Foreign-Function Interface to C

Larceny provides a general foreign-function interface (FFI) substrate on which other FFIs can be built; see Larceny Note #7. The FFI described in this manual section is a simple example of a derived FFI. It is not yet fully evolved, but it is useful.

1. Creating loadable modules
2. Loading and linking foreign functions
3. Foreign data access
4. Heap dumping and the FFI
5. Examples

1. Creating loadable modules

You must first compile your C code and create one or more loadable object modules. These object modules may then be loaded into Larceny, and Scheme foreign functions may link to specific functions in the loaded module. Defining foreign functions in Scheme is covered in a later section.

The method for creating a loadable object module varies from platform to platform. In the following, assume you have to C source files file1.c and file2.c that define functions that you want to make available as foreign functions in Larceny.

1.1. SunOS 4

Compile your source files and create a shared library. Using GCC, the command line might look like this:

  gcc -fPIC -shared file1.c file2.c -o my-library.so

The command creates my-library.so in the current directory. This library can now be loaded into Larceny using foreign-file. Any other shared libraries used by your library files should also be loaded into Larceny using foreign-file before any procedures are linked using foreign-procedure.

By default, /lib/libc.so is made available to the dynamic linker and to the foreign function interface, so there is no need for you to load that library explicitly.

1.2. SunOS 5

Compile your source files and create a shared library, linking with all the necessary libraries. Using GCC, the command line might look like this:

  gcc -fPIC -shared file1.c file2.c -lc -lm -lsocket -o my-library.so

Now you can use foreign-file to load my-library.so into Larceny.

By default, /lib/libc.so is made available to the foreign function interface, so there is no need for you to load that library explicitly.

2. The Interface

2.1. Procedures

Procedure foreign-file (foreign-file filename) => unspecified

Foreign-file loads the named object file into Larceny and makes it available for dynamic linking.

Larceny uses the operating system provided dynamic linker to do dynamic linking. The operation of the dynamic linker varies from platform to platform:

On some versions of SunOS 4, if the linker is given a file that does not exist, it will terminate the process. (Most likely this is a bug.) This means you should never call foreign-file with the name of a file that does not exist.
On SunOS 5, if a foreign file is given to foreign-file without a directory specification, then the dynamic linker will search its load path (the LD_LIBRARY_PATH environment variable) for the file. Hence, a foreign file in the current directory should be "./file.so", not "file.so".

Procedure foreign-procedure (foreign-procedure name (arg-type ...) return-type) => unspecified

Returns a Scheme procedure p that calls the foreign procedure whose name is name. When p is called, it will convert its parameters to representations indicated by the arg-types and invoke the foreign procedure, passing the converted values as parameters. When the foreign procedure returns, its return value is converted to a Scheme value according to return-type.

Types are described below.

The address of the foreign procedure is obtained by searching for name in the symbol tables of the foreign files that have been loaded with foreign-file.

Procedure foreign-null-pointer (foreign-null-pointer) => integer

Returns a foreign null pointer.

Procedure foreign-null-pointer? (foreign-null-pointer? integer) => boolean

Tests whether its argument is a foreign null pointer.

2.2. Types

A type is denoted by a symbol. The following is a list of the accepted types and their conversions at the call-out to the foreign procedure:

int: Any exact integer value in the range [-2^31,2^31-1] is acceptable and is converted to a C "int".
unsigned: Any exact integer value in the range [0,2^32-1] is acceptable and is converted to a C "unsigned".
short: Synonymous with int in the current implementation.
ushort: Synonymous to unsigned in the current implementation.
char: A character is acceptable. It is converted to a C "int" type.
uchar: A character is acceptable. It is converted to a C "unsigned" type.
long: Synonymous with int in the current implementation.
ulong: Synonymous with unsigned in the current implementation.
float: A flonum is acceptable. It is converted to a C "float".
double: A flonum is acceptable. It is converted to a C "double".
bool: Any object is acceptable. It is converted to a C "int": #f is converted to 0, and all other objects to 1.
boxed: Any heap-allocated data structure (pair, bytevector-like, vector-like, procedure) is acceptable. It is converted to a C "void*" to the first element of the structure. The value #f is also acceptable. It is converted to a C "(void*)0" value.
string: A string or #f is acceptable. A string is copied into a NUL-terminated bytevector, and the resulting pointer is passed. #f is converted to a C "(char*)0" value.

Additionally, the types can be used as the return type, where conversions back to Scheme values take place:

int: A C "int" is expected; it is converted to an exact integer.
unsigned: A C "unsigned" is expected; it is converted to an exact integer.
short: Synonymous with int in the current implementation.
ushort: Synonymous with unsigned in the current implementation.
char: A C "int" is expected; it is converted to a character.
uchar: A C "unsigned" is expected; it is converted to a character.
long: Synonymous with int in the current implementation.
ulong: Synonymous with unsigned in the current implementation.
float: A C "float" is expected. It is converted to a flonum.
double: A C "double" is expected. It is converted to a flonum.
bool: A C "int" is expected. 0 is converted to #f, all other values to #t.
void: No return value.
string: A C "char*" is expected. If it is non-null, it is expected to point to a NUL-terminated string, which is copied into a newly allocated Scheme string which is then returned. If the return value is null, then #f is returned.

3. Foreign Data Access

3.1. Raw memory access

The two primitives peek-bytes and poke-bytes are provided for reading and writing memory at specific addresses. These procedures are typically used for copying data from foreign data structures into Scheme bytevectors for subsequent decoding.

(The use of peek-bytes and poke-bytes can often be avoided by keeping foreign data in a Scheme bytevector and passing the bytevector to a call-out using the boxed parameter type. However, this technique is inappropriate if the foreign code retains a pointer to the Scheme datum, which may be moved by the garbage collector.)

Procedure peek-bytes (peek-bytes addr bytevector count) => unspecified: Addr must be an exact nonnegative integer. Count must be a fixnum. The bytes in the range from addr through addr+count-1 are copied into bytevector, which must be long enough to hold that many bytes.
If any address in the range is not an address accessible to the process, unpredictable things may happen. Typically, you'll get a segmentation fault. Larceny does not yet catch segmentation faults.
Procedure poke-bytes (poke-bytes addr bytevector count) => unspecified: Addr must be an exact nonnegative integer. Count must be a fixnum. The count first bytes from bytevector are copied into memory in the range from addr through addr+count-1.
If any address in the range is not an address accessible to the process, unpredictable things may happen. Typically, you'll get a segmentation fault. Larceny does not yet catch segmentation faults.
Also, it's possible to corrupt memory with poke-bytes. Don't do that.

3.2. Foreign data sizes

The following variables constants define the sizes of basic C data types:

sizeof:short The size of a "short int".
sizeof:int The size of an "int".
sizeof:long The size of a "long int".
sizeof:pointer The size of any pointer type.

3.3. Decoding foreign data

Foreign data is visible to a Scheme program either as an object pointed to by a memory address (which is itself represented as an integer), or as a bytevector that contains the bytes of the foreign datum.

A number of utility procedures that make reading and writing data of common C primitive types have been written for both these kinds of foreign objects.

Bytevector accessor procedures (%get16 bv i) => integer (%get16u bv i) => integer (%get32 bv i) => integer (%get32u bv i) => integer (%get-int bv i) => integer (%get-unsigned bv i) => integer (%get-short bv i) => integer (%get-ushort bv i) => integer (%get-long bv i) => integer (%get-ulong bv i) => integer (%get-pointer bv i) => integer

These procedures decode bytevectors that contain the bytes of foreign objects. In each case, bv is a bytevector and i is the offset of the first byte of a field in that bytevector. The field is fetched and returned as an integer (signed or unsigned as appropriate).

Bytevector updater procedures (%set16 bv i val) => unspecified (%set16u bv i val) => unspecified (%set32 bv i val) => unspecified (%set32u bv i val) => unspecified (%set-int bv i val) => unspecified (%set-unsigned bv i val) => unspecified (%set-short bv i val) => unspecified (%set-ushort bv i val) => unspecified (%set-long bv i val) => unspecified (%set-ulong bv i val) => unspecified (%set-pointer bv i val) => unspecified

These procedures update bytevectors that contain the bytes of foreign objects. In each case, bv is a bytevector, i is an offset of the first byte of a field in that bytevector, and val is a value to be stored in that field. The values must be exact integers in a range implied by the data type.

Foreign-pointer accessor procedures (%peek8 addr) => integer (%peek8u addr) => integer (%peek16 addr) => integer (%peek16u addr) => integer (%peek32 addr) => integer (%peek32u addr) => integer (%peek-int addr) => integer (%peek-long addr) => integer (%peek-unsigned addr) => integer (%peek-ulong addr) => integer (%peek-short addr) => integer (%peek-ushort addr) => integer (%peek-pointer addr) => integer (%peek-string addr) => string

These procedures read raw memory. In each case, addr is an address, and the value stored at that address (the size of which is indicated by the name of the procedure) is fetched and returned as an integer.

%Peek-string expects to find a NUL-terminated string of 8-bit bytes at the given address. It is returned as a Scheme string.

Foreign-pointer updater procedures (%poke8 addr val) => unspecified (%poke8u addr val) => unspecified (%poke16 addr val) => unspecified (%poke16u addr val) => unspecified (%poke32 addr val) => unspecified (%poke32u addr val) => unspecified (%poke-int addr val) => unspecified (%poke-long addr val) => unspecified (%poke-unsigned addr val) => unspecified (%poke-ulong addr val) => unspecified (%poke-short addr val) => unspecified (%poke-ushort addr val) => unspecified (%poke-pointer addr val) => unspecified

These procedures update raw memory. In each case, addr is an address, and val is a value to be stored at that address.

4. Heap dumping and the FFI

If foreign functions are linked into Larceny using the FFI, and a Larceny heap image is subsequently dumped (with dump-interactive-heap or dump-heap), then the foreign functions are not saved as part of the heap image. When the heap image is subsequently loaded into Larceny at startup, the FFI will attempt to re-link all the foreign functions in the heap image.

During the relinking phase, foreign files will again be loaded into Larceny, and Larceny's FFI will use the file names as they were originally given to the FFI when it tries to load the files. In particular, if relative pathnames were used, Larceny will not have converted them to absolute pathnames.

An error during relinking will result in Larceny aborting with an error message and returning to the operating system. This is considered a feature.

5. Examples

5.1. Change directory

This procedure uses the chdir() system call to set the process's current working directory. The string parameter type is used to pass a Scheme string to the C procedure.

(define cd
  (let ((chdir (foreign-procedure "chdir" '(string) 'int)))
    (lambda (newdir)
      (if (not (zero? (chdir newdir)))
	  (error "cd: " newdir " is not a valid directory name."))
      (unspecified))))

5.2. Print Working Directory

This procedure uses the getcwd() (get current working directory) system call to retrieve the name of the process's current working directory. A bytevector is created and passed in as a buffer in which to store the return value -- a 0-terminated ASCII string. Then the FFI utility function ffi/asciiz->string is called to convert the bytevector to a string.

(define pwd
  (let ((getcwd (foreign-procedure "getcwd" '(boxed int) 'int)))
    (lambda ()
      (let ((s (make-bytevector 1024)))
	(getcwd s 1024)
	(ffi/asciiz->string s)))))

5.3. Other examples

The Experimental directory contains several examples of use of the FFI. See in particular the files unix.sch (Unix system calls) and socket.sch (procedures for communicating over sockets).

$Id: ffi.html,v 1.5 1999/11/23 23:33:17 lth Exp $
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