diff --git a/bac2/os/chap4/Makefile b/bac2/os/chap4/Makefile
new file mode 100644
index 0000000..65cc495
--- /dev/null
+++ b/bac2/os/chap4/Makefile
@@ -0,0 +1,27 @@
+.PHONY: clean, mrproper, run
+CC = gcc
+CFLAGS = -g -Wall
+
+exos = ex4 group
+
+all: $(exos)
+
+%.o: %.c
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+ex1: ex1.o RingBuffer.o
+	$(CC) $(CFLAGS) -o $@ $+
+
+ex%: ex%.o
+	$(CC) $(CFLAGS) -o $@ $+
+
+group: group.o
+	$(CC) $(CFLAGS) -o $@ $+
+
+clean:
+	rm -f *.o core.*
+	rm -f out group 
+	rm -f ${exos}
+
+run: group 
+	./$<
diff --git a/bac2/os/chap4/RingBuffer.c b/bac2/os/chap4/RingBuffer.c
new file mode 100644
index 0000000..7c24b18
--- /dev/null
+++ b/bac2/os/chap4/RingBuffer.c
@@ -0,0 +1,20 @@
+#include "RingBuffer.h"
+
+void rb_push(rb_t* rb, int el){
+	if(rb->filled)
+		return;
+	rb->data[rb->w_pos] = el; 
+	rb->w_pos = (rb->w_pos + 1) % rb->size;
+	if(rb->w_pos == rb->r_pos)
+		rb->filled = true;
+}
+int rb_pop(rb_t* rb){
+	if(rb->w_pos == rb->r_pos && !rb->filled){
+		rb->error = true;
+		return 0;
+	}
+	int ret = rb->data[rb->r_pos];
+	rb->r_pos = (rb->r_pos + 1) % rb->size;
+	rb->filled = false;
+	return ret;
+}
diff --git a/bac2/os/chap4/RingBuffer.h b/bac2/os/chap4/RingBuffer.h
new file mode 100644
index 0000000..dab22b4
--- /dev/null
+++ b/bac2/os/chap4/RingBuffer.h
@@ -0,0 +1,27 @@
+#pragma once
+#include <pthread.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <stdarg.h>
+
+#define rb_init(size, type)\
+	{(size),\
+	0, 0, false, false,\
+	type,\
+	malloc(sizeof(int)*(size)),\
+	PTHREAD_MUTEX_INITIALIZER};
+
+#define rb_free(rb) free(rb.data)
+
+typedef struct rb{
+	size_t size;
+	size_t w_pos, r_pos;
+	bool filled;
+	bool error;
+	enum {DROP, WAIT} type;
+	int* data;
+	pthread_mutex_t m;
+} rb_t;
+
+void rb_push(rb_t*, int);
+int rb_pop(rb_t*);
diff --git a/bac2/os/chap4/ex1.c b/bac2/os/chap4/ex1.c
new file mode 100644
index 0000000..07b7cd7
--- /dev/null
+++ b/bac2/os/chap4/ex1.c
@@ -0,0 +1,46 @@
+#include "RingBuffer.h"
+#include <stdio.h>
+#include <pthread.h>
+#include <time.h>
+#include <unistd.h>
+
+#define THREADS 10
+
+void* process(void* args);
+
+int main(void)
+{
+	srand(time(NULL));
+	rb_t rb = rb_init(50, DROP);
+	pthread_t threads[THREADS];
+
+	int even, odd = {0};
+
+	for (int i = 0; i < THREADS; ++i) {
+		pthread_create(threads+i, NULL, process, (void*) &rb);
+	}
+	int val;
+	while(true){
+		usleep(250);
+		val = rb_pop(&rb);
+		printf("got -> %d\n", val);
+
+		if(val % 2)
+			odd++;
+		else
+			even++;
+		
+		printf("%d nombres pairs et %d nombre impairs\n", even, odd);
+	}
+
+	rb_free(rb);
+}
+
+void* process(void* args){
+	rb_t *rb = (rb_t*) args;
+
+	while(true){
+		usleep((rand() % 4500) + 500);
+		rb_push(rb, rand());
+	}
+}
diff --git a/bac2/os/chap4/ex2.c b/bac2/os/chap4/ex2.c
new file mode 100644
index 0000000..7cea8a3
--- /dev/null
+++ b/bac2/os/chap4/ex2.c
@@ -0,0 +1,46 @@
+#include "RingBuffer.h"
+#include <stdio.h>
+#include <pthread.h>
+#include <time.h>
+#include <unistd.h>
+
+#define THREADS 10
+
+void* process(void* args);
+
+int main(void)
+{
+	srand(time(NULL));
+	rb_t rb = rb_init(50, WAIT);
+	pthread_t threads[THREADS];
+
+	int even, odd = {0};
+
+	for (int i = 0; i < THREADS; ++i) {
+		pthread_create(threads+i, NULL, process, (void*) &rb);
+	}
+	int val;
+	while(true){
+		usleep(250);
+		val = rb_pop(&rb);
+		printf("got -> %d\n", val);
+
+		if(val % 2)
+			odd++;
+		else
+			even++;
+		
+		printf("%d nombres pairs et %d nombre impairs\n", even, odd);
+	}
+
+	rb_free(rb);
+}
+
+void* process(void* args){
+	rb_t *rb = (rb_t*) args;
+
+	while(true){
+		usleep((rand() % 4500) + 500);
+		rb_push(rb, rand());
+	}
+}
diff --git a/bac2/os/chap4/ex4.c b/bac2/os/chap4/ex4.c
new file mode 100644
index 0000000..ff8063e
--- /dev/null
+++ b/bac2/os/chap4/ex4.c
@@ -0,0 +1,34 @@
+#include <pthread.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#define SIZE 32768
+
+pthread_t child_thread;
+pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
+
+int table[SIZE];
+
+int cmp(const void* a1, const void* a2){
+	return *((int*)a2) - *((int*)a1);
+}
+
+void* child(void* args){
+	pthread_mutex_lock(&mutex);
+	for (int i = 0; i < SIZE; ++i) {
+		table[i] = rand();
+	}
+	pthread_mutex_unlock(&mutex);
+	sleep(1);
+	pthread_mutex_lock(&mutex);
+	return NULL;
+}
+
+int main(void)
+{
+	pthread_create(&child_thread, NULL, child, NULL);
+
+	pthread_mutex_lock(&mutex);
+	qsort(table, SIZE, sizeof(int), cmp);
+	pthread_mutex_unlock(&mutex);
+}
diff --git a/bac2/os/chap4/group.c b/bac2/os/chap4/group.c
new file mode 100644
index 0000000..bbb142a
--- /dev/null
+++ b/bac2/os/chap4/group.c
@@ -0,0 +1,45 @@
+/***************************************************************************
+*  Le dinner des philosophes 											   * 
+*  Two phase locking: 													   *
+*  - array de baguettes													   * 
+*  - manger(): tente l'acquisition des deux. attente passive si non dispo  *
+****************************************************************************/
+#include <pthread.h>
+#include <unistd.h>
+#include <stdlib.h>
+#define TABLE_SIZE 5
+
+#define RIGHT(x) ((x)) % TABLE_SIZE
+#define LEFT(x) ((x)+1) % TABLE_SIZE
+
+typedef unsigned int PHILOSOPHE; // philosophe's id in the table
+
+pthread_mutex_t baguettes[TABLE_SIZE], tableLock;
+int main(void)
+{
+
+}
+
+void penser(){
+	usleep(100 + rand() % 500);
+}
+
+void manger(){
+	usleep(100 + rand() % 500);
+}
+
+void prendre(){
+
+}
+void deposer(){
+
+}
+
+void* thread_philosophe(void* arg){
+	while(42){
+		penser();
+		prendre();
+		manger();
+		deposer();
+	}
+}
diff --git a/bac2/os/chap4/philosophes.h b/bac2/os/chap4/philosophes.h
new file mode 100644
index 0000000..023d2c1
--- /dev/null
+++ b/bac2/os/chap4/philosophes.h
@@ -0,0 +1,11 @@
+#ifndef PHILOSOPHES_H
+#define PHILOSOPHES_H
+
+
+
+
+#ifndef PHILOSPHES_IMPLEMENTATION
+
+#endif /* end of implementation */
+
+#endif /* end of include guard: PHILOSOPHES_H */