cmake generated header causes double build - c++

Question

A very simple scenario - I need to generate header, include it and if generated header is updated while cmake build, all dependent cpp units also must be rebuilt.
A simplified example:
add_custom_command(
OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/test.h
COMMAND ...
DEPENDS "${CMAKE_CURRENT_SOURCE_DIR}/test.in"
add_custom_target(test_target DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/test.h)
add_executable(test_exe
${CMAKE_CURRENT_SOURCE_DIR}/main.cpp
)
add_dependencies(test_exe test_target)
main.cpp is just:
#include "test.h"
int main()
{
return 0;
}
When I do a full rebuild, everything goes fine. But when I change test.in, run just cmake.exe --build . --target all only test.h is regenerated, but main.cpp is not recompiled. But when I run cmake.exe --build . --target all again(second time), main.cpp is recompiled and test_exe is relinked.
What do I wrong?
P.S. If I explicitly use OBJECT_DEPENDS, there is no issue, rebuild works fine, but the docs say it's not required anymore - https://cmake.org/cmake/help/v3.20/prop_sf/OBJECT_DEPENDS.html
Update:
I use Windows 10, CMake 3.19.2 and Ninja 1.10.2
Solution:
Use build directory inside your project dir, then there is no the problem. But if the build directory is outside of our project directory, then there is the problem.

Answer 1

Posting as an answer to be able to show full example
CMakeLists.txt
cmake_minimum_required(VERSION 3.15)
project(example)
add_custom_command(
OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/test.h
COMMAND ${CMAKE_COMMAND} -E copy "${CMAKE_CURRENT_SOURCE_DIR}/test.in" ${CMAKE_CURRENT_SOURCE_DIR}/test.h
DEPENDS "${CMAKE_CURRENT_SOURCE_DIR}/test.in")
add_custom_target(test_target DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/test.h)
add_executable(test_exe
${CMAKE_CURRENT_SOURCE_DIR}/main.cpp
)
add_dependencies(test_exe test_target)
main.cpp
#include "stdio.h"
#include "test.h"
int main()
{
printf("The value is %d\n", FOO);
return 0;
}
test.in
#define FOO 4
This works on windows with CMake 3.19.2 and Ninja 1.10.2, one can change the FOO define in test.in after an initial build and see the resultant executable be rebuilt and see it's value changed.
The commands used to test
$ cmake -B build -G Ninja
-- The C compiler identification is Clang 11.0.0 with GNU-like command-line
-- The CXX compiler identification is Clang 11.0.0 with GNU-like command-line
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: C:/Program Files/LLVM/bin/clang.exe - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/Program Files/LLVM/bin/clang++.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: <some_path>
$ cmake --build build
[3/3] Linking CXX executable test_exe.exe
$ ./build/test_exe.exe
The value is 4
$ echo "#define FOO 3" > test.in
$ cmake --build build
[3/3] Linking CXX executable test_exe.exe
$ ./build/test_exe.exe
The value is 3