(FTS) [fts_dir_signatures]: Add comment.

diff --git a/lib/fts_.h b/lib/fts_.h
index ed0cb1c256083e5570763681306d0f5f5c5275c5..b6ba1b481b604b09c856c6bffa70fcbacd70196d 100644 (file)
--- a/lib/fts_.h
+++ b/lib/fts_.h
@@ -69,8 +69,18 @@ typedef struct {
 # define FTS_NAMEONLY  0x0100          /* (private) child names only */
 # define FTS_STOP      0x0200          /* (private) unrecoverable error */
        int fts_options;                /* fts_open options, global flags */
-       /* Data structure in which to store the fingerprint
-          of each directory we've encountered.  */
+
+       /* This data structure records the directories between a starting
+          point and the current directory.  I.e., a directory is recorded
+          here IFF we have visited it once, but we have not yet completed
+          processing of all its entries.  Every time we visit a new directory,
+          we add that directory to this set.  When we finish with a directory
+          (usually by visiting it a second time), we remove it from this
+          set.  Each entry in this data structure is a device/inode pair.
+          This data structure is used to detect directory cycles efficiently
+          and promptly even when the depth of a hierarchy is in the tens
+          of thousands.  Lazy checking, as done by GNU rm via cycle-check.c,
+          wouldn't be appropriate for du.  */
        void *fts_dir_signatures;
 } FTS;