9PFILE(2)                                               9PFILE(2)

          Tree, alloctree, freetree, File, createfile, closefile,
          removefile, walkfile, opendirfile, readdirfile,
          closedirfile, hasperm - in-memory file hierarchy

          #include <u.h>
          #include <libc.h>
          #include <fcall.h>
          #include <thread.h>
          #include <9p.h>

          typedef struct File
          } File;

          typedef struct Tree
               File *root;
          } Tree;

          Tree*    alloctree(char *uid, char *gid, ulong mode,
                           void (*destroy)(File*))
          void     freetree(Tree *tree)
          File*    createfile(File *dir, char *name, char *uid,
                           ulong mode, void *aux)
          int      removefile(File *file)
          void     closefile(File *file)
          File*    walkfile(File *dir, char *path)
          Readdir* opendirfile(File *dir)
          long     readdirfile(Readdir *rdir, uchar *buf, long n, long o)
          void     closedirfile(Readdir *rdir)
          int      hasperm(File *file, char *uid, int p)

          Files and Trees provide an in-memory file hierarchy intended
          for use in 9P file servers.

          Alloctree creates a new tree of files, and freetree destroys
          it.  The root of the tree (also the root element in the
          structure) will have mode mode and be owned by user uid and
          group gid. Destroy is used when freeing File structures and
          is described later.

          Files (including directories) other than the root are

     9PFILE(2)                                               9PFILE(2)

          created using createfile, which attempts to create a file
          named name in the directory dir. If created, the file will
          have owner uid and have a group inherited from the direc-
          tory.  Mode and the permissions of dir are used to calculate
          the permission bits for the file as described in open(5). It
          is permissible for name to be a slash-separated path rather
          than a single element.

          Removefile removes a file from the file tree.  The file will
          not be freed until the last reference to it has been
          removed.  Directories may only be removed when empty.
          Removefile returns zero on success, -1 on error.  It is cor-
          rect to consider removefile to be closefile with the side
          effect of removing the file when possible.

          Walkfile evaluates path relative to the directory dir,
          returning the resulting file, or zero if the named file or
          any intermediate element does not exist.

          The File structure's aux pointer may be used by the client
          for per-File storage.  Files are reference-counted: if not
          zero, destroy (specified in the call to alloctree) will be
          called for each file when its last reference is removed or
          when the tree is freed.  Destroy should take care of any
          necessary cleanup related to aux.  When creating new file
          references by copying pointers, call incref (see lock(2)) to
          update the reference count.  To note the removal of a refer-
          ence to a file, call closefile. Createfile and walkfile
          return new references.  Removefile, closefile, and walkfile
          (but not createfile) consume the passed reference.

          Directories may be read, yielding a directory entry struc-
          ture (see stat(5)) for each file in the directory.  In order
          to allow concurrent reading of directories, clients must
          obtain a Readdir structure by calling opendirfile on a
          directory.  Subsequent calls to readdirfile will each yield
          an integral number of machine-independent stat buffers,
          until end of directory.  When finished, call closedirfile to
          free the Readdir.

          Hasperm does simplistic permission checking; it assumes only
          one-user groups named by uid and returns non-zero if uid has
          permission p (a bitwise-or of AREAD, AWRITE and AEXEC)
          according to file->mode.  9P servers written using File
          trees will do standard permission checks automatically;
          hasperm may be called explicitly to do additional checks.  A
          9P server may link against a different hasperm implementa-
          tion to provide more complex groups.

          The following code correctly handles references when elemen-
          twise walking a path and creating a file.

     9PFILE(2)                                               9PFILE(2)

               f = tree->root;
               for(i=0; i<n && f!=nil; i++)
                        f = walkfile(f, elem[i]);
               if(f == nil)
                        return nil;
               nf = createfile(f, "foo", "nls", 0666, nil);
               return nf;



          The reference counting is cumbersome.