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@node    The Foreign Function Interface
@comment  node-name,  next,  previous,  up
@chapter The Foreign Function Interface

This chapter describes SBCL's interface to C programs and
libraries (and, since C interfaces are a sort of @emph{ingua
franca} of the Unix world, to other programs and libraries in
general.)

@quotation
Note: In the modern Lisp world, the usual term for this functionality
is Foreign Function Interface, or @acronym{FFI}, where despite the
mention of ``function'' in this term, @acronym{FFI} also
refers to direct manipulation of C data structures as well as
functions. The traditional CMUCL terminology is Alien Interface, and
while that older terminology is no longer used much in the system
documentation, it still reflected in names in the implementation,
notably in the name of the @code{SB-ALIEN} package.
@end quotation

@menu
* Introduction to the Foreign Function Interface::  
* Foreign Types::               
* Operations On Foreign Values::  
* Foreign Variables::           
* Foreign Data Structure Examples::  
* Loading Unix Object Files::   
* Foreign Function Calls::      
* Step-By-Step Example of the Foreign Function Interface::  
@end menu

@node  Introduction to the Foreign Function Interface
@comment  node-name,  next,  previous,  up
@section Introduction to the Foreign Function Interface
@c AKA "Introduction to Aliens" in the CMU CL manual

Because of Lisp's emphasis on dynamic memory allocation and garbage
collection, Lisp implementations use non-C-like memory representations
for objects.  This representation mismatch creates friction when a Lisp
program must share objects with programs which expect C data.  There
are three common approaches to establishing communication:

@itemize
@item
The burden can be placed on the foreign program (and programmer) by
requiring the knowledge and use of the representations used internally
by the Lisp implementation.  This can require a considerable amount of
``glue'' code on the C side, and that code tends to be sensitively
dependent on the internal implementation details of the Lisp system.

@item
The Lisp system can automatically convert objects back and forth
between the Lisp and foreign representations.  This is convenient, but
translation becomes prohibitively slow when large or complex data
structures must be shared. This approach is supported by the SBCL
@acronym{FFI}, and used automatically by the when passing integers and
strings.

@item
The Lisp program can directly manipulate foreign objects through the
use of extensions to the Lisp language.

@end itemize

SBCL, like CMUCL before it, relies primarily on the automatic
conversion and direct manipulation approaches. The @code{SB-ALIEN}
package provices a facility wherein foreign values of simple scalar
types are automatically converted and complex types are directly
manipulated in their foreign representation.  Additionally the
lower-level System Area Pointers (or @acronym{SAP}s) can be used where
necessary to provide untyped access to foreign memory.

Any foreign objects that can't automatically be converted into Lisp
values are represented by objects of type @code{alien-value}.  Since
Lisp is a dynamically typed language, even foreign objects must have a
run-time type; this type information is provided by encapsulating the
raw pointer to the foreign data within an @code{alien-value} object.

The type language and operations on foreign types are
intentionally similar to those of the C language.

@node  Foreign Types
@comment  node-name,  next,  previous,  up
@section Foreign Types
@c AKA "Alien Types" in the CMU CL manual

Alien types have a description language based on nested list
structure. For example the C type

@example
struct foo @{
    int a;
    struct foo *b[100];
@};
@end example

has the corresponding SBCL @acronym{FFI} type

@lisp
(struct foo
  (a int)
  (b (array (* (struct foo)) 100)))
@end lisp


@menu
* Defining Foreign Types::      
* Foreign Types and Lisp Types::  
* Foreign Type Specifiers::     
@end menu

@node  Defining Foreign Types
@comment  node-name,  next,  previous,  up
@subsection Defining Foreign Types

Types may be either named or anonymous.  With structure and union
types, the name is part of the type specifier, allowing recursively
defined types such as:

@lisp
(struct foo (a (* (struct foo))))
@end lisp

An anonymous structure or union type is specified by using the name
@code{nil}.  The @code{with-alien} macro defines a local scope which
``captures'' any named type definitions.  Other types are not
inherently named, but can be given named abbreviations using the
@code{define-alien-type} macro.

@node  Foreign Types and Lisp Types
@comment  node-name,  next,  previous,  up
@subsection Foreign Types and Lisp Types

The foreign types form a subsystem of the SBCL type system.  An
@code{alien} type specifier provides a way to use any foreign type as a
Lisp type specifier.  For example,

@lisp
(typep @var{foo} '(alien (* int)))
@end lisp

can be used to determine whether @var{foo} is a pointer to a foreign
@code{int}. @code{alien} type specifiers can be used in the same ways
as ordinary Lisp type specifiers (like @code{string}.) Alien type
declarations are subject to the same precise type checking as any
other declaration.  @xref{Precise Type Checking}.

Note that the type identifiers used in the foreign type system overlap
with native Lisp type specifiers in some cases.  For example, the type
specifier @code{(alien single-float)} is identical to
@code{single-float}, since foreign floats are automatically converted
to Lisp floats.  When @code{type-of} is called on an alien value that
is not automatically converted to a Lisp value, then it will return an
@code{alien} type specifier.

@node  Foreign Type Specifiers
@comment  node-name,  next,  previous,  up
@subsection Foreign Type Specifiers

Note: All foreign type names are exported from the @code{sb-alien}
package. Some foreign type names are also symbols in
the @code{common-lisp} package, in which case they are
reexported from the @code{sb-alien} package, so that
e.g. it is legal to refer to @code{sb-alien:single-float}.

These are the basic foreign type specifiers: 

@itemize
@item
The foreign type specifier @code{(* @var{foo})} describes a pointer to
an object of type @var{foo}.  A pointed-to type @var{foo} of @code{t}
indicates a pointer to anything, similar to @code{void *} in
ANSI C. A null alien pointer can be detected with the
@code{sb-alien:null-alien} function.

@item
The foreign type specifier @code{(array @var{foo} &optional
dimensions)} describes array of the specified @code{dimensions},
holding elements of type @var{foo}. Note that (unlike in C) @code{(*
@var{foo})} and @code{(array @var{foo})} are considered to be
different types when type checking is done. If equivalence of pointer
and array types is desired, it may be explicitly coerced using
@code{sb-alien:cast}.

Arrays are accessed using @code{sb-alien:deref}, passing the indices
as additional arguments.  Elements are stored in column-major order
(as in C), so the first dimension determines only the size of the
memory block, and not the layout of the higher dimensions.  An array
whose first dimension is variable may be specified by using @code{nil}
as the first dimension.  Fixed-size arrays can be allocated as array
elements, structure slots or @code{sb-alien:with-alien}
variables. Dynamic arrays can only be allocated using
@code{sb-alien:make-alien}.

@item
The foreign type specifier @code{(sb-alien:struct @var{name} &rest
@var{fields})} describes a structure type with the specified
@var{name} and @var{fields}. Fields are allocated at the same offsets
used by the implementation's C compiler. If @var{name} is @code{nil}
then the structure is anonymous.

If a named foreign @code{struct} specifier is passed to
@code{define-alien-type} or @code{with-alien}, then this defines,
respectively, a new global or local foreign structure type.  If no
@var{fields} are specified, then the fields are taken
from the current (local or global) alien structure type definition of
@var{name}.

@item
The foreign type specifier @code{(sb-alien:union @var{name} &rest
@var{fields})} is similar to @code{sb-alien:struct}, but describes a
union type.  All fields are allocated at the same offset, and the size
of the union is the size of the largest field.  The programmer must
determine which field is active from context.

@item
The foreign type specifier @code{(sb-alien:enum @var{name} &rest
@var{specs})} describes an enumeration type that maps between integer
values and keywords. If @var{name} is @code{nil}, then the type is
anonymous.  Each element of the @var{specs} list is either a Lisp
keyword, or a list @code{(@var{keyword} @var{value})}.  @var{value} is
an integer. If @var{value} is not supplied, then it defaults to one
greater than the value for the preceding spec (or to zero if it is the
first spec).

@item
The foreign type specifier @code{(sb-alien:signed &optional
@var{bits})} specifies a signed integer with the specified number of
@var{bits} precision. The upper limit on integer
precision is determined by the machine's word size. If
@var{bits} is not specified, the maximum size will be
used.

@item
The foreign type specifier @code{(integer &optional @var{bits})}
is equivalent to the corresponding type specifier using
@code{sb-alien:signed} instead of @code{integer}.

@item
The foreign type specifier @code{(sb-alien:unsigned &optional
@var{bits})} is like corresponding type specifier using
@code{sb-alien:signed} except that the variable is treated as an
unsigned integer.

@item
The foreign type specifier @code{(boolean &optional @var{bits})} is
similar to an enumeration type, but maps from Lisp @code{nil} and
@code{t} to C @code{0} and @code{1} respectively. @var{bits}
determines the amount of storage allocated to hold the truth value.

@item
The foreign type specifier @code{single-float} describes a
floating-point number in IEEE single-precision format.

@item
The foreign type specifier @code{double-float} describes a
floating-point number in IEEE double-precision format.

@item
The foreign type specifier @code{(function @var{result-type} &rest
@var{arg-types})} describes a foreign function that takes arguments of
the specified @var{arg-types} and returns a result of type
@var{result-type}.  Note that the only context where a foreign
@code{function} type is directly specified is in the argument to
@code{sb-alien:alien-funcall}.  In all other contexts, foreign
functions are represented by foreign function pointer types: @code{(*
(function @dots{}))}.

@item
The foreign type specifier @code{sb-alien:system-area-pointer}
describes a pointer which is represented in Lisp as a
@code{system-area-pointer} object.  SBCL exports this type from
@code{sb-alien} because CMUCL did, but tentatively (as of the first
draft of this section of the manual, SBCL 0.7.6) it is deprecated,
since it doesn't seem to be required by user code.

@item
The foreign type specifier @code{sb-alien:void} is used in function
types to declare that no useful value is returned.  Using
@code{alien-funcall} to call a @code{void} foreign function will
return zero values.

@item
The foreign type specifier @code{sb-alien:c-string} is similar to
@code{(* char)}, but is interpreted as a null-terminated string, and
is automatically converted into a Lisp string when accessed; or if the
pointer is C @code{NULL} or @code{0}, then accessing it gives Lisp
@code{nil}.  Lisp strings are stored with a trailing NUL
termination, so no copying (either by the user or the implementation)
is necessary when passing them to foreign code.

Assigning a Lisp string to a @code{c-string} structure field or
variable stores the contents of the string to the memory already
pointed to by that variable.  When a foreign object of type @code{(*
char)} is assigned to a @code{c-string}, then the
@code{c-string} pointer is assigned to.  This allows
@code{c-string} pointers to be initialized.  For example:

@lisp
(cl:in-package "CL-USER") ; which USEs package "SB-ALIEN"

(define-alien-type nil (struct foo (str c-string)))

(defun make-foo (str)
  (let ((my-foo (make-alien (struct foo))))
    (setf (slot my-foo 'str) (make-alien char (length str))
          (slot my-foo 'str) str)
    my-foo))
@end lisp

Storing Lisp @code{NIL} in a @code{c-string} writes C @code{NULL} to
the variable.

@item
@code{sb-alien} also exports translations of these C type
specifiers as foreign type specifiers: @code{sb-alien:char},
@code{sb-alien:short}, @code{sb-alien:int},
@code{sb-alien:long}, @code{sb-alien:unsigned-char},
@code{sb-alien:unsigned-short},
@code{sb-alien:unsigned-int},
@code{sb-alien:unsigned-long}, @code{sb-alien:float}, and
@code{sb-alien:double}.

@end itemize

@node  Operations On Foreign Values
@comment  node-name,  next,  previous,  up
@section Operations On Foreign Values
@c AKA "Alien Operations" in the CMU CL manual

This section describes how to read foreign values as Lisp values, how
to coerce foreign values to different kinds of foreign values, and how
to dynamically allocate and free foreign variables.

@menu
* Accessing Foreign Values::    
* Coercing Foreign Values::     
* Foreign Dynamic Allocation::  
@end menu

@node  Accessing Foreign Values
@comment  node-name,  next,  previous,  up
@subsection Accessing Foreign Values

@defun sb-alien:deref @var{pointer-or-array} &rest @var{indices}
@findex deref

The @code{sb-alien:deref} function returns the value pointed to by a
foreign pointer, or the value of a foreign array element. When
dereferencing a pointer, an optional single index can be specified to
give the equivalent of C pointer arithmetic; this index is scaled by
the size of the type pointed to. When dereferencing an array, the
number of indices must be the same as the number of dimensions in the
array type. @code{deref} can be set with @code{setf} to assign a new
value.
@end defun

@defun sb-alien:slot @var{struct-or-union} &rest @var{slot-names}
@findex slot

The @code{sb-alien:slot} function extracts the value of the slot named
@var{slot-name} from a foreign @code{struct} or @code{union}. If
@var{struct-or-union} is a pointer to a structure or union, then it is
automatically dereferenced.  @code{sb-alien:slot} can be set with
@code{setf} to assign a new value. Note that @var{slot-name} is
evaluated, and need not be a compile-time constant (but only constant
slot accesses are efficiently compiled).
@end defun


@subsubsection Untyped memory

As noted at the beginning of the chapter, the System Area Pointer
facilities allow untyped access to foreign memory.  @acronym{SAP}s can
be converted to and from the usual typed foreign values using
@code{sap-alien} and @code{alien-sap} (described elsewhere), and also
to and from integers - raw machine addresses.  They should thus be
used with caution; corrupting the Lisp heap or other memory with
@acronym{SAP}s is trivial.

@defun sb-sys:int-sap @var{machine-address}
@findex int-sap

Creates a @acronym{SAP} pointing at the virtual address
@var{machine-address}.
@end defun

@defun sb-sys:sap-ref-32 @var{sap} @var{offset}
@findex sap-ref-32

Access the value of the memory location at @var{offset} bytes from
@var{sap}.  This form may also be used with @code{setf} to alter the
memory at that location.
@end defun

@defun sb-sys:sap= @var{sap1} @var{sap2}
@findex sap=

Compare @var{sap1} and @var{sap2} for equality.
@end defun

Similarly named functions exist for accessing other sizes of word,
other comparisons, and other conversions.  The reader is invited to
use @code{apropos} and @code{describe} for more details

@lisp
(apropos "sap" :sb-sys)
@end lisp


@node  Coercing Foreign Values
@comment  node-name,  next,  previous,  up
@subsection Coercing Foreign Values

@defun sb-alien:addr @var{alien-expr}
@findex addr

The @code{sb-alien:addr} macro returns a pointer to the location
specified by @var{alien-expr}, which must be either a foreign
variable, a use of @code{sb-alien:deref}, a use of
@code{sb-alien:slot}, or a use of @code{sb-alien:extern-alien}.
@end defun

@defun sb-alien:cast @var{foreign-value} @var{new-type}
@findex cast

The @code{sb-alien:cast} macro converts @var{foreign-value} to a new
foreign value with the specified @var{new-type}. Both types, old and
new, must be foreign pointer, array or function types.  Note that the
resulting Lisp foreign variable object is not @code{eq} to the
argument, but it does refer to the same foreign data bits.
@end defun

@defun sb-alien:sap-alien @var{sap} @var{type}
@findex sap-alien

The @code{sb-alien:sap-alien} function converts @var{sap} (a system
area pointer) to a foreign value with the specified
@var{type}. @var{type} is not evaluated.  </para>

The @var{type} must be some foreign pointer, array, or record type.
@end defun

@defun sb-alien:alien-sap @var{foreign-value} @var{type}
@findex alien-sap

The @code{sb-alien:alien-sap} function returns the @acronym{SAP} which
points to @var{alien-value}'s data.

The @var{foreign-value} must be of some foreign pointer, array, or
record type.
@end defun


@node  Foreign Dynamic Allocation
@comment  node-name,  next,  previous,  up
@subsection Foreign Dynamic Allocation

Lisp code can call the C standard library functions @code{malloc} and
@code{free} to dynamically allocate and deallocate foreign
variables. The Lisp code shares the same allocator with foreign C
code, so it's OK for foreign code to call @code{free} on the result of
Lisp @code{sb-alien:make-alien}, or for Lisp code to call
@code{sb-alien:free-alien} on foreign objects allocated by C
code.

@defmac sb-alien:make-alien @var{type} @var{size}
@findex make-alien

The @code{sb-alien:make-alien} macro
returns a dynamically allocated foreign value of the specified
@var{type} (which is not evaluated.)  The allocated memory is not
initialized, and may contain arbitrary junk.  If supplied,
@var{size} is an expression to evaluate to compute the size of the
allocated object.  There are two major cases:

@itemize
@item
When @var{type} is a foreign array type, an array of that type is
allocated and a pointer to it is returned.  Note that you must use
@code{deref} to change the result to an array before you can use
@code{deref} to read or write elements:

@lisp
(cl:in-package "CL-USER") ; which USEs package "SB-ALIEN"
(defvar *foo* (make-alien (array char 10)))
(type-of *foo*) @result{} (alien (* (array (signed 8) 10)))
(setf (deref (deref foo) 0) 10) @result{} 10
@end lisp

If supplied, @var{size} is used as the first dimension for the
    array.

@item
When @var{type} is any other foreign type, then an object for that
type is allocated, and a pointer to it is returned.  So
@code{(make-alien int)} returns a @code{(* int)}.  If @var{size} is
specified, then a block of that many objects is allocated, with the
result pointing to the first one.

@end itemize

@end defmac

@defun sb-alien:free-alien @var{foreign-value}
@findex free-alien

The @code{sb-alien:free-alien} function
frees the storage for @var{foreign-value}, 
which must have been allocated with Lisp @code{make-alien}
or C @code{malloc}.

See also the @code{sb-alien:with-alien} macro, which allocates foreign
values on the stack.
@end defun

@node  Foreign Variables
@comment  node-name,  next,  previous,  up
@section Foreign Variables
@c AKA "Alien Variables" in the CMU CL manual

Both local (stack allocated) and external (C global) foreign variables
are supported.

@menu
* Local Foreign Variables::     
* External Foreign Variables::  
@end menu

@node  Local Foreign Variables
@comment  node-name,  next,  previous,  up
@subsection Local Foreign Variables

@defmac sb-alien:with-alien @var{var-definitions} &body @var{body}
@findex with-alien

The @code{with-alien} macro establishes local foreign variables with
the specified alien types and names.  This form is analogous to
defining a local variable in C: additional storage is allocated, and
the initial value is copied.  This form is less analogous to
@code{LET}-allocated Lisp variables, since the variables can't be
captured in closures: they live only for the dynamic extent of the
body, and referring to them outside is a gruesome error.

The @var{var-definitions} argument is a list of 
variable definitions, each of the form
@lisp
(@var{name} @var{type} &optional @var{initial-value})
@end lisp

The names of the variables are established as symbol-macros; the
bindings have lexical scope, and may be assigned with @code{setq} or
@code{setf}.
 
The @code{with-alien} macro also establishes a new scope for named
structures and unions.  Any @var{type} specified for a variable may
contain named structure or union types with the slots specified.
Within the lexical scope of the binding specifiers and body, a locally
defined foreign structure type @var{foo} can be referenced by its name
using @code{(struct @var{foo})}.
@end defmac

@node  External Foreign Variables
@comment  node-name,  next,  previous,  up
@subsection External Foreign Variables

External foreign names are strings, and Lisp names are symbols. When
an external foreign value is represented using a Lisp variable, there
must be a way to convert from one name syntax into the other. The
macros @code{extern-alien}, @code{define-alien-variable} and
@code{define-alien-routine} use this conversion heuristic:

@itemize

@item
Alien names are converted to Lisp names by uppercasing and replacing
underscores with hyphens.

@item
Conversely, Lisp names are converted to alien names by lowercasing and
replacing hyphens with underscores.

@item
Both the Lisp symbol and alien string names may be separately
specified by using a list of the form

@lisp
(alien-string lisp-symbol)
@end lisp

@end itemize

@defmac sb-alien:define-alien-variable @var{name} @var{type}
@findex define-alien-variable

The @code{define-alien-variable} macro defines @var{name} as an
external foreign variable of the specified foreign @code{type}.
@var{name} and @code{type} are not evaluated.  The Lisp name of the
variable (see above) becomes a global alien variable.  Global alien
variables are effectively ``global symbol macros''; a reference to the
variable fetches the contents of the external variable.  Similarly,
setting the variable stores new contents -- the new contents must be
of the declared @code{type}. Someday, they may well be implemented
using the @acronym{ANSI} @code{define-symbol-macro} mechanism, but as
of SBCL 0.7.5, they are still implemented using an older more-or-less
parallel mechanism inherited from CMUCL.
  
For example, to access a C-level counter @var{foo}, one could write

@lisp
(define-alien-variable "foo" int)
;; Now it is possible to get the value of the C variable foo simply by
;; referencing that Lisp variable:
(print foo)
(setf foo 14)
(incf foo)
@end lisp
@end defmac

@defun sb-alien:get-errno
@findex get-errno

Since in modern C libraries, the @code{errno} ``variable'' is typically
no longer a variable, but some bizarre artificial construct
which behaves superficially like a variable within a given thread,
it can no longer reliably be accessed through the ordinary 
@code{define-alien-variable} mechanism. Instead, SBCL provides
the operator @code{sb-alien:get-errno} to allow Lisp code to read it.
@end defun

@defmac sb-alien:extern-alien @var{name} @var{type}
@findex extern-alien

The @code{extern-alien} macro returns an alien with the specified
@var{type} which points to an externally defined value.  @var{name} is
not evaluated, and may be either a string or a symbol.  @var{type} is
an unevaluated alien type specifier.
@end defmac

@node  Foreign Data Structure Examples
@comment  node-name,  next,  previous,  up
@section Foreign Data Structure Examples
@c AKA "Alien Data Structure Example" in the CMU CL manual

Now that we have alien types, operations and variables, we can
manipulate foreign data structures.  This C declaration

@example
struct foo @{
    int a;
    struct foo *b[100];
@};
@end example

can be translated into the following alien type:

@lisp
(define-alien-type nil
  (struct foo
    (a int)
    (b (array (* (struct foo)) 100))))
@end lisp

Once the @code{foo} alien type has been defined as above, the C
expression

@example
struct foo f;
f.b[7].a;
@end example

can be translated in this way:

@lisp
(with-alien ((f (struct foo)))
  (slot (deref (slot f 'b) 7) 'a)
  ;;
  ;; Do something with f...
  )
@end lisp

Or consider this example of an external C variable and some accesses:

@example
struct c_struct @{
        short x, y;
        char a, b;
        int z;
        c_struct *n;
@};
extern struct c_struct *my_struct;
my_struct->x++;
my_struct->a = 5;
my_struct = my_struct->n;
@end example

which can be manipulated in Lisp like this:

@lisp
(define-alien-type nil
  (struct c-struct
          (x short)
          (y short)
          (a char)
          (b char)
          (z int)
          (n (* c-struct))))
(define-alien-variable "my_struct" (* c-struct))
(incf (slot my-struct 'x))
(setf (slot my-struct 'a) 5)
(setq my-struct (slot my-struct 'n))
@end lisp

@node  Loading Unix Object Files
@comment  node-name,  next,  previous,  up
@section Loading Unix Object Files

Foreign object files can be loaded into the running Lisp process by
calling the functions @code{load-foreign} or @code{load-1-foreign}.

The @code{sb-alien:load-1-foreign} function is the more primitive of
the two operations. It loads a single object file into the currently
running Lisp. The external symbols defining routines and variables are
made available for future external references (e.g. by
@code{extern-alien}). Forward references to foreign symbols aren't
supported: @code{load-1-foreign} must be run before any of the defined
symbols are referenced.

@code{sb-alien:load-foreign} is built in terms of
@code{load-1-foreign} and some other machinery like
@code{sb-ext:run-program}.  It accepts a list of files and libraries,
and runs the linker on the files and libraries, creating an absolute
Unix object file which is then processed by @code{load-1-foreign}.

@quotation
Note: As of SBCL 0.7.5, all foreign code (code loaded with
@code{load-1-function} or @code{load-function}) is lost when a Lisp
core is saved with @code{sb-ext:save-lisp-and-die}, and no attempt is
made to restore it when the core is loaded. Historically this has been
an annoyance both for SBCL users and for CMUCL users.  It's hard to
solve this problem completely cleanly, but some generally-reliable
partial solution might be useful. Once someone in either camp gets
sufficiently annoyed to create it, SBCL is likely to adopt some
mechanism for automatically restoring foreign code when a saved core
is loaded.
@end quotation


@node  Foreign Function Calls
@comment  node-name,  next,  previous,  up
@section Foreign Function Calls

The foreign function call interface allows a Lisp program to call
many functions written in languages that use the C calling convention.

Lisp sets up various signal handling routines and other environment
information when it first starts up, and expects these to be in place
at all times. The C functions called by Lisp should not change the
environment, especially the signal handlers: the signal handlers
installed by Lisp typically have interesting flags set (e.g to request
machine context information, or for signal delivery on an alternate
stack) which the Lisp runtime relies on for correct operation.
Precise details of how this works may change without notice between
versions; the source, or the brain of a friendly SBCL developer, is
the only documentation.  Users of a Lisp built with the
@code{:sb-thread} feature should also read the section about threads,
@ref{Threading}.

@menu
* The alien-funcall Primitive::  
* The define-alien-routine Macro::  
* define-alien-routine Example::  
* Calling Lisp From C::         
@end menu

@node  The alien-funcall Primitive
@comment  node-name,  next,  previous,  up
@subsection The @code{alien-funcall} Primitive

@defun sb-alien:alien-funcall @var{alien-function} &rest @var{arguments}
@findex alien-funcall

The @code{alien-funcall} function is the foreign function call
primitive: @var{alien-function} is called with the supplied
@var{arguments} and its C return value is returned as a Lisp value.
The @var{alien-function} is an arbitrary run-time expression; to refer
to a constant function, use @code{extern-alien} or a value defined by
@code{define-alien-routine}.
  
The type of @code{alien-function} must be @code{(alien (function
...))}  or @code{(alien (* (function ...)))}.  The function type is
used to determine how to call the function (as though it was declared
with a prototype.)  The type need not be known at compile time, but
only known-type calls are efficiently compiled.  Limitations:

@itemize

@item
Structure type return values are not implemented.

@item
Passing of structures by value is not implemented.

@end itemize

@end defun

Here is an example which allocates a @code{(struct foo)}, calls a
foreign function to initialize it, then returns a Lisp vector of all
the @code{(* (struct foo))} objects filled in by the foreign call:

@lisp
;; Allocate a foo on the stack.
(with-alien ((f (struct foo)))
  ;; Call some C function to fill in foo fields.
  (alien-funcall (extern-alien "mangle_foo" (function void (* foo)))
                 (addr f))
  ;; Find how many foos to use by getting the A field.
  (let* ((num (slot f 'a))
         (result (make-array num)))
    ;; Get a pointer to the array so that we don't have to keep extracting it:
    (with-alien ((a (* (array (* (struct foo)) 100)) (addr (slot f 'b))))
      ;; Loop over the first N elements and stash them in the result vector.
      (dotimes (i num)
        (setf (svref result i) (deref (deref a) i)))
      ;; Voila.
      result)))
@end lisp

@node  The define-alien-routine Macro
@comment  node-name,  next,  previous,  up
@subsection The @code{define-alien-routine} Macro

@defmac sb-alien:define-alien-routine @var{name} @var{result-type} &rest @var{arg-specifiers}
@findex define-alien-routine

The @code{define-alien-routine} macro is a convenience for
automatically generating Lisp interfaces to simple foreign functions.
The primary feature is the parameter style specification, which
translates the C pass-by-reference idiom into additional return
values.

@var{name} is usually a string external symbol, but may also be a
symbol Lisp name or a list of the foreign name and the Lisp name.  If
only one name is specified, the other is automatically derived as for
@code{extern-alien}.  @var{result-type} is the alien type of the
return value.

Each element of the @var{arg-specifiers} list 
specifies an argument to the foreign function, and is
of the form
@lisp
(aname atype &amp;optional style)
@end lisp

@var{aname} is the symbol name of the argument to the constructed
function (for documentation). @var{atype} is the alien type of
corresponding foreign argument.  The semantics of the actual call are
the same as for @code{alien-funcall}. @var{style} specifies how this
argument should be handled at call and return time, and should be one
of the following:

@itemize

@item
@code{:in} specifies that the argument is passed by value. This is the
default. @code{:in} arguments have no corresponding return value from
the Lisp function.

@item
@code{:copy} is similar to @code{:in}, but the argument is copied to a
pre-allocated object and a pointer to this object is passed to the
foreign routine.

@item
@code{:out} specifies a pass-by-reference output value.  The type of
the argument must be a pointer to a fixed-sized object (such as an
integer or pointer).  @code{:out} and @code{:in-out} style cannot be
used with pointers to arrays, records or functions.  An object of the
correct size is allocated on the stack, and its address is passed to
the foreign function.  When the function returns, the contents of this
location are returned as one of the values of the Lisp function (and
the location is automatically deallocated).

@item
@code{:in-out} is a combination of @code{:copy} and @code{:out}.  The
argument is copied to a pre-allocated object and a pointer to this
object is passed to the foreign routine.  On return, the contents of
this location is returned as an additional value.

@end itemize

@quotation
Note: Any efficiency-critical foreign interface function should be inline
expanded, which can be done by preceding the
@code{define-alien-routine} call with:

@lisp
(declaim (inline lisp-name))
@end lisp

In addition to avoiding the Lisp call overhead, this allows
pointers, word-integers and floats to be passed using non-descriptor
representations, avoiding consing.)
@end quotation

@end defmac

@node  define-alien-routine Example
@comment  node-name,  next,  previous,  up
@subsection @code{define-alien-routine} Example

Consider the C function @code{cfoo} with the following calling
convention:

@example
void
cfoo (str, a, i)
    char *str;
    char *a; /* update */
    int *i; /* out */
@{
  /* body of cfoo(...) */
@}
@end example

This can be described by the following call to
@code{define-alien-routine}:

@lisp
(define-alien-routine "cfoo" void
  (str c-string)
  (a char :in-out)
  (i int :out))
@end lisp

The Lisp function @code{cfoo} will have two arguments (@var{str} and
@var{a}) and two return values (@var{a} and @var{i}).

@node  Calling Lisp From C
@comment  node-name,  next,  previous,  up
@subsection Calling Lisp From C

Calling Lisp functions from C is sometimes possible, but is extremely
hackish and poorly supported as of SBCL 0.7.5.  See @code{funcall0}
@dots{} @code{funcall3} in the runtime system. The arguments must be
valid SBCL object descriptors (so that e.g. fixnums must be
left-shifted by 2.) As of SBCL 0.7.5, the format of object descriptors
is documented only by the source code and, in parts, by the old CMUCL
@file{INTERNALS} documentation.

Note that the garbage collector moves objects, and won't be
able to fix up any references in C variables.  There are three
mechanisms for coping with this: 

@enumerate
@item
The @code{sb-ext:purify} moves all live Lisp
data into static or read-only areas such that it will never be moved
(or freed) again in the life of the Lisp session

@item
@code{sb-sys:with-pinned-objects} is a macro which arranges for some
set of objects to be pinned in memory for the dynamic extent of its
body forms.  On ports which use the generational garbage collector (as
of SBCL 0.8.3, only the x86) this has a page granularity - i.e. the
entire 4k page or pages containing the objects will be locked down. On
other ports it is implemented by turning off GC for the duration (so
could be said to have a whole-world granularity).

@item
Disable GC, using the @code{without-gcing} macro or @code{gc-off}
call.
@end enumerate

@c <!-- FIXME: This is a "changebar" section from the CMU CL manual.
@c      I (WHN 2002-07-14) am not very familiar with this content, so 
@c      I'm not immediately prepared to try to update it for SBCL, and
@c      I'm not feeling masochistic enough to work to encourage this
@c      kind of low-level hack anyway. However, I acknowledge that callbacks
@c      are sometimes really really necessary, so I include the original
@c      text in case someone is hard-core enough to benefit from it. If
@c      anyone brings the information up to date for SBCL, it belong
@c      either in the main manual or on a CLiki SBCL Internals page.
@c LaTeX \subsection{Accessing Lisp Arrays}
@c LaTeX 
@c LaTeX Due to the way \cmucl{} manages memory, the amount of memory that can
@c LaTeX be dynamically allocated by \code{malloc} or \funref{make-alien} is
@c LaTeX limited\footnote{\cmucl{} mmaps a large piece of memory for it's own
@c LaTeX   use and this memory is typically about 8 MB above the start of the C
@c LaTeX   heap.  Thus, only about 8 MB of memory can be dynamically
@c LaTeX   allocated.}.

@c Empirically determined to be considerably >8Mb on this x86 linux
@c machine, but I don't know what the actual values are - dan 2003.09.01

@c Note that this technique is used in SB-GROVEL in the SBCL contrib

@c LaTeX 
@c LaTeX To overcome this limitation, it is possible to access the content of
@c LaTeX Lisp arrays which are limited only by the amount of physical memory
@c LaTeX and swap space available.  However, this technique is only useful if
@c LaTeX the foreign function takes pointers to memory instead of allocating
@c LaTeX memory for itself.  In latter case, you will have to modify the
@c LaTeX foreign functions.
@c LaTeX 
@c LaTeX This technique takes advantage of the fact that \cmucl{} has
@c LaTeX specialized array types (\pxlref{specialized-array-types}) that match
@c LaTeX a typical C array.  For example, a \code{(simple-array double-float
@c LaTeX   (100))} is stored in memory in essentially the same way as the C
@c LaTeX array \code{double x[100]} would be.  The following function allows us
@c LaTeX to get the physical address of such a Lisp array:
@c LaTeX \begin{example}
@c LaTeX (defun array-data-address (array)
@c LaTeX   "Return the physical address of where the actual data of an array is
@c LaTeX stored.
@c LaTeX 
@c LaTeX ARRAY must be a specialized array type in CMU Lisp.  This means ARRAY
@c LaTeX must be an array of one of the following types:
@c LaTeX 
@c LaTeX                   double-float
@c LaTeX                   single-float
@c LaTeX                   (unsigned-byte 32)
@c LaTeX                   (unsigned-byte 16)
@c LaTeX                   (unsigned-byte  8)
@c LaTeX                   (signed-byte 32)
@c LaTeX                   (signed-byte 16)
@c LaTeX                   (signed-byte  8)
@c LaTeX "
@c LaTeX   (declare (type (or #+signed-array (array (signed-byte 8))
@c LaTeX                      #+signed-array (array (signed-byte 16))
@c LaTeX                      #+signed-array (array (signed-byte 32))
@c LaTeX                      (array (unsigned-byte 8))
@c LaTeX                      (array (unsigned-byte 16))
@c LaTeX                      (array (unsigned-byte 32))
@c LaTeX                      (array single-float)
@c LaTeX                      (array double-float))
@c LaTeX                  array)
@c LaTeX            (optimize (speed 3) (safety 0))
@c LaTeX            (ext:optimize-interface (safety 3)))
@c LaTeX   ;; with-array-data will get us to the actual data.  However, because
@c LaTeX   ;; the array could have been displaced, we need to know where the
@c LaTeX   ;; data starts.
@c LaTeX   (lisp::with-array-data ((data array)
@c LaTeX                           (start)
@c LaTeX                           (end))
@c LaTeX     (declare (ignore end))
@c LaTeX     ;; DATA is a specialized simple-array.  Memory is laid out like this:
@c LaTeX     ;;
@c LaTeX     ;;   byte offset    Value
@c LaTeX     ;;        0         type code (should be 70 for double-float vector)
@c LaTeX     ;;        4         4 * number of elements in vector
@c LaTeX     ;;        8         1st element of vector
@c LaTeX     ;;      ...         ...
@c LaTeX     ;;
@c LaTeX     (let ((addr (+ 8 (logandc1 7 (kernel:get-lisp-obj-address data))))
@c LaTeX           (type-size (let ((type (array-element-type data)))
@c LaTeX                        (cond ((or (equal type '(signed-byte 8))
@c LaTeX                                   (equal type '(unsigned-byte 8)))
@c LaTeX                               1)
@c LaTeX                              ((or (equal type '(signed-byte 16))
@c LaTeX                                   (equal type '(unsigned-byte 16)))
@c LaTeX                               2)
@c LaTeX                              ((or (equal type '(signed-byte 32))
@c LaTeX                                   (equal type '(unsigned-byte 32)))
@c LaTeX                               4)
@c LaTeX                              ((equal type 'single-float)
@c LaTeX                               4)
@c LaTeX                              ((equal type 'double-float)
@c LaTeX                               8)
@c LaTeX                              (t
@c LaTeX                               (error "Unknown specialized array element type"))))))
@c LaTeX       (declare (type (unsigned-byte 32) addr)
@c LaTeX                (optimize (speed 3) (safety 0) (ext:inhibit-warnings 3)))
@c LaTeX       (system:int-sap (the (unsigned-byte 32)
@c LaTeX                         (+ addr (* type-size start)))))))
@c LaTeX \end{example}
@c LaTeX 
@c LaTeX Assume we have the C function below that we wish to use:
@c LaTeX \begin{example}
@c LaTeX   double dotprod(double* x, double* y, int n)
@c LaTeX   \{
@c LaTeX     int k;
@c LaTeX     double sum = 0;
@c LaTeX 
@c LaTeX     for (k = 0; k < n; ++k) \{
@c LaTeX       sum += x[k] * y[k];
@c LaTeX     \}
@c LaTeX   \}
@c LaTeX \end{example}
@c LaTeX The following example generates two large arrays in Lisp, and calls the C
@c LaTeX function to do the desired computation.  This would not have been
@c LaTeX possible using \code{malloc} or \code{make-alien} since we need about
@c LaTeX 16 MB of memory to hold the two arrays.
@c LaTeX \begin{example}
@c LaTeX   (define-alien-routine "dotprod" double
@c LaTeX     (x (* double-float) :in)
@c LaTeX     (y (* double-float) :in)
@c LaTeX     (n int :in))
@c LaTeX     
@c LaTeX   (let ((x (make-array 1000000 :element-type 'double-float))
@c LaTeX         (y (make-array 1000000 :element-type 'double-float)))
@c LaTeX     ;; Initialize X and Y somehow
@c LaTeX     (let ((x-addr (system:int-sap (array-data-address x)))
@c LaTeX           (y-addr (system:int-sap (array-data-address y))))
@c LaTeX       (dotprod x-addr y-addr 1000000)))    
@c LaTeX \end{example}
@c LaTeX In this example, it may be useful to wrap the inner \code{let}
@c LaTeX expression in an \code{unwind-protect} that first turns off garbage
@c LaTeX collection and then turns garbage collection on afterwards.  This will
@c LaTeX prevent garbage collection from moving \code{x} and \code{y} after we
@c LaTeX have obtained the (now erroneous) addresses but before the call to
@c LaTeX \code{dotprod} is made.
@c LaTeX 
@c -->


@node  Step-By-Step Example of the Foreign Function Interface
@comment  node-name,  next,  previous,  up
@section Step-By-Step Example of the Foreign Function Interface

This section presents a complete example of an interface to a somewhat
complicated C function.

Suppose you have the following C function which you want to be able to
call from Lisp in the file @file{test.c}

@example
struct c_struct
@{
  int x;
  char *s;
@};
 
struct c_struct *c_function (i, s, r, a)
    int i;
    char *s;
    struct c_struct *r;
    int a[10];
@{
  int j;
  struct c_struct *r2;
 
  printf("i = %d\n", i);
  printf("s = %s\n", s);
  printf("r->x = %d\n", r->x);
  printf("r->s = %s\n", r->s);
  for (j = 0; j &lt; 10; j++) printf("a[%d] = %d.\n", j, a[j]);
  r2 = (struct c_struct *) malloc (sizeof(struct c_struct));
  r2->x = i + 5;
  r2->s = "a C string";
  return(r2);
@};
@end example

It is possible to call this C function from Lisp using the file
@file{test.lisp} containing

@lisp
(cl:defpackage "TEST-C-CALL" (:use "CL" "SB-ALIEN" "SB-C-CALL"))
(cl:in-package "TEST-C-CALL")

;;; Define the record C-STRUCT in Lisp.
(define-alien-type nil
    (struct c-struct
            (x int)
            (s c-string)))

;;; Define the Lisp function interface to the C routine.  It returns a
;;; pointer to a record of type C-STRUCT.  It accepts four parameters:
;;; I, an int; S, a pointer to a string; R, a pointer to a C-STRUCT
;;; record; and A, a pointer to the array of 10 ints.
;;;
;;; The INLINE declaration eliminates some efficiency notes about heap
;;; allocation of alien values.
(declaim (inline c-function))
(define-alien-routine c-function
    (* (struct c-struct))
  (i int)
  (s c-string)
  (r (* (struct c-struct)))
  (a (array int 10)))

;;; a function which sets up the parameters to the C function and
;;; actually calls it
(defun call-cfun ()
  (with-alien ((ar (array int 10))
               (c-struct (struct c-struct)))
    (dotimes (i 10)                     ; Fill array.
      (setf (deref ar i) i))
    (setf (slot c-struct 'x) 20)
    (setf (slot c-struct 's) "a Lisp string")

    (with-alien ((res (* (struct c-struct))
                      (c-function 5 "another Lisp string" (addr c-struct) ar)))
      (format t "~&amp;back from C function~%")
      (multiple-value-prog1
          (values (slot res 'x)
                  (slot res 's))

        ;; Deallocate result. (after we are done referring to it:
        ;; "Pillage, *then* burn.")
        (free-alien res)))))
@end lisp

To execute the above example, it is necessary to compile the C
routine, e.g.: @samp{cc -c test.c} (In order to enable incremental
loading with some linkers, you may need to say @samp{cc -G 0 -c
test.c})

Once the C code has been compiled, you can start up Lisp and load it
in: @samp{sbcl} Lisp should start up with its normal prompt.

Within Lisp, compile the Lisp file. (This step can be done
separately. You don't have to recompile every time.)
@samp{(compile-file "test.lisp")}

Within Lisp, load the foreign object file to define the necessary
symbols: @samp{(load-foreign "test.o")}.  This must be done before
loading any code that refers to these symbols.

Now you can load the compiled Lisp (``fasl'') file into Lisp:
@samp{(load "test.fasl")}
And once the Lisp file is loaded, you can call the 
Lisp routine that sets up the parameters and calls the C
function:
@samp{(test-c-call::call-cfun)}

The C routine should print the following information to standard output:

@example
i = 5
s = another Lisp string
r->x = 20
r->s = a Lisp string
a[0] = 0.
a[1] = 1.
a[2] = 2.
a[3] = 3.
a[4] = 4.
a[5] = 5.
a[6] = 6.
a[7] = 7.
a[8] = 8.
a[9] = 9.
@end example

After return from the C function,
the Lisp wrapper function should print the following output:

@example
back from C function
@end example

And upon return from the Lisp wrapper function,
before the next prompt is printed, the
Lisp read-eval-print loop should print the following return values:

@example
10
"a C string"
@end example