[CMake] ADD_CUSTOM_COMMAND and Dependencies

[Michael Wild]

On 08/01/2011 09:32 AM, Micha Renner wrote:
> Am Montag, den 25.07.2011, 15:49 +0200 schrieb Michael Wild:
>> On 07/25/2011 03:24 PM, Micha Renner wrote:
>>> There is following sequence:
>>>
>>> ADD_EXECUTEABLE(generator gen.c)
>>>
>>> ADD_CUSTOM_COMMAND(OUTPUT tlib.h
>>> 	COMMAND generator > tlib.h)
>>>
>>> ADD_LIBRARY(tlib tlib.c tlib.h)
>>>
>>> The question: Is it sure that ADD_EXECUTEABLE is invoked before the
>>> library is build or is here an ADD_DEPENDENCIES(tlib generator)
>>> necessary?
>>>
>>> Greetings
>>>
>>> Micha
>>>
>>
>> Yes, CMake should recognize that "generator" is a target and create the
>> dependency of tlib.h on "generator" automagically. And since it knows
>> that tlib.h is GENERATED, it will also add a dependency of tlib on
>> tlib.h. If you want to make really sure that CMake understands that
>> "generator" is a target, use generator expressions, like
>> "$<TARGET_FILE:generator>", but that works only since 2.8.4.
> 
> So really clear is this not.
> After studying the documentation, to get more information for
> target_file, I found this line in the add_custom_command documentation:
> "Additionally, if the target is an executable or library a file-level
> dependency is created to cause the custom command to re-run whenever the
> target is recompiled."
> 
> ADD_CUSTOM_COMMAND(OUTPUT tlib.h COMMAND generator > tlib.h DEPENDS
> generator) should solve my problem.

You don't need the DEPENDS option, since CMake automatically determines
that "generator" is a executable target, and adds the dependency by itself.

> 
> But what is the difference to 
> ADD_CUSTOM_COMMAND(OUTPUT tlib.h COMMAND $<TARGET_FILE:generator>
> generator > tlib.h)?

None, it is just more explicit, and allows for greater flexibility. The
$<...:...> notation also allows one to retrieve other paths, not just
target output files.

Michael