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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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><DIV
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><H1
><A
NAME="FILEIO-FILE-TABLE">Chapter 22. File Table</H1
><P
>Once a file has been opened it is represented by an open file
object. These are allocated from an array of available file
objects. User code accesses these open file objects via a second array
of pointers which is indexed by small integer offsets. This gives the
usual Unix file descriptor functionality, complete with the various
duplication mechanisms.</P
><P
>A file table entry has the following structure:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>struct CYG_FILE_TAG
{
    cyg_uint32	                f_flag;		/* file state                   */
    cyg_uint16                  f_ucount;       /* use count                    */
    cyg_uint16                  f_type;		/* descriptor type              */
    cyg_uint32                  f_syncmode;     /* synchronization protocol     */
    struct CYG_FILEOPS_TAG      *f_ops;         /* file operations              */
    off_t       	        f_offset;       /* current offset               */
    CYG_ADDRWORD	        f_data;		/* file or socket               */
    CYG_ADDRWORD                f_xops;         /* extra type specific ops      */
    cyg_mtab_entry              *f_mte;         /* mount table entry            */
};</PRE
></TD
></TR
></TABLE
><P
>The <TT
CLASS="STRUCTFIELD"
><I
>f_flag</I
></TT
> field contains some FILEIO
control bits and some bits propagated from the
<TT
CLASS="PARAMETER"
><I
>flags</I
></TT
> argument of the
<TT
CLASS="FUNCTION"
>open()</TT
> call (defined by
<TT
CLASS="LITERAL"
>CYG_FILE_MODE_MASK</TT
>).</P
><P
>The <TT
CLASS="STRUCTFIELD"
><I
>f_ucount</I
></TT
> field contains a use count that
controls when a file will be closed. Each duplicate in the file
descriptor array counts for one reference here. It is also
incremented around each I/O operation to ensure that the file cannot
be closed while it has current I/O operations.</P
><P
>The <TT
CLASS="STRUCTFIELD"
><I
>f_type</I
></TT
> field indicates the type of the
underlying file object. Some of the possible values here are
<TT
CLASS="LITERAL"
>CYG_FILE_TYPE_FILE</TT
>,
<TT
CLASS="LITERAL"
>CYG_FILE_TYPE_SOCKET</TT
> or <TT
CLASS="LITERAL"
>CYG_FILE_TYPE_DEVICE</TT
>.</P
><P
>The <TT
CLASS="STRUCTFIELD"
><I
>f_syncmode</I
></TT
> field is copied from the
<TT
CLASS="STRUCTFIELD"
><I
>syncmode</I
></TT
> field of the implementing
filesystem. Its use is described in <A
HREF="fileio-synchronization.html"
>Chapter 24</A
>.</P
><P
>The <TT
CLASS="STRUCTFIELD"
><I
>f_offset</I
></TT
> field records the current file
position. It is the responsibility of the file operation functions to
keep this field up to date.</P
><P
>The <TT
CLASS="STRUCTFIELD"
><I
>f_data</I
></TT
> field contains private data
placed here by the underlying filesystem. Normally this will be a
pointer to, or handle on, the filesystem object that implements this
file.</P
><P
>The <TT
CLASS="STRUCTFIELD"
><I
>f_xops</I
></TT
> field contains a pointer to any
extra type specific operation functions. For example, the socket I/O
system installs a pointer to a table of functions that implement the
standard socket operations.</P
><P
>The <TT
CLASS="STRUCTFIELD"
><I
>f_mte</I
></TT
> field contains a pointer to the
parent mount table entry for this file. It is used mainly to implement
the synchronization protocol. This may contain a pointer to some other
data structure in file objects not derived from a filesystem.</P
><P
>The <TT
CLASS="STRUCTFIELD"
><I
>f_ops</I
></TT
> field contains a pointer to a
table of file I/O operations. This has the following structure:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>struct CYG_FILEOPS_TAG
{
        int	(*fo_read)      (struct CYG_FILE_TAG *fp, struct CYG_UIO_TAG *uio);
        int	(*fo_write)     (struct CYG_FILE_TAG *fp, struct CYG_UIO_TAG *uio);
        int     (*fo_lseek)     (struct CYG_FILE_TAG *fp, off_t *pos, int whence );
        int	(*fo_ioctl)     (struct CYG_FILE_TAG *fp, CYG_ADDRWORD com,
                                 CYG_ADDRWORD data);
        int	(*fo_select)    (struct CYG_FILE_TAG *fp, int which, CYG_ADDRWORD info);
        int     (*fo_fsync)     (struct CYG_FILE_TAG *fp, int mode );        
        int	(*fo_close)     (struct CYG_FILE_TAG *fp);
        int     (*fo_fstat)     (struct CYG_FILE_TAG *fp, struct stat *buf );
        int     (*fo_getinfo)   (struct CYG_FILE_TAG *fp, int key, char *buf, int len );
        int     (*fo_setinfo)   (struct CYG_FILE_TAG *fp, int key, char *buf, int len );
};</PRE
></TD
></TR
></TABLE
><P
>It should be obvious from the names of most of these functions what
their responsibilities are. The <TT
CLASS="FUNCTION"
>fo_getinfo()</TT
>
and <TT
CLASS="FUNCTION"
>fo_setinfo()</TT
> function pointers, like their
counterparts in the filesystem structure, implement minor control and
info functions such as <TT
CLASS="FUNCTION"
>fpathconf()</TT
>.</P
><P
>The second argument to the <TT
CLASS="FUNCTION"
>fo_read()</TT
> and
<TT
CLASS="FUNCTION"
>fo_write()</TT
> function pointers is a pointer to a
UIO structure:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>struct CYG_UIO_TAG
{
    struct CYG_IOVEC_TAG *uio_iov;	/* pointer to array of iovecs */
    int	                 uio_iovcnt;	/* number of iovecs in array */
    off_t       	 uio_offset;	/* offset into file this uio corresponds to */
    ssize_t     	 uio_resid;	/* residual i/o count */
    enum cyg_uio_seg     uio_segflg;    /* see above */
    enum cyg_uio_rw      uio_rw;        /* see above */
};
 
struct CYG_IOVEC_TAG
{
    void           *iov_base;           /* Base address. */
    ssize_t        iov_len;             /* Length. */
};</PRE
></TD
></TR
></TABLE
><P
>This structure encapsulates the parameters of any data transfer
operation. It provides support for scatter/gather operations and
records the progress of any data transfer. It is also compatible with
the I/O operations of any BSD-derived network stacks and filesystems.</P
><P
>When a file is opened (or a file object created by some other means,
such as <TT
CLASS="FUNCTION"
>socket()</TT
> or <TT
CLASS="FUNCTION"
>accept()</TT
>) it is the
responsibility of the filesystem open operation to initialize all the
fields of the object except the <TT
CLASS="STRUCTFIELD"
><I
>f_ucount</I
></TT
>,
<TT
CLASS="STRUCTFIELD"
><I
>f_syncmode</I
></TT
> and
<TT
CLASS="STRUCTFIELD"
><I
>f_mte</I
></TT
> fields. Since the
<TT
CLASS="STRUCTFIELD"
><I
>f_flag</I
></TT
> field will already contain bits belonging to the FILEIO
infrastructure, any changes to it must be made with the appropriate
logical operations.</P
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