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<title>Portability Guide</title>
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<h1>
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<img border="0" src="../../../boost.png" align="center" width="277" height="86">Path
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Name Portability
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Guide</h1>
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<p>
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<a href="#Introduction">Introduction</a><br>
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<a href="#name_check_functions">name_check functions</a><br>
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<a href="#recommendations">File and directory name recommendations</a></p>
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<h2><a name="Introduction">Introduction</a></h2>
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<p>Like any other C++ program which performs I/O operations, there is no
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guarantee that a program using Boost.Filesystem will be portable between
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operating systems. Critical aspects of I/O, such as how the operating system
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interprets paths, are unspecified by the C and C++ Standards.</p>
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<p>It is not possible to know if a file or directory name will be
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valid (and thus portable) for an unknown operating system. There is always the possibility that an operating system could use
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names which are unusual (numbers less than 4096, for example) or very
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limited in size (maximum of six character names, for example). In other words,
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portability is never absolute; it is always relative to specific operating or
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file systems.</p>
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<p>It is possible to know in advance if a directory or file name is likely to be valid for a particular
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operating system. It is also possible to construct names which are
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likely to be portable to a large number of modern and legacy operating systems.</p>
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<p>Almost all modern operating systems support multiple file systems. At the
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minimum, they support a native file system plus a CD-ROM file system (Generally
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ISO-9669, often with Juliet extensions).</p>
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<p>Each file system
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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
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for some of those file systems differ a great deal. Each file system may have
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differing rules for overall path validity, such as a maximum length or number of
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sub-directories.</p>
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<p>As a result, Boost.Filesystem's <i>name_check</i> functions
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cannot guarantee directory and file name portability. Rather, they are intended to
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give the programmer a "fighting chance" to achieve portability by early
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detection of common naming problems.</p>
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<h2><a name="name_check_functions">name_check functions</a></h2>
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<p>A <i>name_check</i> function
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returns true if its argument is a valid name for a particular operating or file
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system. A number of these functions are supplied, and user-supplied <i>
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name_check</i> functions are also allowed.</p>
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<p>The <a href="#portable_name">portable_name</a> function is of particular
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interest because it has been carefully designed to provide wide
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portability yet not overly restrict expressiveness.</p>
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<table border="1" cellpadding="5" cellspacing="0">
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<tr>
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<td align="center" colspan="2"><b>Library Supplied name_check Functions</b></td>
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</tr>
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<tr>
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<td align="center"><b>Name</b></td>
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<td align="center"><b>Description</b></td>
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</tr>
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<tr>
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<td align="left" valign="top"><code><a name="portable_posix_name">portable_posix_name</a></code></td>
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<td>Returns <i>true</i> for names containing only the characters
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specified in<i> Portable Filename Character Set</i> rules as defined in by
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POSIX (<a href="http://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap03.html">www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap03.html</a>).<br>
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Only characters allowed are 0-9, a-z, A-Z, '.', '_', and '-'.<p><b>Use:</b>
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applications which must be portable to any POSIX system.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><code><a name="windows_name">windows_name</a></code></td>
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<td>Returns <i>true</i> for names containing only the characters
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specified by the Windows platform SDK as valid regardless of the file
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system. Allows any character except 0x0-0x1F, '<', '>', ':', '"', '/', '\',
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and '|'. Furthermore, names must not end with a trailing space or period.<p>
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<b>Use:</b> applications which must be portable to Windows.</p>
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<p><b>Note:</b> Reserved device names are not valid as file names, but are
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not being detected because they are still valid as a path. Specifically,
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CON, PRN, AUX, CLOCK$, NUL, COM[1-9], LPT[1-9], and these names followed by
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an extension (for example, NUL.tx7).</td>
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</tr>
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<tr>
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<td align="left" valign="top"><code><a name="portable_name">portable_name</a></code></td>
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<td><code>windows_name(name) && portable_posix_name(name)</code>,
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and the first character not a period or hyphen.<p><b>Use:</b> applications which must be portable to a wide variety of
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modern operating systems, large and small, and to some legacy O/S's.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><code><a name="portable_directory_name">
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portable_directory_name</a></code></td>
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<td><code>portable_name(name)</code>, and no periods.<p><b>Use:</b> applications
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which must be portable to a wide variety of platforms, including OpenVMS.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><code><a name="portable_file_name">
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portable_file_name</a></code></td>
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<td><code>portable_name(name)</code>,except allows one
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period only, and only if followed by one to three additional characters.<p><b>Use:</b>
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applications which must be portable to a wide variety of platforms,
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including OpenVMS and other systems which have a concept of "file extension"
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but limit its length.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><code><a name="native">native</a></code></td>
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<td>Implementation defined name_check. Returns <i>
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true</i> for all names considered valid by the operating system.<p><b>Note:</b> May return <i>true</i> for some names not considered valid
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by the operating system under all conditions (particularly on operating systems which support
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multiple file systems.)</td>
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</tr>
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</table>
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<h2>File and directory name <a name="recommendations">recommendations</a></h2>
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<table border="1" cellpadding="5" cellspacing="0">
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<tr>
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<td align="center" valign="top"><strong>Recommendation</strong></td>
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<td align="center" valign="top"><strong>Rationale</strong></td>
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</tr>
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<tr>
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<td valign="top">Limit file and directory names to the characters A-Z, a-z, 0-9, period, hyphen, and
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underscore.<p>Use any of the "portable_" <a href="#name_check_functions">
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name check functions</a> to enforce this recommendation.</td>
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<td valign="top">These are the characters specified by the POSIX standard for portable directory and
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file names, and are also valid for Windows, Mac, and many other modern file systems.</td>
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</tr>
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<tr>
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<td valign="top">Do not use a period or hyphen as the first
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character of a name. Do not use period as the last character of a name.<p>
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Use <a href="#portable_name">portable_name</a>,
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<a href="#portable_directory_name">portable_directory_name</a>, or
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<a href="#portable_file_name">portable_file_name</a> to enforce this
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recommendation.</td>
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<td valign="top">Some operating systems treat have special rules for the
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first character of names. POSIX, for example. Windows does not permit period
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as the last character.</td>
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</tr>
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<tr>
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<td valign="top">Do not use periods in directory names.<p>Use
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<a href="#portable_directory_name">portable_directory_name</a> to enforce
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this recommendation.</td>
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<td valign="top">Requirement for ISO-9660 without Juliet extensions, OpenVMS filesystem, and other legacy systems.</td>
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</tr>
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<tr>
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<td valign="top">Do not use more that one period in a file name, and limit
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the portion after the period to three characters.<p>Use
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<a href="#portable_file_name">portable_file_name</a> to enforce this
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recommendation.</td>
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<td valign="top">Requirement for ISO-9660 level 1, OpenVMS filesystem, and
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other legacy systems. </td>
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</tr>
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<tr>
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<td valign="top">Do not assume names are case sensitive. For example, do not expected a directory to be
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able to hold separate elements named "Foo" and "foo". </td>
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<td valign="top">Some file systems are case insensitive. For example, Windows
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NTFS is case preserving in the way it stores names, but case insensitive in
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searching for names (unless running under the POSIX sub-system, it which
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case it does case sensitive searches). </td>
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</tr>
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<tr>
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<td valign="top">Do not assume names are case insensitive. For example, do not expect a file
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created with the name of "Foo" to be opened successfully with the name of "foo".</td>
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<td valign="top">Some file systems are case sensitive. For example, POSIX.</td>
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</tr>
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<tr>
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<td valign="top">Don't use hyphens in names.</td>
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<td valign="top">ISO-9660 level 1, and possibly some legacy systems, do not permit
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hyphens.</td>
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</tr>
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<tr>
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<td valign="top">Limit the length of the string returned by path::string() to
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255 characters.
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Note that ISO 9660 has an explicit directory tree depth limit of 8, although
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this depth limit is removed by the Juliet extensions.</td>
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<td valign="top">Some operating systems place limits on the total path length. For example,
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Windows 2000 limits paths to 260 characters total length.</td>
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</tr>
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<tr>
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<td valign="top">Limit the length of any one name in a path. Pick the specific limit according to
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the operating systems and or file systems you wish portability to:<br>
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Not a concern:: POSIX, Windows, MAC OS X.<br>
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31 characters: Classic Mac OS<br>
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8 characters + period + 3 characters: ISO 9660 level 1<br>
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32 characters: ISO 9660 level 2 and 3<br>
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128 characters (64 if Unicode): ISO 9660 with Juliet extensions</td>
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<td valign="top">Limiting name length can markedly reduce the expressiveness of file names, yet placing
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only very high limits on lengths inhibits widest portability.</td>
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</tr>
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</table>
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<hr>
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<p>Revised
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<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->03 June, 2007<!--webbot bot="Timestamp" endspan i-checksum="19946" --></p>
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<p>© Copyright Beman Dawes, 2002, 2003</p>
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<p> Use, modification, and distribution are subject to the Boost Software
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License, Version 1.0. (See accompanying file <a href="../../../LICENSE_1_0.txt">
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LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
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www.boost.org/LICENSE_1_0.txt</a>)</p>
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