[Cmake] EXEC_PROGRAM and subdirs

[Brad King]

klaas.holwerda wrote:
> Indeed i want to use the result, which looks like a normal variable.

You need to save the result in a cache variable.  Then it will be 
available in subdirectories.  It will also prevent the program from 
being executed every time cmake runs.

IF(DEFINED FOO_RESULT)
   # Foo was already run and the result saved.  Just use FOO_RESULT.
ELSE(DEFINED FOO_RESULT)
   # Foo has not been run.  Run it and save the result in the cache.
   EXEC_PROGRAM(foo ARGS bar OUTPUT_VARIABLE OUT)
   SET(FOO_RESULT "${OUT}" CACHE INTERNAL "result of running foo")
ENDIF(DEFINED FOO_RESULT)

You may, of course, add another condition to cause the program to be 
re-executed.

>> All of this is due to historical reasons.  It was not originally 
>> expected that EXEC_PROGRAM would be used in a way that would affect 
>> the flow of CMake code evaluation.  The original version of the 
>> command did not have an OUTPUT_VARIABLE argument.  When the argument 
>> was added the command was not made inherited.
>>
>> Eventually there will be a new methodology in place for listfile 
>> evaluation 
> What is meant by that?

We may introduce a new order in which the listfiles are processed. 
Currently there is no way for a higher level listfile to access a 
variable from one of its subdirectories' listfiles.  Listfiles are 
processed in a pre-order visit of the tree.  We may in the future 
provide a way to have an in-order visit of the tree.

-Brad