How to use CHECK_INCLUDE_FILES macro in cmake? - cmake

Question

I need link my program against Kerberos authentication library (gssapi_krb5) with the corresponding headers gssapi/gssapi.h and gssapi/gssapi_krb5.h included in the source file.
Currently, the compilation will continue if headers are absent and stop with a compile time error saying header files not found.
What I want to implement in the cmake file is to check the existence of the header file and stop compiling if not found.
I add the following code into my CMakeList.txt file.
INCLUDE(CheckIncludeFiles)
CHECK_INCLUDE_FILES(gssapi/gssapi.h;gssapi/gssapi_krb5.h HAVE_KRB_HEADERS)
IF (NOT HAVE_KRB_HEADERS)
RETURN()
ENDIF (NOT HAVE_KRB_HEADERS)
But it still does not act as I expected.
I would like the following lines:
-- Looking for gssapi/gssapi.h - found
-- Looking for gssapi/gssapi_krb5.h - not found
but fail.
Also, the variable HAVE_KRB_HEADERS is empty when output with message macro.
Compile continues until the error described above occurs.
I read somewhere on the Web, this may be because CMake cache.
I'm very new to CMake and not quite clear with that concept.
My CMake version is 2.6.
How could I make this code work? Thank you!

Answer 1

I can't say I'm a huge fan of CheckIncludeFiles because of its difficulty to get right. In principal it's good - it actually creates tiny c files which #include the requested headers and tries to compile them, but it seems to be too easy to get wrong.
I generally prefer just using find_path and/or find_file for this job. This doesn't check the contents of any files found, but usually if you find the required header, its contents are good!
I would use find_path if I needed to know the folder where the header lived. This would usually be because I need to check for other files in the same folder (as in your case), or more commonly because I need to add the folder to an include_directories call.
find_file yields the full path to the file (if found). For headers, normally I don't need the path elsewhere in the CMakeLists - it's just used immediately after the find_file to check the file was actually found.
So, here's how I'd go about checking for "gssapi/gssapi.h" and "gssapi/gssapi_krb5.h"
find_path(GssApiIncludes gssapi.h PATHS <list of folders you'd expect to find it in>)
if(NOT GssApiIncludes)
message(FATAL_ERROR "Can't find folder containing gssapi.h")
endif()
find_file(GssKrb gssapi_krb5.h PATHS ${GssApiIncludes} NO_DEFAULT_PATH)
if(NOT GssKrb)
message(FATAL_ERROR "Can't find gssapi_krb5.h in ${GssApiIncludes}")
endif()
If you do this, then if required you could add
include_directories(${GssApiIncludes})
so that in your source code you can do
#include "gssapi.h"
#include "gssapi_krb5.h"

Answer 2

For anyone who has to work with CHECK_INCLUDE_FILES, the documentation lists a variable called CMAKE_REQUIRED_INCLUDES where you can set additional include paths apart from the default headers.
In a CMake file:
LIST(APPEND CMAKE_REQUIRED_INCLUDES "gssapi")
From the command line:
cmake . --DCMAKE_REQUIRED_INCLUDES="gssapi"
If all else fails, you can set the -I<dir> flag manually. However, this is not recommended as it not portable across compilers.
# note the extra space before `-I`
STRING(APPEND CMAKE_C_FLAGS " -Igssapi")
STRING(APPEND CMAKE_CXX_FLAGS " -Igssapi") # for C++
Also note that C++ headers have a different macro called CheckIncludeFileCXX.