CMake looks for `std.cppm` in wrong directory when using `import std`

Question

Recently I've decided to "modularize" my library. No success. Here is the repo where you can find CMakeLists.txt & CMakePresets.json: kissra (GitHub)

The issue is that CMake cannot find standard std.cppm primary module interface - it is trying to find it in a wrong directory (/lib/share/libc++/v1) whereas the LLVM toolset is in fact installed in /usr/lib/llvm-21/. And I haven't figured out how to force CMake search PMIs in a different directory.

Here is the error I'm getting:

-- Configuring done (7.1s)
CMake Error at build/debug/CMakeFiles/4.2.1/CMakeCXXCompiler.cmake:111 (target_sources):
  Cannot find source file: /lib/share/libc++/v1/std.cppm
Call Stack (most recent call first):
  CMakeLists.txt:8 (project)

I've looked into the generated CMake scripts in the build directory and found this (CMakeCXXCompiler.cmake):

### Imported target for C++26 standard library
if (NOT TARGET "__CMAKE::CXX26")
if (NOT TARGET "__cmake_cxx26")
add_library(__cmake_cxx26 STATIC)
target_sources(__cmake_cxx26 INTERFACE "$<$<STREQUAL:$<TARGET_PROPERTY:TYPE>,STATIC_LIBRARY>:$<TARGET_OBJECTS:__cmake_cxx26>>")
set_property(TARGET __cmake_cxx26 PROPERTY EXCLUDE_FROM_ALL 1)
set_property(TARGET __cmake_cxx26 PROPERTY CXX_SCAN_FOR_MODULES 1)
set_property(TARGET __cmake_cxx26 PROPERTY CXX_MODULE_STD 0)
target_compile_features(__cmake_cxx26 PUBLIC cxx_std_26)
target_compile_options(__cmake_cxx26 PRIVATE -Wno-reserved-module-identifier)
target_include_directories(__cmake_cxx26 PRIVATE  "/lib/x86_64-linux-gnu/../share/libc++/v1")
target_sources(__cmake_cxx26
  PUBLIC
  FILE_SET std TYPE CXX_MODULES
    BASE_DIRS  "/lib/x86_64-linux-gnu/../share/libc++/v1"
    FILES  "/lib/x86_64-linux-gnu/../share/libc++/v1/std.cppm" "/lib/x86_64-linux-gnu/../share/libc++/v1/std.compat.cppm")
endif ()
add_library(__CMAKE::CXX26 ALIAS __cmake_cxx26)
endif ()
if (TARGET "__CMAKE::CXX26")
  list(APPEND CMAKE_CXX_COMPILER_IMPORT_STD "26")
endif ()

As you can see, CMake is trying to find PMIs in /lib/share/libc++/v1. Is there a way to customize the location for PMIs by setting some obscure global CMake variable in CMakePreset.json file?

I commend you on trying to take that step.The pioneers blaze the trail etc! ... myself, I'll probably wait another decade or so before I require C++23 as the minimum standard version for my library, at which time I'll be able to switch to importing the standard library modules. Anyway, +1 and made an edit. — einpoklum
– einpoklum, Commented Dec 22, 2025 at 13:24

Dmitry Katkevich · Accepted Answer · 2025-12-23 02:10:31Z

2

CMAKE_CXX_STDLIB_MODULES_JSON may be helpful

OP update:
Here is a related LLVM issue: [clang] Wrong location for libc++.modules.json #172497
Basically, the issue is that clang returns an invalid path to libc++.modules.json file (CMake uses this file to locate the Standard Modules PMIs).

The fix is to explicitly specify the path to this libc++.modules.json file via CMAKE_CXX_STDLIB_MODULES_JSON CMake variable.
Example:
https://github.com/katkevich/kissra/blob/88f0f54a2b9b0a22a4d6a16816e71145cf13e8ba/CMakePresets.json#L15

edited Dec 23, 2025 at 2:10

Dmitry Katkevich

9877 silver badges27 bronze badges

answered Dec 22, 2025 at 6:10

yjjjyj

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3 Comments

Dmitry Katkevich Dec 22, 2025 at 11:14

Yes, thank you! Apparently, /usr/lib/llvm-21/bin/clang++ -print-file-name=libc++.modules.json returns invalid path. Here is an LLVM issue: https://github.com/llvm/llvm-project/issues/172497. Manually specifying CMAKE_CXX_STDLIB_MODULES_JSON does indeed help!

einpoklum Dec 22, 2025 at 13:04

This answer must really be expanded. Right now it is just a link. What is that phrase? How would OP use it? Why is JSON relevant? etc. See also the FAQ item about using links and also the meta-SO post "Your answer is in another castle".

einpoklum Dec 22, 2025 at 13:11

And if that's not enough - even following the link still doesn't quite tell us what OP should do and how that would work. So, I'm downvoting, but will happily un-downvote and maybe upvote when you edit.

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