noble (7) cmake-buildsystem.7.gz
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
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 the 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 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 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.
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 may
be treated as system include directories. The effects of that are toolchain-dependent, but one common
effect is to omit compiler warnings for headers found in those directories. The SYSTEM property of the
installed target determines this behavior (see the EXPORT_NO_SYSTEM property for how to modify the
installed value for a target). It is also possible to change how consumers interpret the system behavior
of consumed imported targets by setting the NO_SYSTEM_FROM_IMPORTED target property on the consumer.
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 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 target 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 library target created by the
add_library() command with the SHARED option. 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 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 target property is set.
• On macOS: the linker import file (e.g. .tbd) of a shared library target created by the add_library()
command with the SHARED option and when its ENABLE_EXPORTS target 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.
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 configure time by the CMAKE_BUILD_TYPE
variable. For multi-configuration generators like Visual Studio, Xcode, and Ninja Multi-Config, the
configuration is chosen by the user at build time and CMAKE_BUILD_TYPE is ignored. In the
multi-configuration case, the set of available configurations 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 misunderstood, 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 configuration-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 $<CONFIG: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 DEBUG, debug or even DeBuG.
Therefore, you can specify configuration types in CMAKE_BUILD_TYPE and CMAKE_CONFIGURATION_TYPES with any
mixture 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 misunderstanding 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 variables.
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. 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 PUBLIC
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 automatically become include directories in
the usage requirements for the target 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 encapsulated 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 include/Eigen. The install destination
automatically becomes an include directory that is a usage requirement for consumers.
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