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---
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category: development
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date: 2021-05-24T23:56:00
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description: CMake stands for Cross-platform Make. Normally, a build tool like Make
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will parse a configuration file (Makefile) that contains all the commands required
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to build an artifact based on the source files and other resources inside the project.
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image: /images/cmake-office.webp
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slug: cmake-basics
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tags:
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- cmake
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- gsoc
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- fossology
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- gsoc21
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title: Hello CMake
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---
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CMake stands for Cross-platform Make. Normally, a build tool like Make
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will parse a configuration file (Makefile) that contains all the
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commands required to build an artifact based on the source files and
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other resources inside the project.
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I proposed a new architecture for FOSSology that uses CMake instead of
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bare-metal Make as a Google Summer of Code 2021 project. Although these
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tutorials will be useful for anyone interested in learning CMake they
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are specifically tailored to the FOSSology project. This is the first
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blog on CMake in this series. In this blog, I will discuss CMake syntax
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and various features.
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## What is CMake?
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CMake stands for Cross-platform Make. Normally, a build tool like Make
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will parse a configuration file (Makefile) that contains all the
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commands required to build an artifact based on the source files and
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other resources inside the project. On the other hand, CMake will also
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parse a configuration file (CMakeLists.txt), but instead of directly
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build the artifact, it’ll generate another configuration file that will
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build the artifact.
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This approach is very common in Computer Science and is called *adding
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another level of indirection*. In short, you may say that:
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With just one configuration file you’ll be able to generate different configuration files to build your project for different platforms (Make, Visual Studio, etc).
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Another nice CMake feature is the so-called out-of-source build. Any
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file required for the final build, executables included, will be stored
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in a separated build directory (usually called build/). This prevents
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cluttering up the source directory and makes it easy to start over
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again: just remove the build directory and you are done.
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## CMake Syntax
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CMake unlike Make is a configuration language itself. CMake supplies a
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rich library of inherent functions as well as common programming
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language features like conditions, looping, macros, and functions. In
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addition to that CMake is highly modular and you can always write a
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CMake module yourself independent of any project. Specifically for C/C++
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programming, it supplies commands to find and link libraries
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automatically and lot many features.
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### Language Rules
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As mentioned above CMake is a language itself hence there are some
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language rules related to syntax, comments, variables, etc.
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- There are two types of comment in CMake, both start with `#`
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character. The first one is line comments, as clear by name it is
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delimited by a newline. The other one is bracket comment and can
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span until the matching brackets are found.
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```cmake
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# This is a line comment and it ends with the line.
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#[[This is a bracket comment and it can span up to multiple lines.
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But it is only supported in CMake 3.0 or later.]]
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```
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- Variables in CMake are like any other programming language. They are
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case-sensitive and have any alphanumeric characters. In general, it
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is recommended using upper case names as variables. They can be
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assigned and unassigned using `set` and `unset` commands. A variable
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can be referenced using `${VARIABLE_NAME}`.
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> CMake reserves some types of identifers:
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>
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> - begin with **CMAKE_**(upper-, lower-, or mixed-case)
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> - begin with ***CMAKE***(upper-, lower-, or mixed-case)
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> - begin with **_** followed by the name of any CMake Command
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- The CMake commands are case insensitive in the latest version (3.0)
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of CMake. That means `message()`, `Message()` or `MESSAGE()` are all
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same.
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### Basic CMake Commands
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- The `project()` command is used to set the name of the CMake project
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and optionally a language that is used by the project. Every
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top-level CMake configuration must have this option set. The syntax
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is as below:
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```cmake
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project (projectname [CXX] [C] [Java] [NONE])
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```
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If no language is specified then CMake defaults to supporting C/C++.
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If `NONE` is specified then no language support is enabled.
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- The `set()` command is used to set a variable to a value or lists of
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values. It is one of the most used CMake commands. The accompanying
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command is `unset()`. The `unset()` command is used to unset a
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variable or remove a variable from the current scope. The syntax for
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the three commands are:
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```cmake
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set (BLOG_TITLE "CMake Introduction") # assign single value
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set (BLOG_TAGS "gsoc" "cmake" "fossology" "gsoc21") # assign a list of values
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unset (BLOG_TITLE) # unset BLOG_TITLE
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```
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- The `message()` command can be used to display formatted messages
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with different alert modes. There are many
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[modes](https://cmake.org/cmake/help/v3.20/command/message.html#general-messages)
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of displaying messages. The syntax is :
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```cmake
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message ([<mode>] "message text" ...)
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message(NOTICE "Hey this is ${BLOG_TITLE}") # Example message with variable
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```
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- The `cmake_minimum_required()` is used to set the minimum CMake
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version to use to generate the build files. If any older version is
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used than specified then the user gets an error message. It must be
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specified at the top of the *CMakeLists.txt* file.
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```cmake
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cmake_minimum_required (VERSION 3.0)
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```
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- The commands `add_executable()` and `add_library()` specifies what
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executables and libraries to build and what source files must be
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used to build them. One of the two commands must be used for any
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binary generation.
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```cmake
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add_executable(<name> [WIN32] [MACOSX_BUNDLE]
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[EXCLUDE_FROM_ALL]
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[source1] [source2 ...])
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add_library(<name> [STATIC | SHARED | MODULE]
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[EXCLUDE_FROM_ALL]
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[<source>...])
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```
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### Flow Control
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CMake provides three flow control structures. They are conditional
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statements (`if`), looping constructs (`foreach` and `while`) and
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procedure definitions (`function` and `macro`). I will explain each of
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them one by one.
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- **Conditional Statements** The `if` command in CMake is just like
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the `if` command in any other language. It evaluates its expression
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and based on that either executes the code in its body or optionally
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the code in the `else` clause.
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```cmake
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if (FOO)
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# do something here
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elseif (BAR)
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if (NESTED_BAR)
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# do something nested here
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endif(NESTED_BAR)
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# do something else
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else ()
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# do something here
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endif (FOO)
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```
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You can use many operators to form complex conditions. Available
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options are **NOT**, **AND**, **OR**, **COMMAND**, **DEFINED**,
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**EXISTS**, **IS_DIRECTORY**, **IS_ABSOLUTE**, **MATCHES**,
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**IS_NEWER_THAN**, and operators for numerical comparisons
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**EQUAL**, **LESS**, **GREATER**, **STRLESS**, **STREQUAL**,
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**STRGREATER**.
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- **Looping Constructs** The `foreach` command enables you to execute
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a group of CMake commands repeatedly on the members of a list.The
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first argument of the foreach command is the name of the variable
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that will take on a different value with each iteration of the loop.
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The remaining arguments are the list of values over which to loop.
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```cmake
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foreach(<loop_var> <items>)
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<commands>
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endforeach()
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```
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The `while` command provides for looping based on a test condition.
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The format for the test expression in the `while` command is the
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same as that for the `if` command described earlier.
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```cmake
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while(<condition>)
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<commands>
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endwhile()
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```
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It is worth mentioning that foreach loops can be nested and that the
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loop variable is replaced prior to any other variable expansion.
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This means that in the body of a foreach loop you can construct
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variable names using the loop variable.
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- **Procedure Definitions** A function in CMake is very much like a
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function in C or C++. You can pass arguments into it, and the
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arguments passed in become variables within the function. The first
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argument is the name of the function to define. All additional
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arguments are formal parameters to the function.
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```cmake
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function(<name> [<arg1> ...])
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# write the function body here
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<commands>
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endfunction()
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```
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Macros are defined and called in the same manner as functions. The
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main differences are that a macro does not push and pop a new
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variable scope, and the arguments to a macro are not treated as
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variables but are string replaced prior to execution. This is very
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much like the differences between a macro and a function in C or
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C++. The first argument is the name of the macro to create. All
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additional arguments are formal parameters to the macro.
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```cmake
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macro(<name> [<arg1> ...])
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# write macro definition here
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<commands>
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endmacro()
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```
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## A Hello CMake example
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This example compiles a simple *hello_cmake* program. This example and
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the terminal commands are used in Linux context, however there is very
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little difference in different platforms. Make sure to [install
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CMake](https://cmake.org/install/) for your platform.
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- Create a folder and create a file named `hello_cmake.cpp` in that.
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You may add your own code. Here is my example code.
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```cpp
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#include<iostream>
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int main() {
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std::cout << "Hello CMake\n";
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return 0;
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}
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```
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- Create another file named `CMakeLists.txt` and add the following
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script in that file.
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```cmake
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cmake_minimum_required(VERSION 3.0)
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# set project name
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project(hello_cmake)
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# print compiler info
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message("The compiler is ${CMAKE_CXX_COMPILER}")
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# add executable
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add_executable(Hello_cmake hello_cmake.cpp)
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```
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- Create another directory `build` and run the following commands.
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```bash
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# create folder and change directory
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mkdir build && cd build
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# run cmake config
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cmake ..
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# build project
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cmake --build .
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```
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You will be able to see a `Hello_cmake` binary in the *build* folder.
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Hooray you have successfully built a project using CMake. For more [read
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here](https://cmake.org/cmake/help/v3.20/guide/tutorial/index.html). In
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the next blog I will explain how to create CMake configuration for a
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more complex project.
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Thanks!
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## References and Credits
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- [CMake Website](https://cmake.org)
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- [CMake
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Documentation](https://cmake.org/cmake/help/latest/index.html)
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- [Mastering CMake Book](https://www.kitware.com/what-we-offer/#books)
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- [CMake
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Tutorial](https://cmake.org/cmake/help/v3.20/guide/tutorial/index.html)
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- [You (C)Make Me
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Happy](https://prateekvjoshi.com/2014/02/01/cmake-vs-make/)
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- [Compiling xkcd.com](https://xkcd.com/303/)
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