Tests of cross compiling:

g++ library for gfortran

Author: hpjeonGIT

Diff for: cross_compile.md

@@ -0,0 +1,66 @@
+##
+- Ref:https://stackoverflow.com/questions/47957255/cmake-linking-fortran-against-static-c-library-fails
+- ex.c
+```
+int get_key()
+{
+    return 10000;
+}
+```
+- ex1.c
+```
+int get_key1()
+{
+    return 10001;
+}
+```
+- ex.f08
+```
+program main
+    implicit none
+
+    interface
+        function get_key() bind(c, name='get_key')
+            use iso_c_binding
+            integer(kind=c_int) :: get_key
+        end function get_key
+    end interface
+
+    write(*, '(i0)') get_key()
+end program main
+```
+- For c + fortran
+```
+gcc -c ex.c
+gcc -c ex1.c
+ar cr libex.a ex.o ex1.o
+gfortran fex.f08 -L. -lex
+```
+- For C++ + fortran
+```
+g++ -c ex.c
+g++ -c ex1.c
+ar cr libex.a ex.o ex1.o
+gfortran fex.f08 -L. -lex
+```
+- This yields `undefined reference to 'get_key'`
+- Add `extern "C"` to ex.c and ex1.c. Then repeat the process
+```
+extern "C" int get_key()
+{
+    return 10000;
+}
+```
+- If `extern "C"` is not used, then use -static with g++ command
+```
+g++ -static -c ex.c
+g++ -static -c ex1.c
+ar cr libex.a ex.o ex1.o
+gfortran fex.f08 -L. -lex
+```
+
+# In cmake, adjusting archive options
+SET(CMAKE_C_ARCHIVE_CREATE "<CMAKE_AR> <LINK_FLAGS> cr <TARGET> <OBJECTS>")
+SET(CMAKE_C_ARCHIVE_APPEND "<CMAKE_AR> <LINK_FLAGS> r <TARGET> <OBJECTS>")
+SET(CMAKE_CXX_ARCHIVE_CREATE "<CMAKE_AR> <LINK_FLAGS> cr <TARGET> <OBJECTS>")
+SET(CMAKE_CXX_ARCHIVE_APPEND "<CMAKE_AR> <LINK_FLAGS> r <TARGET> <OBJECTS>")