C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  why is only cerr unitbuffered by default?          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Ganny Guest Posted: Thu Nov 24, 2005 8:10 am    Post subject: why is only cerr unitbuffered by default? why is cerr unitbuffered by default (cin and cout are not unitbuffered by default). Thanks! -Ganesh [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Maeder Guest Posted: Fri Nov 25, 2005 6:15 pm    Post subject: Re: why is only cerr unitbuffered by default? "Ganny" <sgganesh (AT) gmail (DOT) com> writes: Quote: why is cerr unitbuffered by default (cin and cout are not unitbuffered by default). If a process terminates unexpectedly, I am usually thankful for everything it wrote to std::cerr and std::clog; if they are unbuffered, nothing will be lost in an unflushed buffer. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Carl Barron Guest Posted: Sat Nov 26, 2005 2:30 pm    Post subject: Re: why is only cerr unitbuffered by default? Ganny <sgganesh (AT) gmail (DOT) com> wrote: Quote: why is cerr unitbuffered by default (cin and cout are not unitbuffered by default). Cerr is meant for error messaging, rather than normal output so is unitbuffered so that the output is presented now incase the stream never flushes or is destructed. cin,cout are not unit buffered for speed reasons. Its also how stderr/stdout of C work. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Sat Nov 26, 2005 2:35 pm    Post subject: Re: why is only cerr unitbuffered by default? Ganny wrote: Quote: why is cerr unitbuffered by default (cin and cout are not unitbuffered by default). Because cerr is used for outputting error text, which presumably has to appear immediately, and should also appear even if the program aborts immediately after. Typically (hopefully?) too, there will be very little output on cerr, so the loss of performance doesn't matter. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Sebastian Redl Guest Posted: Sat Nov 26, 2005 2:43 pm    Post subject: Re: why is only cerr unitbuffered by default? Ganny wrote: Quote: why is cerr unitbuffered by default (cin and cout are not unitbuffered by default). Efficiency vs security. Flushing the buffer can take some time (esp. on slow terminals, e.g. SSH connections over a slow network), so cout flushes as rarely as possible. (cin is an input stream, flushing it ought to have no effect.) However, when your program crashes, all buffered data is lost. This is especially bad for a stream that's supposed to convey error messages. This is why cerr is unitbuffered by default. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Vector Guest Posted: Sat Nov 26, 2005 2:47 pm    Post subject: Re: why is only cerr unitbuffered by default? I guess u meant unbuffered instead of unitbuffered...... cerr is like cout.It is basically an error stream that is attached to the console.You could have used cout there but the advantage with cerr is that it can be re-routed to a logfile and cout only points to the console screen. cerr is unbuffered, where the stream is flushed after every output operation. cout is not required to do this. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alberto Ganesh Barbati Guest Posted: Sat Nov 26, 2005 2:59 pm    Post subject: Re: why is only cerr unitbuffered by default? Thomas Maeder wrote: Quote: "Ganny" <sgganesh (AT) gmail (DOT) com> writes: why is cerr unitbuffered by default (cin and cout are not unitbuffered by default). If a process terminates unexpectedly, I am usually thankful for everything it wrote to std::cerr and std::clog; if they are unbuffered, nothing will be lost in an unflushed buffer. cerr is not unbuffered, it's unit-buffered, that means it will be flushed automatically after every output operation OTOH, clog is probably buffered (although it's not required nor guaranteed to be so) The presence of both of them allows the programmer to choose between two different approaches: clog for more efficient output (flush must be explicitly requested) but unsafe (in case of a crash, data in the buffer might be lost); cerr for safer output (buffer is flushed after every operation) but less efficient (stream may be flushed more often than really needed). For cout the standard gives only one choice: the more efficient one. That's probably because it's more likely that messages produced during a critical situation are output on cerr/clog rather than on cout. Anyway, the programmer can always set cout to unitbuf explicitly if he/she wants. For cin unitbuf doesn't make sense, because you can't flush an input stream. Ganesh [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alberto Ganesh Barbati Guest Posted: Sun Nov 27, 2005 4:04 am    Post subject: Re: why is only cerr unitbuffered by default? Vector wrote: Quote: I guess u meant unbuffered instead of unitbuffered...... No, the OP really meant unitbuffered. He is referring to the fact that cerr has the flag "unitbuf" set. Quote: cerr is like cout. It is basically an error stream that is attached to the console.You could have used cout there but the advantage with cerr is that it can be re-routed to a logfile and cout only points to the console screen. This is inaccurate. First of all cerr is unitbuffered, while cout is not, so they *are* different. Secondly, even cout can be re-routed, by calling the C function freopen on stdout. Quote: cerr is unbuffered, where the stream is flushed after every output operation. cout is not required to do this. That's the definition of unitbuffered. An unbuffered stream would be one where each character is written immediately to the "final" destination (whatever that is) before starting to process the subsequent character. That is roughly equivalent to a stream with only a 1-char buffer. A unitbuffered stream is different because an output operation may require several characters to complete and the stream is flushed as soon as the operation completes (precisely, in the destructor of the sentry object) and not after each single character. Ganesh [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Mon Nov 28, 2005 3:49 pm    Post subject: Re: why is only cerr unitbuffered by default? Alberto Ganesh Barbati wrote: Quote: Vector wrote: I guess u meant unbuffered instead of unitbuffered...... No, the OP really meant unitbuffered. He is referring to the fact that cerr has the flag "unitbuf" set. cerr is like cout. It is basically an error stream that is attached to the console.You could have used cout there but the advantage with cerr is that it can be re-routed to a logfile and cout only points to the console screen. This is inaccurate. First of all cerr is unitbuffered, while cout is not, so they *are* different. Secondly, even cout can be re-routed, by calling the C function freopen on stdout. More to the point, where they go depends on the system and context; I write server software, which runs deconnected from any terminal, and typically, both cout and cerr are connected to /dev/null; in cron jobs, they end up being mailed to the user. I don't know were cout and cerr go in a Windows program (or GUI programs under Unix), but there generally isn't an associated terminal there either to which they can be sent. Secondly, of course, freopen doesn't affect cout and cerr. You can reroute them by using the non-const basic_ios<>::rdbuf(), but typically, it is easier to just open a new file. Most of the time, rerouting is done by the system, before the program is actually started -- both the Windows shell and all of the Unix shells use "> file" in the command line to reroute cout to "file"; the Unix shells I use (ksh and bash) allow "2> file" to reroute cerr, but I don't know if this is possible in the Windows shell, and I don't think it is possible in Unix shells based on the csh. In fact, for programs invoked from the command line, it is fairly rare for cerr to be redirected, even in shells where it is possible, where as redirecting cout is rather common. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Mon Nov 28, 2005 3:58 pm    Post subject: Re: why is only cerr unitbuffered by default? Sebastian Redl wrote: Quote: Ganny wrote: why is cerr unitbuffered by default (cin and cout are not unitbuffered by default). Efficiency vs security. Flushing the buffer can take some time (esp. on slow terminals, e.g. SSH connections over a slow network), so cout flushes as rarely as possible. (cin is an input stream, flushing it ought to have no effect.) Flushing only transfers the data to the OS. It doesn't wait until the data is actually written to the media, unless the OS request to write data waits (which would be rather unusual in a modern OS). In practice, the difference in performance should be very slight, although this depends on the cost of a context switch on the machine. Note too that std::endl flushes the stream. So if you are using std::endl, it is the equivalent of using a line buffered stream, an option which is not otherwise available with iostreams. In C, stderr -- and stdout, if it is connected to an interactive stream, are at most line buffered; they cannot be fully buffered. The equivalent of this in C++ is to use std::endl, rather than 'n'. Personally, I would recommend this as general practice until you fully understand the issues of buffering (including system buffering), and even as a general default later. A quick test on my machine: to write a million records, each with 3 ints, a double and a 20 character string, takes roughly 45 seconds with full buffering, one minute using endl to simulate line buffering, and 2 minutes with unit buffering. And only 45 seconds if request no buffering what so ever -- a quick check, however, shows that the request is ignored by both g++ and Sun CC, this despite §27.8.1.4/10. I'm not sure why line buffering isn't available in C++, at least for output files. It makes far more sense to control this at the file level than in each output statement. Quote: However, when your program crashes, all buffered data is lost. This is especially bad for a stream that's supposed to convey error messages. This is why cerr is unitbuffered by default. And when the system crashes, data buffered by the OS is lost; even the fact that you've flushed it is no guarantee. During debugging, this is not a problem; it's your program which is crashing, and not the system. Depending on what your program does, this *may* be a problem once it is deployed; in my applications (servers with persistent data), it is usual to "write" to an ostringstream (or an ostrstream in older software), and then use a low level system function to do the synchronized physical write to disk. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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