Clarifications.

diff --git a/doc/filesys.texi b/doc/filesys.texi
index 090397c1e4e1fe10330befc771ab837e57ed903a..33cf740a631b49397a5be8a08c5d81801aac5a3a 100644 (file)
--- a/doc/filesys.texi
+++ b/doc/filesys.texi
@@ -111,6 +111,19 @@ root directory file to expand beyond its current limit of ten files.
 Make sure that concurrent accesses to the inode remain properly
 synchronized.
 
 Make sure that concurrent accesses to the inode remain properly
 synchronized.
 

+The user is allowed to seek beyond the current end-of-file (EOF).  The
+seek itself does not extend the file.  Writing at a position past EOF
+extends the file to the position being written, and any gap between the
+previous EOF and the start of the write must be filled with zeros.  A
+read past EOF returns zero bytes.
+
+Writing far beyond EOF can cause many blocks to be entirely zero.  Some
+file systems allocate and write real data blocks for these implicitly
+zeroed blocks.  Other file systems do not allocate these blocks at all
+until they are explicitly written.  The latter file systems are said to
+support ``sparse files.''  You may adopt either allocation strategy in
+your file system.
+

 @node Problem 4-3 Subdirectories
 @section Problem 4-3: Subdirectories
 
 @node Problem 4-3 Subdirectories
 @section Problem 4-3: Subdirectories