FetchContent

Overview

This module enables populating content at configure time via any method supported by the ExternalProject module. Whereas ExternalProject_Add() downloads at build time, the FetchContent module makes content available immediately, allowing the configure step to use the content in commands like add_subdirectory(), include() or file() operations.

Content population details would normally be defined separately from the command that performs the actual population. This separation ensures that all of the dependency details are defined before anything may try to use those details to populate content. This is particularly important in more complex project hierarchies where dependencies may be shared between multiple projects.

The following shows a typical example of declaring content details:

FetchContent_Declare(
  googletest
  GIT_REPOSITORY https://github.com/google/googletest.git
  GIT_TAG        release-1.8.0
)

For most typical cases, populating the content can then be done with a single command like so:

FetchContent_MakeAvailable(googletest)

The above command not only populates the content, it also adds it to the main build (if possible) so that the main build can use the populated project’s targets, etc. In some cases, the main project may need to have more precise control over the population or may be required to explicitly define the population steps (e.g. if CMake versions earlier than 3.14 need to be supported). The typical pattern of such custom steps looks like this:

FetchContent_GetProperties(googletest)
if(NOT googletest_POPULATED)
  FetchContent_Populate(googletest)
  add_subdirectory(${googletest_SOURCE_DIR} ${googletest_BINARY_DIR})
endif()

Regardless of which population method is used, when using the declare-populate pattern with a hierarchical project arrangement, projects at higher levels in the hierarchy are able to override the population details of content specified anywhere lower in the project hierarchy. The ability to detect whether content has already been populated ensures that even if multiple child projects want certain content to be available, the first one to populate it wins. The other child project can simply make use of the already available content instead of repeating the population for itself. See the Examples section which demonstrates this scenario.

The FetchContent module also supports defining and populating content in a single call, with no check for whether the content has been populated elsewhere in the project already. This is a more low level operation and would not normally be the way the module is used, but it is sometimes useful as part of implementing some higher level feature or to populate some content in CMake’s script mode.

Declaring Content Details

FetchContent_Declare
FetchContent_Declare(<name> <contentOptions>...)

The FetchContent_Declare() function records the options that describe how to populate the specified content, but if such details have already been recorded earlier in this project (regardless of where in the project hierarchy), this and all later calls for the same content <name> are ignored. This “first to record, wins” approach is what allows hierarchical projects to have parent projects override content details of child projects.

The content <name> can be any string without spaces, but good practice would be to use only letters, numbers and underscores. The name will be treated case-insensitively and it should be obvious for the content it represents, often being the name of the child project or the value given to its top level project() command (if it is a CMake project). For well-known public projects, the name should generally be the official name of the project. Choosing an unusual name makes it unlikely that other projects needing that same content will use the same name, leading to the content being populated multiple times.

The <contentOptions> can be any of the download or update/patch options that the ExternalProject_Add() command understands. The configure, build, install and test steps are explicitly disabled and therefore options related to them will be ignored. In most cases, <contentOptions> will just be a couple of options defining the download method and method-specific details like a commit tag or archive hash. For example:

FetchContent_Declare(
  googletest
  GIT_REPOSITORY https://github.com/google/googletest.git
  GIT_TAG        release-1.8.0
)

FetchContent_Declare(
  myCompanyIcons
  URL      https://intranet.mycompany.com/assets/iconset_1.12.tar.gz
  URL_HASH 5588a7b18261c20068beabfb4f530b87
)

FetchContent_Declare(
  myCompanyCertificates
  SVN_REPOSITORY svn+ssh://svn.mycompany.com/srv/svn/trunk/certs
  SVN_REVISION   -r12345
)

Populating The Content

For most common scenarios, population means making content available to the main build according to previously declared details for that dependency. There are two main patterns for populating content, one based on calling FetchContent_GetProperties() and FetchContent_Populate() for more precise control and the other on calling FetchContent_MakeAvailable() for a simpler, more automated approach. The former generally follows this canonical pattern:

# Check if population has already been performed
FetchContent_GetProperties(<name>)
string(TOLOWER "<name>" lcName)
if(NOT ${lcName}_POPULATED)
  # Fetch the content using previously declared details
  FetchContent_Populate(<name>)

  # Set custom variables, policies, etc.
  # ...

  # Bring the populated content into the build
  add_subdirectory(${${lcName}_SOURCE_DIR} ${${lcName}_BINARY_DIR})
endif()

The above is such a common pattern that, where no custom steps are needed between the calls to FetchContent_Populate() and add_subdirectory(), equivalent logic can be obtained by calling FetchContent_MakeAvailable() instead (and should be preferred where it meets the needs of the project).

FetchContent_Populate
FetchContent_Populate( <name> )

In most cases, the only argument given to FetchContent_Populate() is the <name>. When used this way, the command assumes the content details have been recorded by an earlier call to FetchContent_Declare(). The details are stored in a global property, so they are unaffected by things like variable or directory scope. Therefore, it doesn’t matter where in the project the details were previously declared, as long as they have been declared before the call to FetchContent_Populate(). Those saved details are then used to construct a call to ExternalProject_Add() in a private sub-build to perform the content population immediately. The implementation of ExternalProject_Add() ensures that if the content has already been populated in a previous CMake run, that content will be reused rather than repopulating them again. For the common case where population involves downloading content, the cost of the download is only paid once.

An internal global property records when a particular content population request has been processed. If FetchContent_Populate() is called more than once for the same content name within a configure run, the second call will halt with an error. Projects can and should check whether content population has already been processed with the FetchContent_GetProperties() command before calling FetchContent_Populate().

FetchContent_Populate() will set three variables in the scope of the caller; <lcName>_POPULATED, <lcName>_SOURCE_DIR and <lcName>_BINARY_DIR, where <lcName> is the lowercased <name>. <lcName>_POPULATED will always be set to True by the call. <lcName>_SOURCE_DIR is the location where the content can be found upon return (it will have already been populated), while <lcName>_BINARY_DIR is a directory intended for use as a corresponding build directory. The main use case for the two directory variables is to call add_subdirectory() immediately after population, i.e.:

FetchContent_Populate(FooBar ...)
add_subdirectory(${foobar_SOURCE_DIR} ${foobar_BINARY_DIR})

The values of the three variables can also be retrieved from anywhere in the project hierarchy using the FetchContent_GetProperties() command.

A number of cache variables influence the behavior of all content population performed using details saved from a FetchContent_Declare() call:

FETCHCONTENT_BASE_DIR

In most cases, the saved details do not specify any options relating to the directories to use for the internal sub-build, final source and build areas. It is generally best to leave these decisions up to the FetchContent module to handle on the project’s behalf. The FETCHCONTENT_BASE_DIR cache variable controls the point under which all content population directories are collected, but in most cases developers would not need to change this. The default location is ${CMAKE_BINARY_DIR}/_deps, but if developers change this value, they should aim to keep the path short and just below the top level of the build tree to avoid running into path length problems on Windows.

FETCHCONTENT_QUIET

The logging output during population can be quite verbose, making the configure stage quite noisy. This cache option (ON by default) hides all population output unless an error is encountered. If experiencing problems with hung downloads, temporarily switching this option off may help diagnose which content population is causing the issue.

FETCHCONTENT_FULLY_DISCONNECTED

When this option is enabled, no attempt is made to download or update any content. It is assumed that all content has already been populated in a previous run or the source directories have been pointed at existing contents the developer has provided manually (using options described further below). When the developer knows that no changes have been made to any content details, turning this option ON can significantly speed up the configure stage. It is OFF by default.

FETCHCONTENT_UPDATES_DISCONNECTED

This is a less severe download/update control compared to FETCHCONTENT_FULLY_DISCONNECTED. Instead of bypassing all download and update logic, the FETCHCONTENT_UPDATES_DISCONNECTED only disables the update stage. Therefore, if content has not been downloaded previously, it will still be downloaded when this option is enabled. This can speed up the configure stage, but not as much as FETCHCONTENT_FULLY_DISCONNECTED. It is OFF by default.

In addition to the above cache variables, the following cache variables are also defined for each content name (<ucName> is the uppercased value of <name>):

FETCHCONTENT_SOURCE_DIR_<ucName>

If this is set, no download or update steps are performed for the specified content and the <lcName>_SOURCE_DIR variable returned to the caller is pointed at this location. This gives developers a way to have a separate checkout of the content that they can modify freely without interference from the build. The build simply uses that existing source, but it still defines <lcName>_BINARY_DIR to point inside its own build area. Developers are strongly encouraged to use this mechanism rather than editing the sources populated in the default location, as changes to sources in the default location can be lost when content population details are changed by the project.

FETCHCONTENT_UPDATES_DISCONNECTED_<ucName>

This is the per-content equivalent of FETCHCONTENT_UPDATES_DISCONNECTED. If the global option or this option is ON, then updates will be disabled for the named content. Disabling updates for individual content can be useful for content whose details rarely change, while still leaving other frequently changing content with updates enabled.

The FetchContent_Populate() command also supports a syntax allowing the content details to be specified directly rather than using any saved details. This is more low-level and use of this form is generally to be avoided in favour of using saved content details as outlined above. Nevertheless, in certain situations it can be useful to invoke the content population as an isolated operation (typically as part of implementing some other higher level feature or when using CMake in script mode):

FetchContent_Populate( <name>
  [QUIET]
  [SUBBUILD_DIR <subBuildDir>]
  [SOURCE_DIR <srcDir>]
  [BINARY_DIR <binDir>]
  ...
)

This form has a number of key differences to that where only <name> is provided:

  • All required population details are assumed to have been provided directly in the call to FetchContent_Populate(). Any saved details for <name> are ignored.

  • No check is made for whether content for <name> has already been populated.

  • No global property is set to record that the population has occurred.

  • No global properties record the source or binary directories used for the populated content.

  • The FETCHCONTENT_FULLY_DISCONNECTED and FETCHCONTENT_UPDATES_DISCONNECTED cache variables are ignored.

The <lcName>_SOURCE_DIR and <lcName>_BINARY_DIR variables are still returned to the caller, but since these locations are not stored as global properties when this form is used, they are only available to the calling scope and below rather than the entire project hierarchy. No <lcName>_POPULATED variable is set in the caller’s scope with this form.

The supported options for FetchContent_Populate() are the same as those for FetchContent_Declare(). Those few options shown just above are either specific to FetchContent_Populate() or their behavior is slightly modified from how ExternalProject_Add() treats them.

QUIET

The QUIET option can be given to hide the output associated with populating the specified content. If the population fails, the output will be shown regardless of whether this option was given or not so that the cause of the failure can be diagnosed. The global FETCHCONTENT_QUIET cache variable has no effect on FetchContent_Populate() calls where the content details are provided directly.

SUBBUILD_DIR

The SUBBUILD_DIR argument can be provided to change the location of the sub-build created to perform the population. The default value is ${CMAKE_CURRENT_BINARY_DIR}/<lcName>-subbuild and it would be unusual to need to override this default. If a relative path is specified, it will be interpreted as relative to CMAKE_CURRENT_BINARY_DIR.

SOURCE_DIR, BINARY_DIR

The SOURCE_DIR and BINARY_DIR arguments are supported by ExternalProject_Add(), but different default values are used by FetchContent_Populate(). SOURCE_DIR defaults to ${CMAKE_CURRENT_BINARY_DIR}/<lcName>-src and BINARY_DIR defaults to ${CMAKE_CURRENT_BINARY_DIR}/<lcName>-build. If a relative path is specified, it will be interpreted as relative to CMAKE_CURRENT_BINARY_DIR.

In addition to the above explicit options, any other unrecognized options are passed through unmodified to ExternalProject_Add() to perform the download, patch and update steps. The following options are explicitly prohibited (they are disabled by the FetchContent_Populate() command):

  • CONFIGURE_COMMAND

  • BUILD_COMMAND

  • INSTALL_COMMAND

  • TEST_COMMAND

If using FetchContent_Populate() within CMake’s script mode, be aware that the implementation sets up a sub-build which therefore requires a CMake generator and build tool to be available. If these cannot be found by default, then the CMAKE_GENERATOR and/or CMAKE_MAKE_PROGRAM variables will need to be set appropriately on the command line invoking the script.

FetchContent_GetProperties

When using saved content details, a call to FetchContent_Populate() records information in global properties which can be queried at any time. This information includes the source and binary directories associated with the content and also whether or not the content population has been processed during the current configure run.

FetchContent_GetProperties( <name>
  [SOURCE_DIR <srcDirVar>]
  [BINARY_DIR <binDirVar>]
  [POPULATED <doneVar>]
)

The SOURCE_DIR, BINARY_DIR and POPULATED options can be used to specify which properties should be retrieved. Each option accepts a value which is the name of the variable in which to store that property. Most of the time though, only <name> is given, in which case the call will then set the same variables as a call to FetchContent_Populate(name). This allows the following canonical pattern to be used, which ensures that the relevant variables will always be defined regardless of whether or not the population has been performed elsewhere in the project already:

FetchContent_GetProperties(foobar)
if(NOT foobar_POPULATED)
  FetchContent_Populate(foobar)
  ...
endif()

The above pattern allows other parts of the overall project hierarchy to re-use the same content and ensure that it is only populated once.

FetchContent_MakeAvailable
FetchContent_MakeAvailable( <name1> [<name2>...] )

This command implements the common pattern typically needed for most dependencies. It iterates over each of the named dependencies in turn and for each one it loosely follows the same canonical pattern as presented at the beginning of this section. One small difference to that pattern is that it will only call add_subdirectory() on the populated content if there is a CMakeLists.txt file in its top level source directory. This allows the command to be used for dependencies that make downloaded content available at a known location but which do not need or support being added directly to the build.

Examples

This first fairly straightforward example ensures that some popular testing frameworks are available to the main build:

include(FetchContent)
FetchContent_Declare(
  googletest
  GIT_REPOSITORY https://github.com/google/googletest.git
  GIT_TAG        release-1.8.0
)
FetchContent_Declare(
  Catch2
  GIT_REPOSITORY https://github.com/catchorg/Catch2.git
  GIT_TAG        v2.5.0
)

# After the following call, the CMake targets defined by googletest and
# Catch2 will be defined and available to the rest of the build
FetchContent_MakeAvailable(googletest Catch2)

In more complex project hierarchies, the dependency relationships can be more complicated. Consider a hierarchy where projA is the top level project and it depends directly on projects projB and projC. Both projB and projC can be built standalone and they also both depend on another project projD. projB additionally depends on projE. This example assumes that all five projects are available on a company git server. The CMakeLists.txt of each project might have sections like the following:

projA:

include(FetchContent)
FetchContent_Declare(
  projB
  GIT_REPOSITORY git@mycompany.com:git/projB.git
  GIT_TAG        4a89dc7e24ff212a7b5167bef7ab079d
)
FetchContent_Declare(
  projC
  GIT_REPOSITORY git@mycompany.com:git/projC.git
  GIT_TAG        4ad4016bd1d8d5412d135cf8ceea1bb9
)
FetchContent_Declare(
  projD
  GIT_REPOSITORY git@mycompany.com:git/projD.git
  GIT_TAG        origin/integrationBranch
)
FetchContent_Declare(
  projE
  GIT_REPOSITORY git@mycompany.com:git/projE.git
  GIT_TAG        origin/release/2.3-rc1
)

# Order is important, see notes in the discussion further below
FetchContent_MakeAvailable(projD projB projC)

projB:

include(FetchContent)
FetchContent_Declare(
  projD
  GIT_REPOSITORY git@mycompany.com:git/projD.git
  GIT_TAG        20b415f9034bbd2a2e8216e9a5c9e632
)
FetchContent_Declare(
  projE
  GIT_REPOSITORY git@mycompany.com:git/projE.git
  GIT_TAG        68e20f674a48be38d60e129f600faf7d
)

FetchContent_MakeAvailable(projD projE)

projC:

include(FetchContent)
FetchContent_Declare(
  projD
  GIT_REPOSITORY git@mycompany.com:git/projD.git
  GIT_TAG        7d9a17ad2c962aa13e2fbb8043fb6b8a
)

# This particular version of projD requires workarounds
FetchContent_GetProperties(projD)
if(NOT projd_POPULATED)
  FetchContent_Populate(projD)

  # Copy an additional/replacement file into the populated source
  file(COPY someFile.c DESTINATION ${projd_SOURCE_DIR}/src)

  add_subdirectory(${projd_SOURCE_DIR} ${projd_BINARY_DIR})
endif()

A few key points should be noted in the above:

  • projB and projC define different content details for projD, but projA also defines a set of content details for projD. Because projA will define them first, the details from projB and projC will not be used. The override details defined by projA are not required to match either of those from projB or projC, but it is up to the higher level project to ensure that the details it does define still make sense for the child projects.

  • In the projA call to FetchContent_MakeAvailable(), projD is listed ahead of projB and projC to ensure that projA is in control of how projD is populated.

  • While projA defines content details for projE, it does not need to explicitly call FetchContent_MakeAvailable(projE) or FetchContent_Populate(projD) itself. Instead, it leaves that to the child projB. For higher level projects, it is often enough to just define the override content details and leave the actual population to the child projects. This saves repeating the same thing at each level of the project hierarchy unnecessarily.

Projects don’t always need to add the populated content to the build. Sometimes the project just wants to make the downloaded content available at a predictable location. The next example ensures that a set of standard company toolchain files (and potentially even the toolchain binaries themselves) is available early enough to be used for that same build.

cmake_minimum_required(VERSION 3.14)

include(FetchContent)
FetchContent_Declare(
  mycom_toolchains
  URL  https://intranet.mycompany.com//toolchains_1.3.2.tar.gz
)
FetchContent_MakeAvailable(mycom_toolchains)

project(CrossCompileExample)

The project could be configured to use one of the downloaded toolchains like so:

cmake -DCMAKE_TOOLCHAIN_FILE=_deps/mycom_toolchains-src/toolchain_arm.cmake /path/to/src

When CMake processes the CMakeLists.txt file, it will download and unpack the tarball into _deps/mycompany_toolchains-src relative to the build directory. The CMAKE_TOOLCHAIN_FILE variable is not used until the project() command is reached, at which point CMake looks for the named toolchain file relative to the build directory. Because the tarball has already been downloaded and unpacked by then, the toolchain file will be in place, even the very first time that cmake is run in the build directory.

Lastly, the following example demonstrates how one might download and unpack a firmware tarball using CMake’s script mode. The call to FetchContent_Populate() specifies all the content details and the unpacked firmware will be placed in a firmware directory below the current working directory.

getFirmware.cmake:

# NOTE: Intended to be run in script mode with cmake -P
include(FetchContent)
FetchContent_Populate(
  firmware
  URL        https://mycompany.com/assets/firmware-1.23-arm.tar.gz
  URL_HASH   MD5=68247684da89b608d466253762b0ff11
  SOURCE_DIR firmware
)