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><H1
><A
NAME="FILEIO-DIRECTORIES"
></A
>Chapter 18. 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 <CODE
CLASS="FUNCTION"
>chdir()</CODE
> filesystem function pointer has two
modes of use. When passed a pointer in the
<CODE
CLASS="PARAMETER"
>dir_out</CODE
> argument, it should locate the named
directory and place a directory pointer there. If the
<CODE
CLASS="PARAMETER"
>dir_out</CODE
> argument is NULL then the
<CODE
CLASS="PARAMETER"
>dir</CODE
> argument is a previously generated
directory pointer that can now be disposed of. When the infrastructure
is implementing the <CODE
CLASS="FUNCTION"
>chdir()</CODE
> function it makes two
calls to filesystem <CODE
CLASS="FUNCTION"
>chdir()</CODE
> 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 <CODE
CLASS="FUNCTION"
>opendir()</CODE
> 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 <CODE
CLASS="STRUCTNAME"
>struct dirent</CODE
>
objects. The only operations that are allowed on this file are
<CODE
CLASS="FUNCTION"
>read</CODE
>, <CODE
CLASS="FUNCTION"
>lseek</CODE
> and
<CODE
CLASS="FUNCTION"
>close</CODE
>. Each read operation on this file should
return a single <CODE
CLASS="STRUCTNAME"
>struct dirent</CODE
> 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 <CODE
CLASS="PARAMETER"
>whence</CODE
>
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
<CODE
CLASS="FUNCTION"
>opendir()</CODE
>, <CODE
CLASS="FUNCTION"
>readdir()</CODE
> and
<CODE
CLASS="FUNCTION"
>closedir()</CODE
> functions. The <CODE
CLASS="STRUCTNAME"
> struct
dirent</CODE
> object returned by <CODE
CLASS="FUNCTION"
>readdir()</CODE
>
will always contain <CODE
CLASS="STRUCTNAME"
>d_name</CODE
> as required by
POSIX. When <TT
CLASS="LITERAL"
>CYGPKG_FILEIO_DIRENT_DTYPE</TT
> is enabled
it will also contain <CODE
CLASS="STRUCTNAME"
>d_type</CODE
>, which is not
part of POSIX, but often implemented by OSes. Currently only the
FATFS, RAMFS, ROMFS and JFFS2 filesystem sets this value. For other
filesystems a value of 0 will be returned in the member.</P
><P
>Support for the <CODE
CLASS="FUNCTION"
>getcwd()</CODE
> 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 <CODE
CLASS="FUNCTION"
>chdir()</CODE
> 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
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