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+\input texinfo @c -*-texinfo-*-
+
+@c $Header$
+@c $Source$
+@c $Locker$
+
+@c Note that although this source file is in texinfo format (more
+@c or less), it is not yet suitable for turning into an ``info''
+@c file. Sorry, maybe next time.
+@c
+@c In order to produce hardcopy documentation from a texinfo file,
+@c run ``tex com_err.texinfo'' which will load in texinfo.tex,
+@c provided in this distribution. (texinfo.tex is from the Free
+@c Software Foundation, and is under different copyright restrictions
+@c from the rest of this package.)
+
+@setfilename com_err.info
+@settitle A Common Error Description Library for UNIX
+
+@ifinfo
+@dircategory Development
+@direntry
+* Com_err: (com_err). A Common Error Description Library for UNIX.
+@end direntry
+@end ifinfo
+
+@c smallbook
+
+@iftex
+@finalout
+@end iftex
+
+@ifinfo
+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.
+
+@ignore
+Permission is granted to process this file through Tex and print the
+results, provided the printed document carries copying permission
+notice identical to this one except for the removal of this paragraph
+(this paragraph not being relevant to the printed manual).
+
+@end ignore
+@end ifinfo
+
+@setchapternewpage odd
+
+@titlepage
+@center @titlefont{A Common Error Description}
+@center @titlefont{Library for UNIX}
+@sp 2
+@center Ken Raeburn
+@center Bill Sommerfeld
+@sp 1
+@center MIT Student Information Processing Board
+@sp 3
+@center last updated 1 January 1989
+@center for version 1.2
+@center ***DRAFT COPY ONLY***
+
+@vskip 2in
+
+@center @b{Abstract}
+
+UNIX has always had a clean and simple system call interface, with a
+standard set of error codes passed between the kernel and user
+programs. Unfortunately, the same cannot be said of many of the
+libraries layered on top of the primitives provided by the kernel.
+Typically, each one has used a different style of indicating errors to
+their callers, leading to a total hodgepodge of error handling, and
+considerable amounts of work for the programmer. This paper describes
+a library and associated utilities which allows a more uniform way for
+libraries to return errors to their callers, and for programs to
+describe errors and exceptional conditions to their users.
+
+@page
+@vskip 0pt plus 1filll
+
+Copyright @copyright{} 1987, 1988 by the 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.
+
+@end titlepage
+
+@node Top, Why com_err?, (dir), (dir)
+
+@top A Common Error Description Library for UNIX
+
+This manual documents the com_err library.
+
+@menu
+* Why com_err?::
+* Error codes::
+* Error table source file::
+* The error-table compiler::
+* Run-time support routines::
+* Coding Conventions::
+* Building and Installation::
+* Bug Reports::
+* Acknowledgements::
+@end menu
+
+@page
+
+@node Why com_err?, Error codes, Top, Top
+@chapter 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 @code{errno},
+@code{h_errno}, the return value from @code{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 @samp{unknown code foo 32}, where @samp{foo} would be the name
+of the table.
+
+@node Error codes, Error table source file, Why com_err?, Top
+@chapter Error codes
+
+Error codes themselves are 32 bit (signed) integers, of which the high
+order 24 bits are an identifier of which error table the error code is
+from, and the low order 8 bits are a sequential error number within
+the table. An error code may thus be easily decomposed into its component
+parts. Only the lowest 32 bits of an error code are considered significant
+on systems which support wider values.
+
+Error table 0 is defined to match the UNIX system call error table
+(@code{sys_errlist}); this allows @code{errno} values to be used directly
+in the library (assuming that @code{errno} is of a type with the same width
+as @t{long}). Other error table numbers are formed by compacting together
+the first four characters of the error table name. The mapping between
+characters in the name and numeric values in the error code are defined in
+a system-independent fashion, so that two systems that can pass integral
+values between them can reliably pass error codes without loss of meaning;
+this should work even if the character sets used are not the same.
+(However, if this is to be done, error table 0 should be avoided, since the
+local system call error tables may differ.)
+
+Any variable which is to contain an error code should be declared @t{long}.
+The draft proposed American National Standard for C (as of May, 1988)
+requires that @t{long} variables be at least 32 bits; any system which does
+not support 32-bit @t{long} values cannot make use of this package (nor
+much other software that assumes an ANSI-C environment base) without
+significant effort.
+
+@node Error table source file, The error-table compiler, Error codes, Top
+@chapter Error table source file
+
+The error table source file begins with the declaration of the table name,
+as
+
+@example
+error_table @var{tablename}
+@end example
+
+Individual error codes are
+specified with
+
+@example
+error_code @var{ERROR_NAME}, @var{"text message"}
+@end example
+
+where @samp{ec} can also be used as a short form of @samp{error_code}. To
+indicate the end of the table, use @samp{end}. Thus, a (short) sample
+error table might be:
+
+@example
+
+ error_table dsc
+
+ error_code DSC_DUP_MTG_NAME,
+ "Meeting already exists"
+
+ ec DSC_BAD_PATH,
+ "A bad meeting pathname was given"
+
+ ec DSC_BAD_MODES,
+ "Invalid mode for this access control list"
+
+ end
+
+@end example
+
+@node The error-table compiler, Run-time support routines, Error table source file, Top
+@chapter The error-table compiler
+
+The error table compiler is named @code{compile_et}. It takes one
+argument, the pathname of a file (ending in @samp{.et}, e.g.,
+@samp{dsc_err.et}) containing an error table source file. It parses the
+error table, and generates two output files -- a C header file
+(@samp{discuss_err.h}) which contains definitions of the numerical values
+of the error codes defined in the error table, and a C source file which
+should be compiled and linked with the executable. The header file must be
+included in the source of a module which wishes to reference the error
+codes defined; the object module generated from the C code may be linked in
+to a program which wishes to use the printed forms of the error codes.
+
+@node Run-time support routines, Coding Conventions, The error-table compiler, Top
+@chapter Run-time support routines
+
+Any source file which uses the routines supplied with or produced by the
+com_err package should include the header file @file{<com_err.h>}. It
+contains declarations and definitions which may be needed on some systems.
+(Some functions cannot be referenced properly without the return type
+declarations in this file. Some functions may work properly on most
+architectures even without the header file, but relying on this is not
+recommended.)
+
+The run-time support routines and variables provided via this package
+include the following:
+
+@example
+void initialize_@var{xxxx}_error_table (void);
+@end example
+
+One of these routines is built by the error compiler for each error table.
+It makes the @var{xxxx} error table ``known'' to the error reporting
+system. By convention, this routine should be called in the initialization
+routine of the @var{xxxx} library. If the library has no initialization
+routine, some combination of routines which form the core of the library
+should ensure that this routine is called. It is not advised to leave it
+the caller to make this call.
+
+There is no harm in calling this routine more than once.
+
+@example
+#define ERROR_TABLE_BASE_@var{xxxx} @var{nnnnn}L
+@end example
+
+This symbol contains the value of the first error code entry in the
+specified table.
+This rarely needs be used by the
+programmer.
+
+@deftypefun const char *error_message (long @var{code});
+
+This routine returns the character string error message associated
+with @code{code}; if this is associated with an unknown error table, or
+if the code is associated with a known error table but the code is not
+in the table, a string of the form @samp{Unknown code @var{xxxx nn}} is
+returned, where @var{xxxx} is the error table name produced by
+reversing the compaction performed on the error table number implied
+by that error code, and @var{nn} is the offset from that base value.
+
+Although this routine is available for use when needed, its use should be
+left to circumstances which render @code{com_err} (below) unusable.
+
+@end deftypefun
+
+@deftypefun void com_err (const char *@var{whoami}, long @var{error_code}, const char *@var{format}, ...);
+
+This routine provides an alternate way to print error messages to
+standard error; it allows the error message to be passed in as a
+parameter, rather than in an external variable. @emph{Provide grammatical
+context for ``message.''}
+
+The module reporting the error should be passed in via @var{whoami}.
+If @var{format} is @code{(char *)NULL}, the formatted message will not be
+printed. @var{format} may not be omitted.
+
+@end deftypefun
+
+@deftypefun void com_err_va (const char *@var{whoami}, long @var{error_code}, const char *@var{format}, va_list @var{args});
+
+This routine provides an interface, equivalent to @code{com_err} above,
+which may be used by higher-level variadic functions (functions which
+accept variable numbers of arguments).
+
+@end deftypefun
+
+@deftypefun void *set_com_err_hook (void (*@var{proc}) (const char *@var{whoami}, long @var{error_code}, va_list @var{args}) (const char *@var{whoami}, long @var{error_code}, va_list @var{args}));
+
+@deftypefunx void reset_com_err_hook ();
+
+These two routines allow a routine to be dynamically substituted for
+@samp{com_err}. After @samp{set_com_err_hook} has been called,
+calls to @samp{com_err} will turn into calls to the new hook routine.
+@samp{reset_com_err_hook} turns off this hook. This may intended to
+be used in daemons (to use a routine which calls @cite{syslog(3)}), or
+in a window system application (which could pop up a dialogue box).
+
+If a program is to be used in an environment in which simply printing
+messages to the @code{stderr} stream would be inappropriate (such as in a
+daemon program which runs without a terminal attached),
+@code{set_com_err_hook} may be used to redirect output from @code{com_err}.
+The following is an example of an error handler which uses @cite{syslog(3)}
+as supplied in BSD 4.3:
+
+@example
+#include <stdio.h>
+#include <stdarg.h>
+#include <syslog.h>
+
+/* extern openlog (const char * name, int logopt, int facility); */
+/* extern syslog (int priority, char * message, ...); */
+
+void hook (const char * whoami, long code,
+ const char * format, va_list args)
+@{
+ char buffer[BUFSIZ];
+ static int initialized = 0;
+ if (!initialized) @{
+ openlog (whoami,
+ LOG_NOWAIT|LOG_CONS|LOG_PID|LOG_NDELAY,
+ LOG_DAEMON);
+ initialized = 1;
+ @}
+ vsprintf (buffer, format, args);
+ syslog (LOG_ERR, "%s %s", error_message (code), buffer);
+@}
+@end example
+
+After making the call
+@code{set_com_err_hook (hook);},
+any calls to @code{com_err} will result in messages being sent to the
+@var{syslogd} daemon for logging.
+The name of the program, @samp{whoami}, is supplied to the
+@samp{openlog()} call, and the message is formatted into a buffer and
+passed to @code{syslog}.
+
+Note that since the extra arguments to @code{com_err} are passed by
+reference via the @code{va_list} value @code{args}, the hook routine may
+place any form of interpretation on them, including ignoring them. For
+consistency, @code{printf}-style interpretation is suggested, via
+@code{vsprintf} (or @code{_doprnt} on BSD systems without full support for
+the ANSI C library).
+
+@end deftypefun
+
+@node Coding Conventions, Building and Installation, Run-time support routines, Top
+@chapter Coding Conventions
+
+The following conventions are just some general stylistic conventions
+to follow when writing robust libraries and programs. Conventions
+similar to this are generally followed inside the UNIX kernel and most
+routines in the Multics operating system. In general, a routine
+either succeeds (returning a zero error code, and doing some side
+effects in the process), or it fails, doing minimal side effects; in
+any event, any invariant which the library assumes must be maintained.
+
+In general, it is not in the domain of non user-interface library
+routines to write error messages to the user's terminal, or halt the
+process. Such forms of ``error handling'' should be reserved for
+failures of internal invariants and consistency checks only, as it
+provides the user of the library no way to clean up for himself in the
+event of total failure.
+
+Library routines which can fail should be set up to return an error
+code. This should usually be done as the return value of the
+function; if this is not acceptable, the routine should return a
+``null'' value, and put the error code into a parameter passed by
+reference.
+
+Routines which use the first style of interface can be used from
+user-interface levels of a program as follows:
+
+@example
+@{
+ if ((code = initialize_world(getuid(), random())) != 0) @{
+ com_err("demo", code,
+ "when trying to initialize world");
+ exit(1);
+ @}
+ if ((database = open_database("my_secrets", &code))==NULL) @{
+ com_err("demo", code,
+ "while opening my_secrets");
+ exit(1);
+ @}
+@}
+@end example
+
+A caller which fails to check the return status is in error. It is
+possible to look for code which ignores error returns by using lint;
+look for error messages of the form ``foobar returns value which is
+sometimes ignored'' or ``foobar returns value which is always
+ignored.''
+
+Since libraries may be built out of other libraries, it is often necessary
+for the success of one routine to depend on another. When a lower level
+routine returns an error code, the middle level routine has a few possible
+options. It can simply return the error code to its caller after doing
+some form of cleanup, it can substitute one of its own, or it can take
+corrective action of its own and continue normally. For instance, a
+library routine which makes a ``connect'' system call to make a network
+connection may reflect the system error code @code{ECONNREFUSED}
+(Connection refused) to its caller, or it may return a ``server not
+available, try again later,'' or it may try a different server.
+
+Cleanup which is typically necessary may include, but not be limited
+to, freeing allocated memory which will not be needed any more,
+unlocking concurrency locks, dropping reference counts, closing file
+descriptors, or otherwise undoing anything which the procedure did up
+to this point. When there are a lot of things which can go wrong, it
+is generally good to write one block of error-handling code which is
+branched to, using a goto, in the event of failure. A common source
+of errors in UNIX programs is failing to close file descriptors on
+error returns; this leaves a number of ``zombied'' file descriptors
+open, which eventually causes the process to run out of file
+descriptors and fall over.
+
+@example
+@{
+ FILE *f1=NULL, *f2=NULL, *f3=NULL;
+ int status = 0;
+
+ if ( (f1 = fopen(FILE1, "r")) == NULL) @{
+ status = errno;
+ goto error;
+ @}
+
+ /*
+ * Crunch for a while
+ */
+
+ if ( (f2 = fopen(FILE2, "w")) == NULL) @{
+ status = errno;
+ goto error;
+ @}
+
+ if ( (f3 = fopen(FILE3, "a+")) == NULL) @{
+ status = errno;
+ goto error;
+ @}
+
+ /*
+ * Do more processing.
+ */
+ fclose(f1);
+ fclose(f2);
+ fclose(f3);
+ return 0;
+
+error:
+ if (f1) fclose(f1);
+ if (f2) fclose(f2);
+ if (f3) fclose(f3);
+ return status;
+@}
+@end example
+
+@node Building and Installation, Bug Reports, Coding Conventions, Top
+@chapter Building and Installation
+
+The distribution of this package will probably be done as a compressed
+``tar''-format file available via anonymous FTP from SIPB.MIT.EDU.
+Retrieve @samp{pub/com_err.tar.Z} and extract the contents. A subdirectory
+@t{profiled} should be created to hold objects compiled for profiling.
+Running ``make all'' should then be sufficient to build the library and
+error-table compiler. The files @samp{libcom_err.a},
+@samp{libcom_err_p.a}, @samp{com_err.h}, and @samp{compile_et} should be
+installed for use; @samp{com_err.3} and @samp{compile_et.1} can also be
+installed as manual pages.
+
+@node Bug Reports, Acknowledgements, Building and Installation, Top
+@chapter Bug Reports
+
+The principal author of this library is: Ken
+Raeburn, @t{raeburn@@MIT.EDU}.
+
+This version of the com_err library is being maintained by Theodore
+Ts'o, and so bugs and comments should be sent to @t{tytso@@thunk.org}.
+
+
+@node Acknowledgements, , Bug Reports, Top
+@chapter Acknowledgements
+
+I would like to thank: Bill Sommerfeld, for his help with some of this
+documentation, and catching some of the bugs the first time around;
+Honeywell Information Systems, for not killing off the @emph{Multics}
+operating system before I had an opportunity to use it; Honeywell's
+customers, who persuaded them not to do so, for a while; Ted Anderson of
+CMU, for catching some problems before version 1.2 left the nest; Stan
+Zanarotti and several others of MIT's Student Information Processing Board,
+for getting us started with ``discuss,'' for which this package was
+originally written; and everyone I've talked into --- I mean, asked to read
+this document and the ``man'' pages.
+
+@contents
+@bye