| <?xml version="1.0" encoding="ISO-8859-1"?> |
| <!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" xml:lang="en" lang="en"> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> |
| <meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" /> |
| <meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL" /> |
| <meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 27." /> |
| <meta name="GENERATOR" content="vi and eight fingers" /> |
| <title>libstdc++-v3 HOWTO: Chapter 27: Input/Output</title> |
| <link rel="StyleSheet" href="../lib3styles.css" type="text/css" /> |
| <link rel="Start" href="../documentation.html" type="text/html" |
| title="GNU C++ Standard Library" /> |
| <link rel="Prev" href="../26_numerics/howto.html" type="text/html" |
| title="Numerics" /> |
| <link rel="Next" href="../ext/howto.html" type="text/html" |
| title="Extensions" /> |
| <link rel="Copyright" href="../17_intro/license.html" type="text/html" /> |
| <link rel="Help" href="../faq/index.html" type="text/html" title="F.A.Q." /> |
| </head> |
| <body> |
| |
| <h1 class="centered"><a name="top">Chapter 27: Input/Output</a></h1> |
| |
| <p>Chapter 27 deals with iostreams and all their subcomponents |
| and extensions. All <em>kinds</em> of fun stuff. |
| </p> |
| |
| |
| <!-- ####################################################### --> |
| <hr /> |
| <h1>Contents</h1> |
| <ul> |
| <li><a href="#1">Copying a file</a></li> |
| <li><a href="#2">The buffering is screwing up my program!</a></li> |
| <li><a href="#3">Binary I/O</a></li> |
| <li><a href="#5">What is this <sstream>/stringstreams thing?</a></li> |
| <li><a href="#6">Deriving a stream buffer</a></li> |
| <li><a href="#7">More on binary I/O</a></li> |
| <li><a href="#8">Pathetic performance? Ditch C.</a></li> |
| <li><a href="#9">Threads and I/O</a></li> |
| <li><a href="#10">Which header?</a></li> |
| <li><a href="#11">Using FILE*s and file descriptors with IOStreams</a></li> |
| </ul> |
| |
| <hr /> |
| |
| <!-- ####################################################### --> |
| |
| <h2><a name="1">Copying a file</a></h2> |
| <p>So you want to copy a file quickly and easily, and most important, |
| completely portably. And since this is C++, you have an open |
| ifstream (call it IN) and an open ofstream (call it OUT): |
| </p> |
| <pre> |
| #include <fstream> |
| |
| std::ifstream IN ("input_file"); |
| std::ofstream OUT ("output_file"); </pre> |
| <p>Here's the easiest way to get it completely wrong: |
| </p> |
| <pre> |
| OUT << IN;</pre> |
| <p>For those of you who don't already know why this doesn't work |
| (probably from having done it before), I invite you to quickly |
| create a simple text file called "input_file" containing |
| the sentence |
| </p> |
| <pre> |
| The quick brown fox jumped over the lazy dog.</pre> |
| <p>surrounded by blank lines. Code it up and try it. The contents |
| of "output_file" may surprise you. |
| </p> |
| <p>Seriously, go do it. Get surprised, then come back. It's worth it. |
| </p> |
| <hr width="60%" /> |
| <p>The thing to remember is that the <code>basic_[io]stream</code> classes |
| handle formatting, nothing else. In particular, they break up on |
| whitespace. The actual reading, writing, and storing of data is |
| handled by the <code>basic_streambuf</code> family. Fortunately, the |
| <code>operator<<</code> is overloaded to take an ostream and |
| a pointer-to-streambuf, in order to help with just this kind of |
| "dump the data verbatim" situation. |
| </p> |
| <p>Why a <em>pointer</em> to streambuf and not just a streambuf? Well, |
| the [io]streams hold pointers (or references, depending on the |
| implementation) to their buffers, not the actual |
| buffers. This allows polymorphic behavior on the part of the buffers |
| as well as the streams themselves. The pointer is easily retrieved |
| using the <code>rdbuf()</code> member function. Therefore, the easiest |
| way to copy the file is: |
| </p> |
| <pre> |
| OUT << IN.rdbuf();</pre> |
| <p>So what <em>was</em> happening with OUT<<IN? Undefined |
| behavior, since that particular << isn't defined by the Standard. |
| I have seen instances where it is implemented, but the character |
| extraction process removes all the whitespace, leaving you with no |
| blank lines and only "Thequickbrownfox...". With |
| libraries that do not define that operator, IN (or one of IN's |
| member pointers) sometimes gets converted to a void*, and the output |
| file then contains a perfect text representation of a hexidecimal |
| address (quite a big surprise). Others don't compile at all. |
| </p> |
| <p>Also note that none of this is specific to o<b>*f*</b>streams. |
| The operators shown above are all defined in the parent |
| basic_ostream class and are therefore available with all possible |
| descendents. |
| </p> |
| <p>Return <a href="#top">to top of page</a> or |
| <a href="../faq/index.html">to the FAQ</a>. |
| </p> |
| |
| <hr /> |
| <h2><a name="2">The buffering is screwing up my program!</a></h2> |
| <!-- |
| This is not written very well. I need to redo this section. |
| --> |
| <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 <em>that</em> 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>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> |
| 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> |
| 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>endl</code> if you also need a newline, or just flush the buffer |
| yourself: |
| </p> |
| <pre> |
| 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 <em>before any I/O operations at |
| all</em> have been done (note that opening counts as an I/O operation): |
| </p> |
| <pre> |
| 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>rdbuf()</code>. |
| Then the public version of <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>streambuf</code> does not specify any actions for its own |
| <code>setbuf()</code>-ish functions; the classes derived from |
| <code>streambuf</code> each define behavior that "makes |
| sense" for that class: an argument of (0,0) turns off buffering |
| for <code>filebuf</code> but does nothing at all for its siblings |
| <code>stringbuf</code> and <code>strstreambuf</code>, and specifying |
| anything other than (0,0) has varying effects. |
| User-defined classes derived from <code>streambuf</code> can |
| do whatever they want. (For <code>filebuf</code> and arguments for |
| <code>(p,s)</code> other than zeros, libstdc++ does what you'd expect: |
| the first <code>s</code> bytes of <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> |
| <p>Return <a href="#top">to top of page</a> or |
| <a href="../faq/index.html">to the FAQ</a>. |
| </p> |
| |
| <hr /> |
| <h2><a name="3">Binary I/O</a></h2> |
| <p>The first and most important thing to remember about binary I/O is |
| that opening a file with <code>ios::binary</code> is not, repeat |
| <em>not</em>, the only thing you have to do. It is not a silver |
| bullet, and will not allow you to use the <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" |
| <a href="#7">covers a lot of ground)</a>, but we will try and clear |
| up a couple of misconceptions and common errors. |
| </p> |
| <p>First, <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><<</code> to write and <code>>></code> to |
| read isn't going to work with the standard file stream classes, even |
| if you use <code>skipws</code> during reading. Why not? Because |
| ifstream and ofstream exist for the purpose of <em>formatting</em>, |
| 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>get()</code> and <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 <em>formatting</em> functions and classes to perform something |
| which <em>requires</em> that formatting not be done? There are a |
| seemingly infinite number of solutions, and a few are listed here: |
| </p> |
| <ul> |
| <li>"Derive your own fstream-type classes and write your own |
| <</>> operators to do binary I/O on whatever data |
| types you're using." 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. |
| </li> |
| <li>"Build the file structure in memory, then <code>mmap()</code> |
| the file and copy the structure." Well, this is easy to |
| make work, and easy to break, and is pretty equivalent to |
| using <code>::read()</code> and <code>::write()</code> directly, and |
| makes no use of the iostream library at all... |
| </li> |
| <li>"Use streambufs, that's what they're there for." |
| 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. |
| </li> |
| </ul> |
| <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>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>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. |
| An instructive thread from comp.lang.c++.moderated delved off into |
| this topic starting more or less at |
| <a href="http://groups.google.com/groups?oi=djq&selm=an_436187505">this</a> |
| article and continuing to the end of the thread. (You'll have to |
| sort through some flames every couple of paragraphs, but the points |
| made are good ones.) |
| </p> |
| |
| <hr /> |
| <h2><a name="5">What is this <sstream>/stringstreams thing?</a></h2> |
| <p>Stringstreams (defined in the header <code><sstream></code>) |
| are in this author's opinion one of the coolest things since |
| sliced time. An example of their use is in the Received Wisdom |
| section for Chapter 21 (Strings), |
| <a href="../21_strings/howto.html#1.1internal"> describing how to |
| format strings</a>. |
| </p> |
| <p>The quick definition is: they are siblings of ifstream and ofstream, |
| and they do for <code>std::string</code> what their siblings do for |
| files. All that work you put into writing <code><<</code> and |
| <code>>></code> functions for your classes now pays off |
| <em>again!</em> Need to format a string before passing the string |
| to a function? Send your stuff via <code><<</code> to an |
| ostringstream. You've read a string as input and need to parse it? |
| Initialize an istringstream with that string, and then pull pieces |
| out of it with <code>>></code>. Have a stringstream and need to |
| get a copy of the string inside? Just call the <code>str()</code> |
| member function. |
| </p> |
| <p>This only works if you've written your |
| <code><<</code>/<code>>></code> functions correctly, though, |
| and correctly means that they take istreams and ostreams as |
| parameters, not i<b>f</b>streams and o<b>f</b>streams. If they |
| take the latter, then your I/O operators will work fine with |
| file streams, but with nothing else -- including stringstreams. |
| </p> |
| <p>If you are a user of the strstream classes, you need to update |
| your code. You don't have to explicitly append <code>ends</code> to |
| terminate the C-style character array, you don't have to mess with |
| "freezing" functions, and you don't have to manage the |
| memory yourself. The strstreams have been officially deprecated, |
| which means that 1) future revisions of the C++ Standard won't |
| support them, and 2) if you use them, people will laugh at you. |
| </p> |
| |
| <hr /> |
| <h2><a name="6">Deriving a stream buffer</a></h2> |
| <p>Creating your own stream buffers for I/O can be remarkably easy. |
| If you are interested in doing so, we highly recommend two very |
| excellent books: |
| <a href="http://www.langer.camelot.de/iostreams.html">Standard C++ |
| IOStreams and Locales</a> by Langer and Kreft, ISBN 0-201-18395-1, and |
| <a href="http://www.josuttis.com/libbook/">The C++ Standard Library</a> |
| by Nicolai Josuttis, ISBN 0-201-37926-0. Both are published by |
| Addison-Wesley, who isn't paying us a cent for saying that, honest. |
| </p> |
| <p>Here is a simple example, io/outbuf1, from the Josuttis text. It |
| transforms everything sent through it to uppercase. This version |
| assumes many things about the nature of the character type being |
| used (for more information, read the books or the newsgroups): |
| </p> |
| <pre> |
| #include <iostream> |
| #include <streambuf> |
| #include <locale> |
| #include <cstdio> |
| |
| class outbuf : public std::streambuf |
| { |
| protected: |
| /* central output function |
| * - print characters in uppercase mode |
| */ |
| virtual int_type overflow (int_type c) { |
| if (c != EOF) { |
| // convert lowercase to uppercase |
| c = std::toupper(static_cast<char>(c),getloc()); |
| |
| // and write the character to the standard output |
| if (putchar(c) == EOF) { |
| return EOF; |
| } |
| } |
| return c; |
| } |
| }; |
| |
| int main() |
| { |
| // create special output buffer |
| outbuf ob; |
| // initialize output stream with that output buffer |
| std::ostream out(&ob); |
| |
| out << "31 hexadecimal: " |
| << std::hex << 31 << std::endl; |
| return 0; |
| } |
| </pre> |
| <p>Try it yourself! More examples can be found in 3.1.x code, in |
| <code>include/ext/*_filebuf.h</code>, and on |
| <a href="http://www.informatik.uni-konstanz.de/~kuehl/c++/iostream/">Dietmar |
| Kühl's IOStreams page</a>. |
| </p> |
| |
| <hr /> |
| <h2><a name="7">More on binary I/O</a></h2> |
| <p>Towards the beginning of February 2001, the subject of |
| "binary" I/O was brought up in a couple of places at the |
| same time. One notable place was Usenet, where James Kanze and |
| Dietmar Kühl separately posted articles on why attempting |
| generic binary I/O was not a good idea. (Here are copies of |
| <a href="binary_iostreams_kanze.txt">Kanze's article</a> and |
| <a href="binary_iostreams_kuehl.txt">Kühl's article</a>.) |
| </p> |
| <p>Briefly, the problems of byte ordering and type sizes mean that |
| the unformatted functions like <code>ostream::put()</code> and |
| <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> |
| <p>The entire Usenet thread is instructive, and took place under the |
| 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. |
| </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: |
| </p> |
| <pre> |
| #include <iostream> |
| #include <cstdio> |
| |
| std::cout << "Hel"; |
| std::printf ("lo, worl"); |
| std::cout << "d!\n"; |
| </pre> |
| <p>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 have been applied |
| which 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 |
| </p> |
| <pre> |
| #include <em>any of the I/O headers such as ios, iostream, etc</em> |
| |
| std::ios::sync_with_stdio(false); |
| </pre> |
| <p>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 <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 /> |
| <h2><a name="10">Which header?</a></h2> |
| <p>To minimize the time you have to wait on the compiler, it's good to |
| only include the headers you really need. Many people simply include |
| <iostream> when they don't need to -- and that can <em>penalize |
| your runtime as well.</em> Here are some tips on which header to use |
| for which situations, starting with the simplest. |
| </p> |
| <p><strong><iosfwd></strong> should be included whenever you simply |
| need the <em>name</em> of an I/O-related class, such as |
| "ofstream" or "basic_streambuf". Like the name |
| implies, these are forward declarations. (A word to all you fellow |
| old school programmers: trying to forward declare classes like |
| "class istream;" won't work. Look in the iosfwd header if |
| you'd like to know why.) For example, |
| </p> |
| <pre> |
| #include <iosfwd> |
| |
| class MyClass |
| { |
| .... |
| std::ifstream input_file; |
| }; |
| |
| extern std::ostream& operator<< (std::ostream&, MyClass&); |
| </pre> |
| <p><strong><ios></strong> declares the base classes for the entire |
| I/O stream hierarchy, std::ios_base and std::basic_ios<charT>, the |
| counting types std::streamoff and std::streamsize, the file |
| positioning type std::fpos, and the various manipulators like |
| std::hex, std::fixed, std::noshowbase, and so forth. |
| </p> |
| <p>The ios_base class is what holds the format flags, the state flags, |
| and the functions which change them (setf(), width(), precision(), |
| etc). You can also store extra data and register callback functions |
| through ios_base, but that has been historically underused. Anything |
| which doesn't depend on the type of characters stored is consolidated |
| here. |
| </p> |
| <p>The template class basic_ios is the highest template class in the |
| hierarchy; it is the first one depending on the character type, and |
| holds all general state associated with that type: the pointer to the |
| polymorphic stream buffer, the facet information, etc. |
| </p> |
| <p><strong><streambuf></strong> declares the template class |
| basic_streambuf, and two standard instantiations, streambuf and |
| wstreambuf. If you need to work with the vastly useful and capable |
| stream buffer classes, e.g., to create a new form of storage |
| transport, this header is the one to include. |
| </p> |
| <p><strong><istream></strong>/<strong><ostream></strong> are |
| the headers to include when you are using the >>/<< |
| interface, or any of the other abstract stream formatting functions. |
| For example, |
| </p> |
| <pre> |
| #include <istream> |
| |
| std::ostream& operator<< (std::ostream& os, MyClass& c) |
| { |
| return os << c.data1() << c.data2(); |
| } |
| </pre> |
| <p>The std::istream and std::ostream classes are the abstract parents of |
| the various concrete implementations. If you are only using the |
| interfaces, then you only need to use the appropriate interface header. |
| </p> |
| <p><strong><iomanip></strong> provides "extractors and inserters |
| that alter information maintained by class ios_base and its dervied |
| classes," such as std::setprecision and std::setw. If you need |
| to write expressions like <code>os << setw(3);</code> or |
| <code>is >> setbase(8);</code>, you must include <iomanip>. |
| </p> |
| <p><strong><sstream></strong>/<strong><fstream></strong> |
| declare the six stringstream and fstream classes. As they are the |
| standard concrete descendants of istream and ostream, you will already |
| know about them. |
| </p> |
| <p>Finally, <strong><iostream></strong> provides the eight standard |
| global objects (cin, cout, etc). To do this correctly, this header |
| also provides the contents of the <istream> and <ostream> |
| headers, but nothing else. The contents of this header look like |
| </p> |
| <pre> |
| #include <ostream> |
| #include <istream> |
| |
| namespace std |
| { |
| extern istream cin; |
| extern ostream cout; |
| .... |
| |
| // this is explained below |
| <strong>static ios_base::Init __foo;</strong> // not its real name |
| } |
| </pre> |
| <p>Now, the runtime penalty mentioned previously: the global objects |
| must be initialized before any of your own code uses them; this is |
| guaranteed by the standard. Like any other global object, they must |
| be initialized once and only once. This is typically done with a |
| construct like the one above, and the nested class ios_base::Init is |
| specified in the standard for just this reason. |
| </p> |
| <p>How does it work? Because the header is included before any of your |
| code, the <strong>__foo</strong> object is constructed before any of |
| your objects. (Global objects are built in the order in which they |
| are declared, and destroyed in reverse order.) The first time the |
| constructor runs, the eight stream objects are set up. |
| </p> |
| <p>The <code>static</code> keyword means that each object file compiled |
| from a source file containing <iostream> will have its own |
| private copy of <strong>__foo</strong>. There is no specified order |
| of construction across object files (it's one of those pesky NP |
| problems that make life so interesting), so one copy in each object |
| file means that the stream objects are guaranteed to be set up before |
| any of your code which uses them could run, thereby meeting the |
| requirements of the standard. |
| </p> |
| <p>The penalty, of course, is that after the first copy of |
| <strong>__foo</strong> is constructed, all the others are just wasted |
| processor time. The time spent is merely for an increment-and-test |
| inside a function call, but over several dozen or hundreds of object |
| files, that time can add up. (It's not in a tight loop, either.) |
| </p> |
| <p>The lesson? Only include <iostream> when you need to use one of |
| the standard objects in that source file; you'll pay less startup |
| time. Only include the header files you need to in general; your |
| compile times will go down when there's less parsing work to do. |
| </p> |
| |
| |
| <hr /> |
| <h2><a name="11">Using FILE*s and file descriptors with IOStreams</a></h2> |
| <!-- referenced by ext/howto.html#2, update link if numbering changes --> |
| <p>The v2 library included non-standard extensions to construct |
| <code>std::filebuf</code>s from C stdio types such as |
| <code>FILE*</code>s and POSIX file descriptors. |
| Today the recommended way to use stdio types with libstdc++-v3 |
| IOStreams is via the <code>stdio_filebuf</code> class (see below), |
| but earlier releases provided slightly different mechanisms. |
| </p> |
| <ul> |
| <li>3.0.x <code>filebuf</code>s have another ctor with this signature: |
| <br /> |
| <code>basic_filebuf(__c_file_type*, ios_base::openmode, int_type);</code> |
| <br />This comes in very handy in a number of places, such as |
| attaching Unix sockets, pipes, and anything else which uses file |
| descriptors, into the IOStream buffering classes. The three |
| arguments are as follows: |
| <ul> |
| <li><code>__c_file_type* F </code> |
| // the __c_file_type typedef usually boils down to stdio's FILE |
| </li> |
| <li><code>ios_base::openmode M </code> |
| // same as all the other uses of openmode |
| </li> |
| <li><code>int_type B </code> |
| // buffer size, defaults to BUFSIZ if not specified |
| </li> |
| </ul> |
| For those wanting to use file descriptors instead of FILE*'s, I |
| invite you to contemplate the mysteries of C's <code>fdopen()</code>. |
| </li> |
| <li>In library snapshot 3.0.95 and later, <code>filebuf</code>s bring |
| back an old extension: the <code>fd()</code> member function. The |
| integer returned from this function can be used for whatever file |
| descriptors can be used for on your platform. Naturally, the |
| library cannot track what you do on your own with a file descriptor, |
| so if you perform any I/O directly, don't expect the library to be |
| aware of it. |
| </li> |
| <li>Beginning with 3.1, the extra <code>filebuf</code> constructor and |
| the <code>fd()</code> function were removed from the standard |
| filebuf. Instead, <code><ext/stdio_filebuf.h></code> contains |
| a derived class called |
| <a href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/class____gnu__cxx_1_1stdio__filebuf.html"><code>__gnu_cxx::stdio_filebuf</code></a>. |
| This class can be constructed from a C <code>FILE*</code> or a file |
| descriptor, and provides the <code>fd()</code> function. |
| </li> |
| </ul> |
| <p>If you want to access a <code>filebuf</code>s file descriptor to |
| implement file locking (e.g. using the <code>fcntl()</code> system |
| call) then you might be interested in Henry Suter's |
| <a href="http://suter.home.cern.ch/suter/RWLock.html">RWLock</a> |
| class. |
| </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> |
| |
| |