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BIND(2) System Calls Manual BIND(2)
bind, mount, unmount - change name space
int bind(char *name, char *old, int flag)
int mount(int fd, int afd, char *old, int flag, char *aname)
int unmount(char *name, char *old)
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 new happens at the time of the
bind, not when the binding is later used.
The fd argument to mount is a file descriptor of an open network con‐
nection 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 directo‐
ries, 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 directo‐
ries (for mount, this condition is true by definition), old
becomes a union directory consisting of one directory (the new
Both the old and new files must be directories. Add the con‐
stituent 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 union.
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 fails.
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 acces‐
sible 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 on the server; the null
string chooses the default tree.
The file descriptor fd is automatically closed by a successful mount
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
bind(1), intro(2), fcall(2), auth(2) (particularly amount), intro(5),
The return value is a positive integer (a unique sequence number) for
success, -1 for failure. These routines set errstr.
Mount will not return until it has successfully attached to the file
server, so the process doing a mount cannot be the one serving.