diff --git a/bac2/os/chap3/Makefile b/bac2/os/chap3/Makefile
index 160b8a9..4cf7ec5 100644
--- a/bac2/os/chap3/Makefile
+++ b/bac2/os/chap3/Makefile
@@ -2,7 +2,10 @@
 CC = gcc
 CFLAGS = -g -Wall
 
-all: ex1 ex2 ex3 ex4 ex5
+all: ex1 ex2 ex3 ex4 ex5 ex6 ex7 group
+
+group: group.o mergeSort.o
+	$(CC) $(CFLAGS) -o $@ $+
 
 %.o: %.c
 	$(CC) $(CFLAGS) -c -o $@ $<
@@ -16,7 +19,7 @@ clean:
 mrproper: clean
 	rm -f ex1
 
-run: ex6
+run: group
 	./$<
 
 graph: run
diff --git a/bac2/os/chap3/ex7.c b/bac2/os/chap3/ex7.c
new file mode 100644
index 0000000..c6ec7c0
--- /dev/null
+++ b/bac2/os/chap3/ex7.c
@@ -0,0 +1,6 @@
+#include <stdio.h>
+
+int main(void)
+{
+	printf("Hello World!\n");
+}
diff --git a/bac2/os/chap3/group.c b/bac2/os/chap3/group.c
new file mode 100644
index 0000000..9ac9c6f
--- /dev/null
+++ b/bac2/os/chap3/group.c
@@ -0,0 +1,23 @@
+#include "mergeSort.h"
+
+#include <stdio.h>
+#include <time.h>
+#include <stdlib.h>
+
+#define printlist(s, x) printf("%s = [", #x); for (int i = 0; i < s; ++i) { printf("%d, ", x[i]); } printf("\b\b]\n");
+
+#define listSize 10000000
+int list[listSize];
+
+int main(void)
+{
+	srand(time(NULL));
+	for (int i = 0; i < listSize; ++i) {
+		list[i] = rand();
+	}
+	/* printlist(listSize, list); */
+
+	margs_t args = {list, 0, 9};
+	merge_sort(&args);
+	printlist(listSize, list);
+}
diff --git a/bac2/os/chap3/mergeSort.c b/bac2/os/chap3/mergeSort.c
new file mode 100644
index 0000000..899baf9
--- /dev/null
+++ b/bac2/os/chap3/mergeSort.c
@@ -0,0 +1,58 @@
+#include "mergeSort.h"
+
+#include <stdlib.h>
+#include <assert.h>
+#include <pthread.h>
+
+#define THREADS
+
+pthread_t threads[THREADS];
+
+void* merge_sort(void* args) {
+	margs_t* f = (margs_t*) args;
+	if(f->start < f->final) {
+		pthread_t t1, t2;
+		index middle = (f->start + f->final) / 2;
+
+		margs_t args1 = {f->array, f->start, middle};
+		assert(!pthread_create(&t1, NULL, merge_sort,  (void*) &args1));
+		margs_t args2 = {f->array, middle+1, f->final};
+		assert(!pthread_create(&t2, NULL, merge_sort,  (void*) &args2));
+
+		assert(pthread_join(t1, NULL));
+		assert(pthread_join(t2, NULL));
+		merge(f->array, f->start, middle, f->final);
+	}
+	int *ret = 0;
+	return ret;
+}
+
+void merge(int array[], index start, index middle, index final) {
+
+	length countL = middle - start + 1;
+	int *arrayL = malloc(countL * sizeof(int));
+	for(index currentL = 0; currentL < countL; currentL ++)
+		arrayL[currentL] = array[start + currentL ];
+
+	length countR = final - middle;
+	int* arrayR = malloc(countR * sizeof(int));
+	for(index currentR = 0; currentR < countR; currentR ++)
+		arrayR[currentR] = array[middle + 1 + currentR ];
+
+	index currentL, currentR, current;
+	for(currentL = 0, currentR = 0, current = start;
+		 	current <= final && currentL < countL && currentR < countR;
+		   	current ++) 
+		if(arrayL[currentL] <= arrayR[currentR])
+			array[current] = arrayL[currentL++];
+		else
+			array[current] = arrayR[currentR++];
+
+	while(currentR < countR)
+		array[current++] = arrayR[currentR++];
+	while(currentL < countL)
+		array[current++] = arrayL[currentL++];
+
+	free(arrayL);
+	free(arrayR);
+}
diff --git a/bac2/os/chap3/mergeSort.h b/bac2/os/chap3/mergeSort.h
new file mode 100644
index 0000000..9453966
--- /dev/null
+++ b/bac2/os/chap3/mergeSort.h
@@ -0,0 +1,15 @@
+#pragma once
+
+typedef unsigned int index;
+typedef unsigned int length;
+
+typedef struct merge_args{
+	int* array;
+	index start;
+	index final;
+} margs_t;
+
+
+void merge(int array[], index start, index middle, index final);
+/* void merge_sort(int array [], index start , index final ); */
+void* merge_sort(void*);
diff --git a/bac2/pgl/usecase/extension_messagerie.tex b/bac2/pgl/usecase/extension_messagerie.tex
new file mode 100644
index 0000000..464b933
--- /dev/null
+++ b/bac2/pgl/usecase/extension_messagerie.tex
@@ -0,0 +1,32 @@
+\documentclass{article}
+
+\usepackage[utf8]{inputenc}
+\usepackage[T1]{fontenc}
+\usepackage{amsmath, amsfonts, amssymb, amsthm}
+\usepackage{tikz}
+
+\author{Debucquoy Anthony}
+\title{Extension messagerie}
+
+\begin{document}
+
+\section{Use case diagram}
+
+\begin{center}
+\resizebox{390pt}{!}{
+	\input{./use_case_messagerie.tex}
+}
+\end{center}
+
+\subsection{Answer topic}
+\subsection{Create discutions}
+\subsubsection{Send messages}
+\subsection{Ask apointment}
+\subsubsection{Export to calendar}
+\subsection{Manage apointment}
+\subsubsection{Propose new apointment}
+\subsection{Create forum}
+\subsubsection{Post topics}
+\subsubsection{Post poll}
+
+\end{document}
diff --git a/bac2/pgl/usecase/use_case_messagerie.uml b/bac2/pgl/usecase/use_case_messagerie.uml
new file mode 100644
index 0000000..c2f39af
--- /dev/null
+++ b/bac2/pgl/usecase/use_case_messagerie.uml
@@ -0,0 +1,39 @@
+@startuml
+
+left to right direction
+
+:Student: as s
+:Teacher: as t
+
+
+package "Messagerie"{
+
+(Answer topics) as at
+(Ask apointement) as aa
+(Create Discution) as cd
+(Create Forum) as cf
+(Manage apointement) as ma
+
+s -- at
+s -- aa
+s -- cd
+
+t -- cd
+t -- cf
+t -- ma
+
+(Export to calendar) as etc
+
+aa <-- etc : << extend >>
+ma <-- etc : << extend >>
+
+(Post topics) as pt
+cf <-- pt : << include >>
+pt <|-- (Post poll)
+cd <-- (Send messages) : << include >>
+
+ma <-- (Propose new appointment) : << exlude>> \n [refuse]
+
+}
+
+@enduml