cmake-buildsystem(7)¶
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
regardless 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 conventions.
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 compiling
sources in those other targets. Furthermore, those usage requirements
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 objects 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 property.
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 downstreams
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 specifying 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 consumers
must use to correctly compile and link with the target they appear on.
For any binary target, the contents of each INTERFACE_ property 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 corresponding
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 targets. The variable CMAKE_INCLUDE_CURRENT_DIR_IN_INTERFACE
can be enabled to add the corresponding directories to the
INTERFACE_INCLUDE_DIRECTORIES of all targets. This makes use of
targets 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 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 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 usage
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
dependency is additionally used in the header files of a library (e.g. for
class inheritance), then it should be specified as a PUBLIC dependency.
A dependency which is not used by the implementation of a library, but only by
its headers should be specified as an INTERFACE dependency. The
target_link_libraries() command may be invoked with multiple 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 relevant 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 specified 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 communicate 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 conflicting,
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
"compatible" with the POSITION_INDEPENDENT_CODE property of
the exe1 target. The library requires that consumers are built as
position-independent-code, while the executable specifies to not built as
position-independent-code, so a diagnostic is issued.
The lib1 and lib2 requirements are not "compatible". One of them
requires that consumers are built as position-independent-code, while
the other requires that consumers are not built as position-independent-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
specified 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 together
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 content 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 responsible for
setting build specifications may make the code more difficult 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
-DCONTAINER_SIZE=200.
Configuration determined build specifications may be conveniently set using
the CONFIG generator expression.
target_compile_definitions(exe1 PRIVATE
$<$<CONFIG:Debug>:DEBUG_BUILD>
)
The CONFIG parameter is compared case-insensitively with the configuration
being built. In the presence of IMPORTED targets, the content of
MAP_IMPORTED_CONFIG_DEBUG is also
accounted for by this expression.
Some buildsystems generated by cmake(1) have a predetermined
build-configuration set in the CMAKE_BUILD_TYPE variable. The
buildsystem for the IDEs such as Visual Studio and Xcode are generated
independent of the build-configuration, and the actual build configuration
is not known until build-time. Therefore, code such as
string(TOLOWER ${CMAKE_BUILD_TYPE} _type)
if (_type STREQUAL debug)
target_compile_definitions(exe1 PRIVATE DEBUG_BUILD)
endif()
may appear to work for Makefile Generators and Ninja
generators, but is not portable to IDE generators. Additionally,
the IMPORTED configuration-mappings are not accounted for
with code like this, so it should be avoided.
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 differently 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() command. The
INSTALL_INTERFACE expression wraps requirements which are only used when
consumed from a target which has been installed and exported 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
-DClimbingStats_FROM_BUILD_LOCATION. The exporting commands generate
IMPORTED targets with either the INSTALL_INTERFACE or the
BUILD_INTERFACE omitted, and the *_INTERFACE marker stripped away.
A separate project consuming 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 compiled
with either -DClimbingStats_FROM_BUILD_LOCATION or
-DClimbingStats_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
INSTALL_PREFIX expression may be used within the argument of an
INSTALL_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_INTERFACE
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 consumed, the entries in the
property are treated as SYSTEM include directories, 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 directories. This behavior for Imported Targets may
be controlled by setting the NO_SYSTEM_FROM_IMPORTED target
property on the consumers of imported targets.
If a binary target is linked transitively to a macOS FRAMEWORK, the
Headers directory of the framework is also treated as a usage requirement.
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 generator expression conditions. However, as consumption of
usage requirements is based on collection from linked dependencies, there is
an additional 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 target is dependent on the linked libraries (lib3), and the
edge of linking 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
generate-time because it can depend on the build-configuration and the
link-language of linked dependencies etc. TARGET_FILE,
TARGET_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 theadd_executable()command.On DLL platforms: the executable file (e.g.
.dll) of a shared library target created by theadd_library()command with theSHAREDoption.
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.
.dllor.so) of a module library target created by theadd_library()command with theMODULEoption.On non-DLL platforms: the shared library file (e.g.
.soor.dylib) of a shared library target created by theadd_library()command with theSHAREDoption.
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.
.libor.a) of a static library target created by theadd_library()command with theSTATICoption.On DLL platforms: the import library file (e.g.
.lib) of a shared library target created by theadd_library()command with theSHAREDoption. 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 executable target created by theadd_executable()command when itsENABLE_EXPORTStarget property is set.On AIX: the linker import file (e.g.
.imp) of an executable target created by theadd_executable()command when itsENABLE_EXPORTStarget 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 directory scope instead of target
scope for convenience.
Pseudo Targets¶
Some target types do not represent outputs of the buildsystem, but only inputs such as external dependencies, aliases or other non-build artifacts. Pseudo targets are not represented in the generated buildsystem.
Imported Targets¶
An IMPORTED target represents a pre-existing dependency. Usually
such targets are defined by an upstream package and should be treated as
immutable. 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
populated 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 executable.
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
accessible 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 binary 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
entirely 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
produce 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.
add_library(Eigen INTERFACE
src/eigen.h
src/vector.h
src/matrix.h
)
target_include_directories(Eigen INTERFACE
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>
$<INSTALL_INTERFACE:include/Eigen>
)
add_executable(exe1 exe1.cpp)
target_link_libraries(exe1 Eigen)
Here, the usage requirements from the Eigen target are consumed and used
when compiling, but it has 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 encapsulated in an
INTERFACE library target.
INTERFACE libraries may be installed and exported. Any content they refer
to must be installed separately:
set(Eigen_headers
src/eigen.h
src/vector.h
src/matrix.h
)
add_library(Eigen INTERFACE ${Eigen_headers})
target_include_directories(Eigen INTERFACE
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>
$<INSTALL_INTERFACE:include/Eigen>
)
install(TARGETS Eigen EXPORT eigenExport)
install(EXPORT eigenExport NAMESPACE Upstream::
DESTINATION lib/cmake/Eigen
)
install(FILES ${Eigen_headers}
DESTINATION include/Eigen
)