Proper Comparisons in CMake

It really saddens me whenever I see code like this:

if(${CMAKE_SYSTEM_NAME} STREQUAL Darwin)
  message("System is Darwin")
else()
  message("System is not Darwin")
endif()

Imagine you run this code on Windows and you get the output “System is Darwin”. How is this possible?

Let’s have a look at the documentation of the if command:

if(<variable|string> STREQUAL <variable|string>)

True if the given string or variable’s value is lexicographically equal to the string or variable on the right.

Both operands are interpreted as a variable when it is defined and as a string literal when it is not. Depending on whether there is a variable with the name Darwin defined, the second operand is evaluated to the value of that variable or as the string literal "Darwin".

The first operand is expanded explicitly by the ${} syntax, so what the if() command sees are the arguments Windows STREQUAL Darwin and it will treat the first argument exaclty like the second one: Depending on whether there is a variable with the name Windows defined, the operand is evaluated to the value of that variable or as the string literal "Windows".

That means, with the following two variable definitions:

set(Darwin 1)
set(Windows 1)

The code above actually compares the two values 1 and 1.

What about properly quoting both sides?

if("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
  message("System is Darwin")
else()
  message("System is not Darwin")
endif()

Depending on what is set as the minimum required CMake version (and way too many maintainers do not set this properly), this might still suffer from the exact same problem.

How to properly compare strings:

1. Make sure that policy CMP0054 is set to NEW, for example by setting the minimum required CMake version to 3.1 or higher.

2. When you want an argument to be interpreted as a string, always put it in quotes.

3. When you want an argument to be interpreted as a variable, don’t explicitly expand it with ${}.

The recommended style is:

cmake_minimum_required(VERSION 3.10)
if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
  message("System is Darwin")
else()
  message("System is not Darwin")
endif()

Now test your own CMake files. If you find a match for if($, then you most likely have a bug.