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B.3 Interfacing with C
1/1
The
facilities relevant to interfacing with 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
The package Interfaces.C contains the basic types,
constants and subprograms that allow an Ada program to pass scalars and
strings to C functions.
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;
21
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;
34
pragma Pack(wchar_array);
35
function Is_Nul_Terminated (Item : in wchar_array) return Boolean;
36
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);
40
Terminator_Error : exception;
41
end Interfaces.C;
42
Each of the types declared in Interfaces.C is
C-compatible.
43
The types int, short, long, unsigned, ptrdiff_t,
size_t, double, char, and wchar_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.
44
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;
46
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;
48
The result of
Is_Nul_Terminated is True if Item contains nul, and is False otherwise.
49
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
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.
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.
60.1/1
A Convention pragma with convention_identifier
C_Pass_By_Copy shall only be applied to a type.
60.2/1
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 a record type that has no discriminants
and that only has components with statically constrained subtypes, and
each component is C-compatible.
60.3/1
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
62.1/1
The constants nul and wide_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.
68.1/1
- An Ada parameter of a 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.
69/1
- An Ada parameter of a record type
T, of any mode, other than an in parameter of a 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.
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.
72
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.
73
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).
74
11 There is no explicit
support for C's union types. Unchecked conversions can be used to obtain
the effect of C unions.
75
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
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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