B.3.1 The Package Interfaces.C.Strings
The package Interfaces.C.Strings declares types and
subprograms allowing an Ada program to allocate, reference, update, and
free C-style strings. In particular, the private type chars_ptr corresponds
to a common use of “char *” in C programs, and an object
of this type can be passed to a subprogram to which
pragma
Import(C,...) has been applied, and for which “char *” is
the type of the argument of the C function.
Static Semantics
The library package
Interfaces.C.Strings has the following declaration:
package Interfaces.C.Strings
is
pragma Preelaborate(Strings);
type char_array_access
is access all char_array;
{
AI95-00161-01}
type chars_ptr
is private;
pragma Preelaborable_Initialization(chars_ptr);
{
AI95-00276-01}
type chars_ptr_array
is array (size_t
range <>)
of aliased chars_ptr;
Null_Ptr :
constant chars_ptr;
function To_Chars_Ptr (Item :
in char_array_access;
Nul_Check :
in Boolean := False)
return chars_ptr;
function New_Char_Array (Chars :
in char_array)
return chars_ptr;
function New_String (Str :
in String)
return chars_ptr;
procedure Free (Item :
in out chars_ptr);
Dereference_Error :
exception;
function Value (Item :
in chars_ptr)
return char_array;
function Value (Item :
in chars_ptr; Length :
in size_t)
return char_array;
function Value (Item :
in chars_ptr)
return String;
function Value (Item :
in chars_ptr; Length :
in size_t)
return String;
function Strlen (Item :
in chars_ptr)
return size_t;
procedure Update (Item :
in chars_ptr;
Offset :
in size_t;
Chars :
in char_array;
Check :
in Boolean := True);
procedure Update (Item :
in chars_ptr;
Offset :
in size_t;
Str :
in String;
Check :
in Boolean := True);
Update_Error :
exception;
private
... -- not specified by the language
end Interfaces.C.Strings;
Discussion: The string manipulation types
and subprograms appear in a child of Interfaces.C versus being there
directly, since it is useful to have Interfaces.C specified as
pragma
Pure.
Differently named functions New_String and New_Char_Array
are declared, since if there were a single overloaded function a call
with a string literal as actual parameter would be ambiguous.
The type chars_ptr is C-compatible and corresponds
to the use of C's “char *” for a pointer to the first char
in a char array terminated by nul. When an object of type chars_ptr is
declared, its value is by default set to Null_Ptr, unless the object
is imported (see
B.1).
Discussion: The type char_array_access
is not necessarily C-compatible, since an object of this type may carry
“dope” information. The programmer should convert from char_array_access
to chars_ptr for objects imported from, exported to, or passed to C.
function To_Chars_Ptr (Item : in char_array_access;
Nul_Check : in Boolean := False)
return chars_ptr;
{
8652/0061}
{
AI95-00140-01}
If Item is
null, then To_Chars_Ptr returns Null_Ptr. If Item is
not
null, Nul_Check is True, and Item.
all does not contain
nul, then the function propagates Terminator_Error; otherwise To_Chars_Ptr
performs a pointer conversion with no allocation of memory.
function New_Char_Array (Chars : in char_array) return chars_ptr;
This function returns a pointer to an allocated
object initialized to Chars(Chars'First .. Index) & nul, where
Index = Chars'Last if Chars does not contain
nul, or
Index is the smallest size_t value I such
that Chars(I+1) = nul.
Storage_Error is
propagated if the allocation fails.
function New_String (Str : in String) return chars_ptr;
This function is
equivalent to New_Char_Array(To_C(Str)).
procedure Free (Item : in out chars_ptr);
If Item is Null_Ptr,
then Free has no effect. Otherwise, Free releases the storage occupied
by Value(Item), and resets Item to Null_Ptr.
function Value (Item : in chars_ptr) return char_array;
If Item = Null_Ptr
then Value propagates Dereference_Error. Otherwise Value returns the
prefix of the array of chars pointed to by Item, up to and including
the first nul. The lower bound of the result is 0. If Item does not point
to a nul-terminated string, then execution of Value is erroneous.
function Value (Item : in chars_ptr; Length : in size_t)
return char_array;
{
8652/0062}
{
AI95-00139-01}
If Item = Null_Ptr then Value propagates Dereference_Error. Otherwise
Value returns the shorter of two arrays, either the first Length chars
pointed to by Item, or Value(Item). The lower bound of the result is
0. If Length is 0, then Value propagates Constraint_Error.
Ramification: Value(New_Char_Array(Chars))
= Chars if Chars does not contain nul; else Value(New_Char_Array( Chars))
is the prefix of Chars up to and including the first nul.
function Value (Item : in chars_ptr) return String;
Equivalent to To_Ada(Value(Item),
Trim_Nul=>True).
function Value (Item : in chars_ptr; Length : in size_t)
return String;
function Strlen (Item : in chars_ptr) return size_t;
Returns Val'Length–1
where Val = Value(Item); propagates Dereference_Error if Item
= Null_Ptr.
Ramification: Strlen returns the number
of chars in the array pointed to by Item, up to and including the char
immediately before the first nul.
Strlen has the same possibility for erroneous
execution as Value, in cases where the string has not been nul-terminated.
Strlen has the effect of C's strlen function.
procedure Update (Item : in chars_ptr;
Offset : in size_t;
Chars : in char_array;
Check : Boolean := True);
{
8652/0064}
{
AI95-00039-01}
If Item = Null_Ptr, then Update propagates Dereference_Error. Otherwise,
this procedure updates the value pointed to by Item, starting at position
Offset, using Chars as the data to be copied into the array. Overwriting
the nul terminator, and skipping with the Offset past the nul terminator,
are both prevented if Check is True, as follows:
Let N = Strlen(Item). If Check is True,
then:
If Offset+Chars'Length>N, propagate
Update_Error.
Otherwise, overwrite the data in
the array pointed to by Item, starting at the char at position Offset,
with the data in Chars.
If Check
is False, then processing is as above, but with no check that Offset+Chars'Length>N.
Ramification: If Chars contains nul,
Update's effect may be to “shorten” the pointed-to char array.
procedure Update (Item : in chars_ptr;
Offset : in size_t;
Str : in String;
Check : in Boolean := True);
{
AI95-00242-01}
Equivalent to Update(Item, Offset, To_C(Str, Append_Nul => False),
Check).
Discussion: {
AI95-00242-01}
To truncate the Item to the length of Str, use Update(Item, Offset, To_C(Str),
Check) instead of Update(Item, Offset, Str, Check). Note that when truncating
Item, Item must be longer than Str.
Erroneous Execution
{erroneous execution
(cause) [partial]} Execution of any of
the following is erroneous if the Item parameter is not null_ptr and
Item does not point to a nul-terminated array of chars.
a Value function not taking a Length parameter,
the Free procedure,
the Strlen function.
{erroneous execution
(cause) [partial]} Execution of Free(X)
is also erroneous if the chars_ptr X was not returned by New_Char_Array
or New_String.
{erroneous execution
(cause) [partial]} Reading or updating
a freed char_array is erroneous.
{erroneous execution
(cause) [partial]} Execution of Update
is erroneous if Check is False and a call with Check equal to True would
have propagated Update_Error.
11 New_Char_Array and New_String might
be implemented either through the allocation function from the C environment
(“malloc”) or through Ada dynamic memory allocation (“new”).
The key points are
the returned value (a chars_ptr)
is represented as a C “char *” so that it may be passed to
C functions;
the allocated object should be freed
by the programmer via a call of Free, not by a called C function.
Inconsistencies With Ada 95
{
AI95-00242-01}
{
inconsistencies with Ada 95}
Amendment
Correction: Update for a String parameter is now defined to not add
a nul character. It did add a nul in Ada 95. This means that programs
that used this behavior of Update to truncate a string will no longer
work (the string will not be truncated). This change makes Update for
a string consistent with Update for a char_array (no implicit nul is
added to the end of a char_array).
Extensions to Ada 95
{
AI95-00161-01}
{
extensions to Ada 95}
Amendment Correction:
Added
pragma
Preelaborable_Initialization to type chars_ptr, so that it can be used
in preelaborated units.
{
AI95-00276-01}
Amendment Correction: The components of chars_ptr_array are aliased
so that it can be used to instantiate Interfaces.C.Pointers (that is
its intended purpose, which is otherwise mysterious as it has no operations).
Wording Changes from Ada 95
Sponsored by Ada-Europe