diff --git a/.gitignore b/.gitignore
index 29d36c8..a43b0c7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,5 @@ linked-list
 mandelbrot
 mandelbrot.bmp
 threads
+structs
+circular-buffer
diff --git a/Makefile b/Makefile
index fb79bd5..f62d539 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 CFLAGS=-std=c99 -Wall -g -O2
 INDENTOPTS=-kr --no-tabs --braces-on-func-def-line --indent-level2
 
-TARGETS=approximate-pi linked-list mandelbrot mandelbrot.bmp threads
+TARGETS=approximate-pi linked-list mandelbrot mandelbrot.bmp threads circular-buffer structs
 
 .PHONY: all
 all: $(TARGETS)
diff --git a/circular-buffer.c b/circular-buffer.c
new file mode 100644
index 0000000..0a5abd7
--- /dev/null
+++ b/circular-buffer.c
@@ -0,0 +1,172 @@
+/* Circular buffer, using a counter to distinguish full from empty buffer. */
+
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <assert.h>
+
+#define DEBUG 0
+#define debug_printf(fmt, ...) \
+          do { if (DEBUG) fprintf(stderr, fmt, __VA_ARGS__); } while (0)
+
+#define CB_SIZE 42
+
+/* Types */
+
+struct elem {
+  int foo;
+};
+typedef struct elem elem_t;
+
+struct cb {
+  int count;
+  int size;
+  int start;
+  int end;
+
+  elem_t * elems;
+};
+typedef struct cb cb_t;
+
+
+/* Helpers */
+
+void dienomem() {
+  fprintf(stderr, "ERROR: No memory?!\n");
+  exit(1);
+}
+
+/* Operations */
+
+void cb_init(cb_t * cb, int size) {
+  cb->count = 0;
+  cb->size = size;
+
+  cb->start = 0;
+
+  cb->elems = malloc(sizeof(elem_t)*size);
+  if (!cb->elems)
+    dienomem();
+}
+
+void cb_free(cb_t * cb) {
+  free(cb->elems);
+}
+
+bool cb_is_full(cb_t * cb) {
+  return (cb->count == cb->size);
+}
+
+bool cb_is_empty(cb_t * cb) {
+  return (cb->count == 0);
+}
+
+int cb_count(cb_t * cb) {
+  return cb->count;
+}
+
+void cb_write(cb_t * cb, elem_t * elem) {
+  int end = (cb->start + cb->count) % cb->size;
+  cb->elems[end] = *elem;
+
+  if (cb_is_full(cb))
+    /* We have just overwritten the first element of the buffer,
+     * therefore move the start. */
+    cb->start = (cb->start + 1) % cb->size;
+  else
+    cb->count++;
+}
+
+bool cb_read(cb_t * cb, elem_t * elem) {
+  if (!cb_is_empty(cb)) {
+    *elem = cb->elems[cb->start];
+    cb->start = (cb->start + 1) % cb->size;
+    cb->count--;
+    return true;
+  } else {
+    return false;
+  }
+}
+
+/* Test it. */
+
+int main(int argc, char *argv[]) {
+  /* Initialize empty CB_SIZE-element circular buffer. */
+  cb_t *test_cb = malloc(sizeof(cb_t));
+  if (!test_cb)
+    dienomem();
+  cb_init(test_cb, CB_SIZE);
+
+  /* Should be empty. */
+  assert(cb_is_empty(test_cb) == true);
+  assert(cb_is_full(test_cb) == false);
+
+  /* Add one element. */
+  elem_t *one = malloc(sizeof(elem_t));
+  one->foo = 23;
+  cb_write(test_cb, one);
+  free(one);
+  assert(cb_is_empty(test_cb) == false);
+
+  /* Read the one element. */
+  elem_t *oneback = malloc(sizeof(elem_t));
+  cb_read(test_cb, oneback);
+  assert(oneback->foo == 23);
+  free(oneback);
+  assert(cb_is_empty(test_cb) == true);
+
+  /* Fill the buffer. */
+  elem_t *e = malloc(sizeof(elem_t));
+  for (int i = 1; i <= CB_SIZE; i++) {
+    e->foo = i;
+    cb_write(test_cb, e);
+  }
+  assert(cb_is_empty(test_cb) == false);
+  assert(cb_is_full(test_cb) == true);
+  assert(cb_count(test_cb) == CB_SIZE);
+
+  /* Add another element. */
+  e->foo = CB_SIZE + 1;
+  cb_write(test_cb, e);
+  assert(cb_is_empty(test_cb) == false);
+  assert(cb_is_full(test_cb) == true);
+  assert(cb_count(test_cb) == CB_SIZE);
+
+  /* Read all elements and check their values. */
+  for (int i = 2; i <= CB_SIZE + 1; i++) {
+    bool rc = cb_read(test_cb, e);
+    assert(rc == true);
+    debug_printf("e->foo == %d\n", e->foo);
+    assert(e->foo == i);
+  }
+  assert(cb_is_empty(test_cb) == true);
+
+  /* Should not be possible to read a value now. */
+  bool rc = cb_read(test_cb, e);
+  assert(rc == false);
+  assert(cb_is_empty(test_cb) == true);
+  assert(cb_count(test_cb) == 0);
+
+  /* Fill the buffer - twice. */
+  for (int i = 1; i <= CB_SIZE * 2; i++) {
+    e->foo = i;
+    cb_write(test_cb, e);
+  }
+  assert(cb_is_empty(test_cb) == false);
+  assert(cb_is_full(test_cb) == true);
+  assert(cb_count(test_cb) == CB_SIZE);
+
+  /* Read all elements and check their values. */
+  for (int i = CB_SIZE + 1; i <= CB_SIZE * 2; i++) {
+    bool rc = cb_read(test_cb, e);
+    assert(rc == true);
+    assert(e->foo == i);
+  }
+  assert(cb_is_empty(test_cb) == true);
+
+  /* Free buffer etc. */
+  free(e);
+  cb_free(test_cb);
+  free(test_cb);
+}
diff --git a/structs.c b/structs.c
new file mode 100644
index 0000000..dfc0013
--- /dev/null
+++ b/structs.c
@@ -0,0 +1,19 @@
+#include <assert.h>
+
+struct test {
+  int foo;
+};
+typedef struct test test_t;
+
+int main(int argc, char * argv[]) {
+  test_t a = { 23 };
+  test_t b = { 42 };
+
+  a = b;
+  assert(a.foo == 42);
+  assert(b.foo == 42);
+
+  a.foo = 69;
+  assert(a.foo == 69);
+  assert(b.foo == 42);
+}