diff --git a/exercises/ex12/counter_array_mutex.c b/exercises/ex12/counter_array_mutex.c
new file mode 100644
index 00000000..ab69ebf8
--- /dev/null
+++ b/exercises/ex12/counter_array_mutex.c
@@ -0,0 +1,130 @@
+/* Example code for Think OS.
+Copyright 2014 Allen Downey
+License: GNU GPLv3
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include "mutex.h"
+
+#define NUM_CHILDREN 2
+
+void perror_exit(char *s)
+{
+    perror(s);
+    exit(-1);
+}
+
+void *check_malloc(int size)
+{
+    void *p = malloc(size);
+    if (p == NULL) {
+        perror_exit("malloc failed");
+    }
+    return p;
+}
+
+typedef struct {
+    int counter;
+    int end;
+    int *array;
+    Mutex *mutex;
+} Shared;
+
+Shared *make_shared(int end)
+{
+    int i;
+    Shared *shared = check_malloc(sizeof(Shared));
+
+    shared->counter = 0;
+    shared->end = end;
+
+    shared->array = check_malloc(shared->end * sizeof(int));
+    for (i=0; i<shared->end; i++) {
+        shared->array[i] = 0;
+    }
+
+    shared->mutex = make_mutex();
+    return shared;
+}
+
+pthread_t make_thread(void *(*entry)(void *), Shared *shared)
+{
+    int ret;
+    pthread_t thread;
+
+    ret = pthread_create(&thread, NULL, entry, (void *) shared);
+    if (ret != 0) {
+        perror_exit("pthread_create failed");
+    }
+    return thread;
+}
+
+void join_thread(pthread_t thread)
+{
+    int ret = pthread_join(thread, NULL);
+    if (ret == -1) {
+        perror_exit("pthread_join failed");
+    }
+}
+
+void child_code(Shared *shared)
+{
+    printf("Starting child at counter %d\n", shared->counter);
+
+    while (1) {
+        mutex_lock(shared->mutex);
+        if (shared->counter >= shared->end) {
+            mutex_unlock(shared->mutex);
+            return;
+        }
+
+        shared->array[shared->counter]++;
+        shared->counter++;
+
+        if (shared->counter % 10000 == 0) {
+            printf("%d\n", shared->counter);
+        }
+        mutex_unlock(shared->mutex);
+    }
+}
+
+void *entry(void *arg)
+{
+    Shared *shared = (Shared *) arg;
+    child_code(shared);
+    printf("Child done.\n");
+    pthread_exit(NULL);
+}
+
+void check_array(Shared *shared)
+{
+    int i, errors=0;
+
+    printf("Checking...\n");
+
+    for (i=0; i<shared->end; i++) {
+        if (shared->array[i] != 1) errors++;
+    }
+    printf("%d errors.\n", errors);
+}
+
+int main()
+{
+    int i;
+    pthread_t child[NUM_CHILDREN];
+
+    Shared *shared = make_shared(1000000);
+
+    for (i=0; i<NUM_CHILDREN; i++) {
+        child[i] = make_thread(entry, shared);
+    }
+
+    for (i=0; i<NUM_CHILDREN; i++) {
+        join_thread(child[i]);
+    }
+
+    check_array(shared);
+    return 0;
+}