re PR libstdc++/45628 (std::fstream::tellg invalidates I/O buffer)

re PR libstdc++/45628 (std::fstream::tellg invalidates I/O buffer)

2010-09-22  David Krauss  <potswa@mac.com>

	PR libstdc++/45628
	* include/bits/fstream.tcc (basic_filebuf::underflow): Add state
	transition to avoid modality requiring seekoff(0,ios::cur).
	(basic_filebuf::pbackfail): Likewise.
	(basic_filebuf::overflow): Likewise.
	(basic_filebuf::_M_seek): Avoid minor unnecessary conversion.
	(basic_filebuf::seekoff): Remove code to _M_get_ext_pos; make
	(0, ios::cur) a special case preserving buffer contents.
	(basic_filebuf::_M_get_ext_pos): New function to obtain status
	about codecvt extern_t buffer for overflow and seekoff.
	* include/std/fstream (basic_filebuf::_M_get_ext_pos): Likewise.
	* config/abi/pre/gnu.ver: Export new symbols.
	* testsuite/27_io/basic_filebuf/seekoff/char/45628-1.cc: New,
	verifies that seekoff(0, ios::cur) preserves buffers.
	* testsuite/27_io/basic_filebuf/seekoff/char/45628-2.cc: Likewise.
	for codecvt case. More lenient as it may still flush put area.
	* testsuite/27_io/basic_filebuf/seekoff/char/4.cc: Modify to
	check that seekoff is not required between read and write.
	* testsuite/27_io/basic_filebuf/seekoff/wchar_t/4.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sync/wchar_t/1.cc: Remove.
	* testsuite/27_io/basic_filebuf/sync/wchar_t/1.cc: Likewise.
	* testsuite/util/testsuite_character.h (codecvt::do_length): Comply
	with 22.2.1.5.2/10 "Returns ... the LARGEST value in the range..."

From-SVN: r164529