Versioned library symbols from Nicolas Boullis. Much appreciated.

lib/Makefile.am

@@ -1,4 +1,4 @@
-#   $Id: Makefile.am,v 1.19 2004/02/09 10:32:35 rocky Exp $
+#   $Id: Makefile.am,v 1.20 2004/02/10 01:38:54 rocky Exp $
 #
 #    Copyright (C) 2003, 2004 Rocky Bernstein <rocky@panix.com>
 #
@@ -29,7 +29,7 @@
 #     faster.
 #
 #  3. If the library source code has changed at all since the last
-#     update, then increment REVISION (`C:R:A' becomes `C:r+1:A').
+#     update, then increment REVISION (`C:R:A' becomes `C:R+1:A').
 #
 #  4. If any interfaces have been added, removed, or changed since the
 #     last update, increment CURRENT, and set REVISION to 0.
@@ -83,7 +83,7 @@ libcdio_sources = \
 lib_LTLIBRARIES    = libcdio.la libiso9660.la
 
 libcdio_la_SOURCES = $(libcdio_sources)
-libcdio_la_LDFLAGS = -version-info $(libcdio_la_CURRENT):$(libcdio_la_REVISION):$(libcdio_la_AGE)
+libcdio_la_LDFLAGS = -version-info $(libcdio_la_CURRENT):$(libcdio_la_REVISION):$(libcdio_la_AGE) -Wl,--version-script=libcdio.la.ver
 
 libiso9660_la_SOURCES = \
 	iso9660.c \
@@ -92,6 +92,80 @@ libiso9660_la_SOURCES = \
 	xa.c
 
 libiso9660_la_LIBADD = $(LIBCDIO_LIBS)
-libiso9660_la_LDFLAGS = -version-info $(libiso9660_la_CURRENT):$(libiso9660_la_REVISION):$(libiso9660_la_AGE)
+libiso9660_la_LDFLAGS = -version-info $(libiso9660_la_CURRENT):$(libiso9660_la_REVISION):$(libiso9660_la_AGE) -Wl,--version-script=libiso9660.la.ver
+
+# An explanation of the versioning problem from Nicolas Boullis and
+# the versioned symbol solution he uses below...
+# 
+# Currently, libvcdinfo uses the cdio_open function from libcdio.
+# Let's imagine a program foobar that uses both the vcdinfo_open
+# function from libvcdinfo and the cdio_open function from libcdio.
+
+# Currently, libcdio has SONAME libcdio.so.0, libvcdinfo has SONAME
+# libvcdinfo.so.0 and requires libcdio.so.0, and foobar requires both
+# libvcdinfo.so.0 and libcdio.so.0. Everything looks fine.
+#
+# Now, for some reason, you decide to change the cdio_open function.
+# That's your right, but you have to bump the CURRENT version and (if I
+# understand it correctly, athough this is not that clear in libtool's
+# documentation) set the AGE to 0. Anyway, this bumps the SONAME, which is
+# sane since the interface changes incompatibly.
+
+# Now, you have a new libcdio with SONAME libcdio.so.1. But libvcdinfo and
+# foobar still require libcdio.so.0. Everything is still fine.
+
+# Now, after some minor changes, the author of foobar recompiles foobar.
+# Then, foobar now requires libvcdinfo.so.0 and libcdio.so.1. And foobar
+# now segfaults...
+
+# What is happening? When you run foobar, if brings both libvcdinfo.so.0
+# and libcdio.so.1, but libvcdinfo.so.0 also brings libcdio.so.0. So you
+# have both libcdio.so.0 and libcdio.so.1 that bring their symbols to the
+# global namespace. Hence, you have to incompatible versions of the
+# cdio_open function in the name space. When foobar calls cdio_open, it
+# may choose the wrong function, and segfaults...
+
+# With versioned symbols, the cdio_open function from libcdio.so.0 may be
+# known as (something that looks like) cdio_open@@CDIO_0. An the cdio_open
+# function from libcdio.so.1 as cdio_open@@CDIO_1. Both versions of
+# libcdio would still be brought in by the most recent foobar, but foobar
+# (and libvcdinfo) know which versioned function to use and then use the
+# good one.
+
+
+# This is some simple versioning where every symbol is versioned with
+# something that looks like the SONAME of the library. More complex (and
+# better) versioning is possible; it is for example what is used by glibc.
+# But good complex versioning is something that requires much more
+# work...
+
+
+# The below is a impliments symbol versioning.  First of all, I
+# compute MAJOR as CURENT - AGE; that is what is used within libtool
+# (at least on GNU/Linux systems) for the number in the SONAME.  The
+# nm command gives the list of symbols known in each of the object
+# files that will be part of the shared library. And the sed command
+# extracts from this list those symbols that will be shared. (This sed
+# command comes from libtool.)
+
+libcdio_la_MAJOR := $(shell expr $(libcdio_la_CURRENT) - $(libcdio_la_AGE))
+libcdio_la_DEPENDENCIES = libcdio.la.ver
+
+libcdio.la.ver: $(libcdio_la_OBJECTS)
+	echo 'CDIO_$(libcdio_la_MAJOR) { ' > $@
+	echo "  global:" >> $@
+	nm $(patsubst %lo,%o,$(libcdio_la_OBJECTS)) | sed -n -e 's/^.*[ 	][ABCDGIRSTW][ABCDGIRSTW]*[ 	][ 	]*\([_A-Za-z][_A-Za-z0-9]*\)$$/    \1;/p' | sort | uniq >> $@
+	echo '};' >> $@
+
+libiso9660_la_MAJOR := $(shell expr $(libiso9660_la_CURRENT) - $(libiso9660_la_AGE))
+libiso9660_la_DEPENDENCIES = $(LIBCDIO_LIBS) libiso9660.la.ver
+
+libiso9660.la.ver: $(libiso9660_la_OBJECTS)
+	echo 'ISO9660_$(libiso9660_la_MAJOR) {' > $@
+	echo "  global:" >> $@
+	nm $(patsubst %lo,%o,$(libiso9660_la_OBJECTS)) | sed -n -e 's/^.*[ 	][ABCDGIRSTW][ABCDGIRSTW]*[ 	][ 	]*\([_A-Za-z][_A-Za-z0-9]*\)$$/    \1;/p' | sort | uniq >> $@
+	echo '};' >> $@
+
+MOSTLYCLEANFILES = libcdio.la.ver libiso9660.la.ver
 
 INCLUDES = -I$(LIBCDIO_CFLAGS)