The crux of the warning is:
/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:1209:11: warning: writing 2 bytes into a region of size 0 [-Wstringop-overflow=]
1209 | memcpy(out, digits2(value), 2);
| ^
/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h: In function ‘write_fixed’:
/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2433:8: note: at offset [-11, -1] into destination object ‘buffer’ of size 11
2433 | Char buffer[digits10<UInt>() + 2];
| ^
Note this only happens since
$ gcc --version
gcc (GCC) 16.1.1 20260430
which has just been pushed into the repos on Arch Linux.
compiling fmtlib as static lib, and then linking with LTO enabled...
set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
FetchContent_Declare(
fmt
GIT_REPOSITORY https://github.com/fmtlib/fmt.git
GIT_TAG 12.1.0
)
FetchContent_MakeAvailable(fmt)
add_executable(app ...)
target_compile_features(app PRIVATE cxx_std_26)
target_link_libraries(app PRIVATE fmt )
and during link stage, I get these odd warnings
cmake -GNinja -S . -B ./build/gcc/release -DCMAKE_COLOR_DIAGNOSTICS=ON -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++ -DCMAKE_BUILD_TYPE=release
....
[2/11] /usr/bin/g++ -I/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include -O3 -DNDEBUG -std=c++20 -flto=auto -fno-fat-lto-objects -fdiagnostics-color=always -MD -MT _deps/fmt-build/CMakeFiles/fmt.dir/src/os.cc.o -MF _deps/fmt-build/CMakeFiles/fmt.dir/src/os.cc.o.d -o _deps/fmt-build/CMakeFiles/fmt.dir/src/os.cc.o -c /home/oliver/psmem/build/gcc/release/_deps/fmt-src/src/os.cc
[3/11] /usr/bin/g++ -I/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include -I/home/oliver/psmem/include -I/home/oliver/psmem/build/gcc/release/_deps/termbox2_src-src -I/home/oliver/psmem/build/gcc/release/_deps/unordered_dense-src/include -O3 -DNDEBUG -std=c++26 -flto=auto -fno-fat-lto-objects -fdiagnostics-color=always -Wall -Wextra -Wshadow -fdiagnostics-color=always -MD -MT CMakeFiles/psmem.dir/src/proc.cpp.o -MF CMakeFiles/psmem.dir/src/proc.cpp.o.d -o CMakeFiles/psmem.dir/src/proc.cpp.o -c /home/oliver/psmem/src/proc.cpp
[4/11] /usr/bin/g++ -I/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include -O3 -DNDEBUG -std=c++20 -flto=auto -fno-fat-lto-objects -fdiagnostics-color=always -MD -MT _deps/fmt-build/CMakeFiles/fmt.dir/src/format.cc.o -MF _deps/fmt-build/CMakeFiles/fmt.dir/src/format.cc.o.d -o _deps/fmt-build/CMakeFiles/fmt.dir/src/format.cc.o -c /home/oliver/psmem/build/gcc/release/_deps/fmt-src/src/format.cc
[5/11] : && /usr/bin/cmake -E rm -f _deps/fmt-build/libfmt.a && "/usr/bin/gcc-ar" qc _deps/fmt-build/libfmt.a _deps/fmt-build/CMakeFiles/fmt.dir/src/format.cc.o _deps/fmt-build/CMakeFiles/fmt.dir/src/os.cc.o && "/usr/bin/gcc-ranlib" _deps/fmt-build/libfmt.a && :
...
[11/11] : && /usr/bin/g++ -O3 -DNDEBUG -flto=auto -fno-fat-lto-objects -fuse-ld=mold -Wl,--dependency-file=CMakeFiles/psmem.dir/link.d CMakeFiles/psmem.dir/app/psmem.cpp.o CMakeFiles/psmem.dir/src/proc.cpp.o -o psmem _deps/fmt-build/libfmt.a libtermbox2.a _deps/fmt-build/libfmt.a && :
In function ‘write2digits’,
inlined from ‘write_significand’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2415:17,
inlined from ‘write_significand’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2407:13,
inlined from ‘write_significand’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2434:31,
inlined from ‘write_significand’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2461:20,
inlined from ‘operator()’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2539:33,
inlined from ‘write_padded’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:1701:9,
inlined from ‘write_padded’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:1710:43,
inlined from ‘write_fixed’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2536:44:
/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:1209:11: warning: writing 2 bytes into a region of size 0 [-Wstringop-overflow=]
1209 | memcpy(out, digits2(value), 2);
| ^
/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h: In function ‘write_fixed’:
/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2433:8: note: at offset [-11, -1] into destination object ‘buffer’ of size 11
2433 | Char buffer[digits10<UInt>() + 2];
| ^Things I have tried:
- disabling LTO removes the warnings
- the above uses the
mold linker but normal ld produces the same warnings
- changing to
-DCMAKE_BUILD_TYPE=debug makes the warnings go away.
- using latest git fmt instead of TAG=12.1.0 produces the same warnings (now line 1208)
These may very well be over eager new gcc....or bizarre UB in my code?
The crux of the warning is:
Note this only happens since
which has just been pushed into the repos on Arch Linux.
compiling fmtlib as static lib, and then linking with LTO enabled...
and during link stage, I get these odd warnings
Things I have tried:
moldlinker but normalldproduces the same warnings-DCMAKE_BUILD_TYPE=debugmakes the warnings go away.These may very well be over eager new gcc....or bizarre UB in my code?