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Building and using shared libraries using gccgo

* Michael Stapelberg:

> Can you please list the full instructions you did to accomplish building
> sparse/index/regexp as a shared library?

Sure. See the Makefile at the end of this mail. Please note that I don't
really know what I'm doing here -- just tried a few things and tried to
make sense of error messages until I got working binaries.

I put symlinks to the index, sparse, regexp directories into
src/code.google.com/p/codesearch and added -Isrc to every gccgo
invocation, so that the compiler looks for the local imports in the
right place. For the shared libraries, I added

1. -shared -fPIC

2. -fno-split-stack
   Otherwise I could not link executables and got the following error
   /usr/bin/ld.bfd.real: cindex: hidden symbol `__morestack' in
   /usr/lib/gcc/x86_64-linux-gnu/4.7/libgcc.a/morestack.o) is referenced
   by DSO
   /usr/bin/ld.bfd.real: final link failed: Bad value

When the libraries are put into
src/code.google.com/codesearch/lib{sparse,regexp,index}.so, they are
subsequently found by the compiler -- and apparently used for imports,
even if the source files are missing.

I have not yet figured out how to specify the full namespace-related
path to the linker yet: -lcode.google.com/p/codesearch/index causes ld
then to look for libcode.google.com/p/codesearch/index.so which is
almost but not quite what is needed.


GCCGO_LIBFLAGS = -shared -fPIC -fno-split-stack

	mkdir -p $(PREFIX)
	ln -sf `pwd`/regexp `pwd`/index `pwd`/sparse $(PREFIX)/

LIBS=sparse regexp index

DYNLIBS=$(patsubst %,$(PREFIX)/lib%.so,$(LIBS))

dynlibs: $(DYNLIBS)

$(PREFIX)/lib%.so: $(PREFIX)/%/*.go
	gccgo $(GCCGO_FLAGS) $(GCCGO_LIBFLAGS) -o $@ $^
$(PREFIX)/libregexp.so: $(PREFIX)/regexp/*.go
	gccgo $(GCCGO_FLAGS) $(GCCGO_LIBFLAGS) -o $@ $^ $(GCCGO_LDFLAGS) -lsparse
$(PREFIX)/libindex.so: $(PREFIX)/index/*.go
	gccgo $(GCCGO_FLAGS) $(GCCGO_LIBFLAGS) -o $@ $^
$(GCCGO_LDFLAGS) -lsparse -lregexp

BINS=cindex csearch cgrep

cindex: cmd/cindex/*.go
	gccgo $(GCCGO_FLAGS) -o $@ $^ $(GCCGO_EXE_LDFLAGS) -lindex
csearch: cmd/csearch/*.go
	gccgo $(GCCGO_FLAGS) -o $@ $^ $(GCCGO_EXE_LDFLAGS) -lregexp -lindex
cgrep: cmd/cgrep/*.go
	gccgo $(GCCGO_FLAGS) -o $@ $^ $(GCCGO_EXE_LDFLAGS) -lregexp

bins: $(DYNLIBS) $(BINS)

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