248 lines
4.6 KiB
Plaintext
248 lines
4.6 KiB
Plaintext
.TH OPEN 5
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.SH NAME
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open, create \- prepare a fid for I/O on an existing or new file
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.SH SYNOPSIS
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.ta \w'\fLTcreate 'u
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.IR size [4]
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.B Topen
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.IR tag [2]
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.IR fid [4]
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.IR mode [1]
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.br
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.IR size [4]
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.B Ropen
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.IR tag [2]
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.IR qid [13]
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.IR iounit [4]
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.PP
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.IR size [4]
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.B Tcreate
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.IR tag [2]
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.IR fid [4]
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.IR name [ s ]
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.IR perm [4]
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.IR mode [1]
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.br
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.IR size [4]
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.B Rcreate
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.IR tag [2]
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.IR qid [13]
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.IR iounit [4]
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.SH DESCRIPTION
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The
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.B open
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request asks the file server to check permissions and
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prepare a fid for I/O with subsequent
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.B read
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and
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.B write
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messages.
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The
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.I mode
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field determines the type of I/O:
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0
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(called
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.BR OREAD
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in
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.BR <libc.h> ),
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1
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.RB ( OWRITE ),
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2
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.RB ( ORDWR ),
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and 3
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.RB ( OEXEC )
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mean
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.I
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read access, write access, read and write access,
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and
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.I
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execute access,
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to be checked against the permissions for the file.
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In addition, if
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.I mode
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has the
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.B OTRUNC
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.RB ( 0x10 )
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bit set,
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the file is to be truncated, which requires write permission
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(if
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the file is append-only, and permission is granted, the
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.B open
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succeeds but the file will not be truncated);
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if the
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.I mode
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has the
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.B ORCLOSE
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.RB ( 0x40 )
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bit set, the file is to be removed when the fid
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is clunked, which requires permission to remove the file from its directory.
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All other bits in
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.I mode
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should be zero.
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It is illegal to write a directory, truncate it, or attempt to remove it on close.
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If the file is marked for exclusive use (see
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.IR stat (5)),
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only one client can have the file open at any time.
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That is, after such a file has been opened,
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further opens will fail until
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.I fid
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has been clunked.
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All these permissions are checked at the time of the
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.B open
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request; subsequent changes to the permissions of files do not affect
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the ability to read, write, or remove an open file.
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.PP
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The
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.B create
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request asks the file server to create a new file
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with the
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.I name
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supplied, in the directory
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.RI ( dir )
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represented by
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.IR fid ,
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and requires write permission in the directory.
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The owner of the file is the implied user id of the request,
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the group of the file is the same as
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.IR dir ,
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and the permissions are the value of
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.ce
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.B "perm & (~0666 | (dir.perm & 0666))"
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if a regular file is being created and
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.ce
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.B "perm & (~0777 | (dir.perm & 0777))"
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if a directory is being created.
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This means, for example, that if the
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.B create
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allows read permission to others, but the containing directory
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does not, then the created file will not allow others to read the file.
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.PP
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Finally, the newly created file is opened according to
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.IR mode ,
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and
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.I fid
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will represent the newly opened file.
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.I Mode
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is not checked against the permissions in
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.IR perm .
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The
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.I qid
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for the new file is returned with the
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.B create
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reply message.
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.PP
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Directories are created by setting the
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.B DMDIR
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bit
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.RB ( 0x80000000 )
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in the
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.IR perm .
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.PP
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The names
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.B .
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and
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.B ..
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are special; it is illegal to create files with these names.
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.PP
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It is an error for either of these messages if the fid
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is already the product of a successful
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.B open
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or
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.B create
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message.
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.PP
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An attempt to
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.B create
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a file in a directory where the given
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.I name
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already exists will be rejected;
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in this case, the
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.I create
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system call
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(see
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.IR open (2))
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uses
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.B open
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with truncation.
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The algorithm used by the
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.IR create
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system call
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is:
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first walk to the directory to contain the file.
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If that fails, return an error.
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Next
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.B walk
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to the specified file.
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If the
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.B walk
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succeeds, send a request to
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.B open
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and truncate the file and return the result, successful or not.
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If the
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.B walk
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fails, send a create message.
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If that fails, it may be because the file was created by another
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process after the previous walk failed, so (once) try the
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.B walk
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and
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.B open
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again.
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.PP
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For the behavior of
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.I create
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on a union directory, see
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.IR bind (2).
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.PP
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The
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.B iounit
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field returned by
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.B open
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and
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.B create
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may be zero.
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If it is not, it is the maximum number of bytes that are guaranteed to
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be read from or written to the file without breaking the I/O transfer
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into multiple 9P messages; see
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.IR read (5).
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.SH ENTRY POINTS
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.I Open
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and
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.I create
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both generate
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.B open
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messages; only
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.I create
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generates a
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.B create
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message.
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The
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.B iounit
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associated with an open file may be discovered by calling
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.IR iounit (2).
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.PP
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For programs that need atomic file creation, without the race
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that exists in the
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.B open-create
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sequence described above,
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the kernel does the following.
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If the
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.B OEXCL
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.RB ( 0x1000 )
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bit is set in the
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.I mode
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for a
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.B create
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system call,
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the
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.B open
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message is not sent; the kernel issues only the
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.BR create .
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Thus, if the file exists,
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.B create
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will draw an error, but if it doesn't and the
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.B create
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system call succeeds, the process issuing the
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.B create
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is guaranteed to be the one that created the file.
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