diff --git a/axl_string.cpp b/axl_string.c
similarity index 96%
rename from axl_string.cpp
rename to axl_string.c
index 0d312a1..fffd90e 100644
--- a/axl_string.cpp
+++ b/axl_string.c
@@ -60,7 +60,9 @@ i8* axl_strcat(i8* dst, const i8* src)
         return dst;
     }
 
-    return axl_strcpy(dst + axl_strlen(dst), src);
+    axl_strcpy(dst + axl_strlen(dst), src);
+
+    return dst;
 }
 
 i8* axl_strncat(i8* dst, const i8* src, u32 n) //n actually means "not more than"
diff --git a/axl_string_test.c b/axl_string_test.c
new file mode 100644
index 0000000..6e38cae
--- /dev/null
+++ b/axl_string_test.c
@@ -0,0 +1,234 @@
+/*
+ * axl_string_tests.c — Unit tests for axl_string.h
+ */
+
+#include "axl_string.h"
+#include "axl_koan.h"
+#include "axl_memory.h"
+
+/* ------------------------------------------------------------------ */
+/* axl_strlen */
+/* ------------------------------------------------------------------ */
+
+KOAN(strlen_empty)
+{
+    i8* s = (i8*)"";
+    ASSERT_UINT_EQ(0, axl_strlen(s));
+}
+
+KOAN(strlen_simple)
+{
+    i8* s = (i8*)"hello";
+    ASSERT_UINT_EQ(5, axl_strlen(s));
+}
+
+KOAN(strlen_with_special_chars)
+{
+    i8* s = (i8*)"hello\n\tworld";
+    ASSERT_UINT_EQ(12, axl_strlen(s));
+}
+
+/* ------------------------------------------------------------------ */
+/* axl_strcpy */
+/* ------------------------------------------------------------------ */
+
+KOAN(strcpy_basic)
+{
+    i8* src = (i8*)"test";
+    i8* dst = (i8*)axl_malloc(10);
+    ASSERT_NOT_NULL(dst);
+    
+    i8* result = axl_strcpy(dst, src);
+    ASSERT_PTR_EQ(dst, result);
+    ASSERT_STR_EQ("test", (const char*)dst);
+    
+    axl_free(dst);
+}
+
+KOAN(strcpy_empty)
+{
+    i8* src = (i8*)"";
+    i8* dst = (i8*)axl_malloc(10);
+    ASSERT_NOT_NULL(dst);
+    axl_memset(dst, 0xFF, 10);
+    
+    i8* result = axl_strcpy(dst, src);
+    ASSERT_PTR_EQ(dst, result);
+    ASSERT_STR_EQ("", (const char*)dst);
+    
+    axl_free(dst);
+}
+
+/* ------------------------------------------------------------------ */
+/* axl_strncpy */
+/* ------------------------------------------------------------------ */
+
+KOAN(strncpy_exact)
+{
+    i8* src = (i8*)"hello";
+    i8* dst = (i8*)axl_malloc(10);
+    ASSERT_NOT_NULL(dst);
+    
+    i8* result = axl_strncpy(dst, src, 5);
+    ASSERT_PTR_EQ(dst, result);
+    ASSERT_TRUE(memcmp(dst, "hello", 5) == 0);
+    
+    axl_free(dst);
+}
+
+KOAN(strncpy_truncates)
+{
+    i8* src = (i8*)"hello world";
+    i8* dst = (i8*)axl_malloc(10);
+    ASSERT_NOT_NULL(dst);
+    
+    i8* result = axl_strncpy(dst, src, 5);
+    ASSERT_PTR_EQ(dst, result);
+    ASSERT_TRUE(memcmp(dst, "hello", 5) == 0);
+    
+    axl_free(dst);
+}
+
+KOAN(strncpy_pads_with_nulls)
+{
+    i8* src = (i8*)"hi";
+    i8* dst = (i8*)axl_malloc(10);
+    ASSERT_NOT_NULL(dst);
+    axl_memset(dst, 0xFF, 10);
+    
+    i8* result = axl_strncpy(dst, src, 5);
+    ASSERT_PTR_EQ(dst, result);
+    ASSERT_TRUE(memcmp(dst, "hi\0\0\0", 5) == 0);
+    
+    axl_free(dst);
+}
+
+/* ------------------------------------------------------------------ */
+/* axl_strcat */
+/* ------------------------------------------------------------------ */
+
+KOAN(strcat_basic)
+{
+    i8* dst = (i8*)axl_malloc(20);
+    ASSERT_NOT_NULL(dst);
+    axl_strcpy(dst, (i8*)"hello ");
+    
+    i8* result = axl_strcat(dst, (i8*)"world");
+    ASSERT_PTR_EQ(dst, result);
+    ASSERT_STR_EQ("hello world", (const char*)dst);
+    
+    axl_free(dst);
+}
+
+KOAN(strcat_to_empty)
+{
+    i8* dst = (i8*)axl_malloc(20);
+    ASSERT_NOT_NULL(dst);
+    axl_strcpy(dst, (i8*)"");
+    
+    i8* result = axl_strcat(dst, (i8*)"test");
+    ASSERT_PTR_EQ(dst, result);
+    ASSERT_STR_EQ("test", (const char*)dst);
+    
+    axl_free(dst);
+}
+
+/* ------------------------------------------------------------------ */
+/* axl_strncat */
+/* ------------------------------------------------------------------ */
+
+KOAN(strncat_full)
+{
+    i8* dst = (i8*)axl_malloc(20);
+    ASSERT_NOT_NULL(dst);
+    axl_strcpy(dst, (i8*)"hello ");
+    
+    i8* result = axl_strncat(dst, (i8*)"world", 5);
+    ASSERT_PTR_EQ(dst, result);
+    ASSERT_STR_EQ("hello world", (const char*)dst);
+    
+    axl_free(dst);
+}
+
+KOAN(strncat_partial)
+{
+    i8* dst = (i8*)axl_malloc(20);
+    ASSERT_NOT_NULL(dst);
+    axl_strcpy(dst, (i8*)"hello ");
+    
+    i8* result = axl_strncat(dst, (i8*)"worldwide", 5);
+    ASSERT_PTR_EQ(dst, result);
+    ASSERT_STR_EQ("hello world", (const char*)dst);
+    
+    axl_free(dst);
+}
+
+KOAN(strncat_always_null_terminates)
+{
+    i8* dst = (i8*)axl_malloc(20);
+    ASSERT_NOT_NULL(dst);
+    axl_strcpy(dst, (i8*)"test");
+    
+    i8* result = axl_strncat(dst, (i8*)"123456789", 3);
+    ASSERT_PTR_EQ(dst, result);
+    ASSERT_STR_EQ("test123", (const char*)dst);
+    ASSERT_INT_EQ(0, (int)dst[7]);  // Ensure null at position 7
+    
+    axl_free(dst);
+}
+
+/* ------------------------------------------------------------------ */
+/* axl_strcmp */
+/* ------------------------------------------------------------------ */
+
+KOAN(strcmp_equal)
+{
+    i8* s1 = (i8*)"hello";
+    i8* s2 = (i8*)"hello";
+    ASSERT_INT_EQ(0, axl_strcmp(s1, s2));
+}
+
+KOAN(strcmp_less)
+{
+    i8* s1 = (i8*)"abc";
+    i8* s2 = (i8*)"def";
+    ASSERT_TRUE(axl_strcmp(s1, s2) < 0);
+}
+
+KOAN(strcmp_greater)
+{
+    i8* s1 = (i8*)"xyz";
+    i8* s2 = (i8*)"abc";
+    ASSERT_TRUE(axl_strcmp(s1, s2) > 0);
+}
+
+KOAN(strcmp_prefix)
+{
+    i8* s1 = (i8*)"test";
+    i8* s2 = (i8*)"testing";
+    ASSERT_TRUE(axl_strcmp(s1, s2) < 0);
+}
+
+KOAN(strcmp_both_empty)
+{
+    i8* s1 = (i8*)"";
+    i8* s2 = (i8*)"";
+    ASSERT_INT_EQ(0, axl_strcmp(s1, s2));
+}
+
+KOAN(strcmp_first_empty)
+{
+    i8* s1 = (i8*)"";
+    i8* s2 = (i8*)"hello";
+    ASSERT_TRUE(axl_strcmp(s1, s2) < 0);
+}
+
+/* ------------------------------------------------------------------ */
+/* Main */
+/* ------------------------------------------------------------------ */
+
+int main(void)
+{
+    axl_init();
+    return koan_run_all();
+}