[CMake] check_include_files() can't "find" header because it can't compile test app

[Alexander Neundorf]

On Tuesday 15 September 2009, Casey Jones wrote:
> On Monday 14 September 2009 3:22:14 pm Alexander Neundorf wrote:
> > On Sunday 13 September 2009, Casey Jones wrote:
> > > Hello, I'm trying to link a Qt4 library into my program as an optional
> > > dependency.  I can get it to find and link the library with a
> > > FindFoo.cmake. But when I use check_include_files( foo.h HAVE_FOO_H )
> > > it says it can't find the header.  So looking in CMakeError.log it says
> > > it found foo.h but since it includes Qt headers like:
> > > #include <QString>
> > > instead of
> > > #include <qt4/QtCore/QString>
> > > it can't compile the test program.
> > >
> > > Is there a way to have check_include_files() not compile the test
> > > program and just make sure the header exists?
> >
> > You could just use find_path(FOO_INCLUDE_DIR foo.h ...), usually this is
> >  good. enough.
> > Or you can set the cmake variable CMAKE_REQUIRED_INCLUDES before calling
> >  the check_include_files() macro so that it lists all necessary include
> >  dirs (should be ${QT_QTCORE_INCLUDE_DIR} or something like this)
> >
> > > Or is there some other way to create the config.h.in and config.h?
> >
> > You don't *have* to compile anything in order to create a config.h from a
> > config.h.in.
> > configure_file() reads the config.h.in and replaces all cmake variables
> >  with their values in that file and writes the output file.
> > It doesn't matter where these values come from, whether a
> > check_include_files() or if you just set them.
> >
> > Alex
>
> I'm trying out FIND_PATH(), but how do I put that variable into
> config.h.in? I have the variable HAVE_FOO_H, and it finds the header, but I
> just don't know what to do with that variable.
>
> Would I need to do something like:
> IF( ${HAVE_FOO_H} )
> 	WRITE_TO_FILE(src/config.h.in "#cmakedefine HAVE_FOO_H"
> ENDIF( ${HAVE_FOO_H} )
>
> I don't know if the write_to_file function exists or not, so thats why I'm
> asking.

You should write a file like e.g. "myheader.h.in", which could look like that:

#define GREETING_TEXT "@MY_MESSAGE@"

#cmakedefine HAVE_FOO_H 1


Then in your CMakeLists.txt you do:

set(MY_MESSAGE "Hello world")
set(HAVE_FOO_H TRUE)

configure_file(myheader.h.in ${CMAKE_CURRENT_BINARY_DIR}/config.h)

This will make cmake read the existing myheader.h.in, replace the @MY_MESSAGE@ 
with the contents of the cmake variable with the same name, and turn the line 
with #cmakedefine either to
#define HAVE_FOO_H 1
(if the cmake variable HAVE_FOO_H is true) or to
#undefine HAVE_FOO_H
(if the cmake variable HAVE_FOO_H is not true).

HTH
Alex