diff --git a/str/str_chr.c b/str/str_chr.c
index 37d24fe..418dd57 100644
--- a/str/str_chr.c
+++ b/str/str_chr.c
@@ -15,3 +15,17 @@ size_t str_chr(const char *in, char needle) {
   }
   return (size_t)(t-in);
 }
+
+#ifdef UNITTEST
+#include <assert.h>
+#include <string.h>
+
+int main() {
+  char buf[100];
+  strcpy(buf,"abcdefghijklmnopqrstuvwxyz");
+  size_t i;
+  for (i=0; i<26; ++i)
+    assert(str_chr(buf,buf[i])==i);
+  assert(str_chr(buf,'X')==26);
+}
+#endif
diff --git a/str/str_diff.c b/str/str_diff.c
index 57ff0b3..3d28b01 100644
--- a/str/str_diff.c
+++ b/str/str_diff.c
@@ -21,3 +21,15 @@ int str_diff(const char* a, const char* b) {
   }
   return j;
 }
+
+#ifdef UNITTEST
+#include <assert.h>
+#include <string.h>
+
+int main() {
+  assert(str_diff("foo","foo")==0);
+  assert(str_diff("foo","fob")==('o'-'b'));
+  assert(str_diff("foo","foox")==(-'x'));
+  assert(str_diff("foox","foo")=='x');
+}
+#endif