malloc serious business A Series of Dire Warnings malloc is hard it s weighted heavily in your final grade you can t start at the last minute a few standard solutions but lots of design room challenging to debug easy to get a zero hard to get help from TAs easy to discover that your solution will never work after you ve spent 20 hours on it easy to break your working solution children will mock you in the street malloc is hard famously so probably the hardest programming assignment in your undergrad career so far can t do malloc will never pass OS if your interviewer is a CMU alum they will ask about this lab subtle bugs can be a massive 50 hours effort can t do malloc at the last minute and get a C you ll probably get a zero no partial credit regardless of what you might have heard or heard from your friends from previous semesters allocator from book doesn t pass the bar no credit broken allocator doesn t pass the bar also no credit if you don t complete at least an explicit allocator implementation you will get a zero subtle bugs two kinds nothing works can be caused by one bad line it almost always works can ALSO be caused by one bad line Debugging is twice as hard as writing the code in the first place Therefore if you write the code as cleverly as possible you are by definition not smart enough to debug it Brian W Kernighan please use version control you will be sad if you dont my allocator worked just five minutes ago just be careful enough to copy working solutions to another directory somewhere or do the symlink thing from the handout malloc c mmmine c or just man up and use svn cvs etc 90 10 rule The first 90 percent of the code will take half the time and the last 10 percent of the code will take the other half of the time if your allocator is 90 percent working you re only half done it happens all the time that a crippling malloc design flaw is discovered hours before the due date leave yourself time to start over ta s are very unhelpful as a rule also we don t like debugging code any more than you do probably less so actually don t expect to hand us your laptop and wander away we don t want to look at your code we can discuss your design we can help troubleshoot VERY specific problems allocations of size blah corrupt my heap after coalescing the previous block at this line number is specific it segfaults is not two ta s are skipping town the week malloc is due come see us now not later set up one on one appointments by email if you d like how to beat malloc plan of attack read malloc section in the textbook look at the textbook allocator copy it out and make it work for 64 bit machines understand how EVERY line works especially the macros look at k r allocator for another slightly different example save this allocator away congratulations you have zero points much too slow if you aren t at least this far you are doomed convert to explicit allocator save this allocator away you should have points now start getting fancy segmented lists other data structures start getting obsessive optimize for the traces etc congratulations you know more than when you started words of wisdom don t use printf for debugging DO use gdb DO write the heap checker as you write your code these should be free points DO start early and often don t feel bad about throwing away broken solutions starting over will clear your head if you re stuck come to office hours this week instead of next week use macros to maximize code readability write other tests besides the really really please don t put this off to the end
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