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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
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><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
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><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
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><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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