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| <h1> |
| <img border="0" src="../../../../boost.png" align="center" width="277" height="86">Path |
| Name Portability |
| Guide</h1> |
| |
| <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 Juliet 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 -->11 January, 2009<!--webbot bot="Timestamp" endspan i-checksum="39335" --></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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