📄 fileio-directories.html
字号:
<!-- 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
>Directories</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="File Table"
HREF="fileio-file-table.html"><LINK
REL="NEXT"
TITLE="Synchronization"
HREF="fileio-synchronization.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-file-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-synchronization.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="CHAPTER"
><H1
><A
NAME="FILEIO-DIRECTORIES">Chapter 23. Directories</H1
><P
>Filesystem operations all take a directory pointer as one of their
arguments. A directory pointer is an opaque handle managed by the
filesystem. It should encapsulate a reference to a specific directory
within the filesystem. For example, it may be a pointer to the data
structure that represents that directory (such as an inode), or a
pointer to a pathname for the directory.</P
><P
>The <TT
CLASS="FUNCTION"
>chdir()</TT
> filesystem function pointer has two
modes of use. When passed a pointer in the
<TT
CLASS="PARAMETER"
><I
>dir_out</I
></TT
> argument, it should locate the named
directory and place a directory pointer there. If the
<TT
CLASS="PARAMETER"
><I
>dir_out</I
></TT
> argument is NULL then the
<TT
CLASS="PARAMETER"
><I
>dir</I
></TT
> argument is a previously generated
directory pointer that can now be disposed of. When the infrastructure
is implementing the <TT
CLASS="FUNCTION"
>chdir()</TT
> function it makes two
calls to filesystem <TT
CLASS="FUNCTION"
>chdir()</TT
> functions. The first
is to get a directory pointer for the new current directory. If this
succeeds the second is to dispose of the old current directory
pointer.</P
><P
>The <TT
CLASS="FUNCTION"
>opendir()</TT
> function is used to open a
directory for reading. This results in an open file object that can be
read to return a sequence of <SPAN
CLASS="STRUCTNAME"
>struct dirent</SPAN
>
objects. The only operations that are allowed on this file are
<TT
CLASS="FUNCTION"
>read</TT
>, <TT
CLASS="FUNCTION"
>lseek</TT
> and
<TT
CLASS="FUNCTION"
>close</TT
>. Each read operation on this file should
return a single <SPAN
CLASS="STRUCTNAME"
>struct dirent</SPAN
> object. When
the end of the directory is reached, zero should be returned. The only
seek operation allowed is a rewind to the start of the directory, by
supplying an offset of zero and a <TT
CLASS="PARAMETER"
><I
>whence</I
></TT
>
specifier of <TT
CLASS="LITERAL"
>SEEK_SET</TT
>.</P
><P
>Most of these considerations are invisible to clients of a filesystem
since they will access directories via the POSIX
<TT
CLASS="FUNCTION"
>opendir()</TT
>, <TT
CLASS="FUNCTION"
>readdir()</TT
> and
<TT
CLASS="FUNCTION"
>closedir()</TT
> functions.</P
><P
>Support for the <TT
CLASS="FUNCTION"
>getcwd()</TT
> function is provided by
three mechanisms. The first is to use the
<TT
CLASS="LITERAL"
>FS_INFO_GETCWD</TT
> getinfo key on the filesystem to use
any internal support that it has for this. If that fails it falls back
on one of the two other mechanisms. If
<TT
CLASS="LITERAL"
>CYGPKG_IO_FILEIO_TRACK_CWD</TT
> is set then the current
directory is tracked textually in <TT
CLASS="FUNCTION"
>chdir()</TT
> and the result of that is
reported in getcwd(). Otherwise an attempt is made to traverse the
directory tree to its root using ".." entries.</P
><P
>This last option is complicated and expensive, and relies on the
filesystem supporting "." and ".." entries. This is not always the
case, particularly if the filesystem has been ported from a
non-UNIX-compatible source. Tracking the pathname textually will
usually work, but might not produce optimum results when symbolic
links are being used.</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-file-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-synchronization.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>File 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"
>Synchronization</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -