friendly messages

[John F Carr]

I include in this article a manual entry for a library function for error
reporting developed by the Student Information Processing Board at MIT.
This is useful because it allows libraries to define their own set of
error codes and messages without adding any user level error processing
code.  Library functions can return an internal error code or a unix error.
The com_err function recognizes the type of error and prints an appropriate
 message.  Each library is given its own range of error codes to avoid
conflict.

It is still possible to print useless or incomprehensible error messages
when using this library, but it encourages a consistent, hopefully good,
style of error reporting.

COM_ERR(3)          UNIX Programmer's Manual           COM_ERR(3)



NAME
     com_err - common error display routine

SYNOPSIS
      #include <com_err.h>

     void com_err (whoami, code, format, ...);
          const char *whoami;
          long code;
          const char *format;

     proc = set_com_err_hook (proc);
     void (* proc ) (const char *, long, const char *, va_list);

     proc = reset_com_err_hook ();

     void initialize_XXXX_error_table ();

DESCRIPTION
     Com_err displays an error message on the standard error
     stream stderr (see stdio(3S)) composed of the whoami string,
     which should specify the program name or some subportion of
     a program, followed by an error message generated from the
     code value (derived from compile_et(1)), and a string pro-
     duced using the format string and any following arguments,
     in the same style as fprintf(3).

     The behavior of com_err can be modified using
     set_com_err_hook; this defines a procedure which is called
     with the arguments passed to com_err, instead of the default
     internal procedure which sends the formatted text to error
     output.  Thus the error messages from a program can all
     easily be diverted to another form of diagnostic logging,
     such as syslog(3).  Reset_com_err_hook may be used to
     restore the behavior of com_err to its default form.  Both
     procedures return the previous ``hook'' value.  These
     ``hook'' procedures must have the declaration given for proc
     above in the synopsis.

     The initialize_XXXX_error_table routine is generated mechan-
     ically by compile_et(1) from a source file containing names
     and associated strings.  Each table has a name of up to four
     characters, which is used in place of the XXXX in the name
     of the routine.  These routines should be called before any
     of the corresponding error codes are used, so that the
     com_err library will recognize error codes from these tables
     when they are used.

     The com_err.h header file should be included in any source
     file that uses routines from the com_err library; executable
     files must be linked using ``-lcom_err'' in order to cause
     the com_err library to be included.

SEE ALSO
     compile_et (1), syslog (3).

     Ken Raeburn, "A Common Error Description Library for UNIX".

-------

Here is an example of its usage (from "zwgc", the main client for the
Zephyr message delivery service):

            if ( (nzqueued = ZPending()) == -1 )
              {
                if(errno == ZERR_EOF)
                  {
                    com_err("zwgc",errno," on select");
                    exit(-1);   /* if eof, we still can't exit_cleanly */
                  }


--
   John Carr             "When they turn the pages of history,
   jfc at Athena.mit.edu     When these days have passed long ago,
   bloom-beacon!          Will they read of us with sadness
   athena.mit.edu!jfc     For the seeds that we let grow?"  --Neil Peart