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This file documents the use of the Common Error Description library.

Copyright (C) 1987, 1988 Student Information Processing Board of the Massachusetts Institute of Technology.

Permission to use, copy, modify, and distribute this software and its documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation, and that the names of M.I.T. and the M.I.T. S.I.P.B. not be used in advertising or publicity pertaining to distribution of the software without specific, written prior permission. M.I.T. and the M.I.T. S.I.P.B. make no representations about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty.

Note that the file texinfo.tex, provided with this distribution, is from the Free Software Foundation, and is under different copyright restrictions from the remainder of this package.

1.0 Why com_err?

In building application software packages, a programmer often has to deal with a number of libraries, each of which can use a different error-reporting mechanism. Sometimes one of two values is returned, indicating simply SUCCESS or FAILURE, with no description of errors encountered. Sometimes it is an index into a table of text strings, where the name of the table used is dependent on the library being used when the error is generated; since each table starts numbering at 0 or 1, additional information as to the source of the error code is needed to determine which table to look at. Sometimes no text messages are supplied at all, and the programmer must supply them at any point at which he may wish to report error conditions. Often, a global variable is assigned some value describing the error, but the programmer has to know in each case whether to look at errno, h_errno, the return value from hes_err(), or whatever other variables or routines are specified. And what happens if something in the procedure of examining or reporting the error changes the same variable?

The package we have developed is an attempt to present a common error-handling mechanism to manipulate the most common form of error code in a fashion that does not have the problems listed above.

A list of up to 256 text messages is supplied to a translator we have written, along with the three- to four-character "name" of the error table. The library using this error table need only call a routine generated from this error-table source to make the table "known" to the com_err library, and any error code the library generates can be converted to the corresponding error message. There is also a default format for error codes accidentally returned before making the table known, which is of the form `unknown code foo 32', where `foo' would be the name of the table.