Category: Ring buffer use cases

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Ring buffer use cases

22.11.2020 Ring buffer use cases

TheoryAlgorithm. Embedded software often involves state machines, circular buffers and queues. This article will give you an overview of the data structure and walks you through the steps involved in implementing circular buffers in low memory devices. If you are already familiar with the basics of such data structures, feel free to jump into the implementation section from the below table of contents.

Choice of a good data structure or algorithm for a given problem comes after a deep understanding of the underlying theory. In this section we will go over some of the key aspects and problems of a circular buffer implementation.

Hopefully, this will allow you to make informed decisions on the choice of data structure. As data is added write to the buffer, the head pointer is incremented and likewise, when the data is being removed read the tail pointer is incremented.

So, for the sake of this discussion, we will consider, that a write is done at head and read at tail.

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Here is a nice GIF from Wikipedia :. The picture says it all. The animation is very fast and may take some iterations before you notice all the cases involved. Do spend the time with this image as it gives a visual representation of the memory and pointers that will be used in later parts of this post. Circular buffers are excessively used to solve the single produce-consumer problem. That is, one thread of execution is responsible for data production and another for consumption.

In the very low to medium embedded devices, the producer is often an Interrupt Service Routine ISR - perhaps data produced from sensors - and consumer is the main event loop.

But why does it have to be circular? It is a very common question that pops out when you hear about circular buffers for the first time. Though the course of this article, the need for these data structures will be made very clear.

On the contrary, they exist there too think of ISRs herein fact, more predominantly there. The nice thing about circular buffers is its elegance.

The down side is its not easy to implement it without race conditions. Another good thing is that they can be implemented without the need for locks for a single producer and single consumer environment. This makes it an ideal data structure for bare-metal embedded programs. The bottom line is that it has to be implemented correctly to be free of a race. The big pain point in circular buffers is that there is no clean way to differentiate the buffer full vs empty cases.

This is because in both the cases, head is equal to tail. That is, initially head and tail pointed point to the same location; when data is filled into the buffer, head is incremented and eventually it wraps around more on this later and catches tail when you fill the N th element into the N element buffer.

At this point, head will point to the same location as tail but now the buffer is actually full, not empty. This article presents a method that gives importance to elegance in design. In this method, we deliberately use only N-1 elements in the N element buffer. The last element is used this of this more like a flag to differentiate between empty and full cases. By this logic. This is the last question that pops up regarding circular buffers. Whether new data has to be discarded or should it overwrite existing data when the buffer is full.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

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A ring buffer is a single contiguous block of memory which contains your items and, when you reach the end, you cycle back to the start:. A circular linked list, due to the linked list nature, does not have to be contiguous at all, so all the elements can be scattered in memory. It simply obeys the property that the elements for a loop:.

The circular linked list has the same advantage over a ring buffer that a linked list has over a fixed array. It can vary in size and you can insert and delete items without shuffling. The disadvantages are similar as well, no O 1 array access and increased work if you're expanding or contracting the list.

A ring buffer would tend to be used when you know the maximum allowed entries in it, or don't mind limiting it. For example, if you have a communications protocol that can throttle the sending side when the buffer becomes full, giving the receiving side time to catch up.

A circular linked list example would be a process list in an operating system where you need to be able to add or delete processes but you don't care about the head of the list, only the current item.

Learn more. Ask Question. Asked 4 years, 7 months ago. Active 4 years, 7 months ago. Viewed 4k times. Active Oldest Votes. That was from implementation point of view. What is the difference from usage point of view? I'll add some more notes. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. The Overflow Blog. The Overflow How many jobs can be done at home?

Using an Array to represent a Circular Queue

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4. Ring Library

Technical site integration observational experiment live on Stack Overflow. Related Due to the resource constrained nature of embedded systems, circular buffer data structures can be found in most projects.

When data is added, the head pointer advances. When data is consumed, the tail pointer advances. If you reach the end of the buffer, the pointers simply wrap around to the beginning. For a more detailed summary of circular buffer operation, please refer to the Wikipedia article. The rest of the article assumes you have an understanding of how circular buffers work. Circular buffers are often used as fixed-sized queues.

The fixed size is beneficial for embedded systems, as developers often try to use static data storage methods rather than dynamic allocations.

Circular buffers are also useful structures for situations where data production and consumption happen at different rates: the most recent data is always available. If the consumer cannot keep up with production, the stale data will be overwritten with more recent data.

By using a circular buffer, we can ensure that we are always consuming the most recent data. For additional use cases, check out Ring Buffer Basics on Embedded. We will start with a C A general-purpose imperative programming language widely used for embedded systems development. Since we are creating a circular buffer library, we want to make sure users work with our library APIs instead of modifying the structure directly.

ring buffer use cases

We also want to keep the implementation contained within our library so we can change it as needed, without requiring end users to update their code.

We will create a handle type that they can use instead. This will prevent us from needing to cast the pointer within our function implementation. Inside of our interface, we would handle the translation to the appropriate pointer type.

We keep the circular buffer type hidden from users, and the only way to interact with the data is through the handle. Using this list, we can put together an API for our library.

Users will interact with the circular buffer library using our opaque handle type, which is created during initialization. You can use any particular type that you like — just be careful to handle the underlying buffer and number of bytes appropriately. Before we proceed, we should take a moment to discuss the method we will use to determine whether or buffer is full or empty.A ring buffer is a data structure that is treated as circular although it its implementation is linear.

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A circular buffer is typically used as a data queue. A circular buffer is a popular way to implement a data stream because the code can be compact.

Creating a Circular Buffer in C and C++

A ring buffer is a common implementation of a queue. It is popular because circular queues are easy to implement. While a ring buffer is represented as a circle, in the underlying code, a ring buffer is linear.

A ring buffer exists as a fixed-length array with two pointers: one that represents the head of a queue, and another that represents the tail.

ring buffer use cases

The first elements of the queue are removed from the head in the order they were added. When the head pointer gets to the end of the array, it wraps around to the first element in the array. Any data in the buffer is overwritten.

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The head of the queue is different from the first element in the actual array and both pointers move as elements are added and removed. One disadvantage of a ring buffer is its fixed size. For queues where elements need to be added and removed in the middle, not just at the start and end of a buffer, an implementation as a linked list is the preferred approach.

Toggle navigation Menu. Home Dictionary Tags Development. Ring Buffer Last Updated: December 27, Definition - What does Ring Buffer mean? A ring buffer is also known as a circular buffer, circular queue or cyclic buffer.

Techopedia explains Ring Buffer A ring buffer is a common implementation of a queue. Share this:. Related Terms. Related Articles. The Most Devastating Computer Viruses. Border Gateway Protocol and Routing Scalability. What is the difference between little endian and big endian data formats?

What is the difference between a virtual machine and a container? What is the difference between alpha testing and beta testing? More of your questions answered by our Experts. Related Tags. Development Memory. Machine Learning and Why It Matters:. Latest Articles. Art Museums and Blockchain: What's the Connection? Cybersecurity Concerns Rise for Remote Work.Jakob Jenkov Last update: A ring buffer is an array which is used as a queue. The ring buffer has a read position and a write position which marks the next position to read from and write to the ring buffer.

When the write position reaches the end of the array, the write position is set back to 0. The same is true for the read position.

Setting the read and write position back to zero when they reach then end of the array is also sometimes referred to as "wrapping around".

It is this behaviour which turns the array into a ring buffer. When the read and write position reaches the end of the array they continue from the beginning of the array, just as if the array was a ring. Hence the name ring buffer.

ring buffer use cases

This ring buffer tutorial will explain how a ring buffer works and show two Java ring buffer implementations. A ring buffer is an array with a fixed size, just like a bounded queue. The array contains the elements stored in the ring buffer. In addition to the array containing the elements, a ring buffer contains a write position which points to the position in the array where the next element is to be inserted into. A ring buffer also needs to keep track of the read position - the next position in the array to read an element from.

A ring buffer also needs to keep track of the free and used space in the element array. When the write position has not wrapped around, the used space is between the write position and the read position. All the rest is free space. The diagram below illustrates a ring buffer in this situation:. When the write position has wrapped around the situation is different. All of a sudden the free space is the space between the write position and the read position.

All the rest is used space. Here is a diagram illustrating the same ring buffer after the write position has wrapped around:.This is a cute little trick that would be way more useful if there was proper OS support for it.

Ring buffers are a really nice data structure — the only problem is that everything that directly interfaces with ring buffers needs to be aware of the fact, to handle wrap-around correctly. A lot of code is written assuming data that is completely linear in memory, and using such code with ring buffers requires copying the data out to a linear block in memory first.

ring buffer use cases

Of course, keeping the multiple copies in sync is both annoying and tricky to get right, and essentially doing every memory access twice is a bad idea. Really the only thing we need is to have the same physical data in two separate logical, or virtual memory locations — and virtual memory paging hardware is, at this point, ubiquitous. If those restrictions are acceptable, then really all we need is some way to get the OS to map the same physical memory multiple times into our address space, using calls available from user space.

And using the right incantations, these OSes can indeed be made to map the same region of physical memory multiple times into a contiguous virtual address range — exactly what we need! The code is available here. The setting had one thread producing variable-sized commands and one thread consuming them, with a SPSC queue inbetween them. With a magic ring buffer, all the logic and special cases just disappear, and so does some amount of wasted memory.

Cool trick. But if you have a ring buffer and you want to hand it off to some other code that assumes a linear buffer, this is a nice trick. Another common case I run into is if you have several buffers and you want to pass them all off to some code that assumes a single linear buffer, it would be awesome if you could set up virtual address maps to make them appear contiguous.

The thing that would be really sweet is if you could map some virtual memory range and associate it with a user callback to service page misses. If you could do that then you could remove all the out-of-range checks from the string matching, which would be a nice speed boost.

If your SPSC is an array of homogeneous items, then it presumably has an integral multiple of the item size, and again wraps will only ever happen at item boundaries — one AND per item inserted, no big deal.

There were lots of commands that were only 4 bytes, but some could contain a kilobyte or more of payload. Similarly on the consumer side.

Both of these are ugly and annoying.

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If you do it using VM, the only cost is a bit of work during init time and however many TLB entries you spend on the wrap-around area usually 1.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information.

I've used it for an in-memory log with a restricted size.

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For example, the application would write log entries while processing user requests. Whenever an exception occurred that would be disruptive to the processing the log records currently in memory would be dumped along with it. The benefit of a circular buffer is, that you don't need infinite amounts of memory, since older entries get overridden automatically.

The "challange" is, that you need to find a suitable size for your usecase. In the example above, it would be very unfortunate when the log record with the most vital information about the exception would have already been overridden.

And on a related note, it conserves harddisk space. Circular buffers are good for serial data streams in embedded systems. Microcontrollers often have a UART to handle a serial byte coming in, these need to be stored in order and dealt with later bytes often come in at a faster rate than they can be handled. The buffer effectively splits the timing-critical response required when the bytes come in, in microseconds to the non timing-critical response to the whole message for example displaying the message which came in, in millisecondse.

As the circular buffer size can be defined pre-compilation the size is then limited. This helps improve space efficiency and should eliminate memory corruption at a trade off to how many bytes can be received before data starts to become lost. One example might be to maintain a sliding average of the last N items. Suppose you want to track the average cost of the last operations of computing some value. To do this, you would need to remove the oldest cost and add in the newest cost.

Without a circular buffer, a costly mechanism for doing this C style would be to have an array of elements. Each time a new cost is computed, you could memmove the 99 elements down and put the new one in at the last position.

This is obviously costly. It would mark the position of the oldest or newest … whichever you choose cost item. After reading the old value for updating the running averageyou replace it with the newest value and increment the buffer position if it is at 99, you set it back to 0 … thus, the circular part. A circular buffer could certainly be implemented with a doubly linked list or even a singly linked list.

But comparing them is a little bit like comparing apples and oranges so to speak. A circular buffer, cyclic buffer or ring buffer is a data structure that uses a single, fixed-size buffer as if it were connected end-to-end.

This structure lends itself easily to buffering data streams. An example that could possibly use an overwriting circular buffer is with multimedia. If the buffer is used as the bounded buffer in the producer-consumer problem then it is probably desired for the producer e. Another example is the digital waveguide synthesis method which uses circular buffers to efficiently simulate the sound of vibrating strings or wind instruments.

With regards comparing to double-linked lists, I imagine it really does depend on what you are using the list for


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