Guide to LaTeX (4th Edition) (Tools and Techniques

446 Appendix D. L AT E X Programming
D.2.7 Issuing errors and warnings
Classes and packages may be programmed to issue their own error messages
and warnings. This is useful to indicate which file is responsible
for the message.
Error messages are generated with
\ClassError{class name}{error text}{help}
\PackageError{package name}{error text}{help}
where error text is the message printed to the monitor and to the transcript
file, and help is additional text printed after the user responds with
H. If the texts contain command names that are to be printed literally,
they must be preceded by \protect; spaces are generated with \space,
and new lines with \MessageBreak. For example,
\PackageError{ghost}{%
The \protect\textwidth\space is too large\MessageBreak
for the paper you have selected}
{Use a smaller width.}
produces the error message
! Package ghost Error: The \textwidth is too large
(ghost) for the paper you have selected.
See the ghost package documentation for explanation.
Type H for immediate help.
Typing H 〈return〉 produces
Use a smaller width.
after which L AT E X halts again to wait for a response as described in Section
C.1.
Warnings may also be issued from classes and packages in a similar
way. The difference is that there is no help text, and the processing does
not stop for a response. The line number of the input file where the
warning occurred may be optionally suppressed.
\ClassWarning{class name}{warning text}
\ClassWarningNoLine{class name}{warning text}
\PackageWarning{package name}{warning text}
\PackageWarningNoLine{package name}{warning text}
For example, with the warning
\PackageWarning{ghost}
{This text is haunted}
one obtains the message
Package ghost Warning: This text is haunted on input line 20.