Memory leak

A memory leak can diminish the performance of the computer by reducing the amount of available memory. Eventually, in the worst case, too much of the available memory may become allocated and all or part of the system or device stops working correctly, the application fails, or the system slows down unacceptably due to thrashing.
Memory leaks may not be serious or even detectable by normal means. In modern operating systems, normal memory used by an application is released when the application terminates. This means that a memory leak in a program that only runs for a short time may not be noticed and is rarely serious.
Leaks that are much more serious include:
Where the program runs for an extended time and consumes additional memory over time, such as background tasks on servers, but especially in embedded devices which may be left running for many years.
Where new memory is allocated frequently for one-time tasks, such as when rendering the frames of a computer game or animated video.
Where the program is able to request memory — such as shared memory — that is not released, even when the program terminates.
Where memory is very limited, such as in an embedded system or portable device.
Where the leak occurs within the operating system or memory manager.
Where the leak is the responsibility of a system device driver.
Where running on an operating systems that does not automatically release memory on program termination. Often on such machines if memory is lost, it can only be reclaimed by a reboot, an example of such a system being AmigaOS.


The following example, written in pseudocode, is intended to show how a memory leak can come about, and its effects, without needing any programming knowledge. The program in this case is part of some very simple software designed to control an elevator. This part of the program is run whenever anyone inside the elevator presses the button for a floor.
When a button is pressed:
  Get some memory, which will be used to remember the floor number
  Put the floor number into the memory
  Are we already on the target floor?
    If so, we have nothing to do: finished
    Otherwise:
      Wait until the lift is idle
      Go to the required floor
      Release the memory we used to remember the floor number
The memory leak would occur if the floor number requested is the same floor that the lift is on; the condition for releasing the memory would be skipped. Each time this case occurs, more memory is leaked.
Cases like this wouldn't usually have any immediate effects. People do not often press the button for the floor they are already on, and in any case, the lift might have enough spare memory that this could happen hundreds or thousands of times. However, the lift will eventually run out of memory. This could take months or years, so it might not be discovered despite thorough testing.
The consequences would be unpleasant; at the very least, the lift would stop responding to requests to move to another floor. If other parts of the program need memory (a part assigned to open and close the door, for example), then someone may be trapped inside, since the software cannot open the door.
The memory leak lasts until the system is reset. For example: if the lift's power were turned off the program would stop running. When power was turned on again, the program would restart and all the memory would be available again, but the slow process of memory leak would restart together with the program, eventually prejudicing the correct running of the system.