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<!-- Copyright (C) 2003 Red Hat, Inc. --> <!-- This material may be distributed only subject to the terms --> <!-- and conditions set forth in the Open Publication License, v1.0 --> <!-- or later (the latest version is presently available at --> <!-- http://www.opencontent.org/openpub/). --> <!-- Distribution of the work or derivative of the work in any --> <!-- standard (paper) book form is prohibited unless prior --> <!-- permission is obtained from the copyright holder. --> <HTML ><HEAD ><TITLE >File Table</TITLE ><meta name="MSSmartTagsPreventParsing" content="TRUE"> <META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+ "><LINK REL="HOME" TITLE="eCos Reference Manual" HREF="ecos-ref.html"><LINK REL="UP" TITLE="File System Support Infrastructure" HREF="fileio.html"><LINK REL="PREVIOUS" TITLE="Mount Table" HREF="fileio-mount-table.html"><LINK REL="NEXT" TITLE="Directories" HREF="fileio-directories.html"></HEAD ><BODY CLASS="CHAPTER" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >eCos Reference Manual</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="fileio-mount-table.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="fileio-directories.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="CHAPTER" ><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" WIDTH="70%" ><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 ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="fileio-mount-table.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="ecos-ref.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="fileio-directories.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Mount Table</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="fileio.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Directories</TD ></TR ></TABLE ></DIV ></BODY ></HTML >
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