.TH OPEN 5 
.SH NAME
open, create \- prepare a fid for I/O on an existing or new file
.SH SYNOPSIS
.ta \w'\fLTcreate 'u
.B
Topen	
.IR tag [2]
.IR fid [2]
.IR mode [1]
.br
.B
Ropen	
.IR tag [2]
.IR fid [2]
.IR qid [8]
.PP
.B
Tcreate	
.IR tag [2]
.IR fid [2]
.IR name [28]
.IR perm [4]
.IR mode [1]
.br
.B
Rcreate	
.IR tag [2]
.IR fid [2]
.IR qid [8]
.SH DESCRIPTION
The
.B open
request asks the file server to check permissions and
prepare a fid for I/O with subsequent
.B read
and
.B write
messages.
The
.I mode
field determines the type of I/O:
0, 1, 2, and 3
mean
.I
read access, write access, read and write access,
and
.I
execute access,
to be checked against the permissions for the file.
In addition, if
.I mode
has the
.B OTRUNC
.RB ( 0x10 )
bit set,
the file is to be truncated, which requires write permission
(if
the file is append-only, and permission is granted, the
.B open
succeeds but the file will not be truncated);
if the
.I mode
has the
.B ORCLOSE
.RB ( 0x40 )
bit set, the file is to be removed when the fid
is clunked, which requires permission to remove the file from its directory.
If other bits are set in
.I mode
they will be ignored.
It is illegal to write a directory, truncate it, or attempt to remove it on close.
If the file is marked for exclusive use (see
.IR stat (5)),
only one client can have the file open at any time.
That is, after such a file has been opened,
no other open will succeed until
.I fid
has been clunked.
All these permissions are checked at the time of the
.B open
request; subsequent changes to the permissions of files do not affect
the ability to read, write, or remove an open file.
.PP
The 
.B create
request asks the file server to create a new file
with the 
.I name
supplied, in the directory
.RI ( dir )
represented by
.IR fid ,
and requires write permission in the directory.
The owner of the file is the implied user id of the request,
the group of the file is the same as
.IR dir ,
and the permissions are the value of
.ce
.B "perm & (~0666 | (dir.perm & 0666))"
if a regular file is being created and
.ce
.B "perm & (~0777 | (dir.perm & 0777))"
if a directory is being created.
This means, for example, that if the
.B create
allows read permission to others, but the containing directory
does not, then the created file will not allow others to read the file.
.PP
Finally, the newly created file is opened according to
.IR mode ,
and
.I fid
will represent the newly opened file.
.I Mode
is not checked against the permissions in
.IR perm .
The
.I qid
for the new file is returned with the
.B create
reply message.
.PP
Directories are created by setting the
.B CHDIR
bit
.RB ( 0x80000000 )
in the
.IR perm .
.PP
The names
.B .
and
.B ..
are special; it is illegal to create files with these names.
.PP
It is an error for either of these messages if the fid
is already the product of a successful
.B open
or
.B create
message.
.PP
An attempt to
.B create
a file in a directory where the given
.I name
already exists will be rejected;
in this case, the
.I create
system call
(see
.IR open (2))
uses
.B open
with truncation.
The algorithm used by
.IR create
is:
first walk to the directory to contain the file.
If that fails, return an error.
Next
.B walk
to the specified file.
If the
.B walk
succeeds, send a request to
.B open
and truncate the file and return the result, successful or not.
If the
.B walk
fails, send a create message.
If that fails, it may be because the file was created by another
process after the previous walk failed, so (once) try the
.B walk
and
.B open
again.
For the behavior of
.I create
on a union directory, see
.IR bind (2).
.SH ENTRY POINTS
.I Open
and
.I create
both generate
.B open
messages; only
.I create
generates a
.B create
message.