OPEN(5) OPEN(5)
NAME
open, create - prepare a fid for I/O on an existing or new
file
SYNOPSIS
Topen tag[2] fid[2] mode[1]
Ropen tag[2] fid[2] qid[8]
Tcreate tag[2] fid[2] name[28] perm[4] mode[1]
Rcreate tag[2] fid[2] qid[8]
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, 1,
2, and 3 (OREAD, OWRITE, ORDWR, OEXEC) mean read access,
write access, read and write access, and execute access, to
be checked against the permissions for the file. In addi-
tion, if mode has the OTRUNC (16r10) bit set, the file is to
be truncated, which requires write permission; if the mode
has the ORCLOSE (16r40) 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 mode
they will be ignored. It is illegal to write a directory,
truncate it, or attempt to remove it on close. 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&(~0777|0111)) | (dir.perm&perm&0666)
if a regular file is being created and
(perm&~0777) | (dir.perm&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,
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 CHDIR bit
(16r80000000) in perm.
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OPEN(5) OPEN(5)
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 sys-open(2)) uses open with trunca-
tion. 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 behaviour of create on a union directory, see sys-
bind(2).
ENTRY POINTS
The open and create calls both generate open messages; only
create generates a create message.
SEE ALSO
sys-bind(2), sys-open(2), stat(5)
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