The Storage Kit: BStatable

Derived from: (none)

Declared in: be/nustorage/Statable.h

Library: libbe.so


Overview

BStatable is a pure abstract class that provides functionality for its two derived class, BEntry and BNode. The BStatable functions let you get and set "statistical" information about a node in the file system. You can...


Nodes and Entries

Technically, BStatable information pertains to nodes, not entries. The fact that BEntry implements the BStatable functions is a (slightly confusing) convenience: When you invoke a BStatable function on a BEntry object, what you're really doing is asking for information about the node that corresponds to the object.

You can't invoke BStatable functions on "abstract" BEntries.

As explained in BEntry, it's possible to create "abstract" BEntry objects; in other words, objects that don't correspond to actual files (nodes) on the disk. You can't get (or set) BStatable information for abstract entries. The BStatable functions return B_BAD_VALUE if the invoked-upon entry is abstract.


Relationship to stat()

The BStatable functions are covers for the POSIX stat() call. stat() retrieves a file-specific stat structure, which records the statistics listed above (and then some). Although BStatable was designed to hide stat details, you can get the stat structure through the GetStat() function. The stat structure is described in The stat Structure at the end of this specification.

stat() is notorious for being expensive. Furthermore, the stat structure is stale as soon as it gets back from the stat() call. So...

If you're concerned with efficiency, be aware that every BStatable function (the "setters" as well as the "getters") performs a stat(). For example, calling GetOwner() and then GetGroup() results in two stat() calls. If you want to look at lot of fields (within the same stat structure) all at once, you might consider using BStatable's GetStat() function.

As for integrity, BStatable info-getting functions are obviously in the same boat as the stat() call itself: The retrieved data isn't guaranteed to be in synch with the actual state of the stat()'d item.

The BDirectory class also defines a stat-retrieving function that, in some cases, can be more efficient than the GetStat() function defined here:

The BDirectory::GetStatFor() function retrieves the stat structure for the node of a named entry within a directory. If you're interested in getting stat information for a series of nodes within the same directory, you should use this function. You have to call it iteratively (once for each named entry), but the accumulation of the iterated calls will be faster than the GetStat() calls made on the analogous BEntry objects.


Accessing Unreadable and Unwritable Entries

BStatable isn't thwarted by file permissions: If you can construct a valid BEntry or BNode to an item, then you can invoke any of the info-getting BStatable functions on that object:

The BStatable functions aren't denied even if the node that you're looking at is read-protected. However, you can only invoke the info-setting functions if the node allows writing.

Similarly, you can get stat info for a locked node, but you won't be able to write the info (through functions such as SetOwner()) unless your object holds the lock. See BNode for more on locking.


Other Details

You rarely set stat information.

In practice, you rarely use BStatable's info-setting functions. Setting information such as when a file was created, who owns it, or how big it is, is the responsibility of the system and the privilege of the user. For example, when you Write() to a BFile object, the system automatically updates the size and modification date for the file.


Member Functions


GetCreationTime(), SetCreationTime(), GetModificationTime(), SetModificationTime(), GetAccessTime(), SetAccessTime()


      status_t GetCreationTime(time_t *ctime) const
      status_t SetCreationTime(time_t ctime)

      status_t GetModificationTime(time_t *mtime) const
      status_t SetModificationTime(time_t mtime)

      status_t GetAccessTime(time_t *atime) const
      status_t SetAccessTime(time_t atime)

Access time is currently unused.

These function let you get and set the time at which the item was created, last modified, and last accessed (opened). The measure of time is given as seconds since (the beginning of ) January 1, 1970.

Note that the time quanta that stat uses is seconds; the rest of the BeOS measures time in microseconds (bigtime_t).

RETURN CODES


GetNodeRef()


      status_t GetNodeRef(node_ref *nref) const

Copies the item's node_ref structure into the nref argument, which must be allocated.

Typically, you use an node's node_ref as a key to the Node Monitor by passing the node_ref structure to the watch_node() function. The Node Monitor watches the node for specific changes; see The Node Monitor for details.

As a convenience, you can use a node_ref structure to initialize a BDirectory object (through the constructor or BDirectory::SetTo() function).

RETURN CODES


GetOwner(), SetOwner(), GetGroup(), SetGroup(), GetPermissions(), SetPermissions()


      status_t GetOwner(uid_t *owner) const
      status_t SetOwner(uid_t owner)

      status_t GetGroup(gid_t *group) const
      status_t SetGroup(gid_t group) 

      status_t GetPermissions(mode_t *perms) const
      status_t SetPermissions(mode_t perms) 

In the Preview Release owner, group, and permissions are mostly ignored by the Be file system. In particular, all users are "root" and there's only one group.

These functions set and get the owner, group, and read/write/execute permissions for the node:

The uid_t, gid_t, and mode_t types used here are standard POSIX types. All three are 32-bit unsigned integers and are defined in posix/sys/types.h.

The owner and group encodings must match values found in the system's user and group files (which are as currently unimplemented).

The permissions value is a combination of the following bitfield constants (defined in posix/sys/stat.h):

For example:

   /* Is a file readable by everybody? */
   mode_t perms;
   if (node.GetPermissions(&perms) < B_NO_ERROR)
      /* handle the error... */
   
   if (perms & S_ISROTH)
      // Yes it is
   else
      // No it isn't

RETURN CODES


GetSize()


      status_t GetSize(off_t *size) const

Sets the size of the node's data portion (in bytes). Only the "used" portions of the node's file blocks are counted; the amount of storage the node actually requires (i.e. the number of blocks the node consumes) may be larger than the size given here.

The size measurement doesn't include the node's attributes.

RETURN CODES


GetStat()


      virtual status_t GetStat(struct stat *st) const

GetStat() returns the stat structure for the node. The structure is copied into the st argument, which must be allocated. The stat structure is described in The stat Structure, below. The BStatable object does not cache the stat structure; every time you call GetStat(), fresh stat information is retrieved.

RETURN CODES


The stat Structure

Declared in: posix/sys/stat.h

The stat structure looks like this:

   struct stat {
       dev_t         st_dev;
       ino_t         st_ino;
       mode_t         st_mode;
       nlink_t         st_nlink; 
       uid_t         st_uid;
       gid_t         st_gid;
       off_t         st_size;
       dev_t         st_rdev;
       size_t         st_blksize;
       time_t         st_atime; 
       time_t         st_mtime;
       time_t         st_ctime;
   } stat;

And the fields are...

By combining st_dev and st_ino you can roll your own node_ref:

   node_ref nref;
   stat st;
   
   if (file.GetStat(&st) == B_NO_ERROR) {
      nref.dev = st.st_dev;
      nref.node = st.st_ino;
   }

Meanwhile...

   if (S_ISREG(st.st_mode))
      /* it's a "regular" file */
      else if (S_ISDIR(st.st_mode))
         /* it's a directory */
         else if (S_ISLINK(st.st_mode))
            /* it's a symbolic link */






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Last modified July 17, 1997.