diff --git a/CMakeLists.txt b/CMakeLists.txt
index 013300a..517b2c3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -18,6 +18,12 @@ project(
     LANGUAGES CXX
 )
 
+# See notes in GNUmakefile about using abi-compliance-checker.
+# soversion is major ABI version.
+set( abi_version 1.0.0 )
+string( REPLACE "." ";" abi_list "${abi_version}" )
+list( GET abi_list 0 soversion )
+
 include( CheckCXXCompilerFlag )
 
 # When built as a sub-project, add a namespace to make targets unique,
@@ -32,6 +38,16 @@ endif()
 
 #-------------------------------------------------------------------------------
 # Options
+if (testsweeper_is_project)
+    set( log "" CACHE STRING "Shorthand for CMAKE_MESSAGE_LOG_LEVEL" )
+    set_property( CACHE log PROPERTY STRINGS
+                  FATAL_ERROR SEND_ERROR WARNING AUTHOR_WARNING DEPRECATION
+                  NOTICE STATUS VERBOSE DEBUG TRACE )
+    if (log)
+        set( CMAKE_MESSAGE_LOG_LEVEL "${log}" )
+    endif()
+endif()
+
 option( BUILD_SHARED_LIBS "Build shared libraries" true )
 option( build_tests "Build test suite" "${testsweeper_is_project}" )
 option( color "Use ANSI color output" true )
@@ -68,6 +84,8 @@ use_openmp             = ${use_openmp}
 testsweeper_install    = ${testsweeper_install}
 testsweeper_is_project = ${testsweeper_is_project}
 testsweeper_           = ${testsweeper_}
+abi_version            = ${abi_version}
+soversion              = ${soversion}
 " )
 
 #-------------------------------------------------------------------------------
@@ -130,6 +148,8 @@ set_target_properties(
     CXX_STANDARD_REQUIRED true  # prohibit < c++17
     CXX_EXTENSIONS false        # prohibit gnu++17
     WINDOWS_EXPORT_ALL_SYMBOLS ON
+    VERSION   "${abi_version}"
+    SOVERSION "${soversion}"
 )
 
 if (CMAKE_VERSION VERSION_GREATER_EQUAL 3.15)
diff --git a/GNUmakefile b/GNUmakefile
index 5b7bc5d..4f55823 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -44,12 +44,41 @@ ifeq ($(origin LD),default)
     LD = $(CXX)
 endif
 
+# Use abi-compliance-checker to compare the ABI (application binary
+# interface) of 2 releases. Changing the ABI does not necessarily change
+# the API (application programming interface). Rearranging a struct or
+# changing a by-value argument from int64 to int doesn't change the
+# API--no source code changes are required, just a recompile.
+#
+# if structs or routines are changed or removed:
+#     bump major version and reset minor, revision = 0;
+# else if structs or routines are added:
+#     bump minor version and reset revision = 0;
+# else (e.g., bug fixes):
+#     bump revision
+#
+# soversion is major ABI version.
+abi_version = 1.0.0
+soversion = ${word 1, ${subst ., ,${abi_version}}}
+
+#-------------------------------------------------------------------------------
 # auto-detect OS
 # $OSTYPE may not be exported from the shell, so echo it
 ostype := $(shell echo $${OSTYPE})
 ifneq ($(findstring darwin, $(ostype)),)
     # MacOS is darwin
     macos = 1
+    # MacOS needs shared library's path set, and shared library version.
+    ldflags_shared = -install_name @rpath/${notdir $@} \
+                     -current_version ${abi_version} \
+                     -compatibility_version ${soversion}
+    so = dylib
+    so2 = .dylib
+    # on macOS, .dylib comes after version: libfoo.4.dylib
+else
+    so = so
+    so1 = .so
+    # on Linux, .so comes before version: libfoo.so.4
 endif
 
 #-------------------------------------------------------------------------------
@@ -57,19 +86,11 @@ endif
 ifneq ($(static),1)
     CXXFLAGS += -fPIC
     LDFLAGS  += -fPIC
-    lib_ext = so
+    lib_ext = ${so}
 else
     lib_ext = a
 endif
 
-#-------------------------------------------------------------------------------
-# MacOS needs shared library's path set
-ifeq ($(macos),1)
-    install_name = -install_name @rpath/$(notdir $@)
-else
-    install_name =
-endif
-
 #-------------------------------------------------------------------------------
 # Files
 
@@ -124,7 +145,7 @@ install: lib
 	mkdir -p $(DESTDIR)$(prefix)/include
 	mkdir -p $(DESTDIR)$(prefix)/lib$(LIB_SUFFIX)
 	cp $(headers) $(DESTDIR)$(prefix)/include
-	cp $(lib) $(DESTDIR)$(prefix)/lib$(LIB_SUFFIX)
+	cp -av libtestsweeper.* $(DESTDIR)$(prefix)/lib$(LIB_SUFFIX)
 
 uninstall:
 	$(RM) $(addprefix $(DESTDIR)$(prefix)/include/, $(headers))
@@ -136,12 +157,21 @@ $(lib_obj) $(tester_obj): make.inc
 
 #-------------------------------------------------------------------------------
 # TestSweeper library
-lib_a  = libtestsweeper.a
-lib_so = libtestsweeper.so
-lib    = libtestsweeper.$(lib_ext)
+lib_name = libtestsweeper
+lib_a  = ${lib_name}.a
+lib_so = ${lib_name}.${so}
+lib    = ${lib_name}.${lib_ext}
+
+# libfoo.so.3 or libfoo.3.dylib
+lib_so_abi_version = ${lib_name}${so1}.${abi_version}${so2}
+lib_so_soversion   = ${lib_name}${so1}.${soversion}${so2}
+
+${lib_so_abi_version}: ${lib_obj}
+	${LD} ${LDFLAGS} -shared ${ldflags_shared} ${lib_obj} ${LIBS} -o $@
 
-$(lib_so): $(lib_obj)
-	$(LD) $(LDFLAGS) -shared $(install_name) $(lib_obj) $(LIBS) -o $@
+${lib_so}: | ${lib_so_abi_version}
+	ln -fs ${lib_so_abi_version} ${lib_so}
+	ln -fs ${lib_so_abi_version} ${lib_so_soversion}
 
 $(lib_a): $(lib_obj)
 	$(RM) $@
@@ -217,10 +247,15 @@ echo:
 	@echo "static        = '$(static)'"
 	@echo "id            = '$(id)'"
 	@echo "last_id       = '$(last_id)'"
+	@echo "abi_version   = '${abi_version}'"
+	@echo "soversion     = '${soversion}'"
 	@echo
+	@echo "lib_name      = ${lib_name}"
 	@echo "lib_a         = $(lib_a)"
 	@echo "lib_so        = $(lib_so)"
 	@echo "lib           = $(lib)"
+	@echo "lib_so_abi_version = ${lib_so_abi_version}"
+	@echo "lib_so_soversion   = ${lib_so_soversion}"
 	@echo
 	@echo "lib_src       = $(lib_src)"
 	@echo
@@ -240,6 +275,7 @@ echo:
 	@echo "LD            = $(LD)"
 	@echo "LDFLAGS       = $(LDFLAGS)"
 	@echo "LIBS          = $(LIBS)"
+	@echo "ldflags_shared = ${ldflags_shared}"
 	@echo
 	@echo "TEST_LDFLAGS  = $(TEST_LDFLAGS)"
 	@echo "TEST_LIBS     = $(TEST_LIBS)"
diff --git a/GNUmakefile.subdir b/GNUmakefile.subdir
new file mode 100644
index 0000000..6f00966
--- /dev/null
+++ b/GNUmakefile.subdir
@@ -0,0 +1,50 @@
+# Copyright (c) 2017-2023, University of Tennessee. All rights reserved.
+# SPDX-License-Identifier: BSD-3-Clause
+# This program is free software: you can redistribute it and/or modify it under
+# the terms of the BSD 3-Clause license. See the accompanying LICENSE file.
+#
+# Subdirectories include this makefile to forward rules to the top level makefile.
+# Define ${top} for where the top level is.
+# Example: src/GNUmakefile:
+#     top = ..
+#     include ${top}/GNUmakefile.subdir
+
+.SUFFIXES:
+
+pwd     := ${shell pwd}
+abs_top := ${abspath ${top}}
+cdir    := ${subst ${abs_top}/,,${pwd}}
+
+.DEFAULT_GOAL: ${cdir}
+.PHONY: ${cdir}
+
+${cdir}:
+	cd ${top} && ${MAKE} $@
+
+DONT_FORWARD += echo_sub
+echo_sub:
+	@echo "pwd     ${pwd}"
+	@echo "abs_top ${abs_top}"
+	@echo "cdir    ${cdir}"
+
+# ------------------------------------------------------------------------------
+ifneq (${MAKECMDGOALS},)
+
+# If arguments are given, presumably files like test.o, forward them to top
+# with cdir prefix.
+# All files are forwarded as one rule, based on first; rest are quietly ignored.
+goals   := ${filter-out echo ${DONT_FORWARD}, ${MAKECMDGOALS}}
+forward := ${addprefix ${cdir}/, ${goals}}
+first   := ${firstword ${goals}}
+rest    := ${wordlist 2, ${words ${goals}}, ${goals}}
+
+${first}: force
+	cd ${top} && ${MAKE} ${forward}
+
+${rest}: force
+	@echo > /dev/null
+
+endif
+# ------------------------------------------------------------------------------
+
+force: ;
diff --git a/test/GNUmakefile b/test/GNUmakefile
new file mode 100644
index 0000000..042d0c6
--- /dev/null
+++ b/test/GNUmakefile
@@ -0,0 +1,2 @@
+top = ..
+include ${top}/GNUmakefile.subdir
diff --git a/testsweeper.hh b/testsweeper.hh
index 081ecd5..d073fb3 100644
--- a/testsweeper.hh
+++ b/testsweeper.hh
@@ -421,13 +421,6 @@ public:
         values_[ index_ ] = value;
     }
 
-    [[deprecated( "use param() instead of param.value(). To be removed 2024-03." )]]
-    T& value()
-    {
-        used_ = true;
-        return values_[ index_ ];
-    }
-
     void set_default( const T& default_value )
     {
         default_value_ = default_value;