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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 pro-
duce 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 oth-
ers. They may also be used when using commands to populate those proper-
ties, such as target_link_libraries(), target_include_directories(),
target_compile_definitions() and others. They enable conditional linking,
conditional definitions used when compiling, conditional include directo-
ries, and more. The conditions may be based on the build configuration,
target properties, platform information, or any other queryable informa-
tion.

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 char-
acters that may be interpreted as command argument separators, the whole
expression should be surrounded by quotes when passed to a command. Fail-
ure 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
)

Finally, the above example can be expressed in a more simple and robust way
using an alternate generator expression:

add_custom_target(run_some_tool
COMMAND some_tool "$<LIST:TRANSFORM,$<TARGET_PROPERTY:tgt,INCLUDE_DIRECTORIES>,PREPEND,-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>
Added in version 3.8.

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

Added in version 3.28: This generator expression short-circuits such
that generator expressions in false_string will not evaluate when
condition is 1, and generator expressions in true_string will not
evaluate when condition is 0.

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, IG-
NORE, 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 pro-
vided 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 con-
ditions 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 con-
dition is 1, else 1.

Added in version 3.28: Logical operators short-circuit such that generator
expressions in the arguments list will not be evaluated once a return value
can be determined.

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 follow-
ing 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>
Added in version 3.7.

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

$<VERSION_GREATER_EQUAL:v1,v2>
Added 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
Most of the expressions in this section are closely associated with the
list() command, providing the same capabilities, but in the form of a gen-
erator expression.

In each of the following list-related generator expressions, the list must
not contain any commas if that generator expression expects something to be
provided after the list. For example, the expression
$<LIST:FIND,list,value> requires a value after the list. Since a comma is
used to separate the list and the value, the list cannot itself contain a
comma. This restriction does not apply to the list() command, it is spe-
cific to the list-handling generator expressions only.

List Comparisons
$<IN_LIST:string,list>
Added in version 3.12.

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

List Queries
$<LIST:LENGTH,list>
Added in version 3.27.

The number of items in the list.

$<LIST:GET,list,index,...>
Added in version 3.27.

Expands to the list of items specified by indices from the list.

$<LIST:SUBLIST,list,begin,length>
Added in version 3.27.

A sublist of the given list. If length is 0, an empty list will be
returned. If length is -1 or the list is smaller than begin +
length, the remaining items of the list starting at begin will be
returned.

$<LIST:FIND,list,value>
Added in version 3.27.

The index of the first item in list with the specified value, or -1
if value is not in the list.

List Transformations
$<LIST:JOIN,list,glue>
Added in version 3.27.

Converts list to a single string with the content of the glue string
inserted between each item. This is conceptually the same operation
as $<JOIN:list,glue>, but the two have different behavior with re-
gard to empty items. $<LIST:JOIN,list,glue> preserves all empty
items, whereas $<JOIN:list,glue> drops all empty items from the
list.

$<LIST:APPEND,list,item,...>
Added in version 3.27.

The list with each item appended. Multiple items should be sepa-
rated by commas.

$<LIST:PREPEND,list,item,...>
Added in version 3.27.

The list with each item inserted at the beginning. If there are
multiple items, they should be separated by commas, and the order of
the prepended items will be preserved.

$<LIST:INSERT,list,index,item,...>
Added in version 3.27.

The list with the item (or multiple items) inserted at the specified
index. Multiple items should be separated by commas.

It is an error to specify an out-of-range index. Valid indexes are 0
to N, where N is the length of the list, inclusive. An empty list
has length 0.

$<LIST:POP_BACK,list>
Added in version 3.27.

The list with the last item removed.

$<LIST:POP_FRONT,list>
Added in version 3.27.

The list with the first item removed.

$<LIST:REMOVE_ITEM,list,value,...>
Added in version 3.27.

The list with all instances of the given value (or values) removed.
If multiple values are given, they should be separated by commas.

$<LIST:REMOVE_AT,list,index,...>
Added in version 3.27.

The list with the item at each given index removed.

$<LIST:REMOVE_DUPLICATES,list>
Added in version 3.27.

The list with all duplicated items removed. The relative order of
items is preserved, but if duplicates are encountered, only the
first instance is preserved. The result is the same as
$<REMOVE_DUPLICATES:list>.

$<LIST:FILTER,list,INCLUDE|EXCLUDE,regex>
Added in version 3.27.

A list of items from the list which match (INCLUDE) or do not match
(EXCLUDE) the regular expression regex. The result is the same as
$<FILTER:list,INCLUDE|EXCLUDE,regex>.

$<LIST:TRANSFORM,list,ACTION[,SELECTOR]>
Added in version 3.27.

The list transformed by applying an ACTION to all or, by specifying
a SELECTOR, to the selected list items.

NOTE:
The TRANSFORM sub-command does not change the number of items in
the list. If a SELECTOR is specified, only some items will be
changed, the other ones will remain the same as before the trans-
formation.

ACTION specifies the action to apply to the items of the list. The
actions have exactly the same semantics as for the list(TRANSFORM)
command. ACTION must be one of the following:

APPEND, PREPEND
Append, prepend specified value to each item of the list.

$<LIST:TRANSFORM,list,(APPEND|PREPEND),value[,SELECTOR]>

TOLOWER, TOUPPER
Convert each item of the list to lower, upper characters.

$<LIST:TRANSFORM,list,(TOLOWER|TOUPPER)[,SELECTOR]>

STRIP Remove leading and trailing spaces from each item of the
list.

$<LIST:TRANSFORM,list,STRIP[,SELECTOR]>

REPLACE:
Match the regular expression as many times as possible and
substitute the replacement expression for the match for
each item of the list.

$<LIST:TRANSFORM,list,REPLACE,regular_expression,replace_expression[,SELECTOR]>

SELECTOR determines which items of the list will be transformed.
Only one type of selector can be specified at a time. When given,
SELECTOR must be one of the following:

AT Specify a list of indexes.

$<LIST:TRANSFORM,list,ACTION,AT,index[,index...]>

FOR Specify a range with, optionally, an increment used to it-
erate over the range.

$<LIST:TRANSFORM,list,ACTION,FOR,start,stop[,step]>

REGEX Specify a regular expression. Only items matching the
regular expression will be transformed.

$<LIST:TRANSFORM,list,ACTION,REGEX,regular_expression>

$<JOIN:list,glue>
Joins the list with the content of the glue string inserted between
each item. This is conceptually the same operation as
$<LIST:JOIN,list,glue>, but the two have different behavior with re-
gard to empty items. $<LIST:JOIN,list,glue> preserves all empty
items, whereas $<JOIN,list,glue> drops all empty items from the
list.

$<REMOVE_DUPLICATES:list>
Added in version 3.15.

Removes duplicated items in the given list. The relative order of
items is preserved, and if duplicates are encountered, only the
first instance is retained. The result is the same as
$<LIST:REMOVE_DUPLICATES,list>.

$<FILTER:list,INCLUDE|EXCLUDE,regex>
Added in version 3.15.

Includes or removes items from list that match the regular expres-
sion regex. The result is the same as
$<LIST:FILTER,list,INCLUDE|EXCLUDE,regex>.

List Ordering
$<LIST:REVERSE,list>
Added in version 3.27.

The list with the items in reverse order.

$<LIST:SORT,list[,(COMPARE:option|CASE:option|ORDER:option)]...>
Added in version 3.27.

The list sorted according to the specified options.

Use one of the COMPARE options to select the comparison method for
sorting:

STRING
Sorts a list of strings alphabetically. This is the de-
fault behavior if the COMPARE option is not given.

FILE_BASENAME
Sorts a list of file paths by their basenames.

NATURAL
Sorts a list of strings using natural order (see the man
page for strverscmp(3)), such that contiguous digits are
compared as whole numbers. For example, the following
list 10.0 1.1 2.1 8.0 2.0 3.1 will be sorted as 1.1 2.0
2.1 3.1 8.0 10.0 if the NATURAL comparison is selected,
whereas it will be sorted as 1.1 10.0 2.0 2.1 3.1 8.0 with
the STRING comparison.

Use one of the CASE options to select a case-sensitive or case-in-
sensitive sort mode:

SENSITIVE
List items are sorted in a case-sensitive manner. This is
the default behavior if the CASE option is not given.

INSENSITIVE
List items are sorted in a case-insensitive manner. The
order of items which differ only by upper/lowercase is not
specified.

To control the sort order, one of the ORDER options can be given:

ASCENDING
Sorts the list in ascending order. This is the default
behavior when the ORDER option is not given.

DESCENDING
Sorts the list in descending order.

Options can be specified in any order, but it is an error to specify
the same option multiple times.

$<LIST:SORT,list,CASE:SENSITIVE,COMPARE:STRING,ORDER:DESCENDING>

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>
Added 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>
Added 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>
Added in version 3.24.

Returns 1 if the path is absolute, 0 otherwise.

$<PATH:IS_RELATIVE,path>
Added in version 3.24.

This will return the opposite of IS_ABSOLUTE.

$<PATH:IS_PREFIX[,NORMALIZE],path,input>
Added 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 ex-
pected to be in cmake-style format.

$<PATH:GET_*,...>
Added 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.

Changed in version 3.27: All operations now accept a list of paths
as argument. When a list of paths is specified, the operation will
be applied to each path.

$<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.

Changed in version 3.27: All operations now accept a list of paths as argu-
ment. When a list of paths is specified, the operation will be applied to
each path.

$<PATH:CMAKE_PATH[,NORMALIZE],path...>
Added 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,...>
Added in version 3.24.

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

See cmake_path(APPEND) for more details.

$<PATH:REMOVE_FILENAME,path...>
Added in version 3.24.

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

See cmake_path(REMOVE_FILENAME) for more details.

$<PATH:REPLACE_FILENAME,path...,input>
Added 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...>
Added 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>
Added 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...>
Added in version 3.24.

Returns path normalized according to the steps described in
Normalization.

$<PATH:RELATIVE_PATH,path...,base_directory>
Added in version 3.24.

Returns path, modified to make it relative to the base_directory ar-
gument.

See cmake_path(RELATIVE_PATH) for more details.

$<PATH:ABSOLUTE_PATH[,NORMALIZE],path...,base_directory>
Added in version 3.24.

Returns path as absolute. If path is a relative path ($<PATH:IS_REL-
ATIVE> returns 1), it is evaluated relative to the given base direc-
tory 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:...>
Added 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.

Added in version 3.14: The ... may be a semicolon-separated list of
paths, in which case each path is converted individually and a re-
sult 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 configura-
tion.

$<OUTPUT_CONFIG:...>
Added 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:...>
Added 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 re-
lated 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 C++ compiler matches version, otherwise 0.

$<CUDA_COMPILER_VERSION>
Added in version 3.15.

The version of the CUDA compiler used.

$<CUDA_COMPILER_VERSION:version>
Added in version 3.15.

1 if the version of the C++ compiler matches version, otherwise 0.

$<OBJC_COMPILER_VERSION>
Added in version 3.16.

The version of the Objective-C compiler used.

$<OBJC_COMPILER_VERSION:version>
Added in version 3.16.

1 if the version of the Objective-C compiler matches version, other-
wise 0.

$<OBJCXX_COMPILER_VERSION>
Added in version 3.16.

The version of the Objective-C++ compiler used.

$<OBJCXX_COMPILER_VERSION:version>
Added in version 3.16.

1 if the version of the Objective-C++ compiler matches version, oth-
erwise 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>
Added in version 3.21.

The version of the HIP compiler used.

$<HIP_COMPILER_VERSION:version>
Added in version 3.21.

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

$<ISPC_COMPILER_VERSION>
Added in version 3.19.

The version of the ISPC compiler used.

$<ISPC_COMPILER_VERSION:version>
Added in version 3.19.

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

Compiler Language, ID, and Frontend-Variant
See also the CMAKE_<LANG>_COMPILER_ID and
CMAKE_<LANG>_COMPILER_FRONTEND_VARIANT variables, which are 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.

Changed in version 3.15: Multiple compiler_ids can be specified.
CMake 3.14 and earlier only accepted a single compiler ID.

$<CXX_COMPILER_ID>
CMake's compiler id of the C++ compiler used.

$<CXX_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 com-
piler_ids, otherwise 0.

Changed in version 3.15: Multiple compiler_ids can be specified.
CMake 3.14 and earlier only accepted a single compiler ID.

$<CUDA_COMPILER_ID>
Added in version 3.15.

CMake's compiler id of the CUDA compiler used.

$<CUDA_COMPILER_ID:compiler_ids>
Added 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 com-
piler_ids, otherwise 0.

$<OBJC_COMPILER_ID>
Added in version 3.16.

CMake's compiler id of the Objective-C compiler used.

$<OBJC_COMPILER_ID:compiler_ids>
Added 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>
Added in version 3.16.

CMake's compiler id of the Objective-C++ compiler used.

$<OBJCXX_COMPILER_ID:compiler_ids>
Added 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 com-
piler_ids, otherwise 0.

Changed in version 3.15: Multiple compiler_ids can be specified.
CMake 3.14 and earlier only accepted a single compiler ID.

$<HIP_COMPILER_ID>
Added in version 3.21.

CMake's compiler id of the HIP compiler used.

$<HIP_COMPILER_ID:compiler_ids>
Added 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 com-
piler_ids, otherwise 0.

$<ISPC_COMPILER_ID>
Added in version 3.19.

CMake's compiler id of the ISPC compiler used.

$<ISPC_COMPILER_ID:compiler_ids>
Added 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 com-
piler_ids, otherwise 0.

$<C_COMPILER_FRONTEND_VARIANT>
Added in version 3.30.

CMake's compiler frontend variant of the C compiler used.

$<C_COMPILER_FRONTEND_VARIANT:compiler_ids>
Added in version 3.30.

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

$<CXX_COMPILER_FRONTEND_VARIANT>
Added in version 3.30.

CMake's compiler frontend variant of the C++ compiler used.

$<CXX_COMPILER_FRONTEND_VARIANT:compiler_ids>
Added in version 3.30.

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

$<CUDA_COMPILER_FRONTEND_VARIANT>
Added in version 3.30.

CMake's compiler id of the CUDA compiler used.

$<CUDA_COMPILER_FRONTEND_VARIANT:compiler_ids>
Added in version 3.30.

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

$<OBJC_COMPILER_FRONTEND_VARIANT>
Added in version 3.30.

CMake's compiler frontend variant of the Objective-C compiler used.

$<OBJC_COMPILER_FRONTEND_VARIANT:compiler_ids>
Added in version 3.30.

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

$<OBJCXX_COMPILER_FRONTEND_VARIANT>
Added in version 3.30.

CMake's compiler frontend variant of the Objective-C++ compiler
used.

$<OBJCXX_COMPILER_FRONTEND_VARIANT:compiler_ids>
Added in version 3.30.

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

$<Fortran_COMPILER_FRONTEND_VARIANT>
Added in version 3.30.

CMake's compiler id of the Fortran compiler used.

$<Fortran_COMPILER_FRONTEND_VARIANT:compiler_ids>
Added in version 3.30.

where compiler_ids is a comma-separated list. 1 if CMake's compiler
frontend variant of the Fortran compiler matches any one of the en-
tries in compiler_ids, otherwise 0.

$<HIP_COMPILER_FRONTEND_VARIANT>
Added in version 3.30.

CMake's compiler id of the HIP compiler used.

$<HIP_COMPILER_FRONTEND_VARIANT:compiler_ids>
Added in version 3.30.

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

$<ISPC_COMPILER_FRONTEND_VARIANT>
Added in version 3.30.

CMake's compiler id of the ISPC compiler used.

$<ISPC_COMPILER_FRONTEND_VARIANT:compiler_ids>
Added in version 3.30.

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

$<COMPILE_LANGUAGE>
Added in version 3.3.

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

$<COMPILE_LANGUAGE:languages>
Added 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 in-
clude 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, COM-
PILING_CXX compile definition, and cxx_headers include directory for
C++ only (compiler id checks elided). It also specifies a COMPIL-
ING_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 Gener-
ators 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>
Added 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 expres-
sion is a short form for the combination of $<COMPILE_LANGUAGE:lan-
guage> 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>
Added 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 re-
quired 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.

Compile Context
$<COMPILE_ONLY:...>
Added in version 3.27.

Content of ..., when collecting transitive compile properties, oth-
erwise it is the empty string. This is intended for use in an
INTERFACE_LINK_LIBRARIES and LINK_LIBRARIES target properties, typi-
cally populated via the target_link_libraries() command. Provides
compilation usage requirements without any linking requirements.

Use cases include header-only usage where all usages are known to
not have linking requirements (e.g., all-inline or C++ template li-
braries).

Note that for proper evaluation of this expression requires policy
CMP0099 to be set to NEW.

Linker Language And ID
$<LINK_LANGUAGE>
Added 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>
Added in version 3.18.

1 when the language used for link step matches any of the comma-sep-
arated 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 lan-
guage.

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:..> expres-
sions will always return 0. The link language computed after
this first pass will be used to do the second pass. To avoid in-
consistency, 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>
Added 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 expres-
sion is a short form for the combination of $<LINK_LANGUAGE:lan-
guage> 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>
Added 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 exam-
ple:

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, num-
bers 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 under-
stands 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 frame-
work targets, even without using the
$<LINK_LIBRARY:FRAMEWORK,...> expression. The generator ex-
pression 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 be-
tween 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 ex-
pected behavior is clear.

Added in version 3.25: The
FRAMEWORK_MULTI_CONFIG_POSTFIX_<CONFIG> target property as
well as the suffix of the framework library name are now sup-
ported 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 -reex-
port_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 plat-
forms 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 -reex-
port-l option as appropriate, and has the same linker con-
straints 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 plat-
forms 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 speci-
fied 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>
Added in version 3.24.

Specify a group of libraries to link to a target, along with a fea-
ture 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, circu-
lar references between libraries typically result in unre-
solved symbols. This feature instructs the linker to search
the specified static libraries repeatedly until no new unde-
fined 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 re-
ferred 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 pos-
sible references are resolved. This will use linker options
like --start-group and --end-group, or on SunOS, -z res-
can-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 ref-
erences 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 fol-
lowing 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 speci-
fied 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:...>
Added in version 3.1.

Content of ..., except while collecting usage requirements from
transitive compile properties, in which case it is the empty string.
This is intended for use in an INTERFACE_LINK_LIBRARIES target prop-
erty, typically populated via the target_link_libraries() command,
to specify private link dependencies without other usage require-
ments such as include directories or compile options.

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

$<DEVICE_LINK:list>
Added in version 3.18.

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

$<HOST_LINK:list>
Added in version 3.18.

Returns the list if it is the normal link step, an empty list other-
wise. 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
Target Meta-Data
These expressions look up information about a target.

$<TARGET_EXISTS:tgt>
Added in version 3.12.

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

$<TARGET_NAME_IF_EXISTS:tgt>
Added 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 ex-
pression is evaluated on.

$<TARGET_NAME:tgt>
The target name tgt as written. This marks tgt as being the name of
a target inside a larger expression, which is required if exporting
targets to multiple dependent export sets. The tgt text must be a
literal name of a target; it may not contain generator expressions.
The target does not have to exist.

$<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 Properties
These expressions look up the values of target properties.

$<TARGET_PROPERTY:tgt,prop>
Value of the property prop on the target tgt, or empty if the prop-
erty is not set.

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

Changed in version 3.26: When encountered during evaluation of
Target Usage Requirements, typically in an INTERFACE_* target prop-
erty, lookup of the tgt name occurs in the directory of the target
specifying the requirement, rather than the directory of the consum-
ing target for which the expression is being evaluated.

$<TARGET_PROPERTY:prop>
Value of the property prop on the target for which the expression is
being evaluated, or empty if the property is not set. Note that for
generator expressions in Target Usage Requirements this is the con-
suming target rather than the target specifying the requirement.

The expressions have special evaluation rules for some properties:

Target Build Specification Properties
These evaluate as a semicolon-separated list representing the union
of the value on the target itself with the values of the correspond-
ing Target Usage Requirements on targets named by the target's
LINK_LIBRARIES:

* For Target Compile Properties, evaluation of corresponding usage
requirements is transitive over the closure of the linked targets'
INTERFACE_LINK_LIBRARIES excluding entries guarded by the
LINK_ONLY generator expression.

* For Target Link Properties, evaluation of corresponding usage re-
quirements is transitive over the closure of the linked targets'
INTERFACE_LINK_LIBRARIES including entries guarded by the
LINK_ONLY generator expression. See policy CMP0166.

Evaluation of LINK_LIBRARIES itself is not transitive.

Target Usage Requirement Properties
These evaluate as a semicolon-separated list representing the union
of the value on the target itself with the values of the same prop-
erties on targets named by the target's INTERFACE_LINK_LIBRARIES:

* For Transitive Compile Properties, evaluation is transitive over
the closure of the target's INTERFACE_LINK_LIBRARIES excluding en-
tries guarded by the LINK_ONLY generator expression.

* For Transitive Link Properties, evaluation is transitive over the
closure of the target's INTERFACE_LINK_LIBRARIES including entries
guarded by the LINK_ONLY generator expression. See policy
CMP0166.

Evaluation of INTERFACE_LINK_LIBRARIES itself is not transitive.

Custom Transitive Properties
Added in version 3.30.

These are processed during evaluation as follows:

* Evaluation of $<TARGET_PROPERTY:tgt,PROP> for some property PROP,
named without an INTERFACE_ prefix, checks the
TRANSITIVE_COMPILE_PROPERTIES and TRANSITIVE_LINK_PROPERTIES prop-
erties on target tgt, on targets named by its LINK_LIBRARIES, and
on the transitive closure of targets named by the linked targets'
INTERFACE_LINK_LIBRARIES.

If PROP is listed by one of those properties, then it evaluates as
a semicolon-separated list representing the union of the value on
the target itself with the values of the corresponding INTER-
FACE_PROP on targets named by the target's LINK_LIBRARIES:

* If PROP is named by TRANSITIVE_COMPILE_PROPERTIES, evaluation of
the corresponding INTERFACE_PROP is transitive over the closure
of the linked targets' INTERFACE_LINK_LIBRARIES, excluding en-
tries guarded by the LINK_ONLY generator expression.

* If PROP is named by TRANSITIVE_LINK_PROPERTIES, evaluation of
the corresponding INTERFACE_PROP is transitive over the closure
of the linked targets' INTERFACE_LINK_LIBRARIES, including en-
tries guarded by the LINK_ONLY generator expression.

* Evaluation of $<TARGET_PROPERTY:tgt,INTERFACE_PROP> for some prop-
erty INTERFACE_PROP, named with an INTERFACE_ prefix, checks the
TRANSITIVE_COMPILE_PROPERTIES and TRANSITIVE_LINK_PROPERTIES prop-
erties on target tgt, and on the transitive closure of targets
named by its INTERFACE_LINK_LIBRARIES.

If the corresponding PROP is listed by one of those properties,
then INTERFACE_PROP evaluates as a semicolon-separated list repre-
senting the union of the value on the target itself with the value
of the same property on targets named by the target's
INTERFACE_LINK_LIBRARIES:

* If PROP is named by TRANSITIVE_COMPILE_PROPERTIES, evaluation of
the corresponding INTERFACE_PROP is transitive over the closure
of the target's INTERFACE_LINK_LIBRARIES, excluding entries
guarded by the LINK_ONLY generator expression.

* If PROP is named by TRANSITIVE_LINK_PROPERTIES, evaluation of
the corresponding INTERFACE_PROP is transitive over the closure
of the target's INTERFACE_LINK_LIBRARIES, including entries
guarded by the LINK_ONLY generator expression.

If a PROP is named by both TRANSITIVE_COMPILE_PROPERTIES and
TRANSITIVE_LINK_PROPERTIES, the latter takes precedence.

Compatible Interface Properties
These evaluate as a single value combined from the target itself,
from targets named by the target's LINK_LIBRARIES, and from the
transitive closure of the linked targets' INTERFACE_LINK_LIBRARIES.
Values of a compatible interface property from multiple targets com-
bine based on the type of compatibility required by the COMPATI-
BLE_INTERFACE_* property defining it.

Target Artifacts
These expressions look up information about artifacts associated with a
given target tgt. Unless otherwise stated, this can be any runtime arti-
fact, 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_FILE:tgt>
Full path to the tgt binary file.

Note that tgt is not added as a dependency of the target this ex-
pression is evaluated on, unless the expression is being used in
add_custom_command() or add_custom_target().

$<TARGET_FILE_BASE_NAME:tgt>
Added 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 ex-
pression is evaluated on.

$<TARGET_FILE_PREFIX:tgt>
Added in version 3.15.

Prefix of the tgt filename (such as lib).

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