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  Variable:  _doserrno, errno, sys_errlist, sys_nerr
 
  Include:   <stdlib.h>, <errno.h>
 
  Syntax:    int _doserrno;
             int errno;
             char * sys_errlist[ ];
             int sys_nerr;
 
     The errno, sys_errlist, and sys_nerr variables are used by the
     perror function to print error information, and are declared in
     the include file STDLIB.H. When an error occurs in a system-level
     call, the errno variable is set to an integer value to reflect the
     type of error.
 
     The perror function uses the errno value to look up the
     corresponding error message in the sys_errlist table. The value
     of the sys_nerr variable is defined as the number of elements in
     the sys_errlist array.
 
     The following errno values are supported:
 
     E2BIG      ECHILD        EINVAL      ENOMEM
     EACCES     EDEADLOCK     EMFILE      ENOSPC
     EAGAIN     EDOM          ENOENT      ERANGE
     EBADF      EEXIST        ENOEXEC     EXDEV
 
     The errno values on DOS and OS/2 are a subset of the values for
     errno on XENIX systems. Therefore, the value assigned to errno in
     case of error does not necessarily correspond to the actual error
     code returned by a DOS or OS/2 system call. Instead, the actual
     DOS and OS/2 error codes are mapped onto the perror values.
 
     Use the _doserrno variable if you want to access the actual
     operating-system error code. When an error occurs in a system
     call, the _doserrno variable is assigned the actual error code
     returned by the corresponding operating-system call.
 
     The value of errno reflects the error value for the last call that
     set errno. The errno value is not automatically cleared by later
     successful calls. Thus, to obtain accurate results, you should
     test for errors and print error messages, if desired, immediately
     after a call.
 
     In general, you should use _doserrno only for error detection in
     operations involving input and output, since the errno values for
     input and output errors have operating-system error-code
     equivalents. Not all of the error values available for errno have
     exact operating-system error-code equivalents, and some may have
     no equivalents, causing the value of _doserrno to be undefined.