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gcc-7.1.0/c++17/old-abi: empty stringstream invalid memory access




I am reproducing here a bug report I filed to gcc's bugzilla against
gcc-7.1.0: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81522 , in
hopes of finding others who observed the same issue or who may have a
solution. The bug only appears on Cygwin and only in c++17 mode with
the old ABI.

-- Irfan

This trivial program:

#include <sstream>
int main() {
std::stringstream os; // same with ("") or (std::string())
os.str();
return 0;
}

causes an invalid memory access with gcc 7.1.0 (compiled from source),
but only on Cygwin in c++17 mode with old ABI, and only if the
stringstream is constructed empty. Changing any of those parameters no
longer causes the issue.

Thread 1 "string" received signal SIGABRT, Aborted.
0x00000001004018a0 in __gnu_cxx::new_allocator<char>::deallocate
(this=0x6, __p=0x3e8000003fc <error: Cannot access memory at address
0x3e8000003fc>)
at /usr/local/gcc-7.1.0/include/c++/7.1.0/ext/new_allocator.h:125
125 ::operator delete(__p);
(gdb) bt
#0 0x00000001004018a0 in __gnu_cxx::new_allocator<char>::deallocate
(this=0x6, __p=0x3e8000003fc <error: Cannot access memory at address
0x3e8000003fc>)
at /usr/local/gcc-7.1.0/include/c++/7.1.0/ext/new_allocator.h:125
#1 0x00000001800b25d2 in CreateEventW () from /usr/bin/cygwin1.dll
#2 0x0000000000000130 in ?? ()

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/local/gcc-7.1.0/libexec/gcc/x86_64-pc-cygwin/7.1.0/lto-wrapper.exe
Target: x86_64-pc-cygwin
Configured with: ../configure --prefix=/usr/local/gcc-7.1.0
--enable-threads=posix --enable-checking=release
--enable-languages=c,c++ --build=x86_64-pc-cygwin --disable-multilib
--with-default-libstdcxx-abi=gcc4-compatible
Thread model: posix
gcc version 7.1.0 (GCC)

Sample program compiled with:

$ g++ -std=c++17 -D_GLIBCXX_USE_CXX11_ABI=0 -ggdb -O0 string.cc -o string

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