[Bug libstdc++/42857] New: std::istream::ignore(std::streamsize n) calls unnecessary underflow

["tommi at tntnet dot org" <gcc-bugzilla at gcc dot gnu dot org>]

When ignoring the number of bytes available in a streambuf using
std::istream::ignore, ignore calls underflow. This should not happen. I feel
the easiest way to reproduce it is to look at strace output when ignoring bytes
from ifstream:

#include <iostream>
#include <fstream>

int main(int argc, char* argv[])
{
  std::ifstream in(argv[0]);
  in.get(); // trigger filling of input buffer by calling underflow
  in.ignore(in.rdbuf()->in_avail());  // this triggers another underflow
}

The strace output shows, that 2 calls to read with 8191 bytes occures. When
ignoring one byte less and then another byte, only one read is done:

#include <iostream>
#include <fstream>

int main(int argc, char* argv[])
{
  std::ifstream in(argv[0]);
  in.get(); // trigger filling of input buffer
  in.ignore(in.rdbuf()->in_avail() - 1);
  in.ignore(1); // this does not trigger underflow
}


-- 
           Summary: std::istream::ignore(std::streamsize n) calls
                    unnecessary underflow
           Product: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tommi at tntnet dot org
GCC target triplet: i486-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42857