Bind and mount modify the file name space of the current process
and other processes in its name space group (see fork(2)). For
both calls, old is the name of an existing file or directory in
the current name space where the modification is to be made. The
name old is evaluated as described in intro(2), except that no
translation of the final path element is done.
For bind, name is the name of another (or possibly the same) existing
file or directory in the current name space. After a successful
bind call, the file name old is an alias for the object originally
named by name; if the modification doesn't hide it, name will
also still refer to its original file. The evaluation of name
happens at the time of the bind, not when the binding is later
The fd argument to mount is a file descriptor of an open network
connection or pipe to a file server, while afd is a authentication
file descriptor as created by fauth(2) and subsequently authenticated.
If authentication is not required, afd should be –1. The old file
must be a directory. After a successful mount the file
tree served (see below) by fd will be visible with its root directory
having name old.
The flag controls details of the modification made to the name
space. In the following, new refers to the file as defined by
name or the root directory served by fd. Either both old and new
files must be directories, or both must not be directories. Flag
can be one of:|
MREPL Replace the old file by the new one. Henceforth, an evaluation
of old will be translated to the new file. If they are directories
(for mount, this condition is true by definition), old becomes
a union directory consisting of one directory (the new file).
MBEFORE Both the old and new files must be directories. Add the
constituent files of the new directory to the union directory
at old so its contents appear first in the union. After an MBEFORE
bind or mount, the new directory will be searched first when evaluating
file names in the union directory.
MAFTER Like MBEFORE but the new directory goes at the end of the
The flags are defined in <libc.h>. In addition, there is an MCREATE
flag that can be OR'd with any of the above. When a create system
call (see open(2)) attempts to create in a union directory, and
the file does not exist, the elements of the union are searched
in order until one is found with MCREATE set. The file
is created in that directory; if that attempt fails, the create
Finally, the MCACHE flag, valid for mount only, turns on caching
for files made available by the mount. By default, file contents
are always retrieved from the server. With caching enabled, the
kernel may instead use a local cache to satisfy read(5) requests
for files accessible through this mount point. The currency of
cached data for a file is verified at each open(5) of the file
from this client machine.
With mount, the file descriptor fd must be open for reading and
writing and prepared to respond to 9P messages (see Section 5).
After the mount, the file tree starting at old is served by a
kernel mnt(3) device. That device will turn operations in the
tree into messages on fd. Aname selects among different file trees
the server; the null string chooses the default tree.
The file descriptor fd is automatically closed by a successful
The effects of bind and mount can be undone by unmount. If name
is zero, everything bound to or mounted upon old is unbound or
unmounted. If name is not zero, it is evaluated as described above
for bind, and the effect of binding or mounting that particular
result on old is undone.