[CMake] Trouble with parallel builds for targets created by add_executable, target_link_libraries, and add_library

[Marcel Loose]

On Tue, 2009-09-29 at 19:46 -0400, Bill Hoffman wrote:
> Alan W. Irwin wrote:
> > On 2009-09-29 16:36-0400 Bill Hoffman wrote:
> > 
> >> make -j N is only supported with the all target.
> > 
> > How difficult would it be to implement parallel build support for more than
> > just the "all" target? Note, if there is some limitation that makes it
> > impractical or inefficient to implement this current "all" target 
> > capability
> > for every target, you could instead implement this capability just for the
> > targets where this capability is wanted (as designated, say, by a
> > PARALLEL_BUILD=ON target property).
> > 
> It is very hard to implement on top of make.  Let me restate what I 
> mean, it is not the all target that is special.  You can do make -j N 
> target.  You just can't do make -jN target1 target2 if target1 and 
> target2 have a common target that they depend on.  You can only specify 
> one target at a time for make -j N.  The /fast targets could be used to 
> avoid the issue you are having maybe...
> 
Hi Alan,

I don't think this is a limitation of CMake. It's rather a limitation of
make. An (in)famous example of doing a parallel build with 'make' that
will likely cause a broken build is:

$ make -j4 all install

In this example, it's possible that 'make' will try to install, e.g., a
library that has yet to be built. The only safe way to run a parallel
'make' with multiple targets is to serialize them (sounds contradictory,
doesn't it?):

$ make -j4 all && make -j4 install

Or better:

$ export MAKEFLAGS=-j4
$ make all && make install

Best regards,
Marcel Loose.