How to test if CMake found a library with find_library

Question

I find the library with the find_library function

find_library(MY_LIB lib PATHS ${MY_PATH}) 

If the library is found, ${MY_LIB} will point to the correct location. If the library is not found ${MY_LIB} will be MY_LIB-NOTFOUND.

But how do I test this?

if(${MY_LIB} EQUAL 'MY_LIB-NOTFOUND')      ... endif() 

always evaluates to false.

Answer 1

You can simply test the variable as such, e.g.:

find_library(LUA_LIB lua) if(NOT LUA_LIB)   message(FATAL_ERROR "lua library not found") endif() 

Example output:

CMake Error at CMakeLists.txt:99 (message):   lua library not found   -- Configuring incomplete, errors occurred! 

Note that we use

if(NOT LUA_LIB) 

and not

if(NOT ${LUA_LIB}) 

because of the different semantics.

With ${}, the variable LUA_LIB is substitued before if() is evaluated. As part of the evaluation the content would then be interpreted as variable name, unless it matches the definition of a constant. And this isn't what we want.