OPEN(5) 							File Formats Manual							   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 mean read access, write access, read and write access, and execute access, to  be  checked
       against	the  permissions  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 truncated); 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.  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.  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, no other open will succeed 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&(~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 direc-
       tory 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 (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 create 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).

ENTRY POINTS

       Open and create both generate open messages; only create generates a create message.

																	   OPEN(5)