Add generic circular buffer

Add a circular buffer implementation, to factorize multiple specific
queues implementation.

app/src/cbuf.h

@@ -0,0 +1,50 @@
+// generic circular buffer (bounded queue) implementation
+#ifndef CBUF_H
+#define CBUF_H
+
+#include <stdbool.h>
+#include <unistd.h>
+
+// To define a circular buffer type of 20 ints:
+//     typedef CBUF(int, 20) my_cbuf_t;
+//
+// data has length CAP + 1 to distinguish empty vs full.
+#define CBUF(TYPE, CAP) { \
+    TYPE data[(CAP) + 1]; \
+    size_t head; \
+    size_t tail; \
+}
+
+#define cbuf_size_(PCBUF) \
+    (sizeof((PCBUF)->data) / sizeof(*(PCBUF)->data))
+
+#define cbuf_is_empty(PCBUF) \
+    ((PCBUF)->head == (PCBUF)->tail)
+
+#define cbuf_is_full(PCBUF) \
+    (((PCBUF)->head + 1) % cbuf_size_(PCBUF) == (PCBUF)->tail)
+
+#define cbuf_init(PCBUF) \
+    (void) ((PCBUF)->head = (PCBUF)->tail = 0)
+
+#define cbuf_push(PCBUF, ITEM) \
+    ({ \
+        bool ok = !cbuf_is_full(PCBUF); \
+        if (ok) { \
+            (PCBUF)->data[(PCBUF)->head] = (ITEM); \
+            (PCBUF)->head = ((PCBUF)->head + 1) % cbuf_size_(PCBUF); \
+        } \
+        ok; \
+    }) \
+
+#define cbuf_take(PCBUF, PITEM) \
+    ({ \
+        bool ok = !cbuf_is_empty(PCBUF); \
+        if (ok) { \
+            *(PITEM) = (PCBUF)->data[(PCBUF)->tail]; \
+            (PCBUF)->tail = ((PCBUF)->tail + 1) % cbuf_size_(PCBUF); \
+        } \
+        ok; \
+    })
+
+#endif