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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> |
| </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" |
| <a class="ulink" href="#7" target="_top">covers a lot of ground)</a>, 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 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 |
| 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 |
| 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> |
| “<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 |
| 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. |
| 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.) |
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