diff --git a/threads-sema/Makefile b/threads-sema/Makefile
index 4619c72..927d969 100644
--- a/threads-sema/Makefile
+++ b/threads-sema/Makefile
@@ -1,9 +1,19 @@
+CC     := gcc
+CFLAGS := -Wall -Werror -I../include 
 
-FLAGS = -Wall -pthread -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
 
-all:
-	gcc $(FLAGS) -o dining_philosophers_deadlock dining_philosophers_deadlock.c 
-	gcc $(FLAGS) -o dining_philosophers_deadlock_print dining_philosophers_deadlock_print.c 
-	gcc $(FLAGS) -o dining_philosophers_no_deadlock dining_philosophers_no_deadlock.c 
-	gcc $(FLAGS) -o dining_philosophers_no_deadlock_print dining_philosophers_no_deadlock_print.c
+OBJS   := ${SRCS:c=o}
+PROGS  := ${SRCS:.c=}
 
+.PHONY: all
+all: ${PROGS}
+
+${PROGS} : % : %.o Makefile
+	${CC} $< -o $@ -pthread
+
+clean:
+	rm -f ${PROGS} ${OBJS}
+
+%.o: %.c Makefile
+	${CC} ${CFLAGS} -c $<
diff --git a/threads-sema/README.md b/threads-sema/README.md
index a6804c4..6095a32 100644
--- a/threads-sema/README.md
+++ b/threads-sema/README.md
@@ -1,4 +1,15 @@
 
+# Fork/Join
+
+Simple example of the fork/join (i.e., waiting for a child) problem
+using semaphores, found in `join.c`.
+
+Run `make` to build the code; run `join` to test it. Fun!
+
+
+
+
+
 # Dining Philosophers
 
 The dining philosophers example from the text is found herein, in a few
diff --git a/threads-sema/join.c b/threads-sema/join.c
new file mode 100644
index 0000000..e9de1ff
--- /dev/null
+++ b/threads-sema/join.c
@@ -0,0 +1,27 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <unistd.h>
+
+#include "common.h"
+#include "common_threads.h"
+
+sem_t s;
+
+void *child(void *arg) {
+    sleep(4);
+    printf("child\n");
+    Sem_post(&s); // signal here: child is done
+    return NULL;
+}
+
+int main(int argc, char *argv[]) {
+    Sem_init(&s, 0); 
+    printf("parent: begin\n");
+    pthread_t c;
+    Pthread_create(&c, NULL, child, NULL);
+    Sem_wait(&s); // wait here for child
+    printf("parent: end\n");
+    return 0;
+}
+