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Ring Buffers

A ring buffer is a circular buffer, whose contents are stored in first-in-first-out order.

For circumstances where an application needs to implement asynchronous “streaming” copying of data, Zephyr provides a struct ring_buf abstraction to manage copies of such data in and out of a shared buffer of memory.

Two content data modes are supported:

  • Byte mode: raw bytes can be enqueued and dequeued.

  • Data item mode: Multiple 32-bit word data items with metadata can be enqueued and dequeued from the ring buffer in chunks of up to 1020 bytes. Each data item also has two associated metadata values: a type identifier and a 16-bit integer value, both of which are application-specific.

While the underlying data structure is the same, it is not legal to mix these two modes on a single ring buffer instance. A ring buffer initialized with a byte count must be used only with the “bytes” API, one initialized with a word count must use the “items” calls.

Concepts

Any number of ring buffers can be defined (limited only by available RAM). Each ring buffer is referenced by its memory address.

A ring buffer has the following key properties:

  • A data buffer of bytes or 32-bit words. The data buffer contains the raw bytes or 32-bit words that have been added to the ring buffer but not yet removed.

  • A data buffer size, measured in bytes or 32-byte words. This governs the maximum amount of data (including possible metadata values) the ring buffer can hold.

A ring buffer must be initialized before it can be used. This sets its data buffer to empty.

A struct ring_buf may be placed anywhere in user-accessible memory, and must be initialized with ring_buf_init() or ring_buf_element_init() before use. This must be provided a region of user-controlled memory for use as the buffer itself. Note carefully that the units of the size of the buffer passed change (either bytes or words) depending on how the ring buffer will be used later. Macros for combining these steps in a single static declaration exist for convenience. RING_BUF_DECLARE will declare and statically initialize a ring buffer with a specified byte count, where RING_BUF_ITEM_DECLARE will declare and statically initialize a buffer with a given count of 32 bit words. RING_BUF_ITEM_SIZEOF will compute the size in 32-bit words corresponding to a type or an expression. Note: rounds up if the size is not a multiple of 32 bits.

“Bytes” data may be copied into the ring buffer using ring_buf_put(), passing a data pointer and byte count. These bytes will be copied into the buffer in order, as many as will fit in the allocated buffer. The total number of bytes copied (which may be fewer than provided) will be returned. Likewise ring_buf_get() will copy bytes out of the ring buffer in the order that they were written, into a user-provided buffer, returning the number of bytes that were transferred.

To avoid multiply-copied-data situations, a “claim” API exists for byte mode. ring_buf_put_claim() takes a byte size value from the user and returns a pointer to memory internal to the ring buffer that can be used to receive those bytes, along with a size of the contiguous internal region (which may be smaller than requested). The user can then copy data into that region at a later time without assembling all the bytes in a single region first. When complete, ring_buf_put_finish() can be used to signal the buffer that the transfer is complete, passing the number of bytes actually transferred. At this point a new transfer can be initiated. Similarly, ring_buf_get_claim() returns a pointer to internal ring buffer data from which the user can read without making a verbatim copy, and ring_buf_get_finish() signals the buffer with how many bytes have been consumed and allows for a new transfer to begin.

“Items” mode works similarly to bytes mode, except that all transfers are in units of 32 bit words and all memory is assumed to be aligned on 32 bit boundaries. The write and read operations are ring_buf_item_put() and ring_buf_item_get(), and work otherwise identically to the bytes mode APIs. There no “claim” API provided for items mode. One important difference is that unlike ring_buf_put(), ring_buf_item_put() will not do a partial transfer; it will return an error in the case where the provided data does not fit in its entirety.

The user can manage the capacity of a ring buffer without modifying it using either ring_buf_space_get() or ring_buf_item_space_get() which returns the number of free bytes or free 32-bit item words respectively, or by testing the ring_buf_is_empty() predicate.

Finally, a ring_buf_reset() call exists to immediately empty a ring buffer, discarding the tracking of any bytes or items already written to the buffer. It does not modify the memory contents of the buffer itself, however.

Byte mode

A byte mode ring buffer instance is declared using RING_BUF_DECLARE() and accessed using: ring_buf_put_claim(), ring_buf_put_finish(), ring_buf_get_claim(), ring_buf_get_finish(), ring_buf_put() and ring_buf_get().

Data can be copied into the ring buffer (see ring_buf_put()) or ring buffer memory can be used directly by the user. In the latter case, the operation is split into three stages:

  1. allocating the buffer (ring_buf_put_claim()) when user requests the destination location where data can be written.

  2. writing the data by the user (e.g. buffer written by DMA).

  3. indicating the amount of data written to the provided buffer (ring_buf_put_finish()). The amount can be less than or equal to the allocated amount.

Data can be retrieved from a ring buffer through copying (see ring_buf_get()) or accessed directly by address. In the latter case, the operation is split into three stages:

  1. retrieving source location with valid data written to a ring buffer (see ring_buf_get_claim()).

  2. processing data

  3. freeing processed data (see ring_buf_get_finish()). The amount freed can be less than or equal or to the retrieved amount.

Data item mode

A data item mode ring buffer instance is declared using RING_BUF_ITEM_DECLARE() and accessed using ring_buf_item_put() and ring_buf_item_get().

A ring buffer data item is an array of 32-bit words from 0 to 1020 bytes in length. When a data item is enqueued (ring_buf_item_put()) its contents are copied to the data buffer, along with its associated metadata values (which occupy one additional 32-bit word). If the ring buffer has insufficient space to hold the new data item the enqueue operation fails.

A data item is dequeued (ring_buf_item_get()) from a ring buffer by removing the oldest enqueued item. The contents of the dequeued data item, as well as its two metadata values, are copied to areas supplied by the retriever. If the ring buffer is empty, or if the data array supplied by the retriever is not large enough to hold the data item’s data, the dequeue operation fails.

Concurrency

The ring buffer APIs do not provide any concurrency control. Depending on usage (particularly with respect to number of concurrent readers/writers) applications may need to protect the ring buffer with mutexes and/or use semaphores to notify consumers that there is data to read.

For the trivial case of one producer and one consumer, concurrency control shouldn’t be needed.

Internal Operation

Data streamed through a ring buffer is always written to the next byte within the buffer, wrapping around to the first element after reaching the end, thus the “ring” structure. Internally, the struct ring_buf contains its own buffer pointer and its size, and also a set of “head” and “tail” indices representing where the next read and write operations may occur.

This boundary is invisible to the user using the normal put/get APIs, but becomes a barrier to the “claim” API, because obviously no contiguous region can be returned that crosses the end of the buffer. This can be surprising to application code, and produce performance artifacts when transfers need to happen close to the end of the buffer, as the number of calls to claim/finish needs to double for such transfers.

Implementation

Defining a Ring Buffer

A ring buffer is defined using a variable of type ring_buf. It must then be initialized by calling ring_buf_init() or ring_buf_item_init().

The following code defines and initializes an empty data item mode ring buffer (which is part of a larger data structure). The ring buffer’s data buffer is capable of holding 64 words of data and metadata information.

#define MY_RING_BUF_WORDS 64

struct my_struct {
    struct ring_buf rb;
    uint32_t buffer[MY_RING_BUF_WORDS];
    ...
};
struct my_struct ms;

void init_my_struct {
    ring_buf_item_init(&ms.rb, MY_RING_BUF_WORDS, ms.buffer);
    ...
}

Alternatively, a ring buffer can be defined and initialized at compile time using one of two macros at file scope. Each macro defines both the ring buffer itself and its data buffer.

The following code defines a data item mode ring buffer:

#define MY_RING_BUF_WORDS 93
RING_BUF_ITEM_DECLARE(my_ring_buf, MY_RING_BUF_WORDS);

The following code defines a ring buffer intended to be used for raw bytes:

#define MY_RING_BUF_BYTES 93
RING_BUF_DECLARE(my_ring_buf, MY_RING_BUF_BYTES);

Enqueuing Data

Bytes are copied to a byte mode ring buffer by calling ring_buf_put().

uint8_t my_data[MY_RING_BUF_BYTES];
uint32_t ret;

ret = ring_buf_put(&ring_buf, my_data, MY_RING_BUF_BYTES);
if (ret != MY_RING_BUF_BYTES) {
    /* not enough room, partial copy. */
    ...
}

Data can be added to a byte mode ring buffer by directly accessing the ring buffer’s memory. For example:

uint32_t size;
uint32_t rx_size;
uint8_t *data;
int err;

/* Allocate buffer within a ring buffer memory. */
size = ring_buf_put_claim(&ring_buf, &data, MY_RING_BUF_BYTES);

/* Work directly on a ring buffer memory. */
rx_size = uart_rx(data, size);

/* Indicate amount of valid data. rx_size can be equal or less than size. */
err = ring_buf_put_finish(&ring_buf, rx_size);
if (err != 0) {
    /* This shouldn't happen unless rx_size > size */
    ...
}

A data item is added to a ring buffer by calling ring_buf_item_put().

uint32_t data[MY_DATA_WORDS];
int ret;

ret = ring_buf_item_put(&ring_buf, TYPE_FOO, 0, data, MY_DATA_WORDS);
if (ret == -EMSGSIZE) {
    /* not enough room for the data item */
    ...
}

If the data item requires only the type or application-specific integer value (i.e. it has no data array), a size of 0 and data pointer of NULL can be specified.

int ret;

ret = ring_buf_item_put(&ring_buf, TYPE_BAR, 17, NULL, 0);
if (ret == -EMSGSIZE) {
    /* not enough room for the data item */
    ...
}

Retrieving Data

Data bytes are copied out from a byte mode ring buffer by calling ring_buf_get(). For example:

uint8_t my_data[MY_DATA_BYTES];
size_t  ret;

ret = ring_buf_get(&ring_buf, my_data, sizeof(my_data));
if (ret != sizeof(my_data)) {
    /* Fewer bytes copied. */
} else {
    /* Requested amount of bytes retrieved. */
    ...
}

Data can be retrieved from a byte mode ring buffer by direct operations on the ring buffer’s memory. For example:

uint32_t size;
uint32_t proc_size;
uint8_t *data;
int err;

/* Get buffer within a ring buffer memory. */
size = ring_buf_get_claim(&ring_buf, &data, MY_RING_BUF_BYTES);

/* Work directly on a ring buffer memory. */
proc_size = process(data, size);

/* Indicate amount of data that can be freed. proc_size can be equal or less
 * than size.
 */
err = ring_buf_get_finish(&ring_buf, proc_size);
if (err != 0) {
    /* proc_size exceeds amount of valid data in a ring buffer. */
    ...
}

A data item is removed from a ring buffer by calling ring_buf_item_get().

uint32_t my_data[MY_DATA_WORDS];
uint16_t my_type;
uint8_t  my_value;
uint8_t  my_size;
int ret;

my_size = MY_DATA_WORDS;
ret = ring_buf_item_get(&ring_buf, &my_type, &my_value, my_data, &my_size);
if (ret == -EMSGSIZE) {
    printk("Buffer is too small, need %d uint32_t\n", my_size);
} else if (ret == -EAGAIN) {
    printk("Ring buffer is empty\n");
} else {
    printk("Got item of type %u value &u of size %u dwords\n",
           my_type, my_value, my_size);
    ...
}

Configuration Options

Related configuration options:

API Reference

The following ring buffer APIs are provided by include/zephyr/sys/ring_buffer.h:

group ring_buffer_apis

Defines

RING_BUF_DECLARE(name, size8)

Define and initialize a ring buffer for byte data.

This macro establishes a ring buffer of an arbitrary size. The basic storage unit is a byte.

The ring buffer can be accessed outside the module where it is defined using:

extern struct ring_buf <name>; 

Parameters
  • name – Name of the ring buffer.

  • size8 – Size of ring buffer (in bytes).

RING_BUF_ITEM_DECLARE(name, size32)

Define and initialize an “item based” ring buffer.

This macro establishes an “item based” ring buffer. Each data item is an array of 32-bit words (from zero to 1020 bytes in length), coupled with a 16-bit type identifier and an 8-bit integer value.

The ring buffer can be accessed outside the module where it is defined using:

extern struct ring_buf <name>; 

Parameters
  • name – Name of the ring buffer.

  • size32 – Size of ring buffer (in 32-bit words).

RING_BUF_ITEM_DECLARE_SIZE(name, size32)

Define and initialize an “item based” ring buffer.

This exists for backward compatibility reasons. RING_BUF_ITEM_DECLARE should be used instead.

Parameters
  • name – Name of the ring buffer.

  • size32 – Size of ring buffer (in 32-bit words).

RING_BUF_ITEM_DECLARE_POW2(name, pow)

Define and initialize a power-of-2 sized “item based” ring buffer.

This macro establishes an “item based” ring buffer by specifying its size using a power of 2. This exists mainly for backward compatibility reasons. RING_BUF_ITEM_DECLARE should be used instead.

Parameters
  • name – Name of the ring buffer.

  • pow – Ring buffer size exponent.

RING_BUF_ITEM_SIZEOF(expr)

Compute the ring buffer size in 32-bit needed to store an element.

The argument can be a type or an expression. Note: rounds up if the size is not a multiple of 32 bits.

Parameters
  • expr – Expression or type to compute the size of

Functions

static inline void ring_buf_init(struct ring_buf *buf, uint32_t size, uint8_t *data)

Initialize a ring buffer for byte data.

This routine initializes a ring buffer, prior to its first use. It is only used for ring buffers not defined using RING_BUF_DECLARE.

Parameters
  • buf – Address of ring buffer.

  • size – Ring buffer size (in bytes).

  • data – Ring buffer data area (uint8_t data[size]).

static inline void ring_buf_item_init(struct ring_buf *buf, uint32_t size, uint32_t *data)

Initialize an “item based” ring buffer.

This routine initializes a ring buffer, prior to its first use. It is only used for ring buffers not defined using RING_BUF_ITEM_DECLARE.

Each data item is an array of 32-bit words (from zero to 1020 bytes in length), coupled with a 16-bit type identifier and an 8-bit integer value.

Each data item is an array of 32-bit words (from zero to 1020 bytes in length), coupled with a 16-bit type identifier and an 8-bit integer value.

Parameters
  • buf – Address of ring buffer.

  • size – Ring buffer size (in 32-bit words)

  • data – Ring buffer data area (uint32_t data[size]).

static inline bool ring_buf_is_empty(struct ring_buf *buf)

Determine if a ring buffer is empty.

Parameters
  • buf – Address of ring buffer.

Returns

true if the ring buffer is empty, or false if not.

static inline void ring_buf_reset(struct ring_buf *buf)

Reset ring buffer state.

Parameters
  • buf – Address of ring buffer.

static inline uint32_t ring_buf_space_get(struct ring_buf *buf)

Determine free space in a ring buffer.

Parameters
  • buf – Address of ring buffer.

Returns

Ring buffer free space (in bytes).

static inline uint32_t ring_buf_item_space_get(struct ring_buf *buf)

Determine free space in an “item based” ring buffer.

Parameters
  • buf – Address of ring buffer.

Returns

Ring buffer free space (in 32-bit words).

static inline uint32_t ring_buf_capacity_get(struct ring_buf *buf)

Return ring buffer capacity.

Parameters
  • buf – Address of ring buffer.

Returns

Ring buffer capacity (in bytes).

static inline uint32_t ring_buf_size_get(struct ring_buf *buf)

Determine used space in a ring buffer.

Parameters
  • buf – Address of ring buffer.

Returns

Ring buffer space used (in bytes).

uint32_t ring_buf_put_claim(struct ring_buf *buf, uint8_t **data, uint32_t size)

Allocate buffer for writing data to a ring buffer.

With this routine, memory copying can be reduced since internal ring buffer can be used directly by the user. Once data is written to allocated area number of bytes written must be confirmed (see ring_buf_put_finish).

Warning

Use cases involving multiple writers to the ring buffer must prevent concurrent write operations, either by preventing all writers from being preempted or by using a mutex to govern writes to the ring buffer.

Warning

Ring buffer instance should not mix byte access and item access (calls prefixed with ring_buf_item_).

Parameters
  • buf[in] Address of ring buffer.

  • data[out] Pointer to the address. It is set to a location within ring buffer.

  • size[in] Requested allocation size (in bytes).

Returns

Size of allocated buffer which can be smaller than requested if there is not enough free space or buffer wraps.

int ring_buf_put_finish(struct ring_buf *buf, uint32_t size)

Indicate number of bytes written to allocated buffers.

The number of bytes must be equal to or lower than the sum corresponding to all preceding ring_buf_put_claim invocations (or even 0). Surplus bytes will be returned to the available free buffer space.

Warning

Use cases involving multiple writers to the ring buffer must prevent concurrent write operations, either by preventing all writers from being preempted or by using a mutex to govern writes to the ring buffer.

Warning

Ring buffer instance should not mix byte access and item access (calls prefixed with ring_buf_item_).

Parameters
  • buf – Address of ring buffer.

  • size – Number of valid bytes in the allocated buffers.

Return values
  • 0 – Successful operation.

  • -EINVAL – Provided size exceeds free space in the ring buffer.

uint32_t ring_buf_put(struct ring_buf *buf, const uint8_t *data, uint32_t size)

Write (copy) data to a ring buffer.

This routine writes data to a ring buffer buf.

Warning

Use cases involving multiple writers to the ring buffer must prevent concurrent write operations, either by preventing all writers from being preempted or by using a mutex to govern writes to the ring buffer.

Warning

Ring buffer instance should not mix byte access and item access (calls prefixed with ring_buf_item_).

Parameters
  • buf – Address of ring buffer.

  • data – Address of data.

  • size – Data size (in bytes).

Return values

Number – of bytes written.

uint32_t ring_buf_get_claim(struct ring_buf *buf, uint8_t **data, uint32_t size)

Get address of a valid data in a ring buffer.

With this routine, memory copying can be reduced since internal ring buffer can be used directly by the user. Once data is processed it must be freed using ring_buf_get_finish.

Warning

Use cases involving multiple reads of the ring buffer must prevent concurrent read operations, either by preventing all readers from being preempted or by using a mutex to govern reads to the ring buffer.

Warning

Ring buffer instance should not mix byte access and item access (calls prefixed with ring_buf_item_).

Parameters
  • buf[in] Address of ring buffer.

  • data[out] Pointer to the address. It is set to a location within ring buffer.

  • size[in] Requested size (in bytes).

Returns

Number of valid bytes in the provided buffer which can be smaller than requested if there is not enough free space or buffer wraps.

int ring_buf_get_finish(struct ring_buf *buf, uint32_t size)

Indicate number of bytes read from claimed buffer.

The number of bytes must be equal or lower than the sum corresponding to all preceding ring_buf_get_claim invocations (or even 0). Surplus bytes will remain available in the buffer.

Warning

Use cases involving multiple reads of the ring buffer must prevent concurrent read operations, either by preventing all readers from being preempted or by using a mutex to govern reads to the ring buffer.

Warning

Ring buffer instance should not mix byte access and item mode (calls prefixed with ring_buf_item_).

Parameters
  • buf – Address of ring buffer.

  • size – Number of bytes that can be freed.

Return values
  • 0 – Successful operation.

  • -EINVAL – Provided size exceeds valid bytes in the ring buffer.

uint32_t ring_buf_get(struct ring_buf *buf, uint8_t *data, uint32_t size)

Read data from a ring buffer.

This routine reads data from a ring buffer buf.

Warning

Use cases involving multiple reads of the ring buffer must prevent concurrent read operations, either by preventing all readers from being preempted or by using a mutex to govern reads to the ring buffer.

Warning

Ring buffer instance should not mix byte access and item mode (calls prefixed with ring_buf_item_).

Parameters
  • buf – Address of ring buffer.

  • data – Address of the output buffer. Can be NULL to discard data.

  • size – Data size (in bytes).

Return values

Number – of bytes written to the output buffer.

uint32_t ring_buf_peek(struct ring_buf *buf, uint8_t *data, uint32_t size)

Peek at data from a ring buffer.

This routine reads data from a ring buffer buf without removal.

Warning

Use cases involving multiple reads of the ring buffer must prevent concurrent read operations, either by preventing all readers from being preempted or by using a mutex to govern reads to the ring buffer.

Warning

Ring buffer instance should not mix byte access and item mode (calls prefixed with ring_buf_item_).

Warning

Multiple calls to peek will result in the same data being ‘peeked’ multiple times. To remove data, use either ring_buf_get or ring_buf_get_claim followed by ring_buf_get_finish with a non-zero size.

Parameters
  • buf – Address of ring buffer.

  • data – Address of the output buffer. Cannot be NULL.

  • size – Data size (in bytes).

Return values

Number – of bytes written to the output buffer.

int ring_buf_item_put(struct ring_buf *buf, uint16_t type, uint8_t value, uint32_t *data, uint8_t size32)

Write a data item to a ring buffer.

This routine writes a data item to ring buffer buf. The data item is an array of 32-bit words (from zero to 1020 bytes in length), coupled with a 16-bit type identifier and an 8-bit integer value.

Warning

Use cases involving multiple writers to the ring buffer must prevent concurrent write operations, either by preventing all writers from being preempted or by using a mutex to govern writes to the ring buffer.

Parameters
  • buf – Address of ring buffer.

  • type – Data item’s type identifier (application specific).

  • value – Data item’s integer value (application specific).

  • data – Address of data item.

  • size32 – Data item size (number of 32-bit words).

Return values
  • 0 – Data item was written.

  • -EMSGSIZE – Ring buffer has insufficient free space.

int ring_buf_item_get(struct ring_buf *buf, uint16_t *type, uint8_t *value, uint32_t *data, uint8_t *size32)

Read a data item from a ring buffer.

This routine reads a data item from ring buffer buf. The data item is an array of 32-bit words (up to 1020 bytes in length), coupled with a 16-bit type identifier and an 8-bit integer value.

Warning

Use cases involving multiple reads of the ring buffer must prevent concurrent read operations, either by preventing all readers from being preempted or by using a mutex to govern reads to the ring buffer.

Parameters
  • buf – Address of ring buffer.

  • type – Area to store the data item’s type identifier.

  • value – Area to store the data item’s integer value.

  • data – Area to store the data item. Can be NULL to discard data.

  • size32 – Size of the data item storage area (number of 32-bit chunks).

Return values
  • 0 – Data item was fetched; size32 now contains the number of 32-bit words read into data area data.

  • -EAGAIN – Ring buffer is empty.

  • -EMSGSIZE – Data area data is too small; size32 now contains the number of 32-bit words needed.