cmake-developer(7)¶
Introduction¶
This manual is intended for reference by developers working with
cmake-language(7)
code, whether writing their own modules,
authoring their own build systems, or working on CMake itself.
See https://cmake.org/get-involved/ to get involved in development of CMake upstream. It includes links to contribution instructions, which in turn link to developer guides for CMake itself.
Accessing Windows Registry¶
CMake offers some facilities to access the registry on Windows
platforms.
Query Windows Registry¶
New in version 3.24.
The cmake_host_system_information()
command offers the possibility to
query the registry on the local computer. See
cmake_host_system(QUERY_WINDOWS_REGISTRY) for
more information.
Find Using Windows Registry¶
Changed in version 3.24.
Options HINTS
and PATHS
of find_file()
,
find_library()
, find_path()
, find_program()
, and
find_package()
commands offer the possibility, on Windows
platform, to query the registry.
The formal syntax, as specified using BNF notation with the regular extensions, for registry query is the following:
registry_query ::= '['sep_definition
?root_key
((key_separator
sub_key
)? (value_separator
value_name
_)?)? ']' sep_definition ::= '{'value_separator
'}' root_key ::= 'HKLM' | 'HKEY_LOCAL_MACHINE' | 'HKCU' | 'HKEY_CURRENT_USER' | 'HKCR' | 'HKEY_CLASSES_ROOT' | 'HKCC' | 'HKEY_CURRENT_CONFIG' | 'HKU' | 'HKEY_USERS' sub_key ::=element
(key_separator
element
)* key_separator ::= '/' | '\\' value_separator ::=element
| ';' value_name ::=element
| '(default)' element ::=character
\+ character ::= <any character exceptkey_separator
andvalue_separator
>
The sep_definition
optional item offers the possibility to specify
the string used to separate the sub_key
from the value_name
item. If not specified, the character ;
is used. Multiple
registry_query
items can be specified as part of a path.
# example using default separator
find_file(... PATHS "/root/[HKLM/Stuff;InstallDir]/lib[HKLM\\\\Stuff;Architecture]")
# example using different specified separators
find_library(... HINTS "/root/[{|}HKCU/Stuff|InstallDir]/lib[{@@}HKCU\\\\Stuff@@Architecture]")
If the value_name
item is not specified or has the special name
(default)
, the content of the default value, if any, will be returned. The
supported types for the value_name
are:
REG_SZ
.REG_EXPAND_SZ
. The returned data is expanded.REG_DWORD
.REG_QWORD
.
When the registry query failed, typically because the key does not exist or
the data type is not supported, the string /REGISTRY-NOTFOUND
is substituted
to the []
query expression.
Find Modules¶
A "find module" is a Find<PackageName>.cmake
file to be loaded by the
find_package()
command when invoked for <PackageName>
.
The primary task of a find module is to determine whether a package is
available, set the <PackageName>_FOUND
variable to reflect this and
provide any variables, macros and imported targets required to use the
package. A find module is useful in cases where an upstream library does
not provide a config file package.
The traditional approach is to use variables for everything, including libraries and executables: see the Standard Variable Names section below. This is what most of the existing find modules provided by CMake do.
The more modern approach is to behave as much like config file packages files as possible, by providing imported target. This has the advantage of propagating usage requirements to consumers.
In either case (or even when providing both variables and imported targets), find modules should provide backwards compatibility with old versions that had the same name.
A FindFoo.cmake module will typically be loaded by the command:
find_package(Foo [major[.minor[.patch[.tweak]]]]
[EXACT] [QUIET] [REQUIRED]
[[COMPONENTS] [components...]]
[OPTIONAL_COMPONENTS components...]
[NO_POLICY_SCOPE])
See the find_package()
documentation for details on what
variables are set for the find module. Most of these are dealt with by
using FindPackageHandleStandardArgs
.
Briefly, the module should only locate versions of the package
compatible with the requested version, as described by the
Foo_FIND_VERSION
family of variables. If Foo_FIND_QUIETLY
is
set to true, it should avoid printing messages, including anything
complaining about the package not being found. If Foo_FIND_REQUIRED
is set to true, the module should issue a FATAL_ERROR
if the package
cannot be found. If neither are set to true, it should print a
non-fatal message if it cannot find the package.
Packages that find multiple semi-independent parts (like bundles of
libraries) should search for the components listed in
Foo_FIND_COMPONENTS
if it is set , and only set Foo_FOUND
to
true if for each searched-for component <c>
that was not found,
Foo_FIND_REQUIRED_<c>
is not set to true. The HANDLE_COMPONENTS
argument of find_package_handle_standard_args()
can be used to
implement this.
If Foo_FIND_COMPONENTS
is not set, which modules are searched for
and required is up to the find module, but should be documented.
For internal implementation, it is a generally accepted convention that variables starting with underscore are for temporary use only.
Standard Variable Names¶
For a FindXxx.cmake
module that takes the approach of setting
variables (either instead of or in addition to creating imported
targets), the following variable names should be used to keep things
consistent between Find modules. Note that all variables start with
Xxx_
, which (unless otherwise noted) must match exactly the name
of the FindXxx.cmake
file, including upper/lowercase.
This prefix on the variable names ensures that they do not conflict with
variables of other Find modules. The same pattern should also be followed
for any macros, functions and imported targets defined by the Find module.
Xxx_INCLUDE_DIRS
The final set of include directories listed in one variable for use by client code. This should not be a cache entry (note that this also means this variable should not be used as the result variable of a
find_path()
command - seeXxx_INCLUDE_DIR
below for that).Xxx_LIBRARIES
The libraries to use with the module. These may be CMake targets, full absolute paths to a library binary or the name of a library that the linker must find in its search path. This should not be a cache entry (note that this also means this variable should not be used as the result variable of a
find_library()
command - seeXxx_LIBRARY
below for that).Xxx_DEFINITIONS
The compile definitions to use when compiling code that uses the module. This really shouldn't include options such as
-DHAS_JPEG
that a client source-code file uses to decide whether to#include <jpeg.h>
Xxx_EXECUTABLE
The full absolute path to an executable. In this case,
Xxx
might not be the name of the module, it might be the name of the tool (usually converted to all uppercase), assuming that tool has such a well-known name that it is unlikely that another tool with the same name exists. It would be appropriate to use this as the result variable of afind_program()
command.Xxx_YYY_EXECUTABLE
Similar to
Xxx_EXECUTABLE
except here theXxx
is always the module name andYYY
is the tool name (again, usually fully uppercase). Prefer this form if the tool name is not very widely known or has the potential to clash with another tool. For greater consistency, also prefer this form if the module provides more than one executable.Xxx_LIBRARY_DIRS
Optionally, the final set of library directories listed in one variable for use by client code. This should not be a cache entry.
Xxx_ROOT_DIR
Where to find the base directory of the module.
Xxx_VERSION_VV
Variables of this form specify whether the
Xxx
module being provided is versionVV
of the module. There should not be more than one variable of this form set to true for a given module. For example, a moduleBarry
might have evolved over many years and gone through a number of different major versions. Version 3 of theBarry
module might set the variableBarry_VERSION_3
to true, whereas an older version of the module might setBarry_VERSION_2
to true instead. It would be an error for bothBarry_VERSION_3
andBarry_VERSION_2
to both be set to true.Xxx_WRAP_YY
When a variable of this form is set to false, it indicates that the relevant wrapping command should not be used. The wrapping command depends on the module, it may be implied by the module name or it might be specified by the
YY
part of the variable.Xxx_Yy_FOUND
For variables of this form,
Yy
is the name of a component for the module. It should match exactly one of the valid component names that may be passed to thefind_package()
command for the module. If a variable of this form is set to false, it means that theYy
component of moduleXxx
was not found or is not available. Variables of this form would typically be used for optional components so that the caller can check whether an optional component is available.Xxx_FOUND
When the
find_package()
command returns to the caller, this variable will be set to true if the module was deemed to have been found successfully.Xxx_NOT_FOUND_MESSAGE
Should be set by config-files in the case that it has set
Xxx_FOUND
to FALSE. The contained message will be printed by thefind_package()
command and byfind_package_handle_standard_args()
to inform the user about the problem. Use this instead of callingmessage()
directly to report a reason for failing to find the module or package.Xxx_RUNTIME_LIBRARY_DIRS
Optionally, the runtime library search path for use when running an executable linked to shared libraries. The list should be used by user code to create the
PATH
on windows orLD_LIBRARY_PATH
on UNIX. This should not be a cache entry.Xxx_VERSION
The full version string of the package found, if any. Note that many existing modules provide
Xxx_VERSION_STRING
instead.Xxx_VERSION_MAJOR
The major version of the package found, if any.
Xxx_VERSION_MINOR
The minor version of the package found, if any.
Xxx_VERSION_PATCH
The patch version of the package found, if any.
The following names should not usually be used in CMakeLists.txt
files.
They are intended for use by Find modules to specify and cache the locations
of specific files or directories. Users are typically able to set and edit
these variables to control the behavior of Find modules (like entering the
path to a library manually):
Xxx_LIBRARY
The path of the library. Use this form only when the module provides a single library. It is appropriate to use this as the result variable in a
find_library()
command.Xxx_Yy_LIBRARY
The path of library
Yy
provided by the moduleXxx
. Use this form when the module provides more than one library or where other modules may also provide a library of the same name. It is also appropriate to use this form as the result variable in afind_library()
command.Xxx_INCLUDE_DIR
When the module provides only a single library, this variable can be used to specify where to find headers for using the library (or more accurately, the path that consumers of the library should add to their header search path). It would be appropriate to use this as the result variable in a
find_path()
command.Xxx_Yy_INCLUDE_DIR
If the module provides more than one library or where other modules may also provide a library of the same name, this form is recommended for specifying where to find headers for using library
Yy
provided by the module. Again, it would be appropriate to use this as the result variable in afind_path()
command.
To prevent users being overwhelmed with settings to configure, try to
keep as many options as possible out of the cache, leaving at least one
option which can be used to disable use of the module, or locate a
not-found library (e.g. Xxx_ROOT_DIR
). For the same reason, mark
most cache options as advanced. For packages which provide both debug
and release binaries, it is common to create cache variables with a
_LIBRARY_<CONFIG>
suffix, such as Foo_LIBRARY_RELEASE
and
Foo_LIBRARY_DEBUG
. The SelectLibraryConfigurations
module
can be helpful for such cases.
While these are the standard variable names, you should provide backwards compatibility for any old names that were actually in use. Make sure you comment them as deprecated, so that no-one starts using them.
A Sample Find Module¶
We will describe how to create a simple find module for a library Foo
.
The top of the module should begin with a license notice, followed by
a blank line, and then followed by a Bracket Comment. The comment
should begin with .rst:
to indicate that the rest of its content is
reStructuredText-format documentation. For example:
# Distributed under the OSI-approved BSD 3-Clause License. See accompanying
# file Copyright.txt or https://cmake.org/licensing for details.
#[=======================================================================[.rst:
FindFoo
-------
Finds the Foo library.
Imported Targets
^^^^^^^^^^^^^^^^
This module provides the following imported targets, if found:
``Foo::Foo``
The Foo library
Result Variables
^^^^^^^^^^^^^^^^
This will define the following variables:
``Foo_FOUND``
True if the system has the Foo library.
``Foo_VERSION``
The version of the Foo library which was found.
``Foo_INCLUDE_DIRS``
Include directories needed to use Foo.
``Foo_LIBRARIES``
Libraries needed to link to Foo.
Cache Variables
^^^^^^^^^^^^^^^
The following cache variables may also be set:
``Foo_INCLUDE_DIR``
The directory containing ``foo.h``.
``Foo_LIBRARY``
The path to the Foo library.
#]=======================================================================]
The module documentation consists of:
An underlined heading specifying the module name.
A simple description of what the module finds. More description may be required for some packages. If there are caveats or other details users of the module should be aware of, specify them here.
A section listing imported targets provided by the module, if any.
A section listing result variables provided by the module.
Optionally a section listing cache variables used by the module, if any.
If the package provides any macros or functions, they should be listed in
an additional section, but can be documented by additional .rst:
comment blocks immediately above where those macros or functions are defined.
The find module implementation may begin below the documentation block. Now the actual libraries and so on have to be found. The code here will obviously vary from module to module (dealing with that, after all, is the point of find modules), but there tends to be a common pattern for libraries.
First, we try to use pkg-config
to find the library. Note that we
cannot rely on this, as it may not be available, but it provides a good
starting point.
find_package(PkgConfig)
if(PKG_CONFIG_FOUND)
pkg_check_modules(PC_Foo QUIET Foo)
endif()
This should define some variables starting PC_Foo_
that contain the
information from the Foo.pc
file.
Now we need to find the libraries and include files; we use the
information from pkg-config
to provide hints to CMake about where to
look.
find_path(Foo_INCLUDE_DIR
NAMES foo.h
PATHS ${PC_Foo_INCLUDE_DIRS}
PATH_SUFFIXES Foo
)
find_library(Foo_LIBRARY
NAMES foo
PATHS ${PC_Foo_LIBRARY_DIRS}
)
Alternatively, if the library is available with multiple configurations, you can
use SelectLibraryConfigurations
to automatically set the
Foo_LIBRARY
variable instead:
find_library(Foo_LIBRARY_RELEASE
NAMES foo
PATHS ${PC_Foo_LIBRARY_DIRS}/Release
)
find_library(Foo_LIBRARY_DEBUG
NAMES foo
PATHS ${PC_Foo_LIBRARY_DIRS}/Debug
)
include(SelectLibraryConfigurations)
select_library_configurations(Foo)
If you have a good way of getting the version (from a header file, for
example), you can use that information to set Foo_VERSION
(although
note that find modules have traditionally used Foo_VERSION_STRING
,
so you may want to set both). Otherwise, attempt to use the information
from pkg-config
set(Foo_VERSION ${PC_Foo_VERSION})
Now we can use FindPackageHandleStandardArgs
to do most of the
rest of the work for us
include(FindPackageHandleStandardArgs)
find_package_handle_standard_args(Foo
FOUND_VAR Foo_FOUND
REQUIRED_VARS
Foo_LIBRARY
Foo_INCLUDE_DIR
VERSION_VAR Foo_VERSION
)
This will check that the REQUIRED_VARS
contain values (that do not
end in -NOTFOUND
) and set Foo_FOUND
appropriately. It will also
cache those values. If Foo_VERSION
is set, and a required version
was passed to find_package()
, it will check the requested version
against the one in Foo_VERSION
. It will also print messages as
appropriate; note that if the package was found, it will print the
contents of the first required variable to indicate where it was found.
At this point, we have to provide a way for users of the find module to link to the library or libraries that were found. There are two approaches, as discussed in the Find Modules section above. The traditional variable approach looks like
if(Foo_FOUND)
set(Foo_LIBRARIES ${Foo_LIBRARY})
set(Foo_INCLUDE_DIRS ${Foo_INCLUDE_DIR})
set(Foo_DEFINITIONS ${PC_Foo_CFLAGS_OTHER})
endif()
If more than one library was found, all of them should be included in these variables (see the Standard Variable Names section for more information).
When providing imported targets, these should be namespaced (hence the
Foo::
prefix); CMake will recognize that values passed to
target_link_libraries()
that contain ::
in their name are
supposed to be imported targets (rather than just library names), and
will produce appropriate diagnostic messages if that target does not
exist (see policy CMP0028
).
if(Foo_FOUND AND NOT TARGET Foo::Foo)
add_library(Foo::Foo UNKNOWN IMPORTED)
set_target_properties(Foo::Foo PROPERTIES
IMPORTED_LOCATION "${Foo_LIBRARY}"
INTERFACE_COMPILE_OPTIONS "${PC_Foo_CFLAGS_OTHER}"
INTERFACE_INCLUDE_DIRECTORIES "${Foo_INCLUDE_DIR}"
)
endif()
One thing to note about this is that the INTERFACE_INCLUDE_DIRECTORIES
and
similar properties should only contain information about the target itself, and
not any of its dependencies. Instead, those dependencies should also be
targets, and CMake should be told that they are dependencies of this target.
CMake will then combine all the necessary information automatically.
The type of the IMPORTED
target created in the
add_library()
command can always be specified as UNKNOWN
type. This simplifies the code in cases where static or shared variants may
be found, and CMake will determine the type by inspecting the files.
If the library is available with multiple configurations, the
IMPORTED_CONFIGURATIONS
target property should also be
populated:
if(Foo_FOUND)
if (NOT TARGET Foo::Foo)
add_library(Foo::Foo UNKNOWN IMPORTED)
endif()
if (Foo_LIBRARY_RELEASE)
set_property(TARGET Foo::Foo APPEND PROPERTY
IMPORTED_CONFIGURATIONS RELEASE
)
set_target_properties(Foo::Foo PROPERTIES
IMPORTED_LOCATION_RELEASE "${Foo_LIBRARY_RELEASE}"
)
endif()
if (Foo_LIBRARY_DEBUG)
set_property(TARGET Foo::Foo APPEND PROPERTY
IMPORTED_CONFIGURATIONS DEBUG
)
set_target_properties(Foo::Foo PROPERTIES
IMPORTED_LOCATION_DEBUG "${Foo_LIBRARY_DEBUG}"
)
endif()
set_target_properties(Foo::Foo PROPERTIES
INTERFACE_COMPILE_OPTIONS "${PC_Foo_CFLAGS_OTHER}"
INTERFACE_INCLUDE_DIRECTORIES "${Foo_INCLUDE_DIR}"
)
endif()
The RELEASE
variant should be listed first in the property
so that the variant is chosen if the user uses a configuration which is
not an exact match for any listed IMPORTED_CONFIGURATIONS
.
Most of the cache variables should be hidden in the ccmake interface unless the user explicitly asks to edit them.
mark_as_advanced(
Foo_INCLUDE_DIR
Foo_LIBRARY
)
If this module replaces an older version, you should set compatibility variables to cause the least disruption possible.
# compatibility variables
set(Foo_VERSION_STRING ${Foo_VERSION})