libxo/libxo/xo_format.5

.\" #
.\" # 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
.\"
.Dd December 4, 2014
.Dt LIBXO 3
.Os
.Sh NAME
.Nm xo_format
.Nd content of format descriptors for xo_emit
.Sh DESCRIPTION
.Pp
.Nm libxo
uses format strings to control the rendering of data into
various output styles, including
.Em text ,
.Em XML ,
.Em JSON ,
and
.Em HTML .
Each format string contains a set of zero or more
.Dq "field descriptions" ,
which describe independent data fields.
Each field description contains a set of
.Dq modifiers ,
a
.Dq "content string" ,
and zero, one, or two
.Dq "format descriptors" .
The modifiers tell
.Nm libxo
what the field is and how to treat it, while the format descriptors are
formatting instructions using
.Xr printf 3 Ns -style
format strings, telling
.Nm libxo
how to format the field.
The field description is placed inside
a set of braces, with a colon
.Ql ( \&: )
after the modifiers and a slash
.Ql ( \&/ )
before each format descriptors.
Text may be intermixed with
field descriptions within the format string.
.Pp
The field description is given as follows:
.Bd -literal -offset indent
    \(aq{\(aq [ role | modifier ]* [\(aq,\(aq long\-names ]* \(aq:\(aq [ content ]
            [ \(aq/\(aq field\-format [ \(aq/\(aq encoding\-format ]] \(aq}\(aq
.Ed
.Pp
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.
.Pp
Braces can be escaped by using double braces, similar to "%%" in
.Xr printf 3 .
The format string "{{braces}}" would emit "{braces}".
.Pp
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
.Xr xo_emit 3 ,
function as an unsigned integer.
.Bd -literal -offset indent
    xo_emit("{P:   }{Lwc:In stock}{:in\-stock/%u}\\n", 65);
.Ed
.Pp
This single line of code can generate text ("In stock: 65\\n"), XML
("<in\-stock>65</in\-stock>"), JSON (\(aq"in\-stock": 65\(aq), or HTML (too
lengthy to be listed here).
.Pp
While roles and modifiers typically use single character for brevity,
there are alternative names for each which allow more verbose
formatting strings.
These names must be preceded by a comma, and may follow any
single\-character values:
.Bd -literal -offset indent
    xo_emit("{L,white,colon:In stock}{,key:in\-stock/%u}\\n", 65);
.Ed
.Ss "Field Roles"
Field roles are optional, and indicate the role and formatting of the
content.
The roles are listed below; only one role is permitted:
.Bl -column "M" "Name12341234"
.It Sy "M" "Name        " "Description"
.It C "color       " "Field is a color or effect"
.It D "decoration  " "Field is non\-text (e.g. colon, comma)"
.It E "error       " "Field is an error message"
.It L "label       " "Field is text that prefixes a value"
.It N "note        " "Field is text that follows a value"
.It P "padding     " "Field is spaces needed for vertical alignment"
.It T "title       " "Field is a title value for headings"
.It U "units       " "Field is the units for the previous value field"
.It V "value       " "Field is the name of field (the default)"
.It W "warning     " "Field is a warning message"
.It \&[ "start\-anchor" "Begin a section of anchored variable\-width text"
.It \&] "stop\-anchor " "End a section of anchored variable\-width text"
.El
.Bd -literal -offset indent
   EXAMPLE:
       xo_emit("{L:Free}{D::}{P:   }{:free/%u} {U:Blocks}\\n",
               free_blocks);
.Ed
.Pp
When a role is not provided, the "value" role is used as the default.
.Pp
Roles and modifiers can also use more verbose names, when preceded by
a comma:
.Bd -literal -offset indent
   EXAMPLE:
        xo_emit("{,label:Free}{,decoration::}{,padding:   }"
               "{,value:free/%u} {,units:Blocks}\\n",
               free_blocks);
.Ed
.Ss "The Color Role ({C:})"
Colors and effects control how text values are displayed; they are
used for display styles (TEXT and HTML).
.Bd -literal -offset indent
    xo_emit("{C:bold}{:value}{C:no\-bold}\\n", value);
.Ed
.Pp
Colors and effects remain in effect until modified by other "C"\-role
fields.
.Bd -literal -offset indent
    xo_emit("{C:bold}{C:inverse}both{C:no\-bold}only inverse\\n");
.Ed
.Pp
If the content is empty, the "reset" action is performed.
.Bd -literal -offset indent
    xo_emit("{C:both,underline}{:value}{C:}\\n", value);
.Ed
.Pp
The content should be a comma\-separated list of zero or more colors or
display effects.
.Bd -literal -offset indent
    xo_emit("{C:bold,underline,inverse}All three{C:no\-bold,no\-inverse}\\n");
.Ed
.Pp
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 ("/"):
.Bd -literal -offset indent
   xo_emit("{C:/%s%s}{:value}{C:}", need_bold ? "bold" : "",
           need_underline ? "underline" : "", value);
.Ed
.Pp
Color names are prefixed with either "fg\-" or "bg\-" to change the
foreground and background colors, respectively.
.Bd -literal -offset indent
    xo_emit("{C:/fg\-%s,bg\-%s}{Lwc:Cost}{:cost/%u}{C:reset}\\n",
            fg_color, bg_color, cost);
.Ed
.Pp
The following table lists the supported effects:
.Bl -column "no\-underline"
.It Sy "Name         " "Description"
.It "bg\-xxxxx     " "Change background color"
.It "bold         " "Start bold text effect"
.It "fg\-xxxxx     " "Change foreground color"
.It "inverse      " "Start inverse (aka reverse) text effect"
.It "no\-bold      " "Stop bold text effect"
.It "no\-inverse   " "Stop inverse (aka reverse) text effect"
.It "no\-underline " "Stop underline text effect"
.It "normal       " "Reset effects (only)"
.It "reset        " "Reset colors and effects (restore defaults)"
.It "underline    " "Start underline text effect"
.El
.Pp
The following color names are supported:
.Bl -column "no\-underline"
.It Sy "Name"
.It black
.It blue
.It cyan
.It default
.It green
.It magenta
.It red
.It white
.It yellow
.El
.Ss "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.
.Bd -literal -offset indent
    xo_emit("{D:((}{:name}{D:))}\\n", name);
.Ed
.Ss "The Gettext Role ({G:})"
.Nm libxo
supports internationalization (i18n) through its use of
.Xr gettext 3 .
Use the "{G:}" role to request that the remaining part of
the format string, following the "{G:}" field, be handled using
.Fn gettext .
Since
.Fn gettext
uses the string as the key into the message catalog,
.Nm libxo
uses a simplified version of the format string that removes
unimportant field formatting and modifiers, stopping minor formatting
changes from impacting the expensive translation process.
A developer
change such as changing "/%06d" to "/%08d" should not force hand
inspection of all .po files.
.Pp
The simplified version can be generated for a single message using the
"xopo \-s <text>" command, or an entire .pot can be translated using
the "xopo \-f <input> \-o <output>" command.
.Bd -literal -offset indent
   xo_emit("{G:}Invalid token\\n");
.Ed
.Pp
The {G:} role allows a domain name to be set.
.Fn gettext
calls will
continue to use that domain name until the current format string
processing is complete, enabling a library function to emit strings
using it\(aqs own catalog.
The domain name can be either static as the
content of the field, or a format can be used to get the domain name
from the arguments.
.Bd -literal -offset indent
   xo_emit("{G:libc}Service unavailable in restricted mode\\n");
.Ed
.Ss "The Label Role ({L:})"
Labels are text that appears before a value.
.Bd -literal -offset indent
    xo_emit("{Lwc:Cost}{:cost/%u}\\n", cost);
.Ed
.Ss "The Note Role ({N:})"
Notes are text that appears after a value.
.Bd -literal -offset indent
    xo_emit("{:cost/%u} {N:per year}\\n", cost);
.Ed
.Ss "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 ("/"):
.Bd -literal -offset indent
    xo_emit("{P:        }{Lwc:Cost}{:cost/%u}\\n", cost);
    xo_emit("{P:/30s}{Lwc:Cost}{:cost/%u}\\n", "", cost);
.Ed
.Ss "The Title Role ({T:})"
Titles 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 ("/"):
.Bd -literal -offset indent
    xo_emit("{T:Interface Statistics}\\n");
    xo_emit("{T:/%20.20s}{T:/%6.6s}\\n", "Item Name", "Cost");
.Ed
.Ss "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.
.Bd -literal -offset indent
    xo_emit("{Lwc:Distance}{:distance/%u}{Uw:miles}\\n", miles);
.Ed
.Pp
Note that the sense of the \(aqw\(aq modifier is reversed for units;
a blank is added before the contents, rather than after it.
.Pp
When the
.Dv XOF_UNITS
flag is set, units are rendered in XML as the
.Dq units
attribute:
.Bd -literal -offset indent
    <distance units="miles">50</distance>
.Ed
.Pp
Units can also be rendered in HTML as the "data\-units" attribute:
.Bd -literal -offset indent
    <div class="data" data\-tag="distance" data\-units="miles"
         data\-xpath="/top/data/distance">50</div>
.Ed
.Ss "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".
.Bd -literal -offset indent
    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);
.Ed
.Ss "The Anchor Roles ({[:} and {]:})"
The anchor roles allow a set of strings by be padded as a group,
but still be visible to
.Xr xo_emit 3
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.
.Pp
To give a width directly, encode it as the content of the anchor tag:
.Bd -literal -offset indent
    xo_emit("({[:10}{:min/%d}/{:max/%d}{]:})\\n", min, max);
.Ed
.Pp
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.
.Bd -literal -offset indent
    xo_emit("({[:/%d}{:min/%d}/{:max/%d}{]:})\\n", width, min, max);
.Ed
.Pp
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.
.Pp
Widths over 8k are considered probable errors and not supported.
If
.Dv XOF_WARN
is set, a warning will be generated.
.Ss "Field Modifiers"
Field modifiers are flags which modify the way content emitted for
particular output styles:
.Bl -column M "Name123456789"
.It Sy M "Name          " "Description"
.It a "argument      " "The content appears as a ""const char *"" argument"
.It c "colon         " "A colon ("":"") is appended after the label"
.It d "display       " "Only emit field for display styles (text/HTML)"
.It e "encoding      " "Only emit for encoding styles (XML/JSON)"
.It h "humanize (hn) " "Format large numbers in human\-readable style"
.It " " "hn\-space     " "Humanize: Place space between numeric and unit"
.It " " "hn\-decimal   " "Humanize: Add a decimal digit, if number < 10"
.It " " "hn\-1000      " "Humanize: Use 1000 as divisor instead of 1024"
.It k "key           " "Field is a key, suitable for XPath predicates"
.It l "leaf\-list    " "Field is a leaf\-list, a list of leaf values"
.It n "no\-quotes    " "Do not quote the field when using JSON style"
.It q "quotes        " "Quote the field when using JSON style"
.It t "trim          " "Trim leading and trailing whitespace"
.It w "white space   " "A blank ("" "") is appended after the label"
.El
.Pp
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 (\(aqc\(aq) and a space (\(aqw\(aq).
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.
.Pp
Roles and modifiers can also use more verbose names, when preceded by
a comma.
For example, the modifier string "Lwc" (or "L,white,colon")
means the field has a label role (text that describes the next field)
and should be followed by a colon (\(aqc\(aq) and a space (\(aqw\(aq).
The modifier string "Vkq" (or ":key,quote") means the field has a value
role (the default role), that it is a key for the current instance,
and that the value should be quoted when encoded for JSON.
.Ss "The Argument Modifier ({a:})"
The argument modifier indicates that the content of the field
descriptor will be placed as a UTF\-8 string (const char *) argument
within the xo_emit parameters.
.Bd -literal -offset indent
    EXAMPLE:
      xo_emit("{La:} {a:}\\n", "Label text", "label", "value");
    TEXT:
      Label text value
    JSON:
      "label": "value"
    XML:
      <label>value</label>
.Ed
.Pp
The argument modifier allows field names for value fields to be passed
on the stack, avoiding the need to build a field descriptor using
.Xr snprintf 1 .
For many field roles, the argument modifier is not needed,
since those roles have specific mechanisms for arguments,
such as "{C:fg\-%s}".
.Ss "The Colon Modifier ({c:})"
The colon modifier appends a single colon to the data value:
.Bd -literal -offset indent
    EXAMPLE:
      xo_emit("{Lc:Name}{:name}\\n", "phil");
    TEXT:
      Name:phil
.Ed
.Pp
The colon modifier is only used for the TEXT and HTML output
styles.
It is commonly combined with the space modifier (\(aq{w:}\(aq).
It is purely a convenience feature.
.Ss "The Display Modifier ({d:})"
The display modifier indicated the field should only be generated for
the display output styles, TEXT and HTML.
.Bd -literal -offset indent
    EXAMPLE:
      xo_emit("{Lcw:Name}{d:name} {:id/%d}\\n", "phil", 1);
    TEXT:
      Name: phil 1
    XML:
      <id>1</id>
.Ed
.Pp
The display modifier is the opposite of the encoding modifier, and
they are often used to give to distinct views of the underlying data.
.Ss "The Encoding Modifier ({e:})"
The encoding modifier indicated the field should only be generated for
the encoding output styles, such as JSON and XML.
.Bd -literal -offset indent
    EXAMPLE:
      xo_emit("{Lcw:Name}{:name} {e:id/%d}\\n", "phil", 1);
    TEXT:
      Name: phil
    XML:
      <name>phil</name><id>1</id>
.Ed
.Pp
The encoding modifier is the opposite of the display modifier, and
they are often used to give to distinct views of the underlying data.
.Ss "The Humanize Modifier ({h:})"
The humanize modifier is used to render large numbers as in a
human\-readable format.
While numbers like "44470272" are completely readable to computers and
savants, humans will generally find "44M" more meaningful.
.Pp
"hn" can be used as an alias for "humanize".
.Pp
The humanize modifier only affects display styles (TEXT and HMTL).
The "no\-humanize" option will block the function of the humanize modifier.
.Pp
There are a number of modifiers that affect details of humanization.
These are only available in as full names, not single characters.
The "hn\-space" modifier places a space between the number and any
multiplier symbol, such as "M" or "K" (ex: "44 K").
The "hn\-decimal" modifier will add a decimal point and a single tenths digit
when the number is less than 10 (ex: "4.4K").
The "hn\-1000" modifier will use 1000 as divisor instead of 1024, following the
JEDEC\-standard instead of the more natural binary powers\-of\-two
tradition.
.Bd -literal -offset indent
    EXAMPLE:
        xo_emit("{h:input/%u}, {h,hn\-space:output/%u}, "
           "{h,hn\-decimal:errors/%u}, {h,hn\-1000:capacity/%u}, "
           "{h,hn\-decimal:remaining/%u}\\n",
            input, output, errors, capacity, remaining);
    TEXT:
        21, 57 K, 96M, 44M, 1.2G
.Ed
.Pp
In the HTML style, the original numeric value is rendered in the
"data\-number" attribute on the <div> element:
.Bd -literal -offset indent
    <div class="data" data\-tag="errors"
         data\-number="100663296">96M</div>
.Ed
.Ss "The Gettext Modifier ({g:})"
The gettext modifier is used to translate individual fields using the
gettext domain (typically set using the "{G:}" role) and current
language settings.
Once libxo renders the field value, it is passed
to
.Xr gettext 3 ,
where it is used as a key to find the native language
translation.
.Pp
In the following example, the strings "State" and "full" are passed
to
.Fn gettext
to find locale\-based translated strings.
.Bd -literal -offset indent
    xo_emit("{Lgwc:State}{g:state}\\n", "full");
.Ed
.Ss "The Key Modifier ({k:})"
The key modifier is used to indicate that a particular field helps
uniquely identify an instance of list data.
.Bd -literal -offset indent
    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");
.Ed
.Pp
Currently the key modifier is only used when generating XPath values
for the HTML output style when
.Dv XOF_XPATH
is set, but other uses are likely in the near future.
.Ss "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.
.Bd -literal -offset indent
    EXAMPLE:
        xo_open_list("user");
        for (i = 0; i < num_users; i++) {
            xo_emit("Member {l:name}\\n", user[i].u_name);
        }
        xo_close_list("user");
    XML:
        <user>phil</user>
        <user>pallavi</user>
    JSON:
        "user": [ "phil", "pallavi" ]
.Ed
.Ss "The No\-Quotes Modifier ({n:})"
The no\-quotes modifier (and its twin, the \(aqquotes\(aq modifier) affect
the quoting of values in the JSON output style.
JSON uses quotes for
string values, but no quotes for numeric, boolean, and null data.
.Xr xo_emit 3
applies a simple heuristic to determine whether quotes are
needed, but often this needs to be controlled by the caller.
.Bd -literal -offset indent
    EXAMPLE:
      const char *bool = is_true ? "true" : "false";
      xo_emit("{n:fancy/%s}", bool);
    JSON:
      "fancy": true
.Ed
.Ss "The Plural Modifier ({p:})"
The plural modifier selects the appropriate plural form of an
expression based on the most recent number emitted and the current
language settings.
The contents of the field should be the singular
and plural English values, separated by a comma:
.Bd -literal -offset indent
    xo_emit("{:bytes} {Ngp:byte,bytes}\\n", bytes);
.Ed
.Pp
The plural modifier is meant to work with the gettext modifier ({g:})
but can work independently.
.Pp
When used without the gettext modifier or when the message does not
appear in the message catalog, the first token is chosen when the last
numeric value is equal to 1; otherwise the second value is used,
mimicking the simple pluralization rules of English.
.Pp
When used with the gettext modifier, the
.Xr ngettext 3
function is
called to handle the heavy lifting, using the message catalog to
convert the singular and plural forms into the native language.
.Ss "The Quotes Modifier ({q:})"
The quotes modifier (and its twin, the \(aqno-quotes\(aq modifier) affect
the quoting of values in the JSON output style.
JSON uses quotes for
string values, but no quotes for numeric, boolean, and null data.
.Xr xo_emit 3
applies a simple heuristic to determine whether quotes are
needed, but often this needs to be controlled by the caller.
.Bd -literal -offset indent
    EXAMPLE:
      xo_emit("{q:time/%d}", 2014);
    JSON:
      "year": "2014"
.Ed
.Ss "The White Space Modifier ({w:})"
The white space modifier appends a single space to the data value:
.Bd -literal -offset indent
    EXAMPLE:
      xo_emit("{Lw:Name}{:name}\\n", "phil");
    TEXT:
      Name phil
.Ed
.Pp
The white space modifier is only used for the TEXT and HTML output
styles.
It is commonly combined with the colon modifier (\(aq{c:}\(aq).
It is purely a convenience feature.
.Pp
Note that the sense of the \(aqw\(aq modifier is reversed for the units role
({Uw:}); a blank is added before the contents, rather than after it.
.Ss "Field Formatting"
The field format is similar to the format string for
.Xr printf 3 .
Its use varies based on the role of the field, but generally is used to
format the field\(aqs contents.
.Pp
If the format string is not provided for a value field, it defaults
to "%s".
.Pp
Note a field definition can contain zero or more printf\-style
.Dq directives ,
which are sequences that start with a \(aq%\(aq and end with
one of following characters: "diouxXDOUeEfFgGaAcCsSp".
Each directive
is matched by one of more arguments to the
.Xr xo_emit 3
function.
.Pp
The format string has the form:
.Bd -literal -offset indent
  \(aq%\(aq format\-modifier * format\-character
.Ed
.Pp
The format\-modifier can be:
.Bl -bullet
.It
a \(aq#\(aq character, indicating the output value should be prefixed with
"0x", typically to indicate a base 16 (hex) value.
.It
a minus sign (\(aq\-\(aq), indicating the output value should be padded on
the right instead of the left.
.It
a leading zero (\(aq0\(aq) indicating the output value should be padded on the
left with zeroes instead of spaces (\(aq \(aq).
.It
one or more digits (\(aq0\(aq \- \(aq9\(aq) indicating the minimum width of the
argument.
If the width in columns of the output value is less than
the minimum width, the value will be padded to reach the minimum.
.It
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.
.Xr xo_emit 3
will never dereference memory beyond the given number of bytes.
.It
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.
.It
one or more \(aqh\(aq characters, indicating shorter input data.
.It
one or more \(aql\(aq characters, indicating longer input data.
.It
a \(aqz\(aq character, indicating a \(aqsize_t\(aq argument.
.It
a \(aqt\(aq character, indicating a \(aqptrdiff_t\(aq argument.
.It
a \(aq \(aq character, indicating a space should be emitted before
positive numbers.
.It
a \(aq+\(aq character, indicating sign should emitted before any number.
.El
.Pp
Note that \(aqq\(aq, \(aqD\(aq, \(aqO\(aq, and \(aqU\(aq are considered deprecated and will be
removed eventually.
.Pp
The format character is described in the following table:
.Bl -column C "Argument Type12"
.It Sy "C" "Argument Type  " "Format"
.It d "int            " "base 10 (decimal)"
.It i "int            " "base 10 (decimal)"
.It o "int            " "base 8 (octal)"
.It u "unsigned       " "base 10 (decimal)"
.It x "unsigned       " "base 16 (hex)"
.It X "unsigned long  " "base 16 (hex)"
.It D "long           " "base 10 (decimal)"
.It O "unsigned long  " "base 8 (octal)"
.It U "unsigned long  " "base 10 (decimal)"
.It e "double         " "[\-]d.ddde+\-dd"
.It E "double         " "[\-]d.dddE+\-dd"
.It f "double         " "[\-]ddd.ddd"
.It F "double         " "[\-]ddd.ddd"
.It g "double         " "as \(aqe\(aq or \(aqf\(aq"
.It G "double         " "as \(aqE\(aq or \(aqF\(aq"
.It a "double         " "[\-]0xh.hhhp[+\-]d"
.It A "double         " "[\-]0Xh.hhhp[+\-]d"
.It c "unsigned char  " "a character"
.It C "wint_t         " "a character"
.It s "char *         " "a UTF\-8 string"
.It S "wchar_t *      " "a unicode/WCS string"
.It p "void *         " "\(aq%#lx\(aq"
.El
.Pp
The \(aqh\(aq and \(aql\(aq modifiers affect the size and treatment of the
argument:
.Bl -column "Mod" "d, i         " "o, u, x, X         "
.It Sy "Mod" "d, i        " "o, u, x, X"
.It "hh " "signed char " "unsigned char"
.It "h  " "short       " "unsigned short"
.It "l  " "long        " "unsigned long"
.It "ll " "long long   " "unsigned long long"
.It "j  " "intmax_t    " "uintmax_t"
.It "t  " "ptrdiff_t   " "ptrdiff_t"
.It "z  " "size_t      " "size_t"
.It "q  " "quad_t      " "u_quad_t"
.El
.Ss "UTF\-8 and Locale Strings"
All strings for
.Nm libxo
must be UTF\-8.
.Nm libxo
will handle turning them
into locale\-based strings for display to the user.
.Pp
For strings, the \(aqh\(aq and \(aql\(aq modifiers affect the interpretation of
the bytes pointed to argument.
The default \(aq%s\(aq string is a \(aqchar *\(aq
pointer to a string encoded as UTF\-8.
Since UTF\-8 is compatible with
.Em ASCII
data, a normal 7\-bit
.Em ASCII
string can be used.
"%ls" expects a
"wchar_t *" pointer to a wide\-character string, encoded as 32\-bit
Unicode values.
"%hs" expects a "char *" pointer to a multi\-byte
string encoded with the current locale, as given by the
.Ev LC_CTYPE ,
.Ev LANG ,
or
.Ev LC_ALL
environment variables.
The first of this list of
variables is used and if none of the variables are set, the locale defaults to
.Em UTF\-8 .
.Pp
.Nm 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.
.Bd -literal -offset indent
   xo_emit("All strings are utf\-8 content {:tag/%ls}",
           L"except for wide strings");
.Ed
.Pp
"%S" is equivalent to "%ls".
.Pp
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.
.Bd -literal -offset indent
    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);
    }
.Ed
.Pp
It is important to note that
.Xr xo_emit 3
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.
.Pp
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.
.Xr xo_emit 3
uses the precision as the former,
and adds a third value for specifying the maximum number of columns.
.Pp
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.
.Bd -literal -offset indent
    xo_emit("{:name/%3.10.6s}", name);
.Ed
.Ss "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".
.Bd -literal -offset indent
  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>
.Ed
.Ss "\(aq%n\(aq is Not Supported"
.Nm libxo
does not support the \(aq%n\(aq directive.
It is a bad idea and we
just do not do it.
.Ss "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".
.Sh EXAMPLE
In this example, the value for the number of items in stock is emitted:
.Bd -literal -offset indent
        xo_emit("{P:   }{Lwc:In stock}{:in\-stock/%u}\\n",
                instock);
.Ed
.Pp
This call will generate the following output:
.Bd -literal -offset indent
  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>
.Ed
.Pp
Clearly HTML wins the verbosity award, and this output does
not include
.Dv XOF_XPATH
or
.Dv XOF_INFO
data, which would expand the penultimate line to:
.Bd -literal -offset indent
       <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>
.Ed
.Sh WHAT MAKES A GOOD FIELD NAME?
To make useful, consistent field names, follow these guidelines:
.Ss "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.
.Ss "Use hyphens, not underscores"
Use of hyphens is traditional in XML, and the
.Dv XOF_UNDERSCORES
flag can be used to generate underscores in JSON, if desired.
But the raw field name should use hyphens.
.Ss "Use full words"
Do not 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".
.Ss "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".
.Ss "Reuse existing field names"
Nothing is worse than writing expressions like:
.Bd -literal -offset indent
    if ($src1/process[pid == $pid]/name ==
        $src2/proc\-table/proc/p[process\-id == $pid]/proc\-name) {
        ...
    }
.Ed
.Pp
Find someone else who is expressing similar data and follow their
fields and hierarchy.
Remember the quote is not
.Dq "Consistency is the hobgoblin of little minds"
but
.Dq "A foolish consistency is the hobgoblin of little minds" .
.Ss "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
.Xr xo_attr 3
calls or "{e:}" fields to make the data useful.
.Ss "Do not 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 is the
difference between errors37 and errors63.
.Ss "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\(aqs 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.
.Pp
Field names constitute the means by which client programmers interact
with our system.
By choosing wise names now, you are making their lives better.
.Pp
After using
.Xr xolint 1
to find errors in your field descriptors, use
.Dq "xolint \-V"
to spell check your field names and to detect different
names for the same data.
.Dq dropped\-short
and
.Dq 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.
.Sh SEE ALSO
.Xr libxo 3 ,
.Xr xolint 1 ,
.Xr xo_emit 3
.Sh HISTORY
The
.Nm libxo
library first appeared in
.Fx 11.0 .
.Sh AUTHORS
.Nm libxo
was written by
.An Phil Shafer Aq Mt phil@freebsd.org .