diff --git a/threads-sema/Makefile b/threads-sema/Makefile
index 927d969..8ab4f2b 100644
--- a/threads-sema/Makefile
+++ b/threads-sema/Makefile
@@ -1,7 +1,12 @@
 CC     := gcc
 CFLAGS := -Wall -Werror -I../include 
 
-SRCS   := dining_philosophers_deadlock.c dining_philosophers_deadlock_print.c dining_philosophers_no_deadlock.c dining_philosophers_no_deadlock_print.c join.c
+SRCS   := dining_philosophers_deadlock.c \
+	dining_philosophers_deadlock_print.c \
+	dining_philosophers_no_deadlock.c \
+	dining_philosophers_no_deadlock_print.c \
+	join.c \
+	zemaphore.c
 
 OBJS   := ${SRCS:c=o}
 PROGS  := ${SRCS:.c=}
diff --git a/threads-sema/zemaphore.c b/threads-sema/zemaphore.c
new file mode 100644
index 0000000..74979e6
--- /dev/null
+++ b/threads-sema/zemaphore.c
@@ -0,0 +1,55 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <unistd.h>
+
+#include "common.h"
+#include "common_threads.h"
+
+typedef struct __Zem_t {
+    int value;
+    pthread_cond_t  cond;
+    pthread_mutex_t lock;
+} Zem_t;
+
+void Zem_init(Zem_t *z, int value) {
+    z->value = value;
+    Cond_init(&z->cond);
+    Mutex_init(&z->lock);
+}
+
+void Zem_wait(Zem_t *z) {
+    Mutex_lock(&z->lock);
+    while (z->value <= 0)
+	Cond_wait(&z->cond, &z->lock);
+    z->value--;
+    Mutex_unlock(&z->lock);
+}
+
+void Zem_post(Zem_t *z) {
+    Mutex_lock(&z->lock);
+    z->value++;
+    Cond_signal(&z->cond);
+    Mutex_unlock(&z->lock);
+}
+
+
+Zem_t s;
+
+void *child(void *arg) {
+    sleep(4);
+    printf("child\n");
+    Zem_post(&s); // signal here: child is done
+    return NULL;
+}
+
+int main(int argc, char *argv[]) {
+    Zem_init(&s, 0); 
+    printf("parent: begin\n");
+    pthread_t c;
+    Pthread_create(&c, NULL, child, NULL);
+    Zem_wait(&s); // wait here for child
+    printf("parent: end\n");
+    return 0;
+}
+