Keep in mind that the concept of a "node" designates the data parts (data and attributes) of a file (a file, directory, or link). Contrast this with an "entry," which designates the entity's location within the file system: For example, you can write to a "node" (but not an entry), and you can rename an "entry" (but not a node).

This isn't just a conceptual crutch, it's the law: Nodes really don't know where they're located. For example, you can't ask a node for its name, or for the identity of its parent. This has some serious implications, the most important of which is…

• If you need to store a reference to a file (or directory, or symbolic link), don't store the node—in other words, don't cache the BNode object. Instead, store the information that you used to create the BNode (typically, a pathname or entry_ref structure).

Now that we've got that straight, we'll relax the rules a bit:

• BDirectory objects are node/entry hybrids. A BDirectory does know its own name (and parent, and so on).

This doesn't really change the "store the info" rule. Even if you're dealing exclusively with BDirectory objects, you should keep the generative information around. The primary reason for this is…

The "Node Pool" is Limited (File Descriptors)

Every BNode object consumes a "file descriptor." Your application can only maintain 256 file descriptors at a time. Because of this limit, you shouldn't keep BNodes around that you don't need. Keep in mind that BEntry objects also consumes file descriptors (one per object).

Note

The file descriptor limit will probably be lifted, or at least settable, in a subsequent release. But even then you should be frugal.

BNode has three derived classes: BFile, BDirectory, and BSymLink. The derived classes define functions that let you access the node's data portion in the appropriate style; for example…

• BFile implements Read() and Write() functions that let you retrieve arbitrary amounts of data from arbitrary positions in the file.

• BDirectory implements functions, such as GetNextEntry() and FindEntry(), that read entries from the directory.

• BSymLink's ReadLink() returns the pathname that it contains.

If you want to (sensibly) look at a node's data portion, you must create an instance of the appropriate derived class. In other words, if you want to browse a directory, you have to create a BDirectory instance; if you want to write to a file, you create a BFile.

Be aware that it's not (always) an error to create an instance of the "wrong" derived class; setting a BFile to a symbolic link, for example, will traverse the link such that the BFile opens the file that the symbolic link is linked to. See the individual derived class specifications for more information.

In practice, you almost always want to create an instance of one of the BNode-derived classes; but if, for whatever reason, you find yourself holding a BNode instance, here's what you'll be able to do with it:

• Read and write attributes. The attribute-accessing functions (ReadAttr(), WriteAttr(), and so on) are general—they work without regard for the node's flavor. Thus, you don't need an instance of a specific derived class to read and write attributes.

• Get stat information. The BStatable functions can be invoked on any flavor of node.

• Lock the node. This prevents other "agents" (other objects, other apps, the user) from accessing reading or writing the node's data and attributes. See "Node Locking".

Note

This section describes situations and presents solutions to problems that are a bit esoteric. If you never create direct instances of BNode (and you never have to), then you should skip this and go to "Node Locking".

There may be times when you find yourself holding on to a BNode (instance) that you want to convert into a BFile, BDirectory, or BSymLink. However, you can't go directly from a BNode instance to an instance of BFile, BDirectory, or BSymLink—you can't tell your BNode to "cast itself" as one of its children.

There are solutions, however…

void Node2Directory(BNode *node, BDirectory *dir)
{
   node_ref nref;

   if (!node || !dir) {
      dir.Unset();
      return;
   }

   node.GetNodeRef(&nref);

   /* Set the BDirectory. If nref isn't a directory node,
   * the SetTo() will fail.
   */
   dir.SetTo(&nref);
}

Another feature provided by the BNode class is "node locking": Through BNode's Lock() function you can restrict access to the node. The lock is removed when Unlock() is called, or when the BNode object is deleted.

• When you lock a node, you prevent other objects (or agents) from reading or writing the node's data and attributes. No other agent can even open the node—other BNode constructions and POSIX open() calls (on that node) will fail while you hold the lock.

• You can only acquire a node lock if there are no file descriptors open on the node (with one exception). This means that no other BNode may be open on the node (locked or not), nor may the node be held open because of a POSIX open() (or opendir()) call.

The one exception to the no-file descriptors rule has to do with BEntrys: Let's say you lock a directory, and then you initialize a BEntry to point to an entry within that directory. Even though the BEntry creates a file descriptor to the directory (as explained in the BEntry class), the initialization will succeed.

Implications

For files (and, less importantly, symlinks), the implications of locking are pretty clear: No one else can read or write the file. For directories, it's worth a closer look:

• Locking a directory means that the contents of the directory can't change: You can't create new nodes in the directory, or rename or remove existing ones. (You can, however, create abstract entries within the directory; see BEntry for more on abstract entries.)

Locking a node does not lock the node's entry: You can't "lock out" entry operations, such as rename, move, and remove. Even if you have a node locked, the entry that acts as the "container" for that node could disappear. If you want to prevent such operations on a node's entry, lock the entry's parent directory.

In general, you should try to avoid locking your nodes. If you must lock, try to make it brief. The primary reason (and, pretty much, the only reason) to lock is if separate elements in the data and/or attributes must be kept in a consistent state. In such a case, you should hold the lock just long enough to ensure consistency.

Warning

You shouldn't use locks to "privatize" data. Locking isn't meant to be used as a heightened permissions bit.