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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>Buffering</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="streambufs.html" title="Chapter 25. Stream Buffers" /><link rel="prev" href="streambufs.html" title="Chapter 25. Stream Buffers" /><link rel="next" href="stringstreams.html" title="Chapter 26. Memory Based Streams" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Buffering</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="streambufs.html">Prev</a> </td><th width="60%" align="center">Chapter 25. Stream Buffers</th><td width="20%" align="right"> <a accesskey="n" href="stringstreams.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Buffering"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="io.streambuf.buffering"></a>Buffering</h2></div></div></div><p>First, are you sure that you understand buffering? Particularly
the fact that C++ may not, in fact, have anything to do with it?
</p><p>The rules for buffering can be a little odd, but they aren't any
different from those of C. (Maybe that's why they can be a bit
odd.) Many people think that writing a newline to an output
stream automatically flushes the output buffer. This is true only
when the output stream is, in fact, a terminal and not a file
or some other device -- and <span class="emphasis"><em>that</em></span> may not even be true
since C++ says nothing about files nor terminals. All of that is
system-dependent. (The "newline-buffer-flushing only occurring
on terminals" thing is mostly true on Unix systems, though.)
</p><p>Some people also believe that sending <code class="code">endl</code> down an
output stream only writes a newline. This is incorrect; after a
newline is written, the buffer is also flushed. Perhaps this
is the effect you want when writing to a screen -- get the text
out as soon as possible, etc -- but the buffering is largely
wasted when doing this to a file:
</p><pre class="programlisting">
output << "a line of text" << endl;
output << some_data_variable << endl;
output << "another line of text" << endl; </pre><p>The proper thing to do in this case to just write the data out
and let the libraries and the system worry about the buffering.
If you need a newline, just write a newline:
</p><pre class="programlisting">
output << "a line of text\n"
<< some_data_variable << '\n'
<< "another line of text\n"; </pre><p>I have also joined the output statements into a single statement.
You could make the code prettier by moving the single newline to
the start of the quoted text on the last line, for example.
</p><p>If you do need to flush the buffer above, you can send an
<code class="code">endl</code> if you also need a newline, or just flush the buffer
yourself:
</p><pre class="programlisting">
output << ...... << flush; // can use std::flush manipulator
output.flush(); // or call a member fn </pre><p>On the other hand, there are times when writing to a file should
be like writing to standard error; no buffering should be done
because the data needs to appear quickly (a prime example is a
log file for security-related information). The way to do this is
just to turn off the buffering <span class="emphasis"><em>before any I/O operations at
all</em></span> have been done (note that opening counts as an I/O operation):
</p><pre class="programlisting">
std::ofstream os;
std::ifstream is;
int i;
os.rdbuf()->pubsetbuf(0,0);
is.rdbuf()->pubsetbuf(0,0);
os.open("/foo/bar/baz");
is.open("/qux/quux/quuux");
...
os << "this data is written immediately\n";
is >> i; // and this will probably cause a disk read </pre><p>Since all aspects of buffering are handled by a streambuf-derived
member, it is necessary to get at that member with <code class="code">rdbuf()</code>.
Then the public version of <code class="code">setbuf</code> can be called. The
arguments are the same as those for the Standard C I/O Library
function (a buffer area followed by its size).
</p><p>A great deal of this is implementation-dependent. For example,
<code class="code">streambuf</code> does not specify any actions for its own
<code class="code">setbuf()</code>-ish functions; the classes derived from
<code class="code">streambuf</code> each define behavior that "makes
sense" for that class: an argument of (0,0) turns off buffering
for <code class="code">filebuf</code> but does nothing at all for its siblings
<code class="code">stringbuf</code> and <code class="code">strstreambuf</code>, and specifying
anything other than (0,0) has varying effects.
User-defined classes derived from <code class="code">streambuf</code> can
do whatever they want. (For <code class="code">filebuf</code> and arguments for
<code class="code">(p,s)</code> other than zeros, libstdc++ does what you'd expect:
the first <code class="code">s</code> bytes of <code class="code">p</code> are used as a buffer,
which you must allocate and deallocate.)
</p><p>A last reminder: there are usually more buffers involved than
just those at the language/library level. Kernel buffers, disk
buffers, and the like will also have an effect. Inspecting and
changing those are system-dependent.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="streambufs.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="streambufs.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="stringstreams.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 25. Stream Buffers </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 26. Memory Based Streams</td></tr></table></div></body></html>
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