Re: Mac Build Error

[David Morsberger <dave () morsberger com>]

I went down a rabbit hole and pulled myself back out. I narrowed it down to the following.

We are using cmake to check if the linker supports ‘-Wl,—as-needed'. The test compile (and link?) command cmake builds 
using ‘cmake -G Xcode’  does not fail.

The command from ‘cmake -G’ has been narrowed down to the following:

$ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang \
 -Qunused-arguments \
 -DWS_LD_FLAG_VALID0 \
 -Wl,--as-needed \
 -c src.c \
 -o src.o 
$

The following modified command generates an error:

$ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang \
 -Qunused-arguments \
 -DWS_LD_FLAG_VALID0 \
 -Wl,--as-needed \
 src.c

ld: unknown option: --as-needed
clang: error: linker command failed with exit code 1 (use -v to see invocation)
$

It appears the linker isn’t called when the -c and the -o options are present. The following warning is generated when 
the ‘-Qunused-arguments’ is removed.

clang: warning: -Wl,--as-needed: 'linker' input unused

The cmake logic in CMakeLists.txt is basically:

   SET(CMAKE_REQUIRED_FLAGS "${_FLAG}")
   CHECK_C_SOURCE_RUNS(“int main() { return 0;}" ${_RESULT})

where _FLAG is set to ‘-DWS_LD_FLAG_VALID0 -Wl,—-as-needed’


So, how/where is the clang compile/link command generated and why isn’t it being setup for clang to link?



> On Jan 20, 2016, at 3:22 PM, Guy Harris <guy () alum mit edu> wrote:
>
> On Jan 20, 2016, at 11:09 AM, David Morsberger <dave () morsberger com> wrote:
>
> > This is what I am also tracking down. It has to do with the way cmake is instantiating the compiler. The 
> > --serialized-diagnostics option is what appears to be hiding the error.
>
> --serialize-diagnostics takes an argument, which is the name of a file to which the diagnostics are written, in a 
> binary format.
>
> Clang documentation sucks.  Srsly.  The man page doesn't enumerate the -W options; the website:
>
>       http://clang.llvm.org/docs/UsersManual.html#options-to-control-error-and-warning-messages
>
> does, but I'm not sure how to get the manual for an *arbitrary* version of Clang (the GCC folks are kind enough to 
> provide online manuals, complete with listings of compiler flags, for multiple versions), and the word "serialize" 
> appears nowhere on that page.
>
> It might be that, when the tests are done, ld is being run directly, rather than by the gcc/clang driver, and that 
> -as-needed isn't getting passed to it.  It obviously is getting passed when the actual *build* is done.
>
> Perhaps when CMake was being designed, its designers should have looked at the documentation for another 
> configuration system - I think it's called "autoconf" or something such as that - and noticed that it offered not 
> only macros to try compiling a file ("compiling" as in "turning a source file into an object file, without trying to 
> link anything"), but macros to try *linking* things, which can come in handy if, say, to pick a purely *hypothetical* 
> example, you're trying to see whether a given linker flag works.
>
> Or perhaps they mistakenly had the attitude that "we take care of all the platform details, so you don't have to".
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