add type(string_type) version

Author: perazz

src/stdlib_kinds.fypp

@@ -4,10 +4,10 @@
 !> The specification of this module is available [here](../page/specs/stdlib_kinds.html).
 module stdlib_kinds
   use iso_fortran_env, only: int8, int16, int32, int64
-  use iso_c_binding, only: c_bool
+  use iso_c_binding, only: c_bool, c_char
   implicit none
   private
-  public :: sp, dp, xdp, qp, int8, int16, int32, int64, lk, c_bool
+  public :: sp, dp, xdp, qp, int8, int16, int32, int64, lk, c_bool, c_char
 
   !> Single precision real numbers
   integer, parameter :: sp = selected_real_kind(6)

src/stdlib_strings.fypp

@@ -7,8 +7,8 @@ module stdlib_strings
     use stdlib_ascii, only: whitespace
     use stdlib_string_type, only: string_type, char, verify, repeat, len, len_trim, move
     use stdlib_optval, only: optval
-    use stdlib_kinds, only: sp, dp, xdp, qp, int8, int16, int32, int64, lk, c_bool
-    use iso_c_binding, only: c_char, c_null_char
+    use stdlib_kinds, only: sp, dp, xdp, qp, int8, int16, int32, int64, lk, c_bool, c_char
+    use iso_c_binding, only: c_null_char
     implicit none
     private
 
@@ -45,6 +45,15 @@ module stdlib_strings
         #:endfor
     end interface to_string
 
+    !> Version: experimental
+    !>
+    !> Format or transfer other types as a string.
+    !> ([Specification](../page/specs/stdlib_strings.html#to_c_string))
+    interface to_c_string
+        module procedure to_c_string_from_char
+        module procedure to_c_string_from_string
+    end interface to_c_string
+
     !> Remove leading and trailing whitespace characters.
     !>
     !> Version: experimental
@@ -959,15 +968,30 @@ contains
     !> Convert a Fortran character string to a C character array
     !>
     !> Version: experimental    
-    pure function to_c_string(value) result(cstr)
+    pure function to_c_string_from_char(value) result(cstr)
        character(len=*), intent(in) :: value
        character(kind=c_char) :: cstr(len(value)+1)
-       integer :: i
-       do concurrent (i=1:len(value)) 
+       integer :: i,lv
+       lv = len(value)
+       do concurrent (i=1:lv) 
            cstr(i) = value(i:i)
        end do 
-       cstr(len(value)+1) = c_null_char
-    end function to_c_string
+       cstr(lv+1) = c_null_char
+    end function to_c_string_from_char
+    
+    !> Convert a Fortran string type to a C character array
+    !>
+    !> Version: experimental    
+    pure function to_c_string_from_string(value) result(cstr)
+       type(string_type), intent(in) :: value
+       character(kind=c_char) :: cstr(len(value)+1)
+       integer :: i,lv
+       lv = len(value)
+       do concurrent (i=1:lv) 
+           cstr(i) = char(value,pos=i)
+       end do 
+       cstr(lv+1) = c_null_char
+    end function to_c_string_from_string    
 
     !> Joins a list of strings with a separator (default: space).
     !> Returns a new string