Relocatable ocamlbuild

Makefile

@@ -31,7 +31,7 @@ endif
 
 CP        ?= cp
 OCB_COMPFLAGS ?= -w @14@29 -w +L -w +R -w +Z -I src -I plugin-lib -I bin -I +unix -safe-string -bin-annot -strict-sequence
-OCB_LINKFLAGS ?= -I +unix -I src
+OCB_LINKFLAGS ?= -I +unix -I src $(OCB_EXTRA_LINKFLAGS)
 
 PACK_CMO= $(addprefix src/,\
   const.cmo \
@@ -179,7 +179,7 @@ configure:
 	$(MAKE) -f configure.make all
 
 # proxy rule for rebuilding configuration files directly from the main Makefile
-Makefile.config src/ocamlbuild_config.ml:
+Makefile.config src/ocamlbuild_config.ml: src/ocamlbuild_config.ml.in
 	$(MAKE) -f configure.make $@
 
 clean::

configure.make

@@ -6,11 +6,11 @@
 # from the main Makefile, but this made it not robust to addition of
 # new variables to this ocaml/Makefile.config that we do not control.
 
-include $(shell ocamlc -where)/Makefile.config
+OCAML_LIBDIR = $(shell ocamlc -where)
+include $(OCAML_LIBDIR)/Makefile.config
 
 OCAML_PREFIX = $(PREFIX)
 OCAML_BINDIR = $(BINDIR)
-OCAML_LIBDIR = $(LIBDIR)
 OCAML_MANDIR = $(MANDIR)
 
 # If you want to affect ocamlbuild's configuration by passing variable
@@ -29,6 +29,35 @@ OCAMLBUILD_MANDIR ?= \
   $(or $(shell opam config var man 2>/dev/null),\
        $(OCAML_MANDIR))
 
+# OCAMLBUILD_RELOCATABLE is true if:
+#   1. The compiler is Relocatable OCaml
+#   2. The ocamlbuild will be installed in the same directory as the compiler
+#   3. OCAMLBUILD_LIBDIR is an explicit relative path (i.e. begins ./ or ../)
+# OCAMLBUILD_RELOCATABLE is empty if any of these things are not true.
+OCAMLBUILD_LIBDIR_RELATIVE = $(filter . .. ./% ../%, $(OCAMLBUILD_LIBDIR))
+OCAML_RELOCATABLE = \
+  $(shell ocamlc -config-var standard_library_relative 2>/dev/null)
+OCAMLC_BIN_DIR = $(abspath $(dir $(shell command -v ocamlc)))
+# On Windows, OCAMLC_BIN_DIR will be a Cygwin-style path but OCAMLBUILD_BINDIR
+# is a native path. The requirement is that the OCAMLBUILD_BINDIR needs to be
+# the same as the compiler - that means it _must_ already exist, so it gets
+# canonicalised using "poor man's realpath" by doing cd+pwd.
+OCAMLBUILD_BINDIR_RESOLVED = $(shell cd '$(OCAMLBUILD_BINDIR)' 2>/dev/null ; pwd)
+OCAMLBUILD_RELOCATABLE := \
+  $(if $(OCAMLBUILD_LIBDIR_RELATIVE),$\
+    $(if $(OCAML_RELOCATABLE),$\
+      $(if $(filter $(abspath $(OCAMLBUILD_BINDIR_RESOLVED)),$(OCAMLC_BIN_DIR)),true)))
+
+# If OCAMLBUILD_LIBDIR is an explicit relative path, but Relocatable ocamlbuild
+# cannot be built (see above), then OCAMLBUILD_LIBDIR_ACTUAL is the absolute
+# path calculated by concatenating OCAML_LIBDIR and OCAMLBUILD_LIBDIR. Otherwise
+# it is just OCAMLBUILD_LIBDIR.
+OCAMLBUILD_LIBDIR_ACTUAL := \
+  $(if $(OCAMLBUILD_RELOCATABLE),$(OCAMLBUILD_LIBDIR),$\
+    $(if $(OCAMLBUILD_LIBDIR_RELATIVE),$\
+      $(if $(filter .., $(OCAMLBUILD_LIBDIR)),$(dir $(OCAML_LIBDIR)),$\
+        $(abspath $(OCAML_LIBDIR)/$(OCAMLBUILD_LIBDIR)),$(OCAMLBUILD_LIBDIR))))
+
 # It is important to distinguish OCAML_LIBDIR, which points to the
 # directory of the ocaml compiler distribution, and OCAMLBUILD_LIBDIR,
 # which should be the general library directory of OCaml projects on
@@ -67,11 +96,6 @@ distclean:
 Makefile.config:
 	(echo "# This file was generated from configure.make"; \
 	echo ;\
-	echo "OCAML_PREFIX=$(OCAML_PREFIX)"; \
-	echo "OCAML_BINDIR=$(OCAML_BINDIR)"; \
-	echo "OCAML_LIBDIR=$(OCAML_LIBDIR)"; \
-	echo "OCAML_MANDIR=$(OCAML_MANDIR)"; \
-	echo ;\
 	echo "EXT_OBJ=$(EXT_OBJ)"; \
 	echo "EXT_ASM=$(EXT_ASM)"; \
 	echo "EXT_LIB=$(EXT_LIB)"; \
@@ -82,20 +106,45 @@ Makefile.config:
 	echo "OCAML_NATIVE_TOOLS=$(OCAML_NATIVE_TOOLS)"; \
 	echo "NATDYNLINK=$(NATDYNLINK)"; \
 	echo "SUPPORT_SHARED_LIBRARIES=$(SUPPORTS_SHARED_LIBRARIES)"; \
+	echo "OCB_EXTRA_LINKFLAGS=$(OCB_EXTRA_LINKFLAGS)"; \
 	echo ;\
 	echo "PREFIX=$(OCAMLBUILD_PREFIX)"; \
 	echo "BINDIR=$(OCAMLBUILD_BINDIR)"; \
-	echo "LIBDIR=$(OCAMLBUILD_LIBDIR)"; \
+	echo "LIBDIR=$(OCAMLBUILD_LIBDIR_ACTUAL)"; \
 	echo "MANDIR=$(OCAMLBUILD_MANDIR)"; \
 	) > $@
 
-src/ocamlbuild_config.ml:
+ifeq ($(OCAMLBUILD_RELOCATABLE), true)
+
+# For Relocatable ocamlbuild, just record the relative path specified for
+# OCAMLBUILD_LIBDIR and the current directory name (".") for BINDIR and the
+# extra code apppended from src/ocamlbuild_config.ml.in will process the correct
+# values at runtime.
+src/ocamlbuild_config.ml: BINDIR = .
+src/ocamlbuild_config.ml: OCAML_LIBDIR =
+src/ocamlbuild_config.ml: LIBDIR_ABS =
+
+OCB_EXTRA_LINKFLAGS = \
+  -set-runtime-default standard_library_default=$(OCAML_RELOCATABLE)
+
+else
+
+# For normal ocamlbuild, record the configured values.
+src/ocamlbuild_config.ml: BINDIR := $(OCAMLBUILD_BINDIR)
+src/ocamlbuild_config.ml: OCAML_LIBDIR := $(abspath $(OCAML_LIBDIR))
+src/ocamlbuild_config.ml: LIBDIR_ABS := $(abspath $(OCAMLBUILD_LIBDIR_ACTUAL))
+
+OCB_EXTRA_LINKFLAGS =
+
+endif
+
+src/ocamlbuild_config.ml: src/ocamlbuild_config.ml.in
 	(echo "(* This file was generated from ../configure.make *)"; \
 	echo ;\
-	echo 'let bindir = {|$(OCAMLBUILD_BINDIR)|}'; \
-	echo 'let libdir = {|$(OCAMLBUILD_LIBDIR)|}'; \
-	echo 'let ocaml_libdir = {|$(abspath $(OCAML_LIBDIR))|}'; \
-	echo 'let libdir_abs = {|$(abspath $(OCAMLBUILD_LIBDIR))|}'; \
+	echo 'let bindir = {|$(BINDIR)|}'; \
+	echo 'let libdir = {|$(OCAMLBUILD_LIBDIR_ACTUAL)|}'; \
+	echo 'let ocaml_libdir = {|$(OCAML_LIBDIR)|}'; \
+	echo 'let libdir_abs = {|$(LIBDIR_ABS)|}'; \
 	echo 'let ocaml_native = $(OCAML_NATIVE)'; \
 	echo 'let ocaml_native_tools = $(OCAML_NATIVE_TOOLS)'; \
 	echo 'let supports_shared_libraries = $(SUPPORTS_SHARED_LIBRARIES)';\
@@ -105,4 +154,5 @@ src/ocamlbuild_config.ml:
 	echo 'let ext_dll = "$(EXT_DLL)"'; \
 	echo 'let exe = "$(EXE)"'; \
 	echo 'let version = "$(shell ocaml scripts/cat.ml VERSION)"'; \
+	$(if $(OCAMLBUILD_RELOCATABLE),cat src/ocamlbuild_config.ml.in;) \
 	) > $@

ocamlbuild.opam

@@ -17,7 +17,7 @@ build: [
     "all"
     "OCAMLBUILD_PREFIX=%{prefix}%"
     "OCAMLBUILD_BINDIR=%{bin}%"
-    "OCAMLBUILD_LIBDIR=%{lib}%"
+    "OCAMLBUILD_LIBDIR=.."
     "OCAMLBUILD_MANDIR=%{man}%"
     "OCAML_NATIVE=%{ocaml:native}%"
     "OCAML_NATIVE_TOOLS=%{ocaml:native}%"

src/ocamlbuild_config.ml.in

@@ -0,0 +1,35 @@
+#2 "ocamlbuild_config.ml.in"
+(***********************************************************************)
+(*                                                                     *)
+(*                             ocamlbuild                              *)
+(*                                                                     *)
+(*                       David Allsopp, Tarides                        *)
+(*                                                                     *)
+(*  Copyright 2025 David Allsopp Ltd.  All rights reserved.  This      *)
+(*  file is distributed under the terms of the GNU Library General     *)
+(*  Public License, with the special exception on linking described    *)
+(*  in file ../LICENSE.                                                *)
+(*                                                                     *)
+(***********************************************************************)
+
+(* Compute the effective default for the Standard Library *)
+let ocaml_libdir, bindir =
+  let module Defs = struct
+    external default : unit -> string = "%standard_library_default"
+    external get : string -> string * string option = "caml_sys_get_stdlib_dirs"
+  end in
+  match Defs.get (Defs.default ()) with
+  | ocaml_libdir, Some bindir ->
+      ocaml_libdir, bindir
+  | ocaml_libdir, None ->
+      (* This code should in theory only be being executed on a relocatable
+         runtime *)
+      ocaml_libdir, Filename.dirname Sys.executable_name
+
+let libdir =
+  if libdir = ".." then
+    (* Cheeky special case, as it happens to match opam *)
+    Filename.dirname ocaml_libdir
+  else
+    (* General case *)
+    Filename.concat ocaml_libdir libdir