@

HTML data can include additional information in attributes that
begin with "data-".  To enable this, three things must occur:

First the application must build an array of xo_info_t structures,
one per tag.  The array must be sorted by name, since libxo uses a
binary search to find the entry that matches names from format
instructions.

Second, the application must inform libxo about this information using
the xo_set_info() call:

    typedef struct xo_info_s {
        const char *xi_name;    /* Name of the element */
        const char *xi_type;    /* Type of field */
        const char *xi_help;    /* Description of field */
    } xo_info_t;

    void xo_set_info (xo_handle_t *xop, xo_info_t *infop, int count);

Like other libxo calls, passing NULL for the handle tells libxo to use
the default handle.

If the count is -1, libxo will count the elements of infop, but there
must be an empty element at the end.  More typically, the number is
known to the application:

    xo_info_t info[] = {
        { "in-stock", "number", "Number of items in stock" },
        { "name", "string", "Name of the item" },
        { "on-order", "number", "Number of items on order" },
        { "sku", "string", "Stock Keeping Unit" },
        { "sold", "number", "Number of items sold" },
    };
    int info_count = (sizeof(info) / sizeof(info[0]));
    ...
    xo_set_info(NULL, info, info_count);

Third, the emission of info must be triggered with the XOF_INFO flag
using either the xo_set_flags() function or the "--libxo=info" command
line argument.

The type and help values, if present, are emitted as the "data-type"
and "data-help" attributes:

  <div class="data" data-tag="sku" data-type="string" 
       data-help="Stock Keeping Unit">GRO-000-533</div>

*** Memory Allocation

The xo_set_allocator function allows libxo to be used in environments
where the standard realloc() and free() functions are not available.

    void xo_set_allocator (xo_realloc_func_t realloc_func,
                           xo_free_func_t free_func);

realloc_func should expect the same arguments as realloc(3) and return
a pointer to memory following the same convention.  free_func will
receive the same argument as free(3) and should release it, as
appropriate for the environment.

By default, the standard realloc() and free() functions are used.

*** LIBXO_OPTIONS @LIBXO_OPTIONS@

The environment variable "LIBXO_OPTIONS" can be set to a string of
options:

|--------+---------------------------------------------|
| Option | Action                                      |
|--------+---------------------------------------------|
| c      | Enable color/effects for TEXT/HTML          |
| F      | Force line-buffered flushing                |
| H      | Enable HTML output (XO_STYLE_HTML)          |
| I      | Enable info output (XOF_INFO)               |
| i<num> | Indent by <number>                          |
| J      | Enable JSON output (XO_STYLE_JSON)          |
| k      | Add keys to XPATH expressions in HTML       |
| n      | Disable humanization (TEXT, HTML)           |
| P      | Enable pretty-printed output (XOF_PRETTY)   |
| T      | Enable text output (XO_STYLE_TEXT)          |
| U      | Add units to HTML output                    |
| u      | Change "-"s to "_"s in element names (JSON) |
| W      | Enable warnings (XOF_WARN)                  |
| X      | Enable XML output (XO_STYLE_XML)            |
| x      | Enable XPath data (XOF_XPATH)               |
|--------+---------------------------------------------|

For example, warnings can be enabled by:

    % env LIBXO_OPTIONS=W my-app

Complete HTML output can be generated with:

    % env LIBXO_OPTIONS=HXI my-app

Since environment variables are inherited, child processes will have
the same options, which may be undesirable, making the use of the
"--libxo" option is preferable in most situations.

*** Errors, Warnings, and Messages

Many programs make use of the standard library functions err() and
warn() to generate errors and warnings for the user.  libxo wants to
pass that information via the current output style, and provides
compatible functions to allow this:

    void xo_warn (const char *fmt, ...);
    void xo_warnx (const char *fmt, ...);
    void xo_warn_c (int code, const char *fmt, ...);
    void xo_warn_hc (xo_handle_t *xop, int code,
                     const char *fmt, ...);
    void xo_err (int eval, const char *fmt, ...);
    void xo_errc (int eval, int code, const char *fmt, ...);
    void xo_errx (int eval, const char *fmt, ...);
    void xo_message (const char *fmt, ...);
    void xo_message_c (int code, const char *fmt, ...);
    void xo_message_hc (xo_handle_t *xop, int code,
                        const char *fmt, ...);
    void xo_message_hcv (xo_handle_t *xop, int code, 
                         const char *fmt, va_list vap);

These functions display the program name, a colon, a formatted message
based on the arguments, and then optionally a colon and an error
message associated with either "errno" or the "code" parameter.

    EXAMPLE:
        if (open(filename, O_RDONLY) < 0)
            xo_err(1, "cannot open file '%s'", filename);

*** xo_error

The xo_error function can be used for generic errors that should be
reported over the handle, rather than to stderr.  The xo_error
function behaves like xo_err for TEXT and HTML output styles, but puts
the error into XML or JSON elements:

    EXAMPLE::
        xo_error("Does not %s", "compute");
    XML::
        <error><message>Does not compute</message></error>
    JSON::
        "error": { "message": "Does not compute" }

*** xo_no_setlocale

libxo automatically initializes the locale based on setting of the
environment variables LC_CTYPE, LANG, and LC_ALL.  The first of this
list of variables is used and if none of the variables, the locale
defaults to "UTF-8".  The caller may wish to avoid this behavior, and
can do so by calling the xo_no_setlocale() function.

    void xo_no_setlocale (void);

** Emitting syslog Messages

syslog is the system logging facility used throughout the unix world.
Messages are sent from commands, applications, and daemons to a
hierarchy of servers, where they are filtered, saved, and forwarded
based on configuration behaviors.

syslog is an older protocol, originally documented only in source
code.  By the time RFC 3164 published, variation and mutation left the
leading "<pri>" string as only common content.  RFC 5424 defines a new
version (version 1) of syslog and introduces structured data into the
messages.  Structured data is a set of name/value pairs transmitted
distinctly alongside the traditional text message, allowing filtering
on precise values instead of regular expressions.

These name/value pairs are scoped by a two-part identifier; an
enterprise identifier names the party responsible for the message
catalog and a name identifying that message.  Enterprise IDs are
defined by IANA, the Internet Assigned Numbers Authority:

https://www.iana.org/assignments/enterprise-numbers/enterprise-numbers

Use the ^xo_set_syslog_enterprise_id^() function to set the Enterprise
ID, as needed.

The message name should follow the conventions in ^good-field-names^,
as should the fields within the message.

    /* Both of these calls are optional */
    xo_set_syslog_enterprise_id(32473);
    xo_open_log("my-program", 0, LOG_DAEMON);

    /* Generate a syslog message */
    xo_syslog(LOG_ERR, "upload-failed",
              "error <%d> uploading file '{:filename}' "
              "as '{:target/%s:%s}'",
              code, filename, protocol, remote);

    xo_syslog(LOG_INFO, "poofd-invalid-state",
              "state {:current/%u} is invalid {:connection/%u}",
	      state, conn);

The developer should be aware that the message name may be used in the
future to allow access to further information, including
documentation.  Care should be taken to choose quality, descriptive
names.

*** Priority, Facility, and Flags @priority@

The xo_syslog, xo_vsyslog, and xo_open_log functions accept a set of
flags which provide the priority of the message, the source facility,
and some additional features.  These values are OR'd together to
create a single integer argument:

    xo_syslog(LOG_ERR | LOG_AUTH, "login-failed",
             "Login failed; user '{:user}' from host '{:address}'",
             user, addr);

These values are defined in <syslog.h>.

The priority value indicates the importance and potential impact of
each message.

|-------------+-------------------------------------------------------|
| Priority    | Description                                           |
|-------------+-------------------------------------------------------|
| LOG_EMERG   | A panic condition, normally broadcast to all users    |
| LOG_ALERT   | A condition that should be corrected immediately      |
| LOG_CRIT    | Critical conditions                                   |
| LOG_ERR     | Generic errors                                        |
| LOG_WARNING | Warning messages                                      |
| LOG_NOTICE  | Non-error conditions that might need special handling |
| LOG_INFO    | Informational messages                                |
| LOG_DEBUG   | Developer-oriented messages                           |
|-------------+-------------------------------------------------------|

The facility value indicates the source of message, in fairly generic
terms. 

|---------------+-------------------------------------------------|
| Facility      | Description                                     |
|---------------+-------------------------------------------------|
| LOG_AUTH      | The authorization system (e.g. login(1))        |
| LOG_AUTHPRIV  | As LOG_AUTH, but logged to a privileged file    |
| LOG_CRON      | The cron daemon: cron(8)                        |
| LOG_DAEMON    | System daemons, not otherwise explicitly listed |
| LOG_FTP       | The file transfer protocol daemons              |
| LOG_KERN      | Messages generated by the kernel                |
| LOG_LPR       | The line printer spooling system                |
| LOG_MAIL      | The mail system                                 |
| LOG_NEWS      | The network news system                         |
| LOG_SECURITY  | Security subsystems, such as ipfw(4)            |
| LOG_SYSLOG    | Messages generated internally by syslogd(8)     |
| LOG_USER      | Messages generated by user processes (default)  |
| LOG_UUCP      | The uucp system                                 |
| LOG_LOCAL0..7 | Reserved for local use                          |
|---------------+-------------------------------------------------|

In addition to the values listed above, xo_open_log accepts a set of
addition flags requesting specific behaviors.

|------------+----------------------------------------------------|
| Flag       | Description                                        |
|------------+----------------------------------------------------|
| LOG_CONS   | If syslogd fails, attempt to write to /dev/console |
| LOG_NDELAY | Open the connection to syslogd(8) immediately      |
| LOG_PERROR | Write the message also to standard error output    |
| LOG_PID    | Log the process id with each message               |
|------------+----------------------------------------------------|

*** xo_syslog

Use the xo_syslog function to generate syslog messages by calling it
with a log priority and facility, a message name, a format string, and
a set of arguments.  The priority/facility argument are discussed
above, as is the message name.

The format string follows the same conventions as xo_emit's format
string, with each field being rendered as an SD-PARAM pair.

    xo_syslog(LOG_ERR, "poofd-missing-file",
              "'{:filename}' not found: {:error/%m}", filename);

    ... [poofd-missing-file@32473 filename="/etc/poofd.conf"
          error="Permission denied"] '/etc/poofd.conf' not
          found: Permission denied

*** Support functions

**** xo_vsyslog

xo_vsyslog is identical in function to xo_syslog, but takes the set of
arguments using a va_list.

    void my_log (const char *name, const char *fmt, ...)
    {
        va_list vap;
        va_start(vap, fmt);
        xo_vsyslog(LOG_ERR, name, fmt, vap);
        va_end(vap);
    }

**** xo_open_log

xo_open_log functions similar to openlog(3), allowing customization of
the program name, the log facility number, and the additional option
flags described in ^priority^.

    void
    xo_open_log (const char *ident, int logopt, int facility);

**** xo_close_log

xo_close_log functions similar to closelog(3), closing the log file
and releasing any associated resources.

    void
    xo_close_log (void);

**** xo_set_logmask

xo_set_logmask function similar to setlogmask(3), restricting the set
of generated log event to those whose associated bit is set in
maskpri.  Use LOG_MASK(pri) to find the appropriate bit, or
LOG_UPTO(toppri) to create a mask for all priorities up to and
including toppri.

    int
    xo_set_logmask (int maskpri);

  Example:
    setlogmask(LOG_UPTO(LOG_WARN));

**** xo_set_syslog_enterprise_id

Use the xo_set_syslog_enterprise_id to supply a platform- or
application-specific enterprise id.  This value is used in any
future syslog messages.

Ideally, the operating system should supply a default value via the
"kern.syslog.enterprise_id" sysctl value.  Lacking that, the
application should provide a suitable value.

    void
    xo_set_syslog_enterprise_id (unsigned short eid);

Enterprise IDs are administered by IANA, the Internet Assigned Number
Authority.  The complete list is EIDs on their web site:

    https://www.iana.org/assignments/enterprise-numbers/enterprise-numbers

New EIDs can be requested from IANA using the following page:

    http://pen.iana.org/pen/PenApplication.page

Each software development organization that defines a set of syslog
messages should register their own EID and use that value in their
software to ensure that messages can be uniquely identified by the
combination of EID + message name.

** Creating Custom Encoders

The number of encoding schemes in current use is staggering, with new
and distinct schemes appearing daily.  While libxo provide XML, JSON,
HMTL, and text natively, there are requirements for other encodings.

Rather than bake support for all possible encoders into libxo, the API
allows them to be defined externally.  libxo can then interfaces with
these encoding modules using a simplistic API.  libxo processes all
functions calls, handles state transitions, performs all formatting,
and then passes the results as operations to a customized encoding
function, which implements specific encoding logic as required.  This
means your encoder doesn't need to detect errors with unbalanced
open/close operations but can rely on libxo to pass correct data.

By making a simple API, libxo internals are not exposed, insulating the
encoder and the library from future or internal changes.

The three elements of the API are:
- loading
- initialization
- operations

The following sections provide details about these topics.

libxo source contain an encoder for Concise Binary Object
Representation, aka CBOR (RFC 7049) which can be used as used as an
example for the API.

*** Loading Encoders

Encoders can be registered statically or discovered dynamically.
Applications can choose to call the xo_encoder_register()
function to explicitly register encoders, but more typically they are
built as shared libraries, placed in the libxo/extensions directory,
and loaded based on name.  libxo looks for a file with the name of the encoder
and an extension of ".enc".  This can be a file or a symlink to the
shared library file that supports the encoder.

    % ls -1 lib/libxo/extensions/*.enc
    lib/libxo/extensions/cbor.enc
    lib/libxo/extensions/test.enc

*** Encoder Initialization

Each encoder must export a symbol used to access the library, which
must have the following signature:

    int xo_encoder_library_init (XO_ENCODER_INIT_ARGS);

XO_ENCODER_INIT_ARGS is a macro defined in xo_encoder.h that defines
an argument called "arg", a pointer of the type
xo_encoder_init_args_t.  This structure contains two fields:

- xei_version is the version number of the API as implemented within
libxo.  This version is currently as 1 using XO_ENCODER_VERSION.  This
number can be checked to ensure compatibility.  The working assumption
is that all versions should be backward compatible, but each side may
need to accurately know the version supported by the other side.  
xo_encoder_library_init can optionally check this value, and must then
set it to the version number used by the encoder, allowing libxo to
detect version differences and react accordingly.  For example, if
version 2 adds new operations, then libxo will know that an encoding
library that set xei_version to 1 cannot be expected to handle those
new operations.

- xei_handler must be set to a pointer to a function of type
xo_encoder_func_t, as defined in xo_encoder.h.  This function
takes a set of parameters:
-- xop is a pointer to the opaque xo_handle_t structure
-- op is an integer representing the current operation
-- name is a string whose meaning differs by operation
-- value is a string whose meaning differs by operation
-- private is an opaque structure provided by the encoder

Additional arguments may be added in the future, so handler functions
should use the XO_ENCODER_HANDLER_ARGS macro.  An appropriate
"extern" declaration is provided to help catch errors.

Once the encoder initialization function has completed processing, it
should return zero to indicate that no error has occurred.  A non-zero
return code will cause the handle initialization to fail.

*** Operations

The encoder API defines a set of operations representing the
processing model of libxo.  Content is formatted within libxo, and
callbacks are made to the encoder's handler function when data is
ready to be processed.

|-----------------------+---------------------------------------|
| Operation             | Meaning  (Base function)              |
|-----------------------+---------------------------------------|
| XO_OP_CREATE          | Called when the handle is created     |
| XO_OP_OPEN_CONTAINER  | Container opened (xo_open_container)  |
| XO_OP_CLOSE_CONTAINER | Container closed (xo_close_container) |
| XO_OP_OPEN_LIST       | List opened (xo_open_list)            |
| XO_OP_CLOSE_LIST      | List closed (xo_close_list)           |
| XO_OP_OPEN_LEAF_LIST  | Leaf list opened (xo_open_leaf_list)  |
| XO_OP_CLOSE_LEAF_LIST | Leaf list closed (xo_close_leaf_list) |
| XO_OP_OPEN_INSTANCE   | Instance opened (xo_open_instance)    |
| XO_OP_CLOSE_INSTANCE  | Instance closed (xo_close_instance)   |
| XO_OP_STRING          | Field with Quoted UTF-8 string        |
| XO_OP_CONTENT         | Field with content                    |
| XO_OP_FINISH          | Finish any pending output             |
| XO_OP_FLUSH           | Flush any buffered output             |
| XO_OP_DESTROY         | Clean up resources                    |
| XO_OP_ATTRIBUTE       | An attribute name/value pair          |
| XO_OP_VERSION         | A version string                      |
|-----------------------+---------------------------------------|

For all the open and close operations, the name parameter holds the
name of the construct.  For string, content, and attribute operations,
the name parameter is the name of the field and the value parameter is
the value.  "string" are differentiated from "content" to allow differing
treatment of true, false, null, and numbers from real strings, though
content values are formatted as strings before the handler is called.
For version operations, the value parameter contains the version.

All strings are encoded in UTF-8.

* The "xo" Utility

The "xo" utility allows command line access to the functionality of
the libxo library.  Using "xo", shell scripts can emit XML, JSON, and
HTML using the same commands that emit text output.

The style of output can be selected using a specific option: "-X" for
XML, "-J" for JSON, "-H" for HTML, or "-T" for TEXT, which is the
default.  The "--style <style>" option can also be used.  The
LIBXO_OPTIONS environment variable can also be used to set the style,
as well as other flags.

The "xo" utility accepts a format string suitable for xo_emit() and a
set of zero or more arguments used to supply data for that string.

    xo "The {k:name} weighs {:weight/%d} pounds.\n" fish 6

  TEXT:
    The fish weighs 6 pounds.
  XML:
    <name>fish</name>
    <weight>6</weight>
  JSON:
    "name": "fish",
    "weight": 6
  HTML:
    <div class="line">
      <div class="text">The </div>
      <div class="data" data-tag="name">fish</div>
      <div class="text"> weighs </div>
      <div class="data" data-tag="weight">6</div>
      <div class="text"> pounds.</div>
    </div>

The "--wrap <path>" option can be used to wrap emitted content in a
specific hierarchy.  The path is a set of hierarchical names separated
by the '/' character.

    xo --wrap top/a/b/c '{:tag}' value

  XML:
    <top>
      <a>
        <b>
          <c>
            <tag>value</tag>
          </c>
        </b>
      </a>
    </top>
  JSON:
    "top": {
      "a": {
        "b": {
          "c": {
            "tag": "value"
          }
        }
      }
    }

The "--open <path>" and "--close <path>" can be used to emit
hierarchical information without the matching close and open
tag.  This allows a shell script to emit open tags, data, and
then close tags.  The "--depth" option may be used to set the
depth for indentation.  The "--leading-xpath" may be used to
prepend data to the XPath values used for HTML output style.

    #!/bin/sh
    xo --open top/data
    xo --depth 2 '{tag}' value
    xo --close top/data
  XML:
    <top>
      <data>
        <tag>value</tag>
      </data>
    </top>
  JSON:
    "top": {
      "data": {
        "tag": "value"
      }
    }

** Command Line Options

Usage: xo [options] format [fields]
  --close <path>        Close tags for the given path
  --depth <num>         Set the depth for pretty printing
  --help                Display this help text
  --html OR -H          Generate HTML output
  --json OR -J          Generate JSON output
  --leading-xpath <path> Add a prefix to generated XPaths (HTML)
  --open <path>         Open tags for the given path
  --pretty OR -p        Make 'pretty' output (add indent, newlines)
  --style <style>       Generate given style (xml, json, text, html)
  --text OR -T          Generate text output (the default style)
  --version             Display version information
  --warn OR -W          Display warnings in text on stderr
  --warn-xml            Display warnings in xml on stdout
  --wrap <path>         Wrap output in a set of containers
  --xml OR -X           Generate XML output
  --xpath               Add XPath data to HTML output);

** Example

  % xo 'The {:product} is {:status}\n' stereo "in route"
  The stereo is in route
  % ./xo/xo -p -X 'The {:product} is {:status}\n' stereo "in route"
  <product>stereo</product>
  <status>in route</status>

* xolint

xolint is a tool for reporting common mistakes in format strings
in source code that invokes xo_emit().  It allows these errors
to be diagnosed at build time, rather than waiting until runtime.

xolint takes the one or more C files as arguments, and reports
and errors, warning, or informational messages as needed.

|------------+---------------------------------------------------|
| Option     | Meaning                                           |
|------------+---------------------------------------------------|
| -c         | Invoke 'cpp' against the input file               |
| -C <flags> | Flags that are passed to 'cpp                     |
| -d         | Enable debug output                               |
| -D         | Generate documentation for all xolint messages    |
| -I         | Generate info table code                          |
| -p         | Print the offending lines after the message       |
| -V         | Print vocabulary of all field names               |
| -X         | Extract samples from xolint, suitable for testing |
|------------+---------------------------------------------------|

The output message will contain the source filename and line number, the
class of the message, the message, and, if -p is given, the
line that contains the error:

    % xolint.pl -t xolint.c
    xolint.c: 16: error: anchor format should be "%d"
    16         xo_emit("{[:/%s}");

The "-I" option will generate a table of xo_info_t structures , 

The "-V" option does not report errors, but prints a complete list of
all field names, sorted alphabetically.  The output can help spot
inconsistencies and spelling errors.

* xohtml @xohtml@

xohtml is a tool for turning the output of libxo-enabled commands into
html files suitable for display in modern HTML web browsers.  It can
be used to test and debug HTML output, as well as to make the user
ache to escape the world of 70s terminal devices.

xohtml is given a command, either on the command line or via the "-c"
option.  If not command is given, standard input is used.  The
command's output is wrapped in HTML tags, with references to
supporting CSS and Javascript files, and written to standard output or
the file given in the "-f" option.  The "-b" option can be used to
provide an alternative base path for the support files.

|--------------+---------------------------------------------------|
| Option       | Meaning                                           |
|--------------+---------------------------------------------------|
| -b <base>    | Base path for finding css/javascript files        |
| -c <command> | Command to execute                                |
| -f <file>    | Output file name                                  |
|--------------+---------------------------------------------------|

The "-c" option takes a full command with arguments, including
any libxo options needed to generate html ("--libxo=html").  This
value must be quoted if it consists of multiple tokens.

* xopo

The "xopo" utility filters ".pot" files generated by the "xgettext"
utility to remove formatting information suitable for use with
the "{G:}" modifier.  This means that when the developer changes the
formatting portion of the field definitions, or the fields modifiers,
the string passed to gettext(3) is unchanged, avoiding the expense of
updating any existing translation files (".po" files).

The syntax for the xopo command is one of two forms; it can be used as
a filter for processing a .po or .pot file, rewriting the "msgid"
strings with a simplified message string.  In this mode, the input is
either standard input or a file given by the "-f" option, and the
output is either standard output or a file given by the "-o" option.

In the second mode, a simple message given using the "-s" option on
the command, and the simplified version of that message is printed on
stdout.

|-----------+---------------------------------|
| Option    | Meaning                         |
|-----------+---------------------------------|
| -o <file> | Output file name                |
| -f <file> | Use the given .po file as input |
| -s <text> | Simplify a format string        |
|-----------+---------------------------------|

    EXAMPLE:
        % xopo -s "There are {:count/%u} {:event/%.6s} events\n"
        There are {:count} {:event} events\n

	% xgettext --default-domain=foo --no-wrap \
	    --add-comments --keyword=xo_emit --keyword=xo_emit_h \
	    --keyword=xo_emit_warn -C -E -n --foreign-user \
	    -o foo.pot.raw foo.c
        % xopo -f foo.pot.raw -o foo.pot

Use of the "--no-wrap" option for xgettext is required to ensure that
incoming msgid strings are not wrapped across multiple lines.

* FAQs

This section contains the set of questions that users typically ask,
along with answers that might be helpful.

!! list-sections

** General

*** Can you share the history of libxo?

In 2001, we added an XML API to the JUNOS operating system, which is
built on top of FreeBSD.  Eventually this API became standardized as
the NETCONF API (RFC 6241).  As part of this effort, we modified many
FreeBSD utilities to emit XML, typically via a "-X" switch.  The
results were mixed.  The cost of maintaining this code, updating it
and carrying it were non-trivial, and contributed to our expense (and
the associated delay) with upgrading the version of FreeBSD on which
each release of JUNOS is based.

A recent (2014) effort within JUNOS aims at removing our modifications
to the underlying FreeBSD code as a means of reducing the expense and
delay.  JUNOS is structured to have system components generate XML
that is rendered by the CLI (think: login shell) into human-readable
text.  This allows the API to use the same plumbing as the CLI, and
ensures that all components emit XML, and that it is emitted with
knowledge of the consumer of that XML, yielding an API that have no
incremental cost or feature delay.

libxo is an effort to mix the best aspects of the JUNOS strategy into
FreeBSD in a seemless way, allowing commands to make printf-like
output calls without needing to care how the output is rendered.

*** Did the complex semantics of format strings evolve over time?

The history is both long and short: libxo's functionality is based
on what JUNOS does in a data modeling language called ODL (output
definition language).  In JUNOS, all subcomponents generate XML,
which is feed to the CLI, where data from the ODL files tell is
how to render that XML into text.  ODL might had a set of tags
like:

     tag docsis-state {
         help "State of the DOCSIS interface";
         type string;
     }

     tag docsis-mode {
         help "DOCSIS mode (2.0/3.0) of the DOCSIS interface";
         type string;
     }

     tag docsis-upstream-speed {
         help "Operational upstream speed of the interface";
         type string;
     }

     tag downstream-scanning {
         help "Result of scanning in downstream direction";
         type string;
     }

     tag ranging {
         help "Result of ranging action";
         type string;
     }

     tag signal-to-noise-ratio {
         help "Signal to noise ratio for all channels";
         type string;
     }

     tag power {
         help "Operational power of the signal on all channels";
         type string;
     }

     format docsis-status-format {
         picture "
   State   : @, Mode: @, Upstream speed: @
   Downstream scanning: @, Ranging: @
   Signal to noise ratio: @
   Power: @
";
         line {
             field docsis-state;
             field docsis-mode;
             field docsis-upstream-speed;
             field downstream-scanning;
             field ranging;
             field signal-to-noise-ratio;
             field power;
         }
     }

These tag definitions are compiled into field definitions
that are triggered when matching XML elements are seen.  ODL
also supports other means of defining output.

The roles and modifiers describe these details.

In moving these ideas to bsd, two things had to happen: the
formatting had to happen at the source since BSD won't have
a JUNOS-like CLI to do the rendering, and we can't depend on
external data models like ODL, which was seen as too hard a
sell to the BSD community.

The results were that the xo_emit strings are used to encode the
roles, modifiers, names, and formats.  They are dense and a bit
cryptic, but not so unlike printf format strings that developers will
be lost.

libxo is a new implementation of these ideas and is distinct from
the previous implementation in JUNOS.

*** What makes a good field name? @good-field-names@

To make useful, consistent field names, follow these guidelines:

= Use lower case, even for TLAs
Lower case is more civilized.  Even TLAs should be lower case
to avoid scenarios where the differences between "XPath" and
"Xpath" drive your users crazy.  Using "xpath" is simpler and better.
= Use hyphens, not underscores
Use of hyphens is traditional in XML, and the XOF_UNDERSCORES
flag can be used to generate underscores in JSON, if desired.
But the raw field name should use hyphens.
= Use full words
Don't abbreviate especially when the abbreviation is not obvious or
not widely used.  Use "data-size", not "dsz" or "dsize".  Use
"interface" instead of "ifname", "if-name", "iface", "if", or "intf".
= Use <verb>-<units>
Using the form <verb>-<units> or <verb>-<classifier>-<units> helps in
making consistent, useful names, avoiding the situation where one app
uses "sent-packet" and another "packets-sent" and another
"packets-we-have-sent".  The <units> can be dropped when it is
obvious, as can obvious words in the classification.
Use "receive-after-window-packets" instead of
"received-packets-of-data-after-window".
= Reuse existing field names
Nothing's worse than writing expressions like:

    if ($src1/process[pid == $pid]/name == 
        $src2/proc-table/proc-list
                   /proc-entry[process-id == $pid]/proc-name) {
        ...
    }

Find someone else who is expressing similar data and follow their
fields and hierarchy.  Remember the quote is not "Consistency is the
hobgoblin of little minds", but "A foolish consistency is the
hobgoblin of little minds".
= Use containment as scoping
In the previous example, all the names are prefixed with "proc-",
which is redundant given that they are nested under the process table.
= Think about your users
Have empathy for your users, choosing clear and useful fields that
contain clear and useful data.  You may need to augment the display
content with xo_attr() calls (^xo_attr^) or "{e:}" fields
(^e-modifier^) to make the data useful.
= Don't use an arbitrary number postfix
What does "errors2" mean?  No one will know.  "errors-after-restart"
would be a better choice.  Think of your users, and think of the
future.  If you make "errors2", the next guy will happily make
"errors3" and before you know it, someone will be asking what's the
difference between errors37 and errors63.
= Be consistent, uniform, unsurprising, and predictable
Think of your field vocabulary as an API.  You want it useful,
expressive, meaningful, direct, and obvious.  You want the client
application's programmer to move between without the need to
understand a variety of opinions on how fields are named.  They should
see the system as a single cohesive whole, not a sack of cats.

Field names constitute the means by which client programmers interact
with our system.  By choosing wise names now, you are making their
lives better.

After using "xolint" to find errors in your field descriptors, use
"xolint -V" to spell check your field names and to detect different
names for the same data.  "dropped-short" and "dropped-too-short" are
both reasonable names, but using them both will lead users to ask the
difference between the two fields.  If there is no difference,
use only one of the field names.  If there is a difference, change the
names to make that difference more obvious.

** What does this message mean?

!!include-file xolint.txt

* Howtos: Focused Directions

This section provides task-oriented instructions for selected tasks.
If you have a task that needs instructions, please open a request as
an enhancement issue on github.

** Howto: Report bugs

libxo uses github to track bugs or request enhancements.  Please use
the following URL:

  https://github.com/Juniper/libxo/issues

** Howto: Install libxo

libxo is open source, under a new BSD license.  Source code is
available on github, as are recent releases.  To get the most
current release, please visit:

  https://github.com/Juniper/libxo/releases

After downloading and untarring the source code, building involves the
following steps:

    sh bin/setup.sh
    cd build
    ../configure
    make
    make test
    sudo make install

libxo uses a distinct "build" directory to keep generated files
separated from source files.

Use "../configure --help" to display available configuration options,
which include the following:

  --enable-warnings      Turn on compiler warnings
  --enable-debug         Turn on debugging
  --enable-text-only     Turn on text-only rendering
  --enable-printflike    Enable use of GCC __printflike attribute
  --disable-libxo-options  Turn off support for LIBXO_OPTIONS
  --with-gettext=PFX     Specify location of gettext installation
  --with-libslax-prefix=PFX  Specify location of libslax config

Compiler warnings are a very good thing, but recent compiler version
have added some very pedantic checks.  While every attempt is made to
keep libxo code warning-free, warnings are now optional.  If you are
doing development work on libxo, it is required that you use
--enable-warnings to keep the code warning free, but most users need
not use this option.

libxo provides the --enable-text-only option to reduce the footprint
of the library for smaller installations.  XML, JSON, and HTML
rendering logic is removed.

The gettext library does not provide a simple means of learning its
location, but libxo will look for it in /usr and /opt/local.  If
installed elsewhere, the installer will need to provide this
information using the --with-gettext=/dir/path option.

libslax is not required by libxo; it contains the "oxtradoc" program
used to format documentation.

For additional information, see ^building-libxo^.

** Howto: Convert command line applications

    How do I convert an existing command line application?

There are three basic steps for converting command line application to
use libxo.

- Setting up the context
- Converting printf calls
- Creating hierarchy
- Converting error functions

*** Setting up the context

To use libxo, you'll need to include the "xo.h" header file in your
source code files:

    #include <libxo/xo.h>

In your main() function, you'll need to call xo_parse_args to handling
argument parsing (^xo_parse_args^).  This function removes
libxo-specific arguments the program's argv and returns either the
number of remaining arguments or -1 to indicate an error.

    int main (int argc, char **argv)
    {
        argc = xo_parse_args(argc, argv);
        if (argc < 0)
            return argc;
        ....
    }

At the bottom of your main(), you'll need to call xo_finish() to
complete output processing for the default handle (^handles^).  libxo
provides the xo_finish_atexit function that is suitable for use with
the atexit(3) function.

    atexit(xo_finish_atexit);

*** Converting printf Calls

The second task is inspecting code for printf(3) calls and replacing
them with xo_emit() calls.  The format strings are similar in task,
but libxo format strings wrap output fields in braces.  The following
two calls produce identical text output:

    printf("There are %d %s events\n", count, etype);
    xo_emit("There are {:count/%d} {:event} events\n", count, etype);

"count" and "event" are used as names for JSON and XML output.  The
"count" field uses the format "%d" and "event" uses the default "%s"
format.  Both are "value" roles, which is the default role.

Since text outside of output fields is passed verbatim, other roles
are less important, but their proper use can help make output more
useful.  The "note" and "label" roles allow HTML output to recognize
the relationship between text and the associated values, allowing
appropriate "hover" and "onclick" behavior.  Using the "units" role
allows the presentation layer to perform conversions when needed.  The
"warning" and "error" roles allows use of color and font to draw
attention to warnings.  The "padding" role makes the use of vital
whitespace more clear (^padding-role^).

The "title" role indicates the headings of table and sections.  This
allows HTML output to use CSS to make this relationship more obvious.

    printf("Statistics:\n");
    xo_emit("{T:Statistics}:\n");

The "color" roles controls foreground and background colors, as well
as effects like bold and underline (see ^color-role^).

    xo_emit("{C:bold}required{C:}\n");

Finally, the start- and stop-anchor roles allow justification and
padding over multiple fields (see ^anchor-role^).

    snprintf(buf, sizeof(buf), "(%u/%u/%u)", min, ave, max);
    printf("%30s", buf);
   
    xo_emit("{[:30}({:minimum/%u}/{:average/%u}/{:maximum/%u}{]:}",
            min, ave, max);

*** Creating Hierarchy

Text output doesn't have any sort of hierarchy, but XML and JSON
require this.  Typically applications use indentation to represent
these relationship:

    printf("table %d\n", tnum);
    for (i = 0; i < tmax; i++) {
        printf("    %s %d\n", table[i].name, table[i].size);
    }

    xo_emit("{T:/table %d}\n", tnum);
    xo_open_list("table");
    for (i = 0; i < tmax; i++) {
        xo_open_instance("table");
        xo_emit("{P:    }{k:name} {:size/%d}\n",
                table[i].name, table[i].size);
        xo_close_instance("table");
    }
    xo_close_list("table");

The open and close list functions are used before and after the list,
and the open and close instance functions are used before and after
each instance with in the list.

Typically these developer looks for a "for" loop as an indication of
where to put these calls.

In addition, the open and close container functions allow for
organization levels of hierarchy.

    printf("Paging information:\n");
    printf("    Free:      %lu\n", free);
    printf("    Active:    %lu\n", active);
    printf("    Inactive:  %lu\n", inactive);

    xo_open_container("paging-information");
    xo_emit("{P:    }{L:Free:      }{:free/%lu}\n", free);
    xo_emit("{P:    }{L:Active:    }{:active/%lu}\n", active);
    xo_emit("{P:    }{L:Inactive:  }{:inactive/%lu}\n", inactive);
    xo_close_container("paging-information");

*** Converting Error Functions

libxo provides variants of the standard error and warning functions,
err(3) and warn(3).  There are two variants, one for putting the
errors on standard error, and the other writes the errors and warnings
to the handle using the appropriate encoding style:

    err(1, "cannot open output file: %s", file);

    xo_err(1, "cannot open output file: %s", file);
    xo_emit_err(1, "cannot open output file: {:filename}", file);

** Howto: Use "xo" in Shell Scripts

** Howto: Internationalization (i18n) @howto-i18n@

    How do I use libxo to support internationalization?

libxo allows format and field strings to be used a keys into message
catalogs to enable translation into a user's native language by
invoking the standard gettext(3) functions.

gettext setup is a bit complicated: text strings are extracted from
source files into "portable object template" (.pot) files using the
"xgettext" command.  For each language, this template file is used as
the source for a message catalog in the "portable object" (.po)
format, which are translated by hand and compiled into "machine
object" (.mo) files using the "msgfmt" command.  The .mo files are
then typically installed in the /usr/share/locale or
/opt/local/share/locale directories.  At run time, the user's language
settings are used to select a .mo file which is searched for matching
messages.  Text strings in the source code are used as keys to look up
the native language strings in the .mo file.

Since the xo_emit format string is used as the key into the message
catalog, libxo removes unimportant field formatting and modifiers from
the format string before use so that minor formatting changes will not
impact the expensive translation process.  We don't want a developer
change such as changing "/%06d" to "/%08d" to force hand inspection of
all .po files.  The simplified version can be generated for a single
message using the "xopo -s <text>" command, or an entire .pot can be
translated using the "xopo -f <input> -o <output>" command.

    EXAMPLE:
        % xopo -s "There are {:count/%u} {:event/%.6s} events\n"
        There are {:count} {:event} events\n

    Recommended workflow:
        # Extract text messages
	xgettext --default-domain=foo --no-wrap \
	    --add-comments --keyword=xo_emit --keyword=xo_emit_h \
	    --keyword=xo_emit_warn -C -E -n --foreign-user \
	    -o foo.pot.raw foo.c

        # Simplify format strings for libxo
        xopo -f foo.pot.raw -o foo.pot

        # For a new language, just copy the file
        cp foo.pot po/LC/my_lang/foo.po

        # For an existing language:
        msgmerge --no-wrap po/LC/my_lang/foo.po \
                foo.pot -o po/LC/my_lang/foo.po.new

        # Now the hard part: translate foo.po using tools
        # like poedit or emacs' po-mode

        # Compile the finished file; Use of msgfmt's "-v" option is
        # strongly encouraged, so that "fuzzy" entries are reported.
        msgfmt -v -o po/my_lang/LC_MESSAGES/foo.mo po/my_lang/foo.po

        # Install the .mo file
        sudo cp po/my_lang/LC_MESSAGES/foo.mo \
                /opt/local/share/locale/my_lang/LC_MESSAGE/

Once these steps are complete, you can use the "gettext" command to
test the message catalog:

    gettext -d foo -e "some text"

*** i18n and xo_emit

There are three features used in libxo used to support i18n:

- The "{G:}" role looks for a translation of the format string.
- The "{g:}" modifier looks for a translation of the field.
- The "{p:}" modifier looks for a pluralized version of the field.

Together these three flags allows a single function call to give
native language support, as well as libxo's normal XML, JSON, and HTML
support.

    printf(gettext("Received %zu %s from {g:server} server\n"),
           counter, ngettext("byte", "bytes", counter),
           gettext("web"));

    xo_emit("{G:}Received {:received/%zu} {Ngp:byte,bytes} "
            "from {g:server} server\n", counter, "web");

libxo will see the "{G:}" role and will first simplify the format
string, removing field formats and modifiers.

    "Received {:received} {N:byte,bytes} from {:server} server\n"

libxo calls gettext(3) with that string to get a localized version.
If your language were Pig Latin, the result might look like:

    "Eceivedray {:received} {N:byte,bytes} omfray "
               "{:server} erversay\n"

Note the field names do not change and they should not be translated.
The contents of the note ("byte,bytes") should also not be translated,
since the "g" modifier will need the untranslated value as the key for
the message catalog.

The field "{g:server}" requests the rendered value of the field be
translated using gettext(3).  In this example, "web" would be used.

The field "{Ngp:byte,bytes}" shows an example of plural form using the
"p" modifier with the "g" modifier.  The base singular and plural
forms appear inside the field, separated by a comma.  At run time,
libxo uses the previous field's numeric value to decide which form to
use by calling ngettext(3).

If a domain name is needed, it can be supplied as the content of the
{G:} role.  Domain names remain in use throughout the format string
until cleared with another domain name.

    printf(dgettext("dns", "Host %s not found: %d(%s)\n"),
        name, errno, dgettext("strerror", strerror(errno)));

    xo_emit("{G:dns}Host {:hostname} not found: "
            "%d({G:strerror}{g:%m})\n", name, errno);

* Examples

** Unit Test

Here is the unit test example:

    int
    main (int argc, char **argv)
    {
        static char base_grocery[] = "GRO";
        static char base_hardware[] = "HRD";
        struct item {
            const char *i_title;
            int i_sold;
            int i_instock;
            int i_onorder;
            const char *i_sku_base;
            int i_sku_num;
        };
        struct item list[] = {
            { "gum", 1412, 54, 10, base_grocery, 415 },
            { "rope", 85, 4, 2, base_hardware, 212 },
            { "ladder", 0, 2, 1, base_hardware, 517 },
            { "bolt", 4123, 144, 42, base_hardware, 632 },
            { "water", 17, 14, 2, base_grocery, 2331 },
            { NULL, 0, 0, 0, NULL, 0 }
        };
        struct item list2[] = {
            { "fish", 1321, 45, 1, base_grocery, 533 },
        };
        struct item *ip;
        xo_info_t info[] = {
            { "in-stock", "number", "Number of items in stock" },
            { "name", "string", "Name of the item" },
            { "on-order", "number", "Number of items on order" },
            { "sku", "string", "Stock Keeping Unit" },
            { "sold", "number", "Number of items sold" },
            { NULL, NULL, NULL },
        };
        int info_count = (sizeof(info) / sizeof(info[0])) - 1;
        
        argc = xo_parse_args(argc, argv);
        if (argc < 0)
            exit(EXIT_FAILURE);
    
        xo_set_info(NULL, info, info_count);
    
        xo_open_container_h(NULL, "top");
    
        xo_open_container("data");
        xo_open_list("item");
    
        for (ip = list; ip->i_title; ip++) {
            xo_open_instance("item");
    
            xo_emit("{L:Item} '{k:name/%s}':\n", ip->i_title);
            xo_emit("{P:   }{L:Total sold}: {n:sold/%u%s}\n",
                    ip->i_sold, ip->i_sold ? ".0" : "");
            xo_emit("{P:   }{Lwc:In stock}{:in-stock/%u}\n", 
                    ip->i_instock);
            xo_emit("{P:   }{Lwc:On order}{:on-order/%u}\n", 
                    ip->i_onorder);
            xo_emit("{P:   }{L:SKU}: {q:sku/%s-000-%u}\n",
                    ip->i_sku_base, ip->i_sku_num);
    
            xo_close_instance("item");
        }
    
        xo_close_list("item");
        xo_close_container("data");
    
        xo_open_container("data");
        xo_open_list("item");
    
        for (ip = list2; ip->i_title; ip++) {
            xo_open_instance("item");
    
            xo_emit("{L:Item} '{:name/%s}':\n", ip->i_title);
            xo_emit("{P:   }{L:Total sold}: {n:sold/%u%s}\n",
                    ip->i_sold, ip->i_sold ? ".0" : "");
            xo_emit("{P:   }{Lwc:In stock}{:in-stock/%u}\n", 
                    ip->i_instock);
            xo_emit("{P:   }{Lwc:On order}{:on-order/%u}\n", 
                    ip->i_onorder);
            xo_emit("{P:   }{L:SKU}: {q:sku/%s-000-%u}\n",
                    ip->i_sku_base, ip->i_sku_num);
    
            xo_close_instance("item");
        }
    
        xo_close_list("item");
        xo_close_container("data");
    
        xo_close_container_h(NULL, "top");
    
        return 0;
    }

Text output:

    % ./testxo --libxo text
    Item 'gum':
       Total sold: 1412.0
       In stock: 54
       On order: 10
       SKU: GRO-000-415
    Item 'rope':
       Total sold: 85.0
       In stock: 4
       On order: 2
       SKU: HRD-000-212
    Item 'ladder':
       Total sold: 0
       In stock: 2
       On order: 1
       SKU: HRD-000-517
    Item 'bolt':
       Total sold: 4123.0
       In stock: 144
       On order: 42
       SKU: HRD-000-632
    Item 'water':
       Total sold: 17.0
       In stock: 14
       On order: 2
       SKU: GRO-000-2331
    Item 'fish':
       Total sold: 1321.0
       In stock: 45
       On order: 1
       SKU: GRO-000-533

JSON output:

    % ./testxo --libxo json,pretty
    "top": {
      "data": {
        "item": [
          {
            "name": "gum",
            "sold": 1412.0,
            "in-stock": 54,
            "on-order": 10,
            "sku": "GRO-000-415"
          },
          {
            "name": "rope",
            "sold": 85.0,
            "in-stock": 4,
            "on-order": 2,
            "sku": "HRD-000-212"
          },
          {
            "name": "ladder",
            "sold": 0,
            "in-stock": 2,
            "on-order": 1,
            "sku": "HRD-000-517"
          },
          {
            "name": "bolt",
            "sold": 4123.0,
            "in-stock": 144,
            "on-order": 42,
            "sku": "HRD-000-632"
          },
          {
            "name": "water",
            "sold": 17.0,
            "in-stock": 14,
            "on-order": 2,
            "sku": "GRO-000-2331"
          }
        ]
      },
      "data": {
        "item": [
          {
            "name": "fish",
            "sold": 1321.0,
            "in-stock": 45,
            "on-order": 1,
            "sku": "GRO-000-533"
          }
        ]
      }
    }

XML output:

    % ./testxo --libxo pretty,xml
    <top>
      <data>
        <item>
          <name>gum</name>
          <sold>1412.0</sold>
          <in-stock>54</in-stock>
          <on-order>10</on-order>
          <sku>GRO-000-415</sku>
        </item>
        <item>
          <name>rope</name>
          <sold>85.0</sold>
          <in-stock>4</in-stock>
          <on-order>2</on-order>
          <sku>HRD-000-212</sku>
        </item>
        <item>
          <name>ladder</name>
          <sold>0</sold>
          <in-stock>2</in-stock>
          <on-order>1</on-order>
          <sku>HRD-000-517</sku>
        </item>
        <item>
          <name>bolt</name>
          <sold>4123.0</sold>
          <in-stock>144</in-stock>
          <on-order>42</on-order>
          <sku>HRD-000-632</sku>
        </item>
        <item>
          <name>water</name>
          <sold>17.0</sold>
          <in-stock>14</in-stock>
          <on-order>2</on-order>
          <sku>GRO-000-2331</sku>
        </item>
      </data>
      <data>
        <item>
          <name>fish</name>
          <sold>1321.0</sold>
          <in-stock>45</in-stock>
          <on-order>1</on-order>
          <sku>GRO-000-533</sku>
        </item>
      </data>
    </top>

HMTL output:

    % ./testxo --libxo pretty,html
    <div class="line">
      <div class="label">Item</div>
      <div class="text"> '</div>
      <div class="data" data-tag="name">gum</div>
      <div class="text">':</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">Total sold</div>
      <div class="text">: </div>
      <div class="data" data-tag="sold">1412.0</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">In stock</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="in-stock">54</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">On order</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="on-order">10</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">SKU</div>
      <div class="text">: </div>
      <div class="data" data-tag="sku">GRO-000-415</div>
    </div>
    <div class="line">
      <div class="label">Item</div>
      <div class="text"> '</div>
      <div class="data" data-tag="name">rope</div>
      <div class="text">':</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">Total sold</div>
      <div class="text">: </div>
      <div class="data" data-tag="sold">85.0</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">In stock</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="in-stock">4</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">On order</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="on-order">2</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">SKU</div>
      <div class="text">: </div>
      <div class="data" data-tag="sku">HRD-000-212</div>
    </div>
    <div class="line">
      <div class="label">Item</div>
      <div class="text"> '</div>
      <div class="data" data-tag="name">ladder</div>
      <div class="text">':</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">Total sold</div>
      <div class="text">: </div>
      <div class="data" data-tag="sold">0</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">In stock</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="in-stock">2</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">On order</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="on-order">1</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">SKU</div>
      <div class="text">: </div>
      <div class="data" data-tag="sku">HRD-000-517</div>
    </div>
    <div class="line">
      <div class="label">Item</div>
      <div class="text"> '</div>
      <div class="data" data-tag="name">bolt</div>
      <div class="text">':</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">Total sold</div>
      <div class="text">: </div>
      <div class="data" data-tag="sold">4123.0</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">In stock</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="in-stock">144</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">On order</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="on-order">42</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">SKU</div>
      <div class="text">: </div>
      <div class="data" data-tag="sku">HRD-000-632</div>
    </div>
    <div class="line">
      <div class="label">Item</div>
      <div class="text"> '</div>
      <div class="data" data-tag="name">water</div>
      <div class="text">':</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">Total sold</div>
      <div class="text">: </div>
      <div class="data" data-tag="sold">17.0</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">In stock</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="in-stock">14</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">On order</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="on-order">2</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">SKU</div>
      <div class="text">: </div>
      <div class="data" data-tag="sku">GRO-000-2331</div>
    </div>
    <div class="line">
      <div class="label">Item</div>
      <div class="text"> '</div>
      <div class="data" data-tag="name">fish</div>
      <div class="text">':</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">Total sold</div>
      <div class="text">: </div>
      <div class="data" data-tag="sold">1321.0</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">In stock</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="in-stock">45</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">On order</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="on-order">1</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">SKU</div>
      <div class="text">: </div>
      <div class="data" data-tag="sku">GRO-000-533</div>
    </div>

HTML output with xpath and info flags:

    % ./testxo --libxo pretty,html,xpath,info
    <div class="line">
      <div class="label">Item</div>
      <div class="text"> '</div>
      <div class="data" data-tag="name"
           data-xpath="/top/data/item/name" data-type="string"
           data-help="Name of the item">gum</div>
      <div class="text">':</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">Total sold</div>
      <div class="text">: </div>
      <div class="data" data-tag="sold"
           data-xpath="/top/data/item/sold" data-type="number"
           data-help="Number of items sold">1412.0</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">In stock</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="in-stock"
           data-xpath="/top/data/item/in-stock" data-type="number"
           data-help="Number of items in stock">54</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">On order</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="on-order"
           data-xpath="/top/data/item/on-order" data-type="number"
           data-help="Number of items on order">10</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">SKU</div>
      <div class="text">: </div>
      <div class="data" data-tag="sku"
           data-xpath="/top/data/item/sku" data-type="string"
           data-help="Stock Keeping Unit">GRO-000-415</div>
    </div>
    <div class="line">
      <div class="label">Item</div>
      <div class="text"> '</div>
      <div class="data" data-tag="name"
           data-xpath="/top/data/item/name" data-type="string"
           data-help="Name of the item">rope</div>
      <div class="text">':</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">Total sold</div>
      <div class="text">: </div>
      <div class="data" data-tag="sold"
           data-xpath="/top/data/item/sold" data-type="number"
           data-help="Number of items sold">85.0</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">In stock</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="in-stock"
           data-xpath="/top/data/item/in-stock" data-type="number"
           data-help="Number of items in stock">4</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">On order</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="on-order"
           data-xpath="/top/data/item/on-order" data-type="number"
           data-help="Number of items on order">2</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">SKU</div>
      <div class="text">: </div>
      <div class="data" data-tag="sku"
           data-xpath="/top/data/item/sku" data-type="string"
           data-help="Stock Keeping Unit">HRD-000-212</div>
    </div>
    <div class="line">
      <div class="label">Item</div>
      <div class="text"> '</div>
      <div class="data" data-tag="name"
           data-xpath="/top/data/item/name" data-type="string"
           data-help="Name of the item">ladder</div>
      <div class="text">':</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">Total sold</div>
      <div class="text">: </div>
      <div class="data" data-tag="sold"
           data-xpath="/top/data/item/sold" data-type="number"
           data-help="Number of items sold">0</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">In stock</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="in-stock"
           data-xpath="/top/data/item/in-stock" data-type="number"
           data-help="Number of items in stock">2</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">On order</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="on-order"
           data-xpath="/top/data/item/on-order" data-type="number"
           data-help="Number of items on order">1</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">SKU</div>
      <div class="text">: </div>
      <div class="data" data-tag="sku"
           data-xpath="/top/data/item/sku" data-type="string"
           data-help="Stock Keeping Unit">HRD-000-517</div>
    </div>
    <div class="line">
      <div class="label">Item</div>
      <div class="text"> '</div>
      <div class="data" data-tag="name"
           data-xpath="/top/data/item/name" data-type="string"
           data-help="Name of the item">bolt</div>
      <div class="text">':</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">Total sold</div>
      <div class="text">: </div>
      <div class="data" data-tag="sold"
           data-xpath="/top/data/item/sold" data-type="number"
           data-help="Number of items sold">4123.0</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">In stock</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="in-stock"
           data-xpath="/top/data/item/in-stock" data-type="number"
           data-help="Number of items in stock">144</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">On order</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="on-order"
           data-xpath="/top/data/item/on-order" data-type="number"
           data-help="Number of items on order">42</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">SKU</div>
      <div class="text">: </div>
      <div class="data" data-tag="sku"
           data-xpath="/top/data/item/sku" data-type="string"
           data-help="Stock Keeping Unit">HRD-000-632</div>
    </div>
    <div class="line">
      <div class="label">Item</div>
      <div class="text"> '</div>
      <div class="data" data-tag="name"
           data-xpath="/top/data/item/name" data-type="string"
           data-help="Name of the item">water</div>
      <div class="text">':</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">Total sold</div>
      <div class="text">: </div>
      <div class="data" data-tag="sold"
           data-xpath="/top/data/item/sold" data-type="number"
           data-help="Number of items sold">17.0</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">In stock</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="in-stock"
           data-xpath="/top/data/item/in-stock" data-type="number"
           data-help="Number of items in stock">14</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">On order</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="on-order"
           data-xpath="/top/data/item/on-order" data-type="number"
           data-help="Number of items on order">2</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">SKU</div>
      <div class="text">: </div>
      <div class="data" data-tag="sku"
           data-xpath="/top/data/item/sku" data-type="string"
           data-help="Stock Keeping Unit">GRO-000-2331</div>
    </div>
    <div class="line">
      <div class="label">Item</div>
      <div class="text"> '</div>
      <div class="data" data-tag="name"
           data-xpath="/top/data/item/name" data-type="string"
           data-help="Name of the item">fish</div>
      <div class="text">':</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">Total sold</div>
      <div class="text">: </div>
      <div class="data" data-tag="sold"
           data-xpath="/top/data/item/sold" data-type="number"
           data-help="Number of items sold">1321.0</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">In stock</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="in-stock"
           data-xpath="/top/data/item/in-stock" data-type="number"
           data-help="Number of items in stock">45</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">On order</div>
      <div class="decoration">:</div>
      <div class="padding"> </div>
      <div class="data" data-tag="on-order"
           data-xpath="/top/data/item/on-order" data-type="number"
           data-help="Number of items on order">1</div>
    </div>
    <div class="line">
      <div class="padding">   </div>
      <div class="label">SKU</div>
      <div class="text">: </div>
      <div class="data" data-tag="sku"
           data-xpath="/top/data/item/sku" data-type="string"
           data-help="Stock Keeping Unit">GRO-000-533</div>
    </div>

{{document:
    name libxo-manual;
    private "The libxo Project";
    ipr none;
    category exp;
    abbreviation LIBXO-MANUAL;
    title "libxo: The Easy Way to Generate text, XML, JSON, and HTML output";
    contributor "author:Phil Shafer:Juniper Networks:phil@juniper.net";
}}