diff --git a/threads-sema/Makefile b/threads-sema/Makefile
index 8ab4f2b..071cf1f 100644
--- a/threads-sema/Makefile
+++ b/threads-sema/Makefile
@@ -6,6 +6,7 @@ SRCS   := dining_philosophers_deadlock.c \
 	dining_philosophers_no_deadlock.c \
 	dining_philosophers_no_deadlock_print.c \
 	join.c \
+	binary.c \
 	zemaphore.c
 
 OBJS   := ${SRCS:c=o}
diff --git a/threads-sema/README.md b/threads-sema/README.md
index 6095a32..0fa6c3e 100644
--- a/threads-sema/README.md
+++ b/threads-sema/README.md
@@ -6,8 +6,23 @@ using semaphores, found in `join.c`.
 
 Run `make` to build the code; run `join` to test it. Fun!
 
+```sh
+prompt> make
+prompt> ./join
+```
 
 
+# Binary Semaphores (Locks)
+
+Simple example of semaphores as locks (binary semaphores).
+Code in `binary.c`.
+
+Run `make` to build the code; run `binary` to test it. Fun!
+
+```sh
+prompt> make
+prompt> ./binary
+```
 
 
 # Dining Philosophers
diff --git a/threads-sema/binary.c b/threads-sema/binary.c
new file mode 100644
index 0000000..93df440
--- /dev/null
+++ b/threads-sema/binary.c
@@ -0,0 +1,32 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <unistd.h>
+
+#include "common.h"
+#include "common_threads.h"
+
+sem_t mutex;
+volatile int counter = 0;
+
+void *child(void *arg) {
+    int i;
+    for (i = 0; i < 10000000; i++) {
+	Sem_wait(&mutex);
+	counter++;
+	Sem_post(&mutex);
+    }
+    return NULL;
+}
+
+int main(int argc, char *argv[]) {
+    Sem_init(&mutex, 1); 
+    pthread_t c1, c2;
+    Pthread_create(&c1, NULL, child, NULL);
+    Pthread_create(&c2, NULL, child, NULL);
+    Pthread_join(c1, NULL);
+    Pthread_join(c2, NULL);
+    printf("result: %d\n", counter);
+    return 0;
+}
+