https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81517
Bug ID: 81517 Summary: Unhelpful what() for uninitialized threads backend (in __gthread_once) Product: gcc Version: 7.1.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: chris.j.leonard at gmx dot com Target Milestone: --- $ gcc -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/lto-wrapper Target: x86_64-pc-linux-gnu Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/ --enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared --enable-threads=posix --enable-libmpx --with-system-zlib --with-isl --enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object --enable-linker-build-id --enable-lto --enable-plugin --enable-install-libiberty --with-linker-hash-style=gnu --enable-gnu-indirect-function --disable-multilib --disable-werror --enable-checking=release --enable-default-pie --enable-default-ssp Thread model: posix gcc version 7.1.1 20170630 (GCC) $ cat example.cpp #include <mutex> int main() { std::once_flag flag; std::call_once(flag, []{}); } $ g++ -Wall -Wextra example.cpp -o example Compiler gives no output and succeeds (as expected) $ ./example terminate called after throwing an instance of 'std::system_error' what(): Unknown error -1 Aborted (core dumped) The what() message, called from __gthread_once, is "Unknown error" and the error code it receives. Reading the header code reveals -1 is returned if a global thread object was not correctly initialized, which should illuminate the mistake: not linking libpthread. This is a typical enough mistake, giving a more useful what() message like "pthread missing / not initialized" would save a programmer time in this instance, or help them where they might have been totally lost.