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Allegro CL version 8.1
Unrevised from 8.0 to 8.1. Minimal update since 8.1 release.
8.0 version

def-foreign-call

Arguments: name-and-options arglist &key call-direct callback convention returning method-index release-heap release-heap-ignorable arg-checking optimize-for-space strings-convert error-value

This macro creates the specification which allows Lisp to correctly call non-Lisp code. Like other defining forms, its macroexpansion clearly shows what will occur and at what eval-when times. The execution of the expanded form always returns the Lisp name being defined. The definition that is installed is a Lisp function that serves as a wrapper and initiates the foreign call.

Macroexpansion of a def-foreign-call form provides useful information about how the call is interpreted. See below.

The following table shows the arguments. The first two entries are the required arguments and the remainder are keyword arguments.

Argument Value and Details Notes
name-and-options A required argument.

Symbol or a list of a symbol and an external-name specification, which can either be:

(1) a symbol naming a function of one argument that returns a string to be used as the foreign name or

(2) a string which will be used as the foreign name.

The symbol represents the lisp-name for which the foreign-call definition will be installed. The external name specification can be either a string specifying a literal external name, or it can be a symbol, which represents the name of a conversion function. That conversion function must take one required argument and at least the language keyword argument, and must be defined at the time the macro expansion is executed. At that time this conversion function will be called and will receive the lisp-name specified, as well as the arguments :language lang where lang is the value of the convention keyword argument to def-foreign-call.
arglist A required argument.

A possibly empty list of argument specifications. nil and (:void) have special meanings.

nil implies much the same as it does in C, that arguments are not checked for type or number.

(:void) means 0 arguments are explicitly required (also as in C).

Note that string-conversion is done automatically when arglist is nil unless strings-convert is specified nil. See Note 4: String conversion when no arguments are specified after the table.

Otherwise a list of argument specifications. See Note 1: Argument Specifications after the table.

See A note on foreign addresses in foreign-functions.htm for a discussion of foreign addresses and what is expected when :foreign-address is specified.

returning Keyword argument.

Default: the foreign type :int, with a conversion to Lisp type integer (a fixnum with a possible overflow to a bignum). See Note 7: Potential problems with foreign functions returning :int in 64-bit Lisps for possible problems with returning :int on 64-bit Lisps.

The value can also be:

A foreign type (defined by def-foreign-type)

A list of a foreign type and a Lisp type (and an optional third element which is not used but may be in a later release). Example: (:double single-float). See also Note 8: Returning booleans.

((* :char)), or ((* :char) string), etc. Causes native-to-string to be called automatically after the return. The alternative specification of defining the returning value as an integer and then to explicitly call with-native-string on the result, still works, and should be used if any external-format other than :default is desired.

:lisp, meaning a Lisp object. No conversion is done. If not actually a Lisp object, gc failure is possible.

:foreign-address

:single-float-from-double, a double is returned and it is converted to a single. This specification is deprecated though it will work. The preferred specification is (:double single-float)

:void, nothing is returned, the Lisp function returns nil (:void is actually a foreign type).

This argument specifies how the value returned from the foreign function will be interpreted. If both foreign and Lisp types are chosen, they will be checked for consistency and a warning might be given.

A common idiom is to use

:returning (:int fixnum)

to specify that the foreign value returned is to be simply shifted into a fixnum value, with no consing and simple truncation of the top two bits on overflow.

If :foreign-address is specified, the return value will be interpreted as an unsigned integer and converted to a positive Lisp integer. The system will not store this value into a foreign-pointer object. The discussion in A note on foreign addresses in foreign-functions.htm does not specifically apply to returning values. Programmers can add code to store the returned value as desired.

convention Keyword argument. Default :c

Other possibilities listed in Note 2: Possible Values for Convention after the table.

This argument allows the specialization of calling conventions due to language or operating-system distinctions. The default convention is :c, and is adequate for most situations. (Note that on Windows the c/stdcall convention distinction is required for callbacks using defun-foreign-callable, but is not required in def-foreign-call).
arg-checking t or nil (default is t) If true, this argument causes the Lisp wrapper function to first check the Lisp types against the Lisp argument type specifications. When nil, no argument checking is done (although the number of arguments might still be checked). If true, the lisp-types specified or implied in the argument specification (provided with the arglist argument) is used to check the actual arguments to the wrapper; if a mismatch occurs, error is called. The default value is t, unless arglist is nil.
call-direct t or nil (default is nil) The argument causes no changes to the Lisp wrapper itself, but, when specified true, allows for other Lisp functions to call the foreign function directly when compiled after the def-foreign-call form is in effect. In order for the compilation of a direct-call to be successful, the argument and return types must imply simple type conversions which the compiler can handle. That list of direct-callable conversions on a platform is constantly changing, but can be examined by calling the function list-call-direct-possibilities.

If for any reason a call to the foreign function cannot be compiled into a direct-call, a warning is issued and a call to the wrapper is generated. When error-value is non-nil, the call cannot be a direct-call so call-direct is ignored (but a warning is printed if call-direct is non-nil).

method-index  [Windows Only] nil (the default) or an index into C++ table, as described at right. This argument allows for calling of C++ style member-methods. The value, if specified, must be an integer index into the virtual table of the C++ class. Symbolic index specifications are not directly supported.

See Note 6: More on the :method-index argument for information on a non-nil value for this argument.

callback t (the default and only allowed value) The callback keyword is non-operative, but is retained in the hopes that its functionality can be revived in future versions. A null value indicates a promise by the programmer that the foreign function will never call-back into lisp. Unfortunately, due to the nature of OS threads implementations, this promise is currently impossible to keep. The value of this keyword is always taken as t, and a warning is issued if specified to nil.
release-heap Only used on platforms that use the :os-threads model for multiprocessing. See Note 3: the release-heap keyword argument below the table. The release-heap keyword allows the foreign function to operate in native-OS threads (so :os-threads is on the *features* list), without causing conflicting demands on the Lisp heap. The values for this keyword are discussed in Note 3: the release-heap keyword argument below the table. Note: the specification of :release-heap on any non-os-threads implementation to any value other than :never will generate a warning, unless the user also specifies :release-heap-ignorable as a non-nil value.
release-heap-ignorable t or nil (default is nil) This argument, when t, tells the system to ignore the release-heap keyword argument, regardless of its value, on non-os-threads implementations. The value on non-os-threads implementations should be :never. If the value of this argument is nil, a warning will be generated if the value of the release-heap is anything other than :never. (The reason is such a value is meaningless and the system wants to be sure that the programmer understands that a meaningless value has been specified. Assuming that the heap can be released on non-os-threads (it cannot be released) may affect behavior or performance.) This argument is provided to allow the same def-foreign-call forms to be used on both os-threads and non-os-threads platforms without warnings being signaled and without conditionalizing the value of the release-heap argument.
optimize-for-space t or nil (default is nil) The optimize-for-space keyword provides for minimal space requirement for foreign-call wrappers. This option is best used in conjunction with the call-direct option. If true, optimize-for-space will ensure that the wrapper definition takes up very little room, usually as a closure. This usually comes at a cost of speed, and so only makes sense when call-direct is used to compile all actual calls to the foreign function directly, so that the Lisp wrapper is not called normally at all.
strings-convert t or nil (default is t) This argument assists in having the foreign-function interface handle Allegro CL's 16-bit strings automatically. When the strings-convert is true, then when any of the specified arguments at def-foreign-call time are declared directly or indirectly as (* :char), def-foreign-call augments the function wrapping the low-level foreign function call so that for each (* :char) declared argument, a check is made at runtime to see if that declaration's corresponding value is a string. If it is, then that value is converted at runtime to native-string format using a dynamic-extent array, and this new array is passed in place of the original string argument to low-level foreign function call. See Foreign-Functions in iacl.htm for full details and examples.

If arglist is nil (meaning that no information about the type or the number of arguments is supplied) and this argument is unspecified, string conversion is enabled and a warning is printed stating that fact. See Note 4: String conversion when no arguments are specified after the table.

error-value nil, :errno, :os-specific, (default is nil) nil causes normal operation of the foreign call and a return of the one value that the foreign call itself returns.

:errno gets the most recent value of the errno variable and returns it as the second return value from the foreign call.

:os-specific gets an architecture/operating-system specific error value and returns it as the second return value from the foreign call. On Windows systems, the GetLastError() function is used to get this error value. Currently, all other architectures retrieve the value of the errno variable.

call-direct is ineffective if error-value is non-nil (a warning is printed and the call-direct argument is ignored if non-nil values are specified for both error-value and call-direct).

See Note 5: More on the error-value argument after the table for more information.

Note 1: Argument Specifications

Argument specifications are available with a rich set of syntax and defaults which allow for a C "feel" while still retaining the Lisp semantics and power.

The basic argument list is simply a list of names, e.g. (a b c d), whose types default to C int types converted in the usual manner from Lisp integer types. Any one argument can be specified more fully, as a list of the name optionally followed by the foreign type optionally followed by the Lisp type (optionally followed by a translation specification, which has not been implemented). Here is an example:

  (a (b :int fixnum) (c :lisp))

The foreign type can either be a built-in foreign type or a type defined by def-foreign-type, or it can be one of the keywords :lisp, :foreign-address, or :single-float-no-proto (deprecated). The value :lisp means the argument will be passed unchanged, analogous to the returning type :lisp. The lisp type (the optional third element of the list) can be any valid lisp type.

If the foreign type is not specified, it defaults to :int (and the Lisp type thus defaults to integer). If the foreign type is specified but the Lisp type is not, a reasonable Lisp type is chosen to correspond to the foreign type. If both foreign and Lisp types are chosen, they will be checked for consistency and a warning might be given. A foreign type specification that includes a reference spec such as (& :int) will be interpreted as a pass-by-reference argument.

For boolean values, specify the argument (argname :int boolean). Then any non-nil Lisp value (including 0) will be converted into a C value of 1, and nil will be converted into a C value of 0. For returned values, a C value of 0 is converted into nil and a non-zero C value is converted into t.

The special Lisp type specification :no-proto is provided for use with a :float foreign type; it is equivalent to the :single-float-no-proto. Note that both of these specifications are deprecated though they will work. The preferred specification is (:double single-float).

Specifying the value (* :float) or (* :double) as the type of an argument is not recommended. The function will expect a foreign address (not a Lisp address) and it will not pass the address of a Lisp float is a Lisp float is given as a value.

See A note on foreign addresses in foreign-functions.htm for a discussion of foreign addresses and what is expected when :foreign-address is specified.

Note 2: Possible Values for Convention

Other than :c, (the default and suitable for most purposes), the :fortran convention is defined. This convention generally causes a conversion of most atomic arguments to pass-by-reference.

:fastcall convention on Windows does not work

:fastcall is a special convention used by some Windows operating systems to speed up some calls, by passing two arguments in registers. However, it does not work with Allegro CL. In Allegro CL on X86 architectures, the first two arguments are usually passed in registers anyway in Lisp, but they are different registers than used in fastcall. Also, since the calling sequence itself overshadows the speed that would be gained by saving a couple of push instructions, the foreign call to a fastcall function would not in fact be very fast at all.

Note 3: the release-heap keyword argument

The native-threads implementation of Allegro CL changes some basic assumptions of the foreign functions user interface. There is always exactly one native thread per Lisp Process, but there is not necessarily a Lisp process for every thread. Threads are free to run whenever they want; however, only one thread at a time can access the Lisp heap (for read or write); a thread cannot access the Lisp heap unless it has "acquired" the heap, which is only possible after another thread has "released" the heap.

See Releasing the heap when calling foreign functions in foreign-functions.htm for more information on this point.

def-foreign-call allows for the specification of whether to release the heap or not during a call. The possibilities for the release-heap keyword argument are:

Note 4: String conversion when no arguments are specified

(ff:def-foreign-call foo ()) has no arguments specified, meaning that any number of arguments of any type will be passed. If any of these arguments are strings, it may be that string conversion should be performed.

String conversion is done by default. The above def-foreign-call form will generate a warning message (to indicate that string arguments will be converted). To suppress the warning but still convert the strings, specify the value t for the strings-convert keyword argument. To suppress the warning and to suppress all automatic strings conversion, specify the value nil for the strings-convert keyword argument.

Note 5: More on the error-value argument

When considering specifying a non-nil value for the error-value argument, care should be taken to assess whether or not the error value is needed. If an error value will not be used, then it should not be specified in the def-foreign-call form, because the foreign call is less efficient when retrieving the error value. However, if the error value from a foreign call is needed, then error-value is the only guaranteed option. One could define a foreign call to GetLastError or to get the current value of errno variable, but this cannot be guaranteed to work. The last errors might be overwritten between the time of the foreign call and the time of the retrieval. The wrong error code will make debugging harder, so the price of the overhead of a non-nil value for the error-value argument will be less than the price of wrong error codes.

There are two non-nil values for error-value: :errno and :os-specific. All UNIX platforms are consistent in always returning the error in or through the errno variable. But many Windows calls (e.g. LoadLibrary()) require you to call GetLastError() in order to get an error value. Other Windows functions, particularly those which emulate libc.a/libc.so functionality (e.g. open()), set errno values like UNIX. When using the argument on Windows, be sure to specify the appropriate value.

Note 6: More on the method-index argument

This argument is supported on Windows only. If a non-nil value is specified for the method-index keyword argument, then the value must be a vector whose first value is the integer index into the virtual table of the C++ class. And then, when the foreign function is called, the first argument of the call must be the vector whose first element is an integer which is a pointer to the table of method addresses. The arguments specified to def-foreign-call follow that first argument. Note that the function name specified in the def-foreign-call form is used only for the Lisp function name, and does not refer to any function in a shared library. Instead, the function's address (as a table address and an index into the table) is passed when the function is called.

Note 7: Potential problems with foreign functions returning :int in 64-bit Lisps

def-foreign-call is defined to default its :returning keyword argument (specifying the expected type of value returned by the foreign call) to :int. :int corresponds to C's int type. However, if the foreign function does not actually return an int, subtle bugs could be introduced in programs, particularly if the C function returns a long, an unsigned long, or a pointer of some sort. In 32-bit Lisps, returning those values is not a problem (when :returning :int is specified or defaulted to) because int is always 32 bits on every architecture we support. But on 64-bit Lisps, if a 64 bit value is returned, the upper 32 bits are lost. If the value was not correctly sign-extended by the foreign code, a negative value in the foreign code could be seen by Lisp as a large positive value. Also, the :long or :unsigned-long type is an inadequate specfication because on Windows long types are always 32 bits. So on either 32 or 64 bit lisps, for portability, use :returning :unsigned-nat when the return value is some kind of pointer. (See The :nat and :unsigned-nat types in implementation.htm for a description of the :nat and :unsigned-nat types.) When the returned value is an integer value, be sure to use the correct type and be sure that the foreign code actually produces that type.

Note 8: Returning booleans

def-foreign-call can accept :returning :boolean as a return type, and it will be automatically translated to a canonical form which is :returning (:int boolean). This works with some foreign languages (where 0 is considered false and anything else is considered true - it translates 0 into nil on the lisp side and anything else to the value t), but in some cases (C++, for instance) the boolean type is equivalent to signed-char type, and so it must be specified in the def-foreign-call as :returning (:char boolean).

Examples:

(def-foreign-call add2 (x y))

The symbol add2 will have a function definition calling the foreign function probably named "add2" in C, whose first arg is named "x" and is an integer in Lisp and which is converted to an int for passing to C. If the integer is larger than can be held in a C int, it is truncated. As with the first arg, the second arg named "y" is an integer converted to a C int. The return value is interpreted as a C int type, and is converted to a Lisp integer (which may either be a fixnum or consed as a bignum).

We say the foreign function is "probably" named "add2" because since no specific name or conversion function is specified, the default system conversion function is used. It depends on the platform and platform-specific rules but typically downcases the symbol name.

(def-foreign-call t_double ((x :double)
                            (y :double single-float)
                            (z :int fixnum))
  :returning :double))

Call a function, probably named "t_double" in C (again "probably" because the actual name depends on platform-dependent defaults), whose first arg is a double-float both in Lisp and in C, and whose second arg is a single-float in Lisp but is converted to double float for passing into C (this is the calling convention used by some non ANSI C compilers and by others when the arguments are not prototyped), and the third argument is a fixnum Lisp passed as an integer to C. The function returns and boxes a double-float value to Lisp.

(def-foreign-call (t-float dash-to-underscore) ((x :double)
                                                (y (:float :no-proto))
                                                (z :int fixnum)
                                                (w (* :char) string))
  :returning #-(or (and sun4 (not svr4)) sun3q) :float
             #+(or (and sun4 (not svr4)) sun3q) (:double single-float)

(def-foreign-call (t-float "t_float") ((x :double)
                                                (y (:float :no-proto))
                                                (z :int fixnum)
                                                (w (* :char) string))
  :returning #-(or (and sun4 (not svr4)) sun3q) :float
             #+(or (and sun4 (not svr4)) sun3q) (:double single-float)

These two examples do the same thing: call a function, named "t_float" in C (assuming in the first case proper conversion by dash-to-underscore, which must already be defined and should downcase the symbol name and replace dashes with underscores), whose first arg is a double-float both in Lisp and in C. Like the previous example, the second arg is a float in Lisp, and is converted to double float for passing into C. The third arg named "z" is a fixnum passed as an int, and "w" is a (null-terminated) Lisp string, whose first-character-address is passed to C (beware, the string may move if a gc is allowed). Depending on the architecture, the C function will return either a double (from older C compilers) or a float, each interpreted and boxed as a Lisp single-float value.

We give both examples to show how a lisp name (the symbol t-float) is converted to a foreign name ("t_float"). You can either specify a function that takes a symbol as an argument and returns the correct string (so (dash-to-underscore 't-float) returns "t_float") or you can simply specify the correct string. Note again that dash-to-underscore must be already defined when the def-foreign-call form is evaluated.

Macroexpansion of a def-foreign-call-form now provides more useful information

def-foreign-call has enhanced the macroexpansion to give useful information for users. Here is an example:

cl-user(11): (pprint 
               (macroexpand '(ff:def-foreign-call foo (x (y (* :char))))))

Warning: A runtime with-native-string call is being generated for argument `y'
         to the foreign-function `foo'.  The with-native-string macro can be
         used for explicit string conversions around the foreign calls.  This
         warning is suppressed when :strings-convert is specified in the
         def-foreign-call.

(progn (eval-when (:compile-toplevel)
         (excl::check-lock-definitions-compile-time 'foo 'function
           'foreign-functions:def-foreign-call (fboundp 'foo))
         (push 'foo excl::.functions-defined.))
       (eval-when (compile load eval) (remprop 'foo 'system::direct-ff-call))
       (setf (fdefinition 'foo)
             (let ((excl::f
                    (named-function foo
                      (lambda (x y)
                        (excl::check-args '((:int (integer * *))
                                            ((* :char) (array character (*))))
                                          'foo x y)
                        (with-native-string (#:g34798
                                             (if*
                                              (stringp y)
                                                then y
                                                else ""))
                                            (unless
                                             (stringp y)
                                             (setq #:g34798 y))
                                            (symbol-macrolet
                                             ((y #:g34798))
                                             (system::ff-funcall
                                              (load-time-value
                                               (excl::determine-foreign-address
                                                '("foo" :language :c)
                                                2
                                                nil))
                                              '(:int (integer * *))
                                              x
                                              '((* :char)
                                                (array character (*)))
                                              y
                                              '(:int (integer * *)))))))))
               (excl::set-func_name excl::f 'foo)
               excl::f))
       (record-source-file 'foo) 'foo)
cl-user(12): 

The effects of the new, longer array implementation on def-foreign-call

The new array implementation is discussed in the Arrays and short arrays section in implementation.htm. In brief, standard Common Lisp arrays now can be significantly larger than in earlier releases, while the new short arrays implement the old arrays (the same size limitations but also the same type codes and structure).

In this discussion, `array' refers to the newly-implemented arrays, while `short array' refers to the old implementation, preserved as short arrays in 7.0.

Foreign calls are made with arrays as arguments by passing the address of the first value. In the new implementation, simple-arrays always have exactly the same first element offset (but some short arrays are aligned to the next higher word boundary so that the elements within are naturally aligned). This sometimes-difference between arrays and short-arrays poses an extra burden on the ff interface, in that the arrays must be distinguished between themselves at runtime.

It is now possible to declare an argument a (short-simple-array ... (*)) and the interface will generate code as it did before for normal arrays, passing the address of the first argument.

For 7.0, a declaration of (simple-array ... (*)) actually generates code that tests at runtime whether the argument is a short array or a normal array. So in effect, a short simple-array passed in as if it were a normal simple-array will be properly passed.

Note that with this setup, if argument checking is specified and a short-array is passed in, the check will fail, because a short-array is not a subtype of simple-array. But for 7.0, if you suppress this argument checking, the interface will pass either array correctly.

However, programmers are urged to provide correct declarations and pass the correct type of array even though 7.0 allows sloppiness. In a future release, we anticipate adding a (dual-simple-array ... (*)) declaration to the direct-call foreign interface, and to move that functionality from its current place in the (simple-array ... (*)) declaration. This would then mean that the (simple-array ... (*)) declaration would only pass simple-arrays, and not short simple-arrays. This change will also make more consistent the arg-checking feature of the direct-call interface, since the dual-simple-array type is defined as an or of simple-array and short-simple-array, so a short-simple-array being passed will pass the test of being a dual-simple-array.

See ftype.htm for information on foreign types in Allegro CL and foreign-functions.htm for general information on foreign functions in Allegro CL. See particularly def-foreign-call in foreign-functions.htm.


Copyright (c) 1998-2009, Franz Inc. Oakland, CA., USA. All rights reserved.
Documentation for Allegro CL version 8.1. This page was not revised from the 8.0 page.
Created 2009.10.15.

ToCDocOverviewCGDocRelNotesFAQIndexPermutedIndex
Allegro CL version 8.1
Unrevised from 8.0 to 8.1. Minimal update since 8.1 release.
8.0 version