|Allegro CL version 10.1|
Moderately revised from 10.0.
Arguments: name-and-options arglist &key call-direct callback convention returning lisp-return-will-not-move method-index release-heap release-heap-ignorable arg-checking optimize-for-space strings-convert error-value pass-structs-by-value allow-gc release-heap-implies-allow-gc documentation
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
|arglist||A required argument.
empty list of
(:void) means 0 arguments are explicitly required (also as in C).
Note that string-conversion is done automatically when
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
Default: the foreign type
The value can also be:
A foreign type (defined by def-foreign-type)
A list of a foreign type and a Lisp type. Example:
A list of a foreign type, a Lisp type, and a symbol naming a
user-defined conversion function. Example:
|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
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.
|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).|
||If true, this argument causes
the Lisp wrapper function to first check the
Lisp types against the Lisp argument type specifications.
An argument specification
||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
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-
|method-index [Windows Only]||
||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
|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
|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
This argument can have the same values as
release-heap, and they roughly mean the same
thing, except here applying to the garbage-collector is allowed to run
either never (with :never), when interrupts are enabled (with
:when-ok) or always (with :always). See Note 3: the release-heap keyword
argument below the table. This argument is only used in an SMP Lisp.
It is planned to add as an allowable value :match-release-heap and eliminate the :release-heap-implies-allow-gc argument.
If in an SMP Lisp the release-heap argument is
given a value other than
It is the intention to remove this argumement and allow an additional value for allow-gc to replace it.
||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.|
||This argument assists in having the
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
If arglist is
ineffective if error-value is
See Note 5: More on the error-value argument after the table for more information.
Supplies a documentation string for the foreign function. The string
can be accessed with
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 elements of an argument list can be:
(name type), where
nameis a sumbol and
typeis either a built-in foreign type, or a type defined by def-foreign-type, or one of the keywords
:aligned. We discuss the keywords below.
(name type lisp-type), where
typeare as above, and
lisp-typecan be any valid Lisp type. It specifies the type of the value passed. If the
lisp-typeis not consistent with the foreign
type, a warning may be signaled.
(name type lisp-type user-conversion-function), where
lisp-typeare as above, and
user-conversion-functionis a symbol naming a function, described in section The user-conversion function in a complex-type-spec in foreign-functions.htm. An example is
The allowable keywords for the foreign type (second list element) are:
:int: the default, the argument will be converted to a C integer.
:lisp: the argument will be passed unchanged. analogous to the returning type
:lisp(in that the argument is passed through unchanged) but the argument value must be a fixnum. If arguments are checked, a value other than a fixnum will be rejected. See Aligned Pointers and the :aligned type in ftype.htm for more information.
:foreign-address: the argument value should be an integer which will be interpreted as a foreign address.
Here is an example:
(a (b :int fixnum) (c :lisp))
A foreign type specification that includes a reference spec such
(& :int) will be interpreted as a
For boolean values, specify the argument
boolean). Then any non-
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
The special Lisp type specification
provided for use with a
:float foreign type (but
its use is deprecated); it is equivalent to
:single-float-no-proto foreign type (not listed
above because it is deprecated). The preferred
(name :double single-float).
Specifying (* :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 if a Lisp float is given as a value.
A note on foreign addresses
in foreign-functions.htm for a discussion of
foreign addresses and what is expected when
:foreign-address is specified.
:c, (the default and suitable for most
:fortran convention is defined.
This convention generally causes a conversion of most atomic arguments
: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.
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:
:never- This is the default and is compatible to the original defforeign interface; the caller necessarily has the lisp heap, this call will not release it. Note that if the foreign code being called spawns any new threads, or if it allows another thread to run, and the other thread attempts to call back into lisp, it will have to wait for the lisp heap. The danger is that the original thread may be waiting for results from its partner thread, but it has not yet given up the heap (this constitutes a deadlock situation). If this situation holds (foreign code does spawn new thread which call back into Lisp),
:when-okis the appropriate value for the release-heap argument.
:always- The foreign call always gives up the heap before making the transition into non-lisp execution. If the form is being called within the dynamic context of a without-interrupts form, an error is signaled.
:when-ok- The current dynamic context is examined, and if without-interrupts is in effect, the heap is not released; otherwise the heap is released.
There is a big difference in dealing with other-thread and garbage collection (gc) action intermingled with foreign calls in all of virtual-threads (processes are managed by Lisp), os-threads (processes are managed by the OS, but only one hardware processor is used for Lisp code), and SMP (multiple hardware processors can be used for Lisp and foreign code) Lisps. All three are different. This makes designing functionality which will run correctly on all three complicated.
The release-heap argument was added for os-threads, and has options :never, :always, and :when-ok. These are still the options. They pertain to whether the thread calling a foreign-function wants to allow another thread to access the Lisp heap during the call (note that amonst other things, this could lead to a garbage-collection).
The virtual-threads Lisps are simpler. Because they are more constrained, :release-heap :never is the only option that makes sense, since there is no concept of releasing the heap by a thread separate from pausing the thread.
If the user is coding for both virtual and os-threads, he can code for os-threads, and then add the :release-heap-ignorable t option to allow virtual-threads calls to compile without warning (and with the implied descriptive value of :never for the :release-heap option).
SMP Lisps are more complicated As with virtual threads, there is again no concept of heap ownership, in this case not because the heap is automatically assigned to the running thread, but because there are no longer any heap management constraints except one (explained later) which means that any thread can access the heap at any time. So the concept of releasing the heap makes even less sense in an SMP Lisp than it does in a virtual-threads Lisp.
In a pure SMP Lisp, there would be no restrictions at all on foreign calls. But, as mentioned above, although the heap itself is no longer a one-at-a-time resource needing gating, the garbage-collector, which is a stop-the-world collector still, must have complete and exclusive access to the heap when it runs, and so threads must specify whether to allow the gc to run.
def-foreign-call has two new keyword arguments: allow-gc and release-heap-implies-allow-gc to deals with these issues.
allow-gc,defaults to :never, to keep it in line
with the release-heap argument. It can have the
same values as release-heap, and they roughly
mean the same thing (with the difference being that the
garbage-collector is allowed to run either never (with :never), when
interrupts are enabled (with :when-ok) or always (with :always). Note
that as with os-threads, in an SMP Lisp,
:never does not lock out the gc if the foreign code performs
a callback to Lisp. So appropriae measures must still be taken, if a
callback is possible, to reacquire Lisp objects after a possible gc,
because they may have moved.
(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
nil for the
strings-convert keyword argument.
Getting an error value from a foreign call is somewhat complicated, so users should not casually use this feature, but instead consider whether the information is necessary and be aware of various ways in which the information may be incorrect.
First note the following:
nilvalue for the error-value argument) if there is reason to believe it may be needed.
nilvalue for for the error-value argument. Foreign function call are complicated and typically additional functionality is wrapped around the actual function call by the system (that is, by Allegro CL). This additional functionality may affect the value of the errno system variable in the current system thread or the value returned by GetLastError() so simply looking at errno or calling GetLastError() after the forreign call has returned will not work: the value seen can be different from the correct value. When error-value has an appropriate non-
nilvalue (appropriate values are discussed below), code is added by the system to get and store the value of errno or the return value of GetLastError() immediately after the actual foreign function returns (and before the additional wrapper code is run). Using error-value is the only way to get the correct value, and even that may not always work, as the next bullet discusses.
The two acceptable non-
for error-value are
The two non-
nil values have the same effect:
the value of errno is stored immediately after the actual
foreign function returns (and before additional wrapper code is
run), and then returned as the second return value of the foreign
The values have different effects.
If you specify the value
:os-specific, then GetLastError() is called
immediately after the actual foreign function returns (and before
additional wrapper code is run), and then returned as the second
return value of the foreign call. This is the standard Windows
procedure, and works for all standard Windows routines, for
example LoadLibrary() (see the description of that function in
the MSDN where it says that you get error information by
:errno, then the value of errno is stored immediately after the actual foreign function returns (and before additional wrapper code is run), and then returned as the second return value of the foreign call. This is not standard Windows procedure, but is how some routines work, particularly those which are provided in UNIX-like libraries. This is often how Windows versions of open() works, for example, (open() is not a standard Windows function).
On Windows, you must specify the appropriate way to get the error
value for the routine you are
:errno will give a wrong value
for LoadLibrary(). Specifying
will give a wrong value for (many, perhaps all implementations
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
def-foreign-call is defined to
default its :returning keyword argument
(specifying the expected type of value returned by the foreign call)
: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
: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
: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
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
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
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).
:lisp return type means that the foreign code
will return a Lisp object. For most Lisp objects (other than
characters, fixnums, and a few others), this means returning a pointer
to the object. If for whatever reason the object has been moved before
the pointer is in a place where it can be seen by the garbage
collector and properly forwarded, the pointer will be invalid and the
result may be Lisp failure because of gc failure. For this reason, a
warning is signaled when
:lisp is specified as a
The warning can be avoided by specifying
:returning value. Although aligned pointers
could also represent objects that move during gc, it is much less
likely so, because they tend to only be used when the objects they
represent are static, like stack-based objects or allocated in
aclmalloc space. Note associating the actual Lisp value with
:aligned value is complicated. See Aligned Pointers and the
:aligned type in ftype.htm for more
If you are sure the item will not move (because it is located in
statis space, for example), you can give
the value of the
argument. That supresses any warning about
(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
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 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
... (*)) 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
... (*)) 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) 1998-2022, Franz Inc. Lafayette, CA., USA. All rights reserved.
This page has had moderate revisions compared to the 10.0 page.
|Allegro CL version 10.1|
Moderately revised from 10.0.