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<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <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 V1.75.2" /><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="io_and_c.html" title="Chapter 28. Interacting with C" /></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="io_and_c.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Binary Input and Output"><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> </p><p>The first and most important thing to remember about binary I/O is that opening a file with <code class="code">ios::binary</code> is not, repeat <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> operators of the normal fstreams to do binary I/O. </p><p>Sorry. Them's the breaks. </p><p>This isn't going to try and be a complete tutorial on reading and writing binary files (because "binary" covers a lot of ground), but we will try and clear up a couple of misconceptions and common errors. </p><p>First, <code class="code">ios::binary</code> has exactly one defined effect, no more and no less. Normal text mode has to be concerned with the newline characters, and the runtime system will translate between (for example) '\n' and the appropriate end-of-line sequence (LF on Unix, 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 if it is doing so in your program then you've discovered a bug in your vendor's compiler (or some other part of the C++ implementation, possibly the runtime system). </p><p>Second, using <code class="code"><<</code> to write and <code class="code">>></code> to 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>, 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 "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 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 class="itemizedlist" type="disc"><li class="listitem"><p><span class="quote">“<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>”</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 have a well-defined meaning (formatting), and this breaks it. </p></li><li class="listitem"><p> <span class="quote">“<span class="quote">Build the file structure in memory, then <code class="code">mmap()</code> the file and copy the structure. </span>”</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 class="listitem"><p> <span class="quote">“<span class="quote">Use streambufs, that's what they're there for.</span>”</span> </p><p> While not trivial for the beginner, this is the best of all solutions. The streambuf/filebuf layer is the layer that is 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. </p><p> 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/group/comp.std.c++/browse_thread/thread/f87b4abd7954a87/946a3eb9921e382d?q=comp.std.c%2B%2B+binary+iostream#946a3eb9921e382d" target="_top">this</a> post and continuing to the end of the thread. (The subject heading is "binary iostreams" on both comp.std.c++ and comp.lang.c++.moderated.) Take special note of the replies by James Kanze and Dietmar Kühl. </p><p>Briefly, the problems of byte ordering and type sizes mean that the unformatted functions like <code class="code">ostream::put()</code> and <code class="code">istream::get()</code> cannot safely be used to communicate between arbitrary programs, or across a network, or from one invocation of a program to another invocation of the same program on a different platform, etc. </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="io_and_c.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"> Chapter 28. Interacting with C</td></tr></table></div></body></html>
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