Provided by: cmake-data_3.16.3-1ubuntu1.20.04.1_all 
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:
• Directories (CMakeLists.txt),
• Scripts (<script>.cmake), and
• 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 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) command-line tool with the -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 to load
a <module>.cmake source file in the scope of the including context. See the
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.
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 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 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 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
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 variable,
or by the empty string if the variable is not set. 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 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>}. See CACHE for more
information.
The if() command has a special condition syntax that allows for variable references in the
short form <variable> instead of ${<variable>}. However, environment and cache variables
always need to be referenced as $ENV{<variable>} or $CACHE{<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()/elseif()/else()/endif() commands delimit code blocks to be executed
conditionally.
Loops
The foreach()/endforeach() and while()/endwhile() commands delimit code blocks to be
executed in a loop. Inside such blocks the break() command may be used to terminate the
loop early whereas the continue() command may be used to start with the next iteration
immediately.
Command Definitions
The macro()/endmacro(), and function()/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() and 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:
Function Scope
Command Definitions created by the 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, 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() and 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 documents the many variables that are provided by CMake or
have meaning to CMake when set by project code.
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>}.
Initialization
Initial values of the CMake environment variables are those of the calling process.
Values can be changed using the set() and 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.
The 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 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"
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