cmake -G: choose the generator (Ninja, Unix Makefiles, Visual Studio)

cmake -G Ninja -S . -B build tells CMake which build system to generate: Ninja files, Makefiles, a Visual Studio solution, an Xcode project. CMake does not compile anything itself; the generator decides which tool cmake --build runs later. Without -G, CMake uses the CMAKE_GENERATOR environment variable or the platform default, which is Unix Makefiles on Linux and macOS and the newest installed Visual Studio on Windows. The choice is stored in the build directory's cache and cannot be changed afterwards for that directory.

Generators side by side

The example uses CMake 4.4.4 and Ninja 1.13.0 from PyPI through uv, with a project that needs no compiler:

cat > CMakeLists.txt <<'EOF'
cmake_minimum_required(VERSION 3.20)
project(demo NONE)
message(STATUS "Generator: ${CMAKE_GENERATOR}")
EOF
cmake() { uv run -q --no-project --python 3.13 --with cmake==4.4.4 --with ninja==1.13.0 cmake "$@"; }
echo "--- -G Ninja";               cmake -S . -B b1 -G Ninja | grep Generator;               ls b1 | grep ninja
echo "--- -G \"Ninja Multi-Config\""; cmake -S . -B b2 -G "Ninja Multi-Config" | grep Generator; ls b2 | grep ninja
echo "--- CMAKE_GENERATOR=Ninja";   CMAKE_GENERATOR=Ninja cmake -S . -B b3 | grep Generator

--- -G Ninja
-- Generator: Ninja
build.ninja
--- -G "Ninja Multi-Config"
-- Generator: Ninja Multi-Config
build-Debug.ninja
build-RelWithDebInfo.ninja
build-Release.ninja
build.ninja
--- CMAKE_GENERATOR=Ninja
-- Generator: Ninja

Ninja writes one build.ninja; Ninja Multi-Config writes one file per configuration, which is why it ignores CMAKE_BUILD_TYPE (see -DCMAKE_BUILD_TYPE). The environment variable has the same effect as -G.

Common generators

The name after -G must be written exactly as cmake --help prints it, including spaces and capitals; quote it when it contains spaces.

Errors -G can cause

printf 'cmake_minimum_required(VERSION 3.20)\nproject(demo NONE)\n' > CMakeLists.txt
cmake() { uv run -q --no-project --python 3.13 --with cmake==4.4.4 --with ninja==1.13.0 cmake "$@"; }
echo "--- wrong case";        cmake -S . -B x -G ninja 2>&1 | head -1
echo "--- tool not installed"; cmake -S . -B m -G "Unix Makefiles" 2>&1 | head -1
echo "--- switch generator"
cmake -S . -B build -G Ninja > /dev/null
cmake -S . -B build -G "Ninja Multi-Config" 2>&1 | head -3

--- wrong case
CMake Error: Could not create named generator ninja
--- tool not installed
CMake Error: CMake was unable to find a build program corresponding to "Unix Makefiles".  CMAKE_MAKE_PROGRAM is not set.  You probably need to select a different build tool.
--- switch generator
CMake Error: Error: generator : Ninja Multi-Config
Does not match the generator used previously: Ninja
Either remove the CMakeCache.txt file and CMakeFiles directory or choose a different binary directory.

• Could not create named generator means the name is unknown: generator names are case-sensitive (ninja fails, Ninja works), and some exist only on one platform.
• unable to find a build program corresponding to "Unix Makefiles" means the generator exists but its tool (make here) is not installed or not on PATH. Install it, or pick a generator whose tool you have.
• Does not match the generator used previously appears when an existing build directory was configured with another generator. Use a new directory or cmake --fresh, as explained in clean a CMake build.

Related options

Using cmake --build build in scripts and CI keeps them independent of the generator: the same command drives Ninja, make or MSBuild.

FAQ