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| -<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Binary Input and Output</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="fstreams.html" title="Chapter 27. File Based Streams" /><link rel="prev" href="fstreams.html" title="Chapter 27. File Based Streams" /><link rel="next" href="bk01pt11ch27s03.html" title="More Binary Input and Output" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Binary Input and Output</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="fstreams.html">Prev</a> </td><th width="60%" align="center">Chapter 27. File Based Streams</th><td width="20%" align="right"> <a accesskey="n" href="bk01pt11ch27s03.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.io.filestreams.binary"></a>Binary Input and Output</h2></div></div></div><p>
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| - </p><p>The first and most important thing to remember about binary I/O is
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| - that opening a file with <code class="code">ios::binary</code> is not, repeat
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| - <span class="emphasis"><em>not</em></span>, the only thing you have to do. It is not a silver
|
| - bullet, and will not allow you to use the <code class="code"><</>></code>
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| - operators of the normal fstreams to do binary I/O.
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| - </p><p>Sorry. Them's the breaks.
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| - </p><p>This isn't going to try and be a complete tutorial on reading and
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| - writing binary files (because "binary"
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| - <a class="ulink" href="#7" target="_top">covers a lot of ground)</a>, but we will try and clear
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| - up a couple of misconceptions and common errors.
|
| - </p><p>First, <code class="code">ios::binary</code> has exactly one defined effect, no more
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| - and no less. Normal text mode has to be concerned with the newline
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| - characters, and the runtime system will translate between (for
|
| - example) '\n' and the appropriate end-of-line sequence (LF on Unix,
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| - CRLF on DOS, CR on Macintosh, etc). (There are other things that
|
| - normal mode does, but that's the most obvious.) Opening a file in
|
| - binary mode disables this conversion, so reading a CRLF sequence
|
| - under Windows won't accidentally get mapped to a '\n' character, etc.
|
| - Binary mode is not supposed to suddenly give you a bitstream, and
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| - if it is doing so in your program then you've discovered a bug in
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| - your vendor's compiler (or some other part of the C++ implementation,
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| - possibly the runtime system).
|
| - </p><p>Second, using <code class="code"><<</code> to write and <code class="code">>></code> to
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| - read isn't going to work with the standard file stream classes, even
|
| - if you use <code class="code">skipws</code> during reading. Why not? Because
|
| - ifstream and ofstream exist for the purpose of <span class="emphasis"><em>formatting</em></span>,
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| - not reading and writing. Their job is to interpret the data into
|
| - text characters, and that's exactly what you don't want to happen
|
| - during binary I/O.
|
| - </p><p>Third, using the <code class="code">get()</code> and <code class="code">put()/write()</code> member
|
| - functions still aren't guaranteed to help you. These are
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| - "unformatted" I/O functions, but still character-based.
|
| - (This may or may not be what you want, see below.)
|
| - </p><p>Notice how all the problems here are due to the inappropriate use
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| - of <span class="emphasis"><em>formatting</em></span> functions and classes to perform something
|
| - which <span class="emphasis"><em>requires</em></span> that formatting not be done? There are a
|
| - seemingly infinite number of solutions, and a few are listed here:
|
| - </p><div class="itemizedlist"><ul type="disc"><li><p>“<span class="quote">Derive your own fstream-type classes and write your own
|
| - <</>> operators to do binary I/O on whatever data
|
| - types you're using.</span>”
|
| - </p><p>
|
| - This is a Bad Thing, because while
|
| - the compiler would probably be just fine with it, other humans
|
| - are going to be confused. The overloaded bitshift operators
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| - have a well-defined meaning (formatting), and this breaks it.
|
| - </p></li><li><p>
|
| - “<span class="quote">Build the file structure in memory, then
|
| - <code class="code">mmap()</code> the file and copy the
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| - structure.
|
| - </span>”
|
| - </p><p>
|
| - Well, this is easy to make work, and easy to break, and is
|
| - pretty equivalent to using <code class="code">::read()</code> and
|
| - <code class="code">::write()</code> directly, and makes no use of the
|
| - iostream library at all...
|
| - </p></li><li><p>
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| - “<span class="quote">Use streambufs, that's what they're there for.</span>”
|
| - </p><p>
|
| - While not trivial for the beginner, this is the best of all
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| - solutions. The streambuf/filebuf layer is the layer that is
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| - responsible for actual I/O. If you want to use the C++
|
| - library for binary I/O, this is where you start.
|
| - </p></li></ul></div><p>How to go about using streambufs is a bit beyond the scope of this
|
| - document (at least for now), but while streambufs go a long way,
|
| - they still leave a couple of things up to you, the programmer.
|
| - As an example, byte ordering is completely between you and the
|
| - operating system, and you have to handle it yourself.
|
| - </p><p>Deriving a streambuf or filebuf
|
| - class from the standard ones, one that is specific to your data
|
| - types (or an abstraction thereof) is probably a good idea, and
|
| - lots of examples exist in journals and on Usenet. Using the
|
| - standard filebufs directly (either by declaring your own or by
|
| - using the pointer returned from an fstream's <code class="code">rdbuf()</code>)
|
| - is certainly feasible as well.
|
| - </p><p>One area that causes problems is trying to do bit-by-bit operations
|
| - with filebufs. C++ is no different from C in this respect: I/O
|
| - must be done at the byte level. If you're trying to read or write
|
| - a few bits at a time, you're going about it the wrong way. You
|
| - must read/write an integral number of bytes and then process the
|
| - bytes. (For example, the streambuf functions take and return
|
| - variables of type <code class="code">int_type</code>.)
|
| - </p><p>Another area of problems is opening text files in binary mode.
|
| - Generally, binary mode is intended for binary files, and opening
|
| - text files in binary mode means that you now have to deal with all of
|
| - those end-of-line and end-of-file problems that we mentioned before.
|
| - An instructive thread from comp.lang.c++.moderated delved off into
|
| - this topic starting more or less at
|
| - <a class="ulink" href="http://groups.google.com/groups?oi=djq&selm=an_436187505" target="_top">this</a>
|
| - article and continuing to the end of the thread. (You'll have to
|
| - sort through some flames every couple of paragraphs, but the points
|
| - made are good ones.)
|
| - </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="fstreams.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="fstreams.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="bk01pt11ch27s03.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 27. File Based Streams </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> More Binary Input and Output</td></tr></table></div></body></html>
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