OPEN(5) OPEN(5)
NAME
open, create - prepare a fid for I/O on an existing or new
file
SYNOPSIS
size[4] Topen tag[2] fid[4] mode[1]
size[4] Ropen tag[2] qid[13] iounit[4]
size[4] Tcreate tag[2] fid[4] name[s] perm[4] mode[1]
size[4] Rcreate tag[2] qid[13] iounit[4]
DESCRIPTION
The open request asks the file server to check permissions
and prepare a fid for I/O with subsequent read and write
messages. The mode field determines the type of I/O: 0
(called OREAD in <libc.h>), 1 (OWRITE), 2 (ORDWR), and 3
(OEXEC) mean read access, write access, read and write
access, and execute access, to be checked against the per-
missions for the file. In addition, if mode has the OTRUNC
(0x10) bit set, the file is to be truncated, which requires
write permission (if the file is append-only, and permission
is granted, the open succeeds but the file will not be trun-
cated); if the mode has the ORCLOSE (0x40) bit set, the file
is to be removed when the fid is clunked, which requires
permission to remove the file from its directory. All other
bits in mode should be zero. 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 stat(5)), only
one client can have the file open at any time. That is,
after such a file has been opened, further opens will fail
until fid has been clunked. All these permissions are
checked at the time of the open request; subsequent changes
to the permissions of files do not affect the ability to
read, write, or remove an open file.
The create request asks the file server to create a new file
with the name supplied, in the directory (dir) represented
by 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 dir, and the permissions
are the value of
perm & (~0666 | (dir.perm & 0666))
if a regular file is being created and
perm & (~0777 | (dir.perm & 0777))
if a directory is being created. This means, for example,
that if the create allows read permission to others, but the
containing directory does not, then the created file will
not allow others to read the file.
Finally, the newly created file is opened according to mode,
Page 1 Plan 9 (printed 11/18/24)
OPEN(5) OPEN(5)
and fid will represent the newly opened file. Mode is not
checked against the permissions in perm. The qid for the new
file is returned with the create reply message.
Directories are created by setting the DMDIR bit
(0x80000000) in the perm.
The names . and .. are special; it is illegal to create
files with these names.
It is an error for either of these messages if the fid is
already the product of a successful open or create message.
An attempt to create a file in a directory where the given
name already exists will be rejected; in this case, the
create system call (see open(2)) uses open with truncation.
The algorithm used by the create system call is: first walk
to the directory to contain the file. If that fails, return
an error. Next walk to the specified file. If the walk
succeeds, send a request to open and truncate the file and
return the result, successful or not. If the 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 walk and open again.
For the behavior of create on a union directory, see
bind(2).
The iounit field returned by open and create may be zero.
If it is not, it is the maximum number of bytes that are
guaranteed to be read from or written to the file without
breaking the I/O transfer into multiple 9P messages; see
read(5).
ENTRY POINTS
Open and create both generate open messages; only create
generates a create message. The iounit associated with an
open file may be discovered by calling iounit(2).
For programs that need atomic file creation, without the
race that exists in the open-create sequence described
above, the kernel does the following. If the OEXCL (0x1000)
bit is set in the mode for a create system call, the open
message is not sent; the kernel issues only the create.
Thus, if the file exists, create will draw an error, but if
it doesn't and the create system call succeeds, the process
issuing the create is guaranteed to be the one that created
the file.
Page 2 Plan 9 (printed 11/18/24)