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B.3.2 The Generic Package Interfaces.C.Pointers

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The generic package Interfaces.C.Pointers allows the Ada programmer to perform C-style operations on pointers. It includes an access type Pointer, Value functions that dereference a Pointer and deliver the designated array, several pointer arithmetic operations, and “copy” procedures that copy the contents of a source pointer into the array designated by a destination pointer. As in C, it treats an object Ptr of type Pointer as a pointer to the first element of an array, so that for example, adding 1 to Ptr yields a pointer to the second element of the array.
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The generic allows two styles of usage: one in which the array is terminated by a special terminator element; and another in which the programmer needs to keep track of the length. 

Static Semantics

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The generic library package Interfaces.C.Pointers has the following declaration: 
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generic
   type Index is (<>);
   type Element is private;
   type Element_Array is array (Index range <>) of aliased Element;
   Default_Terminator : Element;
package Interfaces.C.Pointers is
   pragma Preelaborate(Pointers);
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   type Pointer is access all Element;
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   function Value(Ref        : in Pointer;
                  Terminator : in Element := Default_Terminator)
      return Element_Array;
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   function Value(Ref    : in Pointer;
                  Length : in ptrdiff_t)
      return Element_Array;
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   Pointer_Error : exception;
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   -- C-style Pointer arithmetic
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   function "+" (Left : in Pointer;   Right : in ptrdiff_t) return Pointer;
   function "+" (Left : in ptrdiff_t; Right : in Pointer)   return Pointer;
   function "-" (Left : in Pointer;   Right : in ptrdiff_t) return Pointer;
   function "-" (Left : in Pointer;   Right : in Pointer) return ptrdiff_t;
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   procedure Increment (Ref : in out Pointer);
   procedure Decrement (Ref : in out Pointer);
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   pragma Convention (Intrinsic, "+");
   pragma Convention (Intrinsic, "-");
   pragma Convention (Intrinsic, Increment);
   pragma Convention (Intrinsic, Decrement);
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   function Virtual_Length (Ref        : in Pointer;
                            Terminator : in Element := Default_Terminator)
      return ptrdiff_t;
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   procedure Copy_Terminated_Array
      (Source     : in Pointer;
       Target     : in Pointer;
       Limit      : in ptrdiff_t := ptrdiff_t'Last;
       Terminator : in Element :=  Default_Terminator);
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   procedure Copy_Array (Source  : in Pointer;
                         Target  : in Pointer;
                         Length  : in ptrdiff_t);
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end Interfaces.C.Pointers;
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The type Pointer is C-compatible and corresponds to one use of C's “Element *”. An object of type Pointer is interpreted as a pointer to the initial Element in an Element_Array. Two styles are supported: 
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function Value(Ref        : in Pointer;
               Terminator : in Element := Default_Terminator)
   return Element_Array;
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This function returns an Element_Array whose value is the array pointed to by Ref, up to and including the first Terminator; the lower bound of the array is Index'First. Interfaces.C.Strings.Dereference_Error is propagated if Ref is null.
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function Value(Ref    : in Pointer;
               Length : in ptrdiff_t)
   return Element_Array;
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This function returns an Element_Array comprising the first Length elements pointed to by Ref. The exception Interfaces.C.Strings.Dereference_Error is propagated if Ref is null
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The "+" and "–" functions perform arithmetic on Pointer values, based on the Size of the array elements. In each of these functions, Pointer_Error is propagated if a Pointer parameter is null
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procedure Increment (Ref : in out Pointer);
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Equivalent to Ref := Ref+1.
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procedure Decrement (Ref : in out Pointer);
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Equivalent to Ref := Ref–1.
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function Virtual_Length (Ref        : in Pointer;
                         Terminator : in Element := Default_Terminator)
   return ptrdiff_t;
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Returns the number of Elements, up to the one just before the first Terminator, in Value(Ref, Terminator).
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procedure Copy_Terminated_Array
   (Source     : in Pointer;
    Target     : in Pointer;
    Limit      : in ptrdiff_t := ptrdiff_t'Last;
    Terminator : in Element := Default_Terminator);
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This procedure copies Value(Source, Terminator) into the array pointed to by Target; it stops either after Terminator has been copied, or the number of elements copied is Limit, whichever occurs first. Dereference_Error is propagated if either Source or Target is null
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procedure Copy_Array (Source  : in Pointer;
                      Target  : in Pointer;
                      Length  : in ptrdiff_t);
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This procedure copies the first Length elements from the array pointed to by Source, into the array pointed to by Target. Dereference_Error is propagated if either Source or Target is null.

Erroneous Execution

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It is erroneous to dereference a Pointer that does not designate an aliased Element. 
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Execution of Value(Ref, Terminator) is erroneous if Ref does not designate an aliased Element in an Element_Array terminated by Terminator.
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Execution of Value(Ref, Length) is erroneous if Ref does not designate an aliased Element in an Element_Array containing at least Length Elements between the designated Element and the end of the array, inclusive.
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Execution of Virtual_Length(Ref, Terminator) is erroneous if Ref does not designate an aliased Element in an Element_Array terminated by Terminator.
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Execution of Copy_Terminated_Array(Source, Target, Limit, Terminator) is erroneous in either of the following situations: 
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Execution of Copy_Array(Source, Target, Length) is erroneous if either Value(Source, Length) is erroneous, or copying writes past the end of the array containing the Element designated by Target. 
NOTES
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12  To compose a Pointer from an Element_Array, use 'Access on the first element. For example (assuming appropriate instantiations): 
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Some_Array   : Element_Array(0..5) ;
Some_Pointer : Pointer := Some_Array(0)'Access;

Examples

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Example of Interfaces.C.Pointers:
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with Interfaces.C.Pointers;
with Interfaces.C.Strings;
procedure Test_Pointers is
   package C renames Interfaces.C;
   package Char_Ptrs is
      new C.Pointers (Index              => C.size_t,
                      Element            => C.char,
                      Element_Array      => C.char_array,
                      Default_Terminator => C.nul);
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   use type Char_Ptrs.Pointer;
   subtype Char_Star is Char_Ptrs.Pointer;
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   procedure Strcpy (Target_Ptr, Source_Ptr : Char_Star) is
      Target_Temp_Ptr : Char_Star := Target_Ptr;
      Source_Temp_Ptr : Char_Star := Source_Ptr;
      Element : C.char;
   begin
      if Target_Temp_Ptr = null or Source_Temp_Ptr = null then
         raise C.Strings.Dereference_Error;
      end if;
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      loop
         Element             := Source_Temp_Ptr.all;
         Target_Temp_Ptr.all := Element;
         exit when C."="(Element, C.nul);
         Char_Ptrs.Increment(Target_Temp_Ptr);
         Char_Ptrs.Increment(Source_Temp_Ptr);
      end loop;
   end Strcpy;
begin
   ...
end Test_Pointers;

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