utringbuffer: dynamic ring-buffer macros for C

Here’s a link back to the GitHub project page.

Introduction

The functions in utringbuffer.h are based on the general-purpose array macros provided in utarray.h, so before reading this page you should read that page first.

To use these macros in your own C program, copy both utarray.h and utringbuffer.h into your source directory and use utringbuffer.h in your program.

#include "utringbuffer.h"

The provided operations are based loosely on the C++ STL vector methods. The ring-buffer data type supports construction (with a specified capacity), destruction, iteration, and push, but not pop; once the ring-buffer reaches full capacity, pushing a new element automatically pops and destroys the oldest element. The elements contained in the ring-buffer can be any simple datatype or structure.

Internally the ring-buffer contains a pre-allocated memory region into which the elements are copied, starting at position 0. When the ring-buffer reaches full capacity, the next element to be pushed is pushed at position 0, overwriting the oldest element, and the internal index representing the "start" of the ring-buffer is incremented. A ring-buffer, once full, can never become un-full.

Download

To download the utringbuffer.h header file, follow the links on https://github.com/troydhanson/uthash to clone uthash or get a zip file, then look in the src/ sub-directory.

BSD licensed

This software is made available under the revised BSD license. It is free and open source.

Platforms

The utringbuffer macros have been tested on:

Linux,
Mac OS X,
Windows, using Visual Studio 2008 and Visual Studio 2010

Usage

Declaration

The ring-buffer itself has the data type UT_ringbuffer, regardless of the type of elements to be stored in it. It is declared like,

UT_ringbuffer *history;

New and free

The next step is to create the ring-buffer using utringbuffer_new. Later when you’re done with the ring-buffer, utringbuffer_free will free it and all its elements.

Push, etc

The central features of the ring-buffer involve putting elements into it and iterating over them. There are several operations that deal with either single elements or ranges of elements at a time. In the examples below we will use only the push operation to insert elements.

Elements

Support for dynamic arrays of integers or strings is especially easy. These are best shown by example:

Integers

This example makes a ring-buffer of integers, pushes 0-9 into it, then prints it two different ways. Lastly it frees it.

Integer elements

#include <stdio.h>
#include "utringbuffer.h"

int main() {
  UT_ringbuffer *history;
  int i, *p;

  utringbuffer_new(history, 7, &ut_int_icd);
  for(i=0; i < 10; i++) utringbuffer_push_back(history, &i);

  for (p = (int*)utringbuffer_front(history);
       p != NULL;
       p = (int*)utringbuffer_next(history, p)) {
    printf("%d\n", *p);  /* prints "3 4 5 6 7 8 9" */
  }

  for (i=0; i < utringbuffer_len(history); i++) {
    p = utringbuffer_eltptr(history, i);
    printf("%d\n", *p);  /* prints "3 4 5 6 7 8 9" */
  }

  utringbuffer_free(history);

  return 0;
}

The second argument to utringbuffer_push_back is always a pointer to the type (so a literal cannot be used). So for integers, it is an int*.

Strings

In this example we make a ring-buffer of strings, push two strings into it, print it and free it.

String elements

#include <stdio.h>
#include "utringbuffer.h"

int main() {
  UT_ringbuffer *strs;
  char *s, **p;

  utringbuffer_new(strs, 7, &ut_str_icd);

  s = "hello"; utringbuffer_push_back(strs, &s);
  s = "world"; utringbuffer_push_back(strs, &s);
  p = NULL;
  while ( (p=(char**)utringbuffer_next(strs,p))) {
    printf("%s\n",*p);
  }

  utringbuffer_free(strs);

  return 0;
}

In this example, since the element is a char*, we pass a pointer to it (char**) as the second argument to utringbuffer_push_back. Note that "push" makes a copy of the source string and pushes that copy into the array.

About UT_icd

Arrays can be made of any type of element, not just integers and strings. The elements can be basic types or structures. Unless you’re dealing with integers and strings (which use pre-defined ut_int_icd and ut_str_icd), you’ll need to define a UT_icd helper structure. This structure contains everything that utringbuffer (or utarray) needs to initialize, copy or destruct elements.

typedef struct {
    size_t sz;
    init_f *init;
    ctor_f *copy;
    dtor_f *dtor;
} UT_icd;

The three function pointers init, copy, and dtor have these prototypes:

typedef void (ctor_f)(void *dst, const void *src);
typedef void (dtor_f)(void *elt);
typedef void (init_f)(void *elt);

The sz is just the size of the element being stored in the array.

The init function is used by utarray but is never used by utringbuffer; you may safely set it to any value you want.

The copy function is used whenever an element is copied into the buffer. It is invoked during utringbuffer_push_back. If copy is NULL, it defaults to a bitwise copy using memcpy.

The dtor function is used to clean up an element that is being removed from the buffer. It may be invoked due to utringbuffer_push_back (on the oldest element in the buffer), utringbuffer_clear, utringbuffer_done, or utringbuffer_free. If the elements need no cleanup upon destruction, dtor may be NULL.

Scalar types

The next example uses UT_icd with all its defaults to make a ring-buffer of long elements. This example pushes two longs into a buffer of capacity 1, prints the contents of the buffer (which is to say, the most recent value pushed), and then frees the buffer.

long elements

#include <stdio.h>
#include "utringbuffer.h"

UT_icd long_icd = {sizeof(long), NULL, NULL, NULL };

int main() {
  UT_ringbuffer *nums;
  long l, *p;
  utringbuffer_new(nums, 1, &long_icd);

  l=1; utringbuffer_push_back(nums, &l);
  l=2; utringbuffer_push_back(nums, &l);

  p=NULL;
  while((p = (long*)utringbuffer_next(nums,p))) printf("%ld\n", *p);

  utringbuffer_free(nums);
  return 0;
}

Structures

Structures can be used as utringbuffer elements. If the structure requires no special effort to initialize, copy or destruct, we can use UT_icd with all its defaults. This example shows a structure that consists of two integers. Here we push two values, print them and free the buffer.

Structure (simple)

#include <stdio.h>
#include "utringbuffer.h"

typedef struct {
    int a;
    int b;
} intpair_t;

UT_icd intpair_icd = {sizeof(intpair_t), NULL, NULL, NULL};

int main() {

  UT_ringbuffer *pairs;
  intpair_t ip, *p;
  utringbuffer_new(pairs, 7, &intpair_icd);

  ip.a=1;  ip.b=2;  utringbuffer_push_back(pairs, &ip);
  ip.a=10; ip.b=20; utringbuffer_push_back(pairs, &ip);

  for(p=(intpair_t*)utringbuffer_front(pairs);
      p!=NULL;
      p=(intpair_t*)utringbuffer_next(pairs,p)) {
    printf("%d %d\n", p->a, p->b);
  }

  utringbuffer_free(pairs);
  return 0;
}

The real utility of UT_icd is apparent when the elements stored in the ring-buffer are structures that require special work to initialize, copy or destruct.

For example, when a structure contains pointers to related memory areas that need to be copied when the structure is copied (and freed when the structure is freed), we can use custom init, copy, and dtor members in the UT_icd.

Here we take an example of a structure that contains an integer and a string. When this element is copied (such as when an element is pushed), we want to "deep copy" the s pointer (so the original element and the new element point to their own copies of s). When an element is destructed, we want to "deep free" its copy of s. Lastly, this example is written to work even if s has the value NULL.

Structure (complex)

#include <stdio.h>
#include <stdlib.h>
#include "utringbuffer.h"

typedef struct {
    int a;
    char *s;
} intchar_t;

void intchar_copy(void *_dst, const void *_src) {
  intchar_t *dst = (intchar_t*)_dst, *src = (intchar_t*)_src;
  dst->a = src->a;
  dst->s = src->s ? strdup(src->s) : NULL;
}

void intchar_dtor(void *_elt) {
  intchar_t *elt = (intchar_t*)_elt;
  free(elt->s);
}

UT_icd intchar_icd = {sizeof(intchar_t), NULL, intchar_copy, intchar_dtor};

int main() {
  UT_ringbuffer *intchars;
  intchar_t ic, *p;
  utringbuffer_new(intchars, 2, &intchar_icd);

  ic.a=1; ic.s="hello"; utringbuffer_push_back(intchars, &ic);
  ic.a=2; ic.s="world"; utringbuffer_push_back(intchars, &ic);
  ic.a=3; ic.s="peace"; utringbuffer_push_back(intchars, &ic);

  p=NULL;
  while( (p=(intchar_t*)utringbuffer_next(intchars,p))) {
    printf("%d %s\n", p->a, (p->s ? p->s : "null"));
    /* prints "2 world 3 peace" */
  }

  utringbuffer_free(intchars);
  return 0;
}

Reference

This table lists all the utringbuffer operations. These are loosely based on the C++ vector class.

Operations

`utringbuffer_new(UT_ringbuffer a, int n, UT_icd icd)`	allocate a new ringbuffer
`utringbuffer_free(UT_ringbuffer *a)`	free an allocated ringbuffer
`utringbuffer_init(UT_ringbuffer a, int n, UT_icd icd)`	init a ringbuffer (non-alloc)
`utringbuffer_done(UT_ringbuffer *a)`	dispose of a ringbuffer (non-alloc)
`utringbuffer_clear(UT_ringbuffer *a)`	clear all elements from a, making it empty
`utringbuffer_push_back(UT_ringbuffer a, element p)`	push element p onto a
`utringbuffer_len(UT_ringbuffer *a)`	get length of a
`utringbuffer_empty(UT_ringbuffer *a)`	get whether a is empty
`utringbuffer_full(UT_ringbuffer *a)`	get whether a is full
`utringbuffer_eltptr(UT_ringbuffer *a, int j)`	get pointer of element from index
`utringbuffer_eltidx(UT_ringbuffer a, element e)`	get index of element from pointer
`utringbuffer_front(UT_ringbuffer *a)`	get oldest element of a
`utringbuffer_next(UT_ringbuffer a, element e)`	get element of a following e (front if e is NULL)
`utringbuffer_prev(UT_ringbuffer a, element e)`	get element of a before e (back if e is NULL)
`utringbuffer_back(UT_ringbuffer *a)`	get newest element of a

Notes

utringbuffer_new and utringbuffer_free are used to allocate a new ring-buffer and to free it, while utringbuffer_init and utringbuffer_done can be used if the UT_ringbuffer is already allocated and just needs to be initialized or have its internal resources freed.
Both utringbuffer_new and utringbuffer_init take a second parameter n indicating the capacity of the ring-buffer, that is, the size at which the ring-buffer is considered "full" and begins to overwrite old elements with newly pushed ones.
Once a ring-buffer has become full, it will never again become un-full except by means of utringbuffer_clear. There is no way to "pop" a single old item from the front of the ring-buffer. You can simulate this ability by maintaining a separate integer count of the number of "logically popped elements", and starting your iteration with utringbuffer_eltptr(a, popped_count) instead of with utringbuffer_front(a).
Pointers to elements (obtained using utringbuffer_eltptr, utringbuffer_front, utringbuffer_next, etc.) are not generally invalidated by utringbuffer_push_back, because utringbuffer does not perform reallocation; however, a pointer to the oldest element may suddenly turn into a pointer to the newest element if utringbuffer_push_back is called while the buffer is full.
The elements of a ring-buffer are stored in contiguous memory, but once the ring-buffer has become full, it is no longer true that the elements are contiguously in order from oldest to newest; i.e., (element *)utringbuffer_front(a) + utringbuffer_len(a)-1 is not generally equal to (element *)utringbuffer_back(a).