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C Programming Language: Annotations on K&R II

A better error() function

#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>
#include <errno.h>

 * swerr() *

   PURPOSE: Report errors detected during software execution to the
       standard error device.
   PRE:  An error has been detected, 
         `prog_name' is a global variable with the program's name.
   POST: A detailed error message has been sent to the standard error
       device.  The message includes the paramaters as descibed below.
       If the `error_level' was not 0 then the program has been halted
       with a message and a failure code has been returned to the O/S
       (in the program's exit value).  The program may also exit with
       the same error indication if this function detects an error
       while writing to the standard error device.

   NOTE: Adapted from error() (p.174) of K&R II 
         Advice from Jutta Degener about use of const acknowledged
void swerr(char const *proc,        /* function where the error 
                                       occurred, not necessarily where
                                       swerr() was called from */
           const int  error_level,  /* indicates severity of error */
                 char *format, ...) /* printf()-like */
   extern char *  progname;         
          int     err_num = errno;
          int     status;
          va_list args;

   status = fprintf(stderr, "%s [%s()] ", progname, proc);
   va_start(args, format);
   if (EOF != status) {
      status = vfprintf(stderr, format, args);
   if (err_num) {  /* error reported by library call (includes '\n') */
      status = fprintf(stderr, ", ");
      errno = err_num; /* use stored value in case 
                          fprintf() changes it */
      perror(NULL);  /* error message from library call */
   } else {
      if (EOF != status) {
         status = fprintf(stderr, "\n");
   if ((! error_level) || (EOF == status)) {
}/* swerr() */

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Last updated by J. Blustein on 3 January 1998.

This document is copyright by its author, J. Blustein <>.

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