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B.3 Interfacing with C and C++

1/2
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; support for the Import, Export, and Convention pragmas with convention_identifier C; and support for the Convention pragma with convention_identifier C_Pass_By_Copy.
2/2
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

3
The library package Interfaces.C has the following declaration: 
4
package Interfaces.C is
   pragma Pure(C);
5
   -- Declarations based on C's <limits.h>
6
   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
7
   -- Signed and Unsigned Integers
   type int   is range implementation-defined;
   type short is range implementation-defined;
   type long  is range implementation-defined;
8
   type signed_char is range SCHAR_MIN .. SCHAR_MAX;
   for signed_char'Size use CHAR_BIT;
9
   type unsigned       is mod implementation-defined;
   type unsigned_short is mod implementation-defined;
   type unsigned_long  is mod implementation-defined;
10
   type unsigned_char is mod (UCHAR_MAX+1);
   for unsigned_char'Size use CHAR_BIT;
11
   subtype plain_char is implementation-defined;
12
   type ptrdiff_t is range implementation-defined;
13
   type size_t is mod implementation-defined;
14
   -- Floating Point
15
   type C_float     is digits implementation-defined;
16
   type double      is digits implementation-defined;
17
   type long_double is digits implementation-defined;
18
   -- Characters and Strings 
19
   type char is <implementation-defined character type>;
20/1
   nul : constant char := implementation-defined;
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   function To_C   (Item : in Character) return char;
22
   function To_Ada (Item : in char) return Character;
23
   type char_array is array (size_t range <>) of aliased char;
   pragma Pack(char_array);
   for char_array'Component_Size use CHAR_BIT;
24
   function Is_Nul_Terminated (Item : in char_array) return Boolean;
25
   function To_C   (Item       : in String;
                    Append_Nul : in Boolean := True)
      return char_array;
26
   function To_Ada (Item     : in char_array;
                    Trim_Nul : in Boolean := True)
      return String;
27
   procedure To_C (Item       : in  String;
                   Target     : out char_array;
                   Count      : out size_t;
                   Append_Nul : in  Boolean := True);
28
   procedure To_Ada (Item     : in  char_array;
                     Target   : out String;
                     Count    : out Natural;
                     Trim_Nul : in  Boolean := True);
29
   -- Wide Character and Wide String
30/1
   type wchar_t is <implementation-defined character type>;
31/1
   wide_nul : constant wchar_t := implementation-defined;
32
   function To_C   (Item : in Wide_Character) return wchar_t;
   function To_Ada (Item : in wchar_t       ) return Wide_Character;
33
   type wchar_array is array (size_t range <>) of aliased wchar_t;
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   pragma Pack(wchar_array);
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   function Is_Nul_Terminated (Item : in wchar_array) return Boolean;
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   function To_C   (Item       : in Wide_String;
                    Append_Nul : in Boolean := True)
      return wchar_array;
37
   function To_Ada (Item     : in wchar_array;
                    Trim_Nul : in Boolean := True)
      return Wide_String;
38
   procedure To_C (Item       : in  Wide_String;
                   Target     : out wchar_array;
                   Count      : out size_t;
                   Append_Nul : in  Boolean := True);
39
   procedure To_Ada (Item     : in  wchar_array;
                     Target   : out Wide_String;
                     Count    : out Natural;
                     Trim_Nul : in  Boolean := True);
39.1/2
   -- ISO/IEC 10646:2003 compatible types defined by ISO/IEC TR 19769:2004.
39.2/2
   type char16_t is <implementation-defined character type>;
39.3/2
   char16_nul : constant char16_t := implementation-defined;
39.4/2
   function To_C (Item : in Wide_Character) return char16_t;
   function To_Ada (Item : in char16_t) return Wide_Character;
39.5/2
   type char16_array is array (size_t range <>) of aliased char16_t;
39.6/2
   pragma Pack(char16_array);
39.7/2
   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;
39.8/2
   function To_Ada (Item     : in char16_array;
                    Trim_Nul : in Boolean := True)
      return Wide_String;
39.9/2
   procedure To_C (Item       : in  Wide_String;
                   Target     : out char16_array;
                   Count      : out size_t;
                   Append_Nul : in  Boolean := True);
39.10/2
   procedure To_Ada (Item     : in  char16_array;
                     Target   : out Wide_String;
                     Count    : out Natural;
                     Trim_Nul : in  Boolean := True);
39.11/2
   type char32_t is <implementation-defined character type>;
39.12/2
   char32_nul : constant char32_t := implementation-defined;
39.13/2
   function To_C (Item : in Wide_Wide_Character) return char32_t;
   function To_Ada (Item : in char32_t) return Wide_Wide_Character;
39.14/2
   type char32_array is array (size_t range <>) of aliased char32_t;
39.15/2
   pragma Pack(char32_array);
39.16/2
   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;
39.17/2
   function To_Ada (Item     : in char32_array;
                    Trim_Nul : in Boolean := True)
      return Wide_Wide_String;
39.18/2
   procedure To_C (Item       : in  Wide_Wide_String;
                   Target     : out char32_array;
                   Count      : out size_t;
                   Append_Nul : in  Boolean := True);
39.19/2
   procedure To_Ada (Item     : in  char32_array;
                     Target   : out Wide_Wide_String;
                     Count    : out Natural;
                     Trim_Nul : in  Boolean := True);
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   Terminator_Error : exception;
41
end Interfaces.C;
42
Each of the types declared in Interfaces.C is C-compatible.
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The types int, short, long, unsigned, ptrdiff_t, size_t, double, char, 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.
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The type of the subtype plain_char is either signed_char or unsigned_char, depending on the C implementation. 
45
function To_C   (Item : in Character) return char;
function To_Ada (Item : in char     ) return Character;
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The functions To_C and To_Ada map between the Ada type Character and the C type char.
47
function Is_Nul_Terminated (Item : in char_array) return Boolean;
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The result of Is_Nul_Terminated is True if Item contains nul, and is False otherwise.
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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;
50/2
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.
51
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.
52
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);
53
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.
54
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.
55
function Is_Nul_Terminated (Item : in wchar_array) return Boolean;
56
The result of Is_Nul_Terminated is True if Item contains wide_nul, and is False otherwise.
57
function To_C   (Item : in Wide_Character) return wchar_t;
function To_Ada (Item : in wchar_t       ) return Wide_Character;
58
To_C and To_Ada provide the mappings between the Ada and C wide character types.
59
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);
60
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.
60.1/2
function Is_Nul_Terminated (Item : in char16_array) return Boolean;
60.2/2
The result of Is_Nul_Terminated is True if Item contains char16_nul, and is False otherwise.
60.3/2
function To_C (Item : in Wide_Character) return char16_t;
function To_Ada (Item : in char16_t ) return Wide_Character;
60.4/2
To_C and To_Ada provide mappings between the Ada and C 16-bit character types.
60.5/2
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);
60.6/2
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.
60.7/2
function Is_Nul_Terminated (Item : in char32_array) return Boolean;
60.8/2
The result of Is_Nul_Terminated is True if Item contains char16_nul, and is False otherwise.
60.9/2
function To_C (Item : in Wide_Wide_Character) return char32_t;
function To_Ada (Item : in char32_t ) return Wide_Wide_Character;
60.10/2
To_C and To_Ada provide mappings between the Ada and C 32-bit character types.
60.11/2
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);
60.12/2
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.
60.13/1
   A Convention pragma with convention_identifier C_Pass_By_Copy shall only be applied to a type.
60.14/2
   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.
60.15/1
   If a type is C_Pass_By_Copy-compatible then it is also C-compatible.

Implementation Requirements

61/1
An implementation shall support pragma Convention with a C convention_identifier for a C-eligible type (see B.1). An implementation shall support pragma Convention with a C_Pass_By_Copy convention_identifier for a C_Pass_By_Copy-eligible type. 

Implementation Permissions

62
An implementation may provide additional declarations in the C interface packages. 

Implementation Advice

62.1/2
  The constants nul, wide_nul, char16_nul, and char32_nul should have a representation of zero. 
63
An implementation should support the following interface correspondences between Ada and C. 
64
An Ada procedure corresponds to a void-returning C function. 
65
An Ada function corresponds to a non-void C function.
66
An Ada in scalar parameter is passed as a scalar argument to a C function.
67
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.
68
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.
68.1/2
An Ada parameter of a (record) type T of convention C_Pass_By_Copy, 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.
69/2
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, is passed as a t* argument to a C function, where t is the C struct corresponding to the Ada type T.
70
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.
71
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. 
71.1/2
  An Ada parameter of a private type is passed as specified for the full view of the type.
NOTES
72
8  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.
73
9  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).
74/2
This paragraph was deleted.
75
10  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

76
Example of using the Interfaces.C package: 
77
--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.
78
   -- 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);
79
   pragma Import(C, Strcpy, "strcpy");
80
   Chars1 :  C.char_array(1..20);
   Chars2 :  C.char_array(1..20);
81
begin
   Chars2(1..6) := "qwert" & C.nul;
82
   Strcpy(Chars1, Chars2);
83
-- Now Chars1(1..6) = "qwert" & C.Nul
84
end Test;
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