B.3.2 The Generic Package Interfaces.C.Pointers
1
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.
2
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
3
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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- Explicit termination of an array value
with Default_Terminator (a special terminator value);
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- Programmer-managed
length, with Default_Terminator treated simply as a data element.
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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.
32.a
Ramification: It is the programmer's
responsibility to ensure that elements are not copied beyond the logical
length of the target array.
32.b
Implementation Note: The implementation
has to take care to check the Limit first.
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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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{erroneous execution
(cause) [partial]} It is erroneous to
dereference a Pointer that does not designate an aliased Element.
35.a
Discussion: Such a Pointer could arise
via "+", "–", Increment, or Decrement.
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{erroneous execution
(cause) [partial]} 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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{erroneous execution
(cause) [partial]} 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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{erroneous execution
(cause) [partial]} 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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{erroneous
execution (cause) [partial]} Execution
of Copy_Terminated_Array(Source, Target, Limit, Terminator) is erroneous
in either of the following situations:
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- Execution of both Value(Source, Terminator)
and Value(Source, Limit) are erroneous, or
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- Copying writes past the end of the
array containing the Element designated by Target.
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{erroneous execution
(cause) [partial]} 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.
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14 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;
49/1
{
8652/0065}
{
AI95-00142-01}
loop
Element := Source_Temp_Ptr.
all;
Target_Temp_Ptr.
all := Element;
exit when C."="(Element, C.nul) 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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