cffi-sys
********

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

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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:

  * Functions in CFFI-SYS that are low-level versions of functions exported from the CFFI package begin with a leading percent-sign (eg. %mem-ref).

  * Where ``foreign type'' is mentioned as the kind of an argument, the meaning is restricted to that subset of all foreign types defined in Built-In Foreign Types.  Support for higher-level types is always defined in terms of those lower-level types in CFFI proper.

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.

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.

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.

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.

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.

Example
=======

     ;; 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))

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.

Examples
========

     ;; 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.
       )

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.

Foreign Globals
***************

 -- Function: foreign-symbol-pointer name => pointer
     Return a pointer to a foreign symbol NAME.