Unable to link RISC-V linux-gnu and linux-musl executables using zig cc and CMake

[fwsGonzo]

Zig Version

0.14.0-dev.2384+cbc05e0b1

Steps to Reproduce and Observed Behavior

Simple toolchain file:

set(CMAKE_SYSTEM_NAME "Linux")
set(CMAKE_SYSTEM_VERSION 1)
set(CMAKE_SYSTEM_PROCESSOR "riscv64")
set(CMAKE_CROSSCOMPILING TRUE)
#set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
set(CMAKE_C_COMPILER "zig" cc -target riscv64-linux-gnu)
set(CMAKE_CXX_COMPILER "zig" c++ -target riscv64-linux-gnu)

The try-static-library thing is just to avoid the compiler checks. The linking step fails, so we want the compiler check.

Simple build script:

mkdir -p .zig
pushd .zig
cmake .. -DCMAKE_BUILD_TYPE=Release -DCMAKE_TOOLCHAIN_FILE="$TOOLCHAIN"
make -j
popd

Compiler test:

$ bash zig.sh 
~/github/godot-sandbox-programs/.zig ~/github/godot-sandbox-programs
-- The C compiler identification is Clang 19.1.0
-- The CXX compiler identification is Clang 19.1.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - failed
-- Check for working C compiler: /home/gonzo/zig/zig
-- Check for working C compiler: /home/gonzo/zig/zig - broken
CMake Error at /usr/share/cmake-3.22/Modules/CMakeTestCCompiler.cmake:69 (message):
  The C compiler

    "/home/gonzo/zig/zig"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: /home/gonzo/github/godot-sandbox-programs/.zig/CMakeFiles/CMakeTmp
    
    Run Build Command(s):/usr/bin/gmake -f Makefile cmTC_aaa0d/fast && /usr/bin/gmake  -f CMakeFiles/cmTC_aaa0d.dir/build.make CMakeFiles/cmTC_aaa0d.dir/build
    gmake[1]: Entering directory '/home/gonzo/github/godot-sandbox-programs/.zig/CMakeFiles/CMakeTmp'
    Building C object CMakeFiles/cmTC_aaa0d.dir/testCCompiler.c.o
    /home/gonzo/zig/zig cc -target riscv64-linux-gnu    -MD -MT CMakeFiles/cmTC_aaa0d.dir/testCCompiler.c.o -MF CMakeFiles/cmTC_aaa0d.dir/testCCompiler.c.o.d -o CMakeFiles/cmTC_aaa0d.dir/testCCompiler.c.o -c /home/gonzo/github/godot-sandbox-programs/.zig/CMakeFiles/CMakeTmp/testCCompiler.c
    Linking C executable cmTC_aaa0d
    /usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_aaa0d.dir/link.txt --verbose=1
    /home/gonzo/zig/zig cc -target riscv64-linux-gnu CMakeFiles/cmTC_aaa0d.dir/testCCompiler.c.o -o cmTC_aaa0d   -latomic 
    error: unable to find dynamic system library 'atomic' using strategy 'paths_first'. searched paths: none
    gmake[1]: *** [CMakeFiles/cmTC_aaa0d.dir/build.make:100: cmTC_aaa0d] Error 1
    gmake[1]: Leaving directory '/home/gonzo/github/godot-sandbox-programs/.zig/CMakeFiles/CMakeTmp'
    gmake: *** [Makefile:127: cmTC_aaa0d/fast] Error 2

It's erroneously trying to link against libatomic: -latomic. Same behavior on both linux-gnu and linux-musl.

As a sanity check I tried to compile manually on the command-line, and that works as -latomic is not being added. But, maybe this is a CMake bug? And is -latomic supposed to be present? Can it be an empty library?

Expected Behavior

Don't link against libatomic.

[fwsGonzo]

For those looking for a work-around, if you enable static-library set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY), and then add your own empty library called atomic, it will link just fine.

CMake bug?

[alexrp]

You'll have to figure out why CMake is deciding that it needs libatomic. That requirement is definitely not coming from Zig.

[fwsGonzo]

Alright, well all's well that ends well anyway. All my programs are linking, so there is definitely no actual need for -latomic.

if (ZIG_COMPILER)
	add_library(atomic STATIC "${CMAKE_SOURCE_DIR}/ext/atomic.c")
endif()

I added an empty atomic.c file to a folder, and created a library from it. That was enough to make everything work.

[alexrp]

Reopening this actually. I think that, regardless of whatever CMake is doing here, it makes sense for us to recognize and treat libatomic specially as it's a compiler-provided library. Our compiler-rt implementation should be able to satisfy it (as evidenced by your workaround actually working).