fstream bug ??

[Jonathan Wakely]

On 12 May 2010 17:10, Theodore Papadopoulo wrote:
> I have a behaviour with g++-4.4.3 that I do not understand...
> The following program does not do what I would expect:
>
> #include <string>
> #include <iostream>
> #include <fstream>
> #include <unistd.h>
>
> int main() {
>    std::string str;
>    std::fstream rw("/tmp/toto");
>    rw >> str;
>    std::cout << str << std::endl;
>    rw << "Streams are complicated." << std::endl;
>    sleep(1);
>    return 0;
> }
>
> This program reads whatever is the first word in the file /tmp/toto but
> never writes
> the "Streams are complicated." (checked with strace).
>
> If I swap the read and the write, ie:
>
> int main() {
>    std::string str;
>    std::fstream rw("/tmp/toto");
>    rw << "Streams are complicated." << std::endl;
>    rw >> str;
>    std::cout << str << std::endl;
>    sleep(1);
>    return 0;
> }
>
> then it writes but never reads...
> I use streams a lot, but it's probably the first time I use bidirectional
> streams, so
> there might be sthg I do not understand, but at first sight it is really
> surprising
> and looks like a bug. Can someone confirm this before I file it in bugzilla
> ??

This is a property of file streams, not streams in general. Filebufs
need a seek (either pubseekoff or pubseekpos) when switching between
reading and writing

e.g.

   rw >> str;
   std::cout << str << std::endl;
   rw.rdbuf()->pubseekoff(0, std::ios::end);
   rw << "Streams are complicated." << std::endl;

>
>    Theo.
>