Memory leak facts for kids

Imagine your computer has a special storage area called 'memory'. A memory leak happens when a computer program uses some of this memory but then forgets to give it back when it's done. It's like borrowing a toy and never returning it! Sometimes, a program might even store something in memory and then lose track of where it put it. So, that memory can't be used again.

Memory leaks can be tricky to find. Usually, only the person who wrote the program can figure out if one is happening. Because they can use up all the available memory, memory leaks often make programs run slower over time.

What Happens When Memory Leaks?

A memory leak makes your computer slower by using up its available memory. It can cause programs to use more memory than they should, making everything run slower. In the worst cases, too much memory might be taken, and parts of your computer or device could stop working. The program might crash, or the whole system could become very slow.

Memory leaks might not always be a big deal. In modern computer systems, any memory a program uses is usually given back when the program closes. This means a leak in a program that only runs for a short time might not even be noticed.

However, some leaks are much more serious:

• When a program runs for a very long time, like background tasks on servers or in devices that stay on for years.
• When new memory is needed very often for quick tasks, such as making the frames of a video game or an animated movie.
• When a program asks for special memory that isn't given back, even after the program closes.
• When a device has very little memory to begin with, like a small embedded system or a portable device.
• When the leak happens inside the computer's main operating system.
• When a special program called a 'device driver' causes a leak.
• When the computer's operating system doesn't automatically give back memory when a program finishes.

An Example: The Elevator Program

Let's look at a simple example using an elevator program. This part of the program runs every time someone presses a floor button inside the elevator.

When a button is pressed: Get some memory to remember the floor number Put the floor number into that memory Are we already on the target floor? If yes, we don't need to do anything: finished Otherwise: Wait until the elevator is not busy Go to the floor needed Release the memory we used for the floor number

The memory leak would happen if someone presses the button for the floor the elevator is already on. In this case, the program skips the step where it releases the memory. Each time this happens, a little more memory is leaked.

Leaks like this usually don't cause problems right away. People don't often press the button for the floor they are already on. Also, the elevator might have enough spare memory that this could happen hundreds of times before it's an issue. However, the elevator will eventually run out of memory. This could take months or even years, so it might not be found during testing.

If the elevator runs out of memory, it would be a problem. At the very least, it might stop responding to requests to move. If other parts of the program need memory (like the part that opens and closes the door), then people might not be able to get in or out.

The memory leak lasts until the system is reset. For example, if the elevator's power was turned off, the program would stop. When the power comes back on, the program restarts, and all the memory is available again. But the slow memory leak process would also start again.

The leak in the example above can be fixed by always releasing the memory, no matter what:

When a button is pressed: Get some memory to remember the floor number Put the floor number into that memory Are we already on the target floor? If not: Wait until the elevator is not busy Go to the floor needed Release the memory we used for the floor number

Programming Challenges

Memory leaks are a common mistake in programming. This is especially true when using programming languages like C and C++ that don't automatically clean up unused memory. Usually, a memory leak happens because memory that was set aside for a program can no longer be reached or used. Many special tools have been made to help programmers find these hidden memory problems.

Even though special programs can find memory that can't be reached, they can't free memory that is still "reachable" and might seem useful. Modern memory systems let programmers mark memory with different levels of importance. This helps the system know what memory can be safely removed.

In general, systems that automatically manage memory are easier for programmers to use. They don't have to worry about cleaning up memory themselves. However, automatic memory management can sometimes make programs run a little slower, and it doesn't stop all types of memory leaks.

How Memory Leaks Affect Your Computer

If a program has a memory leak, its memory use will slowly grow. There won't usually be an immediate sign of a problem. Every computer has a limited amount of memory. If the leak isn't stopped (for example, by restarting the program), it will eventually cause problems.

Most modern computers use both main memory (RAM chips) and secondary storage (like a hard drive). Memory is given out as needed. Active parts of programs are kept in main memory for fast access. Less active parts are moved to secondary storage to make room. When one program starts using a lot of memory, it pushes other programs out to secondary storage. This usually makes the whole system run much slower. Even if the leaking program is closed, it might take some time for other programs to move back into main memory, and for performance to return to normal.

When all the memory on a system is used up, any attempt to get more memory will fail. This usually causes the program trying to get memory to crash. Some programs are designed to recover from this. The first program to run out of memory might not even be the one causing the leak.

Some operating systems have special ways to deal with running out of memory. They might close programs randomly, or close the program using the most memory. Some operating systems also limit how much memory each program can use. This stops one program from using all the memory. The downside is that sometimes the system needs to be changed to let programs that truly need a lot of memory (like those for graphics or science) run properly.

The "sawtooth" pattern of memory use: the sudden drop in used memory can mean a memory leak.

If the memory leak is in the computer's core operating system (the 'kernel'), the whole computer will likely crash. Simpler computers, like many embedded systems, can also completely fail from a memory leak.

Public systems like web servers can be attacked if someone finds a way to make them leak memory. This is called an 'exploit'.

A "sawtooth" pattern of memory use (where memory use goes up, then suddenly drops, then goes up again) can be a sign of a memory leak. The drops often happen when the program is restarted.

Other Reasons for High Memory Use

It's important to know that memory use that keeps going up isn't always a memory leak. Some programs store more and more information in memory on purpose, like a cache (a place to store frequently used data for quick access). If the cache gets too big and causes problems, it might be a design mistake, but it's not a memory leak because the information is still technically being used.

In other cases, programs might need a lot of memory because the programmer thought there would always be enough. For example, a program that works with image files might try to load an entire huge image into memory at once. This won't work if the image is too big for the available memory.

So, a memory leak is a specific type of programming mistake. If you just see a program using more and more memory, you can only guess that there *might* be a memory leak. It's better to say "constantly increasing memory use" unless you know for sure it's a leak.

A Simple C++ Example

Here's a simple example in a programming language called C++. This program purposely leaks memory by losing track of where it put the memory it asked for.

int main() {
     int* a = new int(5);
     a = nullptr;
    /* The pointer 'a' no longer exists, so the memory cannot be freed.
     But the system still has that memory set aside.
     If the program keeps doing this without freeing memory,
     it will use up more and more memory.
     This causes a leak. */
}

See also

• Buffer overflow
• Memory management
• Memory debugger