Interfacing with C and C++

{8652/0059} {AI95-00131-01} {AI95-00376-01} {AI05-0229-1} The facilities relevant to interfacing with the C language and the corresponding subset of the C++ language are the package Interfaces.C and its children, and ; and support for specifying the Import, Export, and Convention aspect pragmas with convention_identifiers convention_identifier C and; and support for the Convention pragma with convention_identifier C_Pass_By_Copy.

{AI95-00376-01} {AI95-0262-1} The package Interfaces.C contains the basic types, constants, and subprograms that allow an Ada program to pass scalars and strings to C and C++ functions. When this clause mentions a C entity, the reference also applies to the corresponding entity in C++.

Static Semantics

package Interfaces.C is
pragma Pure(C);

-- Declarations based on C's <limits.h>

   CHAR_BIT  : constant := implementation-defined;  -- typically 8
   SCHAR_MIN : constant := implementation-defined;  -- typically –128
   SCHAR_MAX : constant := implementation-defined;  -- typically 127
   UCHAR_MAX : constant := implementation-defined;  -- typically 255

   -- Signed and Unsigned Integers
   type int   is range implementation-defined;
   type short is range implementation-defined;
   type long  is range implementation-defined;

type signed_char is range SCHAR_MIN .. SCHAR_MAX;
for signed_char'Size use CHAR_BIT;

   type unsigned       is mod implementation-defined;
   type unsigned_short is mod implementation-defined;
   type unsigned_long  is mod implementation-defined;

type unsigned_char is mod (UCHAR_MAX+1);
for unsigned_char'Size use CHAR_BIT;

subtype plain_char is implementation-defined;

type ptrdiff_t is range implementation-defined;

type size_t is mod implementation-defined;

type C_float is digits implementation-defined;

type double is digits implementation-defined;

type long_double is digits implementation-defined;

-- Characters and Strings

type char is <implementation-defined character type>;

{8652/0060} {AI95-00037-01} nul : constant char := implementation-defined char'First;

function To_C (Item : in Character) return char;

function To_Ada (Item : in char) return Character;

{AI05-0229-1}    type char_array is array (size_t range <>) of aliased char;
   pragma Pack(char_array);
   for char_array'Component_Size use CHAR_BIT;

function Is_Nul_Terminated (Item : in char_array) return Boolean;

   function To_C   (Item       : in String;
                    Append_Nul : in Boolean := True)
      return char_array;

   function To_Ada (Item     : in char_array;
                    Trim_Nul : in Boolean := True)
      return String;

   procedure To_C (Item       : in  String;
                   Target     : out char_array;
                   Count      : out size_t;
                   Append_Nul : in  Boolean := True);

   procedure To_Ada (Item     : in  char_array;
                     Target   : out String;
                     Count    : out Natural;
                     Trim_Nul : in  Boolean := True);

-- Wide Character and Wide String

{8652/0060} {AI95-00037-01} type wchar_t is <implementation-defined character type> implementation-defined;

{8652/0060} {AI95-00037-01} wide_nul : constant wchar_t := implementation-defined wchar_t'First;

function To_C (Item : in Wide_Character) return wchar_t;
function To_Ada (Item : in wchar_t ) return Wide_Character;

{AI05-0229-1} type wchar_array is array (size_t range <>) of aliased wchar_t
with Pack;

This paragraph was deleted.{AI05-0229-1} pragma Pack(wchar_array);

function Is_Nul_Terminated (Item : in wchar_array) return Boolean;

   function To_C   (Item       : in Wide_String;
                    Append_Nul : in Boolean := True)
      return wchar_array;

   function To_Ada (Item     : in wchar_array;
                    Trim_Nul : in Boolean := True)
      return Wide_String;

   procedure To_C (Item       : in  Wide_String;
                   Target     : out wchar_array;
                   Count      : out size_t;
                   Append_Nul : in  Boolean := True);

   procedure To_Ada (Item     : in  wchar_array;
                     Target   : out Wide_String;
                     Count    : out Natural;
                     Trim_Nul : in  Boolean := True);

{AI95-00285-01} -- ISO/IEC 10646:2003 compatible types defined by ISO/IEC TR 19769:2004.

{AI95-00285-01} type char16_t is <implementation-defined character type>;

char16_nul : constant char16_t := implementation-defined;

function To_C (Item : in Wide_Character) return char16_t;
function To_Ada (Item : in char16_t) return Wide_Character;

{AI05-0229-1} type char16_array is array (size_t range <>) of aliased char16_t
with Pack;

This paragraph was deleted. {AI05-0229-1} pragma Pack(char16_array);

   function Is_Nul_Terminated (Item : in char16_array) return Boolean;
   function To_C (Item       : in Wide_String;
                  Append_Nul : in Boolean := True)
      return char16_array;

   function To_Ada (Item     : in char16_array;
                    Trim_Nul : in Boolean := True)
      return Wide_String;

   procedure To_C (Item       : in  Wide_String;
                   Target     : out char16_array;
                   Count      : out size_t;
                   Append_Nul : in  Boolean := True);

   procedure To_Ada (Item     : in  char16_array;
                     Target   : out Wide_String;
                     Count    : out Natural;
                     Trim_Nul : in  Boolean := True);

{AI95-00285-01} type char32_t is <implementation-defined character type>;

char32_nul : constant char32_t := implementation-defined;

function To_C (Item : in Wide_Wide_Character) return char32_t;
function To_Ada (Item : in char32_t) return Wide_Wide_Character;

{AI05-0229-1} type char32_array is array (size_t range <>) of aliased char32_t
with Pack;

This paragraph was deleted. {AI05-0229-1} pragma Pack(char32_array);

   function Is_Nul_Terminated (Item : in char32_array) return Boolean;
   function To_C (Item       : in Wide_Wide_String;
                  Append_Nul : in Boolean := True)
      return char32_array;

   function To_Ada (Item     : in char32_array;
                    Trim_Nul : in Boolean := True)
      return Wide_Wide_String;

   procedure To_C (Item       : in  Wide_Wide_String;
                   Target     : out char32_array;
                   Count      : out size_t;
                   Append_Nul : in  Boolean := True);

   procedure To_Ada (Item     : in  char32_array;
                     Target   : out Wide_Wide_String;
                     Count    : out Natural;
                     Trim_Nul : in  Boolean := True);

Terminator_Error : exception;

end Interfaces.C;

{AI95-00285-01} The types int, short, long, unsigned, ptrdiff_t, size_t, double, char, and wchar_t, char16_t, and char32_t correspond respectively to the C types having the same names. The types signed_char, unsigned_short, unsigned_long, unsigned_char, C_float, and long_double correspond respectively to the C types signed char, unsigned short, unsigned long, unsigned char, float, and long double.

Discussion: The C types wchar_t and char16_t seem to be the same. However, wchar_t has an implementation-defined size, whereas char16_t is guaranteed to be an unsigned type of at least 16 bits. Also, char16_t and char32_t are encouraged to have UTF-16 and UTF-32 representations; that means that they are not directly the same as the Ada types, which most likely don't use any UTF encoding.

function To_C (Item : in Character) return char;
function To_Ada (Item : in char ) return Character;

Implementation Note: {8652/0114} {AI95-00038-01} The To_C and To_Ada functions map between corresponding characters, not necessarily between characters with the same internal representation. Corresponding characters are characters defined by the same enumeration literal, if such exist; otherwise, the correspondence is unspecified.

To_C (Latin_1_Char) = char'Value(Character'Image(Latin_1_Char))
provided that char'Value does not raise an exception; otherwise the result is unspecified.

To_Ada (Native_C_Char) = Character'Value(char'Image(Native_C_Char))
provided that Character'Value does not raise an exception; otherwise the result is unspecified.

function Is_Nul_Terminated (Item : in char_array) return Boolean;

function To_C   (Item : in String;     Append_Nul : in Boolean := True)
   return char_array;

function To_Ada (Item : in char_array; Trim_Nul   : in Boolean := True)
   return String;

{AI95-00258-01} The result of To_C is a char_array value of length Item'Length (if Append_Nul is False) or Item'Length+1 (if Append_Nul is True). The lower bound is 0. For each component Item(I), the corresponding component in the result is To_C applied to Item(I). The value nul is appended if Append_Nul is True. If Append_Nul is False and Item'Length is 0, then To_C propagates Constraint_Error.

The result of To_Ada is a String whose length is Item'Length (if Trim_Nul is False) or the length of the slice of Item preceding the first nul (if Trim_Nul is True). The lower bound of the result is 1. If Trim_Nul is False, then for each component Item(I) the corresponding component in the result is To_Ada applied to Item(I). If Trim_Nul is True, then for each component Item(I) before the first nul the corresponding component in the result is To_Ada applied to Item(I). The function propagates Terminator_Error if Trim_Nul is True and Item does not contain nul.

procedure To_C (Item       : in  String;
                Target     : out char_array;
                Count      : out size_t;
                Append_Nul : in  Boolean := True);

procedure To_Ada (Item     : in  char_array;
                  Target   : out String;
                  Count    : out Natural;
                  Trim_Nul : in  Boolean := True);

For procedure To_C, each element of Item is converted (via the To_C function) to a char, which is assigned to the corresponding element of Target. If Append_Nul is True, nul is then assigned to the next element of Target. In either case, Count is set to the number of Target elements assigned. If Target is not long enough, Constraint_Error is propagated.

For procedure To_Ada, each element of Item (if Trim_Nul is False) or each element of Item preceding the first nul (if Trim_Nul is True) is converted (via the To_Ada function) to a Character, which is assigned to the corresponding element of Target. Count is set to the number of Target elements assigned. If Target is not long enough, Constraint_Error is propagated. If Trim_Nul is True and Item does not contain nul, then Terminator_Error is propagated.

function Is_Nul_Terminated (Item : in wchar_array) return Boolean;

function To_C (Item : in Wide_Character) return wchar_t;
function To_Ada (Item : in wchar_t ) return Wide_Character;

function To_C   (Item       : in Wide_String;
                 Append_Nul : in Boolean := True)
   return wchar_array;

function To_Ada (Item     : in wchar_array;
                 Trim_Nul : in Boolean := True)
   return Wide_String;

procedure To_C (Item       : in  Wide_String;
                Target     : out wchar_array;
                Count      : out size_t;
                Append_Nul : in  Boolean := True);

procedure To_Ada (Item     : in  wchar_array;
                  Target   : out Wide_String;
                  Count    : out Natural;
                  Trim_Nul : in  Boolean := True);

The To_C and To_Ada subprograms that convert between Wide_String and wchar_array have analogous effects to the To_C and To_Ada subprograms that convert between String and char_array, except that wide_nul is used instead of nul.

function Is_Nul_Terminated (Item : in char16_array) return Boolean;

function To_C (Item : in Wide_Character) return char16_t;
function To_Ada (Item : in char16_t ) return Wide_Character;

function To_C (Item       : in Wide_String;
               Append_Nul : in Boolean := True)
   return char16_array;

function To_Ada (Item     : in char16_array;
                 Trim_Nul : in Boolean := True)
   return Wide_String;

procedure To_C (Item       : in  Wide_String;
                Target     : out char16_array;
                Count      : out size_t;
                Append_Nul : in  Boolean := True);

procedure To_Ada (Item     : in  char16_array;
                  Target   : out Wide_String;
                  Count    : out Natural;
                  Trim_Nul : in  Boolean := True);

{AI95-00285-01} The To_C and To_Ada subprograms that convert between Wide_String and char16_array have analogous effects to the To_C and To_Ada subprograms that convert between String and char_array, except that char16_nul is used instead of nul.

function Is_Nul_Terminated (Item : in char32_array) return Boolean;

function To_C (Item : in Wide_Wide_Character) return char32_t;
function To_Ada (Item : in char32_t ) return Wide_Wide_Character;

function To_C (Item       : in Wide_Wide_String;
               Append_Nul : in Boolean := True)
   return char32_array;

function To_Ada (Item     : in char32_array;
                 Trim_Nul : in Boolean := True)
   return Wide_Wide_String;

procedure To_C (Item       : in  Wide_Wide_String;
                Target     : out char32_array;
                Count      : out size_t;
                Append_Nul : in  Boolean := True);

procedure To_Ada (Item     : in  char32_array;
                  Target   : out Wide_Wide_String;
                  Count    : out Natural;
                  Trim_Nul : in  Boolean := True);

{AI95-00285-01} The To_C and To_Ada subprograms that convert between Wide_Wide_String and char32_array have analogous effects to the To_C and To_Ada subprograms that convert between String and char_array, except that char32_nul is used instead of nul.

Discussion: The Interfaces.C package provides an implementation-defined character type, char, designed to model the C run-time character set, and mappings between the types char and Character.

One application of the C interface package is to compose a C string and pass it to a C function. One way to do this is for the programmer to declare an object that will hold the C array, and then pass this array to the C function. This is realized via the type char_array:

The programmer can declare an Ada String, convert it to a char_array, and pass the char_array as actual parameter to the C function that is expecting a char *.

An alternative approach is for the programmer to obtain a C char pointer from an Ada String (or from a char_array) by invoking an allocation function. The package Interfaces.C.Strings (see below) supplies the needed facilities, including a private type chars_ptr that corresponds to C's char *, and two allocation functions. To avoid storage leakage, a Free procedure releases the storage that was allocated by one of these allocate functions.

It is typical for a C function that deals with strings to adopt the convention that the string is delimited by a nul char. The C interface packages support this convention. A constant nul of type Char is declared, and the function Value(Chars_Ptr) in Interfaces.C.Strings returns a char_array up to and including the first nul in the array that the chars_ptr points to. The Allocate_Chars function allocates an array that is nul terminated.

Some C functions that deal with strings take an explicit length as a parameter, thus allowing strings to be passed that contain nul as a data element. Other C functions take an explicit length that is an upper bound: the prefix of the string up to the char before nul, or the prefix of the given length, is used by the function, whichever is shorter. The C Interface packages support calling such functions.

{8652/0059} {AI95-00131-01} {AI05-0229-1} The A Convention aspect pragma with convention_identifier C_Pass_By_Copy shall only be specified for applied to a type.

{8652/0059} {AI95-00131-01} {AI95-00216-01} The eligibility rules in B.1 do not apply to convention C_Pass_By_Copy. Instead, a type T is eligible for convention C_Pass_By_Copy if T is an unchecked union type or if T is a record type that has no discriminants and that only has components with statically constrained subtypes, and each component is C-compatible.

Implementation Requirements

{8652/0059} {AI95-00131-01} {AI05-0229-1} An implementation shall support specifying aspect pragma Convention with a C convention_identifier for a C-eligible type (see B.1). An implementation shall support specifying aspect pragma Convention with a C_Pass_By_Copy convention_identifier for a C_Pass_By_Copy-eligible type.

Implementation Permissions

{AI05-0002-1} {AI05-0229-1} An implementation need not support specifying the Convention aspect with convention_identifier C in the following cases:

{AI05-0248-1} for a subprogram that has a parameter of an unconstrained array subtype, unless the Import aspect has the value True for the subprogram;

Implementation Note: {AI05-0002-1} These rules ensure that an implementation never needs to create bounds for an unconstrained array that originates in C (and thus does not have bounds). An implementation can do so if it wishes, of course. Note that these permissions do not extend to passing an unconstrained array as a parameter to a C function; in this case, the bounds can simply be dropped and thus support is required.

Implementation Advice

{8652/0060} {AI95-00037-01} {AI95-00285-01} The constants nul, and wide_nul, char16_nul, and char32_nul should have a representation of zero.

Implementation Advice: The constants nul, wide_nul, char16_nul, and char32_nul in package Interfaces.C should have a representation of zero.

An Ada in parameter of an access-to-object type with designated type T is passed as a t* argument to a C function, where t is the C type corresponding to the Ada type T.

An Ada access T parameter, or an Ada out or in out parameter of an elementary type T, is passed as a t* argument to a C function, where t is the C type corresponding to the Ada type T. In the case of an elementary out or in out parameter, a pointer to a temporary copy is used to preserve by-copy semantics.

{8652/0059} {AI95-00131-01} {AI95-00343-01} An Ada parameter of a (record) type T of convention C_Pass_By_Copy -compatible (record) type T, of mode in, is passed as a t argument to a C function, where t is the C struct corresponding to the Ada type T.

{8652/0059} {AI95-00131-01} {AI95-00343-01} An Ada parameter of a record type T, of any mode, other than an in parameter of a type of convention C_Pass_By_Copy -compatible type, is passed as a t* argument to a C function, where t is the C struct corresponding to the Ada type T.

An Ada parameter of an array type with component type T, of any mode, is passed as a t* argument to a C function, where t is the C type corresponding to the Ada type T.

An Ada parameter of an access-to-subprogram type is passed as a pointer to a C function whose prototype corresponds to the designated subprogram's specification.

This paragraph was deleted.{AI95-00337-01} {AI05-0002-1} An Ada parameter of a private type is passed as specified for the full view of the type.

9 Values of type char_array are not implicitly terminated with nul. If a char_array is to be passed as a parameter to an imported C function requiring nul termination, it is the programmer's responsibility to obtain this effect.

10 To obtain the effect of C's sizeof(item_type), where Item_Type is the corresponding Ada type, evaluate the expression: size_t(Item_Type'Size/CHAR_BIT).

This paragraph was deleted.11 {AI95-00216-01} There is no explicit support for C's union types. Unchecked conversions can be used to obtain the effect of C unions.

12 A C function that takes a variable number of arguments can correspond to several Ada subprograms, taking various specific numbers and types of parameters.

Examples

--Calling the C Library Function strcpy
with Interfaces.C;
procedure Test is
   package C renames Interfaces.C;
   use type C.char_array;
   -- Call <string.h>strcpy:
   -- C definition of strcpy:  char *strcpy(char *s1, const char *s2);
   --    This function copies the string pointed to by s2 (including the terminating null character)
   --     into the array pointed to by s1. If copying takes place between objects that overlap,
   --     the behavior is undefined. The strcpy function returns the value of s1.

{AI05-0229-1}    -- Note: since the C function's return value is of no interest, the Ada interface is a procedure
   procedure Strcpy (Target : out C.char_array;
                     Source : in  C.char_array)
      with Import => True, Convention => C, External_Name => "strcpy";

This paragraph was deleted.{AI05-0229-1} pragma Import(C, Strcpy, "strcpy");

Chars1 : C.char_array(1..20);
Chars2 : C.char_array(1..20);

begin
Chars2(1..6) := "qwert" & C.nul;

Strcpy(Chars1, Chars2);

-- Now Chars1(1..6) = "qwert" & C.Nul

Incompatibilities With Ada 95

{AI95-00285-01} Types char16_t and char32_t and their related types and operations are newly added to Interfaces.C. If Interfaces.C is referenced in a use_clause, and an entity E with the same defining_identifier as a new entity in Interfaces.C is defined in a package that is also referenced in a use_clause, the entity E may no longer be use-visible, resulting in errors. This should be rare and is easily fixed if it does occur.

Extensions to Ada 95

Wording Changes from Ada 95

{AI95-00337-01} Clarified that the interface correspondences also apply to private types whose full types have the specified characteristics.

{AI95-00343-01} Clarified that a type must have convention C_Pass_By_Copy in order to be passed by copy (not just a type that could have that convention).

Incompatibilities With Ada 2005

{AI05-0002-1} Correction: Added a definition of correspondences for function results. Also added wording to make it clear that we do not expect the implementation to conjure bounds for unconstrained arrays out of thin air. These changes allow (but don't require) compilers to reject unreasonable uses of array types. Such uses probably didn't work anyway (and probably were rejected, no matter what the language definition said), so little existing code should be impacted.

B.3 Interfacing with C and C++Interfacing with C