Memory leak in C,C++; forgot to do free,delete

Question

We allocate memory in C using malloc and in C++ using new. I know that memory allocated must be freed or given back to OS using free in C and delete in C++. If I forgot to use free/delete after allocating memory, it means there will be memory leak.

Now, my question is, is this memory leak only during the time period of execution of program; or is it permanent leak/loss or is it gained again once I restart the system ? What is the internal process actually ? What does memory leak/loss mean exactly?

I would be very grateful if someone could explain this in detail or provide me with some nice references.

UPDATE 1

After reading some answers, I learnt that memory is given back to OS/system after program terminates, if that is so, why everyone needs to care so much about memory leak, and why preventing memory leak is very important ?

UPDATE 2

So, memory leak should be prevented so that the system doesn't crash due to lack of sufficient memory for allocation purposes ??

UPDATE 3

So, after reading all answers, I realized that memory leak is quiet important issues to prevent system crashes. But, for a beginner like me, how can I be sure if my program is completely free from memory leak. I try to do free,delete if I am using malloc,new but sometimes, it gets messy. Is there any tool or method which I can use to know if my program is doing any memory leak ?

UPDATE 4

After reading answers, I have now understood the importance of memory leak free code, less use of new/delete, more use of STL, learnt new stuffs like RAII,valgrind and good programming practices. Thanks to all :)

Answer 1

It's per-process. Once your process exits, the allocated memory is returned to the OS for use by other processes (new or existing).

To answer your edited question, there's only a finite amount of memory in your machine. So if you have a memory leak, then the major problem is that the memory isn't available for other processes to use. A secondary, but not negligible, effect is that your process image grows, you'll swap to disc and performance will be hit. Finally your program will exhaust all the memory in the system and fail, since it's unable to allocate any memory for itself.

It's arguable that for a small process with a short lifetime, memory leaks are tolerable, since the leaked memory will be small in quantity and short-lived.

Take a look at this resource, for possibly more info than you'll ever need. What we're discussing here is dynamic or heap allocation.