From CMake setup 'make' to use '-j' option by default - cmake

Question

I want my CMake project to be built by make -j N, whenever I call make from the terminal. I don't want to set -j option manually every time.
For that, I set CMAKE_MAKE_PROGRAM variable to the specific command line. I use the ProcessorCount() function, which gives the number of procesors to perform build in parallel.
When I do make, I do not see any speed up. However if I do make -j N, then it is built definitely faster.
Would you please help me on this issue? (I am developing this on Linux.)
Here is the snippet of the code that I use in CMakeList.txt:
include(ProcessorCount)
ProcessorCount(N)
message("number of processors: " ${N})
if(NOT N EQUAL 0)
set(CTEST_BUILD_FLAGS -j${N})
set(ctest_test_args ${ctest_test_args} PARALLEL_LEVEL ${N})
set(CMAKE_MAKE_PROGRAM "${CMAKE_MAKE_PROGRAM} -j ${N}")
endif()
message("cmake make program" ${CMAKE_MAKE_PROGRAM})
Thank you very much.

Answer 1

In case you want to speed up the build you can run multiple make processes in parallel but not cmake.
To perform every build with predefined number of parallel processes you can define this in MAKEFLAGS.
Set MAKEFLAGS in your environment script, e.g. ~/.bashrc as you want:
export MAKEFLAGS=-j8
On Linux the following sets MAKEFLAGS to the number of CPUs - 1: (Keep one CPU free for other tasks while build) and is useful in environments with dynamic ressources, e.g. VMware:
export MAKEFLAGS=-j$(($(grep -c "^processor" /proc/cpuinfo) - 1))
New from cmake v3.12 on:
The command line has a new option --parallel <JOBS>.
Example:
cmake --build build_arm --parallel 4 --target all
Example with number of CPUs- 1 using nproc:
cmake --build build_arm --parallel $(($(nproc) - 1)) --target all

Answer 2

Via setting the CMAKE_MAKE_PROGRAM variable you want to affect the build process. But:
This variable affects only the build via cmake --build, not on native tool (make) call:
The CMAKE_MAKE_PROGRAM variable is set for use by project code. The value is also used by the cmake(1) --build and ctest(1) --build-and-test tools to launch the native build process.
This variable should be a CACHEd one. It is used in such way by make-like generators:
These generators store CMAKE_MAKE_PROGRAM in the CMake cache so that it may be edited by the user.
That is, you need to set this variable with
set(CMAKE_MAKE_PROGRAM <program> CACHE PATH "Path to build tool" FORCE)
This variable should refer to the executable itself, not to a program with arguments:
The value may be the full path to an executable or just the tool name if it is expected to be in the PATH.
That is, value "make -j 2" cannot be used for that variable (splitting arguments as list
set(CMAKE_MAKE_PROGRAM make -j 2 CACHE PATH "Path to build tool" FORCE)
wouldn't help either).
In summary, you may redefine the behavior of cmake --build calls with setting the CMAKE_MAKE_PROGRAM variable to the script, which calls make with parallel options. But you may not affect the behavior of direct make calls.

Answer 3

You may set the env variable MAKEFLAGS using this command
export MAKEFLAGS=-j$(nproc)

Answer 4

My solution is to have a small script which will run make include all sorts of other features, not just the number of CPUs.
I call my script mk and I do a chmod 755 mk so I can run it with ./mk in the root of my project. I also have a few flags to be able to run various things with a simple command line. For example, while working on the code and I get many errors, I like to pipe the output to less. I can do that with ./mk -l without having to retype all the heavy duty Unix stuff...
As you can see, I have the -j4 in a couple of places where it makes sense. For the -l option, I don't want it because in this case it would eventually cause multiple errors to be printed at the same time (I tried that before!)
#!/bin/sh -e
#
# Execute make
case "$1" in
"-l")
make -C ../BUILD/Debug 2>&1 | less -R
;;
"-r")
make -j4 -C ../BUILD/Release
;;
"-d")
rm -rf ../BUILD/Debug/doc/lpp-doc-?.*.tar.gz \
../BUILD/Debug/doc/lpp-doc-?.*
make -C ../BUILD/Debug
;;
"-t")
make -C ../BUILD/Debug
../BUILD/Debug/src/lpp tests/suite/syntax-print.logo
g++ -std=c++14 -I rt l.cpp rt/*.cpp
;;
*)
make -j4 -C ../BUILD/Debug
;;
esac
# From the https://github.com/m2osw/lpp project
With CMake, it wouldn't work unless, as Tsyvarev mentioned, you create your own script. But I personally don't think it's sensible to call make from your make script. Plus it could break a build process which would not expect that strange script. Finally, my script, as I mentioned, allows me to vary the options depending on the situation.

Answer 5

I usually use alias in linux to set cm equal to cmake .. && make -j12. Or write a shell to specify make and clean progress ...
alias cm='cmake .. && make -j12'
Then use cm to make in a single command.