12.1 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.

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.