BIND(2) BIND(2)
NAME
bind, mount, unmount - change name space
SYNOPSIS
#include <u.h>
#include <libc.h>
int bind(char *name, char *old, int flag)
int mount(int fd, char *old, int flag, char *aname)
int unmount(char *name, char *old)
DESCRIPTION
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 modi-
fication 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 connection or pipe to a file server. 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
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BIND(2) BIND(2)
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 accessible through this
mount point. The currency of cached data for a file is ver-
ified 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 Sec-
tion 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 success-
ful mount call.
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.
SOURCE
/sys/src/libc/9syscall
SEE ALSO
bind(1), intro(2), fcall(2), auth(2) (particularly amount),
intro(5), mnt(3), srv(3)
DIAGNOSTICS
The return value is a positive integer (a unique sequence
number) for success, -1 for failure. These routines set
errstr.
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BIND(2) BIND(2)
BUGS
Mount will not return until it has successfully attached to
the file server, so the process doing a mount cannot be the
one serving.
The file system cache enabled by MCACHE mishandles (in fact,
doesn't know about) append-only files. The fix requires a
change to 9P. /lib/namespace mounts mailboxes, the most
important append-only files, uncached.
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