What REALLY happens when you don't free after malloc?

Question

This has been something that has bothered me for ages now.

We are all taught in school (at least, I was) that you MUST free every pointer that is allocated. I\'m a

Answer 1

I typically free every allocated block once I'm sure that I'm done with it. Today, my program's entry point might be main(int argc, char *argv[]) , but tomorrow it might be foo_entry_point(char **args, struct foo *f) and typed as a function pointer.

So, if that happens, I now have a leak.

Regarding your second question, if my program took input like a=5, I would allocate space for a, or re-allocate the same space on a subsequent a="foo". This would remain allocated until:

1. The user typed 'unset a'
2. My cleanup function was entered, either servicing a signal or the user typed 'quit'

I can not think of any modern OS that does not reclaim memory after a process exits. Then again, free() is cheap, why not clean up? As others have said, tools like valgrind are great for spotting leaks that you really do need to worry about. Even though the blocks you example would be labeled as 'still reachable' , its just extra noise in the output when you're trying to ensure you have no leaks.

Another myth is "If its in main(), I don't have to free it", this is incorrect. Consider the following:

char *t;

for (i=0; i < 255; i++) {
    t = strdup(foo->name);
    let_strtok_eat_away_at(t);
}

If that came prior to forking / daemonizing (and in theory running forever), your program has just leaked an undetermined size of t 255 times.

A good, well written program should always clean up after itself. Free all memory, flush all files, close all descriptors, unlink all temporary files, etc. This cleanup function should be reached upon normal termination, or upon receiving various kinds of fatal signals, unless you want to leave some files laying around so you can detect a crash and resume.

Really, be kind to the poor soul who has to maintain your stuff when you move on to other things .. hand it to them 'valgrind clean' :)