On this page:

3.1 Type Constructors

(make-ctype type scheme-to-c c-to-scheme)  ctype?
  type : ctype?
  scheme-to-c : (or/c #f (any/c . -> . any))
  c-to-scheme : (or/c #f (any/c . -> . any))
Creates a new C type value whose representation for foreign code is the same as type’s. The given conversions functions convert to and from the Racket representation of type. Either conversion function can be #f, meaning that the conversion for the corresponding direction is the identity function. If both functions are #f, type is returned.

(ctype? v)  boolean?
  v : any/c
Returns #t if v is a C type, #f otherwise.

Returns the size or alignment of a given type for the current platform.

(ctype->layout type)
  (flat-rec-contract rep symbol? (listof rep))
  type : ctype?
Returns a value to describe the eventual C representation of the type. It can be any of the following symbols:

'int8 'uint8 'int16 'uint16 'int32 'uint32 'int64 'uint64
'float 'double 'bool 'void 'pointer 'fpointer
'bytes 'string/ucs-4 'string/utf-16

The result can also be a list, which describes a C struct whose element representations are provided in order within the list. Finally, the result can be a vector of size 2 containing an element representation followed by an exact-integer count.

Possible values for sym are 'int, 'char, 'short, 'long, '*, 'void, 'float, 'double, or lists of symbols, such as '(long long). The result is the size of the corresponding type according to the C sizeof operator for the current platform. The compiler-sizeof operation should be used to gather information about the current platform, such as defining alias type like _int to a known type like _int32.