cmake-language(7) CMake cmake-language(7)
NAME
cmake-language - CMake Language Reference
ORGANIZATION
CMake input files are written in the "CMake Language" in source files
named CMakeLists.txt or ending in a .cmake file name extension.
CMake Language source files in a project are organized into:
o Directories (CMakeLists.txt),
o Scripts (<script>.cmake), and
o Modules (<module>.cmake).
Directories
When CMake processes a project source tree, the entry point is a source
file called CMakeLists.txt in the top-level source directory. This
file may contain the entire build specification or use the
add_subdirectory() <#command:add_subdirectory> command to add
subdirectories to the build. Each subdirectory added by the command
must also contain a CMakeLists.txt file as the entry point to that
directory. For each source directory whose CMakeLists.txt file is
processed CMake generates a corresponding directory in the build tree
to act as the default working and output directory.
Scripts
An individual <script>.cmake source file may be processed in script
mode by using the cmake(1) <#manual:cmake(1)> command-line tool with
the -P <#cmdoption-cmake-P> option. Script mode simply runs the
commands in the given CMake Language source file and does not generate
a build system. It does not allow CMake commands that define build
targets or actions.
Modules
CMake Language code in either Directories or Scripts may use the
include() <#command:include> command to load a <module>.cmake source
file in the scope of the including context. See the cmake-modules(7)
<#manual:cmake-modules(7)> manual page for documentation of modules
included with the CMake distribution. Project source trees may also
provide their own modules and specify their location(s) in the
CMAKE_MODULE_PATH <#variable:CMAKE_MODULE_PATH> variable.
SYNTAX
Encoding
A CMake Language source file may be written in 7-bit ASCII text for
maximum portability across all supported platforms. Newlines may be
encoded as either \n or \r\n but will be converted to \n as input files
are read.
Note that the implementation is 8-bit clean so source files may be
encoded as UTF-8 on platforms with system APIs supporting this
encoding. In addition, CMake 3.2 and above support source files
encoded in UTF-8 on Windows (using UTF-16 to call system APIs).
Furthermore, CMake 3.0 and above allow a leading UTF-8 Byte-Order Mark
<https://en.wikipedia.org/wiki/Byte_order_mark> in source files.
Source Files
A CMake Language source file consists of zero or more Command
Invocations separated by newlines and optionally spaces and Comments:
file ::= file_element*
file_element ::= command_invocation line_ending |
(bracket_comment|space)* line_ending
line_ending ::= line_comment? newline
space ::= <match '[ \t]+'>
newline ::= <match '\n'>
Note that any source file line not inside Command Arguments or a
Bracket Comment can end in a Line Comment.
Command Invocations
A command invocation is a name followed by paren-enclosed arguments
separated by whitespace:
command_invocation ::= space* identifier space* '(' arguments ')'
identifier ::= <match '[A-Za-z_][A-Za-z0-9_]*'>
arguments ::= argument? separated_arguments*
separated_arguments ::= separation+ argument? |
separation* '(' arguments ')'
separation ::= space | line_ending
For example:
add_executable(hello world.c)
Command names are case-insensitive. Nested unquoted parentheses in the
arguments must balance. Each ( or ) is given to the command invocation
as a literal Unquoted Argument. This may be used in calls to the if()
<#command:if> command to enclose conditions. For example:
if(FALSE AND (FALSE OR TRUE)) # evaluates to FALSE
Note:
CMake versions prior to 3.0 require command name identifiers to be
at least 2 characters.
CMake versions prior to 2.8.12 silently accept an Unquoted Argument
or a Quoted Argument immediately following a Quoted Argument and not
separated by any whitespace. For compatibility, CMake 2.8.12 and
higher accept such code but produce a warning.
Command Arguments
There are three types of arguments within Command Invocations:
argument ::= bracket_argument | quoted_argument | unquoted_argument
Bracket Argument
A bracket argument, inspired by Lua <https://www.lua.org/> long bracket
syntax, encloses content between opening and closing "brackets" of the
same length:
bracket_argument ::= bracket_open bracket_content bracket_close
bracket_open ::= '[' '='* '['
bracket_content ::= <any text not containing a bracket_close with
the same number of '=' as the bracket_open>
bracket_close ::= ']' '='* ']'
An opening bracket is written [ followed by zero or more = followed by
[. The corresponding closing bracket is written ] followed by the same
number of = followed by ]. Brackets do not nest. A unique length may
always be chosen for the opening and closing brackets to contain
closing brackets of other lengths.
Bracket argument content consists of all text between the opening and
closing brackets, except that one newline immediately following the
opening bracket, if any, is ignored. No evaluation of the enclosed
content, such as Escape Sequences or Variable References, is performed.
A bracket argument is always given to the command invocation as exactly
one argument.
For example:
message([=[
This is the first line in a bracket argument with bracket length 1.
No \-escape sequences or ${variable} references are evaluated.
This is always one argument even though it contains a ; character.
The text does not end on a closing bracket of length 0 like ]].
It does end in a closing bracket of length 1.
]=])
Note:
CMake versions prior to 3.0 do not support bracket arguments. They
interpret the opening bracket as the start of an Unquoted Argument.
Quoted Argument
A quoted argument encloses content between opening and closing
double-quote characters:
quoted_argument ::= '"' quoted_element* '"'
quoted_element ::= <any character except '\' or '"'> |
escape_sequence |
quoted_continuation
quoted_continuation ::= '\' newline
Quoted argument content consists of all text between opening and
closing quotes. Both Escape Sequences and Variable References are
evaluated. A quoted argument is always given to the command invocation
as exactly one argument.
For example:
message("This is a quoted argument containing multiple lines.
This is always one argument even though it contains a ; character.
Both \\-escape sequences and ${variable} references are evaluated.
The text does not end on an escaped double-quote like \".
It does end in an unescaped double quote.
")
The final \ on any line ending in an odd number of backslashes is
treated as a line continuation and ignored along with the immediately
following newline character. For example:
message("\
This is the first line of a quoted argument. \
In fact it is the only line but since it is long \
the source code uses line continuation.\
")
Note:
CMake versions prior to 3.0 do not support continuation with \.
They report errors in quoted arguments containing lines ending in an
odd number of \ characters.
Unquoted Argument
An unquoted argument is not enclosed by any quoting syntax. It may not
contain any whitespace, (, ), #, ", or \ except when escaped by a
backslash:
unquoted_argument ::= unquoted_element+ | unquoted_legacy
unquoted_element ::= <any character except whitespace or one of '()#"\'> |
escape_sequence
unquoted_legacy ::= <see note in text>
Unquoted argument content consists of all text in a contiguous block of
allowed or escaped characters. Both Escape Sequences and Variable
References are evaluated. The resulting value is divided in the same
way Lists divide into elements. Each non-empty element is given to the
command invocation as an argument. Therefore an unquoted argument may
be given to a command invocation as zero or more arguments.
For example:
foreach(arg
NoSpace
Escaped\ Space
This;Divides;Into;Five;Arguments
Escaped\;Semicolon
)
message("${arg}")
endforeach()
Note:
To support legacy CMake code, unquoted arguments may also contain
double-quoted strings ("...", possibly enclosing horizontal
whitespace), and make-style variable references ($(MAKEVAR)).
Unescaped double-quotes must balance, may not appear at the
beginning of an unquoted argument, and are treated as part of the
content. For example, the unquoted arguments -Da="b c", -Da=$(v),
and a" "b"c"d are each interpreted literally. They may instead be
written as quoted arguments "-Da=\"b c\"", "-Da=$(v)", and "a\"
\"b\"c\"d", respectively.
Make-style references are treated literally as part of the content
and do not undergo variable expansion. They are treated as part of
a single argument (rather than as separate $, (, MAKEVAR, and )
arguments).
The above "unquoted_legacy" production represents such arguments.
We do not recommend using legacy unquoted arguments in new code.
Instead use a Quoted Argument or a Bracket Argument to represent the
content.
Escape Sequences
An escape sequence is a \ followed by one character:
escape_sequence ::= escape_identity | escape_encoded | escape_semicolon
escape_identity ::= '\' <match '[^A-Za-z0-9;]'>
escape_encoded ::= '\t' | '\r' | '\n'
escape_semicolon ::= '\;'
A \ followed by a non-alphanumeric character simply encodes the literal
character without interpreting it as syntax. A \t, \r, or \n encodes a
tab, carriage return, or newline character, respectively. A \; outside
of any Variable References encodes itself but may be used in an
Unquoted Argument to encode the ; without dividing the argument value
on it. A \; inside Variable References encodes the literal ;
character. (See also policy CMP0053 <#policy:CMP0053> documentation
for historical considerations.)
Variable References
A variable reference has the form ${<variable>} and is evaluated inside
a Quoted Argument or an Unquoted Argument. A variable reference is
replaced by the value of the specified variable or cache entry, or if
neither is set, by the empty string. Variable references can nest and
are evaluated from the inside out, e.g.
${outer_${inner_variable}_variable}.
Literal variable references may consist of alphanumeric characters, the
characters /_.+-, and Escape Sequences. Nested references may be used
to evaluate variables of any name. See also policy CMP0053 <#
policy:CMP0053> documentation for historical considerations and reasons
why the $ is also technically permitted but is discouraged.
The Variables section documents the scope of variable names and how
their values are set.
An environment variable reference has the form $ENV{<variable>}. See
the Environment Variables section for more information.
A cache variable reference has the form $CACHE{<variable>}, and is
replaced by the value of the specified cache entry without checking for
a normal variable of the same name. If the cache entry does not exist,
it is replaced by the empty string. See CACHE <#variable:CACHE> for
more information.
The if() <#command:if> command has a special condition syntax that
allows for variable references in the short form <variable> instead of
${<variable>}. However, environment variables always need to be
referenced as $ENV{<variable>}.
Comments
A comment starts with a # character that is not inside a Bracket
Argument, Quoted Argument, or escaped with \ as part of an Unquoted
Argument. There are two types of comments: a Bracket Comment and a
Line Comment.
Bracket Comment
A # immediately followed by a bracket_open forms a bracket comment
consisting of the entire bracket enclosure:
bracket_comment ::= '#' bracket_argument
For example:
#[[This is a bracket comment.
It runs until the close bracket.]]
message("First Argument\n" #[[Bracket Comment]] "Second Argument")
Note:
CMake versions prior to 3.0 do not support bracket comments. They
interpret the opening # as the start of a Line Comment.
Line Comment
A # not immediately followed by a bracket_open forms a line comment
that runs until the end of the line:
line_comment ::= '#' <any text not starting in a bracket_open
and not containing a newline>
For example:
# This is a line comment.
message("First Argument\n" # This is a line comment :)
"Second Argument") # This is a line comment.
CONTROL STRUCTURES
Conditional Blocks
The if() <#command:if>/elseif() <#command:elseif>/else() <#
command:else>/endif() <#command:endif> commands delimit code blocks to
be executed conditionally.
Loops
The foreach() <#command:foreach>/endforeach() <#command:endforeach> and
while() <#command:while>/endwhile() <#command:endwhile> commands
delimit code blocks to be executed in a loop. Inside such blocks the
break() <#command:break> command may be used to terminate the loop
early whereas the continue() <#command:continue> command may be used to
start with the next iteration immediately.
Command Definitions
The macro() <#command:macro>/endmacro() <#command:endmacro>, and
function() <#command:function>/endfunction() <#command:endfunction>
commands delimit code blocks to be recorded for later invocation as
commands.
VARIABLES
Variables are the basic unit of storage in the CMake Language. Their
values are always of string type, though some commands may interpret
the strings as values of other types. The set() <#command:set> and
unset() <#command:unset> commands explicitly set or unset a variable,
but other commands have semantics that modify variables as well.
Variable names are case-sensitive and may consist of almost any text,
but we recommend sticking to names consisting only of alphanumeric
characters plus _ and -.
Variables have dynamic scope. Each variable "set" or "unset" creates a
binding in the current scope:
Block Scope
The block() <#command:block> command may create a new scope for
variable bindings.
Function Scope
Command Definitions created by the function() <#
command:function> command create commands that, when invoked,
process the recorded commands in a new variable binding scope.
A variable "set" or "unset" binds in this scope and is visible
for the current function and any nested calls within it, but not
after the function returns.
Directory Scope
Each of the Directories in a source tree has its own variable
bindings. Before processing the CMakeLists.txt file for a
directory, CMake copies all variable bindings currently defined
in the parent directory, if any, to initialize the new directory
scope. CMake Scripts, when processed with cmake -P <#cmdoption-
cmake-P>, bind variables in one "directory" scope.
A variable "set" or "unset" not inside a function call binds to
the current directory scope.
Persistent Cache
CMake stores a separate set of "cache" variables, or "cache
entries", whose values persist across multiple runs within a
project build tree. Cache entries have an isolated binding
scope modified only by explicit request, such as by the CACHE
option of the set() <#command:set> and unset() <#command:unset>
commands.
When evaluating Variable References, CMake first searches the function
call stack, if any, for a binding and then falls back to the binding in
the current directory scope, if any. If a "set" binding is found, its
value is used. If an "unset" binding is found, or no binding is found,
CMake then searches for a cache entry. If a cache entry is found, its
value is used. Otherwise, the variable reference evaluates to an empty
string. The $CACHE{VAR} syntax can be used to do direct cache entry
lookups.
The cmake-variables(7) <#manual:cmake-variables(7)> manual documents
the many variables that are provided by CMake or have meaning to CMake
when set by project code.
Note:
CMake reserves identifiers that:
o begin with CMAKE_ (upper-, lower-, or mixed-case), or
o begin with _CMAKE_ (upper-, lower-, or mixed-case), or
o begin with _ followed by the name of any CMake Command <#
manual:cmake-commands(7)>.
ENVIRONMENT VARIABLES
Environment Variables are like ordinary Variables, with the following
differences:
Scope Environment variables have global scope in a CMake process.
They are never cached.
References
Variable References have the form $ENV{<variable>}, using the
ENV <#variable:ENV> operator.
Initialization
Initial values of the CMake environment variables are those of
the calling process. Values can be changed using the set() <#
command:set> and unset() <#command:unset> commands. These
commands only affect the running CMake process, not the system
environment at large. Changed values are not written back to
the calling process, and they are not seen by subsequent build
or test processes.
See the cmake -E env <#cmdoption-cmake-E-arg-env> command-line
tool to run a command in a modified environment.
Inspection
See the cmake -E environment <#cmdoption-cmake-E-arg-
environment> command-line tool to display all current
environment variables.
The cmake-env-variables(7) <#manual:cmake-env-variables(7)> manual
documents environment variables that have special meaning to CMake.
LISTS
Although all values in CMake are stored as strings, a string may be
treated as a list in certain contexts, such as during evaluation of an
Unquoted Argument. In such contexts, a string is divided into list
elements by splitting on ; characters not following an unequal number
of [ and ] characters and not immediately preceded by a \. The
sequence \; does not divide a value but is replaced by ; in the
resulting element.
A list of elements is represented as a string by concatenating the
elements separated by ;. For example, the set() <#command:set> command
stores multiple values into the destination variable as a list:
set(srcs a.c b.c c.c) # sets "srcs" to "a.c;b.c;c.c"
Lists are meant for simple use cases such as a list of source files and
should not be used for complex data processing tasks. Most commands
that construct lists do not escape ; characters in list elements, thus
flattening nested lists:
set(x a "b;c") # sets "x" to "a;b;c", not "a;b\;c"
In general, lists do not support elements containing ; characters. To
avoid problems, consider the following advice:
o The interfaces of many CMake commands, variables, and properties
accept semicolon-separated lists. Avoid passing lists with elements
containing semicolons to these interfaces unless they document either
direct support or some way to escape or encode semicolons.
o When constructing a list, substitute an otherwise-unused placeholder
for ; in elements when. Then substitute ; for the placeholder when
processing list elements. For example, the following code uses | in
place of ; characters:
set(mylist a "b|c")
foreach(entry IN LISTS mylist)
string(REPLACE "|" ";" entry "${entry}")
# use "${entry}" normally
endforeach()
The ExternalProject <#module:ExternalProject> module's LIST_SEPARATOR
option is an example of an interface built using this approach.
o In lists of generator expressions <#manual:cmake-generator-
expressions(7)>, use the $<SEMICOLON> <#genex:SEMICOLON> generator
expression.
o In command calls, use Quoted Argument syntax whenever possible. The
called command will receive the content of the argument with
semicolons preserved. An Unquoted Argument will be split on
semicolons.
o In function() <#command:function> implementations, avoid ARGV and
ARGN, which do not distinguish semicolons in values from those
separating values. Instead, prefer using named positional arguments
and the ARGC and ARGV# variables. When using cmake_parse_arguments()
<#command:cmake_parse_arguments> to parse arguments, prefer its
PARSE_ARGV signature, which uses the ARGV# variables.
Note that this approach does not apply to macro() <#command:macro>
implementations because they reference arguments using placeholders,
not real variables.
Copyright
2000-2024 Kitware, Inc. and Contributors
3.31.10 January 26, 2026 cmake-language(7)
cmake 3.31.10 - Generated Tue Jan 27 19:15:39 CST 2026