[Search for users] [Overall Top Noters] [List of all Conferences] [Download this site]

Conference turris::decc

Title:	DECC
Notice:	General DEC C discussions
Moderator:	TLE::D_SMITHNTE

Created:	Fri Nov 13 1992
Last Modified:	Fri Jun 06 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	2212
Total number of notes:	11045

2181.0. "behavior of increment operator" by MILORD::BISHOP (The punishment that brought us peace was upon Him) Wed May 07 1997 11:53

    In the following statement, which value of tim is passed to SYS$CANTIM?
    The value before incrementing or the value after?
    
    	SYS$CANTIM (tim++, 0);
    
    Reason for asking is that DEC C T5.2-006 (as used in the build of
    OpenVMS Alpha V7.0) passes the post-increment value, but DEC C V5.2-003
    (in use for V7.1 and current Raven builds) passes the pre-increment
    value.
    
    My reading of my old C course notes is that the new behavior is
    correct, but I'd like to confirm this (i.e. was a bug fixed between
    those versions, or was it introduced then?)
    
    btw I intend to make this a moot point by changing the code to 
    
    	SYS$CANTIM (tim, 0);
    	tim++;
    
    It's safer and clearer and by the time GEM is done optimizing, the 
    code is much the same.
    
    Thanks
    
    - Richard.

T.R	Title	User	Personal Name	Date	Lines
2181.1	pre-increment uses ++prefix, post-increment uses postfix++	XDELTA::HOFFMAN	Steve, OpenVMS Engineering	`Wed May 07 1997 12:05`	10
	With ++tim, the value before the increment is used, while with tim++, the value after the increment is used. In either case, the value is incremented. The compiler-specific order of evaluation -- which doesn't factor in with this particular code fragment -- is also an important consideration. This appears to be a bug in T5.2-006, and your workaround should operate as expected on both DEC C T5.2-006, DEC C V5.2-003, and other C compilers
2181.2		WIBBIN::NOYCE	Pulling weeds, pickin' stones	`Wed May 07 1997 12:21`	7
	No, Steve, you've got it backwards. ++tim increments tim, then uses the new result. tim++ uses the old value, then increments tim. It looks as if the "T" version is broken, but the "V" version works.
2181.3	ignoranaleptic seizures	XDELTA::HOFFMAN	Steve, OpenVMS Engineering	`Wed May 07 1997 12:38`	4
	:No, Steve, you've got it backwards. Whoops -- brain cramp.
2181.4	thanks	MILORD::BISHOP	The punishment that brought us peace was upon Him	`Wed May 07 1997 13:24`	7
	thanks guys...what I expected (except for Steve's open-foot-insert-mouth routine. :-) Right now, there are some very happy devos in EDO who are no longer suffering from horrendous hangs in the System Code Debugger... - Richard.