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OPEN(9P)							      OPEN(9P)

       open, create - prepare a	fid for	I/O on an existing or new file

       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]

       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 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 ap-
       pend-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  permis-
       sion  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(9P)), only one	client can have	the file  open	at  any	 time.
       That is,	after such a file has been opened, further opens will fail un-
       til 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  im-
       plied 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,  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

       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 fscreate call	(see 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, success-
       ful 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.

       Fsopen and fscreate (see	both generate  open  messages;	only  fscreate
       generates  a  create  message.  The iounit associated with an open file
       may be discovered by calling fsiounit.

       For programs that need atomic file creation, without the	race that  ex-
       ists  in	 the  open-create  sequence described above, fscreate does the
       following.  If the OEXCL	(0x1000) bit is	set in the mode	for a fscreate
       call,  the open message is not sent; the	kernel issues only the create.
       Thus, if	the file exists, fscreate  will	 draw  an  error,  but	if  it
       doesn't	and the	fscreate call succeeds,	the process issuing the	fscre-
       ate is guaranteed to be the one that created the	file.



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