+++ /dev/null
-<?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>
OSDN Git Service
