gcc 3.4.3, stdio_filebuf problem

[Ralf Fassel]

gcc 3.4.3, HP-UX 10.20

I got a weird problem with the following program on HP-10.20 w/ gcc
3.4.3 (but not with gcc 3.4.1):

    #include <iostream>
    #include <ext/stdio_filebuf.h>

    typedef __gnu_cxx::stdio_filebuf<char> my_strbuf;

    class mstream : public std::iostream
    {
    public:
      mstream() : std::iostream(NULL) {
        my_strbuf *strbuf = new my_strbuf(stdin, std::ios::in);
        rdbuf(strbuf);
      }
    };

    int
    main(int argc, char *argv[]) {

      size_t count = 2048, total = 0, blocks = 0;
      if (argc > 1) count = atoi(argv[1]);

      char buffer[count];
      mstream inp;

      while (!inp.eof()) {
        inp.read(buffer, count);
        total += inp.gcount();
        blocks++;
        if (count != inp.gcount()) {
          std::cerr << "wanted " << count << ", got " << inp.gcount() << 
std::endl;
        }
      }
      std::cerr << "total bytes read " << total << ", blocks " << blocks << 
std::endl;
    }

The idea is to attach stdin (or any other FILE*) to the iostream via
the rdbuf() created from the FILE*.

Now this runs ok _except_ when I try to read buffers using an
`unusual' byte count in a pipe:

redirected from file, ok:
    % ./t343 < big_file
    ./t343 < big_file
    wanted 2048, got 1712
    total bytes read 11601584, blocks 5665

redirected from file, byte count 2084, ok:
    % ./t343 2084 < big_file
    ./t343 2084 < big_file
    wanted 2084, got 2040
    total bytes read 11601584, blocks 5567

in pipe, ok:
    % cat big_file | ./t343
    cat big_file | ./t343
    wanted 2048, got 1712
    total bytes read 11601584, blocks 5665

in pipe, byte count 2084, NOT OK:
    % cat big_file | ./t343 2084
    cat big_file | ./t343 2084
    wanted 2084, got 1940
    total bytes read 8192, blocks 4

Note the short read in the last case.

I had seen the same effect on Linux, but only once.  When trying to
reproduce it, the error went away :-(.  But on HP, it is definitely
reproducable.  The same program compiled with gcc 3.4.1 runs ok on the
very same machine.

Any ideas?

R'