cmake and make with -j flag produces error in docker

Question

In my dockerfile, I have this:

RUN cmake -j$(nproc) -S . -B build

When I run it, it produces this error: CMake Error: Unknown argument -j16

I have cmake > 3.12. What is the issue here? It runs perfectly when I do it outside of the docker in the host machine.

Answer 1

If you want to use the -j option with cmake then try something like this:

🗎 Dockerfile

FROM ubuntu:latest

RUN apt-get update -qq && apt-get install -y -qq cmake g++

COPY . .

RUN cmake -S . -B build
RUN cmake --build build -j$(nproc)

Loosely speaking, the first time cmake is run it generates the build directory and writes a suitable Makefile. The second time cmake is run with the --build argument it actually does the build. It effectively uses the make command and that accepts the -j argument, specifying the number of concurrent processes. Obviously for the example code below this is overkill but will make a difference for a large project.

Files required to complete the example:

🗎 CMakeLists.txt

cmake_minimum_required(VERSION 3.10)
project(project)

add_executable(executable main.cpp)

install(TARGETS executable DESTINATION bin)

🗎 main.cpp

#include <iostream>

int main() {
    std::cout << "Hello, CMake world!" << std::endl;
    return 0;
}

Logs for relevant part of build process:

Step 4/5 : RUN cmake -S . -B build
 ---> Running in 1fceddb78831
-- The C compiler identification is GNU 11.4.0
-- The CXX compiler identification is GNU 11.4.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - 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: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /build
Removing intermediate container 1fceddb78831
 ---> 73e99ae72e90
Step 5/5 : RUN cmake --build build -j$(nproc)
 ---> Running in ac07d5ad7e0e
[ 50%] Building CXX object CMakeFiles/executable.dir/main.cpp.o
[100%] Linking CXX executable executable
[100%] Built target executable
Removing intermediate container ac07d5ad7e0e
 ---> e196a5849f75

Check executable.