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>Directories</TITLE
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>Directories</TITLE
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><H1
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><H1
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><A
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><A
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NAME="FILEIO-DIRECTORIES">Chapter 23. Directories</H1
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NAME="FILEIO-DIRECTORIES">Chapter 23. Directories</H1
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><P
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><P
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>Filesystem operations all take a directory pointer as one of their
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>Filesystem operations all take a directory pointer as one of their
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arguments. A directory pointer is an opaque handle managed by the
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arguments. A directory pointer is an opaque handle managed by the
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filesystem. It should encapsulate a reference to a specific directory
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filesystem. It should encapsulate a reference to a specific directory
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within the filesystem. For example, it may be a pointer to the data
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within the filesystem. For example, it may be a pointer to the data
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structure that represents that directory (such as an inode), or a
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structure that represents that directory (such as an inode), or a
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pointer to a pathname for the directory.</P
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pointer to a pathname for the directory.</P
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><P
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><P
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>The <TT
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>The <TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>chdir()</TT
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>chdir()</TT
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> filesystem function pointer has two
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> filesystem function pointer has two
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modes of use. When passed a pointer in the
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modes of use. When passed a pointer in the
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<TT
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<TT
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CLASS="PARAMETER"
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CLASS="PARAMETER"
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><I
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><I
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>dir_out</I
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>dir_out</I
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></TT
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></TT
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> argument, it should locate the named
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> argument, it should locate the named
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directory and place a directory pointer there. If the
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directory and place a directory pointer there. If the
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<TT
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<TT
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CLASS="PARAMETER"
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CLASS="PARAMETER"
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><I
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><I
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>dir_out</I
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>dir_out</I
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></TT
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></TT
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> argument is NULL then the
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> argument is NULL then the
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<TT
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<TT
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CLASS="PARAMETER"
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CLASS="PARAMETER"
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><I
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><I
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>dir</I
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>dir</I
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></TT
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></TT
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> argument is a previously generated
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> argument is a previously generated
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directory pointer that can now be disposed of. When the infrastructure
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directory pointer that can now be disposed of. When the infrastructure
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is implementing the <TT
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is implementing the <TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>chdir()</TT
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>chdir()</TT
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> function it makes two
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> function it makes two
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calls to filesystem <TT
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calls to filesystem <TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>chdir()</TT
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>chdir()</TT
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> functions. The first
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> functions. The first
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is to get a directory pointer for the new current directory. If this
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is to get a directory pointer for the new current directory. If this
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succeeds the second is to dispose of the old current directory
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succeeds the second is to dispose of the old current directory
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pointer.</P
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pointer.</P
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><P
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><P
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>The <TT
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>The <TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>opendir()</TT
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>opendir()</TT
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> function is used to open a
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> function is used to open a
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directory for reading. This results in an open file object that can be
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directory for reading. This results in an open file object that can be
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read to return a sequence of <SPAN
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read to return a sequence of <SPAN
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CLASS="STRUCTNAME"
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CLASS="STRUCTNAME"
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>struct dirent</SPAN
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>struct dirent</SPAN
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>
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>
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objects. The only operations that are allowed on this file are
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objects. The only operations that are allowed on this file are
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>read</TT
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>read</TT
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>, <TT
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>, <TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>lseek</TT
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>lseek</TT
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> and
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> and
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>close</TT
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>close</TT
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>. Each read operation on this file should
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>. Each read operation on this file should
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return a single <SPAN
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return a single <SPAN
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CLASS="STRUCTNAME"
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CLASS="STRUCTNAME"
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>struct dirent</SPAN
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>struct dirent</SPAN
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> object. When
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> object. When
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the end of the directory is reached, zero should be returned. The only
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the end of the directory is reached, zero should be returned. The only
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seek operation allowed is a rewind to the start of the directory, by
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seek operation allowed is a rewind to the start of the directory, by
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supplying an offset of zero and a <TT
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supplying an offset of zero and a <TT
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CLASS="PARAMETER"
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CLASS="PARAMETER"
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><I
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><I
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>whence</I
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>whence</I
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></TT
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></TT
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>
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>
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specifier of <TT
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specifier of <TT
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CLASS="LITERAL"
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CLASS="LITERAL"
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>SEEK_SET</TT
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>SEEK_SET</TT
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>.</P
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>.</P
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><P
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><P
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>Most of these considerations are invisible to clients of a filesystem
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>Most of these considerations are invisible to clients of a filesystem
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since they will access directories via the POSIX
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since they will access directories via the POSIX
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>opendir()</TT
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>opendir()</TT
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>, <TT
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>, <TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>readdir()</TT
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>readdir()</TT
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> and
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> and
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>closedir()</TT
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>closedir()</TT
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> functions.</P
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> functions.</P
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><P
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><P
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>Support for the <TT
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>Support for the <TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>getcwd()</TT
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>getcwd()</TT
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> function is provided by
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> function is provided by
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three mechanisms. The first is to use the
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three mechanisms. The first is to use the
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<TT
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<TT
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CLASS="LITERAL"
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CLASS="LITERAL"
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>FS_INFO_GETCWD</TT
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>FS_INFO_GETCWD</TT
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> getinfo key on the filesystem to use
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> getinfo key on the filesystem to use
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any internal support that it has for this. If that fails it falls back
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any internal support that it has for this. If that fails it falls back
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on one of the two other mechanisms. If
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on one of the two other mechanisms. If
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<TT
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<TT
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CLASS="LITERAL"
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CLASS="LITERAL"
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>CYGPKG_IO_FILEIO_TRACK_CWD</TT
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>CYGPKG_IO_FILEIO_TRACK_CWD</TT
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> is set then the current
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> is set then the current
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directory is tracked textually in <TT
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directory is tracked textually in <TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>chdir()</TT
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>chdir()</TT
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> and the result of that is
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> and the result of that is
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reported in getcwd(). Otherwise an attempt is made to traverse the
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reported in getcwd(). Otherwise an attempt is made to traverse the
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directory tree to its root using ".." entries.</P
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directory tree to its root using ".." entries.</P
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><P
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><P
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>This last option is complicated and expensive, and relies on the
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>This last option is complicated and expensive, and relies on the
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filesystem supporting "." and ".." entries. This is not always the
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filesystem supporting "." and ".." entries. This is not always the
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case, particularly if the filesystem has been ported from a
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case, particularly if the filesystem has been ported from a
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non-UNIX-compatible source. Tracking the pathname textually will
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non-UNIX-compatible source. Tracking the pathname textually will
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usually work, but might not produce optimum results when symbolic
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usually work, but might not produce optimum results when symbolic
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links are being used.</P
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links are being used.</P
|
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