guid_new() returns GUIDs compatible with RFC4122

common/guid.c

@@ -30,11 +30,27 @@
 #include "os_calls.h"
 #include "string_calls.h"
 
+enum
+{
+    /* Field offsets in the UUID */
+    E_CLOCK_SEQ_HI_AND_RESERVED = 8,
+    E_TIME_HI_AND_VERSION_MSB = 7,
+    /* UUID versions from RFC4122 section 4.1.3 */
+    E_UUID_VERSION_RANDOM = 4
+};
+
 struct guid
 guid_new(void)
 {
     struct guid guid = {0};
     g_random(guid.g, sizeof(guid.g));
+    /* Show this UUID as conforming to RFC4122 (section 4.1.1) */
+    guid.g[E_CLOCK_SEQ_HI_AND_RESERVED] &= ~0x40; /* Clear bit 6 */
+    guid.g[E_CLOCK_SEQ_HI_AND_RESERVED] |= (char)0x80; /* Set bit 7 */
+
+    guid.g[E_TIME_HI_AND_VERSION_MSB] &= ~0xf0;
+    guid.g[E_TIME_HI_AND_VERSION_MSB] |= (E_UUID_VERSION_RANDOM << 4);
+
     return guid;
 }
 

common/guid.h

@@ -32,8 +32,23 @@
 
 
 /**
- * Use a struct for the guid so we can easily copy by assignment
- */
+ * Use a struct for the guid so we can easily copy by assignment.
+ * We use an array of char so that
+ * we can compare GUIDs with a straight memcmp()
+ *
+ * Some fields of the GUID are in little-endian-order as specified by
+ * [MS-DTYP]. This is at odds with RFC4122 which specifies big-endian
+ * order for all fields.
+ *
+ * Octets RFC4122 field
+ * ------ -------------
+ * 0-3    time_low (little-endian)
+ * 4-5    time_mid (little-endian)
+ * 6-7    time_hi_and_version (little-endian)
+ * 8      clock_seq_hi_and_reserved
+ * 9      clock_seq_low (in order)
+ * 10-15  node
+  */
 struct guid
 {
     char g[GUID_SIZE];
@@ -42,6 +57,8 @@ struct guid
 /**
  * Get an initialised GUID
  *
+ * The GUID is compatible with RFC4122 section 4.4.
+ *
  * @return new GUID
  */
 struct guid guid_new(void);

tests/common/test_guid.c

@@ -57,6 +57,42 @@ START_TEST(test_guid_to_str_ignore)
 }
 END_TEST
 
+START_TEST(test_guid_to_str_random)
+{
+    /* setup */
+    char dest[GUID_STR_SIZE];
+    struct guid guid;
+    unsigned int i;
+    for (i = 0 ; i < 100; ++i)
+    {
+        guid = guid_new();
+        /* test */
+        guid_to_str(&guid, dest);
+
+        /* Check all the '-' signs are in the right places */
+        ck_assert_int_eq(dest[8], '-');
+        ck_assert_int_eq(dest[13], '-');
+        ck_assert_int_eq(dest[18], '-');
+        ck_assert_int_eq(dest[23], '-');
+
+        /* Check the variant is RFC4122 */
+        char c = dest[18 + 1]; /* char after 3rd dash */
+        if (c != '8' && c != '9' && c != 'A' && c != 'B')
+        {
+            ck_abort_msg("Generated UUID is not RFC4122 compliant");
+        }
+
+        /* Check the version is 'randomly generated' */
+        c = dest[13 + 1]; /* char after 2nd dash */
+        if (c != '4')
+        {
+            ck_abort_msg("Generated UUID is not RFC4122 randomly-generated");
+        }
+
+    }
+}
+END_TEST
+
 /******************************************************************************/
 
 Suite *
@@ -72,6 +108,7 @@ make_suite_test_guid(void)
     tcase_add_test(tc_guid, test_guid_to_str_remotefx);
     tcase_add_test(tc_guid, test_guid_to_str_nscodec);
     tcase_add_test(tc_guid, test_guid_to_str_ignore);
+    tcase_add_test(tc_guid, test_guid_to_str_random);
 
     return s;
 }