circular buffer or ring buffer 工作原理（From Wikipedia, the free encyclopedia）转

Circular buffer

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A ring showing, conceptually, a circular buffer. This visually shows that the buffer has no real end and it can loop around the buffer. However, since memory is never physically created as a ring, a linear representation is generally used as is done below.

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.

[hide]

[edit] Uses

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.g., an audio generator) to overwrite old data if the consumer (e.g., the sound card) is unable to momentarily keep up. Another example is the digital waveguide synthesis method which uses circular buffers to efficiently simulate the sound of vibrating strings or wind instruments.

The "prized" attribute of a circular buffer is that it does not need to have its elements shuffled around when one is consumed. (If a non-circular buffer were used then it would be necessary to shift all elements when one is consumed.) In other words, the circular buffer is well suited as a FIFO buffer while a standard, non-circular buffer is well suited as a LIFO buffer.

Circular buffering makes a good implementation strategy for a queue that has fixed maximum size. Should a maximum size be adopted for a queue, then a circular buffer is a completely ideal implementation; all queue operations are constant time. However, expanding a circular buffer requires shifting memory, which is comparatively costly. For arbitrarily expanding queues, a Linked list approach may be preferred instead.

[edit] How it works

A circular buffer first starts empty and of some predefined length. For example, this is a 7-element buffer:

Assume that a 1 is written into the middle of the buffer (exact starting location does not matter in a circular buffer):

Then assume that two more elements are added — 2 & 3 — which get appended after the 1:

If two elements are then removed from the buffer, the oldest values inside the buffer are removed. The two elements removed, in this case, are 1 & 2 leaving the buffer with just a 3:

If the buffer has 7 elements then it is completely full:

A consequence of the circular buffer is that when it is full and a subsequent write is performed, then it starts overwriting the oldest data. In this case, two more elements — A & B — are added and they overwrite the 3 & 4:

Alternatively, the routines that manage the buffer could prevent overwriting the data and return an error or raise an exception. Whether or not data is overwritten is up to the semantics of the buffer routines or the application using the circular buffer.

Finally, if two elements are now removed then what would be returned is not 3 & 4 but 5 & 6 because A & B overwrote the 3 & the 4 yielding the buffer with:

[edit] Circular buffer mechanics

What is not shown in the example above is the mechanics of how the circular buffer is managed.

[edit] Start / End Pointers

Generally, a circular buffer requires four pointers:

one to the actual buffer in memory
one to the buffer end in memory (or alternately: the size of the buffer)
one to point to the start of valid data (or alternately: amount of data written to the buffer)
one to point to the end of valid data (or alternately: amount of data read from the buffer)

Alternatively, a fixed-length buffer with two integers to keep track of indices can be used in languages that do not have pointers.

Taking a couple of examples from above. (While there are numerous ways to label the pointers and exact semantics can vary, this is one way to do it.)

This image shows a partially full buffer:

This image shows a full buffer with two elements having been overwritten:

What to note about the second one is that after each element is overwritten then the start pointer is incremented as well.

[edit] Difficulties

[edit] Full / Empty Buffer Distinction

A small disadvantage of relying on pointers or relative indices of the start and end of data is, that in the case the buffer is entirely full, both pointers point to the same element:

This is exactly the same situation as when the buffer is empty:

To solve this confusion there are a number of solutions:

[edit] Always Keep One Slot Open

This design always keeps one slot unallocated. A full buffer has at most $\text{(size}-1)$ slots. If both pointers refer to the same slot, the buffer is empty. If the end (write) pointer refers to the slot preceding the one referred to by the start (read) pointer, the buffer is full. This is a simple, robust, approach that only requires two pointers, at the expense of one buffer slot.

Example implementation, 'C' language

/* Circular buffer example, keeps one slot open */
 
#include <stdio.h>
#include <malloc.h>
 
/* Opaque buffer element type.  This would be defined by the application. */
typedef struct { int value; } ElemType;
 
/* Circular buffer object */
typedef struct {
    int         size;   /* maximum number of elements           */
    int         start;  /* index of oldest element              */
    int         end;    /* index at which to write new element  */
    ElemType   *elems;  /* vector of elements                   */
} CircularBuffer;
 
void cbInit(CircularBuffer *cb, int size) {
    cb->size  = size + 1; /* include empty elem */
    cb->start = 0;
    cb->end   = 0;
    cb->elems = (ElemType *)calloc(cb->size, sizeof(ElemType));
}
 
void cbFree(CircularBuffer *cb) {
    free(cb->elems); /* OK if null */ }
 
int cbIsFull(CircularBuffer *cb) {
    return (cb->end + 1) % cb->size == cb->start; }
 
int cbIsEmpty(CircularBuffer *cb) {
    return cb->end == cb->start; }
 
/* Write an element, overwriting oldest element if buffer is full. App can
   choose to avoid the overwrite by checking cbIsFull(). */
void cbWrite(CircularBuffer *cb, ElemType *elem) {
    cb->elems[cb->end] = *elem;
    cb->end = (cb->end + 1) % cb->size;
    if (cb->end == cb->start)
        cb->start = (cb->start + 1) % cb->size; /* full, overwrite */
}
 
/* Read oldest element. App must ensure !cbIsEmpty() first. */
void cbRead(CircularBuffer *cb, ElemType *elem) {
    *elem = cb->elems[cb->start];
    cb->start = (cb->start + 1) % cb->size;
}
 
int main(int argc, char **argv) {
    CircularBuffer cb;
    ElemType elem = {0};
 
    int testBufferSize = 10; /* arbitrary size */
    cbInit(&cb, testBufferSize);
 
    /* Fill buffer with test elements 3 times */
    for (elem.value = 0; elem.value < 3 * testBufferSize; ++ elem.value)
        cbWrite(&cb, &elem);
 
    /* Remove and print all elements */
    while (!cbIsEmpty(&cb)) {
        cbRead(&cb, &elem);
        printf("%d\n", elem.value);
    }
 
    cbFree(&cb);
    return 0;
}

[edit] Use a Fill Count

This approach replaces the end pointer with a counter that tracks the number of readable items in the buffer. This unambiguously indicates when the buffer is empty or full and allows use of all buffer slots.

The performance impact should be negligible, since this approach adds the costs of maintaining the counter and computing the tail slot on writes but eliminates the need to maintain the end pointer and simplifies the fullness test.

Note: When using semaphores in a Producer-consumer model, the semaphores act as a fill count.

Differences from previous example

/* This approach replaces the CircularBuffer 'end' field with the
   'count' field and changes these functions: */
 
void cbInit(CircularBuffer *cb, int size) {
    cb->size  = size;
    cb->start = 0;
    cb->count = 0;
    cb->elems = (ElemType *)calloc(cb->size, sizeof(ElemType));
}
 
int cbIsFull(CircularBuffer *cb) {
    return cb->count == cb->size; }
 
int cbIsEmpty(CircularBuffer *cb) {
    return cb->count == 0; }
 
void cbWrite(CircularBuffer *cb, ElemType *elem) {
    int end = (cb->start + cb->count) % cb->size;
    cb->elems[end] = *elem;
    if (cb->count == cb->size)
        cb->start = (cb->start + 1) % cb->size; /* full, overwrite */
    else
        ++ cb->count;
}
 
void cbRead(CircularBuffer *cb, ElemType *elem) {
    *elem = cb->elems[cb->start];
    cb->start = (cb->start + 1) % cb->size;
    -- cb->count;
}

[edit] Read / Write Counts

Another solution is to keep counts of the number of items written to and read from the circular buffer. Both counts are stored in signed integer variables with numerical limits larger than the number of items that can be stored and are allowed to wrap freely.

The unsigned difference (write_count - read_count) always yields the number of items placed in the buffer and not yet retrieved. This can indicate that the buffer is empty, partially full, completely full (without waste of a storage location) or in a state of overrun.

The advantage is:

The source and sink of data can implement independent policies for dealing with a full buffer and overrun while adhering to the rule that only the source of data modifies the write count and only the sink of data modifies the read count. This can result in elegant and robust circular buffer implementations even in multi-threaded environments.

The disadvantage is:

You need two additional variables.

[edit] Record last operation

Another solution is to keep a flag indicating whether the most recent operation was a read or a write. If the two pointers are equal, then the flag will show whether the buffer is full or empty: if the most recent operation was a write, the buffer must be full, and conversely if it was a read, it must be empty.

The advantages are:

Only a single bit needs to be stored (which may be particularly useful if the algorithm is implemented in hardware)
The test for full/empty is simple

The disadvantage is:

You need an extra variable

[edit] Absolute indices

If indices are used instead of pointers, indices can store read/write counts instead of the offset from start of the buffer. This is similar to the above solution, except that there are no separate variables, and relative indices are obtained on the fly by division modulo the buffer's length.

The advantage is:

No extra variables are needed.

The disadvantages are:

Every access needs an additional modulo operation.
If counter wrap is possible, complex logic can be needed if the buffer's length is not a divisor of the counter's capacity.

On binary computers, both of these disadvantages disappear if the buffer's length is a power of two—at the cost of a constraint on possible buffers lengths.

[edit] Multiple Read Pointers

A little bit more complex are multiple read pointers on the same circular buffer. This is useful if you have n threads, which are reading from the same buffer, but one thread writing to the buffer.

[edit] Chunked Buffer

Much more complex are different chunks of data in the same circular buffer. The writer is not only writing elements to the buffer, it also assigns these elements to chunks^{[citation needed]}.

The reader should not only be able to read from the buffer, it should also get informed about the chunk borders.

Example: The writer is reading data from small files, writing them into the same circular buffer. The reader is reading the data, but needs to know when and which file is starting at a given position.

[edit] Optimization

A circular-buffer implementation may be optimized by mapping the underlying buffer to two contiguous regions of virtual memory. (Naturally, the underlying buffer‘s length must then equal some multiple of the system’s page size.) Reading from and writing to the circular buffer may then be carried out with greater efficiency by means of direct memory access; those accesses which fall beyond the end of the first virtual-memory region will automatically wrap around to the beginning of the underlying buffer. When the read offset is advanced into the second virtual-memory region, both offsets—read and write—are decremented by the length of the underlying buffer.

[edit] Optimized POSIX Implementation

#include <sys/mman.h>
#include <stdlib.h>
#include <unistd.h>
 
#define report_exceptional_condition() abort ()
 
struct ring_buffer
{
  void *address;
 
  unsigned long count_bytes;
  unsigned long write_offset_bytes;
  unsigned long read_offset_bytes;
};
 
//Warning order should be at least 12 for Linux
void
ring_buffer_create (struct ring_buffer *buffer, unsigned long order)
{
  char path[] = "/dev/shm/ring-buffer-XXXXXX";
  int file_descriptor;
  void *address;
  int status;
 
  file_descriptor = mkstemp (path);
  if (file_descriptor < 0)
    report_exceptional_condition ();
 
  status = unlink (path);
  if (status)
    report_exceptional_condition ();
 
  buffer->count_bytes = 1UL << order;
  buffer->write_offset_bytes = 0;
  buffer->read_offset_bytes = 0;
 
  status = ftruncate (file_descriptor, buffer->count_bytes);
  if (status)
    report_exceptional_condition ();
 
  buffer->address = mmap (NULL, buffer->count_bytes << 1, PROT_NONE,
                          MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
 
  if (buffer->address == MAP_FAILED)
    report_exceptional_condition ();
 
  address =
    mmap (buffer->address, buffer->count_bytes, PROT_READ | PROT_WRITE,
          MAP_FIXED | MAP_SHARED, file_descriptor, 0);
 
  if (address != buffer->address)
    report_exceptional_condition ();
 
  address = mmap (buffer->address + buffer->count_bytes,
                  buffer->count_bytes, PROT_READ | PROT_WRITE,
                  MAP_FIXED | MAP_SHARED, file_descriptor, 0);
 
  if (address != buffer->address + buffer->count_bytes)
    report_exceptional_condition ();
 
  status = close (file_descriptor);
  if (status)
    report_exceptional_condition ();
}
 
void
ring_buffer_free (struct ring_buffer *buffer)
{
  int status;
 
  status = munmap (buffer->address, buffer->count_bytes << 1);
  if (status)
    report_exceptional_condition ();
}
 
void *
ring_buffer_write_address (struct ring_buffer *buffer)
{
  /*** void pointer arithmetic is a constraint violation. ***/
  return buffer->address + buffer->write_offset_bytes;
}
 
void
ring_buffer_write_advance (struct ring_buffer *buffer,
                           unsigned long count_bytes)
{
  buffer->write_offset_bytes += count_bytes;
}
 
void *
ring_buffer_read_address (struct ring_buffer *buffer)
{
  return buffer->address + buffer->read_offset_bytes;
}
 
void
ring_buffer_read_advance (struct ring_buffer *buffer,
                          unsigned long count_bytes)
{
  buffer->read_offset_bytes += count_bytes;
 
  if (buffer->read_offset_bytes >= buffer->count_bytes)
    {
      buffer->read_offset_bytes -= buffer->count_bytes;
      buffer->write_offset_bytes -= buffer->count_bytes;
    }
}
 
unsigned long
ring_buffer_count_bytes (struct ring_buffer *buffer)
{
  return buffer->write_offset_bytes - buffer->read_offset_bytes;
}
 
unsigned long
ring_buffer_count_free_bytes (struct ring_buffer *buffer)
{
  return buffer->count_bytes - ring_buffer_count_bytes (buffer);
}
 
void
ring_buffer_clear (struct ring_buffer *buffer)
{
  buffer->write_offset_bytes = 0;
  buffer->read_offset_bytes = 0;
}
 
/*Note, that initial anonymous mmap() can be avoided - after initial mmap() for descriptor fd,
you can try mmap() with hinted address as (buffer->address + buffer->count_bytes) and if it fails -
another one with hinted address as (buffer->address - buffer->count_bytes).
Make sure MAP_FIXED is not used in such case, as under certain situations it could end with segfault.
The advantage of such approach is, that it avoids requirement to map twice the amount you need initially
(especially useful e.g. if you want to use hugetlbfs and the allowed amount is limited)
and in context of gcc/glibc - you can avoid certain feature macros
(MAP_ANONYMOUS usually requires one of: _BSD_SOURCE, _SVID_SOURCE or _GNU_SOURCE).*/

posted @ 2012-09-20 17:01 我要飞4324 阅读(696) 评论(0) 编辑收藏举报

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