diff --git a/bac2/os/.gitignore b/bac2/os/.gitignore
new file mode 100644
index 0000000..7cc3bf8
--- /dev/null
+++ b/bac2/os/.gitignore
@@ -0,0 +1,3 @@
+ex[0-9]
+group
+*.o
diff --git a/bac2/os/chap2/Makefile b/bac2/os/chap2/Makefile
index ac91e76..0ca1240 100644
--- a/bac2/os/chap2/Makefile
+++ b/bac2/os/chap2/Makefile
@@ -2,7 +2,7 @@
 CC = gcc
 CFLAGS = -g -Wall
 
-all: ex3 ex4
+all: ex3 ex4 ex5 group
 
 %.o: %.c
 	$(CC) $(CFLAGS) -c -o $@ $<
@@ -16,11 +16,14 @@ ex4: ex4.o
 ex5: ex5.o
 	$(CC) $(CFLAGS) -o $@ $+
 
+group: group.o
+	$(CC) $(CFLAGS) -o $@ $+
+
 clean:
 	rm -f *.o core.*
 
 mrproper: clean
-	rm -f ex3 ex4 ex5
+	rm -f ex3 ex4 ex5 group
 
-run: ex5
-	./ex5
+run: group
+	./group
diff --git a/bac2/os/chap2/group.c b/bac2/os/chap2/group.c
new file mode 100644
index 0000000..e3751cd
--- /dev/null
+++ b/bac2/os/chap2/group.c
@@ -0,0 +1,90 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <assert.h>
+
+typedef struct node {
+	struct node* next;
+	int data;
+} node_t;
+
+//could be only `last`; by definition, `head = last->next;`
+typedef struct{
+	node_t* head;
+	node_t* last;
+} CIRC;
+
+//exo
+CIRC mkcircular();
+void insert(CIRC* cycle, int el);
+node_t* extract(CIRC* cycle);
+node_t* rotateToEven(CIRC* cycle);
+node_t* rotateToOdd(CIRC* cycle);
+
+void rotate(CIRC* cycle);
+
+int main(void)
+{
+	CIRC circ = mkcircular();
+	insert(&circ, 42);
+	insert(&circ, 21);
+	insert(&circ, 12);
+
+	printf("rotateToEven -> %d\n", rotateToEven(&circ)->data);
+	printf("rotateToOdd -> %d\n", rotateToOdd(&circ)->data);
+	printf("rotateToOdd -> %d\n", rotateToOdd(&circ)->data);
+	printf("rotateToEven -> %d\n", rotateToEven(&circ)->data);
+
+	printf("--------------------------\n");
+	printf("extract -> %d\n", extract(&circ)->data);
+	printf("extract -> %d\n", extract(&circ)->data);
+	printf("extract -> %d\n", extract(&circ)->data);
+	assert(!extract(&circ));
+}
+
+CIRC mkcircular(){
+	return (CIRC){};
+}
+
+void insert(CIRC* cycle, int el){
+	node_t* new = malloc(sizeof(node_t));
+	if(cycle->head == NULL)
+		cycle->head = cycle->last = new;
+	new->next = cycle->head;
+	new->data = el;
+
+	cycle->last->next = new;
+	cycle->last = new;
+}
+
+//problem with return node_t, it is the user that has to free the memory
+//that he did not explicitly allocated...
+node_t* extract(CIRC* cycle){
+	if(cycle->head == NULL || cycle->last == NULL)
+		return NULL;
+	node_t* ret = cycle->head;
+	if(cycle->head == cycle->last){
+		cycle->head = cycle->last = NULL;
+		return ret;
+	}
+	cycle->head = cycle->head->next;
+	cycle->last->next = cycle->head; 
+	return ret;
+}
+
+void rotate(CIRC* cycle){
+	cycle->last = cycle->head;
+	cycle->head = cycle->head->next;
+}
+
+node_t* rotateToEven(CIRC* cycle){
+	while(cycle->head->data % 2 != 0)
+		rotate(cycle);
+	return cycle->head;
+}
+
+node_t* rotateToOdd(CIRC* cycle ){
+	while(cycle->head->data % 2 == 0)
+		rotate(cycle);
+	return cycle->head;
+
+}