User Interaction Guide — CMake 4.2.6 Documentation

# User Interaction Guide

## Introduction

Where a software package supplies a CMake-based buildsystem with the source of their software, the consumer of the software is required to run a CMake user interaction tool in order to build it.

Well-behaved CMake-based buildsystems do not create any output in the source directory, so typically, the user performs an out-of-source build and performs the build there. First, CMake must be instructed to generate a suitable buildsystem, then the user invokes a build tool to process that generated buildsystem. The generated buildsystem is specific to the machine used to generate it and is not redistributable. Each consumer of a provided source software package is required to use CMake to generate a buildsystem specific to their system.

Generated buildsystems should generally be treated as read-only. The CMake files as a primary artifact should completely specify the buildsystem and there should be no reason to populate properties manually in an IDE for example after generating the buildsystem. CMake will periodically rewrite the generated buildsystem, so modifications by users will be overwritten.

The features and user interfaces described in this manual are available for all CMake-based build systems by virtue of providing CMake files.

The CMake tooling may report errors to the user when processing provided CMake files, such as reporting that the compiler is not supported, or the compiler does not support a required compile option, or a dependency can not be found. These errors must be resolved by the user by choosing a different compiler, `installing dependencies`, or instructing CMake where to find them, etc.

### Command Line cmake tool

A simple but typical use of `cmake(1)` with a fresh copy of software source code is to create a build directory and invoke cmake there:

    $ cd some_software-1.4.2
    $ mkdir build
    $ cd build
    $ cmake .. -DCMAKE_INSTALL_PREFIX=/opt/the/prefix
    $ cmake --build .
    $ cmake --build . --target install

It is recommended to build in a separate directory to the source because that keeps the source directory pristine, allows for building a single source with multiple toolchains, and allows easy clearing of build artifacts by simply deleting the build directory.

The CMake tooling may report warnings which are intended for the provider of the software, not intended for the consumer of the software. Such warnings end with "This warning is for project developers". Users may disable such warnings by passing the `-Wno-dev` flag to `cmake(1)`.

### cmake-gui tool

Users more accustomed to GUI interfaces may use the `cmake-gui(1)` tool to invoke CMake and generate a buildsystem.

The source and binary directories must first be populated. It is always advised to use different directories for the source and the build.

## Generating a Buildsystem

There are several user interface tools which may be used to generate a buildsystem from CMake files. The `ccmake(1)` and `cmake-gui(1)` tools guide the user through setting the various necessary options. The `cmake(1)` tool can be invoked to specify options on the command line. This manual describes options which may be set using any of the user interface tools, though the mode of setting an option is different for each tool.

### Command line environment

When invoking `cmake(1)` with a command line buildsystem such as `Makefiles` or `Ninja`, it is necessary to use the correct build environment to ensure that build tools are available. CMake must be able to find the appropriate `build tool`, compiler, linker and other tools as needed.

On Linux systems, the appropriate tools are often provided in system-wide locations and may be readily installed through the system package manager. Other toolchains provided by the user or installed in non-default locations can also be used.

When cross-compiling, some platforms may require environment variables to be set or may provide scripts to set the environment.

Visual Studio ships multiple command prompts and `vcvarsall.bat` scripts for setting up the correct environments for command line buildsystems. While not strictly necessary to use a corresponding command line environment when using a Visual Studio generator, doing so has no disadvantages.

When using `Xcode`, there can be more than one Xcode version installed. Which one to use can be selected in a number of different ways, but the most common methods are:

- Setting the default version in the preferences of the Xcode IDE.

- Setting the default version via the `xcode-select` command line tool.

- Overriding the default version by setting the `DEVELOPER_DIR` environment variable when running CMake and the build tool.

For convenience, `cmake-gui(1)` provides an environment variable editor.

### Command line `-G` option

CMake chooses a generator by default based on the platform. Usually, the default generator is sufficient to allow the user to proceed to build the software.

The user may override the default generator with the `-G` option:

    $ cmake .. -G Ninja

The output of `cmake --help` includes a list of `generators` available for the user to choose from. Note that generator names are case sensitive.

On Unix-like systems (including Mac OS X), the `Unix Makefiles` generator is used by default. A variant of that generator can also be used on Windows in various environments, such as the `NMake Makefiles` and `MinGW Makefiles` generator. These generators generate a `Makefile` variant which can be executed with `make`, `gmake`, `nmake` or similar tools. See the individual generator documentation for more information on targeted environments and tools.

The `Ninja` generator is available on all major platforms. `ninja` is a build tool similar in use-cases to `make`, but with a focus on performance and efficiency.

On Windows, `cmake(1)` can be used to generate solutions for the Visual Studio IDE. Visual Studio versions may be specified by the product name of the IDE, which includes a four-digit year. Aliases are provided for other means by which Visual Studio versions are sometimes referred to, such as two digits which correspond to the product version of the VisualC++ compiler, or a combination of the two:

    $ cmake .. -G "Visual Studio 2019"
    $ cmake .. -G "Visual Studio 16"
    $ cmake .. -G "Visual Studio 16 2019"

Visual Studio Generators can target different architectures. One can specify the target architecture using the `-A` option:

    cmake .. -G "Visual Studio 2019" -A x64
    cmake .. -G "Visual Studio 16" -A ARM
    cmake .. -G "Visual Studio 16 2019" -A ARM64

On Apple, the `Xcode` generator may be used to generate project files for the Xcode IDE.

Some IDEs such as KDevelop4, QtCreator and CLion have native support for CMake-based buildsystems. Those IDEs provide user interface for selecting an underlying generator to use, typically a choice between a `Makefile` or a `Ninja` based generator.

Note that it is not possible to change the generator with `-G` after the first invocation of CMake. To change the generator, the build directory must be deleted and the build must be started from scratch.

When generating Visual Studio project and solutions files several other options are available to use when initially running `cmake(1)`.

The Visual Studio toolset can be specified with the `cmake -T` option:

    $ # Build with the clang-cl toolset
    $ cmake.exe .. -G "Visual Studio 16 2019" -A x64 -T ClangCL
    $ # Build targeting Windows XP
    $ cmake.exe .. -G "Visual Studio 16 2019" -A x64 -T v120_xp

Whereas the `-A` option specifies the _target_ architecture, the `-T` option can be used to specify details of the toolchain used. For example, `-Thost=x64` can be given to select the 64-bit version of the host tools. The following demonstrates how to use 64-bit tools and also build for a 64-bit target architecture:

    $ cmake .. -G "Visual Studio 16 2019" -A x64 -Thost=x64

### Choosing a generator in cmake-gui

The "Configure" button triggers a new dialog to select the CMake generator to use.

All generators available on the command line are also available in `cmake-gui(1)`.

When choosing a Visual Studio generator, further options are available to set an architecture to generate for.

## Setting Build Variables

Software projects often require variables to be set on the command line when invoking CMake. Some of the most commonly used CMake variables are listed in the table below:

Variable | Meaning
--- | ---
`CMAKE_PREFIX_PATH` | Path to search for `dependent packages`
`CMAKE_MODULE_PATH` | Path to search for additional CMake modules
`CMAKE_BUILD_TYPE` | Build configuration, such as `Debug` or `Release`, determining debug/optimization flags. This is only relevant for single-configuration buildsystems such as Makefile Generators and Ninja Generators. Multi-configuration buildsystems such as those for Visual Studio Generators and `Xcode` ignore this setting.
`CMAKE_INSTALL_PREFIX` | Location to install the software to with the `install` build target
`CMAKE_TOOLCHAIN_FILE` | File containing cross-compiling data such as `toolchains and sysroots`.
`BUILD_SHARED_LIBS` | Whether to build shared instead of static libraries for `add_library()` commands used without a type
`CMAKE_EXPORT_COMPILE_COMMANDS` | Generate a `compile_commands.json` file for use with clang-based tools
`CMAKE_EXPORT_BUILD_DATABASE` | Generate a `build_database.json` file for use with clang-based tools

Other project-specific variables may be available to control builds, such as enabling or disabling components of the project.

There is no convention provided by CMake for how such variables are named between different provided buildsystems, except that variables with the prefix `CMAKE_` usually refer to options provided by CMake itself and should not be used in third-party options, which should use their own prefix instead. The `cmake-gui(1)` tool can display options in groups defined by their prefix, so it makes sense for third parties to ensure that they use a self-consistent prefix.

### Setting variables on the command line

CMake variables can be set on the command line either when creating the initial build:

    $ mkdir build
    $ cd build
    $ cmake .. -G Ninja -DCMAKE_BUILD_TYPE=Debug

or later on a subsequent invocation of `cmake(1)`:

    $ cd build
    $ cmake . -DCMAKE_BUILD_TYPE=Debug

The `-U` flag may be used to unset variables on the `cmake(1)` command line:

    $ cd build
    $ cmake . -UMyPackage_DIR

A CMake buildsystem which was initially created on the command line can be modified using the `cmake-gui(1)` and vice-versa.

The `cmake(1)` tool allows specifying a file to use to populate the initial cache using the `-C` option. This can be useful to simplify commands and scripts which repeatedly require the same cache entries.

### Setting variables with cmake-gui

Variables may be set in the cmake-gui using the "Add Entry" button. This triggers a new dialog to set the value of the variable.

The main view of the `cmake-gui(1)` user interface can be used to edit existing variables.

### The CMake Cache

When CMake is executed, it needs to find the locations of compilers, tools and dependencies. It also needs to be able to consistently re-generate a buildsystem to use the same compile/link flags and paths to dependencies. Such parameters are also required to be configurable by the user because they are paths and options specific to the users system.

When it is first executed, CMake generates a `CMakeCache.txt` file in the build directory containing key-value pairs for such artifacts. The cache file can be viewed or edited by the user by running the `cmake-gui(1)` or `ccmake(1)` tool. The tools provide an interactive interface for re-configuring the provided software and re-generating the buildsystem, as is needed after editing cached values. Each cache entry may have an associated short help text which is displayed in the user interface tools.

The cache entries may also have a type to signify how it should be presented in the user interface. For example, a cache entry of type `BOOL` can be edited by a checkbox in a user interface, a `STRING` can be edited in a text field, and a `FILEPATH` while similar to a `STRING` should also provide a way to locate filesystem paths using a file dialog. An entry of type `STRING` may provide a restricted list of allowed values which are then provided in a drop-down menu in the `cmake-gui(1)` user interface (see the `STRINGS` cache property).

The CMake files shipped with a software package may also define boolean toggle options using the `option()` command. The command creates a cache entry which has a help text and a default value. Such cache entries are typically specific to the provided software and affect the configuration of the build, such as whether tests and examples are built, whether to build with exceptions enabled etc.

## Presets

CMake understands a file, `CMakePresets.json`, and its user-specific counterpart, `CMakeUserPresets.json`, for saving presets for commonly-used configure settings. These presets can set the build directory, generator, cache variables, environment variables, and other command-line options. All of these options can be overridden by the user. The full details of the `CMakePresets.json` format are listed in the `cmake-presets(7)` manual.

### Using presets on the command-line

When using the `cmake(1)` command line tool, a preset can be invoked by using the `--preset` option. If `--preset` is specified, the generator and build directory are not required, but can be specified to override them. For example, if you have the following `CMakePresets.json` file:

    {
      "version": 1,
      "configurePresets": [
        {
          "name": "ninja-release",
          "binaryDir": "${sourceDir}/build/${presetName}",
          "generator": "Ninja",
          "cacheVariables": {
            "CMAKE_BUILD_TYPE": "Release"
          }
        }
      ]
    }

and you run the following:

    cmake -S /path/to/source --preset=ninja-release

This will generate a build directory in `/path/to/source/build/ninja-release` with the `Ninja` generator, and with `CMAKE_BUILD_TYPE` set to `Release`.

If you want to see the list of available presets, you can run:

    cmake -S /path/to/source --list-presets

This will list the presets available in `/path/to/source/CMakePresets.json` and `/path/to/source/CMakeUsersPresets.json` without generating a build tree.

### Using presets in cmake-gui

If a project has presets available, either through `CMakePresets.json` or `CMakeUserPresets.json`, the list of presets will appear in a drop-down menu in `cmake-gui(1)` between the source directory and the binary directory. Choosing a preset sets the binary directory, generator, environment variables, and cache variables, but all of these options can be overridden after a preset is selected.

## Invoking the Buildsystem

After generating the buildsystem, the software can be built by invoking the particular build tool. In the case of the IDE generators, this can involve loading the generated project file into the IDE to invoke the build.

CMake is aware of the specific build tool needed to invoke a build so in general, to build a buildsystem or project from the command line after generating, the following command may be invoked in the build directory:

    $ cmake --build .

The `--build` flag enables a particular mode of operation for the `cmake(1)` tool. It invokes the `CMAKE_MAKE_PROGRAM` command associated with the `generator`, or the build tool configured by the user.

The `--build` mode also accepts the parameter `--target` to specify a particular target to build, for example a particular library, executable or custom target, or a particular special target like `install`:

    $ cmake --build . --target myexe

The `--build` mode also accepts a `--config` parameter in the case of multi-config generators to specify which particular configuration to build:

    $ cmake --build . --target myexe --config Release

The `--config` option has no effect if the generator generates a buildsystem specific to a configuration which is chosen when invoking cmake with the `CMAKE_BUILD_TYPE` variable.

Some buildsystems omit details of command lines invoked during the build. The `--verbose` flag can be used to cause those command lines to be shown:

    $ cmake --build . --target myexe --verbose

The `--build` mode can also pass particular command line options to the underlying build tool by listing them after `--`. This can be useful to specify options to the build tool, such as to continue the build after a failed job, where CMake does not provide a high-level user interface.

For all generators, it is possible to run the underlying build tool after invoking CMake. For example, `make` may be executed after generating with the `Unix Makefiles` generator to invoke the build, or `ninja` after generating with the `Ninja` generator etc. The IDE buildsystems usually provide command line tooling for building a project which can also be invoked.

### Selecting a Target

Each executable and library described in the CMake files is a build target, and the buildsystem may describe custom targets, either for internal use, or for user consumption, for example to create documentation.

CMake provides some built-in targets for all buildsystems providing CMake files.

`all`

  The default target used by Makefile Generators and Ninja Generators. Builds all targets in the buildsystem, except those which are excluded by their `EXCLUDE_FROM_ALL` target property or `EXCLUDE_FROM_ALL` directory property. The name `ALL_BUILD` is used for this purpose for the `Xcode` and Visual Studio Generators.

`help`

  Lists the targets available for build. This target is available when using the Makefile Generators or Ninja Generators, and the exact output is tool-specific.

`clean`

  Delete built object files and other output files. The Makefile Generators create a `clean` target per directory, so that an individual directory can be cleaned. The `Ninja` tool provides its own granular `-t clean` system.

`test`

  Runs tests. This target is only automatically available if the CMake files provide CTest-based tests. See also Running Tests.

`install`

  Installs the software. This target is only automatically available if the software defines install rules with the `install()` command. See also Software Installation.

`package`

  Creates a binary package. This target is only automatically available if the CMake files provide CPack-based packages.

`package_source`

  Creates a source package. This target is only automatically available if the CMake files provide CPack-based packages.

For Makefile Generators, `/fast` variants of binary build targets are provided. The `/fast` variants are used to build the specified target without regard for its dependencies. The dependencies are not checked and are not rebuilt if out of date. The Ninja Generators are sufficiently fast at dependency checking that such targets are not provided for that generator.

Makefile Generators also provide build-targets to preprocess, assemble and compile individual files in a particular directory.

    $ make foo.cpp.i
    $ make foo.cpp.s
    $ make foo.cpp.o

The file extension is built into the name of the target because another file with the same name but a different extension may exist. However, build-targets without the file extension are also provided.

    $ make foo.i
    $ make foo.s
    $ make foo.o

In buildsystems which contain `foo.c` and `foo.cpp`, building the `foo.i` target will preprocess both files.

### Specifying a Build Program

The program invoked by the `--build` mode is determined by the `CMAKE_MAKE_PROGRAM` variable. For most generators, the particular program does not need to be configured.

Generator | Default make program | Alternatives
--- | --- | ---
XCode | `xcodebuild` |
Unix Makefiles | `make` |
NMake Makefiles | `nmake` | `jom`
NMake Makefiles JOM | `jom` | `nmake`
MinGW Makefiles | `mingw32-make` |
MSYS Makefiles | `make` |
Ninja | `ninja` |
Visual Studio | `msbuild` |
Watcom WMake | `wmake` |

The `jom` tool is capable of reading makefiles of the `NMake` flavor and building in parallel, while the `nmake` tool always builds serially. After generating with the `NMake Makefiles` generator a user can run `jom` instead of `nmake`. The `--build` mode would also use `jom` if the `CMAKE_MAKE_PROGRAM` was set to `jom` while using the `NMake Makefiles` generator, and as a convenience, the `NMake Makefiles JOM` generator is provided to find `jom` in the normal way and use it as the `CMAKE_MAKE_PROGRAM`. For completeness, `nmake` is an alternative tool which can process the output of the `NMake Makefiles JOM` generator, but doing so would be a pessimization.

## Software Installation

The `CMAKE_INSTALL_PREFIX` variable can be set in the CMake cache to specify where to install the provided software. If the provided software has install rules, specified using the `install()` command, they will install artifacts into that prefix. On Windows, the default installation location corresponds to the `ProgramFiles` system directory which may be architecture specific. On Unix hosts, `/usr/local` is the default installation location.

The `CMAKE_INSTALL_PREFIX` variable always refers to the installation prefix on the target filesystem.

In cross-compiling or packaging scenarios where the sysroot is read-only or where the sysroot should otherwise remain pristine, the `CMAKE_STAGING_PREFIX` variable can be set to a location to actually install the files.

The commands:

    $ cmake .. -DCMAKE_INSTALL_PREFIX=/usr/local \
      -DCMAKE_SYSROOT=$HOME/root \
      -DCMAKE_STAGING_PREFIX=/tmp/package
    $ cmake --build .
    $ cmake --build . --target install

result in files being installed to paths such as `/tmp/package/lib/libfoo.so` on the host machine. The `/usr/local` location on the host machine is not affected.

Some provided software may specify `uninstall` rules, but CMake does not generate such rules by default itself.

## Running Tests

The `ctest(1)` tool is shipped with the CMake distribution to execute provided tests and report results. The `test` build-target is provided to run all available tests, but the `ctest(1)` tool allows granular control over which tests to run, how to run them, and how to report results. Executing `ctest(1)` in the build directory is equivalent to running the `test` target:

    $ ctest

A regular expression can be passed to run only tests which match the expression. To run only tests with `Qt` in their name:

    $ ctest -R Qt

Tests can be excluded by regular expression too. To run only tests without `Qt` in their name:

    $ ctest -E Qt

Tests can be run in parallel by passing `-j` arguments to `ctest(1)`:

    $ ctest -R Qt -j8

The environment variable `CTEST_PARALLEL_LEVEL` can alternatively be set to avoid the need to pass `-j`.

By default `ctest(1)` does not print the output from the tests. The command line argument `-V` (or `--verbose`) enables verbose mode to print the output from all tests. The `--output-on-failure` option prints the test output for failing tests only. The environment variable `CTEST_OUTPUT_ON_FAILURE` can be set to `1` as an alternative to passing the `--output-on-failure` option to `ctest(1)`.