6.20.5.1 Foreign Types

The first impedance mismatch that one sees between C and Scheme is that in C, the storage locations (variables) are typed, but in Scheme types are associated with values, not variables. See section Values and Variables.

So when describing a C function or a C structure so that it can be accessed from Scheme, the data types of the parameters or fields must be passed explicitly.

These “C type values” may be constructed using the constants and procedures from the (system foreign) module, which may be loaded like this:

(use-modules (system foreign))

(system foreign) exports a number of values expressing the basic C types:

Scheme Variable: int8

Scheme Variable: uint8

Scheme Variable: uint16

Scheme Variable: int16

Scheme Variable: uint32

Scheme Variable: int32

Scheme Variable: uint64

Scheme Variable: int64

Scheme Variable: float

Scheme Variable: double

These values represent the C numeric types of the specified sizes and signednesses.

In addition there are some convenience bindings for indicating types of platform-dependent size:

Scheme Variable: int

Scheme Variable: unsigned-int

Scheme Variable: long

Scheme Variable: unsigned-long

Scheme Variable: size_t

Scheme Variable: ssize_t

Scheme Variable: ptrdiff_t

Values exported by the (system foreign) module, representing C numeric types. For example, long may be equal? to int64 on a 64-bit platform.

Scheme Variable: void

The void type. It can be used as the first argument to pointer->procedure to wrap a C function that returns nothing.

In addition, the symbol * is used by convention to denote pointer types. Procedures detailed in the following sections, such as pointer->procedure, accept it as a type descriptor.

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