LIBXO(3) BSD Library Functions Manual LIBXO(3)
NAME
xo_format -- content of format descriptors for xo_emit
DESCRIPTION
libxo uses format strings to control the rendering of data into various output styles, including text, XML, and HTML. 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(3)-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(3), function as an unsigned integer.
xo_emit("{P: }{Lwc:In stock}{:in-stock/%u}
", 65);
This single line of code can generate text ("In stock: 65
"), 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
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 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:))}
", name);
The Label Role ({L:})
Labels are text that appears before a value.
xo_emit("{Lwc:Cost}{:cost/%u}
", cost);
The Note Role ({N:})
Notes are text that appears after a value.
xo_emit("{:cost/%u} {N:per year}
", 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}
", cost);
xo_emit("{P:/30s}{Lwc:Cost}{:cost/%u}
", "", cost);
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 ("/"):
xo_emit("{T:Interface Statistics}
");
xo_emit("{T:/%20.20s}{T:/%6.6s}
", "Item Name", "Cost");
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}
", 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}
",
length, width, height);
xo_emit("{:author} wrote
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(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.
To give a width directly, encode it as the content of the anchor tag:
xo_emit("({[:10}{:min/%d}/{:max/%d}{]:})
", 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}{]:})
", 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 num-
ber 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
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}
", "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}
", "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:})
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}
", "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
",
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 values 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 sin-
gle elements, where JSON renders them as arrays.
EXAMPLE:
xo_open_list("user");
for (i = 0; i < num_users; i++) {
xo_emit("Member {l:name}0, user[i].u_name);
}
xo_close_list("user");
XML:
<user>phil</user>
<user>pallavi</user>
JSON:
"user": [ "phil", "pallavi" ]
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 values, but no quotes for numeric, boolean, and null data. xo_emit(3) 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 values, but no quotes for numeric, boolean, and null data. xo_emit(3) 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}
", "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 for-
mat 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(3) function.
The format string has the form:
'%' format-modifier * format-character
The format- modifier can be:
o a '#' character, indicating the output value should be prefixed with
o a minus sign ('-'), indicating the output value should be padded on the right instead of the left.
o a leading zero ('0') indicating the output value should be padded on the left with zeroes instead of spaces (' ').
o one or more digits ('0' - '9') 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.
o a period followed by one or more digits indicating the maximum number of bytes which will be examined for a string argument, or the maxi-
mum width for a non-string argument. When handling ASCII strings this functions as the field width but for multi-byte characters, a sin-
gle character may be composed of multiple bytes. xo_emit(3) will never dereference memory beyond the given number of bytes.
o 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.
o one or more 'h' characters, indicating shorter input data.
o one or more 'l' characters, indicating longer input data.
o a 'z' character, indicating a 'size_t' argument.
o a 't' character, indicating a 'ptrdiff_t' argument.
o a ' ' character, indicating a space should be emitted before positive numbers.
o 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:
C 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
All strings for libxo must be UTF-8. libxo will handle turning them into locale-based strings for display to the user.
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. Unicode values.
string encoded with the current locale, as given by the LC_CTYPE, LANG, or 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 UTF-8.
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("All strings are utf-8 content {:tag/%ls}",
L"except for wide strings");
"%S" is equivalent to "%ls".
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}.
",
name, size_val);
xo_emit("It was ordered on {:order-time/%ls}.
",
when);
}
It is important to note that 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.
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 num-
ber of columns to emit. xo_emit(3) 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 col-
umns.
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}.
", 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 is a bad idea and we just do not 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".
EXAMPLE
In this example, the value for the number of items in stock is emitted:
xo_emit("{P: }{Lwc:In stock}{:in-stock/%u}
",
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>
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
Do not abbreviate especially when the abbreviation is not obvious or not widely used. Use "data-size", not "dsz" or "dsize". Use "inter-
face" 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 is 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 con-
tent with xo_attr(3) calls or "{e:}" fields to make the data useful.