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