[CMake] --debug-trycompile causes false positives from check_function_exists

[David Cole]

To use "--debug-trycompile" effectively, you have to:

- run *without* "--debug-trycompile" all the way through once to establish
all the cache settings
- remove/delete the *single* cache entry whose try_compile result you are
trying to debug
- run again with "--debug-trycompile"

The reasons are:
- try_compile operations generate a project and a project build tree, and
then clean up after themselves (delete the generated project source and
build tree)
- only one try_compile operation may be executed at a time because they all
share the same source and build trees
- clean up must occur in between runs so that each one is correct and
independent of all the other ones
- --debug-trycompile *leaves* *the* try_compile project source and build
tree on disk so that you can inspect them, and presumably "debug" the
problem

So..... given this view into the implementation details of try_compile, you
cannot count on the results of --debug-trycompile to be accurate for more
than one try_compile in a row. Leaving the build tree from a previous
try_compile operation affects the results of subsequent try_compiler
operations. Only one at a time may be "debugged".

(I realize this belongs in the documentation somewhere, but my question to
you is: where would you have looked first for such documentation? If you can
answer that, or several people answer approximately the same way, then I
could add documentation for this in such a spot. -- Thanks.)


Hope this helps,
David


On Fri, Feb 11, 2011 at 2:57 AM, Jack Poulson <jack.poulson at gmail.com>wrote:

> On Fri, Feb 11, 2011 at 1:47 AM, Michael Wild <themiwi at gmail.com> wrote:
>
>> On 02/11/2011 04:30 AM, Jack Poulson wrote:
>> > This is occurring with CMake 2.8.3 on OSX for several different
>> > check_function_exists calls. Is this expected behavior?
>> >
>> > Jack
>>
>> Seriously, we're going to need some more information than just "I get
>> false positives"...
>>
>>
> I did not give any more information because, well, it seems to _always_
> give false positives with those settings. I don't really care if it's fixed
> because I can easily work around it, but I thought I'd give you guys a heads
> up.
>
> If it helps, this is the relevant snippet:
>
> include(CheckFunctionExists)
> set(CMAKE_REQUIRED_LIBRARIES ${MATH_LIBS})
> check_function_exists(vdSin MKL)
> check_function_exists(vsin MASS)
>
> If I run without --debug-trycompile when I am not linking in the MKL or
> MASS libraries, the routines vdSin and vsin are not found. When I run with
> --debug-trycompile, they are found. That's all I have time to give. Use it
> or not. I'm not asking for help.
>
> Jack
>
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