C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Unexpected blocking and streambuf::in_avail()          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message lhyatt@princeton.edu Guest Posted: Thu Feb 09, 2006 11:06 am    Post subject: Unexpected blocking and streambuf::in_avail() Hello- I am having some strange difficulties getting interprocess communication to work. (The details are probably off topic here but its related to using named pipes on Linux). The short question is, if streambuf::in_avail() returns nonzero, is that supposed to guarantee that a call to get() will return immediately? I have code along these lines: ifstream f(filename); char c; if(f.rdbuf()->in_avail()>0) f.get(c); I would expect this to either do nothing or fill c immediately, which is the case on most systems, but on particular compilers the call to get() will block until the process writing to the FIFO terminates, in which case all the output it sent becomes available at once. (Even though the output process was sending unbuffered output). Curiously, if I do this: char buffer; f.rdbuf()->pubsetbuf(&buffer,1); before the read, then everything works as expected. (It doesn't work if I set the buffer size to 0 though, even though it is supposed to). Anyway, sorry if this was too much OS-specific information, but the bottom line question is, according to the standard, does in_avail() > 0 imply that get() should return immediately? Thanks to anyone who has any advice. -Lewis [ 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: Thu Feb 09, 2006 3:06 pm    Post subject: Re: Unexpected blocking and streambuf::in_avail() In article <1139439823.436995.77410 (AT) g47g2000cwa (DOT) googlegroups.com>, <"lhyatt (AT) princeton (DOT) edu"> wrote: Quote: Hello- I am having some strange difficulties getting interprocess communication to work. (The details are probably off topic here but its related to using named pipes on Linux). The short question is, if streambuf::in_avail() returns nonzero, is that supposed to guarantee that a call to get() will return immediately? I have code along these lines: ifstream f(filename); char c; if(f.rdbuf()->in_avail()>0) f.get(c); I would expect this to either do nothing or fill c immediately, which is the case on most systems, but on particular compilers the call to get() will block until the process writing to the FIFO terminates, in which case all the output it sent becomes available at once. (Even though the output process was sending unbuffered output). Curiously, if I do this: char buffer; f.rdbuf()->pubsetbuf(&buffer,1); before the read, then everything works as expected. (It doesn't work if I set the buffer size to 0 though, even though it is supposed to). Anyway, sorry if this was too much OS-specific information, but the bottom line question is, according to the standard, does in_avail() > 0 imply that get() should return immediately? Thanks to anyone who has any advice. -Lewis in_avail() should see if there is any data in the stream buffer's buffer and if any return the # characters therein. If not it calls showmanyc(). All input from your device in a buffered streambuf is done in underflow(). In_available() does if there is a buffer if the # of unread chars in the buffer > 0 return this number. else return showmanyc() showmanyc() is a virtual protected function that defaults to return 0. [supposed to use a device dependent estimate of the # of chars to read without blocking] If you are interested see Langer and Kreft's book on the iostream library [includes streambuf] and locales. It is worth reading by anyone with more than a passing interests of what the IO stream and/or locale libraries provide. Sorry I don't have it handy to quote title or isbn... [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top lhyatt@princeton.edu Guest Posted: Thu Feb 09, 2006 6:06 pm    Post subject: Re: Unexpected blocking and streambuf::in_avail() OK, thanks for the reply. I gather from what you've said that if in_avail() returns nonzero, but input blocks anyway, it must be the case that the nonzero value is coming from showmanyc()... so it appears that showmanyc() on this particular platform is not reliable. It still doesn't explain why the input from a named pipe blocks when read with iostreams, but not with cstdio, though, although I suppose that is a UNIX question. [ 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: Fri Feb 10, 2006 3:06 am    Post subject: Re: Unexpected blocking and streambuf::in_avail() Carl Barron ha scritto: Quote: All input from your device in a buffered streambuf is done in underflow(). In_available() does if there is a buffer if the # of unread chars in the buffer > 0 return this number. else return showmanyc() showmanyc() is a virtual protected function that defaults to return 0. [supposed to use a device dependent estimate of the # of chars to read without blocking] Your interpretation of showmanyc() is not correct. showmanyc() returns "an estimate of the number of characters available in the sequence, or -1. If it returns a positive value, then successive calls to underflow() will not return traits::eof() until at least that number of characters have been extracted from the stream." So if the buffer is empty and in_avail() == showmanyc() returns a positive number, get() will eventually call underflow(), which is potentially blocking, in order to extract the next character. I'm afraid but AFAIK there's no way to do a non-blocking get or peek that relies only on the public interface of basic_streambuf. That's really a pity, indeed. HTH, Ganesh [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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