.TH BIND 2
.SH NAME
bind, mount, unmount \- change name space
.SH SYNOPSIS
.B #include <u.h>
.br
.B #include <libc.h>
.PP
.B
int bind(char *name, char *old, int flag)
.PP
.B
int mount(int fd, int afd, char *old, int flag, char *aname)
.PP
.B
int unmount(char *name, char *old)
.SH DESCRIPTION
.I Bind
and
.I mount
modify the file name space of the current process and other
processes in its name space group
(see
.IR fork (2)).
For both calls,
.I old
is the name of an existing file or directory in the
current name space where the modification is to be made.
The name
.I old
is
.I evaluated
as described in
.IR intro (2),
except that no translation of the final path element is done.
.PP
For
.IR bind ,
.I name
is the name of another (or possibly the same)
existing file or directory in
the current name space.
After a successful
.I bind
call, the file name
.I old
is an alias for the object originally named by
.IR name ;
if the modification doesn't hide it,
.I name
will also still refer to its original file.
The evaluation of
.I new
happens at the time of the
.IR bind ,
not when the binding is later used.
.PP
The
.I fd
argument to
.I mount
is a file descriptor of an open network connection
or pipe to a file server, while
.I afd
is a authentication file descriptor as created by
.IR fauth (2)
and subsequently authenticated.
If authentication is not required,
.I afd
should be -1.
The
.I old
file must be a directory.
After a successful
.I mount
the file tree
.I served
(see below) by
.I fd
will be visible with its root directory having name
.IR old .
.PP
The
.I flag
controls details of the modification made to the name space.
In the following,
.I new
refers to the file
as defined by
.I name
or the root directory served by
.IR fd .
Either both
.I old
and new files must be directories,
or both must not be directories.
.I Flag
can be one of:
.TF MBEFORE
.TP 
.B MREPL
Replace the
.I old
file by the new one.
Henceforth, an evaluation of
.I old
will be translated to the new file.
If they are directories (for
.IR mount ,
this condition is true by definition),
.I old
becomes a
.I "union directory"
consisting of one directory (the new file).
.TP
.B MBEFORE
Both the
.I old
and new files must be directories.
Add the constituent files of the new directory
to the union directory at
.I old
so its contents appear first in the union.
After an
.B MBEFORE
.I bind
or
.IR mount ,
the new directory will be searched first when evaluating file names
in the union directory.
.TP
.B MAFTER
Like
.B MBEFORE
but the new directory goes at the end of the union.
.PD
.PP
The flags are defined in
.BR <libc.h> .
In addition, there is an
.B MCREATE
flag that can be OR'd with any of the above.
When a
.I create
system call (see
.IR 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
.B MCREATE
set.
The file is created in that directory; if that attempt fails,
the
.I create
fails.
.PP
Finally, the
.B MCACHE
flag, valid for
.I 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
.IR read (5)
requests for files accessible through this mount point.
The currency of cached data for a file is verified at each
.IR open (5)
of the file from this client machine.
.PP
With
.IR mount ,
the file descriptor
.I fd
must be open for reading and writing
and prepared to respond to 9P messages
(see Section 5).
After the
.IR mount ,
the file tree starting at
.I old
is served by a kernel
.IR mnt (3)
device.
That device will turn operations in the tree into messages on
.IR fd .
.I Aname
selects among different
file trees on the server; the null string chooses the default tree.
.PP
The file descriptor
.I fd
is automatically closed by a successful
.I mount
call.
.PP
The effects of
.I bind
and
.I mount
can be undone by
.IR unmount .
If
.I name
is zero, everything bound to or mounted upon
.I old
is unbound or unmounted.
If
.I name
is not zero, it is evaluated as described above for
.IR bind ,
and the effect of binding or mounting that particular result on
.I old
is undone.
.SH SOURCE
.B /sys/src/libc/9syscall
.SH SEE ALSO
.IR bind (1),
.IR intro (2),
.IR fcall (2),
.IR auth (2)
(particularly
.BR amount ),
.IR intro (5),
.IR mnt (3),
.IR srv (3)
.SH DIAGNOSTICS
The return value is a positive integer (a unique sequence number) for
success, -1 for failure.
These routines set
.IR errstr .
.SH BUGS
.I Mount
will not return until it has successfully attached
to the file server, so the process doing a
.I mount
cannot be the one serving.