Field Roles

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

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

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

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

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

The Color Role ({C:})

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

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

Colors and effects remain in effect until modified by other “C”-role fields:

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

If the content is empty, the “reset” action is performed:

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

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


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

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

Color names are prefixed with either “fg-” or “bg-” to change the foreground and background colors, respectively:

        fg_color, bg_color, cost);

The following table lists the supported effects:

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

The following color names are supported:

Name Description
default Default color for foreground or background

When using colors, the developer should remember that users will change the foreground and background colors of terminal session according to their own tastes, so assuming that “blue” looks nice is never safe, and is a constant annoyance to your dear author. In addition, a significant percentage of users (1 in 12) will be color blind. Depending on color to convey critical information is not a good idea. Color should enhance output, but should not be used as the sole means of encoding information.

The Decoration Role ({D:})

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

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

The Gettext Role ({G:})

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

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

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

xo_emit(“{G:}Invalid tokenn”);

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

xo_emit(“{G:libc}Service unavailable in restricted moden”);

See Howto: Internationalization (i18n) for additional details.

The Label Role ({L:})

Labels are text that appears before a value:

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

If a label needs to include a slash, it must be escaped using two backslashes, one for the C compiler and one for libxo:

xo_emit("{Lc:Low\\/warn level}{:level/%s}\n", level);

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:


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"

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”:

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

The Anchor Roles ({[:} 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.