diff --git a/library/changes b/library/changes
index a418499..33f18b1 100644
--- a/library/changes
+++ b/library/changes
@@ -1,3 +1,12 @@
+- The internal 'struct fd' file descriptor table entry data structure
+  now has a user data field entry.
+
+- Rearranged the contents of the 'struct fd' file descriptor table entry
+  data structure in preparation for making it public. Also added a version
+  field so that user code can handle changes to it gracefully. The default
+  file is no longer a BCPL pointer to a file handle by default, but
+  both a BPTR and a socket identifier, wrapped into a union.
+
 - Added experimental tilde expansion in Unix path names. This still needs
   some more work.
 
diff --git a/library/stdio_duplicate_fd.c b/library/stdio_duplicate_fd.c
index 28c42c5..b37f406 100644
--- a/library/stdio_duplicate_fd.c
+++ b/library/stdio_duplicate_fd.c
@@ -1,5 +1,5 @@
 /*
- * $Id: stdio_duplicate_fd.c,v 1.5 2006-01-08 12:04:24 obarthel Exp $
+ * $Id: stdio_duplicate_fd.c,v 1.6 2006-10-10 13:39:26 obarthel Exp $
  *
  * :ts=4
  *
@@ -55,8 +55,8 @@ __duplicate_fd(struct fd * duplicate_fd,struct fd * original_fd)
 		duplicate_fd->fd_Original = original_fd;
 
 	/* Add the duplicate at the beginning of the list. */
-	duplicate_fd->fd_NextLink				= duplicate_fd->fd_Original->fd_NextLink;
-	duplicate_fd->fd_Original->fd_NextLink	= duplicate_fd;
+	duplicate_fd->fd_NextAlias				= duplicate_fd->fd_Original->fd_NextAlias;
+	duplicate_fd->fd_Original->fd_NextAlias	= duplicate_fd;
 
 	__fd_unlock(original_fd);
 }
diff --git a/library/stdio_headers.h b/library/stdio_headers.h
index 947a08e..b6190b0 100644
--- a/library/stdio_headers.h
+++ b/library/stdio_headers.h
@@ -1,5 +1,5 @@
 /*
- * $Id: stdio_headers.h,v 1.27 2006-09-25 14:05:31 obarthel Exp $
+ * $Id: stdio_headers.h,v 1.28 2006-10-10 13:39:26 obarthel Exp $
  *
  * :ts=4
  *
@@ -365,6 +365,7 @@ struct iob
 											   standard input/output/error streams. */
 #define FDF_TERMIOS				(1UL<<13)	/* File is under termios control.
 											   FDF_IS_INTERACTIVE should also be set. */
+
 /****************************************************************************/
 
 /* The file action function for unbuffered files. */
@@ -379,25 +380,39 @@ typedef void (*fd_cleanup_t)(struct fd * fd);
 
 struct fd
 {
-	file_action_fd_t	fd_Action;			/* Function to invoke to perform actions */
-	ULONG				fd_Flags;			/* File properties */
-	struct fd *			fd_Original;		/* NULL if this is not a dup()ed file
-											   descriptor; points to original
-											   descriptor if non-NULL */
-	struct fd *			fd_NextLink;		/* Points to next duplicate of this
-											   file descriptor; NULL for none */
-	BPTR				fd_DefaultFile;		/* A dos.library file handle */
-	ULONG				fd_Position;		/* Cached file position (seek offset). */
-	fd_cleanup_t		fd_Cleanup;			/* Cleanup function, if any. */
+	int							fd_Version;			/* Version number of this definition
+													   of the 'fd' data structure. */
+	file_action_fd_t			fd_Action;			/* Function to invoke to perform actions */
+	void *						fd_UserData;		/* To be used by custom file action
+													   functions */
+	ULONG						fd_Flags;			/* File properties */
 
-	struct SignalSemaphore * fd_Lock;		/* For thread locking */
+	union
+	{
+		BPTR					fdu_File;			/* A dos.library file handle */
+		LONG					fdu_Socket;			/* A socket identifier */
+	} fdu_Default;
 
-	void *				fd_Aux;				/* Auxilliary data for "special" files,
-											   e.g. termios support. */
+	struct SignalSemaphore *	fd_Lock;			/* For thread locking */
+	ULONG						fd_Position;		/* Cached file position (seek offset). */
+	fd_cleanup_t				fd_Cleanup;			/* Cleanup function, if any. */
+
+	struct fd *					fd_Original;		/* NULL if this is not a dup()ed file
+													   descriptor; points to original
+													   descriptor if non-NULL */
+	struct fd *					fd_NextAlias;		/* Points to next duplicate of this
+													   file descriptor; NULL for none */
+	void *						fd_Aux;				/* Auxilliary data for "special" files,
+													   e.g. termios support. */
 };
 
 /****************************************************************************/
 
+#define fd_DefaultFile	fdu_Default.fdu_File
+#define fd_Socket		fdu_Default.fdu_Socket
+
+/****************************************************************************/
+
 /* Files and directories to be unlinked when the program exits. */
 struct UnlinkNode
 {
@@ -481,7 +496,7 @@ extern BOOL NOCOMMON __no_standard_io;
 /****************************************************************************/
 
 #define __fd_is_aliased(fd) \
-	((fd)->fd_Original != NULL || (fd)->fd_NextLink != NULL)
+	((fd)->fd_Original != NULL || (fd)->fd_NextAlias != NULL)
 
 /****************************************************************************/
 
diff --git a/library/stdio_initializefd.c b/library/stdio_initializefd.c
index 0ab9f84..50585c5 100644
--- a/library/stdio_initializefd.c
+++ b/library/stdio_initializefd.c
@@ -1,5 +1,5 @@
 /*
- * $Id: stdio_initializefd.c,v 1.6 2006-01-08 12:04:24 obarthel Exp $
+ * $Id: stdio_initializefd.c,v 1.7 2006-10-10 13:39:26 obarthel Exp $
  *
  * :ts=4
  *
@@ -49,6 +49,7 @@ __initialize_fd(
 
 	memset(fd,0,sizeof(*fd));
 
+	fd->fd_Version		= 1;
 	fd->fd_DefaultFile	= default_file;
 	fd->fd_Flags		= flags;
 	fd->fd_Action		= action_function;
diff --git a/library/stdio_record_locking.c b/library/stdio_record_locking.c
index 0f7d623..761add9 100644
--- a/library/stdio_record_locking.c
+++ b/library/stdio_record_locking.c
@@ -1,5 +1,5 @@
 /*
- * $Id: stdio_record_locking.c,v 1.17 2006-09-25 14:51:15 obarthel Exp $
+ * $Id: stdio_record_locking.c,v 1.18 2006-10-10 13:39:26 obarthel Exp $
  *
  * :ts=4
  *
@@ -342,7 +342,7 @@ remove_locked_region_node(struct FileLockSemaphore * fls,struct fd * fd,LONG sta
 				{
 					CLEAR_FLAG(fd->fd_Flags,FDF_IS_LOCKED);
 				}
-				while((fd = fd->fd_NextLink) != NULL);
+				while((fd = fd->fd_NextAlias) != NULL);
 			}
 		}
 	}
@@ -725,7 +725,7 @@ cleanup_locked_records(struct fd * fd)
 				{
 					CLEAR_FLAG(fd->fd_Flags,FDF_IS_LOCKED);
 				}
-				while((fd = fd->fd_NextLink) != NULL);
+				while((fd = fd->fd_NextAlias) != NULL);
 			}
 
 			release_file_lock_semaphore(fls);
@@ -1086,7 +1086,7 @@ __handle_record_locking(int cmd,struct flock * l,struct fd * fd,int * error_ptr)
 		{
 			SET_FLAG(fd->fd_Flags,FDF_IS_LOCKED);
 		}
-		while((fd = fd->fd_NextLink) != NULL);
+		while((fd = fd->fd_NextAlias) != NULL);
 	}
 	else if (cmd == F_SETLK)
 	{
@@ -1164,7 +1164,7 @@ __handle_record_locking(int cmd,struct flock * l,struct fd * fd,int * error_ptr)
 		{
 			SET_FLAG(fd->fd_Flags,FDF_IS_LOCKED);
 		}
-		while((fd = fd->fd_NextLink) != NULL);
+		while((fd = fd->fd_NextAlias) != NULL);
 	}
 	else if (cmd == F_GETLK)
 	{
diff --git a/library/stdio_remove_fd_alias.c b/library/stdio_remove_fd_alias.c
index 0ac89b7..0de2c14 100644
--- a/library/stdio_remove_fd_alias.c
+++ b/library/stdio_remove_fd_alias.c
@@ -1,5 +1,5 @@
 /*
- * $Id: stdio_remove_fd_alias.c,v 1.4 2006-01-08 12:04:25 obarthel Exp $
+ * $Id: stdio_remove_fd_alias.c,v 1.5 2006-10-10 13:39:26 obarthel Exp $
  *
  * :ts=4
  *
@@ -54,36 +54,36 @@ __remove_fd_alias(struct fd * fd)
 		/* Remove this alias from the list. */
 		for(list_fd = fd->fd_Original ;
 		    list_fd != NULL ;
-		    list_fd = list_fd->fd_NextLink)
+		    list_fd = list_fd->fd_NextAlias)
 		{
-			if(list_fd->fd_NextLink == fd)
+			if(list_fd->fd_NextAlias == fd)
 			{
-				list_fd->fd_NextLink = fd->fd_NextLink;
+				list_fd->fd_NextAlias = fd->fd_NextAlias;
 				break;
 			}
 		}
 	}
-	else if (fd->fd_NextLink != NULL) /* this one has aliases attached; it is the 'original' resource */
+	else if (fd->fd_NextAlias != NULL) /* this one has aliases attached; it is the 'original' resource */
 	{
 		struct fd * first_alias;
 		struct fd * next_alias;
 		struct fd * list_fd;
 
 		/* The first link now becomes the original resource */
-		first_alias = fd->fd_NextLink;
-		next_alias = first_alias->fd_NextLink;
+		first_alias = fd->fd_NextAlias;
+		next_alias = first_alias->fd_NextAlias;
 
 		/* Structure copy... */
 		(*first_alias) = (*fd);
 
 		/* Fix up the linkage. */
-		first_alias->fd_NextLink = next_alias;
+		first_alias->fd_NextAlias = next_alias;
 		first_alias->fd_Original = NULL;
 
 		/* The resources are migrated to the first link. */
 		for(list_fd = next_alias ;
 		    list_fd != NULL ;
-		    list_fd = list_fd->fd_NextLink)
+		    list_fd = list_fd->fd_NextAlias)
 		{
 			list_fd->fd_Original = first_alias;
 		}