CMAKE-GENERATOR-EXPRESSIONS(7)

CMake

CMAKE-GENERATOR-EXPRESSIONS(7)

NAME¶

cmake-generator-expressions - CMake Generator Expressions

INTRODUCTION¶

Generator expressions are evaluated during build system generation to produce information specific to each build configuration. They have the form $<...>. For example:

target_include_directories(tgt PRIVATE /opt/include/$<CXX_COMPILER_ID>)

This would expand to /opt/include/GNU, /opt/include/Clang, etc. depending on the C++ compiler used.

Generator expressions are allowed in the context of many target properties, such as LINK_LIBRARIES, INCLUDE_DIRECTORIES, COMPILE_DEFINITIONS and others. They may also be used when using commands to populate those properties, such as target_link_libraries(), target_include_directories(), target_compile_definitions() and others. They enable conditional linking, conditional definitions used when compiling, conditional include directories, and more. The conditions may be based on the build configuration, target properties, platform information, or any other queryable information.

Generator expressions can be nested:

target_compile_definitions(tgt PRIVATE


  $<$<VERSION_LESS:$<CXX_COMPILER_VERSION>,4.2.0>:OLD_COMPILER>
)

The above would expand to OLD_COMPILER if the CMAKE_CXX_COMPILER_VERSION is less than 4.2.0.

WHITESPACE AND QUOTING¶

Generator expressions are typically parsed after command arguments. If a generator expression contains spaces, new lines, semicolons or other characters that may be interpreted as command argument separators, the whole expression should be surrounded by quotes when passed to a command. Failure to do so may result in the expression being split and it may no longer be recognized as a generator expression.

When using add_custom_command() or add_custom_target(), use the VERBATIM and COMMAND_EXPAND_LISTS options to obtain robust argument splitting and quoting.

# WRONG: Embedded space will be treated as an argument separator.
# This ends up not being seen as a generator expression at all.
add_custom_target(run_some_tool


  COMMAND some_tool -I$<JOIN:$<TARGET_PROPERTY:tgt,INCLUDE_DIRECTORIES>, -I>


  VERBATIM
)

# Better, but still not robust. Quotes prevent the space from splitting the
# expression. However, the tool will receive the expanded value as a single
# argument.
add_custom_target(run_some_tool


  COMMAND some_tool "-I$<JOIN:$<TARGET_PROPERTY:tgt,INCLUDE_DIRECTORIES>, -I>"


  VERBATIM
)

# Nearly correct. Using a semicolon to separate arguments and adding the
# COMMAND_EXPAND_LISTS option means that paths with spaces will be handled
# correctly. Quoting the whole expression ensures it is seen as a generator
# expression. But if the target property is empty, we will get a bare -I
# with nothing after it.
add_custom_target(run_some_tool


  COMMAND some_tool "-I$<JOIN:$<TARGET_PROPERTY:tgt,INCLUDE_DIRECTORIES>,;-I>"


  COMMAND_EXPAND_LISTS


  VERBATIM
)

Using variables to build up a more complex generator expression is also a good way to reduce errors and improve readability. The above example can be improved further like so:

# The $<BOOL:...> check prevents adding anything if the property is empty,
# assuming the property value cannot be one of CMake's false constants.
set(prop "$<TARGET_PROPERTY:tgt,INCLUDE_DIRECTORIES>")
add_custom_target(run_some_tool


  COMMAND some_tool "$<$<BOOL:${prop}>:-I$<JOIN:${prop},;-I>>"


  COMMAND_EXPAND_LISTS


  VERBATIM
)

A common mistake is to try to split a generator expression across multiple lines with indenting:

# WRONG: New lines and spaces all treated as argument separators, so the
# generator expression is split and not recognized correctly.
target_compile_definitions(tgt PRIVATE


  $<$<AND:


      $<CXX_COMPILER_ID:GNU>,


      $<VERSION_GREATER_EQUAL:$<CXX_COMPILER_VERSION>,5>


    >:HAVE_5_OR_LATER>
)

Again, use helper variables with well-chosen names to build up a readable expression instead:

set(is_gnu "$<CXX_COMPILER_ID:GNU>")
set(v5_or_later "$<VERSION_GREATER_EQUAL:$<CXX_COMPILER_VERSION>,5>")
set(meet_requirements "$<AND:${is_gnu},${v5_or_later}>")
target_compile_definitions(tgt PRIVATE


  "$<${meet_requirements}:HAVE_5_OR_LATER>"
)

DEBUGGING¶

Since generator expressions are evaluated during generation of the buildsystem, and not during processing of CMakeLists.txt files, it is not possible to inspect their result with the message() command. One possible way to generate debug messages is to add a custom target:

add_custom_target(genexdebug COMMAND ${CMAKE_COMMAND} -E echo "$<...>")

After running cmake, you can then build the genexdebug target to print the result of the $<...> expression (i.e. run the command cmake --build ... --target genexdebug).

Another way is to write debug messages to a file with file(GENERATE):

file(GENERATE OUTPUT filename CONTENT "$<...>")

GENERATOR EXPRESSION REFERENCE¶

NOTE:

This reference deviates from most of the CMake documentation in that it omits angular brackets <...> around placeholders like condition, string, target, etc. This is to prevent an opportunity for those placeholders to be misinterpreted as generator expressions.

Conditional Expressions¶

A fundamental category of generator expressions relates to conditional logic. Two forms of conditional generator expressions are supported:

$<condition:true_string>: Evaluates to true_string if condition is 1, or an empty string if condition evaluates to 0. Any other value for condition results in an error.

$<IF:condition,true_string,false_string>

New in version 3.8.

Evaluates to true_string if condition is 1, or false_string if condition is 0. Any other value for condition results in an error.

Typically, the condition is itself a generator expression. For instance, the following expression expands to DEBUG_MODE when the Debug configuration is used, and the empty string for all other configurations:

$<$<CONFIG:Debug>:DEBUG_MODE>

Boolean-like condition values other than 1 or 0 can be handled by wrapping them with the $<BOOL:...> generator expression:

$<BOOL:string>: Converts string to 0 or 1. Evaluates to 0 if any of the following is true:

string is empty,
string is a case-insensitive equal of 0, FALSE, OFF, N, NO, IGNORE, or NOTFOUND, or
string ends in the suffix -NOTFOUND (case-sensitive).

Otherwise evaluates to 1.

The $<BOOL:...> generator expression is often used when a condition is provided by a CMake variable:

$<$<BOOL:${HAVE_SOME_FEATURE}>:-DENABLE_SOME_FEATURE>

Logical Operators¶

The common boolean logic operators are supported:

$<AND:conditions>: where conditions is a comma-separated list of boolean expressions, all of which must evaluate to either 1 or 0. The whole expression evaluates to 1 if all conditions are 1. If any condition is 0, the whole expression evaluates to 0.

$<OR:conditions>: where conditions is a comma-separated list of boolean expressions. all of which must evaluate to either 1 or 0. The whole expression evaluates to 1 if at least one of the conditions is 1. If all conditions evaluate to 0, the whole expression evaluates to 0.

$<NOT:condition>: condition must be 0 or 1. The result of the expression is 0 if condition is 1, else 1.

Primary Comparison Expressions¶

CMake supports a variety of generator expressions that compare things. This section covers the primary and most widely used comparison types. Other more specific comparison types are documented in their own separate sections further below.

String Comparisons¶

$<STREQUAL:string1,string2>: 1 if string1 and string2 are equal, else 0. The comparison is case-sensitive. For a case-insensitive comparison, combine with a string transforming generator expression. For example, the following evaluates to 1 if ${foo} is any of BAR, Bar, bar, etc.

$<STREQUAL:$<UPPER_CASE:${foo}>,BAR>

$<EQUAL:value1,value2>: 1 if value1 and value2 are numerically equal, else 0.

Version Comparisons¶

$<VERSION_LESS:v1,v2>: 1 if v1 is a version less than v2, else 0.

$<VERSION_GREATER:v1,v2>: 1 if v1 is a version greater than v2, else 0.

$<VERSION_EQUAL:v1,v2>: 1 if v1 is the same version as v2, else 0.

$<VERSION_LESS_EQUAL:v1,v2>

New in version 3.7.

1 if v1 is a version less than or equal to v2, else 0.

$<VERSION_GREATER_EQUAL:v1,v2>

New in version 3.7.

1 if v1 is a version greater than or equal to v2, else 0.

String Transformations¶

$<LOWER_CASE:string>: Content of string converted to lower case.

$<UPPER_CASE:string>: Content of string converted to upper case.

$<MAKE_C_IDENTIFIER:...>: Content of ... converted to a C identifier. The conversion follows the same behavior as string(MAKE_C_IDENTIFIER).

List Expressions¶

$<IN_LIST:string,list>

New in version 3.12.

1 if string is an item in the semicolon-separated list, else 0. It uses case-sensitive comparisons.

$<JOIN:list,string>: Joins the list with the content of string inserted between each item.

$<REMOVE_DUPLICATES:list>

New in version 3.15.

Removes duplicated items in the given list. The relative order of items is preserved, but if duplicates are encountered, only the first instance is preserved.

$<FILTER:list,INCLUDE|EXCLUDE,regex>

New in version 3.15.

Includes or removes items from list that match the regular expression regex.

Path Expressions¶

Most of the expressions in this section are closely associated with the cmake_path() command, providing the same capabilities, but in the form of a generator expression.

For all generator expressions in this section, paths are expected to be in cmake-style format. The $<PATH:CMAKE_PATH> generator expression can be used to convert a native path to a cmake-style one.

Path Comparisons¶

$<PATH_EQUAL:path1,path2>

New in version 3.24.

Compares the lexical representations of two paths. No normalization is performed on either path. Returns 1 if the paths are equal, 0 otherwise.

See cmake_path(COMPARE) for more details.

Path Queries¶

These expressions provide the generation-time capabilities equivalent to the Query options of the cmake_path() command. All paths are expected to be in cmake-style format.

$<PATH:HAS_*,path>

New in version 3.24.

The following operations return 1 if the particular path component is present, 0 otherwise. See Path Structure And Terminology for the meaning of each path component.

$<PATH:HAS_ROOT_NAME,path>
$<PATH:HAS_ROOT_DIRECTORY,path>
$<PATH:HAS_ROOT_PATH,path>
$<PATH:HAS_FILENAME,path>
$<PATH:HAS_EXTENSION,path>
$<PATH:HAS_STEM,path>
$<PATH:HAS_RELATIVE_PART,path>
$<PATH:HAS_PARENT_PATH,path>

Note the following special cases:

For HAS_ROOT_PATH, a true result will only be returned if at least one of root-name or root-directory is non-empty.
For HAS_PARENT_PATH, the root directory is also considered to have a parent, which will be itself. The result is true except if the path consists of just a filename.

$<PATH:IS_ABSOLUTE,path>

New in version 3.24.

Returns 1 if the path is absolute, 0 otherwise.

$<PATH:IS_RELATIVE,path>

New in version 3.24.

This will return the opposite of IS_ABSOLUTE.

$<PATH:IS_PREFIX[,NORMALIZE],path,input>

New in version 3.24.

Returns 1 if path is the prefix of input, 0 otherwise.

When the NORMALIZE option is specified, path and input are normalized before the check.

Path Decomposition¶

These expressions provide the generation-time capabilities equivalent to the Decomposition options of the cmake_path() command. All paths are expected to be in cmake-style format.

$<PATH:GET_*,...>

New in version 3.24.

The following operations retrieve a different component or group of components from a path. See Path Structure And Terminology for the meaning of each path component.

$<PATH:GET_ROOT_NAME,path>
$<PATH:GET_ROOT_DIRECTORY,path>
$<PATH:GET_ROOT_PATH,path>
$<PATH:GET_FILENAME,path>
$<PATH:GET_EXTENSION[,LAST_ONLY],path>
$<PATH:GET_STEM[,LAST_ONLY],path>
$<PATH:GET_RELATIVE_PART,path>
$<PATH:GET_PARENT_PATH,path>

If a requested component is not present in the path, an empty string is returned.

Path Transformations¶

These expressions provide the generation-time capabilities equivalent to the Modification and Generation options of the cmake_path() command. All paths are expected to be in cmake-style format.

$<PATH:CMAKE_PATH[,NORMALIZE],path>

New in version 3.24.

Returns path. If path is a native path, it is converted into a cmake-style path with forward-slashes (/). On Windows, the long filename marker is taken into account.

When the NORMALIZE option is specified, the path is normalized after the conversion.

$<PATH:APPEND,path,input,...>

New in version 3.24.

Returns all the input arguments appended to path using / as the directory-separator. Depending on the input, the value of path may be discarded.

See cmake_path(APPEND) for more details.

$<PATH:REMOVE_FILENAME,path>

New in version 3.24.

Returns path with filename component (as returned by $<PATH:GET_FILENAME>) removed. After removal, any trailing directory-separator is left alone, if present.

See cmake_path(REMOVE_FILENAME) for more details.

$<PATH:REPLACE_FILENAME,path,input>

New in version 3.24.

Returns path with the filename component replaced by input. If path has no filename component (i.e. $<PATH:HAS_FILENAME> returns 0), path is unchanged.

See cmake_path(REPLACE_FILENAME) for more details.

$<PATH:REMOVE_EXTENSION[,LAST_ONLY],path>

New in version 3.24.

Returns path with the extension removed, if any.

See cmake_path(REMOVE_EXTENSION) for more details.

$<PATH:REPLACE_EXTENSION[,LAST_ONLY],path,input>

New in version 3.24.

Returns path with the extension replaced by input, if any.

See cmake_path(REPLACE_EXTENSION) for more details.

$<PATH:NORMAL_PATH,path>

New in version 3.24.

Returns path normalized according to the steps described in Normalization.

$<PATH:RELATIVE_PATH,path,base_directory>

New in version 3.24.

Returns path, modified to make it relative to the base_directory argument.

See cmake_path(RELATIVE_PATH) for more details.

$<PATH:ABSOLUTE_PATH[,NORMALIZE],path,base_directory>

New in version 3.24.

Returns path as absolute. If path is a relative path ($<PATH:IS_RELATIVE> returns 1), it is evaluated relative to the given base directory specified by base_directory argument.

When the NORMALIZE option is specified, the path is normalized after the path computation.

See cmake_path(ABSOLUTE_PATH) for more details.

Shell Paths¶

$<SHELL_PATH:...>

New in version 3.4.

Content of ... converted to shell path style. For example, slashes are converted to backslashes in Windows shells and drive letters are converted to posix paths in MSYS shells. The ... must be an absolute path.

New in version 3.14: The ... may be a semicolon-separated list of paths, in which case each path is converted individually and a result list is generated using the shell path separator (: on POSIX and ; on Windows). Be sure to enclose the argument containing this genex in double quotes in CMake source code so that ; does not split arguments.

Configuration Expressions¶

$<CONFIG>: Configuration name. Use this instead of the deprecated CONFIGURATION generator expression.

$<CONFIG:cfgs>

1 if config is any one of the entries in comma-separated list cfgs, else 0. This is a case-insensitive comparison. The mapping in MAP_IMPORTED_CONFIG_<CONFIG> is also considered by this expression when it is evaluated on a property of an IMPORTED target.

Changed in version 3.19: Multiple configurations can be specified for cfgs. CMake 3.18 and earlier only accepted a single configuration.

$<OUTPUT_CONFIG:...>

New in version 3.20.

Only valid in add_custom_command() and add_custom_target() as the outer-most generator expression in an argument. With the Ninja Multi-Config generator, generator expressions in ... are evaluated using the custom command's "output config". With other generators, the content of ... is evaluated normally.

$<COMMAND_CONFIG:...>

New in version 3.20.

Only valid in add_custom_command() and add_custom_target() as the outer-most generator expression in an argument. With the Ninja Multi-Config generator, generator expressions in ... are evaluated using the custom command's "command config". With other generators, the content of ... is evaluated normally.

Toolchain And Language Expressions¶

Platform¶

$<PLATFORM_ID>: The current system's CMake platform id. See also the CMAKE_SYSTEM_NAME variable.

$<PLATFORM_ID:platform_ids>: 1 if CMake's platform id matches any one of the entries in comma-separated list platform_ids, otherwise 0. See also the CMAKE_SYSTEM_NAME variable.

Compiler Version¶

See also the CMAKE_<LANG>_COMPILER_VERSION variable, which is closely related to the expressions in this sub-section.

$<C_COMPILER_VERSION>: The version of the C compiler used.

$<C_COMPILER_VERSION:version>: 1 if the version of the C compiler matches version, otherwise 0.

$<CXX_COMPILER_VERSION>: The version of the CXX compiler used.

$<CXX_COMPILER_VERSION:version>: 1 if the version of the CXX compiler matches version, otherwise 0.

$<CUDA_COMPILER_VERSION>

New in version 3.15.

The version of the CUDA compiler used.

$<CUDA_COMPILER_VERSION:version>

New in version 3.15.

1 if the version of the CXX compiler matches version, otherwise 0.

$<OBJC_COMPILER_VERSION>

New in version 3.16.

The version of the OBJC compiler used.

$<OBJC_COMPILER_VERSION:version>

New in version 3.16.

1 if the version of the OBJC compiler matches version, otherwise 0.

$<OBJCXX_COMPILER_VERSION>

New in version 3.16.

The version of the OBJCXX compiler used.

$<OBJCXX_COMPILER_VERSION:version>

New in version 3.16.

1 if the version of the OBJCXX compiler matches version, otherwise 0.

$<Fortran_COMPILER_VERSION>: The version of the Fortran compiler used.

$<Fortran_COMPILER_VERSION:version>: 1 if the version of the Fortran compiler matches version, otherwise 0.

$<HIP_COMPILER_VERSION>

New in version 3.21.

The version of the HIP compiler used.

$<HIP_COMPILER_VERSION:version>

New in version 3.21.

1 if the version of the HIP compiler matches version, otherwise 0.

$<ISPC_COMPILER_VERSION>

New in version 3.19.

The version of the ISPC compiler used.

$<ISPC_COMPILER_VERSION:version>

New in version 3.19.

1 if the version of the ISPC compiler matches version, otherwise 0.

Compiler Language And ID¶

See also the CMAKE_<LANG>_COMPILER_ID variable, which is closely related to most of the expressions in this sub-section.

$<C_COMPILER_ID>: CMake's compiler id of the C compiler used.

$<C_COMPILER_ID:compiler_ids>: where compiler_ids is a comma-separated list. 1 if CMake's compiler id of the C compiler matches any one of the entries in compiler_ids, otherwise 0.

$<CXX_COMPILER_ID>: CMake's compiler id of the CXX compiler used.

$<CXX_COMPILER_ID:compiler_ids>: where compiler_ids is a comma-separated list. 1 if CMake's compiler id of the CXX compiler matches any one of the entries in compiler_ids, otherwise 0.

$<CUDA_COMPILER_ID>

New in version 3.15.

CMake's compiler id of the CUDA compiler used.

$<CUDA_COMPILER_ID:compiler_ids>

New in version 3.15.

where compiler_ids is a comma-separated list. 1 if CMake's compiler id of the CUDA compiler matches any one of the entries in compiler_ids, otherwise 0.

$<OBJC_COMPILER_ID>

New in version 3.16.

CMake's compiler id of the OBJC compiler used.

$<OBJC_COMPILER_ID:compiler_ids>

New in version 3.16.

where compiler_ids is a comma-separated list. 1 if CMake's compiler id of the Objective-C compiler matches any one of the entries in compiler_ids, otherwise 0.

$<OBJCXX_COMPILER_ID>

New in version 3.16.

CMake's compiler id of the OBJCXX compiler used.

$<OBJCXX_COMPILER_ID:compiler_ids>

New in version 3.16.

where compiler_ids is a comma-separated list. 1 if CMake's compiler id of the Objective-C++ compiler matches any one of the entries in compiler_ids, otherwise 0.

$<Fortran_COMPILER_ID>: CMake's compiler id of the Fortran compiler used.

$<Fortran_COMPILER_ID:compiler_ids>: where compiler_ids is a comma-separated list. 1 if CMake's compiler id of the Fortran compiler matches any one of the entries in compiler_ids, otherwise 0.

$<HIP_COMPILER_ID>

New in version 3.21.

CMake's compiler id of the HIP compiler used.

$<HIP_COMPILER_ID:compiler_ids>

New in version 3.21.

where compiler_ids is a comma-separated list. 1 if CMake's compiler id of the HIP compiler matches any one of the entries in compiler_ids, otherwise 0.

$<ISPC_COMPILER_ID>

New in version 3.19.

CMake's compiler id of the ISPC compiler used.

$<ISPC_COMPILER_ID:compiler_ids>

New in version 3.19.

where compiler_ids is a comma-separated list. 1 if CMake's compiler id of the ISPC compiler matches any one of the entries in compiler_ids, otherwise 0.

$<COMPILE_LANGUAGE>

New in version 3.3.

The compile language of source files when evaluating compile options. See the related boolean expression $<COMPILE_LANGUAGE:language> for notes about the portability of this generator expression.

$<COMPILE_LANGUAGE:languages>

New in version 3.3.

Changed in version 3.15: Multiple languages can be specified for languages. CMake 3.14 and earlier only accepted a single language.

1 when the language used for compilation unit matches any of the comma-separated entries in languages, otherwise 0. This expression may be used to specify compile options, compile definitions, and include directories for source files of a particular language in a target. For example:

add_executable(myapp main.cpp foo.c bar.cpp zot.cu)
target_compile_options(myapp


  PRIVATE $<$<COMPILE_LANGUAGE:CXX>:-fno-exceptions>
)
target_compile_definitions(myapp


  PRIVATE $<$<COMPILE_LANGUAGE:CXX>:COMPILING_CXX>


          $<$<COMPILE_LANGUAGE:CUDA>:COMPILING_CUDA>
)
target_include_directories(myapp


  PRIVATE $<$<COMPILE_LANGUAGE:CXX,CUDA>:/opt/foo/headers>
)

This specifies the use of the -fno-exceptions compile option, COMPILING_CXX compile definition, and cxx_headers include directory for C++ only (compiler id checks elided). It also specifies a COMPILING_CUDA compile definition for CUDA.

Note that with Visual Studio Generators and Xcode there is no way to represent target-wide compile definitions or include directories separately for C and CXX languages. Also, with Visual Studio Generators there is no way to represent target-wide flags separately for C and CXX languages. Under these generators, expressions for both C and C++ sources will be evaluated using CXX if there are any C++ sources and otherwise using C. A workaround is to create separate libraries for each source file language instead:

add_library(myapp_c foo.c)
add_library(myapp_cxx bar.cpp)
target_compile_options(myapp_cxx PUBLIC -fno-exceptions)
add_executable(myapp main.cpp)
target_link_libraries(myapp myapp_c myapp_cxx)

$<COMPILE_LANG_AND_ID:language,compiler_ids>

New in version 3.15.

1 when the language used for compilation unit matches language and CMake's compiler id of the language compiler matches any one of the comma-separated entries in compiler_ids, otherwise 0. This expression is a short form for the combination of $<COMPILE_LANGUAGE:language> and $<LANG_COMPILER_ID:compiler_ids>. This expression may be used to specify compile options, compile definitions, and include directories for source files of a particular language and compiler combination in a target. For example:

add_executable(myapp main.cpp foo.c bar.cpp zot.cu)
target_compile_definitions(myapp


  PRIVATE $<$<COMPILE_LANG_AND_ID:CXX,AppleClang,Clang>:COMPILING_CXX_WITH_CLANG>


          $<$<COMPILE_LANG_AND_ID:CXX,Intel>:COMPILING_CXX_WITH_INTEL>


          $<$<COMPILE_LANG_AND_ID:C,Clang>:COMPILING_C_WITH_CLANG>
)

This specifies the use of different compile definitions based on both the compiler id and compilation language. This example will have a COMPILING_CXX_WITH_CLANG compile definition when Clang is the CXX compiler, and COMPILING_CXX_WITH_INTEL when Intel is the CXX compiler. Likewise, when the C compiler is Clang, it will only see the COMPILING_C_WITH_CLANG definition.

Without the COMPILE_LANG_AND_ID generator expression, the same logic would be expressed as:

target_compile_definitions(myapp


  PRIVATE $<$<AND:$<COMPILE_LANGUAGE:CXX>,$<CXX_COMPILER_ID:AppleClang,Clang>>:COMPILING_CXX_WITH_CLANG>


          $<$<AND:$<COMPILE_LANGUAGE:CXX>,$<CXX_COMPILER_ID:Intel>>:COMPILING_CXX_WITH_INTEL>


          $<$<AND:$<COMPILE_LANGUAGE:C>,$<C_COMPILER_ID:Clang>>:COMPILING_C_WITH_CLANG>
)

Compile Features¶

$<COMPILE_FEATURES:features>

New in version 3.1.

where features is a comma-separated list. Evaluates to 1 if all of the features are available for the 'head' target, and 0 otherwise. If this expression is used while evaluating the link implementation of a target and if any dependency transitively increases the required C_STANDARD or CXX_STANDARD for the 'head' target, an error is reported. See the cmake-compile-features(7) manual for information on compile features and a list of supported compilers.

Linker Language And ID¶

$<LINK_LANGUAGE>

New in version 3.18.

The link language of the target when evaluating link options. See the related boolean expression $<LINK_LANGUAGE:languages> for notes about the portability of this generator expression.

NOTE:

This generator expression is not supported by the link libraries properties to avoid side-effects due to the double evaluation of these properties.

$<LINK_LANGUAGE:languages>

New in version 3.18.

1 when the language used for link step matches any of the comma-separated entries in languages, otherwise 0. This expression may be used to specify link libraries, link options, link directories and link dependencies of a particular language in a target. For example:

add_library(api_C ...)
add_library(api_CXX ...)
add_library(api INTERFACE)
target_link_options(api   INTERFACE $<$<LINK_LANGUAGE:C>:-opt_c>


                                    $<$<LINK_LANGUAGE:CXX>:-opt_cxx>)
target_link_libraries(api INTERFACE $<$<LINK_LANGUAGE:C>:api_C>


                                    $<$<LINK_LANGUAGE:CXX>:api_CXX>)
add_executable(myapp1 main.c)
target_link_options(myapp1 PRIVATE api)
add_executable(myapp2 main.cpp)
target_link_options(myapp2 PRIVATE api)

This specifies to use the api target for linking targets myapp1 and myapp2. In practice, myapp1 will link with target api_C and option -opt_c because it will use C as link language. And myapp2 will link with api_CXX and option -opt_cxx because CXX will be the link language.

NOTE:

To determine the link language of a target, it is required to collect, transitively, all the targets which will be linked to it. So, for link libraries properties, a double evaluation will be done. During the first evaluation, $<LINK_LANGUAGE:..> expressions will always return 0. The link language computed after this first pass will be used to do the second pass. To avoid inconsistency, it is required that the second pass do not change the link language. Moreover, to avoid unexpected side-effects, it is required to specify complete entities as part of the $<LINK_LANGUAGE:..> expression. For example:

add_library(lib STATIC file.cxx)
add_library(libother STATIC file.c)
# bad usage
add_executable(myapp1 main.c)
target_link_libraries(myapp1 PRIVATE lib$<$<LINK_LANGUAGE:C>:other>)
# correct usage
add_executable(myapp2 main.c)
target_link_libraries(myapp2 PRIVATE $<$<LINK_LANGUAGE:C>:libother>)

In this example, for myapp1, the first pass will, unexpectedly, determine that the link language is CXX because the evaluation of the generator expression will be an empty string so myapp1 will depends on target lib which is C++. On the contrary, for myapp2, the first evaluation will give C as link language, so the second pass will correctly add target libother as link dependency.

$<LINK_LANG_AND_ID:language,compiler_ids>

New in version 3.18.

1 when the language used for link step matches language and the CMake's compiler id of the language linker matches any one of the comma-separated entries in compiler_ids, otherwise 0. This expression is a short form for the combination of $<LINK_LANGUAGE:language> and $<LANG_COMPILER_ID:compiler_ids>. This expression may be used to specify link libraries, link options, link directories and link dependencies of a particular language and linker combination in a target. For example:

add_library(libC_Clang ...)
add_library(libCXX_Clang ...)
add_library(libC_Intel ...)
add_library(libCXX_Intel ...)
add_executable(myapp main.c)
if (CXX_CONFIG)


  target_sources(myapp PRIVATE file.cxx)
endif()
target_link_libraries(myapp


  PRIVATE $<$<LINK_LANG_AND_ID:CXX,Clang,AppleClang>:libCXX_Clang>


          $<$<LINK_LANG_AND_ID:C,Clang,AppleClang>:libC_Clang>


          $<$<LINK_LANG_AND_ID:CXX,Intel>:libCXX_Intel>


          $<$<LINK_LANG_AND_ID:C,Intel>:libC_Intel>)

This specifies the use of different link libraries based on both the compiler id and link language. This example will have target libCXX_Clang as link dependency when Clang or AppleClang is the CXX linker, and libCXX_Intel when Intel is the CXX linker. Likewise when the C linker is Clang or AppleClang, target libC_Clang will be added as link dependency and libC_Intel when Intel is the C linker.

See the note related to $<LINK_LANGUAGE:language> for constraints about the usage of this generator expression.

Link Features¶

$<LINK_LIBRARY:feature,library-list>

New in version 3.24.

Specify a set of libraries to link to a target, along with a feature which provides details about how they should be linked. For example:

add_library(lib1 STATIC ...)
add_library(lib2 ...)
target_link_libraries(lib2 PRIVATE "$<LINK_LIBRARY:WHOLE_ARCHIVE,lib1>")

This specifies that lib2 should link to lib1 and use the WHOLE_ARCHIVE feature when doing so.

Feature names are case-sensitive and may only contain letters, numbers and underscores. Feature names defined in all uppercase are reserved for CMake's own built-in features. The pre-defined built-in library features are:

DEFAULT: This feature corresponds to standard linking, essentially equivalent to using no feature at all. It is typically only used with the LINK_LIBRARY_OVERRIDE and LINK_LIBRARY_OVERRIDE_<LIBRARY> target properties.
WHOLE_ARCHIVE: Force inclusion of all members of a static library. This feature is only supported for the following platforms, with limitations as noted:

Linux.
All BSD variants.
SunOS.
All Apple variants. The library must be specified as a CMake target name, a library file name (such as libfoo.a), or a library file path (such as /path/to/libfoo.a). Due to a limitation of the Apple linker, it cannot be specified as a plain library name like foo, where foo is not a CMake target.
Windows. When using a MSVC or MSVC-like toolchain, the MSVC version must be greater than 1900.
Cygwin.
MSYS.

FRAMEWORK: This option tells the linker to search for the specified framework using the -framework linker option. It can only be used on Apple platforms, and only with a linker that understands the option used (i.e. the linker provided with Xcode, or one compatible with it).
The framework can be specified as a CMake framework target, a bare framework name, or a file path. If a target is given, that target must have the FRAMEWORK target property set to true. For a file path, if it contains a directory part, that directory will be added as a framework search path.

add_library(lib SHARED ...)
target_link_libraries(lib PRIVATE "$<LINK_LIBRARY:FRAMEWORK,/path/to/my_framework>")
# The constructed linker command line will contain:
#   -F/path/to -framework my_framework

File paths must conform to one of the following patterns (* is a wildcard, and optional parts are shown as [...]):

[/path/to/]FwName[.framework]
[/path/to/]FwName.framework/FwName[suffix]
[/path/to/]FwName.framework/Versions/*/FwName[suffix]

Note that CMake recognizes and automatically handles framework targets, even without using the $<LINK_LIBRARY:FRAMEWORK,...> expression. The generator expression can still be used with a CMake target if the project wants to be explicit about it, but it is not required to do so. The linker command line may have some differences between using the generator expression or not, but the final result should be the same. On the other hand, if a file path is given, CMake will recognize some paths automatically, but not all cases. The project may want to use $<LINK_LIBRARY:FRAMEWORK,...> for file paths so that the expected behavior is clear.

New in version 3.25: The FRAMEWORK_MULTI_CONFIG_POSTFIX_<CONFIG> target property as well as the suffix of the framework library name are now supported by the FRAMEWORK features.

NEEDED_FRAMEWORK: This is similar to the FRAMEWORK feature, except it forces the linker to link with the framework even if no symbols are used from it. It uses the -needed_framework option and has the same linker constraints as FRAMEWORK.
REEXPORT_FRAMEWORK: This is similar to the FRAMEWORK feature, except it tells the linker that the framework should be available to clients linking to the library being created. It uses the -reexport_framework option and has the same linker constraints as FRAMEWORK.
WEAK_FRAMEWORK: This is similar to the FRAMEWORK feature, except it forces the linker to mark the framework and all references to it as weak imports. It uses the -weak_framework option and has the same linker constraints as FRAMEWORK.
NEEDED_LIBRARY: This is similar to the NEEDED_FRAMEWORK feature, except it is for use with non-framework targets or libraries (Apple platforms only). It uses the -needed_library or -needed-l option as appropriate, and has the same linker constraints as NEEDED_FRAMEWORK.
REEXPORT_LIBRARY: This is similar to the REEXPORT_FRAMEWORK feature, except it is for use with non-framework targets or libraries (Apple platforms only). It uses the -reexport_library or -reexport-l option as appropriate, and has the same linker constraints as REEXPORT_FRAMEWORK.
WEAK_LIBRARY: This is similar to the WEAK_FRAMEWORK feature, except it is for use with non-framework targets or libraries (Apple platforms only). It uses the -weak_library or -weak-l option as appropriate, and has the same linker constraints as WEAK_FRAMEWORK.

Built-in and custom library features are defined in terms of the following variables:

CMAKE_<LANG>_LINK_LIBRARY_USING_<FEATURE>_SUPPORTED
CMAKE_<LANG>_LINK_LIBRARY_USING_<FEATURE>
CMAKE_LINK_LIBRARY_USING_<FEATURE>_SUPPORTED
CMAKE_LINK_LIBRARY_USING_<FEATURE>

The value used for each of these variables is the value as set at the end of the directory scope in which the target was created. The usage is as follows:

1.: If the language-specific CMAKE_<LANG>_LINK_LIBRARY_USING_<FEATURE>_SUPPORTED variable is true, the feature must be defined by the corresponding CMAKE_<LANG>_LINK_LIBRARY_USING_<FEATURE> variable.
2.: If no language-specific feature is supported, then the CMAKE_LINK_LIBRARY_USING_<FEATURE>_SUPPORTED variable must be true and the feature must be defined by the corresponding CMAKE_LINK_LIBRARY_USING_<FEATURE> variable.

The following limitations should be noted:

The library-list can specify CMake targets or libraries. Any CMake target of type OBJECT or INTERFACE will ignore the feature aspect of the expression and instead be linked in the standard way.
The $<LINK_LIBRARY:...> generator expression can only be used to specify link libraries. In practice, this means it can appear in the LINK_LIBRARIES, INTERFACE_LINK_LIBRARIES, and INTERFACE_LINK_LIBRARIES_DIRECT target properties, and be specified in target_link_libraries() and link_libraries() commands.
If a $<LINK_LIBRARY:...> generator expression appears in the INTERFACE_LINK_LIBRARIES property of a target, it will be included in the imported target generated by a install(EXPORT) command. It is the responsibility of the environment consuming this import to define the link feature used by this expression.
Each target or library involved in the link step must have at most only one kind of library feature. The absence of a feature is also incompatible with all other features. For example:

add_library(lib1 ...)
add_library(lib2 ...)
add_library(lib3 ...)
# lib1 will be associated with feature1
target_link_libraries(lib2 PUBLIC "$<LINK_LIBRARY:feature1,lib1>")
# lib1 is being linked with no feature here. This conflicts with the
# use of feature1 in the line above and would result in an error.
target_link_libraries(lib3 PRIVATE lib1 lib2)

Where it isn't possible to use the same feature throughout a build for a given target or library, the LINK_LIBRARY_OVERRIDE and LINK_LIBRARY_OVERRIDE_<LIBRARY> target properties can be used to resolve such incompatibilities.

•: The $<LINK_LIBRARY:...> generator expression does not guarantee that the list of specified targets and libraries will be kept grouped together. To manage constructs like --start-group and --end-group, as supported by the GNU ld linker, use the LINK_GROUP generator expression instead.

$<LINK_GROUP:feature,library-list>

New in version 3.24.

Specify a group of libraries to link to a target, along with a feature which defines how that group should be linked. For example:

add_library(lib1 STATIC ...)
add_library(lib2 ...)
target_link_libraries(lib2 PRIVATE "$<LINK_GROUP:RESCAN,lib1,external>")

This specifies that lib2 should link to lib1 and external, and that both of those two libraries should be included on the linker command line according to the definition of the RESCAN feature.

RESCAN

Some linkers are single-pass only. For such linkers, circular references between libraries typically result in unresolved symbols. This feature instructs the linker to search the specified static libraries repeatedly until no new undefined references are created.

Normally, a static library is searched only once in the order that it is specified on the command line. If a symbol in that library is needed to resolve an undefined symbol referred to by an object in a library that appears later on the command line, the linker would not be able to resolve that reference. By grouping the static libraries with the RESCAN feature, they will all be searched repeatedly until all possible references are resolved. This will use linker options like --start-group and --end-group, or on SunOS, -z rescan-start and -z rescan-end.

Using this feature has a significant performance cost. It is best to use it only when there are unavoidable circular references between two or more static libraries.

This feature is available when using toolchains that target Linux, BSD, and SunOS. It can also be used when targeting Windows platforms if the GNU toolchain is used.

Built-in and custom group features are defined in terms of the following variables:

CMAKE_<LANG>_LINK_GROUP_USING_<FEATURE>_SUPPORTED
CMAKE_<LANG>_LINK_GROUP_USING_<FEATURE>
CMAKE_LINK_GROUP_USING_<FEATURE>_SUPPORTED
CMAKE_LINK_GROUP_USING_<FEATURE>

The value used for each of these variables is the value as set at the end of the directory scope in which the target was created. The usage is as follows:

1.: If the language-specific CMAKE_<LANG>_LINK_GROUP_USING_<FEATURE>_SUPPORTED variable is true, the feature must be defined by the corresponding CMAKE_<LANG>_LINK_GROUP_USING_<FEATURE> variable.
2.: If no language-specific feature is supported, then the CMAKE_LINK_GROUP_USING_<FEATURE>_SUPPORTED variable must be true and the feature must be defined by the corresponding CMAKE_LINK_GROUP_USING_<FEATURE> variable.

The LINK_GROUP generator expression is compatible with the LINK_LIBRARY generator expression. The libraries involved in a group can be specified using the LINK_LIBRARY generator expression.

Each target or external library involved in the link step is allowed to be part of multiple groups, but only if all the groups involved specify the same feature. Such groups will not be merged on the linker command line, the individual groups will still be preserved. Mixing different group features for the same target or library is forbidden.

add_library(lib1 ...)
add_library(lib2 ...)
add_library(lib3 ...)
add_library(lib4 ...)
add_library(lib5 ...)
target_link_libraries(lib3 PUBLIC  "$<LINK_GROUP:feature1,lib1,lib2>")
target_link_libraries(lib4 PRIVATE "$<LINK_GROUP:feature1,lib1,lib3>")
# lib4 will be linked with the groups {lib1,lib2} and {lib1,lib3}.
# Both groups specify the same feature, so this is fine.
target_link_libraries(lib5 PRIVATE "$<LINK_GROUP:feature2,lib1,lib3>")
# An error will be raised here because both lib1 and lib3 are part of two
# groups with different features.

When a target or an external library is involved in the link step as part of a group and also as not part of any group, any occurrence of the non-group link item will be replaced by the groups it belongs to.

add_library(lib1 ...)
add_library(lib2 ...)
add_library(lib3 ...)
add_library(lib4 ...)
target_link_libraries(lib3 PUBLIC lib1)
target_link_libraries(lib4 PRIVATE lib3 "$<LINK_GROUP:feature1,lib1,lib2>")
# lib4 will only be linked with lib3 and the group {lib1,lib2}

Because lib1 is part of the group defined for lib4, that group then gets applied back to the use of lib1 for lib3. The end result will be as though the linking relationship for lib3 had been specified as:

target_link_libraries(lib3 PUBLIC "$<LINK_GROUP:feature1,lib1,lib2>")

Be aware that the precedence of the group over the non-group link item can result in circular dependencies between groups. If this occurs, a fatal error is raised because circular dependencies are not allowed for groups.

add_library(lib1A ...)
add_library(lib1B ...)
add_library(lib2A ...)
add_library(lib2B ...)
add_library(lib3 ...)
# Non-group linking relationships, these are non-circular so far
target_link_libraries(lib1A PUBLIC lib2A)
target_link_libraries(lib2B PUBLIC lib1B)
# The addition of these groups creates circular dependencies
target_link_libraries(lib3 PRIVATE


  "$<LINK_GROUP:feat,lib1A,lib1B>"


  "$<LINK_GROUP:feat,lib2A,lib2B>"
)

Because of the groups defined for lib3, the linking relationships for lib1A and lib2B effectively get expanded to the equivalent of:

target_link_libraries(lib1A PUBLIC "$<LINK_GROUP:feat,lib2A,lib2B>")
target_link_libraries(lib2B PUBLIC "$<LINK_GROUP:feat,lib1A,lib1B>")

This creates a circular dependency between groups: lib1A --> lib2B --> lib1A.

The following limitations should also be noted:

The library-list can specify CMake targets or libraries. Any CMake target of type OBJECT or INTERFACE will ignore the feature aspect of the expression and instead be linked in the standard way.
The $<LINK_GROUP:...> generator expression can only be used to specify link libraries. In practice, this means it can appear in the LINK_LIBRARIES, INTERFACE_LINK_LIBRARIES,and INTERFACE_LINK_LIBRARIES_DIRECT target properties, and be specified in target_link_libraries() and link_libraries() commands.
If a $<LINK_GROUP:...> generator expression appears in the INTERFACE_LINK_LIBRARIES property of a target, it will be included in the imported target generated by a install(EXPORT) command. It is the responsibility of the environment consuming this import to define the link feature used by this expression.

Link Context¶

$<LINK_ONLY:...>

New in version 3.1.

Content of ..., except while collecting Transitive Usage Requirements, in which case it is the empty string. This is intended for use in an INTERFACE_LINK_LIBRARIES target property, typically populated via the target_link_libraries() command, to specify private link dependencies without other usage requirements.

New in version 3.24: LINK_ONLY may also be used in a LINK_LIBRARIES target property. See policy CMP0131.

$<DEVICE_LINK:list>

New in version 3.18.

Returns the list if it is the device link step, an empty list otherwise. The device link step is controlled by CUDA_SEPARABLE_COMPILATION and CUDA_RESOLVE_DEVICE_SYMBOLS properties and policy CMP0105. This expression can only be used to specify link options.

$<HOST_LINK:list>

New in version 3.18.

Returns the list if it is the normal link step, an empty list otherwise. This expression is mainly useful when a device link step is also involved (see $<DEVICE_LINK:list> generator expression). This expression can only be used to specify link options.

Target-Dependent Expressions¶

These queries refer to a target tgt. Unless otherwise stated, this can be any runtime artifact, namely:

An executable target created by add_executable().
A shared library target (.so, .dll but not their .lib import library) created by add_library().
A static library target created by add_library().

In the following, the phrase "the tgt filename" means the name of the tgt binary file. This has to be distinguished from the phrase "the target name", which is just the string tgt.

$<TARGET_EXISTS:tgt>

New in version 3.12.

1 if tgt exists as a CMake target, else 0.

$<TARGET_NAME_IF_EXISTS:tgt>

New in version 3.12.

The target name tgt if the target exists, an empty string otherwise.

Note that tgt is not added as a dependency of the target this expression is evaluated on.

$<TARGET_NAME:...>: Marks ... as being the name of a target. This is required if exporting targets to multiple dependent export sets. The ... must be a literal name of a target, it may not contain generator expressions.

$<TARGET_PROPERTY:tgt,prop>: Value of the property prop on the target tgt.
Note that tgt is not added as a dependency of the target this expression is evaluated on.

$<TARGET_PROPERTY:prop>: Value of the property prop on the target for which the expression is being evaluated. Note that for generator expressions in Transitive Usage Requirements this is the consuming target rather than the target specifying the requirement.

$<TARGET_OBJECTS:tgt>

New in version 3.1.

List of objects resulting from building tgt. This would typically be used on object library targets.

$<TARGET_POLICY:policy>: 1 if the policy was NEW when the 'head' target was created, else 0. If the policy was not set, the warning message for the policy will be emitted. This generator expression only works for a subset of policies.

$<TARGET_FILE:tgt>: Full path to the tgt binary file.
Note that tgt is not added as a dependency of the target this expression is evaluated on, unless the expression is being used in add_custom_command() or add_custom_target().

$<TARGET_FILE_BASE_NAME:tgt>

New in version 3.15.

Base name of tgt, i.e. $<TARGET_FILE_NAME:tgt> without prefix and suffix. For example, if the tgt filename is libbase.so, the base name is base.

See also the OUTPUT_NAME, ARCHIVE_OUTPUT_NAME, LIBRARY_OUTPUT_NAME and RUNTIME_OUTPUT_NAME target properties and their configuration specific variants OUTPUT_NAME_<CONFIG>, ARCHIVE_OUTPUT_NAME_<CONFIG>, LIBRARY_OUTPUT_NAME_<CONFIG> and RUNTIME_OUTPUT_NAME_<CONFIG>.

The <CONFIG>_POSTFIX and DEBUG_POSTFIX target properties can also be considered.

Note that tgt is not added as a dependency of the target this expression is evaluated on.

$<TARGET_FILE_PREFIX:tgt>

New in version 3.15.

Prefix of the tgt filename (such as lib).

Export And Install Expressions¶

$<INSTALL_INTERFACE:...>: Content of ... when the property is exported using install(EXPORT), and empty otherwise.

$<BUILD_INTERFACE:...>: Content of ... when the property is exported using export(), or when the target is used by another target in the same buildsystem. Expands to the empty string otherwise.

$<INSTALL_PREFIX>: Content of the install prefix when the target is exported via install(EXPORT), or when evaluated in the INSTALL_NAME_DIR property or the INSTALL_NAME_DIR argument of install(RUNTIME_DEPENDENCY_SET), and empty otherwise.

Multi-level Expression Evaluation¶

$<GENEX_EVAL:expr>

New in version 3.12.

Content of expr evaluated as a generator expression in the current context. This enables consumption of generator expressions whose evaluation results itself in generator expressions.

$<TARGET_GENEX_EVAL:tgt,expr>

New in version 3.12.

Content of expr evaluated as a generator expression in the context of tgt target. This enables consumption of custom target properties that themselves contain generator expressions.

Having the capability to evaluate generator expressions is very useful when you want to manage custom properties supporting generator expressions. For example:

add_library(foo ...)
set_property(TARGET foo PROPERTY


  CUSTOM_KEYS $<$<CONFIG:DEBUG>:FOO_EXTRA_THINGS>
)
add_custom_target(printFooKeys


  COMMAND ${CMAKE_COMMAND} -E echo $<TARGET_PROPERTY:foo,CUSTOM_KEYS>
)

This naive implementation of the printFooKeys custom command is wrong because CUSTOM_KEYS target property is not evaluated and the content is passed as is (i.e. $<$<CONFIG:DEBUG>:FOO_EXTRA_THINGS>).

To have the expected result (i.e. FOO_EXTRA_THINGS if config is Debug), it is required to evaluate the output of $<TARGET_PROPERTY:foo,CUSTOM_KEYS>:

add_custom_target(printFooKeys


  COMMAND ${CMAKE_COMMAND} -E


    echo $<TARGET_GENEX_EVAL:foo,$<TARGET_PROPERTY:foo,CUSTOM_KEYS>>
)

Escaped Characters¶

These expressions evaluate to specific string literals. Use them in place of the actual string literal where you need to prevent them from having their special meaning.

$<ANGLE-R>: A literal >. Used for example to compare strings that contain a >.

$<COMMA>: A literal ,. Used for example to compare strings which contain a ,.

$<SEMICOLON>: A literal ;. Used to prevent list expansion on an argument with ;.

Deprecated Expressions¶

$<CONFIGURATION>: Configuration name. Deprecated since CMake 3.0. Use CONFIG instead.

COPYRIGHT¶

2000-2022 Kitware, Inc. and Contributors

November 30, 2022

3.25.1

Source file:	cmake-generator-expressions.7.en.gz (from cmake-data 3.25.1-1)
Source last updated:	2022-11-30T19:57:47Z
Converted to HTML:	2024-11-12T15:06:21Z