set_current_image_info:
     proc;					/* passively determines the transaction code
						   and indirect descriptor which currently apply to this record */
						/* The validity of this algorithm while records are subject to asynch
						   changes is guaranteed by the strict observance of a standard modification
						   protocol */

	do while ("1"b);				/* loops on detecting asynch changes */
	     time_stamp = fixed (time_stamp_struct.time_last_modified);
						/* to detect asynch change */
	     cur_mod = stat_struct.prev_mod;
	     ref_cnt = block_ptr -> stat_struct.ref_count;
	     if cur_mod = -3			/* creation in progress */
	     then ind_desc = -1;
	     else if block_ptr -> record_block.indirect
	     then ind_desc = ind_struct.prev_desc;
	     else if block_ptr -> record_block.after_applies
	     then do;
		     ind_des.comp = stat_struct.ind_comp;
		     ind_des.offset = time_stamp_struct.ind_offset;
		     mod = stat_struct.modifier;
		     cur_mod = mod;
		     ref_cnt = stat_struct.ref_count_after;
		     go to check_time;		/* validate snapshot */
		end;
	     else ind_desc = current_descrip;		/* compact case */
	     mod = stat_struct.modifier;
	     if block_ptr -> record_block.lock_flag	/* before image may not be valid */
	     then if mod > 0			/* transaction may be in progress on this record */
		then if mod ^= cur_mod		/* before and after are different */
		     then do;
			     new_ind_des.comp = stat_struct.ind_comp;
			     new_ind_des.offset = time_stamp_struct.ind_offset;
			     if cur_mod = stat_struct.prev_mod
						/* in case of asynch change */
			     then do;		/* must look in tcf to see which applies */
				     if ^transaction/* no tcf available */
				     then do;	/* abort with non-zero code */
					     code = error_table_$higher_inconsistency;
						/* can't tell which image
						   is the one to use */
					     return;
						/* abort */
					end;
				     unspec (gk_inf.flags) = "0"b;
				     gk_inf.input_key = "1"b;
				     gk_inf.key_len = 4;
				     gk_inf.head_size = 4;
				     gk_key = mod;	/* current transaction using this record */
				     call iox_$control (tcfp, "get_key", addr (gk_inf), er_code);
				     if er_code = 0
				     then if gk_inf.descrip = -1
						/* past checkpoint */
					then do;	/* use after image info */
						cur_mod = mod;
						ind_desc = new_ind_desc;
						ref_cnt = stat_struct.ref_count_after;
						return;
						/* done */
					     end;
					else ;
				     else if er_code = error_table_$no_key
				     then code = error_table_$higher_inconsistency;
				end;
			end;
	     if cur_mod = stat_struct.prev_mod		/* same as at previous reference */
	     then if cur_mod > 0			/* modifier is unique for this record image */
		then return;
		else do;
check_time:
			if fixed (time_stamp_struct.time_last_modified) = time_stamp
			then return;		/* will use time_stamp for verification */
		     end;
	     if clock () > timeout			/* time limit is exhausted */
	     then do;
		     code = error_table_$record_busy;
		     return;
		end;
	end;					/* keep trying until reference succeeds */

	dcl     er_code		 fixed (35);
	dcl     1 new_ind_des	 based (addr (new_ind_desc)),
		2 comp		 fixed (17) unal,
		2 offset		 bit (18) unal;
	dcl     new_ind_desc	 fixed (35);
	dcl     gk_key		 fixed (35) based (addr (gk_inf.key));
	dcl     1 gk_inf,
		2 header		 like gk_header,
		2 key		 char (4);
     end set_current_image_info;

     dcl	   1 ind_des	      like ind_des_structure based (addr (ind_desc));
     dcl	   1 stat_struct	      like stat_structure based (block_ptr);
     dcl	   1 ind_struct	      like ind_structure based (block_ptr);
     dcl	   1 time_stamp_struct    like time_stamp_structure based (addr (stat_struct.time_stamp_words));
     dcl	   1 record_block	      like record_block_structure based (block_ptr);


*/
                                          -----------------------------------------------------------


Historical Background

This edition of the Multics software materials and documentation is provided and donated
to Massachusetts Institute of Technology by Group Bull including Bull HN Information Systems Inc. 
as a contribution to computer science knowledge.  
This donation is made also to give evidence of the common contributions of Massachusetts Institute of Technology,
Bell Laboratories, General Electric, Honeywell Information Systems Inc., Honeywell Bull Inc., Groupe Bull
and Bull HN Information Systems Inc. to the development of this operating system. 
Multics development was initiated by Massachusetts Institute of Technology Project MAC (1963-1970),
renamed the MIT Laboratory for Computer Science and Artificial Intelligence in the mid 1970s, under the leadership
of Professor Fernando Jose Corbato. Users consider that Multics provided the best software architecture for 
managing computer hardware properly and for executing programs. Many subsequent operating systems 
incorporated Multics principles.
Multics was distributed in 1975 to 2000 by Group Bull in Europe , and in the U.S. by Bull HN Information Systems Inc., 
as successor in interest by change in name only to Honeywell Bull Inc. and Honeywell Information Systems Inc. .

                                          -----------------------------------------------------------

Permission to use, copy, modify, and distribute these programs and their documentation for any purpose and without
fee is hereby granted,provided that the below copyright notice and historical background appear in all copies
and that both the copyright notice and historical background and this permission notice appear in supporting
documentation, and that the names of MIT, HIS, Bull or Bull HN not be used in advertising or publicity pertaining
to distribution of the programs without specific prior written permission.
    Copyright 1972 by Massachusetts Institute of Technology and Honeywell Information Systems Inc.
    Copyright 2006 by Bull HN Information Systems Inc.
    Copyright 2006 by Bull SAS
    All Rights Reserved

*/