On 2013-01-24 23:07+0100 Wojciech Knapik wrote:
Anyway - yeah, the dependencies are there now and I don't think I'm
missing any. If I create a target foo, that depends on bar and qux and
both bar and qux do target_link_libraries(target fred), then when I try
to build foo in parallel, I get a race when building fred as a
dependency for bar and qux.
Like I said above though, this is probably about the location of the
targets foo, bar, baz, qux and fred in the source tree relative to each
other.
If you are just using standard targets (libraries and executables)
with _no_ custom commands or targets, and you are using
target_link_libraries appropriately, then parallel builds should just
work. Also, the subdirectly location of where targets are configured
should not matter. Really!
So I am virtually positive the race conditions you are seeing are due
to some dependency screwup in your build system for add_custom_command
and/or add_custom_target. Typically, the issue is that two different
custom targets file-depend on the same OUTPUT file from an
add_custom_command. That's OK, but if those two targets have no
add_dependencies command relating them, then they can both be built at
the same time with a parallel-build ==> two instances of the custom
command (or one of its dependencies such as a build of a library) run
simultaneously ==> race. So very often the solution is to add a
add_dependencies command as appropriate to insure the two targets with
common custom_command OUTPUT file dependencies aren't built at the
same time.
I don't feel like researching this further, I've spent enough time on
it, so I've just moved on to other options.
Note, CMake has both file dependencies and target dependencies, and
both have to be set up correctly in order for parallel builds to work
properly. CMake often does the right thing here, but there are also
cases where you have to explicitly tell it about dependencies. For
more detail on the two kinds of dependencies look extremely carefully
at the documentation of add_custom_command, add_custom_target, and
add_dependencies. I tend to carefully re-read that documentation
every time I implement a new test target.
Ok, so let's say I have a custom command that generates code from
.idl file for instance. If I give all the targets that require that
generated code a dependency on a target that depends on the generated
files - is that enough ?
Yes. add_custom_command generates OUTPUT files. What I tend to do in
my own build systems is immediately after any add_custom command I use
add_custom_target to create a target that _file_ depends on those
OUTPUT files. Then I use add_dependencies to establish what other
targets depend on that custom_target. You will see that pattern often
for ephcom-3.0.0 and te_gen-2.0.0, and as a result parallel builds
just work for those projects without issues.
Alan
__________________________
Alan W. Irwin
Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).
Programming affiliations with the FreeEOS equation-of-state
implementation for stellar interiors (freeeos.sf.net); the Time
Ephemerides project (timeephem.sf.net); PLplot scientific plotting
software package (plplot.sf.net); the libLASi project
(unifont.org/lasi); the Loads of Linux Links project (loll.sf.net);
and the Linux Brochure Project (lbproject.sf.net).
__________________________
Linux-powered Science
__________________________
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