man(1) Manual page archive


     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.

     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)