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 Sys), 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 (16r10) bit
          set, the file is to be truncated, which requires write per-
          mission (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 (16r40) 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 per-
          missions are checked at the time of the open request; subse-
          quent 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 & (~8r666 | (dir.perm & 8r666))
          if a regular file is being created and
                   perm & (~8r777 | (dir.perm & 8r777))
          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,

     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
          (16r80000000) 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 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 behavior of create on a union directory, see sys-
          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 Styx 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 sys-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
          (16r1000) 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 suc-
          ceeds, the process issuing the create is guaranteed to be
          the one that created the file.

     SEE ALSO
          sys-bind(2), sys-open(2), stat(5)