On Jan 21, 2016, at 6:08 AM, David Morsberger <dave@xxxxxxxxxxxxxx> wrote:
> It appears the linker isn’t called when the -c and the -o options are present.
Correct. It's not *supposed* to be called. In UNIX C compilers, the "-c" flag means "compile to an object file, but don't link the resulting object file into an executable file"; it's the standard option used if you have a program that's built from more than one object file.
> The cmake logic in CMakeLists.txt is basically:
>
> SET(CMAKE_REQUIRED_FLAGS "${_FLAG}")
> CHECK_C_SOURCE_RUNS(“int main() { return 0;}" ${_RESULT})
The documentation for the CHECK_C_SOURCE_RUNS() macro is
* CheckCSourceRuns: Check if the given C source code compiles and runs.
CHECK_C_SOURCE_RUNS(<code> <var>)
<code> - source code to try to compile
<var> - variable to store the result
(1 for success, empty for failure)
Presumably what it's doing is:
1) compiling the source file into an object file;
2) linking the object file into an executable;
3) trying to run the resulting executable.
(As per my previous CMake complaint, they ***REALLY*** need CHECK_C_SOURCE_LINKS(), for the benefit of, for example, projects that are cross-compiling. But I digress.)
The documentation for CHECK_C_SOURCE_RUNS() continues:
The following variables may be set before calling this macro to modify the way the check is run:
CMAKE_REQUIRED_FLAGS = string of compile command line flags
CMAKE_REQUIRED_DEFINITIONS = list of macros to define (-DFOO=bar)
CMAKE_REQUIRED_INCLUDES = list of include directories
CMAKE_REQUIRED_LIBRARIES = list of libraries to link
Note the lack of a "string of *LINKER* command line flags" in that list; *that* is what we'd need to test this.
I guess, with Makefiles, the linking is done with a compiler command (another UNIX tradition) and the -Wl,--as-needed flag is passed on the link command line, but, with Xcode, it's not done with a compiler command, and the C flags aren't passed to the link phase at all.
