Interfacing to C (GNAT Reference Manual) (original) (raw)

13.1 Interfacing to C ¶

Interfacing to C with GNAT can use one of two approaches:

The types in the package Interfaces.C may be used.
Standard Ada types may be used directly. This may be less portable to other compilers, but will work on all GNAT compilers, which guarantee correspondence between the C and Ada types.

Pragma Convention C may be applied to Ada types, but mostly has no effect, since this is the default. The following table shows the correspondence between Ada scalar types and the corresponding C types.

Ada Type	C Type
Integer	int
Short_Integer	short
Short_Short_Integer	signed char
Long_Integer	long
Long_Long_Integer	long long
Short_Float	float
Float	float
Long_Float	double
Long_Long_Float	This is the longest floating-point type supported by the hardware.

Additionally, there are the following general correspondences between Ada and C types:

Ada enumeration types map to C enumeration types directly if pragmaConvention C is specified, which causes them to have a length of 32 bits, except for boolean types which map to C99 bool and for which the length is 8 bits. Without pragma Convention C, Ada enumeration types map to 8, 16, or 32 bits (i.e., C types signed char, short,int, respectively) depending on the number of values passed. This is the only case in which pragma Convention C affects the representation of an Ada type.
Ada access types map to C pointers, except for the case of pointers to unconstrained types in Ada, which have no direct C equivalent.
Ada arrays map directly to C arrays.
Ada records map directly to C structures.
Packed Ada records map to C structures where all members are bit fields of the length corresponding to the type'Size value in Ada.