Redlance Compat Shims

Backport library that supplies missing-on-old-macOS symbols and ObjC selectors, so applications built against modern SDKs can still load and run on legacy systems (Mavericks 10.9 onward).

Pairs well with macports-legacy-support which handles a different layer (POSIX/libc/Darwin headers and helpers). RCS specifically covers:

• Modern ObjC ABI fast-paths (objc_alloc_init, objc_opt_new, etc.)
• Compiler-emitted runtime helpers (__availability_version_check)
• libc++ low-level ulock primitives (__ulock_wait2, __ulock_wake)
• Tracing/logging APIs (os_log_create, os_signpost_*)
• Quality-of-Service APIs (pthread_get_qos_class_np, etc.)
• C++17 typeinfo/vtable for std::bad_variant_access, std::bad_optional_access
• ObjC selector backports via +load categories

Build

make                                          # x86_64, deployment 10.9
make ARCHS="x86_64 arm64"                     # universal
make DEPLOYMENT_TARGET=10.7                   # bump back further
make CC=/opt/local/bin/clang-mp-19            # MacPorts clang
make SHARED=no                                # only static archive
make print-config                             # show resolved variables
make test                                     # run smoke tests

Outputs land in build/: libRedlanceCompatShims.a and libRedlanceCompatShims.dylib.

Install

make install PREFIX=/opt/local
make install PREFIX=/opt/local DESTDIR=/tmp/staging  # for packagers

Architecture

Every shim is gated by an #if RL_RCS_DEPLOYMENT < <version> guard. The RL_RCS_DEPLOYMENT macro is defined by the Makefile from the DEPLOYMENT_TARGET variable in standard form (e.g. 10.9 → 100900, 10.14.4 → 101404). On systems where the symbol exists natively, no code is emitted.

Sources:

src/objc-abi-shims.m         — objc_alloc, objc_alloc_init, objc_opt_new, ...
src/availability-check.c     — __availability_version_check via sysctl
src/ulock.c                  — __ulock_wait2, __ulock_wake (no-op fallback)
src/filesystem-shims.c       — renameatx_np
src/os-log.c                 — os_log_create, os_signpost_* (no-op)
src/qos.c                    — pthread_*_qos_*_np (no-op, return DEFAULT)
src/cxx17-polyfill.cpp       — std::bad_variant_access, std::bad_optional_access
src/NSToolbarItemGroup+Compat.m — example +load category backport

Coverage cheat-sheet

Symbol	Native since	Source
objc_alloc	10.10	objc-abi-shims.m
objc_allocWithZone	10.10	objc-abi-shims.m
objc_opt_new	10.10	objc-abi-shims.m
objc_unsafeClaimAutoreleasedReturnValue	10.11	objc-abi-shims.m
objc_alloc_init	10.14.4	objc-abi-shims.m
qos_class_self	10.10	qos.c
pthread_get_qos_class_np	10.10	qos.c
pthread_override_qos_class_*_np	10.10	qos.c
os_log_create	10.12	os-log.c
__ulock_wake	10.12	ulock.c
renameatx_np	10.12	filesystem-shims.c
os_signpost_*	10.14	os-log.c
std::bad_variant_access	10.14 libc++	cxx17-polyfill.cpp
std::bad_optional_access	10.14 libc++	cxx17-polyfill.cpp
__availability_version_check	10.15	availability-check.c
__ulock_wait2	11.0	ulock.c

Linker integration

For the ObjC ABI / C / C++ shims (anything with C-callable symbols), normal static linkage is enough. The linker picks them up through unresolved references just like any other library.

For ObjC +load categories (currently NSToolbarItemGroup+Compat.m), you MUST use -force_load because there are no C symbol references that could pull the .o in:

clang myapp.m -framework AppKit -framework Foundation \
  -Wl,-force_load,/opt/local/lib/libRedlanceCompatShims.a \
  -o myapp

In Kotlin/Native (Gradle):

binaries.all {
    linkerOpts(
        "-Wl,-force_load,/opt/local/lib/libRedlanceCompatShims.a",
    )
}

Adding a new shim

For a missing C function: add to an existing .c file (or create one), wrap in #if RL_RCS_DEPLOYMENT < <version> based on when it appeared natively, mark exported via __attribute__((visibility("default"))).

For a missing ObjC selector on an existing class: add a category file under src/. Inside +load, do class_respondsToSelector check, then class_addMethod with the right type encoding.

For a missing class entirely: harder — see project docs.

License

MIT.