@node sb-grovel
@section sb-grovel
@cindex Foreign Function Interface, generation

The @code{sb-grovel} module helps in generation of foreign function
interfaces. It aids in extracting constants' values from the C
compiler and in generating SB-ALIEN structure and union types,
@pxref{Defining Foreign Types}.

The ASDF(@uref{http://www.cliki.net/ASDF}) component type
GROVEL-CONSTANTS-FILE has its PERFORM
@c @xref for PERFORM when asdf manual is included?
operation defined to write out a C source file, compile it, and run
it.  The output from this program is Lisp, which is then itself
compiled and loaded.

sb-grovel is used in a few contributed modules, and it is currently
compatible only to SBCL. However, if you want to use it, here are a
few directions.

@subsection Using sb-grovel in your own ASDF system

@enumerate

@item
Create a Lisp package for the foreign constants/functions to go into.

@item
Make your system depend on the 'sb-grovel system.

@item
Create a grovel-constants data file - for an example, see
example-constants.lisp in the contrib/sb-grovel/ directory in the SBCL
source distribution.

@item
Add it as a component in your system. e.g.

@lisp
(eval-when (:compile-toplevel :load-toplevel :execute)
  (require :sb-grovel))

(defpackage :example-package.system
            (:use :cl :asdf :sb-grovel :sb-alien))

(in-package :example-package.system)
            
(defsystem example-system
    :depends-on (sb-grovel)
    :components
    ((:module "sbcl"
              :components
              ((:file "defpackage")
               (grovel-constants-file "example-constants"
                                      :package :example-package)))))
@end lisp

Make sure to specify the package you chose in step 1

@item
Build stuff.

@end enumerate

@subsection Contents of a grovel-constants-file

The grovel-constants-file, typically named @code{constants.lisp},
comprises lisp expressions describing the foreign things that you want
to grovel for. A @code{constants.lisp} file contains two sections:

@itemize
@item
a list of headers to include in the C program, for example:
@lisp
("sys/types.h" "sys/socket.h" "sys/stat.h" "unistd.h" "sys/un.h"
 "netinet/in.h" "netinet/in_systm.h" "netinet/ip.h" "net/if.h"
 "netdb.h" "errno.h" "netinet/tcp.h" "fcntl.h" "signal.h" )
@end lisp

@item
A list of sb-grovel clauses describing the things you want to grovel
from the C compiler, for example:
@lisp
((:integer af-local
           #+(or sunos solaris) "AF_UNIX"
           #-(or sunos solaris) "AF_LOCAL"
           "Local to host (pipes and file-domain).")
 (:structure stat ("struct stat"
                   (integer dev "dev_t" "st_dev")
                   (integer atime "time_t" "st_atime")))
 (:function getpid ("getpid" int )))
@end lisp
@end itemize

There are two types of things that sb-grovel can sensibly extract from
the C compiler: constant integers and structure layouts. It is also
possible to define foreign functions in the constants.lisp file, but
these definitions don't use any information from the C program; they
expand directly to @code{sb-alien:define-alien-routine}
(@pxref{The define-alien-routine Macro}) forms.

Here's how to use the grovel clauses:

@itemize
@item
@code{:integer} - constant expressions in C. Used in this form:
@lisp
 (:integer lisp-variable-name "C expression" &optional doc export)
@end lisp

@code{"C expression"} will be typically be the name of a constant. But
other forms are possible.

@item
@code{:enum}
@lisp
 (:enum lisp-type-name ((lisp-enumerated-name c-enumerated-name) ...)))
@end lisp

An @code{sb-alien:enum} type with name @code{lisp-type-name} will be defined.
The symbols are the @code{lisp-enumerated-name}s, and the values
are grovelled from the @code{c-enumerated-name}s.

@item
@code{:structure} - alien structure definitions look like this:
@lisp
 (:structure lisp-struct-name ("struct c_structure"
                               (type-designator lisp-element-name
                                "c_element_type" "c_element_name"
                                :distrust-length nil)
                               ; ...
                               ))
@end lisp

@code{type-designator} is a reference to a type whose size (and type
constraints) will be groveled for. sb-grovel accepts a form of type
designator that doesn't quite conform to either lisp nor sb-alien's
type specifiers. Here's a list of type designators that sb-grovel
currently accepts:
@itemize
@item
@code{integer} - a C integral type; sb-grovel will infer the exact
type from size information extracted from the C program. All common C
integer types can be grovelled for with this type designator, but it
is not possible to grovel for bit fields yet.

@item
@code{(unsigned n)} - an unsigned integer variable that is @code{n}
bytes long. No size information from the C program will be used.
@item
@code{(signed n)} - an signed integer variable that is @code{n} bytes
long. No size information from the C program will be used.

@item
@code{c-string} - an array of @code{char} in the structure. sb-grovel
will use the array's length from the C program, unless you pass it the
@code{:distrust-length} keyword argument with non-@code{nil} value
(this might be required for structures such as solaris's @code{struct
dirent}).

@item
@code{c-string-pointer} - a pointer to a C string, corresponding to
the @code{sb-alien:c-string} type (@pxref{Foreign Type Specifiers}).
@item
@code{(array alien-type)} - An array of the previously-declared alien
type. The array's size will be determined from the output of the C
program and the alien type's size.
@item
@code{(array alien-type n)} - An array of the previously-declared alien
type. The array's size will be assumed as being @code{n}.
@end itemize


Note that @code{c-string} and @code{c-string-pointer} do not have the
same meaning. If you declare that an element is of type
@code{c-string}, it will be treated as if the string is a part of the
structure, whereas if you declare that the element is of type
@code{c-string-pointer}, a @emph{pointer to a string} will be the
structure member.

@item
@code{:function} - alien function definitions are similar to
@code{define-alien-routine} definitions, because they expand to such
forms when the lisp program is loaded. @xref{Foreign Function Calls}.

@lisp
(:function lisp-function-name ("alien_function_name" alien-return-type
                                                     (argument alien-type)
                                                     (argument2 alien-type)))
@end lisp
@end itemize


@subsection Programming with sb-grovel's structure types

Let us assume that you have a grovelled structure definition:
@lisp
 (:structure mystruct ("struct my_structure"
                       (integer myint "int" "st_int")
                       (c-string mystring "char[]" "st_str")))
@end lisp

What can you do with it? Here's a short interface document:

@itemize
@item
Creating and destroying objects:
@itemize
@item
Function @code{(allocate-mystruct)} - allocates an object of type @code{mystruct}and
returns a system area pointer to it.
@item
Function @code{(free-mystruct var)} - frees the alien object pointed to by
@var{var}.
@item
Macro @code{(with-mystruct var ((member init) [...]) &body body)} -
allocates an object of type @code{mystruct} that is valid in
@var{body}. If @var{body} terminates or control unwinds out of
@var{body}, the object pointed to by @var{var} will be deallocated.
@end itemize

@item
Accessing structure members:
@itemize
@item
@code{(mystruct-myint var)} and @code{(mystruct-mystring var)} return
the value of the respective fields in @code{mystruct}.
@item
@code{(setf (mystruct-myint var) new-val)} and
@code{(setf (mystruct-mystring var) new-val)} sets the value of the respective
structure member to the value of @var{new-val}. Notice that in
@code{(setf (mystruct-mystring var) new-val)}'s case, new-val is a lisp
string.
@end itemize
@end itemize

@subsubsection Traps and Pitfalls
Basically, you can treat functions and data structure definitions that
sb-grovel spits out as if they were alien routines and types. This has
a few implications that might not be immediately obvious (especially
if you have programmed in a previous version of sb-grovel that didn't
use alien types):

@itemize
@item
You must take care of grovel-allocated structures yourself. They are
alien types, so the garbage collector will not collect them when you
drop the last reference.

@item
If you use the @code{with-mystruct} macro, be sure that no references
to the variable thus allocated leaks out. It will be deallocated when
the block exits.
@end itemize
