Keywords: ISO C++, library

Interacting with C
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## Interacting with C

### Using FILE* and file descriptors

See the extensions for using FILE and file descriptors with `ofstream` and `ifstream`.

### Performance

Pathetic Performance? Ditch C.

It sounds like a flame on C, but it isn't. Really. Calm down. I'm just saying it to get your attention.

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:

         #include <iostream>
         #include <cstdio>

         std::cout << "Hel";
         std::printf ("lo, worl");
         std::cout << "d!\n";

This must do what you think it does.

Alert members of the audience will immediately notice that buffering is going to make a hash of the output unless special steps are taken.

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 tricky to get right.) The upside is that correctness is ensured. The downside is that writing through `cout` 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 have been applied which improve the situation for 3.1.

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

         #include any of the I/O headers such as ios, iostream, etc

         std::ios::sync_with_stdio(false);

You must do this before performing any I/O via the C++ stream objects. Once you call this, the C++ streams will operate independently of the (unused) C streams. For GCC 3.x, this means that `cout` and company will become fully buffered on their own.

Note, by the way, that the synchronization requirement only applies to the standard streams (`cin`, `cout`, `cerr`, `clog`, and their wide-character counterchapters). File stream objects that you declare yourself have no such requirement and are fully buffered.

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