diff --git a/bac2/os/chap3/Makefile b/bac2/os/chap3/Makefile
index 959a532..160b8a9 100644
--- a/bac2/os/chap3/Makefile
+++ b/bac2/os/chap3/Makefile
@@ -2,12 +2,12 @@
 CC = gcc
 CFLAGS = -g -Wall
 
-all: ex1
+all: ex1 ex2 ex3 ex4 ex5
 
 %.o: %.c
 	$(CC) $(CFLAGS) -c -o $@ $<
 
-ex1: ex1.o
+%: %.o
 	$(CC) $(CFLAGS) -o $@ $+
 
 clean:
@@ -16,5 +16,9 @@ clean:
 mrproper: clean
 	rm -f ex1
 
-run: ex1
+run: ex6
 	./$<
+
+graph: run
+	graph -Tpng plot > plot.png
+	nsxiv plot.png
diff --git a/bac2/os/chap3/ex2.c b/bac2/os/chap3/ex2.c
new file mode 100644
index 0000000..79a55d7
--- /dev/null
+++ b/bac2/os/chap3/ex2.c
@@ -0,0 +1,14 @@
+#include <unistd.h>
+#include <stdio.h>
+
+#define printvi(X) printf("%s = %d\n", #X, X);
+
+int main(void)
+{
+	printvi(getpid());
+
+	pid_t f = fork();
+	if(f != 0)
+		getchar();
+}
+
diff --git a/bac2/os/chap3/ex3.c b/bac2/os/chap3/ex3.c
new file mode 100644
index 0000000..e4aa541
--- /dev/null
+++ b/bac2/os/chap3/ex3.c
@@ -0,0 +1,25 @@
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/wait.h>
+#include <time.h>
+
+#define printvi(X) printf("%s = %d\n", #X, X);
+
+int main(void)
+{
+	pid_t f = fork();
+	if(f == 0){
+		srand(time(NULL));
+		int delay = 1 + rand() % 10;
+		int value = rand() % 256;
+		printvi(delay);
+		printvi(value);
+		sleep(delay);
+		return value;
+	}
+
+	int child_ret = 0;
+	wait(&child_ret);
+	printvi(WEXITSTATUS(child_ret));
+}
diff --git a/bac2/os/chap3/ex4.c b/bac2/os/chap3/ex4.c
new file mode 100644
index 0000000..b6f1480
--- /dev/null
+++ b/bac2/os/chap3/ex4.c
@@ -0,0 +1,29 @@
+#include <sys/time.h>
+#include <stdio.h>
+#include <time.h>
+#include <unistd.h>
+
+#define printvi(X) printf("%s = %d\n", #X, X);
+#define printvf(X) printf("%s = %lld\n", #X, X);
+
+typedef struct timespec timespec_t;
+
+timespec_t start, end;
+
+long long int dtime(timespec_t *start, timespec_t *end){
+	return (end->tv_sec - start->tv_sec) * 1e9 + end->tv_nsec - start->tv_nsec;
+}
+
+int main(void)
+{
+	FILE* fd = fopen("plot", "w");
+	long long int time, time_sum;
+	for (int i = 0; i < 10000; ++i) {
+		clock_gettime(CLOCK_REALTIME, &start);
+		usleep(1);
+		clock_gettime(CLOCK_REALTIME, &end);
+		time = dtime(&start, &end);
+		time_sum += time;
+		fprintf(fd, "%d	%lld\n", i, time);
+	}
+}
diff --git a/bac2/os/chap3/ex5.c b/bac2/os/chap3/ex5.c
new file mode 100644
index 0000000..4d423bf
--- /dev/null
+++ b/bac2/os/chap3/ex5.c
@@ -0,0 +1,29 @@
+#include <sys/time.h>
+#include <stdio.h>
+#include <time.h>
+#include <unistd.h>
+
+#define printvi(X) printf("%s = %d\n", #X, X);
+#define printvf(X) printf("%s = %lld\n", #X, X);
+
+typedef struct timespec timespec_t;
+
+timespec_t start, end, s = {0, 1};
+
+long long int dtime(timespec_t *start, timespec_t *end){
+	return (end->tv_sec - start->tv_sec) * 1e9 + end->tv_nsec - start->tv_nsec;
+}
+
+int main(void)
+{
+	FILE* fd = fopen("plot", "w");
+	long long int time, time_sum;
+	for (int i = 0; i < 10000; ++i) {
+		clock_gettime(CLOCK_REALTIME, &start);
+		nanosleep(&s, NULL);
+		clock_gettime(CLOCK_REALTIME, &end);
+		time = dtime(&start, &end);
+		time_sum += time;
+		fprintf(fd, "%d	%lld\n", i, time);
+	}
+}
diff --git a/bac2/os/chap3/ex6.c b/bac2/os/chap3/ex6.c
new file mode 100644
index 0000000..82c789a
--- /dev/null
+++ b/bac2/os/chap3/ex6.c
@@ -0,0 +1,44 @@
+#include <stdlib.h>
+#include <pthread.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <time.h>
+
+#define THREADS 9
+
+int cmp(const void * first, const void *second){
+	return (*(int*)first) - (*(int*)second);
+}
+
+void* threaded(){
+	int delay = rand() % 6;
+	int value = rand() % 256;
+	int *ret = malloc(sizeof(int));
+	*ret = value;
+	sleep(delay);
+	return ret;
+}
+
+int main(void)
+{
+	srand(time(NULL));
+	pthread_t threads[THREADS];
+	int returned[THREADS];
+	for (int i = 0; i < THREADS; ++i) {
+		pthread_create(threads+i, NULL, threaded, NULL);
+	}
+
+	int *ret= NULL;
+	for (int i = 0; i < THREADS; ++i) {
+		pthread_join(threads[i], (void*)&ret);
+		returned[i] = *ret;
+	}
+
+	qsort((void* ) returned, THREADS, sizeof(int), cmp);
+
+	for (int i = 0; i < THREADS; ++i) {
+		printf("%d\n", returned[i]);
+	}
+
+	return 0;
+}