GNAT Reference Manual: Interfacing to C

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 pragma Convention C is specified, which causes them to have int length. 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.