Okay you got me curious so I did some further digging and testing.
Michael's example works because the generated header is listed as a source
file in the add_executable() call. Therefore CMake automatically sets up
the required dependency for you. If you don't want to do that, two other
approaches also achieve your end goal. The wording of the OBJECT_DEPENDS
source property documentation gives a clue for one method, although it's
easy to miss (emphasis added by me):
This property need not be used to specify the dependency of a source file
on a generated header file that it includes. Although the property was
originally introduced for this purpose, it is no longer necessary. If the
generated header file is created by a custom command *in the same target as
the source file*, the automatic dependency scanning process will recognize
the dependency. If the generated header file is created *by another target*,
an inter-target dependency should be created with the add_dependencies()
command (if one does not already exist due to linking relationships).
As far as I can tell, the only way to create a custom command in the same
target and have that custom command execute before the source file compiles
is to use a PRE_BUILD custom command, but these are only supported with the
Visual Studio generator and therefore are not so useful. Instead, you can
create another target with add_custom_target() which depends on the
generated header and then use add_dependencies() to specify the executable
target depends on the new custom target, something like this:
cmake_minimum_required(VERSION 3.0)
project(simple)
add_custom_command(
OUTPUT ${CMAKE_BINARY_DIR}/tab.h
COMMAND "mktab > ${CMAKE_BINARY_DIR}/tab.h"
)
add_custom_target(gentab DEPENDS ${CMAKE_BINARY_DIR}/tab.h)
include_directories(${CMAKE_BINARY_DIR})
add_executable(foo foo.c)
add_dependencies(foo gentab)
Another choice is to set the OBJECT_DEPENDS property on foo.c, even though
the documentation for that source property says it shouldn't be needed. The
OBJECT_DEPENDS property sets up a dependency between files rather than
targets and can be used like this:
cmake_minimum_required(VERSION 3.0)
project(simple)
add_custom_command(
OUTPUT ${CMAKE_BINARY_DIR}/tab.h
COMMAND "mktab > ${CMAKE_BINARY_DIR}/tab.h"
)
include_directories(${CMAKE_BINARY_DIR})
add_executable(foo foo.c)
set_source_files_properties(foo.c PROPERTIES OBJECT_DEPENDS
${CMAKE_BINARY_DIR}/tab.h)
Either approach or Michael's would seem to achieve the goal of your
original question.
On Sat, May 20, 2017 at 9:10 AM, Michael Ellery <mellery...@gmail.com>
wrote:
>
> > On May 19, 2017, at 3:48 PM, Urs Thuermann <u...@isnogud.escape.de>
> wrote:
> >
> > Craig Scott <craig.sc...@crascit.com> writes:
> >
> >> A bit of a long-shot, have you tried generating the file with the
> extension
> >> .h instead of .c? That might allow it to be picked up by the dependency
> >> scanner (I don't know if it treats file extensions differently). Also,
> the
> >> add_custom_command(OUTPUT...) call needs to be in the same
> >> CMakeLists.txt
> >
> > No, I have tried that before in several variations, and nothing
> > worked.
> >
>
>
> Here:
>
> https://github.com/mellery451/gen_header
>
> works for with me with makefile generator.
>
> -Mike
> --
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--
Craig Scott
Melbourne, Australia
https://crascit.com
--
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