How do you pass the --start-group and --end-group flags to gcc when using scons?

In scons, how do you implement the --start-group and --end-group gcc flags? These should wrap a particular set of LIBS, but I couldn't find a PREFIX/SUFFIX option that wraps the entire (or partial set of) input to LIBS.

In case you're wondering what gcc flags I'm talking about, read more here:

GCC: what are the --start-group and --end-group command line options?

Solution

You're right that there is no built in prefix/suffix for this wrapper. The command line options specifying the list of libraries passed to the link command is computed based on the LIBS environment variable and stored in the _LIBFLAGS environment variable. _LIBFLAGS is then used as a part of the complete linker command line.

The defaults, defined in Tool/link.py in the SCons library, look something like:

env['LINKCOM']  = '$LINK -o $TARGET $LINKFLAGS $__RPATH $SOURCES $_LIBDIRFLAGS $_LIBFLAGS'
env['_LIBFLAGS']='${_stripixes(LIBLINKPREFIX, LIBS, LIBLINKSUFFIX, LIBPREFIXES, LIBSUFFIXES, __env__)}'

Assuming that you want to wrap the entire set of libraries, then in your environment, you can edit either the LINKCOM or the _LIBFLAGS variables to add the options:

# SConstruct

env = Environment(
    CC = 'gcc',
    LINKCOM = '$LINK -o $TARGET $LINKFLAGS $__RPATH $SOURCES $_LIBDIRFLAGS -Wl,--start-group $_LIBFLAGS -Wl,--end-group'
)

#env.Prepend(_LIBFLAGS="-Wl,--start-group ")  # the trailing space is required
#env.Append(_LIBFLAGS=" -Wl,--end-group")     # the leading space is required
env.Program('foo', ['foo.c'], LIBS='m')

I think editing LINKCOM is a little less invasive. In my trivial example, this generates:

$ scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
gcc -o foo.o -c foo.c
gcc -o foo foo.o -Wl,--start-group -lm -Wl,--end-group
scons: done building targets.

If you want to group just a subset of libraries, then you'll have to use separate LIBS variables for the "wrapped" and "unwrapped" libraries and duplicate the _LIBFLAGS function in a separate variable appended to LINKCOM.