[CMake] adding a custom command with the file name as a target

[Jim Newsome]

That doesn't quite work either. I get some errors about circular
dependencies. Strangely, doing it this way hello.bin _does_ get built,
but it gets built every time, even if 'hello' hasn't changed.

I imagine this behavior could depend on the build system- I'm using
GNU make 3.81.

Here's what I get:

$ make clean
$ make
[ 50%] Building C object CMakeFiles/hello.dir/hello.o
Linking C executable hello
[ 50%] Built target hello
make[2]: Circular CMakeFiles/hello.bin <- hello.bin dependency dropped.
make[2]: Circular hello.bin <- hello.bin dependency dropped.
[ 50%] objcopying hello to hello.bin
[100%] Built target hello.bin
$ make
[ 50%] Built target hello
make[2]: Circular CMakeFiles/hello.bin <- hello.bin dependency dropped.
make[2]: Circular hello.bin <- hello.bin dependency dropped.
[ 50%] objcopying hello to hello.bin
[100%] Built target hello.bin


On Fri, Mar 11, 2011 at 2:30 PM, Michael Hertling <mhertling at online.de> wrote:
> On 03/11/2011 08:00 PM, David Cole wrote:
>> On Fri, Mar 11, 2011 at 12:05 PM, Jim Newsome <jnewsome at cmu.edu> wrote:
>>> Unfortunately it seems that the target needs to have a different name
>>> from its dependencies. When building I get:
>>> make[2]: Circular CMakeFiles/hello.bin <- hello.bin dependency dropped.
>>>
>>> Another workaround I thought of is to change the output filename to,
>>> e.g., _hello.bin and have a command in the target to move _hello.bin
>>> to hello.bin. Unfortunately this forces _hello.bin to be regenerated
>>> every time. I suppose I could work around _that_ by copying instead of
>>> moving, but that's getting fairly ugly.
>>>
>>> On Fri, Mar 11, 2011 at 11:56 AM, Tyler <tyler at cryptio.net> wrote:
>>>> The target in the Makefile is created with the add_custom_target()
>>>> call. So just change yours to:
>>>>
>>>> add_custom_target(hello.bin ALL DEPENDS hello.bin)
>>>>
>>>> hth,
>>>> tyler
>>>>
>>>> On Fri, Mar 11, 2011 at 7:58 AM, Jim Newsome <jnewsome at cmu.edu> wrote:
>>>>> I'd like to do something like add_custom_command, with the output file
>>>>> name as a target in the generated makefile. Is there an elegant way of
>>>>> doing this?
>>>>>
>>>>> The closest I've come is:
>>>>> add_executable (hello hello.c)
>>>>> add_custom_command(OUTPUT hello.bin
>>>>>                    COMMAND objcopy --output-format=binary hello hello.bin
>>>>>                   DEPENDS hello
>>>>>                   COMMENT "objcopying hello to hello.bin")
>>>>> add_custom_target(bin ALL DEPENDS hello.bin)
>>>>>
>>>>> However, the target name in the generated makefile is 'bin' rather
>>>>> than 'hello.bin'. Is there a way to make 'hello.bin' itself a target
>>>>> in the generated makefile?
>>>>
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>>
>> How about making the *named output* of your custom command simply be a
>> sentinel file (with a different name) that just happens to produce the
>> real file (with the same name as the target)...?
>>
>> Something like this:
>>
>> add_executable (hello hello.c)
>> add_custom_command(OUTPUT hello.bin.sentinel
>>                   COMMAND objcopy --output-format=binary hello hello.bin
>>                   COMMAND ${CMAKE_COMMAND} -E touch hello.bin.sentinel
>>                   DEPENDS hello
>>                   COMMENT "objcopying hello to hello.bin")
>> add_custom_target(hello.bin ALL DEPENDS hello.bin.sentinel)
>>
>> Does that work?
>
> No, you won't have a dependency of the hello.bin custom target on the
> hello.bin file, i.e. if the latter is removed, it won't be rebuilt.
> Instead, use absolute paths for the custom target's dependencies:
>
> add_executable (hello hello.c)
> add_custom_command(OUTPUT ${CMAKE_BINARY_DIR}/hello.bin
>                   COMMAND <hello-to-${CMAKE_BINARY_DIR}/hello.bin>
>                   DEPENDS hello
>                   COMMENT "objcopying hello to hello.bin")
> add_custom_target(hello.bin ALL DEPENDS ${CMAKE_BINARY_DIR}/hello.bin)
>                  ^^^^^^^^^             ^^^^^^^^^^^^^^^^^^^
>
> Apparently, that's sufficient to distinguish the hello.bin custom
> target from the equally named output file of the custom command.
>
> Regards,
>
> Michael
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