add predefined macro PROJECT_ROOT

example_packages/preprocess_project_root/app/main.f90

@@ -0,0 +1,18 @@
+!> from https://stackoverflow.com/a/46342008
+#ifdef __GFORTRAN__
+# define STRINGIFY_START(X) "&
+# define STRINGIFY_END(X) &X"
+#else /* default stringification */
+# define STRINGIFY_(X) #X
+# define STRINGIFY_START(X) &
+# define STRINGIFY_END(X) STRINGIFY_(X)
+#endif
+
+program preprocess_project_root
+    implicit none
+    character(len=*), parameter :: root = STRINGIFY_START(PROJECT_ROOT)
+                                          STRINGIFY_END(PROJECT_ROOT)
+
+    print *, root
+
+end program preprocess_project_root

example_packages/preprocess_project_root/fpm.toml

@@ -0,0 +1,4 @@
+name = "preprocess_project_root"
+
+[preprocess]
+[preprocess.cpp]

example_packages/preprocess_project_root/readme.md

@@ -0,0 +1,11 @@
+# Preprocess_project_root
+If you have activated preprocess option in `fpm.toml` via
+```toml
+[preprocess]
+[preprocess.cpp]
+```
+some predefined variables will be added to macro definitions. In this example, the macro `PROJECT_ROOT=\path\to\project` is passed to the Fortran source code.
+
+In practice, the value of `PROJECT_ROOT`, which is literally `\path\to\project`, is simply pasted without quoted as a valid string literal. So the code using `PROJECT_ROOT` macro may not compile. You can refer to [Stringify macro with GNU gfortran](https://stackoverflow.com/a/46342008) to solve this problem.
+
+

src/fpm_compiler.f90

@@ -42,6 +42,7 @@ module fpm_compiler
     & getline, run
 use fpm_strings, only: split, string_cat, string_t, str_ends_with, str_begins_with_str
 use fpm_manifest, only : package_config_t
+use fpm_os, only: get_current_directory
 use fpm_error, only: error_t
 implicit none
 public :: compiler_t, new_compiler, archiver_t, new_archiver, get_macros
@@ -485,6 +486,23 @@ function get_macros(id, macros_list, version) result(macros)
 
     end do
 
+    !> add predefined variables
+    call set_predefined_macro()
+
+contains
+
+    !> append predefined variable as macro definition
+    subroutine set_predefined_macro()
+        type(error_t), allocatable :: error
+        character(len=:), allocatable :: project_root
+
+        !> add PROJECT_ROOT
+        call get_current_directory(project_root, error)
+        if (.not. allocated(project_root)) project_root = ''
+        macros = macros//' '//macro_definition_symbol//"PROJECT_ROOT="//project_root
+
+    end subroutine set_predefined_macro
+
 end function get_macros
 
 function get_include_flag(self, path) result(flags)