10.1 Interfacing to C

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

1. The types in the package Interfaces.C may be used.
2. 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 maybe 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.

• 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.