已分配但未释放的内存

Question

I have read this-

Memory that is allocated by malloc(for example) and that is not freed using free() function is released when the program terminates.And that it is done by the opearting system. So when does having or not having a garbage collector come into picture?

Or is it that not all operating systems do this automatic release of memory on program termination?

Answer 1

That claim about malloc and free is correct for all modern computing operating systems. But the statement as a whole reflects a complete misunderstanding of the purpose of garbage collection.

The reason you call free is not to clean things up for after your program terminates. The reason you call free is to permit the memory to be re-used during the subsequent execution of a long-running program.

Consider a message server that handles a hundred messages per second. You call malloc when you receive a new message. And then you have to do lots of things with it. You may have to log it. You may have to send it to other clients. You may have to write it to a database. When you are done, if you don't free it, after a few days you'll have millions of messages stuck in memory. So you have to call free .

But when do you call free ? When one client is done sending a message, another client might still be using it. And maybe the database still needs it.

The purpose of garbage collection is to ensure that object's used memory is released (so it can be re-used to hold a new message during the application's lifetime) without having to burden the application programmer with the duty (and risks) associated with tracking exactly when the object is no longer required by any code that might be using it.

If an application doesn't run for very long or doesn't have any objects whose lifetimes are difficult to figure out, then garbage collection doesn't do very much good. And there are other techniques (such as reference-counted pointers) that can provide many of the same benefits as garbage collection. But there is a real problem that garbage collection does solve.

Answer 2

Most modern operating systems will indeed free everything you're allocated on program termination. However, a garbage collector will free unused memory before program termination. This allows your program to skip the frees, but still manage to keep allocating memory indefinitely, as long as it lets go to references to memory that isn't being used anymore, and as long as your total working set size doesn't exceed physical memory limits.

Answer 3

All OS do free memory when the program quits. Memory leaks are 'only' a problem because they waste memory on the machine, or cause the program to crash. So you'd garbage collect to prevent these things from happening, but without worrying about actually freeing your own pointers when you're done with them.

They're two solutions to the same problem, really. But, it's because the problem happens during runtime that you're worried about it.

Answer 4

Imagine a long running process like a web server that malloc() sa bunch of data structures for every connection it services. If it never free() s any of that memory, the process memory usage will continually grow, possibly consuming everything available on the system.