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- <h1>
- <img border="0" src="../../../boost.png" align="center" width="277" height="86">Filesystem
- Library Design</h1>
- <p><a href="#Introduction">Introduction</a><br>
- <a href="#Requirements">Requirements</a><br>
- <a href="#Realities">Realities</a><br>
- <a href="#Rationale">Rationale</a><br>
- <a href="#Abandoned_Designs">Abandoned_Designs</a><br>
- <a href="#References">References</a></p>
- <h2><a name="Introduction">Introduction</a></h2>
- <p>The primary motivation for beginning work on the Filesystem Library was
- frustration with Boost administrative tools. Scripts were written in
- Python, Perl, Bash, and Windows command languages. There was no single
- scripting language familiar and acceptable to all Boost administrators. Yet they
- were all skilled C++ programmers - why couldn't C++ be used as the scripting
- language?</p>
- <p>The key feature C++ lacked for script-like applications was the ability to
- perform portable filesystem operations on directories and their contents. The
- Filesystem Library was developed to fill that void.</p>
- <p>The intent is not to compete with traditional scripting languages, but to
- provide a solution for situations where C++ is already the language
- of choice..</p>
- <h2><a name="Requirements">Requirements</a></h2>
- <ul>
- <li>Be able to write portable script-style filesystem operations in modern
- C++.<br>
- <br>
- Rationale: This is a common programming need. It is both an
- embarrassment and a hardship that this is not possible with either the current
- C++ or Boost libraries. The need is particularly acute
- when C++ is the only toolset allowed in the tool chain. File system
- operations are provided by many languages used on multiple platforms,
- such as Perl and Python, as well as by many platform specific scripting
- languages. All operating systems provide some form of API for filesystem
- operations, and the POSIX bindings are increasingly available even on
- operating systems not normally associated with POSIX, such as the Mac, z/OS,
- or OS/390.<br>
- </li>
- <li>Work within the <a href="#Realities">realities</a> described below.<br>
- <br>
- Rationale: This isn't a research project. The need is for something that works on
- today's platforms, including some of the embedded operating systems
- with limited file systems. Because of the emphasis on portability, such a
- library would be much more useful if standardized. That means being able to
- work with a much wider range of platforms that just Unix or Windows and their
- clones.<br>
- </li>
- <li>Avoid dangerous programming practices. Particularly, all-too-easy-to-ignore error notifications
- and use of global variables. If a dangerous feature is provided, identify it as such.<br>
- <br>
- Rationale: Normally this would be covered by "the usual Boost requirements...",
- but it is mentioned explicitly because the equivalent native platform and
- scripting language interfaces often depend on all-too-easy-to-ignore error
- notifications and global variables like "current
- working directory".<br>
- </li>
- <li>Structure the library so that it is still useful even if some functionality
- does not map well onto a given platform or directory tree. Particularly, much
- useful functionality should be portable even to flat
- (non-hierarchical) filesystems.<br>
- <br>
- Rationale: Much functionality which does not
- require a hierarchical directory structure is still useful on flat-structure
- filesystems. There are many systems, particularly embedded systems,
- where even very limited functionality is still useful.</li>
- </ul>
- <ul>
- <li>Interface smoothly with current C++ Standard Library input/output
- facilities. For example, paths should be
- easy to use in std::basic_fstream constructors.<br>
- <br>
- Rationale: One of the most common uses of file system functionality is to
- manipulate paths for eventual use in input/output operations.
- Thus the need to interface smoothly with standard library I/O.<br>
- </li>
- <li>Suitable for eventual standardization. The implication of this requirement
- is that the interface be close to minimal, and that great care be take
- regarding portability.<br>
- <br>
- Rationale: The lack of file system operations is a serious hole
- in the current standard, with no other known candidates to fill that hole.
- Libraries with elaborate interfaces and difficult to port specifications are much less likely to be accepted for
- standardization.<br>
- </li>
- <li>The usual Boost <a href="http://www.boost.org/more/lib_guide.htm">requirements and
- guidelines</a> apply.<br>
- </li>
- <li>Encourage, but do not require, portability in path names.<br>
- <br>
- Rationale: For paths which originate from user input it is unreasonable to
- require portable path syntax.<br>
- </li>
- <li>Avoid giving the illusion of portability where portability in fact does not
- exist.<br>
- <br>
- Rationale: Leaving important behavior unspecified or "implementation defined" does a
- great disservice to programmers using a library because it makes it appear
- that code relying on the behavior is portable, when in fact there is nothing
- portable about it. The only case where such under-specification is acceptable is when both users and implementors know from
- other sources exactly what behavior is required, yet for some reason it isn't
- possible to specify it exactly.</li>
- </ul>
- <h2><a name="Realities">Realities</a></h2>
- <ul>
- <li>Some operating systems have a single directory tree root, others have
- multiple roots.<br>
- </li>
- <li>Some file systems provide both a long and short form of filenames.<br>
- </li>
- <li>Some file systems have different syntax for file paths and directory
- paths.<br>
- </li>
- <li>Some file systems have different rules for valid file names and valid
- directory names.<br>
- </li>
- <li>Some file systems (ISO-9660, level 1, for example) use very restricted
- (so-called 8.3) file names.<br>
- </li>
- <li>Some operating systems allow file systems with different
- characteristics to be "mounted" within a directory tree. Thus an
- ISO-9660 or Windows
- file system may end up as a sub-tree of a POSIX directory tree.<br>
- </li>
- <li>Wide-character versions of directory and file operations are available on some operating
- systems, and not available on others.<br>
- </li>
- <li>There is no law that says directory hierarchies have to be specified in
- terms of left-to-right decent from the root.<br>
- </li>
- <li>Some file systems have a concept of file "version number" or "generation
- number". Some don't.<br>
- </li>
- <li>Not all operating systems use single character separators in path names. Some use
- paired notations. A typical fully-specified OpenVMS filename
- might look something like this:<br>
- <br>
- <code> DISK$SCRATCH:[GEORGE.PROJECT1.DAT]BIG_DATA_FILE.NTP;5<br>
- </code><br>
- The general OpenVMS format is:<br>
- <br>
-
- <i>Device:[directories.dot.separated]filename.extension;version_number</i><br>
- </li>
- <li>For common file systems, determining if two descriptors are for same
- entity is extremely difficult or impossible. For example, the concept of
- equality can be different for each portion of a path - some portions may be
- case or locale sensitive, others not. Case sensitivity is a property of the
- pathname itself, and not the platform. Determining collating sequence is even
- worse.<br>
- </li>
- <li>Race-conditions may occur. Directory trees, directories, files, and file attributes are in effect shared between all threads, processes, and computers which have access to the
- filesystem. That may well include computers on the other side of the
- world or in orbit around the world. This implies that file system operations
- may fail in unexpected ways. For example:<br>
- <br>
- <code> assert( exists("foo") == exists("foo") );
- // may fail!<br>
- assert( is_directory("foo") == is_directory("foo");
- // may fail!<br>
- </code><br>
- In the first example, the file may have been deleted between calls to
- exists(). In the second example, the file may have been deleted and then
- replaced by a directory of the same name between the calls to is_directory().<br>
- </li>
- <li>Even though an application may be portable, it still will have to traffic
- in system specific paths occasionally; user provided input is a common
- example.<br>
- </li>
- <li><a name="symbolic-link-use-case">Symbolic</a> links cause canonical and
- normal form of some paths to represent different files or directories. For
- example, given the directory hierarchy <code>/a/b/c</code>, with a symbolic
- link in <code>/a</code> named <code>x</code> pointing to <code>b/c</code>,
- then under POSIX Pathname Resolution rules a path of <code>"/a/x/.."</code>
- should resolve to <code>"/a/b"</code>. If <code>"/a/x/.."</code> were first
- normalized to <code>"/a"</code>, it would resolve incorrectly. (Case supplied
- by Walter Landry.)</li>
- </ul>
- <h2><a name="Rationale">Rationale</a></h2>
- <p>The <a href="#Requirements">Requirements</a> and <a href="#Realities">
- Realities</a> above drove much of the C++ interface design. In particular,
- the desire to make script-like code straightforward caused a great deal of
- effort to go into ensuring that apparently simple expressions like <i>exists( "foo"
- )</i> work as expected.</p>
- <p>See the <a href="faq.htm">FAQ</a> for the rationale behind many detailed
- design decisions.</p>
- <p>Several key insights went into the <i>path</i> class design:</p>
- <ul>
- <li>Decoupling of the input formats, internal conceptual (<i>vector<string></i>
- or other sequence)
- model, and output formats.</li>
- <li>Providing two input formats (generic and O/S specific) broke a major
- design deadlock.</li>
- <li>Providing several output formats solved another set of previously
- intractable problems.</li>
- <li>Several non-obvious functions (particularly decomposition and composition)
- are required to support portable code. (Peter Dimov, Thomas Witt, Glen
- Knowles, others.)</li>
- </ul>
- <p>Error checking was a particularly difficult area. One key insight was that
- with file and directory names, portability isn't a universal truth.
- Rather, the programmer must think out the question "What operating systems do I
- want this path to be portable to?" By providing support for several
- answers to that question, the Filesystem Library alerts programmers of the need
- to ask it in the first place.</p>
- <h2><a name="Abandoned_Designs">Abandoned Designs</a></h2>
- <h3>operations.hpp</h3>
- <p>Dietmar Kühl's original dir_it design and implementation supported
- wide-character file and directory names. It was abandoned after extensive
- discussions among Library Working Group members failed to identify portable
- semantics for wide-character names on systems not providing native support. See
- <a href="faq.htm#wide-character_names">FAQ</a>.</p>
- <p>Previous iterations of the interface design used explicitly named functions providing a
- large number of convenience operations, with no compile-time or run-time
- options. There were so many function names that they were very confusing to use,
- and the interface was much larger. Any benefits seemed theoretical rather than
- real. </p>
- <p>Designs based on compile time (rather than runtime) flag and option selection
- (via policy, enum, or int template parameters) became so complicated that they
- were abandoned, often after investing quite a bit of time and effort. The need
- to qualify attribute or option names with namespaces, even aliases, made use in
- template parameters ugly; that wasn't fully appreciated until actually writing
- real code.</p>
- <p>Yet another set of convenience functions ( for example, <i>remove</i> with
- permissive, prune, recurse, and other options, plus predicate, and possibly
- other, filtering features) were abandoned because the details became both
- complex and contentious.</p>
- <p>What is left is a toolkit of low-level operations from which the user can
- create more complex convenience operations, plus a very small number of
- convenience functions which were found to be useful enough to justify inclusion.</p>
- <h3>path.hpp</h3>
- <p>There were so many abandoned path designs, I've lost track. Policy-based
- class templates in several flavors, constructor supplied runtime policies,
- operation specific runtime policies, they were all considered, often
- implemented, and ultimately abandoned as far too complicated for any small
- benefits observed.</p>
- <p>Additional design considerations apply to <a href="v3_design.html">Internationalization</a>. </p>
- <h3>error checking</h3>
- <p>A number of designs for the error checking machinery were abandoned, some
- after experiments with implementations. Totally automatic error checking was
- attempted in particular. But automatic error checking tended to make the overall
- library design much more complicated.</p>
- <p>Some designs associated error checking mechanisms with paths. Some with
- operations functions. A policy-based error checking template design was
- partially implemented, then abandoned as too complicated for everyday
- script-like programs.</p>
- <p>The final design, which depends partially on explicit error checking function
- calls, is much simpler and straightforward, although it does depend to
- some extent on programmer discipline. But it should allow programmers who
- are concerned about portability to be reasonably sure that their programs will
- work correctly on their choice of target systems.</p>
- <h2><a name="References">References</a></h2>
- <table border="0" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" width="100%">
- <tr>
- <td width="13%" valign="top">[<a name="IBM-01">IBM-01</a>]</td>
- <td width="87%">IBM Corporation, <i>z/OS V1R3.0 C/C++ Run-Time
- Library Reference</i>, SA22-7821-02, 2001,
- <a href="http://www-1.ibm.com/servers/eserver/zseries/zos/bkserv/">
- www-1.ibm.com/servers/eserver/zseries/zos/bkserv/</a></td>
- </tr>
- <tr>
- <td width="13%" valign="top">[<a name="ISO-9660">ISO-9660</a>]</td>
- <td width="87%">International Standards Organization, 1988</td>
- </tr>
- <tr>
- <td width="13%" valign="top">[<a name="Kuhn">Kuhn</a>]</td>
- <td width="87%">UTF-8 and Unicode FAQ for Unix/Linux,
- <a href="http://www.cl.cam.ac.uk/~mgk25/unicode.html">
- www.cl.cam.ac.uk/~mgk25/unicode.html</a></td>
- </tr>
- <tr>
- <td width="13%" valign="top">[<a name="MSDN">MSDN</a>] </td>
- <td width="87%">Microsoft Platform SDK for Windows, Storage Start
- Page,
- <a href="http://msdn.microsoft.com/library/en-us/fileio/base/storage_start_page.asp">
- msdn.microsoft.com/library/en-us/fileio/base/storage_start_page.asp</a></td>
- </tr>
- <tr>
- <td width="13%" valign="top">[<a name="POSIX-01">POSIX-01</a>]</td>
- <td width="87%">IEEE Std 1003.1-2001, ISO/IEC 9945:2002, and The Open Group Base Specifications, Issue 6. Also known as The
- Single Unix<font face="Times New Roman">® Specification, Version 3.
- Available from each of the organizations involved in its creation. For
- example, read online or download from
- <a href="http://www.unix.org/single_unix_specification/">
- www.unix.org/single_unix_specification/</a>.</font> The ISO JTC1/SC22/WG15 - POSIX
- homepage is <a href="http://www.open-std.org/jtc1/sc22/WG15/">
- www.open-std.org/jtc1/sc22/WG15/</a></td>
- </tr>
- <tr>
- <td width="13%" valign="top">[<a name="URI">URI</a>]</td>
- <td width="87%">RFC-2396, Uniform Resource Identifiers (URI): Generic
- Syntax, <a href="http://www.ietf.org/rfc/rfc2396.txt">
- www.ietf.org/rfc/rfc2396.txt</a></td>
- </tr>
- <tr>
- <td width="13%" valign="top">[<a name="UTF-16">UTF-16</a>]</td>
- <td width="87%">Wikipedia, UTF-16,
- <a href="http://en.wikipedia.org/wiki/UTF-16">
- en.wikipedia.org/wiki/UTF-16</a></td>
- </tr>
- <tr>
- <td width="13%" valign="top">[<a name="Wulf-Shaw-73">Wulf-Shaw-73</a>]</td>
- <td width="87%">William Wulf, Mary Shaw, <i>Global
- Variable Considered Harmful</i>, ACM SIGPLAN Notices, 8, 2, 1973, pp. 23-34</td>
- </tr>
- </table>
- <hr>
- <p>Revised
- <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->26 December, 2014<!--webbot bot="Timestamp" endspan i-checksum="38646" --></p>
- <p>© Copyright Beman Dawes, 2002</p>
- <p> Use, modification, and distribution are subject to the Boost Software
- License, Version 1.0. (See accompanying file <a href="../../../LICENSE_1_0.txt">
- LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
- www.boost.org/LICENSE_1_0.txt</a>)</p>
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