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B.3.1 The Package Interfaces.C.Strings

1/3
{AI05-0229-1} 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 with Import => True, Convention => C has been specified, and for which “char *” is the type of the argument of the C function. 

Static Semantics

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The library package Interfaces.C.Strings has the following declaration: 
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package Interfaces.C.Strings is
   pragma Preelaborate(Strings);
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   type char_array_access is access all char_array;
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{AI95-00161-01}    type chars_ptr is private;
   pragma Preelaborable_Initialization(chars_ptr);
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{AI95-00276-01}    type chars_ptr_array is array (size_t range <>) of aliased chars_ptr;
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   Null_Ptr : constant chars_ptr;
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   function To_Chars_Ptr (Item      : in char_array_access;
                          Nul_Check : in Boolean := False)
      return chars_ptr;
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   function New_Char_Array (Chars   : in char_array) return chars_ptr;
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   function New_String (Str : in String) return chars_ptr;
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   procedure Free (Item : in out chars_ptr);
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   Dereference_Error : exception;
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   function Value (Item : in chars_ptr) return char_array;
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   function Value (Item : in chars_ptr; Length : in size_t)
      return char_array;
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   function Value (Item : in chars_ptr) return String;
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   function Value (Item : in chars_ptr; Length : in size_t)
      return String;
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   function Strlen (Item : in chars_ptr) return size_t;
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   procedure Update (Item   : in chars_ptr;
                     Offset : in size_t;
                     Chars  : in char_array;
                     Check  : in Boolean := True);
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   procedure Update (Item   : in chars_ptr;
                     Offset : in size_t;
                     Str    : in String;
                     Check  : in Boolean := True);
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   Update_Error : exception;
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private
   ... -- not specified by the language
end Interfaces.C.Strings;
21.a
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.
21.b
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. 
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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). 
22.a
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.
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function To_Chars_Ptr (Item      : in char_array_access;
                       Nul_Check : in Boolean := False)
   return chars_ptr;
24/3
{8652/0061} {AI95-00140-01} {AI05-0264-1} 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.
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function New_Char_Array (Chars   : in char_array) return chars_ptr;
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This function returns a pointer to an allocated object initialized to Chars(Chars'First .. Index) & nul, where 
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Index = Chars'Last if Chars does not contain nul, or
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Index is the smallest size_t value I such that Chars(I+1) = nul. 
28.1
Storage_Error is propagated if the allocation fails.
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function New_String (Str : in String) return chars_ptr;
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This function is equivalent to New_Char_Array(To_C(Str)).
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procedure Free (Item : in out chars_ptr);
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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.
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function Value (Item : in chars_ptr) return char_array;
34/3
{AI05-0264-1} 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.
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function Value (Item : in chars_ptr; Length : in size_t)
   return char_array;
36/3
{8652/0062} {AI95-00139-01} {AI05-0264-1} 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. 
36.a
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. 
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function Value (Item : in chars_ptr) return String;
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Equivalent to To_Ada(Value(Item), Trim_Nul=>True).
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function Value (Item : in chars_ptr; Length : in size_t)
   return String;
40/1
{8652/0063} {AI95-00177-01} Equivalent to To_Ada(Value(Item, Length) & nul, Trim_Nul=>True).
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function Strlen (Item : in chars_ptr) return size_t;
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Returns Val'Length–1 where Val = Value(Item); propagates Dereference_Error if Item = Null_Ptr. 
42.a
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.
42.b
Strlen has the same possibility for erroneous execution as Value, in cases where the string has not been nul-terminated.
42.c
Strlen has the effect of C's strlen function.
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procedure Update (Item   : in chars_ptr;
                  Offset : in size_t;
                  Chars  : in char_array;
                  Check  : Boolean := True);
44/1
{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: 
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Let N = Strlen(Item). If Check is True, then: 
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If Offset+Chars'Length>N, propagate Update_Error.
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Otherwise, overwrite the data in the array pointed to by Item, starting at the char at position Offset, with the data in Chars. 
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If Check is False, then processing is as above, but with no check that Offset+Chars'Length>N.
48.a
Ramification: If Chars contains nul, Update's effect may be to “shorten” the pointed-to char array.
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procedure Update (Item   : in chars_ptr;
                  Offset : in size_t;
                  Str    : in String;
                  Check  : in Boolean := True);
50/2
{AI95-00242-01} Equivalent to Update(Item, Offset, To_C(Str, Append_Nul => False), Check). 
50.a/2
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

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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. 
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a Value function not taking a Length parameter,
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the Free procedure,
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the Strlen function. 
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Execution of Free(X) is also erroneous if the chars_ptr X was not returned by New_Char_Array or New_String.
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Reading or updating a freed char_array is erroneous.
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Execution of Update is erroneous if Check is False and a call with Check equal to True would have propagated Update_Error. 
NOTES
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9  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 
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the returned value (a chars_ptr) is represented as a C “char *” so that it may be passed to C functions;
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the allocated object should be freed by the programmer via a call of Free, not by a called C function. 

Inconsistencies With Ada 95

60.a/2
{AI95-00242-01} 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

60.b/2
{AI95-00161-01} Amendment Correction: Added pragma Preelaborable_Initialization to type chars_ptr, so that it can be used in preelaborated units.
60.c/2
{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

60.d/2
{8652/0061} {AI95-00140-01} Corrigendum: Fixed the missing semantics of To_Char_Ptr when Nul_Check is False.
60.e/2
{8652/0062} {AI95-00139-01} Corrigendum: Fixed the missing semantics of Value when the Length is 0.
60.f/2
{8652/0063} {AI95-00177-01} Corrigendum: Corrected the definition of Value to avoid raising Terminator_Error.
60.g/2
{8652/0064} {AI95-00039-01} Corrigendum: Fixed the missing semantics of Update when Item is Null_Ptr. 

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