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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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><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 &quot;..&quot; entries.</P
><P
>This last option is complicated and expensive, and relies on the
filesystem supporting &quot;.&quot; and &quot;..&quot;  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
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Line No.	Rev	Author	Line
1	28	unneback	`<!-- Copyright (C) 2003 Red Hat, Inc. -->`
2			`<!-- This material may be distributed only subject to the terms -->`
3			`<!-- and conditions set forth in the Open Publication License, v1.0 -->`
4			`<!-- or later (the latest version is presently available at -->`
5			`<!-- http://www.opencontent.org/openpub/). -->`
6			`<!-- Distribution of the work or derivative of the work in any -->`
7			`<!-- standard (paper) book form is prohibited unless prior -->`
8			`<!-- permission is obtained from the copyright holder. -->`
9			`<HTML`
10			`><HEAD`
11			`><TITLE`
12			`>Directories</TITLE`
13			`><meta name="MSSmartTagsPreventParsing" content="TRUE">`
14			`<META`
15			`NAME="GENERATOR"`
16			`CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+`
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18			`REL="HOME"`
19			`TITLE="eCos Reference Manual"`
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21			`REL="UP"`
22			`TITLE="File System Support Infrastructure"`
23			`HREF="fileio.html"><LINK`
24			`REL="PREVIOUS"`
25			`TITLE="File Table"`
26			`HREF="fileio-file-table.html"><LINK`
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28			`TITLE="Synchronization"`
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45			`><TR`
46			`><TH`
47			`COLSPAN="3"`
48			`ALIGN="center"`
49			`>eCos Reference Manual</TH`
50			`></TR`
51			`><TR`
52			`><TD`
53			`WIDTH="10%"`
54			`ALIGN="left"`
55			`VALIGN="bottom"`
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79			`WIDTH="100%"></DIV`
80			`><DIV`
81			`CLASS="CHAPTER"`
82			`><H1`
83			`><A`
84			`NAME="FILEIO-DIRECTORIES">Chapter 23. Directories</H1`
85			`><P`
86			`>Filesystem operations all take a directory pointer as one of their`
87			`arguments. A directory pointer is an opaque handle managed by the`
88			`filesystem. It should encapsulate a reference to a specific directory`
89			`within the filesystem. For example, it may be a pointer to the data`
90			`structure that represents that directory (such as an inode), or a`
91			`pointer to a pathname for the directory.</P`
92			`><P`
93			`>The <TT`
94			`CLASS="FUNCTION"`
95			`>chdir()</TT`
96			`> filesystem function pointer has two`
97			`modes of use. When passed a pointer in the`
98			`<TT`
99			`CLASS="PARAMETER"`
100			`><I`
101			`>dir_out</I`
102			`></TT`
103			`> argument, it should locate the named`
104			`directory and place a directory pointer there. If the`
105			`<TT`
106			`CLASS="PARAMETER"`
107			`><I`
108			`>dir_out</I`
109			`></TT`
110			`> argument is NULL then the`
111			`<TT`
112			`CLASS="PARAMETER"`
113			`><I`
114			`>dir</I`
115			`></TT`
116			`> argument is a previously generated`
117			`directory pointer that can now be disposed of. When the infrastructure`
118			`is implementing the <TT`
119			`CLASS="FUNCTION"`
120			`>chdir()</TT`
121			`> function it makes two`
122			`calls to filesystem <TT`
123			`CLASS="FUNCTION"`
124			`>chdir()</TT`
125			`> functions. The first`
126			`is to get a directory pointer for the new current directory. If this`
127			`succeeds the second is to dispose of the old current directory`
128			`pointer.</P`
129			`><P`
130			`>The <TT`
131			`CLASS="FUNCTION"`
132			`>opendir()</TT`
133			`> function is used to open a`
134			`directory for reading. This results in an open file object that can be`
135			`read to return a sequence of <SPAN`
136			`CLASS="STRUCTNAME"`
137			`>struct dirent</SPAN`
138			`>`
139			`objects. The only operations that are allowed on this file are`
140			`<TT`
141			`CLASS="FUNCTION"`
142			`>read</TT`
143			`>, <TT`
144			`CLASS="FUNCTION"`
145			`>lseek</TT`
146			`> and`
147			`<TT`
148			`CLASS="FUNCTION"`
149			`>close</TT`
150			`>. Each read operation on this file should`
151			`return a single <SPAN`
152			`CLASS="STRUCTNAME"`
153			`>struct dirent</SPAN`
154			`> object. When`
155			`the end of the directory is reached, zero should be returned. The only`
156			`seek operation allowed is a rewind to the start of the directory, by`
157			`supplying an offset of zero and a <TT`
158			`CLASS="PARAMETER"`
159			`><I`
160			`>whence</I`
161			`></TT`
162			`>`
163			`specifier of <TT`
164			`CLASS="LITERAL"`
165			`>SEEK_SET</TT`
166			`>.</P`
167			`><P`
168			`>Most of these considerations are invisible to clients of a filesystem`
169			`since they will access directories via the POSIX`
170			`<TT`
171			`CLASS="FUNCTION"`
172			`>opendir()</TT`
173			`>, <TT`
174			`CLASS="FUNCTION"`
175			`>readdir()</TT`
176			`> and`
177			`<TT`
178			`CLASS="FUNCTION"`
179			`>closedir()</TT`
180			`> functions.</P`
181			`><P`
182			`>Support for the <TT`
183			`CLASS="FUNCTION"`
184			`>getcwd()</TT`
185			`> function is provided by`
186			`three mechanisms. The first is to use the`
187			`<TT`
188			`CLASS="LITERAL"`
189			`>FS_INFO_GETCWD</TT`
190			`> getinfo key on the filesystem to use`
191			`any internal support that it has for this. If that fails it falls back`
192			`on one of the two other mechanisms. If`
193			`<TT`
194			`CLASS="LITERAL"`
195			`>CYGPKG_IO_FILEIO_TRACK_CWD</TT`
196			`> is set then the current`
197			`directory is tracked textually in <TT`
198			`CLASS="FUNCTION"`
199			`>chdir()</TT`
200			`> and the result of that is`
201			`reported in getcwd(). Otherwise an attempt is made to traverse the`
202			`directory tree to its root using ".." entries.</P`
203			`><P`
204			`>This last option is complicated and expensive, and relies on the`
205			`filesystem supporting "." and ".." entries. This is not always the`
206			`case, particularly if the filesystem has been ported from a`
207			`non-UNIX-compatible source. Tracking the pathname textually will`
208			`usually work, but might not produce optimum results when symbolic`
209			`links are being used.</P`
210			`></DIV`
211			`><DIV`
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[/] [openrisc/] [trunk/] [rtos/] [ecos-2.0/] [doc/] [html/] [ref/] [fileio-directories.html] - Blame information for rev 510