CMAKE-BUILDSYSTEM(7) CMake CMAKE-BUILDSYSTEM(7)
NAME
cmake-buildsystem - CMake Buildsystem Reference
INTRODUCTION
A CMake-based buildsystem is organized as a set of high-level logical
targets. Each target corresponds to an executable or library, or is a
custom target containing custom commands. Dependencies between the
targets are expressed in the buildsystem to determine the build order
and the rules for regeneration in response to change.
BINARY TARGETS
Executables and libraries are defined using the add_executable() and
add_library() commands. The resulting binary files have appropriate
PREFIX, SUFFIX and extensions for the platform targeted. Dependencies
between binary targets are expressed using the target_link_libraries()
command:
add_library(archive archive.cpp zip.cpp lzma.cpp)
add_executable(zipapp zipapp.cpp)
target_link_libraries(zipapp archive)
archive is defined as a STATIC library -- an archive containing objects
compiled from archive.cpp, zip.cpp, and lzma.cpp. zipapp is defined as
an executable formed by compiling and linking zipapp.cpp. When linking
the zipapp executable, the archive static library is linked in.
Binary Executables
The add_executable() command defines an executable target:
add_executable(mytool mytool.cpp)
Commands such as add_custom_command(), which generates rules to be run
at build time can transparently use an EXECUTABLE target as a COMMAND
executable. The buildsystem rules will ensure that the executable is
built before attempting to run the command.
Binary Library Types
Normal Libraries
By default, the add_library() command defines a STATIC library, unless
a type is specified. A type may be specified when using the command:
add_library(archive SHARED archive.cpp zip.cpp lzma.cpp)
add_library(archive STATIC archive.cpp zip.cpp lzma.cpp)
The BUILD_SHARED_LIBS variable may be enabled to change the behavior of
add_library() to build shared libraries by default.
In the context of the buildsystem definition as a whole, it is largely
irrelevant whether particular libraries are SHARED or STATIC -- the
commands, dependency specifications and other APIs work similarly re-
gardless of the library type. The MODULE library type is dissimilar in
that it is generally not linked to -- it is not used in the
right-hand-side of the target_link_libraries() command. It is a type
which is loaded as a plugin using runtime techniques. If the library
does not export any unmanaged symbols (e.g. Windows resource DLL,
C++/CLI DLL), it is required that the library not be a SHARED library
because CMake expects SHARED libraries to export at least one symbol.
add_library(archive MODULE 7z.cpp)
Apple Frameworks
A SHARED library may be marked with the FRAMEWORK target property to
create an macOS or iOS Framework Bundle. A library with the FRAMEWORK
target property should also set the FRAMEWORK_VERSION target property.
This property is typically set to the value of "A" by macOS conven-
tions. The MACOSX_FRAMEWORK_IDENTIFIER sets CFBundleIdentifier key and
it uniquely identifies the bundle.
add_library(MyFramework SHARED MyFramework.cpp)
set_target_properties(MyFramework PROPERTIES
FRAMEWORK TRUE
FRAMEWORK_VERSION A # Version "A" is macOS convention
MACOSX_FRAMEWORK_IDENTIFIER org.cmake.MyFramework
)
Object Libraries
The OBJECT library type defines a non-archival collection of object
files resulting from compiling the given source files. The object
files collection may be used as source inputs to other targets by using
the syntax $<TARGET_OBJECTS:name>. This is a generator expression that
can be used to supply the OBJECT library content to other targets:
add_library(archive OBJECT archive.cpp zip.cpp lzma.cpp)
add_library(archiveExtras STATIC $<TARGET_OBJECTS:archive> extras.cpp)
add_executable(test_exe $<TARGET_OBJECTS:archive> test.cpp)
The link (or archiving) step of those other targets will use the object
files collection in addition to those from their own sources.
Alternatively, object libraries may be linked into other targets:
add_library(archive OBJECT archive.cpp zip.cpp lzma.cpp)
add_library(archiveExtras STATIC extras.cpp)
target_link_libraries(archiveExtras PUBLIC archive)
add_executable(test_exe test.cpp)
target_link_libraries(test_exe archive)
The link (or archiving) step of those other targets will use the object
files from OBJECT libraries that are directly linked. Additionally,
usage requirements of the OBJECT libraries will be honored when compil-
ing sources in those other targets. Furthermore, those usage require-
ments will propagate transitively to dependents of those other targets.
Object libraries may not be used as the TARGET in a use of the
add_custom_command(TARGET) command signature. However, the list of ob-
jects can be used by add_custom_command(OUTPUT) or file(GENERATE) by
using $<TARGET_OBJECTS:objlib>.
BUILD SPECIFICATION AND USAGE REQUIREMENTS
The target_include_directories(), target_compile_definitions() and
target_compile_options() commands specify the build specifications and
the usage requirements of binary targets. The commands populate the
INCLUDE_DIRECTORIES, COMPILE_DEFINITIONS and COMPILE_OPTIONS target
properties respectively, and/or the INTERFACE_INCLUDE_DIRECTORIES,
INTERFACE_COMPILE_DEFINITIONS and INTERFACE_COMPILE_OPTIONS target
properties.
Each of the commands has a PRIVATE, PUBLIC and INTERFACE mode. The
PRIVATE mode populates only the non-INTERFACE_ variant of the target
property and the INTERFACE mode populates only the INTERFACE_ variants.
The PUBLIC mode populates both variants of the respective target prop-
erty. Each command may be invoked with multiple uses of each keyword:
target_compile_definitions(archive
PRIVATE BUILDING_WITH_LZMA
INTERFACE USING_ARCHIVE_LIB
)
Note that usage requirements are not designed as a way to make down-
streams use particular COMPILE_OPTIONS or COMPILE_DEFINITIONS etc for
convenience only. The contents of the properties must be requirements,
not merely recommendations or convenience.
See the Creating Relocatable Packages section of the cmake-packages(7)
manual for discussion of additional care that must be taken when speci-
fying usage requirements while creating packages for redistribution.
Target Properties
The contents of the INCLUDE_DIRECTORIES, COMPILE_DEFINITIONS and
COMPILE_OPTIONS target properties are used appropriately when compiling
the source files of a binary target.
Entries in the INCLUDE_DIRECTORIES are added to the compile line with
-I or -isystem prefixes and in the order of appearance in the property
value.
Entries in the COMPILE_DEFINITIONS are prefixed with -D or /D and added
to the compile line in an unspecified order. The DEFINE_SYMBOL target
property is also added as a compile definition as a special convenience
case for SHARED and MODULE library targets.
Entries in the COMPILE_OPTIONS are escaped for the shell and added in
the order of appearance in the property value. Several compile options
have special separate handling, such as POSITION_INDEPENDENT_CODE.
The contents of the INTERFACE_INCLUDE_DIRECTORIES,
INTERFACE_COMPILE_DEFINITIONS and INTERFACE_COMPILE_OPTIONS target
properties are Usage Requirements -- they specify content which con-
sumers must use to correctly compile and link with the target they ap-
pear on. For any binary target, the contents of each INTERFACE_ prop-
erty on each target specified in a target_link_libraries() command is
consumed:
set(srcs archive.cpp zip.cpp)
if (LZMA_FOUND)
list(APPEND srcs lzma.cpp)
endif()
add_library(archive SHARED ${srcs})
if (LZMA_FOUND)
# The archive library sources are compiled with -DBUILDING_WITH_LZMA
target_compile_definitions(archive PRIVATE BUILDING_WITH_LZMA)
endif()
target_compile_definitions(archive INTERFACE USING_ARCHIVE_LIB)
add_executable(consumer)
# Link consumer to archive and consume its usage requirements. The consumer
# executable sources are compiled with -DUSING_ARCHIVE_LIB.
target_link_libraries(consumer archive)
Because it is common to require that the source directory and corre-
sponding build directory are added to the INCLUDE_DIRECTORIES, the
CMAKE_INCLUDE_CURRENT_DIR variable can be enabled to conveniently add
the corresponding directories to the INCLUDE_DIRECTORIES of all tar-
gets. The variable CMAKE_INCLUDE_CURRENT_DIR_IN_INTERFACE can be en-
abled to add the corresponding directories to the
INTERFACE_INCLUDE_DIRECTORIES of all targets. This makes use of tar-
gets in multiple different directories convenient through use of the
target_link_libraries() command.
Transitive Usage Requirements
The usage requirements of a target can transitively propagate to the
dependents. The target_link_libraries() command has PRIVATE, INTERFACE
and PUBLIC keywords to control the propagation.
add_library(archive archive.cpp)
target_compile_definitions(archive INTERFACE USING_ARCHIVE_LIB)
add_library(serialization serialization.cpp)
target_compile_definitions(serialization INTERFACE USING_SERIALIZATION_LIB)
add_library(archiveExtras extras.cpp)
target_link_libraries(archiveExtras PUBLIC archive)
target_link_libraries(archiveExtras PRIVATE serialization)
# archiveExtras is compiled with -DUSING_ARCHIVE_LIB
# and -DUSING_SERIALIZATION_LIB
add_executable(consumer consumer.cpp)
# consumer is compiled with -DUSING_ARCHIVE_LIB
target_link_libraries(consumer archiveExtras)
Because the archive is a PUBLIC dependency of archiveExtras, the usage
requirements of it are propagated to consumer too.
Because serialization is a PRIVATE dependency of archiveExtras, the us-
age requirements of it are not propagated to consumer.
Generally, a dependency should be specified in a use of
target_link_libraries() with the PRIVATE keyword if it is used by only
the implementation of a library, and not in the header files. If a de-
pendency is additionally used in the header files of a library (e.g.
for class inheritance), then it should be specified as a PUBLIC depen-
dency. A dependency which is not used by the implementation of a li-
brary, but only by its headers should be specified as an INTERFACE de-
pendency. The target_link_libraries() command may be invoked with mul-
tiple uses of each keyword:
target_link_libraries(archiveExtras
PUBLIC archive
PRIVATE serialization
)
Usage requirements are propagated by reading the INTERFACE_ variants of
target properties from dependencies and appending the values to the
non-INTERFACE_ variants of the operand. For example, the
INTERFACE_INCLUDE_DIRECTORIES of dependencies is read and appended to
the INCLUDE_DIRECTORIES of the operand. In cases where order is rele-
vant and maintained, and the order resulting from the
target_link_libraries() calls does not allow correct compilation, use
of an appropriate command to set the property directly may update the
order.
For example, if the linked libraries for a target must be specified in
the order lib1 lib2 lib3 , but the include directories must be speci-
fied in the order lib3 lib1 lib2:
target_link_libraries(myExe lib1 lib2 lib3)
target_include_directories(myExe
PRIVATE $<TARGET_PROPERTY:lib3,INTERFACE_INCLUDE_DIRECTORIES>)
Note that care must be taken when specifying usage requirements for
targets which will be exported for installation using the
install(EXPORT) command. See Creating Packages for more.
Compatible Interface Properties
Some target properties are required to be compatible between a target
and the interface of each dependency. For example, the
POSITION_INDEPENDENT_CODE target property may specify a boolean value
of whether a target should be compiled as position-independent-code,
which has platform-specific consequences. A target may also specify
the usage requirement INTERFACE_POSITION_INDEPENDENT_CODE to communi-
cate that consumers must be compiled as position-independent-code.
add_executable(exe1 exe1.cpp)
set_property(TARGET exe1 PROPERTY POSITION_INDEPENDENT_CODE ON)
add_library(lib1 SHARED lib1.cpp)
set_property(TARGET lib1 PROPERTY INTERFACE_POSITION_INDEPENDENT_CODE ON)
add_executable(exe2 exe2.cpp)
target_link_libraries(exe2 lib1)
Here, both exe1 and exe2 will be compiled as position-independent-code.
lib1 will also be compiled as position-independent-code because that is
the default setting for SHARED libraries. If dependencies have con-
flicting, non-compatible requirements cmake(1) issues a diagnostic:
add_library(lib1 SHARED lib1.cpp)
set_property(TARGET lib1 PROPERTY INTERFACE_POSITION_INDEPENDENT_CODE ON)
add_library(lib2 SHARED lib2.cpp)
set_property(TARGET lib2 PROPERTY INTERFACE_POSITION_INDEPENDENT_CODE OFF)
add_executable(exe1 exe1.cpp)
target_link_libraries(exe1 lib1)
set_property(TARGET exe1 PROPERTY POSITION_INDEPENDENT_CODE OFF)
add_executable(exe2 exe2.cpp)
target_link_libraries(exe2 lib1 lib2)
The lib1 requirement INTERFACE_POSITION_INDEPENDENT_CODE is not "com-
patible" with the POSITION_INDEPENDENT_CODE property of the exe1 tar-
get. The library requires that consumers are built as position-inde-
pendent-code, while the executable specifies to not built as posi-
tion-independent-code, so a diagnostic is issued.
The lib1 and lib2 requirements are not "compatible". One of them re-
quires that consumers are built as position-independent-code, while the
other requires that consumers are not built as position-indepen-
dent-code. Because exe2 links to both and they are in conflict, a
CMake error message is issued:
CMake Error: The INTERFACE_POSITION_INDEPENDENT_CODE property of "lib2" does
not agree with the value of POSITION_INDEPENDENT_CODE already determined
for "exe2".
To be "compatible", the POSITION_INDEPENDENT_CODE property, if set must
be either the same, in a boolean sense, as the
INTERFACE_POSITION_INDEPENDENT_CODE property of all transitively speci-
fied dependencies on which that property is set.
This property of "compatible interface requirement" may be extended to
other properties by specifying the property in the content of the
COMPATIBLE_INTERFACE_BOOL target property. Each specified property
must be compatible between the consuming target and the corresponding
property with an INTERFACE_ prefix from each dependency:
add_library(lib1Version2 SHARED lib1_v2.cpp)
set_property(TARGET lib1Version2 PROPERTY INTERFACE_CUSTOM_PROP ON)
set_property(TARGET lib1Version2 APPEND PROPERTY
COMPATIBLE_INTERFACE_BOOL CUSTOM_PROP
)
add_library(lib1Version3 SHARED lib1_v3.cpp)
set_property(TARGET lib1Version3 PROPERTY INTERFACE_CUSTOM_PROP OFF)
add_executable(exe1 exe1.cpp)
target_link_libraries(exe1 lib1Version2) # CUSTOM_PROP will be ON
add_executable(exe2 exe2.cpp)
target_link_libraries(exe2 lib1Version2 lib1Version3) # Diagnostic
Non-boolean properties may also participate in "compatible interface"
computations. Properties specified in the COMPATIBLE_INTERFACE_STRING
property must be either unspecified or compare to the same string among
all transitively specified dependencies. This can be useful to ensure
that multiple incompatible versions of a library are not linked to-
gether through transitive requirements of a target:
add_library(lib1Version2 SHARED lib1_v2.cpp)
set_property(TARGET lib1Version2 PROPERTY INTERFACE_LIB_VERSION 2)
set_property(TARGET lib1Version2 APPEND PROPERTY
COMPATIBLE_INTERFACE_STRING LIB_VERSION
)
add_library(lib1Version3 SHARED lib1_v3.cpp)
set_property(TARGET lib1Version3 PROPERTY INTERFACE_LIB_VERSION 3)
add_executable(exe1 exe1.cpp)
target_link_libraries(exe1 lib1Version2) # LIB_VERSION will be "2"
add_executable(exe2 exe2.cpp)
target_link_libraries(exe2 lib1Version2 lib1Version3) # Diagnostic
The COMPATIBLE_INTERFACE_NUMBER_MAX target property specifies that con-
tent will be evaluated numerically and the maximum number among all
specified will be calculated:
add_library(lib1Version2 SHARED lib1_v2.cpp)
set_property(TARGET lib1Version2 PROPERTY INTERFACE_CONTAINER_SIZE_REQUIRED 200)
set_property(TARGET lib1Version2 APPEND PROPERTY
COMPATIBLE_INTERFACE_NUMBER_MAX CONTAINER_SIZE_REQUIRED
)
add_library(lib1Version3 SHARED lib1_v3.cpp)
set_property(TARGET lib1Version3 PROPERTY INTERFACE_CONTAINER_SIZE_REQUIRED 1000)
add_executable(exe1 exe1.cpp)
# CONTAINER_SIZE_REQUIRED will be "200"
target_link_libraries(exe1 lib1Version2)
add_executable(exe2 exe2.cpp)
# CONTAINER_SIZE_REQUIRED will be "1000"
target_link_libraries(exe2 lib1Version2 lib1Version3)
Similarly, the COMPATIBLE_INTERFACE_NUMBER_MIN may be used to calculate
the numeric minimum value for a property from dependencies.
Each calculated "compatible" property value may be read in the consumer
at generate-time using generator expressions.
Note that for each dependee, the set of properties specified in each
compatible interface property must not intersect with the set specified
in any of the other properties.
Property Origin Debugging
Because build specifications can be determined by dependencies, the
lack of locality of code which creates a target and code which is re-
sponsible for setting build specifications may make the code more dif-
ficult to reason about. cmake(1) provides a debugging facility to
print the origin of the contents of properties which may be determined
by dependencies. The properties which can be debugged are listed in
the CMAKE_DEBUG_TARGET_PROPERTIES variable documentation:
set(CMAKE_DEBUG_TARGET_PROPERTIES
INCLUDE_DIRECTORIES
COMPILE_DEFINITIONS
POSITION_INDEPENDENT_CODE
CONTAINER_SIZE_REQUIRED
LIB_VERSION
)
add_executable(exe1 exe1.cpp)
In the case of properties listed in COMPATIBLE_INTERFACE_BOOL or
COMPATIBLE_INTERFACE_STRING, the debug output shows which target was
responsible for setting the property, and which other dependencies also
defined the property. In the case of COMPATIBLE_INTERFACE_NUMBER_MAX
and COMPATIBLE_INTERFACE_NUMBER_MIN, the debug output shows the value
of the property from each dependency, and whether the value determines
the new extreme.
Build Specification with Generator Expressions
Build specifications may use generator expressions containing content
which may be conditional or known only at generate-time. For example,
the calculated "compatible" value of a property may be read with the
TARGET_PROPERTY expression:
add_library(lib1Version2 SHARED lib1_v2.cpp)
set_property(TARGET lib1Version2 PROPERTY
INTERFACE_CONTAINER_SIZE_REQUIRED 200)
set_property(TARGET lib1Version2 APPEND PROPERTY
COMPATIBLE_INTERFACE_NUMBER_MAX CONTAINER_SIZE_REQUIRED
)
add_executable(exe1 exe1.cpp)
target_link_libraries(exe1 lib1Version2)
target_compile_definitions(exe1 PRIVATE
CONTAINER_SIZE=$<TARGET_PROPERTY:CONTAINER_SIZE_REQUIRED>
)
In this case, the exe1 source files will be compiled with -DCON-
TAINER_SIZE=200.
The unary TARGET_PROPERTY generator expression and the TARGET_POLICY
generator expression are evaluated with the consuming target context.
This means that a usage requirement specification may be evaluated dif-
ferently based on the consumer:
add_library(lib1 lib1.cpp)
target_compile_definitions(lib1 INTERFACE
$<$<STREQUAL:$<TARGET_PROPERTY:TYPE>,EXECUTABLE>:LIB1_WITH_EXE>
$<$<STREQUAL:$<TARGET_PROPERTY:TYPE>,SHARED_LIBRARY>:LIB1_WITH_SHARED_LIB>
$<$<TARGET_POLICY:CMP0041>:CONSUMER_CMP0041_NEW>
)
add_executable(exe1 exe1.cpp)
target_link_libraries(exe1 lib1)
cmake_policy(SET CMP0041 NEW)
add_library(shared_lib shared_lib.cpp)
target_link_libraries(shared_lib lib1)
The exe1 executable will be compiled with -DLIB1_WITH_EXE, while the
shared_lib shared library will be compiled with -DLIB1_WITH_SHARED_LIB
and -DCONSUMER_CMP0041_NEW, because policy CMP0041 is NEW at the point
where the shared_lib target is created.
The BUILD_INTERFACE expression wraps requirements which are only used
when consumed from a target in the same buildsystem, or when consumed
from a target exported to the build directory using the export() com-
mand. The INSTALL_INTERFACE expression wraps requirements which are
only used when consumed from a target which has been installed and ex-
ported with the install(EXPORT) command:
add_library(ClimbingStats climbingstats.cpp)
target_compile_definitions(ClimbingStats INTERFACE
$<BUILD_INTERFACE:ClimbingStats_FROM_BUILD_LOCATION>
$<INSTALL_INTERFACE:ClimbingStats_FROM_INSTALLED_LOCATION>
)
install(TARGETS ClimbingStats EXPORT libExport ${InstallArgs})
install(EXPORT libExport NAMESPACE Upstream::
DESTINATION lib/cmake/ClimbingStats)
export(EXPORT libExport NAMESPACE Upstream::)
add_executable(exe1 exe1.cpp)
target_link_libraries(exe1 ClimbingStats)
In this case, the exe1 executable will be compiled with -DClimb-
ingStats_FROM_BUILD_LOCATION. The exporting commands generate IMPORTED
targets with either the INSTALL_INTERFACE or the BUILD_INTERFACE omit-
ted, and the *_INTERFACE marker stripped away. A separate project con-
suming the ClimbingStats package would contain:
find_package(ClimbingStats REQUIRED)
add_executable(Downstream main.cpp)
target_link_libraries(Downstream Upstream::ClimbingStats)
Depending on whether the ClimbingStats package was used from the build
location or the install location, the Downstream target would be com-
piled with either -DClimbingStats_FROM_BUILD_LOCATION or -DClimb-
ingStats_FROM_INSTALL_LOCATION. For more about packages and exporting
see the cmake-packages(7) manual.
Include Directories and Usage Requirements
Include directories require some special consideration when specified
as usage requirements and when used with generator expressions. The
target_include_directories() command accepts both relative and absolute
include directories:
add_library(lib1 lib1.cpp)
target_include_directories(lib1 PRIVATE
/absolute/path
relative/path
)
Relative paths are interpreted relative to the source directory where
the command appears. Relative paths are not allowed in the
INTERFACE_INCLUDE_DIRECTORIES of IMPORTED targets.
In cases where a non-trivial generator expression is used, the IN-
STALL_PREFIX expression may be used within the argument of an IN-
STALL_INTERFACE expression. It is a replacement marker which expands
to the installation prefix when imported by a consuming project.
Include directories usage requirements commonly differ between the
build-tree and the install-tree. The BUILD_INTERFACE and INSTALL_IN-
TERFACE generator expressions can be used to describe separate usage
requirements based on the usage location. Relative paths are allowed
within the INSTALL_INTERFACE expression and are interpreted relative to
the installation prefix. For example:
add_library(ClimbingStats climbingstats.cpp)
target_include_directories(ClimbingStats INTERFACE
$<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/generated>
$<INSTALL_INTERFACE:/absolute/path>
$<INSTALL_INTERFACE:relative/path>
$<INSTALL_INTERFACE:$<INSTALL_PREFIX>/$<CONFIG>/generated>
)
Two convenience APIs are provided relating to include directories usage
requirements. The CMAKE_INCLUDE_CURRENT_DIR_IN_INTERFACE variable may
be enabled, with an equivalent effect to:
set_property(TARGET tgt APPEND PROPERTY INTERFACE_INCLUDE_DIRECTORIES
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR};${CMAKE_CURRENT_BINARY_DIR}>
)
for each target affected. The convenience for installed targets is an
INCLUDES DESTINATION component with the install(TARGETS) command:
install(TARGETS foo bar bat EXPORT tgts ${dest_args}
INCLUDES DESTINATION include
)
install(EXPORT tgts ${other_args})
install(FILES ${headers} DESTINATION include)
This is equivalent to appending ${CMAKE_INSTALL_PREFIX}/include to the
INTERFACE_INCLUDE_DIRECTORIES of each of the installed IMPORTED targets
when generated by install(EXPORT).
When the INTERFACE_INCLUDE_DIRECTORIES of an imported target is con-
sumed, the entries in the property are treated as SYSTEM include direc-
tories, as if they were listed in the
INTERFACE_SYSTEM_INCLUDE_DIRECTORIES of the dependency. This can result
in omission of compiler warnings for headers found in those directo-
ries. This behavior for Imported Targets may be controlled by setting
the NO_SYSTEM_FROM_IMPORTED target property on the consumers of im-
ported targets, or by setting the IMPORTED_NO_SYSTEM target property on
the imported targets themselves.
If a binary target is linked transitively to a macOS FRAMEWORK, the
Headers directory of the framework is also treated as a usage require-
ment. This has the same effect as passing the framework directory as
an include directory.
Link Libraries and Generator Expressions
Like build specifications, link libraries may be specified with genera-
tor expression conditions. However, as consumption of usage require-
ments is based on collection from linked dependencies, there is an ad-
ditional limitation that the link dependencies must form a "directed
acyclic graph". That is, if linking to a target is dependent on the
value of a target property, that target property may not be dependent
on the linked dependencies:
add_library(lib1 lib1.cpp)
add_library(lib2 lib2.cpp)
target_link_libraries(lib1 PUBLIC
$<$<TARGET_PROPERTY:POSITION_INDEPENDENT_CODE>:lib2>
)
add_library(lib3 lib3.cpp)
set_property(TARGET lib3 PROPERTY INTERFACE_POSITION_INDEPENDENT_CODE ON)
add_executable(exe1 exe1.cpp)
target_link_libraries(exe1 lib1 lib3)
As the value of the POSITION_INDEPENDENT_CODE property of the exe1 tar-
get is dependent on the linked libraries (lib3), and the edge of link-
ing exe1 is determined by the same POSITION_INDEPENDENT_CODE property,
the dependency graph above contains a cycle. cmake(1) issues an error
message.
Output Artifacts
The buildsystem targets created by the add_library() and
add_executable() commands create rules to create binary outputs. The
exact output location of the binaries can only be determined at gener-
ate-time because it can depend on the build-configuration and the
link-language of linked dependencies etc. TARGET_FILE, TAR-
GET_LINKER_FILE and related expressions can be used to access the name
and location of generated binaries. These expressions do not work for
OBJECT libraries however, as there is no single file generated by such
libraries which is relevant to the expressions.
There are three kinds of output artifacts that may be build by targets
as detailed in the following sections. Their classification differs
between DLL platforms and non-DLL platforms. All Windows-based systems
including Cygwin are DLL platforms.
Runtime Output Artifacts
A runtime output artifact of a buildsystem target may be:
• The executable file (e.g. .exe) of an executable target created by
the add_executable() command.
• On DLL platforms: the executable file (e.g. .dll) of a shared library
target created by the add_library() command with the SHARED option.
The RUNTIME_OUTPUT_DIRECTORY and RUNTIME_OUTPUT_NAME target properties
may be used to control runtime output artifact locations and names in
the build tree.
Library Output Artifacts
A library output artifact of a buildsystem target may be:
• The loadable module file (e.g. .dll or .so) of a module library tar-
get created by the add_library() command with the MODULE option.
• On non-DLL platforms: the shared library file (e.g. .so or .dylib) of
a shared library target created by the add_library() command with the
SHARED option.
The LIBRARY_OUTPUT_DIRECTORY and LIBRARY_OUTPUT_NAME target properties
may be used to control library output artifact locations and names in
the build tree.
Archive Output Artifacts
An archive output artifact of a buildsystem target may be:
• The static library file (e.g. .lib or .a) of a static library target
created by the add_library() command with the STATIC option.
• On DLL platforms: the import library file (e.g. .lib) of a shared li-
brary target created by the add_library() command with the SHARED op-
tion. This file is only guaranteed to exist if the library exports
at least one unmanaged symbol.
• On DLL platforms: the import library file (e.g. .lib) of an exe-
cutable target created by the add_executable() command when its
ENABLE_EXPORTS target property is set.
• On AIX: the linker import file (e.g. .imp) of an executable target
created by the add_executable() command when its ENABLE_EXPORTS tar-
get property is set.
The ARCHIVE_OUTPUT_DIRECTORY and ARCHIVE_OUTPUT_NAME target properties
may be used to control archive output artifact locations and names in
the build tree.
Directory-Scoped Commands
The target_include_directories(), target_compile_definitions() and
target_compile_options() commands have an effect on only one target at
a time. The commands add_compile_definitions(), add_compile_options()
and include_directories() have a similar function, but operate at di-
rectory scope instead of target scope for convenience.
BUILD CONFIGURATIONS
Configurations determine specifications for a certain type of build,
such as Release or Debug. The way this is specified depends on the
type of generator being used. For single configuration generators like
Makefile Generators and Ninja, the configuration is specified at con-
figure time by the CMAKE_BUILD_TYPE variable. For multi-configuration
generators like Visual Studio, Xcode, and Ninja Multi-Config, the con-
figuration is chosen by the user at build time and CMAKE_BUILD_TYPE is
ignored. In the multi-configuration case, the set of available config-
urations is specified at configure time by the
CMAKE_CONFIGURATION_TYPES variable, but the actual configuration used
cannot be known until the build stage. This difference is often misun-
derstood, leading to problematic code like the following:
# WARNING: This is wrong for multi-config generators because they don't use
# and typically don't even set CMAKE_BUILD_TYPE
string(TOLOWER ${CMAKE_BUILD_TYPE} build_type)
if (build_type STREQUAL debug)
target_compile_definitions(exe1 PRIVATE DEBUG_BUILD)
endif()
Generator expressions should be used instead to handle configura-
tion-specific logic correctly, regardless of the generator used. For
example:
# Works correctly for both single and multi-config generators
target_compile_definitions(exe1 PRIVATE
$<$<CONFIG:Debug>:DEBUG_BUILD>
)
In the presence of IMPORTED targets, the content of
MAP_IMPORTED_CONFIG_DEBUG is also accounted for by the above $<CON-
FIG:Debug> expression.
Case Sensitivity
CMAKE_BUILD_TYPE and CMAKE_CONFIGURATION_TYPES are just like other
variables in that any string comparisons made with their values will be
case-sensitive. The $<CONFIG> generator expression also preserves the
casing of the configuration as set by the user or CMake defaults. For
example:
# NOTE: Don't use these patterns, they are for illustration purposes only.
set(CMAKE_BUILD_TYPE Debug)
if(CMAKE_BUILD_TYPE STREQUAL DEBUG)
# ... will never get here, "Debug" != "DEBUG"
endif()
add_custom_target(print_config ALL
# Prints "Config is Debug" in this single-config case
COMMAND ${CMAKE_COMMAND} -E echo "Config is $<CONFIG>"
VERBATIM
)
set(CMAKE_CONFIGURATION_TYPES Debug Release)
if(DEBUG IN_LIST CMAKE_CONFIGURATION_TYPES)
# ... will never get here, "Debug" != "DEBUG"
endif()
In contrast, CMake treats the configuration type case-insensitively
when using it internally in places that modify behavior based on the
configuration. For example, the $<CONFIG:Debug> generator expression
will evaluate to 1 for a configuration of not only Debug, but also DE-
BUG, debug or even DeBuG. Therefore, you can specify configuration
types in CMAKE_BUILD_TYPE and CMAKE_CONFIGURATION_TYPES with any mix-
ture of upper and lowercase, although there are strong conventions (see
the next section). If you must test the value in string comparisons,
always convert the value to upper or lowercase first and adjust the
test accordingly.
Default And Custom Configurations
By default, CMake defines a number of standard configurations:
• Debug
• Release
• RelWithDebInfo
• MinSizeRel
In multi-config generators, the CMAKE_CONFIGURATION_TYPES variable will
be populated with (potentially a subset of) the above list by default,
unless overridden by the project or user. The actual configuration
used is selected by the user at build time.
For single-config generators, the configuration is specified with the
CMAKE_BUILD_TYPE variable at configure time and cannot be changed at
build time. The default value will often be none of the above standard
configurations and will instead be an empty string. A common misunder-
standing is that this is the same as Debug, but that is not the case.
Users should always explicitly specify the build type instead to avoid
this common problem.
The above standard configuration types provide reasonable behavior on
most platforms, but they can be extended to provide other types. Each
configuration defines a set of compiler and linker flag variables for
the language in use. These variables follow the convention
CMAKE_<LANG>_FLAGS_<CONFIG>, where <CONFIG> is always the uppercase
configuration name. When defining a custom configuration type, make
sure these variables are set appropriately, typically as cache vari-
ables.
PSEUDO TARGETS
Some target types do not represent outputs of the buildsystem, but only
inputs such as external dependencies, aliases or other non-build arti-
facts. Pseudo targets are not represented in the generated buildsys-
tem.
Imported Targets
An IMPORTED target represents a pre-existing dependency. Usually such
targets are defined by an upstream package and should be treated as im-
mutable. After declaring an IMPORTED target one can adjust its target
properties by using the customary commands such as
target_compile_definitions(), target_include_directories(),
target_compile_options() or target_link_libraries() just like with any
other regular target.
IMPORTED targets may have the same usage requirement properties popu-
lated as binary targets, such as INTERFACE_INCLUDE_DIRECTORIES,
INTERFACE_COMPILE_DEFINITIONS, INTERFACE_COMPILE_OPTIONS,
INTERFACE_LINK_LIBRARIES, and INTERFACE_POSITION_INDEPENDENT_CODE.
The LOCATION may also be read from an IMPORTED target, though there is
rarely reason to do so. Commands such as add_custom_command() can
transparently use an IMPORTED EXECUTABLE target as a COMMAND exe-
cutable.
The scope of the definition of an IMPORTED target is the directory
where it was defined. It may be accessed and used from subdirectories,
but not from parent directories or sibling directories. The scope is
similar to the scope of a cmake variable.
It is also possible to define a GLOBAL IMPORTED target which is acces-
sible globally in the buildsystem.
See the cmake-packages(7) manual for more on creating packages with
IMPORTED targets.
Alias Targets
An ALIAS target is a name which may be used interchangeably with a bi-
nary target name in read-only contexts. A primary use-case for ALIAS
targets is for example or unit test executables accompanying a library,
which may be part of the same buildsystem or built separately based on
user configuration.
add_library(lib1 lib1.cpp)
install(TARGETS lib1 EXPORT lib1Export ${dest_args})
install(EXPORT lib1Export NAMESPACE Upstream:: ${other_args})
add_library(Upstream::lib1 ALIAS lib1)
In another directory, we can link unconditionally to the Upstream::lib1
target, which may be an IMPORTED target from a package, or an ALIAS
target if built as part of the same buildsystem.
if (NOT TARGET Upstream::lib1)
find_package(lib1 REQUIRED)
endif()
add_executable(exe1 exe1.cpp)
target_link_libraries(exe1 Upstream::lib1)
ALIAS targets are not mutable, installable or exportable. They are en-
tirely local to the buildsystem description. A name can be tested for
whether it is an ALIAS name by reading the ALIASED_TARGET property from
it:
get_target_property(_aliased Upstream::lib1 ALIASED_TARGET)
if(_aliased)
message(STATUS "The name Upstream::lib1 is an ALIAS for ${_aliased}.")
endif()
Interface Libraries
An INTERFACE library target does not compile sources and does not pro-
duce a library artifact on disk, so it has no LOCATION.
It may specify usage requirements such as
INTERFACE_INCLUDE_DIRECTORIES, INTERFACE_COMPILE_DEFINITIONS,
INTERFACE_COMPILE_OPTIONS, INTERFACE_LINK_LIBRARIES, INTERFACE_SOURCES,
and INTERFACE_POSITION_INDEPENDENT_CODE. Only the INTERFACE modes of
the target_include_directories(), target_compile_definitions(),
target_compile_options(), target_sources(), and target_link_libraries()
commands may be used with INTERFACE libraries.
Since CMake 3.19, an INTERFACE library target may optionally contain
source files. An interface library that contains source files will be
included as a build target in the generated buildsystem. It does not
compile sources, but may contain custom commands to generate other
sources. Additionally, IDEs will show the source files as part of the
target for interactive reading and editing.
A primary use-case for INTERFACE libraries is header-only libraries.
Since CMake 3.23, header files may be associated with a library by
adding them to a header set using the target_sources() command:
add_library(Eigen INTERFACE)
target_sources(Eigen INTERFACE
FILE_SET HEADERS
BASE_DIRS src
FILES src/eigen.h src/vector.h src/matrix.h
)
add_executable(exe1 exe1.cpp)
target_link_libraries(exe1 Eigen)
When we specify the FILE_SET here, the BASE_DIRS we define automati-
cally become include directories in the usage requirements for the tar-
get Eigen. The usage requirements from the target are consumed and
used when compiling, but have no effect on linking.
Another use-case is to employ an entirely target-focussed design for
usage requirements:
add_library(pic_on INTERFACE)
set_property(TARGET pic_on PROPERTY INTERFACE_POSITION_INDEPENDENT_CODE ON)
add_library(pic_off INTERFACE)
set_property(TARGET pic_off PROPERTY INTERFACE_POSITION_INDEPENDENT_CODE OFF)
add_library(enable_rtti INTERFACE)
target_compile_options(enable_rtti INTERFACE
$<$<OR:$<COMPILER_ID:GNU>,$<COMPILER_ID:Clang>>:-rtti>
)
add_executable(exe1 exe1.cpp)
target_link_libraries(exe1 pic_on enable_rtti)
This way, the build specification of exe1 is expressed entirely as
linked targets, and the complexity of compiler-specific flags is encap-
sulated in an INTERFACE library target.
INTERFACE libraries may be installed and exported. We can install the
default header set along with the target:
add_library(Eigen INTERFACE)
target_sources(Eigen INTERFACE
FILE_SET HEADERS
BASE_DIRS src
FILES src/eigen.h src/vector.h src/matrix.h
)
install(TARGETS Eigen EXPORT eigenExport
FILE_SET HEADERS DESTINATION include/Eigen)
install(EXPORT eigenExport NAMESPACE Upstream::
DESTINATION lib/cmake/Eigen
)
Here, the headers defined in the header set are installed to in-
clude/Eigen. The install destination automatically becomes an include
directory that is a usage requirement for consumers.
COPYRIGHT
2000-2022 Kitware, Inc. and Contributors
3.25.1 November 30, 2022 CMAKE-BUILDSYSTEM(7)
Generated by dwww version 1.15 on Tue Jun 25 12:20:16 CEST 2024.