Added in version 3.11.
Note
The Using Dependencies Guide
provides a high-level
introduction to this general topic. It provides a broader overview of
where the FetchContent
module fits into the bigger picture,
including its relationship to the find_package()
command.
The guide is recommended pre-reading before moving on to the details below.
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 should be defined separately from the command that performs the actual population. This separation ensures that all the dependency details are defined before anything might try to use them 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 for some dependencies and then ensuring they are populated with a separate call:
FetchContent_Declare(
googletest
GIT_REPOSITORY https://github.com/google/googletest.git
GIT_TAG 703bd9caab50b139428cea1aaff9974ebee5742e # release-1.10.0
)
FetchContent_Declare(
myCompanyIcons
URL https://intranet.mycompany.com/assets/iconset_1.12.tar.gz
URL_HASH MD5=5588a7b18261c20068beabfb4f530b87
)
FetchContent_MakeAvailable(googletest myCompanyIcons)
The FetchContent_MakeAvailable()
command ensures the named
dependencies have been populated, either by an earlier call or by populating
them itself. When performing the population, it will also add them to the
main build, if possible, so that the main build can use the populated
projects' targets, etc. See the command's documentation for how these steps
are performed.
When using a hierarchical project arrangement, projects at higher levels in the hierarchy are able to override the declared details of content specified anywhere lower in the project hierarchy. The first details to be declared for a given dependency take precedence, regardless of where in the project hierarchy that occurs. Similarly, the first call that tries to populate a dependency "wins", with subsequent populations reusing the result of the first instead of repeating the population again. See the Examples which demonstrate this scenario.
In some cases, the main project may need to have more precise control over
the population, or it may be required to explicitly define the population
steps in a way that cannot be captured by the declared details alone.
For such situations, the lower level FetchContent_GetProperties()
and
FetchContent_Populate()
commands can be used. These lack the richer
features provided by FetchContent_MakeAvailable()
though, so their
direct use should be considered a last resort. The typical pattern of such
custom steps looks like this:
# NOTE: Where possible, prefer to use FetchContent_MakeAvailable()
# instead of custom logic like this
# Check if population has already been performed
FetchContent_GetProperties(depname)
if(NOT depname_POPULATED)
# Fetch the content using previously declared details
FetchContent_Populate(depname)
# Set custom variables, policies, etc.
# ...
# Bring the populated content into the build
add_subdirectory(${depname_SOURCE_DIR} ${depname_BINARY_DIR})
endif()
The FetchContent
module also supports defining and populating
content in a single call, with no check for whether the content has been
populated elsewhere already. This should not be done in projects, but may
be appropriate for populating content in CMake's script mode.
See FetchContent_Populate()
for details.
FetchContent_Declare(
<name>
<contentOptions>...
[EXCLUDE_FROM_ALL]
[SYSTEM]
[OVERRIDE_FIND_PACKAGE |
FIND_PACKAGE_ARGS args...]
)
The FetchContent_Declare()
function records the options that describe
how to populate the specified content. 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, update or 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. The SOURCE_SUBDIR
option is an
exception, see FetchContent_MakeAvailable()
for details on how that
affects behavior.
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 703bd9caab50b139428cea1aaff9974ebee5742e # release-1.10.0
)
FetchContent_Declare(
myCompanyIcons
URL https://intranet.mycompany.com/assets/iconset_1.12.tar.gz
URL_HASH MD5=5588a7b18261c20068beabfb4f530b87
)
FetchContent_Declare(
myCompanyCertificates
SVN_REPOSITORY svn+ssh://svn.mycompany.com/srv/svn/trunk/certs
SVN_REVISION -r12345
)
Where contents are being fetched from a remote location and you do not
control that server, it is advisable to use a hash for GIT_TAG
rather
than a branch or tag name. A commit hash is more secure and helps to
confirm that the downloaded contents are what you expected.
Changed in version 3.14: Commands for the download, update or patch steps can access the terminal. This may be needed for things like password prompts or real-time display of command progress.
Added in version 3.22: The CMAKE_TLS_VERIFY
, CMAKE_TLS_CAINFO
,
CMAKE_NETRC
and CMAKE_NETRC_FILE
variables now
provide the defaults for their corresponding content options, just like
they do for ExternalProject_Add()
. Previously, these variables
were ignored by the FetchContent
module.
Added in version 3.24:
FIND_PACKAGE_ARGS
This option is for scenarios where the
FetchContent_MakeAvailable()
command may first try a call to
find_package()
to satisfy the dependency for <name>
.
By default, such a call would be simply find_package(<name>)
, but
FIND_PACKAGE_ARGS
can be used to provide additional arguments to be
appended after the <name>
. FIND_PACKAGE_ARGS
can also be given
with nothing after it, which indicates that find_package()
can
still be called if FETCHCONTENT_TRY_FIND_PACKAGE_MODE
is
set to OPT_IN
or is not set.
It would not normally be appropriate to specify REQUIRED
as one of
the additional arguments after FIND_PACKAGE_ARGS
. Doing so would
mean the find_package()
call must succeed, so none of the other
details specified in the FetchContent_Declare()
call would get a
chance to be used as a fall-back.
Everything after the FIND_PACKAGE_ARGS
keyword is appended to the
find_package()
call, so all other <contentOptions>
must
come before the FIND_PACKAGE_ARGS
keyword. If the
CMAKE_FIND_PACKAGE_TARGETS_GLOBAL
variable is set to true
at the time FetchContent_Declare()
is called, a GLOBAL
keyword
will be appended to the find_package()
arguments if it was
not already specified. It will also be appended if
FIND_PACKAGE_ARGS
was not given, but
FETCHCONTENT_TRY_FIND_PACKAGE_MODE
was set to ALWAYS
.
OVERRIDE_FIND_PACKAGE
cannot be used when FIND_PACKAGE_ARGS
is
given.
Dependency Providers discusses another way that
FetchContent_MakeAvailable()
calls can be redirected.
FIND_PACKAGE_ARGS
is intended for project control, whereas
dependency providers allow users to override project behavior.
OVERRIDE_FIND_PACKAGE
When a FetchContent_Declare(<name> ...)
call includes this option,
subsequent calls to find_package(<name> ...)
will ensure that
FetchContent_MakeAvailable(<name>)
has been called, then use the
config package files in the CMAKE_FIND_PACKAGE_REDIRECTS_DIR
directory (which are usually created by FetchContent_MakeAvailable()
).
This effectively makes FetchContent_MakeAvailable()
override
find_package()
for the named dependency, allowing the former to
satisfy the package requirements of the latter. FIND_PACKAGE_ARGS
cannot be used when OVERRIDE_FIND_PACKAGE
is given.
If a dependency provider has been set
and the project calls find_package()
for the <name>
dependency, OVERRIDE_FIND_PACKAGE
will not prevent the provider
from seeing that call. Dependency providers always have the opportunity
to intercept any direct call to find_package()
, except if that
call contains the BYPASS_PROVIDER
option.
Added in version 3.25:
SYSTEM
If the SYSTEM
argument is provided, the SYSTEM
directory
property of a subdirectory added by
FetchContent_MakeAvailable()
will be set to true. This will
affect non-imported targets created as part of that command.
See the SYSTEM
target property documentation for a more
detailed discussion of the effects.
Added in version 3.28:
EXCLUDE_FROM_ALL
If the EXCLUDE_FROM_ALL
argument is provided, then targets in the
subdirectory added by FetchContent_MakeAvailable()
will not be
included in the ALL
target by default, and may be excluded from IDE
project files. See the add_subdirectory()
EXCLUDE_FROM_ALL
argument documentation for a more detailed discussion of the effects.
Added in version 3.14.
FetchContent_MakeAvailable(<name1> [<name2>...])
This command ensures that each of the named dependencies are made available
to the project by the time it returns. There must have been a call to
FetchContent_Declare()
for each dependency, and the first such call
will control how that dependency will be made available, as described below.
If <lowercaseName>_SOURCE_DIR
is not set:
Added in version 3.24: If a dependency provider is set, call the
provider's command with FETCHCONTENT_MAKEAVAILABLE_SERIAL
as the
first argument, followed by the arguments of the first call to
FetchContent_Declare()
for <name>
. If SOURCE_DIR
or
BINARY_DIR
were not part of the original declared arguments, they
will be added with their default values.
If FETCHCONTENT_TRY_FIND_PACKAGE_MODE
was set to NEVER
when the details were declared, any FIND_PACKAGE_ARGS
will be
omitted. The OVERRIDE_FIND_PACKAGE
keyword is also always omitted.
If the provider fulfilled the request, FetchContent_MakeAvailable()
will consider that dependency handled, skip the remaining steps below
and move on to the next dependency in the list.
Added in version 3.24: If permitted, find_package(<name> [<args>...])
will be called, where <args>...
may be provided by the
FIND_PACKAGE_ARGS
option in FetchContent_Declare()
.
The value of the FETCHCONTENT_TRY_FIND_PACKAGE_MODE
variable
at the time FetchContent_Declare()
was called determines whether
FetchContent_MakeAvailable()
can call find_package()
.
If the CMAKE_FIND_PACKAGE_TARGETS_GLOBAL
variable is set to
true when FetchContent_MakeAvailable()
is called, it still affects
any imported targets created when that in turn calls
find_package()
, even if that variable was false when the
corresponding details were declared.
If the dependency was not satisfied by a provider or a
find_package()
call, FetchContent_MakeAvailable()
then uses
the following logic to make the dependency available:
If the dependency has already been populated earlier in this run, set
the <lowercaseName>_POPULATED
, <lowercaseName>_SOURCE_DIR
and
<lowercaseName>_BINARY_DIR
variables in the same way as a call to
FetchContent_GetProperties()
, then skip the remaining steps
below and move on to the next dependency in the list.
Call FetchContent_Populate()
to populate the dependency using
the details recorded by an earlier call to FetchContent_Declare()
.
Halt with a fatal error if no such details have been recorded.
FETCHCONTENT_SOURCE_DIR_<uppercaseName>
can be used to override
the declared details and use content provided at the specified location
instead.
Added in version 3.24: Ensure the CMAKE_FIND_PACKAGE_REDIRECTS_DIR
directory
contains a <lowercaseName>-config.cmake
and a
<lowercaseName>-config-version.cmake
file (or equivalently
<name>Config.cmake
and <name>ConfigVersion.cmake
).
The directory that the CMAKE_FIND_PACKAGE_REDIRECTS_DIR
variable points to is cleared at the start of every CMake run.
If no config file exists when FetchContent_Populate()
returns,
a minimal one will be written which includes
any
<lowercaseName>-extra.cmake
or <name>Extra.cmake
file with the
OPTIONAL
flag (so the files can be missing and won't generate a
warning). Similarly, if no config version file exists, a very simple
one will be written which sets PACKAGE_VERSION_COMPATIBLE
and
PACKAGE_VERSION_EXACT
to true. This ensures all future calls to
find_package()
for the dependency will use the redirected
config file, regardless of any version requirements.
CMake cannot automatically determine an arbitrary dependency's version,
so it cannot set PACKAGE_VERSION
.
When a dependency is pulled in via add_subdirectory()
in the
next step, it may choose to overwrite the generated config version file
in CMAKE_FIND_PACKAGE_REDIRECTS_DIR
with one that also sets
PACKAGE_VERSION
.
The dependency may also write a <lowercaseName>-extra.cmake
or
<name>Extra.cmake
file to perform custom processing or define any
variables that their normal (installed) package config file would
otherwise usually define (many projects don't do any custom processing
or set any variables and therefore have no need to do this).
If required, the main project can write these files instead if the
dependency project doesn't do so. This allows the main project to
add missing details from older dependencies that haven't or can't be
updated to support this functionality.
See Integrating With find_package() for examples.
If the top directory of the populated content contains a CMakeLists.txt
file, call add_subdirectory()
to add it to the main build.
It is not an error for there to be no CMakeLists.txt
file, which
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.
Added in version 3.18: The SOURCE_SUBDIR
option can be given in the declared details to
look somewhere below the top directory instead (i.e. the same way that
SOURCE_SUBDIR
is used by the ExternalProject_Add()
command). The path provided with SOURCE_SUBDIR
must be relative
and will be treated as relative to the top directory. It can also
point to a directory that does not contain a CMakeLists.txt
file
or even to a directory that doesn't exist. This can be used to avoid
adding a project that contains a CMakeLists.txt
file in its top
directory.
Added in version 3.25: If the SYSTEM
keyword was included in the call to
FetchContent_Declare()
, the SYSTEM
keyword will be
added to the add_subdirectory()
command as well.
Added in version 3.28: If the EXCLUDE_FROM_ALL
keyword was included in the call to
FetchContent_Declare()
, the EXCLUDE_FROM_ALL
keyword will
be added to the add_subdirectory()
command as well.
Added in version 3.29: CMAKE_EXPORT_FIND_PACKAGE_NAME
is set to the dependency name
before calling add_subdirectory()
.
Projects should aim to declare the details of all dependencies they might
use before they call FetchContent_MakeAvailable()
for any of them.
This ensures that if any of the dependencies are also sub-dependencies of
one or more of the others, the main project still controls the details
that will be used (because it will declare them first before the
dependencies get a chance to). In the following code samples, assume that
the uses_other
dependency also uses FetchContent
to add the other
dependency internally:
# WRONG: Should declare all details first
FetchContent_Declare(uses_other ...)
FetchContent_MakeAvailable(uses_other)
FetchContent_Declare(other ...) # Will be ignored, uses_other beat us to it
FetchContent_MakeAvailable(other) # Would use details declared by uses_other
# CORRECT: All details declared first, so they will take priority
FetchContent_Declare(uses_other ...)
FetchContent_Declare(other ...)
FetchContent_MakeAvailable(uses_other other)
Note that CMAKE_VERIFY_INTERFACE_HEADER_SETS
is explicitly set
to false upon entry to FetchContent_MakeAvailable()
, and is restored to
its original value before the command returns. Developers typically only
want to verify header sets from the main project, not those from any
dependencies. This local manipulation of the
CMAKE_VERIFY_INTERFACE_HEADER_SETS
variable provides that
intuitive behavior. You can use variables like
CMAKE_PROJECT_INCLUDE
or
CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE
to turn verification back
on for all or some dependencies. You can also set the
VERIFY_INTERFACE_HEADER_SETS
property of individual targets.
Note
Where possible, prefer to use FetchContent_MakeAvailable()
instead of implementing population manually with this command.
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:
<lowercaseName>_POPULATED
This will always be set to TRUE
by the call.
<lowercaseName>_SOURCE_DIR
The location where the populated content can be found upon return.
<lowercaseName>_BINARY_DIR
A directory intended for use as a corresponding build directory.
The main use case for the <lowercaseName>_SOURCE_DIR
and
<lowercaseName>_BINARY_DIR
variables is to call
add_subdirectory()
immediately after population:
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.
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 favor 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 <lowercaseName>_SOURCE_DIR
and <lowercaseName>_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 <lowercaseName>_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}/<lowercaseName>-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
.
This option should not be confused with the SOURCE_SUBDIR
option which
only affects the FetchContent_MakeAvailable()
command.
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}/<lowercaseName>-src
and BINARY_DIR
defaults to ${CMAKE_CURRENT_BINARY_DIR}/<lowercaseName>-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.
Added in version 3.18: Added support for the DOWNLOAD_NO_EXTRACT
option.
When using saved content details, a call to
FetchContent_MakeAvailable()
or FetchContent_Populate()
records information in global properties which can be queried at any time.
This information may include 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_MakeAvailable(name)
or
FetchContent_Populate(name)
.
Note that the SOURCE_DIR
and BINARY_DIR
values can be empty if the
call is fulfilled by a dependency provider.
This command is rarely needed when using
FetchContent_MakeAvailable()
. It is more commonly used as part of
implementing the following pattern with FetchContent_Populate()
,
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:
# Check if population has already been performed
FetchContent_GetProperties(depname)
if(NOT depname_POPULATED)
# Fetch the content using previously declared details
FetchContent_Populate(depname)
# Set custom variables, policies, etc.
# ...
# Bring the populated content into the build
add_subdirectory(${depname_SOURCE_DIR} ${depname_BINARY_DIR})
endif()
Added in version 3.24.
Note
This command should only be called by dependency providers. Calling it in any other context is unsupported and future CMake versions may halt with a fatal error in such cases.
FetchContent_SetPopulated(
<name>
[SOURCE_DIR <srcDir>]
[BINARY_DIR <binDir>]
)
If a provider command fulfills a FETCHCONTENT_MAKEAVAILABLE_SERIAL
request, it must call this function before returning. The SOURCE_DIR
and BINARY_DIR
arguments can be used to specify the values that
FetchContent_GetProperties()
should return for its corresponding
arguments. Only provide SOURCE_DIR
and BINARY_DIR
if they have
the same meaning as if they had been populated by the built-in
FetchContent_MakeAvailable()
implementation.
A number of cache variables can influence the behavior where details from a
FetchContent_Declare()
call are used to populate content.
Note
All of these variables are intended for the developer to customize behavior. They should not normally be set by the project.
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.
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.
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.
Note
The FETCHCONTENT_FULLY_DISCONNECTED
variable is not an appropriate way
to prevent any network access on the first run in a build directory.
Doing so can break projects, lead to misleading error messages, and hide
subtle population failures. This variable is specifically intended to
only be turned on after the first time CMake has been run.
If you want to prevent network access even on the first run, use a
dependency provider and populate the
dependency from local content instead.
This is a less severe download/update control compared to
FETCHCONTENT_FULLY_DISCONNECTED
. Instead of bypassing all
download and update logic, FETCHCONTENT_UPDATES_DISCONNECTED
only
prevents the update step from making connections to remote servers
when using the git or hg download methods. Updates still occur if details
about the update step change, but the update is attempted with only the
information already available locally (so switching to a different tag or
commit that is already fetched locally will succeed, but switching to an
unknown commit hash will fail). The download step is not affected, so if
content has not been downloaded previously, it will still be downloaded
when this option is enabled. This can speed up the configure step, but
not as much as FETCHCONTENT_FULLY_DISCONNECTED
.
FETCHCONTENT_UPDATES_DISCONNECTED
is OFF
by default.
Added in version 3.24.
This variable modifies the details that FetchContent_Declare()
records for a given dependency. While it ultimately controls the behavior
of FetchContent_MakeAvailable()
, it is the variable's value when
FetchContent_Declare()
is called that gets used. It makes no
difference what the variable is set to when
FetchContent_MakeAvailable()
is called. Since the variable should
only be set by the user and not by projects directly, it will typically have
the same value throughout anyway, so this distinction is not usually
noticeable.
FETCHCONTENT_TRY_FIND_PACKAGE_MODE
ultimately controls whether
FetchContent_MakeAvailable()
is allowed to call
find_package()
to satisfy a dependency. The variable can be set
to one of the following values:
OPT_IN
FetchContent_MakeAvailable()
will only call
find_package()
if the FetchContent_Declare()
call
included a FIND_PACKAGE_ARGS
keyword. This is also the default
behavior if FETCHCONTENT_TRY_FIND_PACKAGE_MODE
is not set.
ALWAYS
find_package()
can be called by
FetchContent_MakeAvailable()
regardless of whether the
FetchContent_Declare()
call included a FIND_PACKAGE_ARGS
keyword or not. If no FIND_PACKAGE_ARGS
keyword was given, the
behavior will be as though FIND_PACKAGE_ARGS
had been provided,
with no additional arguments after it.
NEVER
FetchContent_MakeAvailable()
will not call
find_package()
. Any FIND_PACKAGE_ARGS
given to the
FetchContent_Declare()
call will be ignored.
As a special case, if the FETCHCONTENT_SOURCE_DIR_<uppercaseName>
variable has a non-empty value for a dependency, it is assumed that the
user is overriding all other methods of making that dependency available.
FETCHCONTENT_TRY_FIND_PACKAGE_MODE
will have no effect on that
dependency and FetchContent_MakeAvailable()
will not try to call
find_package()
for it.
In addition to the above, the following variables are also defined for each content name:
If this is set, no download or update steps are performed for the specified
content and the <lowercaseName>_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 <lowercaseName>_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.
This is the per-content equivalent of
FETCHCONTENT_UPDATES_DISCONNECTED
. If the global option or
this option is ON
, then updates for the git and hg methods will not
contact any remote for the named content. They will only use information
already available locally. Disabling updates for individual content can
be useful for content whose details rarely change, while still leaving
other frequently changing content with updates enabled.
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 703bd9caab50b139428cea1aaff9974ebee5742e # release-1.10.0
)
FetchContent_Declare(
Catch2
GIT_REPOSITORY https://github.com/catchorg/Catch2.git
GIT_TAG 605a34765aa5d5ecbf476b4598a862ada971b0cc # v3.0.1
)
# After the following call, the CMake targets defined by googletest and
# Catch2 will be available to the rest of the build
FetchContent_MakeAvailable(googletest Catch2)
For the previous example, if the user wanted to try to find googletest
and Catch2
via find_package()
first before trying to download
and build them from source, they could set the
FETCHCONTENT_TRY_FIND_PACKAGE_MODE
variable to ALWAYS
.
This would also affect any other calls to FetchContent_Declare()
throughout the project, which might not be acceptable. The behavior can be
enabled for just these two dependencies instead by adding FIND_PACKAGE_ARGS
to the declared details and leaving
FETCHCONTENT_TRY_FIND_PACKAGE_MODE
unset, or set to OPT_IN
:
include(FetchContent)
FetchContent_Declare(
googletest
GIT_REPOSITORY https://github.com/google/googletest.git
GIT_TAG 703bd9caab50b139428cea1aaff9974ebee5742e # release-1.10.0
FIND_PACKAGE_ARGS NAMES GTest
)
FetchContent_Declare(
Catch2
GIT_REPOSITORY https://github.com/catchorg/Catch2.git
GIT_TAG 605a34765aa5d5ecbf476b4598a862ada971b0cc # v3.0.1
FIND_PACKAGE_ARGS
)
# This will try calling find_package() first for both dependencies
FetchContent_MakeAvailable(googletest Catch2)
For Catch2
, no additional arguments to find_package()
are needed,
so no additional arguments are provided after the FIND_PACKAGE_ARGS
keyword. For googletest
, its package is more commonly called GTest
,
so arguments are added to support it being found by that name.
If the user wanted to disable FetchContent_MakeAvailable()
from
calling find_package()
for any dependency, even if it provided
FIND_PACKAGE_ARGS
in its declared details, they could set
FETCHCONTENT_TRY_FIND_PACKAGE_MODE
to NEVER
.
If the project wanted to indicate that these two dependencies should be
downloaded and built from source and that find_package()
calls
should be redirected to use the built dependencies, the
OVERRIDE_FIND_PACKAGE
option should be used when declaring the content
details:
include(FetchContent)
FetchContent_Declare(
googletest
GIT_REPOSITORY https://github.com/google/googletest.git
GIT_TAG 703bd9caab50b139428cea1aaff9974ebee5742e # release-1.10.0
OVERRIDE_FIND_PACKAGE
)
FetchContent_Declare(
Catch2
GIT_REPOSITORY https://github.com/catchorg/Catch2.git
GIT_TAG 605a34765aa5d5ecbf476b4598a862ada971b0cc # v3.0.1
OVERRIDE_FIND_PACKAGE
)
# The following will automatically forward through to FetchContent_MakeAvailable()
find_package(googletest)
find_package(Catch2)
CMake provides a FindGTest module which defines some variables that older
projects may use instead of linking to the imported targets. To support
those cases, we can provide an extra file. In keeping with the
"first to define, wins" philosophy of FetchContent
, we only write out
that file if something else hasn't already done so.
FetchContent_MakeAvailable(googletest)
if(NOT EXISTS ${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/googletest-extra.cmake AND
NOT EXISTS ${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/googletestExtra.cmake)
file(WRITE ${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/googletest-extra.cmake
[=[
if("${GTEST_LIBRARIES}" STREQUAL "" AND TARGET GTest::gtest)
set(GTEST_LIBRARIES GTest::gtest)
endif()
if("${GTEST_MAIN_LIBRARIES}" STREQUAL "" AND TARGET GTest::gtest_main)
set(GTEST_MAIN_LIBRARIES GTest::gtest_main)
endif()
if("${GTEST_BOTH_LIBRARIES}" STREQUAL "")
set(GTEST_BOTH_LIBRARIES ${GTEST_LIBRARIES} ${GTEST_MAIN_LIBRARIES})
endif()
]=])
endif()
Projects will also likely be using find_package(GTest)
rather than
find_package(googletest)
, but it is possible to make use of the
CMAKE_FIND_PACKAGE_REDIRECTS_DIR
area to pull in the latter as
a dependency of the former. This is likely to be sufficient to satisfy
a typical find_package(GTest)
call.
FetchContent_MakeAvailable(googletest)
if(NOT EXISTS ${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/gtest-config.cmake AND
NOT EXISTS ${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/GTestConfig.cmake)
file(WRITE ${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/gtest-config.cmake
[=[
include(CMakeFindDependencyMacro)
find_dependency(googletest)
]=])
endif()
if(NOT EXISTS ${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/gtest-config-version.cmake AND
NOT EXISTS ${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/GTestConfigVersion.cmake)
file(WRITE ${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/gtest-config-version.cmake
[=[
include(${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/googletest-config-version.cmake OPTIONAL)
if(NOT PACKAGE_VERSION_COMPATIBLE)
include(${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/googletestConfigVersion.cmake OPTIONAL)
endif()
]=])
endif()
If the sub-project's CMakeLists.txt
file is not at the top level of its
source tree, the SOURCE_SUBDIR
option can be used to tell FetchContent
where to find it. The following example shows how to use that option, and
it also sets a variable which is meaningful to the subproject before pulling
it into the main build (set as an INTERNAL
cache variable to avoid
problems with policy CMP0077
):
include(FetchContent)
FetchContent_Declare(
protobuf
GIT_REPOSITORY https://github.com/protocolbuffers/protobuf.git
GIT_TAG ae50d9b9902526efd6c7a1907d09739f959c6297 # v3.15.0
SOURCE_SUBDIR cmake
)
set(protobuf_BUILD_TESTS OFF CACHE INTERNAL "")
FetchContent_MakeAvailable(protobuf)
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:
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 v2.3-rc1
)
# Order is important, see notes in the discussion further below
FetchContent_MakeAvailable(projD projB projC)
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)
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.
This last 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
)