------------------------------------------------------------------------------
--                                                                          --
--                         GNAT COMPILER COMPONENTS                         --
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--                 I N T E R F A C E S . C . S T R I N G S                  --
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------------------------------------------------------------------------------

--  This package declares types and subprograms that allow the allocation,
--  reference, update and deallocation of C-style strings, as defined by
--  ARM B.3.1.

--  Preconditions in this unit are meant for analysis only, not for run-time
--  checking, so that the expected exceptions are raised. This is enforced by
--  setting the corresponding assertion policy to Ignore. These preconditions
--  protect from Constraint_Error, Dereference_Error and Update_Error, but not
--  from Storage_Error.

pragma Assertion_Policy (Pre => Ignore);

with System.Parameters;

package Interfaces.C.Strings with
  SPARK_Mode     => On,
  Abstract_State => (C_Memory, (C_Addresses with Synchronous)),
  Initializes    => (C_Memory),
  Always_Terminates
is
   pragma Preelaborate;

   --  Definitions for C character arrays
   type char_array_access is access all char_array;
   for char_array_access'Size use System.Parameters.ptr_bits;

   pragma No_Strict_Aliasing (char_array_access);
   --  Since this type is used for external interfacing, with the pointer
   --  coming from who knows where, it seems a good idea to turn off any
   --  strict aliasing assumptions for this type.

   type chars_ptr is private
   with
     Preelaborable_Initialization;

   type chars_ptr_array is array (size_t range <>) of aliased chars_ptr;

   Null_Ptr : constant chars_ptr;
   --  Null value for private type chars_ptr

   function To_Chars_Ptr
     (Item      : char_array_access;
      Nul_Check : Boolean := False) return chars_ptr
   with
     SPARK_Mode => Off;  --  To_Chars_Ptr'Result is aliased with Item
   --  Extract raw chars_ptr from char_array access type

   function New_Char_Array (Chars : char_array) return chars_ptr with
     Volatile_Function,
     Post   => New_Char_Array'Result /= Null_Ptr,
     Global => (Input => (C_Memory, C_Addresses));
   --  Copy the contents of Chars into a newly allocated chars_ptr

   function New_String (Str : String) return chars_ptr with
     Volatile_Function,
     Post   => New_String'Result /= Null_Ptr,
     Global => (Input => (C_Memory, C_Addresses));
   --  Copy the contents of Str into a newly allocated chars_ptr

   procedure Free (Item : in out chars_ptr) with
     SPARK_Mode => Off;
   --  When deallocation is prohibited (eg: cert runtimes) this routine
   --  will raise Program_Error.

   Dereference_Error : exception;
   --  This exception is raised when a subprogram of this unit tries to
   --  dereference a chars_ptr with the value Null_Ptr.

   --  The Value functions copy the contents of a chars_ptr object
   --  into a char_array/String.
   --  There is a guard for a storage error on an object declaration for
   --  an array type with a modular index type with the size of
   --  Long_Long_Integer. The special processing is needed in this case
   --  to compute reliably the size of the object, and eventually, to
   --  raise Storage_Error, when wrap-around arithmetic might compute
   --  a meangingless size for the object.
   --
   --  The guard raises Storage_Error when
   --
   --    (Arr'Last / 2 - Arr'First / 2) > (2 ** 30)
   --
   function Value (Item : chars_ptr) return char_array with
     Pre    => Item /= Null_Ptr,
     Global => (Input => C_Memory);

   function Value
     (Item   : chars_ptr;
      Length : size_t) return char_array
   with
     Pre    => Item /= Null_Ptr and then Length /= 0,
     Global => (Input => C_Memory);

   function Value (Item : chars_ptr) return String with
     Pre    => Item /= Null_Ptr,
     Global => (Input => C_Memory);

   function Value
     (Item   : chars_ptr;
      Length : size_t) return String
   with
     Pre    => Item /= Null_Ptr and then Length /= 0,
     Global => (Input => C_Memory);

   function Strlen (Item : chars_ptr) return size_t with
     Pre    => Item /= Null_Ptr,
     Global => (Input => C_Memory);
   --  Return the length of a string contained in a chars_ptr

   --  Update the contents of a chars_ptr with a char_array/String. If the
   --  update exceeds the original length of the chars_ptr the Update_Error
   --  exception is raised.
   procedure Update
     (Item   : chars_ptr;
      Offset : size_t;
      Chars  : char_array;
      Check  : Boolean := True)
   with
     Pre    =>
       Item /= Null_Ptr
         and then (Chars'First /= 0 or else Chars'Last /= size_t'Last)
         and then Chars'Length <= size_t'Last - Offset
         and then Chars'Length + Offset <= Strlen (Item),
     Global => (In_Out => C_Memory);

   procedure Update
     (Item   : chars_ptr;
      Offset : size_t;
      Str    : String;
      Check  : Boolean := True)
   with
     Pre    =>
       Item /= Null_Ptr
         and then Str'Length <= size_t'Last - Offset
         and then Str'Length + Offset <= Strlen (Item),
     Global => (In_Out => C_Memory);

   Update_Error : exception;

private
   pragma SPARK_Mode (Off);
   type chars_ptr is access all Character;
   for chars_ptr'Size use System.Parameters.ptr_bits;

   pragma No_Strict_Aliasing (chars_ptr);
   --  Since this type is used for external interfacing, with the pointer
   --  coming from who knows where, it seems a good idea to turn off any
   --  strict aliasing assumptions for this type.

   Null_Ptr : constant chars_ptr := null;
end Interfaces.C.Strings;
