Inform the CSPICE error handling mechanism of entry into a
   routine.

   ERROR

   VARIABLE  I/O  DESCRIPTION
   --------  ---  ---------------------------------------------------
   module     I   The name of the calling routine.

   module         is the name of the routine calling chkin_c.  The
                  named routine is supposed to be `checking in'
                  when it calls chkin_c; that is, the call should be
                  the first executable statement following the
                  reference to the function return_c() (which should be
                  the first executable statement).

   None.

   None.

   This routine is part of the CSPICE error handling mechanism.

   Conceptually, the effect of this routine is to `push' the
   supplied module name onto a stack.  The routine chkout_c performs
   the inverse, or `pop', operation.

   Every routine that participates in the traceback scheme should
   have a call to chkin_c as the second executable statement.  The
   first executable statements should be:

      if ( return_c() )
         {
         return;
         }
      else
         {
         chkin_c ( module );
         }


   Here module is the name of the routine in which this code appears.

   The line of code preceding the exit or any return statement should
   be

       chkout_c ( module );


   All CSPICE routines should call chkin_c and chkout_c, unless they
   are classified as `error free'.  Programs linked with CSPICE
   may also use chkin_c and chkout_c.

   Routines that don't call chkin_c and chkout_c won't appear in the
   traceback.

   All routines that call chkin_c must also call chkout_c, or else the
   trace mechanism will become very confused and require therapy.

   It is possible to disable check-ins (and check-outs) by calling
   the trcoff_c.  chkin_c and chkout_c will return immediately
   upon entry after trcoff_c has been called.  It is not possible to
   re-enable check-ins and check-outs after calling trcoff_c. Routines
   that don't call chkin_c and chkout_c won't appear in the traceback.

   See `Particulars' for an example of how to call this routine.

   Routines that call this routine must call chkout_c immediately
   prior to any return or exit statement.

   module names are assumed to have no embedded blanks.

   1) The error SPICE(EMPTYSTRING) is signalled if the input
      string does not contain at least one character, since the
      input string cannot be converted to a Fortran-style string
      in this case.

   2) The error SPICE(NULLPOINTER) is signalled if the input string
      pointer is null.

   The underlying f2c'd CSPICE routine chkin_ does not signal errors;
   rather it writes error messages, so as to avoid recursion.  The
   errors detected by chkin_ are:

   3)  If the traceback storage area overflows, the short error
       message "SPICE(TRACEBACKOVERFLOW)" is written to the error
       output device.

   4)  If the input argument module is blank, the short error message
       SPICE(BLANKMODULENAME) is written to the error output device.

   None.

   K.R. Gehringer  (JPL)
   N.J. Bachman    (JPL)

   None.

   -CSPICE Version 2.0.3, 23-JUL-2001 (NJB)

      Tabs removed from source file.

   -CSPICE Version 2.0.2, 25-MAR-1998 (EDW)

      Minor corrections to header.

   -CSPICE Version 2.0.1, 08-FEB-1998 (EDW)

      Corrected and clarified header entries.

   -CSPICE Version 2.0.0, 09-JAN-1998 (NJB)

      Input argument filename was changed to type ConstSpiceChar *.

      Re-implemented routine without dynamically allocated, temporary
      strings.

   -CSPICE Version 1.0.0, 25-OCT-1997 (EDW)

   module check in