From ba9134e3d0a4c1fc4afd62a6a368c59f026d68f5 Mon Sep 17 00:00:00 2001
From: Andre Noll <maan@tuebingen.mpg.de>
Date: Sun, 25 Aug 2024 16:30:07 +0200
Subject: [PATCH] Support cross build

The build system was not designed for cross compiling, so this
never worked. One problem is that we run both lopsubgen-stage1 and
lopsubgen during the build, so these executables must be built for
both the build and the host architecture.

This patch enables cross building by setting CC and CC_FOR_BUILD
like this:

	make CC=/usr/bin/arm-linux-gnueabihf-gcc-11 CC_FOR_BUILD=cc

Object files and executables for the build system go to build
directory while files for the host system stay in the top level
directory as before.
---
 Makefile | 44 ++++++++++++++++++++++++++++----------------
 README   |  4 ++++
 2 files changed, 32 insertions(+), 16 deletions(-)

diff --git a/Makefile b/Makefile
index 6fbca9c..5418ef9 100644
--- a/Makefile
+++ b/Makefile
@@ -6,6 +6,9 @@ MAKEFLAGS += -Rr
 ifeq ("$(origin CC)", "default")
         CC := cc
 endif
+ifeq ($(CC_FOR_BUILD),)
+	CC_FOR_BUILD := $(CC)
+endif
 .ONESHELL:
 .SHELLFLAGS := -ec
 
@@ -48,7 +51,9 @@ ifdef SOURCE_DATE_EPOCH
 else
 	DATE := $(shell date '$(DATE_FMT)')
 endif
-GIT_VERSION := $(shell ./version-gen.sh)
+
+B := build
+GIT_VERSION := $(shell mkdir -p $(B) && ./version-gen.sh)
 PLAIN_VERSION := $(firstword $(subst -, , $(GIT_VERSION)))
 MAJOR_VERSION := $(firstword $(subst ., , $(PLAIN_VERSION)))
 SONAME := liblopsub.so.$(MAJOR_VERSION)
@@ -56,7 +61,7 @@ REALNAME := liblopsub.so.$(PLAIN_VERSION)
 LINKERNAME:=liblopsub.so
 
 m4_man_pages := lopsub-suite.5.gz lopsub.7.gz
-all := $(m4_man_pages) $(REALNAME) lopsubgen lopsubgen.1.gz \
+all := $(m4_man_pages) $(REALNAME) lopsubgen $(B)/lopsubgen lopsubgen.1.gz \
 	lopsubex lopsubex.1.gz
 all: $(all)
 
@@ -80,20 +85,20 @@ $(m4_man_pages): %.gz: %.m4 version.c
 	$(LEX) -o $@ $<
 
 # lopsubgen
-lopsubgen.lsg.c lopsubgen.lsg.h: lopsubgen.suite lopsubgen-stage1 \
+lopsubgen.lsg.c lopsubgen.lsg.h: lopsubgen.suite $(B)/lopsubgen-stage1 \
 		lopsub-internal.h
-	./lopsubgen-stage1 < $<
+	$(B)/lopsubgen-stage1 < $<
 %.lsg.c: %.suite lopsubgen
-	./lopsubgen --gen-c < $<
+	$(B)/./lopsubgen --gen-c < $<
 %.lsg.h: %.suite lopsubgen
-	./lopsubgen --gen-header < $<
+	$(B)/./lopsubgen --gen-header < $<
 %.1.gz: %.suite lopsubgen
-	./lopsubgen --gen-man=${@:.gz=} --version-string $(GIT_VERSION) < $<
+	$(B)/./lopsubgen --gen-man=${@:.gz=} --version-string $(GIT_VERSION) < $<
 	$(GZIP) ${@:.gz=}
 
-# compiling
-lsg1_objs := lopsubgen.o lsg1.o version.o
+lsg1_objs := $(addprefix $(B)/, lopsubgen.o lsg1.o version.o)
 lsg_objs := lopsubgen.o lsg.o lopsubgen.lsg.o lopsub.o version.o
+blsg_objs := $(addprefix $(B)/, $(lsg_objs))
 liblopsub_objs := config_file.o lopsub.o version.o
 lopsubex_objs := lopsubex.o lopsubex.lsg.o $(liblopsub_objs)
 
@@ -102,18 +107,24 @@ STRICT_CFLAGS := -Wall
 STRICT_CFLAGS += -Werror-implicit-function-declaration
 
 $(lsg_objs) $(liblopsub_objs) $(lopsubex_objs): %.o: %.c
+$(blsg_objs): $(B)/%.o: %.c
 lopsubgen.o config_file.o:
 	$(CC) $(CPPFLAGS) $(LLS_CFLAGS) $(CFLAGS) -c -o $@ ${@:.o=.c}
-lsg1.o: lsg.c lsg.h
-	$(CC) $(CPPFLAGS) $(LLS_CFLAGS) $(STRICT_CFLAGS) $(CFLAGS) -DSTAGE1 -c -o $@ $<
+$(B)/lopsubgen.o: lopsubgen.c
+	$(CC_FOR_BUILD) $(CPPFLAGS) $(LLS_CFLAGS) $(CFLAGS) -c -o $@ $<
+$(B)/lsg1.o: lsg.c lsg.h
+	$(CC_FOR_BUILD) $(CPPFLAGS) $(LLS_CFLAGS) $(STRICT_CFLAGS) $(CFLAGS) -DSTAGE1 -c -o $@ $<
 %.o: %.c
 	$(CC) -I. $(CPPFLAGS) $(LLS_CFLAGS) $(STRICT_CFLAGS) $(CFLAGS) -c -o $@ $<
-
-# linking
-lopsubgen-stage1: $(lsg1_objs)
-	$(CC) -Wall -g $(lsg1_objs) -o $@
+$(B)/%.o: %.c
+	$(CC_FOR_BUILD) -I. $(CPPFLAGS) $(LLS_CFLAGS) $(STRICT_CFLAGS) $(CFLAGS) -c -o $@ $<
+$(B)/lopsubgen-stage1: $(lsg1_objs)
+	$(CC_FOR_BUILD) -Wall -g $(lsg1_objs) -o $@
 lopsubgen: $(lsg_objs)
 	$(CC) -Wall -g $(LDFLAGS) -o $@ $(lsg_objs)
+$(B)/lopsubgen: $(blsg_objs)
+	$(CC_FOR_BUILD) -Wall -g $(LDFLAGS) -o $@ $(blsg_objs)
+
 $(REALNAME): $(liblopsub_objs)
 	$(CC) --shared -Wl,-soname,liblopsub.so.$(MAJOR_VERSION) \
 		$(LDFLAGS) -o $@ $^
@@ -151,8 +162,9 @@ install: $(all)
 	$(INSTALL) -m 644 lopsub.7.gz $(DESTDIR)$(PREFIX)/share/man/man7
 
 clean:
-	$(RM) $(all) $(html) *.o *.man
+	$(RM) $(all) $(html) *.o *.man $(B)/*.o
 distclean: clean
 	$(RM) *.lsg.c *.lsg.h lopsubgen.c config_file.c lopsubgen-stage1 \
 		lopsub.h lopsub.7 lopsub-suite.5 version.c
+	$(RM) -r $(B)
 -include Makefile.local
diff --git a/README b/README
index 2f0bc51..d9f67ca 100644
--- a/README
+++ b/README
@@ -34,3 +34,7 @@ compile the source files of your application and -L/somewhere/else/lib
 for linking. Alternatively, don't run make install at all and specify
 the path to the top level directory of the repository for both -I
 and -L.
+
+To cross-compile the package, run a command like
+
+	make CC=/usr/bin/arm-linux-gnueabihf-gcc-11 CC_FOR_BUILD=cc
-- 
2.39.5