add examples for dynamic linking / loading

one example by doing it manually with an ugly Makefile,
another one by using autotools.

Signed-off-by: Manuel Traut <manut@linutronix.de>

6 files changed, 88 insertions, 0 deletions

diff --git a/schulung_tools/linking/hello/Makefile b/schulung_tools/linking/hello/Makefile
new file mode 100644
index 0000000..a5b4c26
--- /dev/null
+++ b/schulung_tools/linking/hello/Makefile
@@ -0,0 +1,33 @@
+hello: hello.c libfunc.so
+	$(CROSS_COMPILE)gcc -g -ohello hello.c -L. -lfunc
+
+hello2: hello.c libfunc.a
+	rm -f libfunc.so
+	$(CROSS_COMPILE)gcc -g -ohello2 hello.c -L. -lfunc
+
+libfunc.so: libfunc.so.0.0.1
+	ln -s libfunc.so.0.0.1 libfunc.so
+
+libfunc.so.0.0.1: func1.o func2.o
+	$(CROSS_COMPILE)gcc -shared -olibfunc.so.0.0.1 -Wl,-soname,libfunc.so.0 func1.o func2.o
+
+libfunc.a: func1.o func2.o
+	$(CROSS_COMPILE)ar cr libfunc.a func1.o func2.o
+	$(CROSS_COMPILE)ranlib libfunc.a
+
+func1.o: func1.c
+	$(CROSS_COMPILE)gcc -fPIC -g -c func1.c
+
+func2.o: func2.c
+	$(CROSS_COMPILE)gcc -fPIC -g -c func2.c
+
+install_lib: libfunc.so.0.0.1
+	echo "/opt/acme/lib" > /etc/ld.so.conf.d/acme.conf
+	mkdir -p /opt/acme/lib
+	cp libfunc.so.0.0.1 /opt/acme/lib
+	ldconfig
+
+clean:
+	rm -f hello hello2 *.o lib*
+
+.PHONY: clean
diff --git a/schulung_tools/linking/hello/func1.c b/schulung_tools/linking/hello/func1.c
new file mode 100644
index 0000000..ab006bd
--- /dev/null
+++ b/schulung_tools/linking/hello/func1.c
@@ -0,0 +1,6 @@
+#include <stdio.h>
+
+void func1(void)
+{
+	printf("Hello from %s!\n", __func__);
+}
diff --git a/schulung_tools/linking/hello/func2.c b/schulung_tools/linking/hello/func2.c
new file mode 100644
index 0000000..9dc05ab
--- /dev/null
+++ b/schulung_tools/linking/hello/func2.c
@@ -0,0 +1,6 @@
+#include <stdio.h>
+
+void func2(void)
+{
+	printf("Hello from %s!\n", __func__);
+}
diff --git a/schulung_tools/linking/hello/hello.c b/schulung_tools/linking/hello/hello.c
new file mode 100644
index 0000000..82b8d53
--- /dev/null
+++ b/schulung_tools/linking/hello/hello.c
@@ -0,0 +1,12 @@
+#include <stdio.h>
+
+void func1(void);
+void func2(void);
+
+int main(void)
+{
+	func1();
+	printf("Hello, world!\n");
+	func2();
+	return 0;
+}
diff --git a/schulung_tools/linking/hello/objdump.txt b/schulung_tools/linking/hello/objdump.txt
new file mode 100644
index 0000000..9c77720
--- /dev/null
+++ b/schulung_tools/linking/hello/objdump.txt
@@ -0,0 +1,11 @@
+# identify dependencies
+objdump -x some_elf_file | grep NEEDED
+
+# identify SONAME used by dynamic loader
+objdump -x some_lib | grep SONAME
+
+# identify runtime library search paths
+objdump -x some_elf_file | grep PATH
+
+# identify path of dynamic loader (search for "interp")
+objdump -s some_elf_file | less
diff --git a/schulung_tools/linking/hello/shared_library.txt b/schulung_tools/linking/hello/shared_library.txt
new file mode 100644
index 0000000..1dd0f9f
--- /dev/null
+++ b/schulung_tools/linking/hello/shared_library.txt
@@ -0,0 +1,20 @@
+# build shared library
+gcc -fPIC -shared -Wl,-soname,libhello.so.1 -olibhello.so.1.0.3 libhello.c
+
+# install shared library
+sudo mkdir -p /opt/myfirma/lib
+sudo cp libhello.so.1.0.3 /opt/myfirma/lib
+
+# add custom path to dynamic loader search path
+echo "/opt/myfirma/lib" | sudo tee /etc/ld.so.conf.d/myfirma.conf
+
+# refresh dynamic loader cache
+sudo ldconfig
+
+# verify custom library is in cache
+/sbin/ldconfig -p | grep libhello.so.1
+
+# compile program using custom library
+ln -s /opt/myfirma/lib/libhello.so.1.0.3 libhello.so
+gcc -L. -ohello hello.c -lhello
+rm libhello.so