Issue about the build order of meson.

[Nima0908]

Hello,
iam using meson for my project and i have a problem with meson wanting to compile the exe first before the dependencie it depends on. Does anyone know how to fix that?

meson.build

  'dockage',
  'cpp',
  default_options : ['cpp_std=c++26']
)

add_project_arguments('-fmodules', language: 'cpp')

subdir('dockage-core')
subdir('dockage-cli')

dockage-core/meson.build

cpp_sources = []

all_sources = cppm_sources + cpp_sources

core_lib = static_library(
  'dockage-core',
  all_sources,
)

core_dep = declare_dependency(
  link_with : core_lib,
)

dockage-cli/meson.build

cpp_sources = ['src/main.cpp']

all_sources = cppm_sources + cpp_sources

cli_exe = executable(
  'dockage-cli',
  all_sources,
  dependencies: [core_dep]  # always link to dockage-core
)

[bonzini]

It's not clear what's the problem also because the cppm_sources variable is not defined.

[Nima0908]

Oh, sorry i should have provided the output. The cppm_sources is not defined right now, but it will be in the future. Iam very early in the development process at the moment.

Output:

INFO: autodetecting backend as ninja
INFO: calculating backend command to run: /usr/bin/ninja -C /home/marius/Dokumente/Projects/dockage/build
ninja: Entering directory `/home/marius/Dokumente/Projects/dockage/build'
[1/4] Compiling C++ object dockage-cli/dockage-cli.p/src_main.cpp.o
FAILED: [code=1] dockage-cli/dockage-cli.p/src_main.cpp.o 
clang++ -Idockage-cli/dockage-cli.p -Idockage-cli -I../dockage-cli -fdiagnostics-color=always -D_GLIBCXX_ASSERTIONS=1 -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch
 -std=c++26 -O0 -g -fmodules -MD -MQ dockage-cli/dockage-cli.p/src_main.cpp.o -MF dockage-cli/dockage-cli.p/src_main.cpp.o.d -o dockage-cli/dockage-cli.p/src_ma
in.cpp.o -c ../dockage-cli/src/main.cpp
../dockage-cli/src/main.cpp:1:8: fatal error: module 'dockage.core.logger' not found
    1 | import dockage.core.logger;
      | ~~~~~~~^~~~~~~
1 error generated.
[2/4] Compiling C++ object dockage-core/libdockage-core.a.p/modules_dockage.core.logger.cppm.o
ninja: build stopped: subcommand failed.

The dockage.core.logger.cppm module needs to be compiled first for my code to link correctly, but meson is compiling it second. Ive set dore_dep as a dependencie under the impression that meson will build it first, but i was wrong. I also tried link_with but that didnt work either.

[dcbaker]

Okay, so our modules implementation checks the command line arguments to a target for modules, I'm wondering if it doesn't take add_project_arguments() into account.

A related question, what version of Meson are you using?

[Nima0908]

Iam using meson version 1.9.1. I also tried adding cpp_args but this doesnt change the outcome

[dcbaker]

Dang, there goes my first idea.

Do you have any ninja steps for depscan and depaccumulate, if you don't then Meson is deciding for some reason not to scan your sources for modules. That seems like a bug we should fix.

[Nima0908]

You are spot on. The ninja.build file generated by meson is just doing normal header checking and is completely forgetting about dyndep and modules. It generates a comment that says module specific stuff should follow, but there is just nothing. Here is the code from the ninja.build file if you are interested:

# It is autogenerated by the Meson build system.
# Do not edit by hand.

ninja_required_version = 1.8.2

# Rules for module scanning.

# Rules for compiling.

rule cpp_COMPILER
 command = clang++ $ARGS -MD -MQ $out -MF $DEPFILE -o $out -c $in
 deps = gcc
 depfile = $DEPFILE_UNQUOTED
 description = Compiling C++ object $out

# Rules for linking.

rule STATIC_LINKER
 command = rm -f $out && llvm-ar $LINK_ARGS $out $in
 description = Linking static target $out

rule cpp_LINKER
 command = clang++ $ARGS -o $out $in $LINK_ARGS
 description = Linking target $out

# Other rules

rule CUSTOM_COMMAND
 command = $COMMAND
 description = $DESC
 restat = 1

rule REGENERATE_BUILD
 command = /usr/lib/python-exec/python3.13/meson --internal regenerate /home/marius/Dokumente/Projects/dockage .
 description = Regenerating build files
 generator = 1

# Phony build target, always out of date

build PHONY: phony 

# Build rules for targets

build dockage-core/libdockage-core.a.p/modules_dockage.core.logger.cppm.o: cpp_COMPILER ../dockage-core/modules/dockage.core.logger.cppm
 DEPFILE = dockage-core/libdockage-core.a.p/modules_dockage.core.logger.cppm.o.d
 DEPFILE_UNQUOTED = dockage-core/libdockage-core.a.p/modules_dockage.core.logger.cppm.o.d
 ARGS = -Idockage-core/libdockage-core.a.p -Idockage-core -I../dockage-core -fdiagnostics-color=always -D_GLIBCXX_ASSERTIONS=1 -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -std=c++26 -O0 -g -fmodules -fmodules-cache-path=modules -fPIC

build dockage-core/libdockage-core.a: STATIC_LINKER dockage-core/libdockage-core.a.p/modules_dockage.core.logger.cppm.o
 LINK_ARGS = csrDT

build dockage-cli/dockage-cli.p/src_main.cpp.o: cpp_COMPILER ../dockage-cli/src/main.cpp
 DEPFILE = dockage-cli/dockage-cli.p/src_main.cpp.o.d
 DEPFILE_UNQUOTED = dockage-cli/dockage-cli.p/src_main.cpp.o.d
 ARGS = -Idockage-cli/dockage-cli.p -Idockage-cli -I../dockage-cli -fdiagnostics-color=always -D_GLIBCXX_ASSERTIONS=1 -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -std=c++26 -O0 -g -fmodules -fmodules-cache-path=modules

build dockage-cli/dockage-cli: cpp_LINKER dockage-cli/dockage-cli.p/src_main.cpp.o | dockage-core/libdockage-core.a
 LINK_ARGS = -Wl,--as-needed -Wl,--no-undefined dockage-core/libdockage-core.a

# Test rules

build test: phony meson-internal__test

build meson-internal__test: CUSTOM_COMMAND all meson-test-prereq PHONY
 COMMAND = /usr/lib/python-exec/python3.13/meson test --no-rebuild --print-errorlogs
 DESC = Running$ all$ tests
 pool = console

build benchmark: phony meson-internal__benchmark

build meson-internal__benchmark: CUSTOM_COMMAND all meson-benchmark-prereq PHONY
 COMMAND = /usr/lib/python-exec/python3.13/meson test --benchmark --logbase benchmarklog --num-processes=1 --no-rebuild
 DESC = Running$ benchmark$ suite
 pool = console

# Install rules

build install: phony meson-internal__install

build meson-internal__install: CUSTOM_COMMAND PHONY | all
 DESC = Installing$ files
 COMMAND = /usr/lib/python-exec/python3.13/meson install --no-rebuild
 pool = console

build dist: phony meson-internal__dist

build meson-internal__dist: CUSTOM_COMMAND PHONY
 DESC = Creating$ source$ packages
 COMMAND = /usr/lib/python-exec/python3.13/meson dist
 pool = console

# Suffix

build scan-build: phony meson-internal__scan-build

build meson-internal__scan-build: CUSTOM_COMMAND PHONY
 COMMAND = /usr/lib/python-exec/python3.13/meson --internal scanbuild /home/marius/Dokumente/Projects/dockage /home/marius/Dokumente/Projects/dockage/build subprojects /usr/lib/python-exec/python3.13/meson setup
 pool = console

build uninstall: phony meson-internal__uninstall

build meson-internal__uninstall: CUSTOM_COMMAND PHONY
 COMMAND = /usr/lib/python-exec/python3.13/meson --internal uninstall
 pool = console

build all: phony dockage-core/libdockage-core.a dockage-cli/dockage-cli

build meson-test-prereq: phony 

build meson-benchmark-prereq: phony 

build clean: phony meson-internal__clean

build meson-internal__clean: CUSTOM_COMMAND PHONY
 COMMAND = /usr/bin/ninja -t clean
 description = Cleaning

build build.ninja: REGENERATE_BUILD ../meson.build ../dockage-core/meson.build ../dockage-cli/meson.build /home/marius/Dokumente/Projects/dockage/clang-native.ini meson-private/coredata.dat
 pool = console

build reconfigure: REGENERATE_BUILD PHONY
 pool = console

build ../meson.build ../dockage-core/meson.build ../dockage-cli/meson.build /home/marius/Dokumente/Projects/dockage/clang-native.ini meson-private/coredata.dat: phony 

default all

Please let me know if there is something i can do to help fix this bug :)

[dcbaker]

Apparently we've had two people recently discover the same issue!: #15370

[eli-schwartz]

iam using meson for my project and i have a problem with meson wanting to compile the exe first before the dependencie it depends on.

It is intended behavior for C or C++ programs, that compiling source code to object code can happen in parallel across libraries and executables. An executable doesn't need to compile against a library, only against the library's headers. It does need to link against the library, so meson enforces that order.

You specifically mention C++20 modules though which act more like headers than like sources. Unfortunately, half a decade later compiler tooling for modules is still anemic (and uses the same compilation model as PCH, which is scary and also means no real sharing between targets).

Modules support is still pretty experimental. The most recent release has some support for compiling std, though.

[Nima0908]

Thanks, then its probaply an issue of the sparse adaption of the cpp modules