https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93672
--- Comment #5 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:2d694414ada8e3b58f504c1b175d31088529632e commit r14-9978-g2d694414ada8e3b58f504c1b175d31088529632e Author: Jonathan Wakely <jwak...@redhat.com> Date: Thu Apr 4 10:33:33 2024 +0100 libstdc++: Fix infinite loop in std::istream::ignore(n, delim) [PR93672] A negative delim value passed to std::istream::ignore can never match any character in the stream, because the comparison is done using traits_type::eq_int_type(sb->sgetc(), delim) and sgetc() never returns negative values (except at EOF). The optimized version of ignore for the std::istream specialization uses traits_type::find to locate the delim character in the streambuf, which _can_ match a negative delim on platforms where char is signed, but then we do another comparison using eq_int_type which fails. The code then keeps looping forever, with traits_type::find locating the character and traits_type::eq_int_type saying it's not a match, so traits_type::find is used again and finds the same character again. A possible fix would be to check with eq_int_type after a successful find, to see whether we really have a match. However, that would be suboptimal since we know that a negative delimiter will never match using eq_int_type. So a better fix is to adjust the check at the top of the function that handles delim==eof(), so that we treat all negative delim values as equivalent to EOF. That way we don't bother using find to search for something that will never match with eq_int_type. The version of ignore in the primary template doesn't need a change, because it doesn't use traits_type::find, instead characters are extracted one-by-one and always matched using eq_int_type. That avoids the inconsistency between find and eq_int_type. The specialization for std::wistream does use traits_type::find, but traits_type::to_int_type is equivalent to an implicit conversion from wchar_t to wint_t, so passing a wchar_t directly to ignore without using to_int_type works. libstdc++-v3/ChangeLog: PR libstdc++/93672 * src/c++98/istream.cc (istream::ignore(streamsize, int_type)): Treat all negative delimiter values as eof(). * testsuite/27_io/basic_istream/ignore/char/93672.cc: New test. * testsuite/27_io/basic_istream/ignore/wchar_t/93672.cc: New test.
