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| <H2>30.4 Binary and Text Mode</H2> |
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| <P>The representation of text files varies among operating systems. For example, the end of a line in a UNIX environment is represented by the <I>linefeed</I> character <SAMP>`\n'</SAMP>. On some other systems, such as Microsoft Windows, the end of the line consists of two characters, <I>carriage</I> <I>return</I> <SAMP>'\r'</SAMP> and <I>linefeed</I> <SAMP>'\n'</SAMP>. The end of the file differs as well on these two operating systems. Peculiarities on other operating systems are also conceivable.</P> |
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| <P>To make programs more portable among operating systems, an automatic conversion can be done on input and output. The carriage return or linefeed sequence, for example, can be converted to a single <SAMP>`\n'</SAMP> character on input; the <SAMP>`\n'</SAMP> can be expanded to <SAMP>"\r\n"</SAMP> on output. This conversion mode is called <I>text mode</I>, as opposed to<I> binary mode</I>. In binary mode, no such conversions are performed.</P> |
| <P>The mode flag <SAMP>std::ios_base::binary</SAMP> has the effect of opening a file in binary mode. This has the effect described above; in other words, all automatic conversions, such as converting <SAMP>"\r\n"</SAMP> to <SAMP>`\n',</SAMP> are suppressed. [Basically, the binary mode flag is passed on to the respective operating system's service function, which means that in principle all system-specific conversions are suppressed, not only the carriage return / linefeed handling.]</P> |
| <P>If you must process a binary file, you should always set the <SAMP>binary</SAMP> mode flag, because most likely you do not want any kind of implicit, system-specific conversion performed.</P> |
| <P>The effect of the binary open mode is frequently misunderstood. It does <I>not </I>put the inserters and extractors into a binary mode, and hence suppress the formatting they usually perform. Binary input and output is done solely by <SAMP>basic_istream<>::read()</SAMP> and <SAMP>basic_ostream<>::write()</SAMP>.</P> |
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