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
#

* libxo

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:

    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>
        <filename>/etc/motd</filename>
        <lines>25</lines>
        <words>165</words>
        <characters>1140</characters>
      </file>
    </wc>
    % wc --libxo json,pretty,warn /etc/motd
    {
      "wc": {
        "file": [
          {
            "filename": "/etc/motd",
            "lines": 25,
            "words": 165,
            "characters": 1140
          }
        ]
      }
    }
    % 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
is shipped as part of FreeBSD 11.0.

* Overview

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:
      Connection to my-box.example.com...
    XML:
      <host>my-box</host>
      <domain>example.com</domain>
    JSON:
      "host": "my-box",
      "domain": "example.com"
      
For brevity, the HTML output is emitted.

** Encoding Styles

There are four encoding styles supported by libxo: TEXT, HTML, JSON,
and XML.  JSON and XML are suitable for encoding data, while TEXT and
HTML are suited for display to the user.  TEXT output can be display
on a terminal session, allowing compatibility with traditional usage.
HTML can be matched with a small CSS file to permit rendering in any
HTML5 browser.  XML output is suitable for tools like XPath and
protocols like NETCONF.  JSON output can be used for RESTful APIs.

*** 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 ]* ':' [ 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).

*** Modifier Roles

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

|---+--------------+-------------------------------------------------|
| M | Name         | Description                                     |
|---+--------------+-------------------------------------------------|
| C | color/effect | Field has color and effect controls             |
| D | decoration   | Field is non-text (e.g., colon, comma)          |
| E | error        | Field is an error message                       |
| 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   |
|---+--------------+-------------------------------------------------|

**** The Color Role ({C:})

Colors and effects control how text values are displayed; they are
used for display styles (TEXT and HTML).  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:bold}{Lwc:Cost}{:cost/%u}{C:reset}\n", cost);
    xo_emit("{C:/fg-%s,bg-%s}{Lwc:Cost}{:cost/%u}{C:reset}\n",
            fg_color, bg_color, cost);

The content should be a comma-separated list of zero or more colors or
display effects.  Colors and effects remain in effect until
modified by other "C" roles.  If the content is empty, the "reset"
action is performed.

|---------------+-------------------------------------------------|
|  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         |
|---------------|
| black         |
| blue          |
| cyan          |
| default       |
| green         |
| magenta       |
| red           |
| white         |
| yellow        |
|---------------|

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

**** 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 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 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 Modifiers ({[:} and {]:})

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.

*** Modifier Flags

The modifiers can also include the following flags, which modify the
content emitted for some 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)        |
| 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    |
| q | quotes       | Quote the field when using JSON style           |
| w | white space  | A blank (" ") is appended after the label       |
|---+--------------+-------------------------------------------------|

For example, the modifier string "Lwc" 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" means the
field has a value 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 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 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 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>

*** "%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.

*** 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                                                |
|-----------+-------------------------------------------------------|
| 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)                 |
| 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                                      |
| 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

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_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_PRETTY      | Make 'pretty printed' output          |
| XOF_UNDERSCORES | Replaces hyphens with underscores     |
| XOF_UNITS       | Display units (XML and 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_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.

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

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

** 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",
            }
        ]

** Additional Functionality

*** Parsing Command-line Arguments (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(1);

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                                    |
|--------+-------------------------------------------|
| 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)        |
| P      | Enable pretty-printed output (XOF_PRETTY) |
| T      | Enable text output (XO_STYLE_TEXT)        |
| 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);

* 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 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.

* 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?

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/p[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".
= 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

* 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(1);
    
        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";
}}