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561 lines
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)">
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<meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL">
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<meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 27.">
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<meta name="GENERATOR" content="vi and eight fingers">
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<title>libstdc++-v3 HOWTO: Chapter 27</title>
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<link rel="StyleSheet" href="../lib3styles.css">
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</head>
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<body>
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<h1 class="centered"><a name="top">Chapter 27: Input/Output</a></h1>
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<p>Chapter 27 deals with iostreams and all their subcomponents
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and extensions. All <em>kinds</em> of fun stuff.
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</p>
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<!-- ####################################################### -->
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<hr>
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<h1>Contents</h1>
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<ul>
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<li><a href="#1">Copying a file</a>
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<li><a href="#2">The buffering is screwing up my program!</a>
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<li><a href="#3">Binary I/O</a>
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<li><a href="#5">What is this <sstream>/stringstreams thing?</a>
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<li><a href="#6">Deriving a stream buffer</a>
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<li><a href="#7">More on binary I/O</a>
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<li><a href="#8">Pathetic performance? Ditch C.</a>
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<li><a href="#9">Threads and I/O</a>
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</ul>
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<hr>
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<!-- ####################################################### -->
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<h2><a name="1">Copying a file</a></h2>
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<p>So you want to copy a file quickly and easily, and most important,
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completely portably. And since this is C++, you have an open
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ifstream (call it IN) and an open ofstream (call it OUT):
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<pre>
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#include <fstream>
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std::ifstream IN ("input_file");
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std::ofstream OUT ("output_file"); </pre>
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</p>
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<p>Here's the easiest way to get it completely wrong:
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<pre>
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OUT << IN;</pre>
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For those of you who don't already know why this doesn't work
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(probably from having done it before), I invite you to quickly
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create a simple text file called "input_file" containing
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the sentence
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<pre>
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The quick brown fox jumped over the lazy dog.</pre>
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surrounded by blank lines. Code it up and try it. The contents
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of "output_file" may surprise you.
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</p>
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<p>Seriously, go do it. Get surprised, then come back. It's worth it.
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</p>
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<hr width="60%">
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<p>The thing to remember is that the <code>basic_[io]stream</code> classes
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handle formatting, nothing else. In particular, they break up on
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whitespace. The actual reading, writing, and storing of data is
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handled by the <code>basic_streambuf</code> family. Fortunately, the
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<code>operator<<</code> is overloaded to take an ostream and
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a pointer-to-streambuf, in order to help with just this kind of
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"dump the data verbatim" situation.
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</p>
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<p>Why a <em>pointer</em> to streambuf and not just a streambuf? Well,
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the [io]streams hold pointers (or references, depending on the
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implementation) to their buffers, not the actual
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buffers. This allows polymorphic behavior on the part of the buffers
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as well as the streams themselves. The pointer is easily retrieved
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using the <code>rdbuf()</code> member function. Therefore, the easiest
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way to copy the file is:
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<pre>
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OUT << IN.rdbuf();</pre>
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</p>
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<p>So what <em>was</em> happening with OUT<<IN? Undefined
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behavior, since that particular << isn't defined by the Standard.
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I have seen instances where it is implemented, but the character
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extraction process removes all the whitespace, leaving you with no
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blank lines and only "Thequickbrownfox...". With
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libraries that do not define that operator, IN (or one of IN's
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member pointers) sometimes gets converted to a void*, and the output
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file then contains a perfect text representation of a hexidecimal
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address (quite a big surprise). Others don't compile at all.
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</p>
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<p>Also note that none of this is specific to o<B>*f*</B>streams.
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The operators shown above are all defined in the parent
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basic_ostream class and are therefore available with all possible
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descendents.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr>
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<h2><a name="2">The buffering is screwing up my program!</a></h2>
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<!--
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This is not written very well. I need to redo this section.
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-->
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<p>First, are you sure that you understand buffering? Particularly
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the fact that C++ may not, in fact, have anything to do with it?
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</p>
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<p>The rules for buffering can be a little odd, but they aren't any
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different from those of C. (Maybe that's why they can be a bit
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odd.) Many people think that writing a newline to an output
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stream automatically flushes the output buffer. This is true only
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when the output stream is, in fact, a terminal and not a file
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or some other device -- and <em>that</em> may not even be true
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since C++ says nothing about files nor terminals. All of that is
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system-dependant. (The "newline-buffer-flushing only occuring
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on terminals" thing is mostly true on Unix systems, though.)
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</p>
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<p>Some people also believe that sending <code>endl</code> down an
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output stream only writes a newline. This is incorrect; after a
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newline is written, the buffer is also flushed. Perhaps this
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is the effect you want when writing to a screen -- get the text
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out as soon as possible, etc -- but the buffering is largely
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wasted when doing this to a file:
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<pre>
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output << "a line of text" << endl;
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output << some_data_variable << endl;
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output << "another line of text" << endl; </pre>
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The proper thing to do in this case to just write the data out
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and let the libraries and the system worry about the buffering.
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If you need a newline, just write a newline:
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<pre>
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output << "a line of text\n"
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<< some_data_variable << '\n'
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<< "another line of text\n"; </pre>
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I have also joined the output statements into a single statement.
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You could make the code prettier by moving the single newline to
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the start of the quoted text on the thing line, for example.
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</p>
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<p>If you do need to flush the buffer above, you can send an
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<code>endl</code> if you also need a newline, or just flush the buffer
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yourself:
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<pre>
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output << ...... << flush; // can use std::flush manipulator
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output.flush(); // or call a member fn </pre>
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</p>
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<p>On the other hand, there are times when writing to a file should
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be like writing to standard error; no buffering should be done
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because the data needs to appear quickly (a prime example is a
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log file for security-related information). The way to do this is
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just to turn off the buffering <em>before any I/O operations at
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all</em> have been done, i.e., as soon as possible after opening:
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<pre>
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std::ofstream os ("/foo/bar/baz");
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std::ifstream is ("/qux/quux/quuux");
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int i;
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os.rdbuf()->pubsetbuf(0,0);
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is.rdbuf()->pubsetbuf(0,0);
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...
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os << "this data is written immediately\n";
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is >> i; // and this will probably cause a disk read </pre>
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</p>
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<p>Since all aspects of buffering are handled by a streambuf-derived
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member, it is necessary to get at that member with <code>rdbuf()</code>.
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Then the public version of <code>setbuf</code> can be called. The
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arguments are the same as those for the Standard C I/O Library
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function (a buffer area followed by its size).
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</p>
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<p>A great deal of this is implementation-dependant. For example,
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<code>streambuf</code> does not specify any actions for its own
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<code>setbuf()</code>-ish functions; the classes derived from
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<code>streambuf</code> each define behavior that "makes
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sense" for that class: an argument of (0,0) turns off buffering
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for <code>filebuf</code> but has undefined behavior for its sibling
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<code>stringbuf</code>, and specifying anything other than (0,0) has
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varying effects. Other user-defined class derived from streambuf can
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do whatever they want. (For <code>filebuf</code> and arguments for
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<code>(p,s)</code> other than zeros, libstdc++ does what you'd expect:
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the first <code>s</code> bytes of <code>p</code> are used as a buffer,
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which you must allocate and deallocate.)
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</p>
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<p>A last reminder: there are usually more buffers involved than
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just those at the language/library level. Kernel buffers, disk
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buffers, and the like will also have an effect. Inspecting and
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changing those are system-dependant.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr>
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<h2><a name="3">Binary I/O</a></h2>
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<p>The first and most important thing to remember about binary I/O is
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that opening a file with <code>ios::binary</code> is not, repeat
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<em>not</em>, the only thing you have to do. It is not a silver
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bullet, and will not allow you to use the <code><</>></code>
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operators of the normal fstreams to do binary I/O.
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</p>
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<p>Sorry. Them's the breaks.
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</p>
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<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 href="#7">covers a lot of ground)</a>, but we will try and clear
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up a couple of misconceptions and common errors.
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</p>
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<p>First, <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
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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
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normal mode does, but that's the most obvious.) Opening a file in
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binary mode disables this conversion, so reading a CRLF sequence
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under Windows won't accidentally get mapped to a '\n' character, etc.
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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).
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</p>
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<p>Second, using <code><<</code> to write and <code>>></code> to
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read isn't going to work with the standard file stream classes, even
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if you use <code>skipws</code> during reading. Why not? Because
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ifstream and ofstream exist for the purpose of <em>formatting</em>,
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not reading and writing. Their job is to interpret the data into
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text characters, and that's exactly what you don't want to happen
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during binary I/O.
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</p>
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<p>Third, using the <code>get()</code> and <code>put()/write()</code> member
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functions still aren't guaranteed to help you. These are
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"unformatted" I/O functions, but still character-based.
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(This may or may not be what you want, see below.)
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</p>
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<p>Notice how all the problems here are due to the inappropriate use
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of <em>formatting</em> functions and classes to perform something
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which <em>requires</em> that formatting not be done? There are a
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seemingly infinite number of solutions, and a few are listed here:
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<ul>
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<li>"Derive your own fstream-type classes and write your own
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<</>> operators to do binary I/O on whatever data
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types you're using." This is a Bad Thing, because while
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the compiler would probably be just fine with it, other humans
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are going to be confused. The overloaded bitshift operators
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have a well-defined meaning (formatting), and this breaks it.
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<li>"Build the file structure in memory, then <code>mmap()</code>
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the file and copy the structure." Well, this is easy to
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make work, and easy to break, and is pretty equivalent to
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using <code>::read()</code> and <code>::write()</code> directly, and
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makes no use of the iostream library at all...
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<li>"Use streambufs, that's what they're there for."
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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++
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library for binary I/O, this is where you start.
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</ul>
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</p>
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<p>How to go about using streambufs is a bit beyond the scope of this
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document (at least for now), but while streambufs go a long way,
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they still leave a couple of things up to you, the programmer.
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As an example, byte ordering is completely between you and the
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operating system, and you have to handle it yourself.
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</p>
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<p>Deriving a streambuf or filebuf
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class from the standard ones, one that is specific to your data
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types (or an abstraction thereof) is probably a good idea, and
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lots of examples exist in journals and on Usenet. Using the
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standard filebufs directly (either by declaring your own or by
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using the pointer returned from an fstream's <code>rdbuf()</code>)
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is certainly feasible as well.
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</p>
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<p>One area that causes problems is trying to do bit-by-bit operations
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with filebufs. C++ is no different from C in this respect: I/O
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must be done at the byte level. If you're trying to read or write
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a few bits at a time, you're going about it the wrong way. You
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must read/write an integral number of bytes and then process the
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bytes. (For example, the streambuf functions take and return
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variables of type <code>int_type</code>.)
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</p>
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<p>Another area of problems is opening text files in binary mode.
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Generally, binary mode is intended for binary files, and opening
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text files in binary mode means that you now have to deal with all of
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those end-of-line and end-of-file problems that we mentioned before.
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An instructive thread from comp.lang.c++.moderated delved off into
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this topic starting more or less at
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<a href="http://www.deja.com/getdoc.xp?AN=436187505">this</a>
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article and continuing to the end of the thread. (You'll have to
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sort through some flames every couple of paragraphs, but the points
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made are good ones.)
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</p>
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<hr>
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<h2><a name="5">What is this <sstream>/stringstreams thing?</a></h2>
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<p>Stringstreams (defined in the header <code><sstream></code>)
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are in this author's opinion one of the coolest things since
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sliced time. An example of their use is in the Received Wisdom
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section for Chapter 21 (Strings),
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<a href="../21_strings/howto.html#1.1internal"> describing how to
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format strings</a>.
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</p>
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<p>The quick definition is: they are siblings of ifstream and ofstream,
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and they do for <code>std::string</code> what their siblings do for
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files. All that work you put into writing <code><<</code> and
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<code>>></code> functions for your classes now pays off
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<em>again!</em> Need to format a string before passing the string
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to a function? Send your stuff via <code><<</code> to an
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ostringstream. You've read a string as input and need to parse it?
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Initialize an istringstream with that string, and then pull pieces
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out of it with <code>>></code>. Have a stringstream and need to
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get a copy of the string inside? Just call the <code>str()</code>
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member function.
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</p>
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<p>This only works if you've written your
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<code><<</code>/<code>>></code> functions correctly, though,
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and correctly means that they take istreams and ostreams as
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parameters, not i<B>f</B>streams and o<B>f</B>streams. If they
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take the latter, then your I/O operators will work fine with
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file streams, but with nothing else -- including stringstreams.
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</p>
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<p>If you are a user of the strstream classes, you need to update
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your code. You don't have to explicitly append <code>ends</code> to
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terminate the C-style character array, you don't have to mess with
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"freezing" functions, and you don't have to manage the
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memory yourself. The strstreams have been officially deprecated,
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which means that 1) future revisions of the C++ Standard won't
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support them, and 2) if you use them, people will laugh at you.
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</p>
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<hr>
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<h2><a name="6">Deriving a stream buffer</a></h2>
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<p>Creating your own stream buffers for I/O can be remarkably easy.
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If you are interested in doing so, we highly recommend two very
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excellent books: <em>Standard C++ IOStreams and Locales</em> by
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Langer and Kreft, ISBN 0-201-18395-1, and
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<a href="http://www.josuttis.com/libbook/">The C++ Standard Library</a>
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by Nicolai Josuttis, ISBN 0-201-37926-0. Both are published by
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Addison-Wesley, who isn't paying us a cent for saying that, honest.
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</p>
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<p>Here is a simple example, io/outbuf1, from the Josuttis text. It
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transforms everything sent through it to uppercase. This version
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assumes many things about the nature of the character type being
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used (for more information, read the books or the newsgroups):
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<pre>
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#include <iostream>
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#include <streambuf>
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#include <locale>
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#include <cstdio>
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class outbuf : public std::streambuf
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{
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protected:
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/* central output function
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* - print characters in uppercase mode
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*/
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virtual int_type overflow (int_type c) {
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if (c != EOF) {
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// convert lowercase to uppercase
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c = std::toupper(static_cast<char>(c),getloc());
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// and write the character to the standard output
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if (putchar(c) == EOF) {
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return EOF;
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}
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}
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return c;
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}
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};
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int main()
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{
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// create special output buffer
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outbuf ob;
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// initialize output stream with that output buffer
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std::ostream out(&ob);
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out << "31 hexadecimal: "
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<< std::hex << 31 << std::endl;
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return 0;
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}
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</pre>
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Try it yourself!
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</p>
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<hr>
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<h2><a name="7">More on binary I/O</a></h2>
|
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<p>Towards the beginning of February 2001, the subject of
|
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"binary" I/O was brought up in a couple of places at the
|
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same time. One notable place was Usenet, where James Kanze and
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Dietmar Kühl separately posted articles on why attempting
|
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generic binary I/O was not a good idea. (Here are copies of
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<a href="binary_iostreams_kanze.txt">Kanze's article</a> and
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<a href="binary_iostreams_kuehl.txt">Kühl's article</a>.)
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</p>
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<p>Briefly, the problems of byte ordering and type sizes mean that
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the unformatted functions like <code>ostream::put()</code> and
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<code>istream::get()</code> cannot safely be used to communicate
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between arbitrary programs, or across a network, or from one
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invocation of a program to another invocation of the same program
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on a different platform, etc.
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</p>
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<p>The entire Usenet thread is instructive, and took place under the
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subject heading "binary iostreams" on both comp.std.c++
|
|
and comp.lang.c++.moderated in parallel. Also in that thread,
|
|
Dietmar Kühl mentioned that he had written a pair of stream
|
|
classes that would read and write XDR, which is a good step towards
|
|
a portable binary format.
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|
</p>
|
|
|
|
<hr>
|
|
<h2><a name="8">Pathetic performance? Ditch C.</a></h2>
|
|
<p>It sounds like a flame on C, but it isn't. Really. Calm down.
|
|
I'm just saying it to get your attention.
|
|
</p>
|
|
<p>Because the C++ library includes the C library, both C-style and
|
|
C++-style I/O have to work at the same time. For example:
|
|
<pre>
|
|
#include <iostream>
|
|
#include <cstdio>
|
|
|
|
std::cout << "Hel";
|
|
std::printf ("lo, worl");
|
|
std::cout << "d!\n";
|
|
</pre>
|
|
This must do what you think it does.
|
|
</p>
|
|
<p>Alert members of the audience will immediately notice that buffering
|
|
is going to make a hash of the output unless special steps are taken.
|
|
</p>
|
|
<p>The special steps taken by libstdc++, at least for version 3.0,
|
|
involve doing very little buffering for the standard streams, leaving
|
|
most of the buffering to the underlying C library. (This kind of
|
|
thing is <a href="../explanations.html#cstdio">tricky to get right</a>.)
|
|
The upside is that correctness is ensured. The downside is that
|
|
writing through <code>cout</code> can quite easily lead to awful
|
|
performance when the C++ I/O library is layered on top of the C I/O
|
|
library (as it is for 3.0 by default). Some patches are in the
|
|
works which should improve the situation for 3.1.
|
|
</p>
|
|
<p>However, the C and C++ standard streams only need to be kept in sync
|
|
when both libraries' facilities are in use. If your program only uses
|
|
C++ I/O, then there's no need to sync with the C streams. The right
|
|
thing to do in this case is to call
|
|
<pre>
|
|
#include <em>any of the I/O headers such as ios, iostream, etc</em>
|
|
|
|
std::ios::sync_with_stdio(false);
|
|
</pre>
|
|
</p>
|
|
<p>You must do this before performing any I/O via the C++ stream objects.
|
|
Once you call this, the C++ streams will operate independantly of the
|
|
(unused) C streams. For GCC 3.0, this means that <code>cout</code> and
|
|
company will become fully buffered on their own.
|
|
</p>
|
|
<p>Note, by the way, that the synchronization requirement only applies to
|
|
the standard streams (<code>cin</code>, <code>cout</code>,
|
|
<code>cerr</code>,
|
|
<code>clog</code>, and their wide-character counterparts). File stream
|
|
objects that you declare yourself have no such requirement and are fully
|
|
buffered.
|
|
</p>
|
|
|
|
<hr>
|
|
<h2><a name="9">Threads and I/O</a></h2>
|
|
<p>I'll assume that you have already read the
|
|
<a href="../17_intro/howto.html#3">general notes on library threads</a>,
|
|
and the
|
|
<a href="../23_containers/howto.html#3">notes on threaded container
|
|
access</a> (you might not think of an I/O stream as a container, but
|
|
the points made there also hold here). If you have not read them,
|
|
please do so first.
|
|
</p>
|
|
<p>This gets a bit tricky. Please read carefully, and bear with me.
|
|
</p>
|
|
<h3>Structure</h3>
|
|
<p>As described <a href="../explanations.html#cstdio">here</a>, a wrapper
|
|
type called <code>__basic_file</code> provides our abstraction layer
|
|
for the <code>std::filebuf</code> classes. Nearly all decisions dealing
|
|
with actual input and output must be made in <code>__basic_file</code>.
|
|
</p>
|
|
<p>A generic locking mechanism is somewhat in place at the filebuf layer,
|
|
but is not used in the current code. Providing locking at any higher
|
|
level is akin to providing locking within containers, and is not done
|
|
for the same reasons (see the links above).
|
|
</p>
|
|
<h3>The defaults for 3.0.x</h3>
|
|
<p>The __basic_file type is simply a collection of small wrappers around
|
|
the C stdio layer (again, see the link under Structure). We do no
|
|
locking ourselves, but simply pass through to calls to <code>fopen</code>,
|
|
<code>fwrite</code>, and so forth.
|
|
</p>
|
|
<p>So, for 3.0, the question of "is multithreading safe for I/O"
|
|
must be answered with, "is your platform's C library threadsafe
|
|
for I/O?" Some are by default, some are not; many offer multiple
|
|
implementations of the C library with varying tradeoffs of threadsafety
|
|
and efficiency. You, the programmer, are always required to take care
|
|
with multiple threads.
|
|
</p>
|
|
<p>(As an example, the POSIX standard requires that C stdio FILE*
|
|
operations are atomic. POSIX-conforming C libraries (e.g, on Solaris
|
|
and GNU/Linux) have an internal mutex to serialize operations on
|
|
FILE*s. However, you still need to not do stupid things like calling
|
|
<code>fclose(fs)</code> in one thread followed by an access of
|
|
<code>fs</code> in another.)
|
|
</p>
|
|
<p>So, if your platform's C library is threadsafe, then your
|
|
<code>fstream</code> I/O operations will be threadsafe at the lowest
|
|
level. For higher-level operations, such as manipulating the data
|
|
contained in the stream formatting classes (e.g., setting up callbacks
|
|
inside an <code>std::ofstream</code>), you need to guard such accesses
|
|
like any other critical shared resource.
|
|
</p>
|
|
<h3>The future</h3>
|
|
<p>As already mentioned <a href="../explanations.html#cstdio">here</a>, a
|
|
second choice is available for I/O implementations: libio. This is
|
|
disabled by default, and in fact will not currently work due to other
|
|
issues. It will be revisited, however.
|
|
</p>
|
|
<p>The libio code is a subset of the guts of the GNU libc (glibc) I/O
|
|
implementation. When libio is in use, the <code>__basic_file</code>
|
|
type is basically derived from FILE. (The real situation is more
|
|
complex than that... it's derived from an internal type used to
|
|
implement FILE. See libio/libioP.h to see scary things done with
|
|
vtbls.) The result is that there is no "layer" of C stdio
|
|
to go through; the filebuf makes calls directly into the same
|
|
functions used to implement <code>fread</code>, <code>fwrite</code>,
|
|
and so forth, using internal data structures. (And when I say
|
|
"makes calls directly," I mean the function is literally
|
|
replaced by a jump into an internal function. Fast but frightening.
|
|
*grin*)
|
|
</p>
|
|
<p>Also, the libio internal locks are used. This requires pulling in
|
|
large chunks of glibc, such as a pthreads implementation, and is one
|
|
of the issues preventing widespread use of libio as the libstdc++
|
|
cstdio implementation.
|
|
</p>
|
|
<p>But we plan to make this work, at least as an option if not a future
|
|
default. Platforms running a copy of glibc with a recent-enough
|
|
version will see calls from libstdc++ directly into the glibc already
|
|
installed. For other platforms, a copy of the libio subsection will
|
|
be built and included in libstdc++.
|
|
</p>
|
|
<h3>Alternatives</h3>
|
|
<p>Don't forget that other cstdio implemenations are possible. You could
|
|
easily write one to perform your own forms of locking, to solve your
|
|
"interesting" problems.
|
|
</p>
|
|
|
|
|
|
<!-- ####################################################### -->
|
|
|
|
<hr>
|
|
<p class="fineprint"><em>
|
|
See <a href="../17_intro/license.html">license.html</a> for copying conditions.
|
|
Comments and suggestions are welcome, and may be sent to
|
|
<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
|
|
</em></p>
|
|
|
|
|
|
</body>
|
|
</html>
|
|
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