ICU-770 Add some documentation.

X-SVN-Rev: 5850

icu4c/source/common/ucln.h

@@ -19,18 +19,42 @@
 
 #include "unicode/utypes.h"
 
-enum ECleanupLibraryType {
+/** These are the functions used to register a library's memory cleanup
+ * functions.  Each library should define a single library register function
+ * to call this API.  In the i18n library, it is ucln_i18n_registerCleanup().
+ *
+ * None of the cleanup functions should not use a mutex to clean up an API's
+ * allocated memory because a cleanup function is not meant to be thread safe,
+ * and plenty of data cannot be reference counted in order to make sure that
+ * no one else needs the allocated data.
+ *
+ * In order to make a cleanup function get called when u_cleanup is called,
+ * You should add your function to the library specific cleanup function.
+ * If the cleanup function is not in the common library, the code that
+ * allocates the memory should call the library specific cleanup function.
+ * For instance, in the i18n library, any memory allocated statically must
+ * call ucln_i18n_registerCleanup() from the ucln_in.h header.  These library
+ * cleanup functions are needed in order to prevent a circular dependency
+ * between the common library and any other library.
+ *
+ * The order of the cleanup is very important.  In general, an API that
+ * depends on a second API should be cleaned up before the second API.
+ * For instance, the default converter in ustring depends upon the converter
+ * API.  So the default converter should be closed before the converter API
+ * has its cache flushed.  This will prevent any memory leaks due to
+ * reference counting.
+ *
+ * Please see common/ucln_cmn.{h,c} and i18n/ucln_in.{h,c} for examples.
+ */
+
+typedef enum ECleanupLibraryType {
     UCLN_START = -1,
     UCLN_CUSTOM,    /* Custom is for anyone else. */
     UCLN_LAYOUT,
     UCLN_USTDIO,
     UCLN_I18N,
     UCLN_COMMON /* This must be the last one to cleanup. */
-};
-
-#ifndef XP_CPLUSPLUS
-typedef enum ECleanupLibraryType ECleanupLibraryType;
-#endif
+} ECleanupLibraryType;
 
 typedef UBool cleanupFunc(void);