cmake DEFINED does not seems to recognize variable

问题:

I have this code in cmake:

#mandatory
SET(BOOST_DIR "$ENV{BOOST_HOME}")
if (DEFINED ${BOOST_DIR})
    #global include directories
    include_directories(${BOOST_DIR})
else(DEFINED ${BOOST_DIR})
    message( STATUS "BOOST_HOME at ${BOOST_DIR}")

    message( FATAL_ERROR "Undefined BOOST_HOME env var.")
endif(DEFINED ${BOOST_DIR})

The env var BOOST_HOME is defined (/home/ferran/boost). The behavior is very odd because the output is:

-- BOOST_HOME at /home/ferran/boost
CMake Error at CMakeLists.txt:14 (message):
  Undefined BOOST_HOME env var.


-- Configuring incomplete, errors occurred!

So, the var exists and prints its contents and at the same time does not exists?

回答1:

You need to avoid dereferencing the variable BOOST_DIR in the if statement:

set(BOOST_DIR "$ENV{BOOST_HOME}")
if(BOOST_DIR)               # <--- Use 'BOOST_DIR', not 'DEFINED ${BOOST_DIR}'
    #global include directories
    include_directories(${BOOST_DIR})
else()
    message(STATUS "BOOST_HOME at ${BOOST_DIR}")
    message(FATAL_ERROR "Undefined BOOST_HOME env var.")
endif()

By dereferencing BOOST_DIR, you're effectively querying if CMake has a variable called /home/ferran/boost defined.