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<p>This text is a work in progress—highly subject to
change—and may not accurately describe any released
version of the Apache™ Subversion® software.
Bookmarking or otherwise referring others to this page is
probably not such a smart idea. Please visit
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<h2 class="title" style="clear: both"><a id="svn.forcvs.binary-and-trans"></a>Binary Files and Translation</h2>
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<p>In the most general sense, Subversion handles binary files
more gracefully than CVS does. Because CVS uses RCS, it can
only store successive full copies of a changing binary file.
Subversion, however, expresses differences between files using a
binary differencing algorithm, regardless of whether they
contain textual or binary data. That means all files are
stored differentially (compressed) in the repository.</p>
<p>CVS users have to mark binary files with
<code class="option">-kb</code> flags to prevent data from being
garbled (due to keyword expansion and line-ending translations).
They sometimes forget to do this.</p>
<p>Subversion takes the more paranoid route. First, it never
performs any kind of keyword or line-ending translation unless
you explicitly ask it to do so (see
<a class="xref" href="svn.advanced.props.special.keywords.html" title="Keyword Substitution">the section called “Keyword Substitution”</a> and
<a class="xref" href="svn.advanced.props.file-portability.html#svn.advanced.props.special.eol-style" title="End-of-Line Character Sequences">the section called “End-of-Line Character Sequences”</a> for more
details). By default, Subversion treats all file data as
literal byte strings, and files are always stored in the
repository in an untranslated state.</p>
<p>Second, Subversion maintains an internal notion of whether a
file is <span class="quote">“<span class="quote">text</span>”</span> or <span class="quote">“<span class="quote">binary</span>”</span> data, but
this notion is <span class="emphasis"><em>only</em></span> extant in the working
copy. During an <span class="command"><strong>svn update</strong></span>, Subversion will
perform contextual merges on locally modified text files, but
will not attempt to do so for binary files.</p>
<p>To determine whether a contextual merge is possible,
Subversion examines the <code class="literal">svn:mime-type</code>
property. If the file has no <code class="literal">svn:mime-type</code>
property, or has a MIME type that is textual (e.g.,
<code class="literal">text/*</code>),
Subversion assumes it is text. Otherwise, Subversion assumes
the file is binary. Subversion also helps users by running a
binary-detection algorithm in the <span class="command"><strong>svn import</strong></span>
and <span class="command"><strong>svn add</strong></span> commands. These commands will
make a good guess and then (possibly) set a binary
<code class="literal">svn:mime-type</code> property on the file being
added. (If Subversion guesses wrong, the user can always remove
or hand-edit the property.)</p>
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