Warning diagnostics emitted by GNU troff are divided into named,
numbered categories. The name associated with each warning category is
used by the -w and -W options. Each category is also
assigned a power of two; the sum of enabled category values is used by
the warn request and the .warn register.
Warnings of each category are produced under the following circumstances.
No mounted font defines a glyph for the requested character. This category is enabled by default.
An invalid numeric expression was encountered. This category is enabled by default. See Numeric Expressions.
A filled output line could not be broken such that its length was less than the output line length ‘\n[.l]’. This category is enabled by default.
The closing delimiter in an escape sequence was missing or mismatched.
The el request was encountered with no prior corresponding
ie request. See if-else.
A scaling unit inappropriate to its context was used in a numeric expression.
A numeric expression was out of range for its context.
A self-contradictory hyphenation mode was requested; an empty or
incomplete numeric expression was encountered; an operand to a numeric
operator was missing; an attempt was made to define a recursive, empty,
or nonsensical character class; or a groff extension conditional
expression operator was used while in compatibility mode.
A di, da, box, or boxa request was invoked
without an argument when there was no current diversion.
An undefined string, macro, or diversion was used. When such an object is dereferenced, an empty one of that name is automatically created. So, unless it is later deleted, at most one warning is given for each.
This warning is also emitted upon an attempt to move an unplanted trap macro (see Page Location Traps). In such cases, the unplanted macro is not dereferenced, so it is not created if it does not exist.
An undefined register was used. When an undefined register is dereferenced, it is automatically defined with a value of 0. So, unless it is later deleted, at most one warning is given for each.
A tab character was encountered where a number was expected, or appeared in an unquoted macro argument.
A right brace escape sequence \} was encountered where a number
was expected.
A request was invoked with a mandatory argument absent.
An invalid character occurred on the input stream.
An unsupported escape sequence was encountered.
A space was missing between a request or macro and its argument. This warning is produced when an undefined name longer than two characters is encountered and the first two characters of the name constitute a defined name. No request is invoked, no macro called, and an empty macro is not defined. This category is enabled by default. It never occurs in compatibility mode.
A non-existent font was selected, or the selection was ignored because a font selection escape sequence was used after the output line continuation escape sequence on an input line. This category is enabled by default.
An invalid escape sequence occurred in input ignored using the ig
request. This warning category diagnoses a condition that is an error
when it occurs in non-ignored input.
An undefined color was selected, an attempt was made to define a color using an unrecognized color space, an invalid component in a color definition was encountered, or an attempt was made to redefine a default color.
An attempt was made to load a file that does not exist. This category is enabled by default.
Two warning names group other warning categories for convenience.
All warning categories except ‘di’, ‘mac’ and ‘reg’.
This shorthand is intended to produce all warnings that are useful with
macro packages written for AT&T troff and its
descendants, which have less fastidious diagnostics than GNU
troff.
All warning categories. Authors of documents and macro packages
targeting groff are encouraged to use this setting.