Types

This page contains a canonical list of type symbols recognized by attach_function and Struct#layout

Types

The following types can be used as arguments and return types:

Symbol	Description	Expected/returned type from FFI	Notes
:char	8-bit character	Integer
:uchar	8-bit unsigned character	Integer
:int8	8-bit signed integer	Integer
:uint8	8-bit unsigned integer	Integer
:short	16-bit signed integer	Integer
:ushort	16-bit unsigned integer	Integer
:int16	16-bit signed integer	Integer
:uint16	16-bit unsigned integer	Integer
:int	signed integer	Integer	platform specific size
:uint	unsigned integer	Integer	platform specific size
:int32	32-bit signed integer	Integer
:uint32	32-bit unsigned integer	Integer
:long	long int	Integer	platform-specific size
:ulong	unsigned long int	Integer	platform-specific size
:int64	64-bit signed integer	Integer
:uint64	64-bit unsigned integer	Integer
:long_long	64-bit signed integer	Integer
:ulong_long	64-bit unsigned integer	Integer
:float	32-bit floating point	Float
:double	64-bit floating point (double-precision)	Float
:pointer	pointer with platform-specific size	out: FFI::Pointer in: FFI::Pointer, String, Integer	Data may contain zero bytes and may not be zero-terminated. Ruby Strings can be passed directly to C, but should be considered read-only on both Ruby and C side.
:string	C-style (NULL-terminated) character string.	String	:string should be considered to be const char * and the Ruby string must not be changed as long as it’s accessed by the library. If the string buffer shall be modified from C or Ruby side, use a FFI::MemoryPointer with the :pointer type instead.
:bool	boolean	true/false
Enums	your own custom enum group (or its symbol) as a type	Symbol/Integer

For function return type only:

Symbol	Description	Returned type from FFI	Notes
:void	return type void	Pointer	for functions that return nothing
:strptr	Ruby String and a Pointer to C memory	Array (e.g. ["foo", ptr])	useful to free the memory from within Ruby

For function argument type only:

Symbol	Description	Expected by FFI	Notes
:buffer_in	Similar to :pointer, but optimized for Buffers that the function can only read (not write).	FFI::Pointer
:buffer_out	Similar to :pointer, but optimized for Buffers that the function can only write (not read).	FFI::Pointer
:buffer_inout	Similar to :pointer, but may be optimized for Buffers.	FFI::Pointer
:varargs	variadic arguments	anything	see examples

See also Core-Concepts for details about the memory management of :pointer, :string and :buffer types.

More Types

Most common C types like :size_t , :uint32_t and :in_addr_t are predefined. For a full list see lib/ffi/platform/<your platform>/types.conf in the git repository.

Furthermore it’s possible to define your own types. This is particular useful to distinguish between different :pointer types like so:

typedef :pointer, :libusb_device_handle
attach_function 'libusb_reset_device', [:libusb_device_handle], :int  

There are even more nice possibilities to map values:

• Mapping of structs with optional type safety
• Definition of your own data converters

See this blog post for more information.