add nullptr c_associated example

Author: scivision

CMakeLists.txt

@@ -47,6 +47,7 @@ foreach(a IN ITEMS
   array bool error exception
   iterator
   glibc malloc
+  nullptr
   opaque pointer poly_function poly_type real struct
   sleep string submodule
   time vector)

test/nullptr/CMakeLists.txt

@@ -0,0 +1,14 @@
+set_property(DIRECTORY PROPERTY LABELS nullptr)
+
+add_library(c_nullptr OBJECT lib.c)
+target_include_directories(c_nullptr PRIVATE .)
+add_library(cpp_nullptr OBJECT lib.cpp)
+target_include_directories(cpp_nullptr PRIVATE .)
+
+add_executable(C_Fortran_nullptr main.f90 $<TARGET_OBJECTS:c_nullptr>)
+add_test(NAME C_Nullptr COMMAND C_Fortran_nullptr)
+
+add_executable(Cpp_Fortran_nullptr main.f90 $<TARGET_OBJECTS:cpp_nullptr>)
+set_property(TARGET Cpp_Fortran_nullptr PROPERTY LINKER_LANGUAGE ${linker_lang})
+
+add_test(NAME Cpp_Nullptr COMMAND Cpp_Fortran_nullptr)

test/nullptr/Readme.md

@@ -0,0 +1,8 @@
+# NULL / nullptr between C, C++, and Fortran
+
+Unlike in C / C++, do not check equality with C_NULL_PTR in Fortran.
+Instead, by definition the
+[c_associated()](https://fortranwiki.org/fortran/show/c_associated)
+logical function returns ".false." if the C_PTR is C_NULL_PTR, and ".true." otherwise.
+See [strptime.f90](./strptime.f90)
+for an example of checking in Fortran for a C null pointer returned from a function.

test/nullptr/lib.c

@@ -0,0 +1,6 @@
+#include <nullptr.h>
+#include <stddef.h>
+
+const char* nullchar(bool b) {
+  return b ? NULL : "hello";
+}

test/nullptr/lib.cpp

@@ -0,0 +1,5 @@
+#include <nullptr.h>
+
+const char* nullchar(bool b) {
+  return b ? nullptr : "hello";
+}

test/nullptr/main.f90

@@ -0,0 +1,43 @@
+program main
+
+use, intrinsic :: iso_c_binding
+
+implicit none
+
+interface
+  function nullchar(b) bind(C)
+    import
+    logical(c_bool), value :: b
+    type(C_PTR) :: nullchar
+  end function
+end interface
+
+integer, parameter :: n = 5
+character(n) :: s
+
+logical(C_BOOL), parameter :: t = .true., f = .false.
+
+type(c_ptr) :: ptr
+
+if (.not. c_associated(nullchar(f))) error stop "nullptr should not be detected"
+
+ptr = nullchar(t)
+if (c_associated(ptr)) error stop "nullptr not detected"
+
+! for simplicity, assume we know strlen is 5.
+block
+  character(kind=c_char), pointer :: c_string(:)
+  integer :: i
+
+  s = "" !< ensure s has no garbage characters
+
+  ptr = nullchar(f)
+  call c_f_pointer(ptr, c_string, [n])
+  do i = 1, n
+    s(i:i) = c_string(i)
+  end do
+end block
+
+print '(a)', s
+
+end program

test/nullptr/nullptr.h

@@ -0,0 +1,12 @@
+
+#ifdef __cplusplus
+extern "C" {
+#else
+#include <stdbool.h>
+#endif
+
+const char* nullchar(bool b);
+
+#ifdef __cplusplus
+}
+#endif