Chapter 8  Alien Objects

• Introduction to Aliens
• Alien Types
  • Defining Alien Types
  • Alien Types and Lisp Types
  • Alien Type Specifiers
  • The C-Call Package
• Alien Operations
  • Alien Access Operations
  • Alien Coercion Operations
  • Alien Dynamic Allocation
• Alien Variables
  • Local Alien Variables
  • External Alien Variables
• Alien Data Structure Example
• Loading Unix Object Files
• Alien Function Calls
  • The alien-funcall Primitive
  • The def-alien-routine Macro
  • def-alien-routine Example
  • Calling Lisp from C
  • Callback Example
  • Accessing Lisp Arrays
• Step-by-Step Alien Example

8.1  Introduction to Aliens

• The burden can be placed on the foreign program (and programmer) by requiring the use of Lisp object representations. The main difficulty with this approach is that either the foreign program must be written with Lisp interaction in mind, or a substantial amount of foreign “glue” code must be written to perform the translation.
  
  
• 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.
  
  
• The Lisp program can directly manipulate foreign objects through the use of extensions to the Lisp language. Most Lisp systems make use of this approach, but the language for describing types and expressing accesses is often not powerful enough for complex objects to be easily manipulated.

8.2  Alien Types

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

(struct foo
  (a int)
  (b (array (* (struct foo)) 100)))

8.2.1  Defining Alien Types

(struct foo (a (* (struct foo))))

This macro globally defines name as a shorthand for the Alien type type. When introducing global structure and union type definitions, name may be nil, in which case the name to define is taken from the type's name.

8.2.2  Alien Types and Lisp Types

(typep foo '(alien (* int)))

8.2.3  Alien Type Specifiers

A pointer to an object of the specified type. If type is t, then it means a pointer to anything, similar to “void *” in ANSI C. Currently, the only way to detect a null pointer is:

  (zerop (sap-int (alien-sap ptr)))

See section 6.5

An array of the specified dimensions, holding elements of type type. Note that (* int) and (array int) are considered to be different types when type checking is done; pointer and array types must be explicitly coerced using cast.

Arrays are accessed using 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 nil as the first dimension. Fixed-size arrays can be allocated as array elements, structure slots or with-alien variables. Dynamic arrays can only be allocated using make-alien.

A structure type with the specified name and fields. Fields are allocated at the same positions used by the implementation's C compiler. bits is intended for C-like bit field support, but is currently unused. If name is nil, then the type is anonymous.

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

Similar to struct, but defines 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.

An enumeration type that maps between integer values and keywords. If name is nil, then the type is anonymous. Each spec is either a keyword, or a list (keyword value). If integer 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.)

A signed integer with the specified number of bits precision. The upper limit on integer precision is determined by the machine's word size. If no size is specified, the maximum size will be used.

Identical to signed—the distinction between signed and integer is purely stylistic.

Like signed, but specifies an unsigned integer.

Similar to an enumeration type that maps 0 to nil and all other values to t. bits determines the amount of storage allocated to hold the truth value.

A floating-point number in IEEE single format.

A floating-point number in IEEE double format.

A Alien function that takes arguments of the specified arg-types and returns a result of type result-type. Note that the only context where a function type is directly specified is in the argument to alien-funcall (see section alien-funcall.) In all other contexts, functions are represented by function pointer types: (* (function ...)).

A pointer which is represented in Lisp as a system-area-pointer object (see section 6.5.)

8.2.4  The C-Call Package

This type is used in function types to declare that no useful value is returned. Evaluation of an alien-funcall form will return zero values.

This type is similar to (* char), but is interpreted as a null-terminated string, and is automatically converted into a Lisp string when accessed. If the pointer is C NULL (or 0), then accessing gives Lisp nil.

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

  (def-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))) (setf (slot
  my-foo 'str) str) my-foo))

Storing Lisp nil writes C NULL to the c-string pointer.

8.3  Alien Operations

8.3.1  Alien Access Operations

This function returns the value pointed to by an Alien pointer or the value of an Alien array element. If 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. If an array, the number of indices must be the same as the number of dimensions in the array type. deref can be set with setf to assign a new value.

This function extracts the value of slot slot-name from the an Alien struct or union. If struct-or-union is a pointer to a structure or union, then it is automatically dereferenced. This can be set with setf to assign a new value. Note that slot-name is evaluated, and need not be a compile-time constant (but only constant slot accesses are efficiently compiled.)

8.3.2  Alien Coercion Operations

This macro returns a pointer to the location specified by alien-expr, which must be either an Alien variable, a use of deref, a use of slot, or a use of extern-alien.

This macro converts alien to a new Alien with the specified new-type. Both types must be an Alien pointer, array or function type. Note that the result is not eq to the argument, but does refer to the same data bits.

[Function]
alien:alien-sap alien-value    

sap-alien converts sap (a system area pointer see section 6.5) to an Alien value with the specified type. type is not evaluated.

alien-sap returns the SAP which points to alien-value's data.

The type to sap-alien and the type of the alien-value to alien-sap must some Alien pointer, array or record type.

8.3.3  Alien Dynamic Allocation

This macro returns a dynamically allocated Alien of the specified type (which is not evaluated.) The allocated memory is not initialized, and may contain arbitrary junk. If supplied, size is an expression to evaluate to compute the size of the allocated object. There are two major cases:

• When type is an array type, an array of that type is allocated and a pointer to it is returned. Note that you must use deref to change the result to an array before you can use deref to read or write elements:

  (defvar *foo* (make-alien (array char 10)))
  
  (type-of *foo*) ==> (alien (* (array (signed 8) 10)))
  
  (setf (deref (deref foo) 0) 10) ==> 10
  

  If supplied, size is used as the first dimension for the array.
  
  
• When type is any other type, then then an object for that type is allocated, and a pointer to it is returned. So (make-alien int) returns a (* int). If size is specified, then a block of that many objects is allocated, with the result pointing to the first one.

This function frees the storage for alien (which must have been allocated with make-alien or malloc.)

8.4  Alien Variables

8.4.1  Local Alien Variables

This macro establishes local alien variables with the specified Alien types and names for dynamic extent of the body. The variable names are established as symbol-macros; the bindings have lexical scope, and may be assigned with setq or setf. This form is analogous to defining a local variable in C: additional storage is allocated, and the initial value is copied.

with-alien also establishes a new scope for named structures and unions. Any type specified for a variable may contain name structure or union types with the slots specified. Within the lexical scope of the binding specifiers and body, a locally defined structure type foo can be referenced by its name using:

  (struct foo)

8.4.2  External Alien Variables

• Alien names are converted to Lisp names by uppercasing and replacing underscores with hyphens.
  
  
• Conversely, Lisp names are converted to Alien names by lowercasing and replacing hyphens with underscores.
  
  
• Both the Lisp symbol and Alien string names may be separately specified by using a list of the form:

    (alien-string lisp-symbol)
  

This macro defines name as an external Alien variable of the specified Alien type. name and type are not evaluated. The Lisp name of the variable (see above) becomes a global Alien variable in the Lisp namespace. 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 type.

For example, it is often necessary to read the global C variable errno to determine why a particular function call failed. It is possible to define errno and make it accessible from Lisp by the following:

(def-alien-variable "errno" int)

;; Now it is possible to get the value of the C variable errno simply by
;; referencing that Lisp variable:
;;
(print errno)

This macro returns an Alien with the specified type which points to an externally defined value. name is not evaluated, and may be specified either as a string or a symbol. type is an unevaluated Alien type specifier.

8.5  Alien Data Structure Example

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

 <==>

(def-alien-type nil
  (struct foo
    (a int)
    (b (array (* (struct foo)) 100))))

struct foo f;
f.b[7].a

 <==>

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

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;

(def-alien-type nil
  (struct c-struct
          (x short)
          (y short)
          (a char)
          (b char)
          (z int)
          (n (* c-struct))))

(def-alien-variable "my_struct" (* c-struct))

(incf (slot my-struct 'x))
(setf (slot my-struct 'a) 5)
(setq my-struct (slot my-struct 'n))

8.6  Loading Unix Object Files

files is a simple-string or list of simple-strings specifying the names of the object files. If files is a simple-string, the file that it designates is loaded using the platform's dlopen mechanism. If it is a list of strings, the platform linker ld is invoked to transform the object files into a loadable object file. libraries is a list of simple-strings specifying libraries in a format that the platform linker expects. The default value for libraries is ("-lc") (i.e., the standard C library). base-file is the file to use for the initial symbol table information. The default is the Lisp start up code: path:lisp. env should be a list of simple strings in the format of Unix environment variables (i.e., A=B, where A is an environment variable and B is its value). The default value for env is the environment information available at the time Lisp was invoked. Unless you are certain that you want to change this, you should just use the default.

8.7  Alien Function Calls

8.7.1  The alien-funcall Primitive

This function is the foreign function call primitive: alien-function is called with the supplied arguments and its value is returned. The alien-function is an arbitrary run-time expression; to call a constant function, use extern-alien or def-alien-routine.

The type of alien-function must be (alien (function ...)) or (alien (* (function ...))), See section 8.2.3. 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:

• Structure type return values are not implemented.
• Passing of structures by value is not implemented.

;; 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)))
      result)))

8.7.2  The def-alien-routine Macro

This 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.

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, (see section 8.4.2.)

result-type is the Alien type of the return value. Each remaining subform specifies an argument to the foreign function. aname is the symbol name of the argument to the constructed function (for documentation) and atype is the Alien type of corresponding foreign argument. The semantics of the actual call are the same as for alien-funcall. style should be one of the following:

:in

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

: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). :out and :in-out cannot be used with pointers to arrays, records or functions. An object of the correct size is allocated, 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.

:copy

is similar to :in, but the argument is copied to a pre-allocated object and a pointer to this object is passed to the foreign routine.

:in-out

is a combination of :copy and :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.

Any efficiency-critical foreign interface function should be inline expanded by preceding def-alien-routine with:

(declaim (inline lisp-name))

In addition to avoiding the Lisp call overhead, this allows pointers, word-integers and floats to be passed using non-descriptor representations, avoiding consing (see section 5.11.2.)

8.7.3  def-alien-routine Example

/* a for update
 * i out
 */
void cfoo (char *str, char *a, int *i);

(def-alien-routine "cfoo" void
  (str c-string)
  (a char :in-out)
  (i int :out))

8.7.4  Calling Lisp from C

This macro defines a Lisp function that can be called from C and a Lisp variable. The arguments to the function must be alien types, and the return type must also be an alien type. This Lisp function can be accessed via the callback macro.

name is the name of the Lisp function. It is also the name of a variable to be used by the callback macro.

return-type is the return type of the function. This must be a recognized alien type.

arg-name specifies the name of the argument to the function, and the argument has type arg-type, which must be an alien type.

This macro extracts the appropriate information for the function named callback-symbol so that it can be called by a C function. callback-symbol must be a symbol created by the def-callback macro.

This macro does the necessary stuff to call the callback named callback-name with the given arguments.

8.7.5  Callback Example

(use-package :alien)
(use-package :c-call)

(def-callback foo (int (arg1 int) (arg2 int))
  (format t "~&foo: ~S, ~S~%" arg1 arg2)
  (+ arg1 arg2))

(defun test-foo ()
  (callback-funcall foo 555 444444))

(use-package :alien)
(use-package :c-call)

(def-alien-routine qsort void
  (base (* t))
  (nmemb int)
  (size int)
  (compar (* (function int (* t) (* t)))))

(def-callback my< (int (arg1 (* double))
                       (arg2 (* double)))
  (let ((a1 (deref arg1))
        (a2 (deref arg2)))
    (cond ((= a1 a2)  0)
          ((< a1 a2) -1)
          (t         +1))))

(defun test-qsort ()
  (let ((a (make-array 10 :element-type 'double-float
                       :initial-contents '(0.1d0 0.5d0 0.2d0 1.2d0 1.5d0
                                           2.5d0 0.0d0 0.1d0 0.2d0 0.3d0))))
    (print a)
    (qsort (sys:vector-sap a)
           (length a)
           (alien-size double :bytes)
           (alien:callback my<))
    (print a)))

8.7.6  Accessing Lisp Arrays

(defun array-data-address (array)
  "Return the physical address of where the actual data of an array is
stored.

ARRAY must be a specialized array type in CMUCL.  This means ARRAY
must be an array of one of the following types:

                  double-float
                  single-float
                  (unsigned-byte 32)
                  (unsigned-byte 16)
                  (unsigned-byte  8)
                  (signed-byte 32)
                  (signed-byte 16)
                  (signed-byte  8)
"
  (declare (type (or (simple-array (signed-byte 8))
                     (simple-array (signed-byte 16))
                     (simple-array (signed-byte 32))
                     (simple-array (unsigned-byte 8))
                     (simple-array (unsigned-byte 16))
                     (simple-array (unsigned-byte 32))
                     (simple-array single-float)
                     (simple-array double-float)
                     (simple-array (complex single-float))
                     (simple-array (complex double-float)))
                 array)
           (optimize (speed 3) (safety 0))
           (ext:optimize-interface (safety 3)))
  ;; with-array-data will get us to the actual data.  However, because
  ;; the array could have been displaced, we need to know where the
  ;; data starts.
  (lisp::with-array-data ((data array)
                          (start)
                          (end))
    (declare (ignore end))
    ;; DATA is a specialized simple-array.  Memory is laid out like this:
    ;;
    ;;   byte offset    Value
    ;;        0         type code (should be 70 for double-float vector)
    ;;        4         4 * number of elements in vector
    ;;        8         1st element of vector
    ;;      ...         ...
    ;;
    (let ((addr (+ 8 (logandc1 7 (kernel:get-lisp-obj-address data))))
          (type-size
           (let ((type (array-element-type data)))
             (cond ((or (equal type '(signed-byte 8))
                        (equal type '(unsigned-byte 8)))
                    1)
                   ((or (equal type '(signed-byte 16))
                        (equal type '(unsigned-byte 16)))
                    2)
                   ((or (equal type '(signed-byte 32))
                        (equal type '(unsigned-byte 32)))
                    4)
                   ((equal type 'single-float)
                    4)
                   ((equal type 'double-float)
                    8)
                   (t
                    (error "Unknown specialized array element type"))))))
      (declare (type (unsigned-byte 32) addr)
               (optimize (speed 3) (safety 0) (ext:inhibit-warnings 3)))
      (system:int-sap (the (unsigned-byte 32)
                        (+ addr (* type-size start)))))))

  double dotprod(double* x, double* y, int n)
  {
    int k;
    double sum = 0;

    for (k = 0; k < n; ++k) {
      sum += x[k] * y[k];
    }
    return sum;
  }

  (alien:def-alien-routine "dotprod" c-call:double
    (x (* double-float) :in)
    (y (* double-float) :in)
    (n c-call:int :in))
    
  (defun test-dotprod ()
    (let ((x (make-array 10000 :element-type 'double-float :initial-element 2d0))
          (y (make-array 10000 :element-type 'double-float :initial-element 10d0)))
        (sys:without-gcing
          (let ((x-addr (sys:vector-sap x))
                (y-addr (sys:vector-sap y)))
            (dotprod x-addr y-addr 10000)))))

8.8  Step-by-Step Alien 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 < 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);
};

;;; -*- Package: test-c-call -*-
(in-package "TEST-C-CALL")
(use-package "ALIEN")
(use-package "C-CALL")

;;; Define the record c-struct in Lisp.
(def-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))
(def-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 "Returned from C function.~%")
      (multiple-value-prog1
          (values (slot res 'x)
                  (slot res 's))
        ;;              
        ;; Deallocate result  after we are done using it.
        (free-alien res)))))

cc -c test.c

cc -G 0 -c test.c

% lisp
;;; Lisp should start up with its normal prompt.

;;; Compile the Lisp file.  This step can be done separately.  You don't have
;;; to recompile every time.
* (compile-file "test.lisp")

;;; Load the foreign object file to define the necessary symbols.  This must
;;; be done before loading any code that refers to these symbols.  next block
;;; of comments are actually the output of LOAD-FOREIGN.  Different linkers
;;; will give different warnings, but some warning about redefining the code
;;; size is typical.
* (load-foreign "test.o")

;;; Running library:load-foreign.csh...
;;; Loading object file...
;;; Parsing symbol table...
Warning:  "_gp" moved from #x00C082C0 to #x00C08460.
Warning:  "end" moved from #x00C00340 to #x00C004E0.

;;; o.k. now load the compiled Lisp object file.
* (load "test")

;;; Now we can call the routine that sets up the parameters and calls the C
;;; function.
* (test-c-call::call-cfun)

;;; The C routine prints the following information to standard output.
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.
;;; Lisp prints out the following information.
Returned from C function.
;;; Return values from the call to test-c-call::call-cfun.
10
"A C string"
*

1

CMUCL mmaps a large piece of memory for its own use and this memory is typically about 256 MB above the start of the C heap. Thus, only about 256 MB of memory can be dynamically allocated. In earlier versions, this limit was closer to 8 MB.