CFFI-SYS Interface Specification

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Copyright © 2005-2006, James Bielman <jamesjb at>

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1 Introduction

CFFI, the Common Foreign Function Interface, purports to be a portable foreign function interface for Common Lisp.

This specification defines a set of low-level primitives that must be defined for each Lisp implementation supported by CFFI. These operators are defined in the CFFI-SYS package.

The CFFI package uses the CFFI-SYS interface to implement an extensible foreign type system with support for typedefs, structures, and unions, a declarative interface for defining foreign function calls, and automatic conversion of foreign function arguments to/from Lisp types.

Please note the following conventions that apply to everything in CFFI-SYS:

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2 Built-In Foreign Types

— Foreign Type: :char
— Foreign Type: :unsigned-char
— Foreign Type: :short
— Foreign Type: :unsigned-short
— Foreign Type: :int
— Foreign Type: :unsigned-int
— Foreign Type: :long
— Foreign Type: :unsigned-long
— Foreign Type: :long-long
— Foreign Type: :unsigned-long-long

These types correspond to the native C integer types according to the ABI of the system the Lisp implementation is compiled against.

— Foreign Type: :int8
— Foreign Type: :uint8
— Foreign Type: :int16
— Foreign Type: :uint16
— Foreign Type: :int32
— Foreign Type: :uint32
— Foreign Type: :int64
— Foreign Type: :uint64

Foreign integer types of specific sizes, corresponding to the C types defined in stdint.h.

— Foreign Type: :size
— Foreign Type: :ssize
— Foreign Type: :ptrdiff
— Foreign Type: :time

Foreign integer types corresponding to the standard C types (without the _t suffix).

Implementor's note: I'm sure there are more of these that could be useful, let's add any types that can't be defined portably to this list as necessary.

— Foreign Type: :float
— Foreign Type: :double

The :float type represents a C float and a Lisp single-float. :double represents a C double and a Lisp double-float.

— Foreign Type: :pointer

A foreign pointer to an object of any type, corresponding to void *.

— Foreign Type: :void

No type at all. Only valid as the return type of a function.

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3 Operations on Built-in Foreign Types

— Function: %foreign-type-size type => size

Return the size, in bytes, of objects having foreign type type. An error is signalled if type is not a known built-in foreign type.

— Function: %foreign-type-alignment type => alignment

Return the default alignment in bytes for structure members of foreign type type. An error is signalled if type is not a known built-in foreign type.

Implementor's note: Maybe this should take an optional keyword argument specifying an alternate alignment system, eg. :mac68k for 68000-compatible alignment on Darwin.

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4 Basic Pointer Operations

— Function: pointerp ptr => boolean

Return true if ptr is a foreign pointer.

— Function: null-pointer => pointer

Return a null foreign pointer.

— Function: null-pointer-p ptr => boolean

Return true if ptr is a null foreign pointer.

— Function: make-pointer address => pointer

Return a pointer corresponding to the numeric integer address.

— Function: inc-pointer ptr offset => pointer

Return the result of numerically incrementing ptr by offset.

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5 Foreign Memory Allocation

— Function: foreign-alloc size => pointer

Allocate size bytes of foreign-addressable memory and return a pointer to the allocated block. An implementation-specific error is signalled if the memory cannot be allocated.

— Function: foreign-free ptr => unspecified

Free a pointer ptr allocated by foreign-alloc. The results are undefined if ptr is used after being freed.

— Macro: with-foreign-pointer (var size &optional size-var) &body body

Bind var to a pointer to size bytes of foreign-accessible memory during body. Both ptr and the memory block it points to have dynamic extent and may be stack allocated if supported by the implementation. If size-var is supplied, it will be bound to size during body.

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6 Memory Access

— Accessor: %mem-ref ptr type &optional offset

Dereference a pointer offset bytes from ptr to an object for reading (or writing when used with setf) of built-in type type.


     ;; An impractical example, since time returns the time as well,
     ;; but it demonstrates %MEM-REF. Better (simple) examples wanted!
     (with-foreign-pointer (p (foreign-type-size :time))
       (foreign-funcall "time" :pointer p :time)
       (%mem-ref p :time))

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7 Foreign Function Calling

— Macro: %foreign-funcall name {arg-type arg}* &optional result-type => object
— Macro: %foreign-funcall-pointer ptr {arg-type arg}* &optional result-type => object

Invoke a foreign function called name in the foreign source code.

Each arg-type is a foreign type specifier, followed by arg, Lisp data to be converted to foreign data of type arg-type. result-type is the foreign type of the function's return value, and is assumed to be :void if not supplied.

%foreign-funcall-pointer takes a pointer ptr to the function, as returned by foreign-symbol-pointer, rather than a string name.


     ;; Calling a standard C library function:
     (%foreign-funcall "sqrtf" :float 16.0 :float) => 4.0
     ;; Dynamic allocation of a buffer and passing to a function:
     (with-foreign-ptr (buf 255 buf-size)
       (%foreign-funcall "gethostname" :pointer buf :size buf-size :int)
       ;; Convert buf to a Lisp string using MAKE-STRING and %MEM-REF or
       ;; a portable CFFI function such as CFFI:FOREIGN-STRING-TO-LISP.

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8 Loading Foreign Libraries

— Function: %load-foreign-library name => unspecified

Load the foreign shared library name.

Implementor's note: There is a lot of behavior to decide here. Currently I lean toward not requiring NAME to be a full path to the library so we can search the system library directories (maybe even get LD_LIBRARY_PATH from the environment) as necessary.

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9 Foreign Globals

— Function: foreign-symbol-pointer name => pointer

Return a pointer to a foreign symbol name.

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Symbol Index