FindFLEX and CACHE INTERNAL

[EosPengwern (Stephen Morris)]

I’m having trouble with setting CACHE INTERNAL variables, and must be getting the syntax wrong. The context, in case it’s relevant, is trying to work round a problem with FindFLEX.

I have flex.exe installed in C:\cygwin\bin. On running

find_package(FLEX ${required_version})

… I find that FLEX_FOUND and FLEX_EXECUTABLE are set correctly, but FLEX_INCLUDE_DIRS is not. That’s irritating, but I can live with it; after all, I know that FlexLexer.h is in C:\cygwin\usr\inlcude, so I say so:

set(FLEX_INCLUDE_DIRS "C:/cygwin/usr/include" CACHE INTERNAL "")

However, CMake seems to completely ignore this, so that if in the very next line I write

message("FLEX_INCLUDE_DIRS = ${FLEX_INCLUDE_DIRS}")

…then the response is:

FLEX_INCLUDE_DIRS = FLEX_INCLUDE_DIR-NOTFOUND

What am I doing wrong?

[ben.boeckel (Ben Boeckel (Kitware))]

Scoped variables shadow cache variables, so if FLEX_INCLUDE_DIRS is set as a non-cache variable, it will “shadow” the cache setting.

[EosPengwern (Stephen Morris)]

So if I understand correctly…

• FindFLEX sets FLEX_INCLUDE_DIRS as a non-cache variable;

• I’m then setting it as a cache variable (because I want to use it globally without having to promote it up a couple of levels using PARENT_SCOPE)

• But when I try to access the variable normally using ${FLEX_INCLUDE_DIRS}, I’m seeing the non-cache variable defined by FindFLEX, rather than the cache variable I defined myself?

If so I’m still puzzled, because I can’t see anything in FindFLEX.cmake that raises FLEX_INCLUDE_DIRS to the parent scope, and the place where I actually want to invoke it is another level up again.

But am I on the right track?

[ben.boeckel (Ben Boeckel (Kitware))]

@EosPengwern

FindFLEX sets FLEX_INCLUDE_DIRS as a non-cache variable;

Looks like it does.

@EosPengwern

I’m then setting it as a cache variable (because I want to use it globally without having to promote it up a couple of levels using PARENT_SCOPE)

find_package is scoped. Unless you set the targets’ GLOBAL property, they don’t really do much to parent scopes (except function and macro definitions…those are always global).

@EosPengwern

But when I try to access the variable normally using ${FLEX_INCLUDE_DIRS}, I’m seeing the non-cache variable defined by FindFLEX, rather than the cache variable I defined myself?

If you’re in the same scope and after the find_package call, yes.

@EosPengwern

But am I on the right track?

To understanding, yes. To a fix…I think the find_package call just needs to be called in a high enough scope that anything that wants to use it is “after” it and under a relevant directory.

[EosPengwern (Stephen Morris)]

Thank you, yes. The solution was to make sure that find_package(FLEX) was run in the global namespace, rather than inside a function, and to get rid of all of the cache variables.