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- <h1>
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- Name Portability
- Guide</h1>
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- <p>
- <a href="#Introduction">Introduction</a><br>
- <a href="#name_check_functions">name_check functions</a><br>
- <a href="#recommendations">File and directory name recommendations</a></p>
- <h2><a name="Introduction">Introduction</a></h2>
- <p>Like any other C++ program which performs I/O operations, there is no
- guarantee that a program using Boost.Filesystem will be portable between
- operating systems. Critical aspects of I/O such as how the operating system
- interprets paths are unspecified by the C and C++ Standards.</p>
- <p>It is not possible to know if a file or directory name will be
- valid (and thus portable) for an unknown operating system. There is always the possibility that an operating system could use
- names which are unusual (numbers less than 4096, for example) or very
- limited in size (maximum of six character names, for example). In other words,
- portability is never absolute; it is always relative to specific operating
- systems or
- file systems.</p>
- <p>It is possible, however, to know in advance if a directory or file name is likely to be valid for a particular
- operating system. It is also possible to construct names which are
- likely to be portable to a large number of modern and legacy operating systems.</p>
- <p>Almost all modern operating systems support multiple file systems. At the
- minimum, they support a native file system plus a CD-ROM file system (Generally
- ISO-9669, often with Joliet extensions).</p>
- <p>Each file system
- may have its own naming rules. For example, modern versions of Windows support NTFS, FAT, FAT32, and ISO-9660 file systems, among others, and the naming rules
- for those file systems differ. Each file system may also have
- differing rules for overall path validity, such as a maximum length or number of
- sub-directories. Some legacy systems have different rules for directory names
- versus regular file names.</p>
- <p>As a result, Boost.Filesystem's <i>name_check</i> functions
- cannot guarantee directory and file name portability. Rather, they are intended to
- give the programmer a "fighting chance" to achieve portability by early
- detection of common naming problems.</p>
- <h2><a name="name_check_functions">name_check functions</a></h2>
- <p>A <i>name_check</i> function
- returns true if its argument is valid as a directory and regular file name for a
- particular operating or file system. A number of these functions are provided.</p>
- <p>The <a href="#portable_name">portable_name</a> function is of particular
- interest because it has been carefully designed to provide wide
- portability yet not overly restrict expressiveness.</p>
- <table border="1" cellpadding="5" cellspacing="0">
- <tr>
- <td align="center" colspan="2"><b>Library Supplied name_check Functions</b></td>
- </tr>
- <tr>
- <td align="center"><b>Function</b></td>
- <td align="center"><b>Description</b></td>
- </tr>
- <tr>
- <td align="left" valign="top"><code><a name="portable_posix_name">portable_posix_name</a>(const
- std::string&<i> name</i>)</code></td>
- <td><b>Returns:</b> <i>true</i> if <code>!name.empty() && name</code> contains only the characters
- specified in<i> Portable Filename Character Set</i> rules as defined in by
- POSIX (<a href="http://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap03.html">www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap03.html</a>).<br>
- The allowed characters are <code>0-9</code>, <code>a-z</code>, <code>A-Z</code>,
- <code>'.'</code>, <code>'_'</code>, and <code>'-'</code>.<p><b>Use:</b>
- applications which must be portable to any POSIX system.</td>
- </tr>
- <tr>
- <td align="left" valign="top"><code><a name="windows_name">windows_name</a>(const
- std::string&<i> name</i>)</code></td>
- <td><b>Returns:</b> <i>true</i> if <code>!name.empty() && name</code> contains
- only the characters specified by the Windows platform SDK as valid
- regardless of the file system <code>&& (name</code> is <code>"."</code> or
- <code>".."</code> or does not end with a trailing space or period<code>)</code>.
- The allowed characters are anything except <code>0x0-0x1F</code>, <code>'<'</code>,
- <code>'>'</code>, <code>':'</code>, <code>'"'</code>, <code>'/'</code>,
- <code>'\'</code>, and <code>'|'</code>.<p>
- <b>Use:</b> applications which must be portable to Windows.</p>
- <p><b>Note:</b> Reserved device names are not valid as file names, but are
- not being detected because they are still valid as a path. Specifically,
- CON, PRN, AUX, CLOCK$, NUL, COM[1-9], LPT[1-9], and these names followed by
- an extension (for example, NUL.tx7).</td>
- </tr>
- <tr>
- <td align="left" valign="top"><code><a name="portable_name">portable_name</a>(const
- std::string&<i> name</i>)</code></td>
- <td><b>Returns:</b> <code> windows_name(name) && portable_posix_name(name)
- && (name</code> is <code>"."</code> or <code>".."</code>, and the first character not a period or hyphen<code>)</code>.<p><b>Use:</b> applications which must be portable to a wide variety of
- modern operating systems, large and small, and to some legacy O/S's. The
- first character not a period or hyphen restriction is a requirement of
- several older operating systems.</td>
- </tr>
- <tr>
- <td align="left" valign="top"><code><a name="portable_directory_name">
- portable_directory_name</a>(const std::string&<i> name</i>)</code></td>
- <td><b>Returns:</b> <code>portable_name(name) && (name</code> is <code>"."</code>
- or <code>".."</code> or contains no periods<code>)</code>.<p><b>Use:</b> applications
- which must be portable to a wide variety of platforms, including OpenVMS.</td>
- </tr>
- <tr>
- <td align="left" valign="top"><code><a name="portable_file_name">
- portable_file_name</a>(const std::string&<i> name</i>)</code></td>
- <td><b>Returns:</b> <code>portable_name(name) && </code>any period is followed by one to three additional
- non-period characters.<p><b>Use:</b>
- applications which must be portable to a wide variety of platforms,
- including OpenVMS and other systems which have a concept of "file extension"
- but limit its length.</td>
- </tr>
- <tr>
- <td align="left" valign="top"><code><a name="native">native</a>(const
- std::string&<i> name</i>)</code></td>
- <td><b>Returns:</b> Implementation defined. Returns <i>
- true</i> for names considered valid by the operating system's native file
- systems.<p><b>Note:</b> May return <i>true</i> for some names not considered valid
- by the operating system under all conditions (particularly on operating systems which support
- multiple file systems.)</td>
- </tr>
- </table>
- <h2>File and directory name <a name="recommendations">recommendations</a></h2>
- <table border="1" cellpadding="5" cellspacing="0">
- <tr>
- <td align="center" valign="top"><strong>Recommendation</strong></td>
- <td align="center" valign="top"><strong>Rationale</strong></td>
- </tr>
- <tr>
- <td valign="top">Limit file and directory names to the characters A-Z, a-z, 0-9, period, hyphen, and
- underscore.<p>Use any of the "portable_" <a href="#name_check_functions">
- name check functions</a> to enforce this recommendation.</td>
- <td valign="top">These are the characters specified by the POSIX standard for portable directory and
- file names, and are also valid for Windows, Mac, and many other modern file systems.</td>
- </tr>
- <tr>
- <td valign="top">Do not use a period or hyphen as the first
- character of a name. Do not use period as the last character of a name.<p>
- Use <a href="#portable_name">portable_name</a>,
- <a href="#portable_directory_name">portable_directory_name</a>, or
- <a href="#portable_file_name">portable_file_name</a> to enforce this
- recommendation.</td>
- <td valign="top">Some operating systems treat have special rules for the
- first character of names. POSIX, for example. Windows does not permit period
- as the last character.</td>
- </tr>
- <tr>
- <td valign="top">Do not use periods in directory names.<p>Use
- <a href="#portable_directory_name">portable_directory_name</a> to enforce
- this recommendation.</td>
- <td valign="top">Requirement for ISO-9660 without Juliet extensions, OpenVMS filesystem, and other legacy systems.</td>
- </tr>
- <tr>
- <td valign="top">Do not use more that one period in a file name, and limit
- the portion after the period to three characters.<p>Use
- <a href="#portable_file_name">portable_file_name</a> to enforce this
- recommendation.</td>
- <td valign="top">Requirement for ISO-9660 level 1, OpenVMS filesystem, and
- other legacy systems. </td>
- </tr>
- <tr>
- <td valign="top">Do not assume names are case sensitive. For example, do not expected a directory to be
- able to hold separate elements named "Foo" and "foo". </td>
- <td valign="top">Some file systems are case insensitive. For example, Windows
- NTFS is case preserving in the way it stores names, but case insensitive in
- searching for names (unless running under the POSIX sub-system, it which
- case it does case sensitive searches). </td>
- </tr>
- <tr>
- <td valign="top">Do not assume names are case insensitive. For example, do not expect a file
- created with the name of "Foo" to be opened successfully with the name of "foo".</td>
- <td valign="top">Some file systems are case sensitive. For example, POSIX.</td>
- </tr>
- <tr>
- <td valign="top">Don't use hyphens in names.</td>
- <td valign="top">ISO-9660 level 1, and possibly some legacy systems, do not permit
- hyphens.</td>
- </tr>
- <tr>
- <td valign="top">Limit the length of the string returned by path::string() to
- 255 characters.
- Note that ISO 9660 has an explicit directory tree depth limit of 8, although
- this depth limit is removed by the Juliet extensions.</td>
- <td valign="top">Some operating systems place limits on the total path length. For example,
- Windows 2000 limits paths to 260 characters total length.</td>
- </tr>
- <tr>
- <td valign="top">Limit the length of any one name in a path. Pick the specific limit according to
- the operating systems and or file systems you wish portability to:<br>
- Not a concern:: POSIX, Windows, MAC OS X.<br>
- 31 characters: Classic Mac OS<br>
- 8 characters + period + 3 characters: ISO 9660 level 1<br>
- 32 characters: ISO 9660 level 2 and 3<br>
- 128 characters (64 if Unicode): ISO 9660 with Juliet extensions</td>
- <td valign="top">Limiting name length can markedly reduce the expressiveness of file names, yet placing
- only very high limits on lengths inhibits widest portability.</td>
- </tr>
- </table>
- <hr>
- <p>Revised
- <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->29 December, 2014<!--webbot bot="Timestamp" endspan i-checksum="38652" --></p>
- <p>© Copyright Beman Dawes, 2002, 2003</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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