term% ls -F
term% cat index.txt
OPEN(5)                       File Formats Manual                      OPEN(5)

       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(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 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 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  mes‐
       sage.  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

       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 mul‐
       tiple 9P messages; see read(5).

       Open and create both generate open messages; only  create  generates  a
       create message.  The iounit associated with an open file may be discov‐
       ered by calling iounit(2).

       For programs that need atomic file creation, without the race that  ex‐
       ists  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.