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);