FFIGEN is a program system which facilitates the writing of translators from C header files to foreign function interfaces for particular programming language implementations. This document describes its structure and use. The discussion is aimed at translator writers; everyone else should confine themselves to section 3. A companion document, FFIGEN Manifesto and Overview, motivates the work, and other companion documents describe specific translator implementations. In particular, the document FFIGEN Back-end for Chez Scheme Version 5 describes one translator in detail.
FFIGEN is based on the lcc C compiler, which is copyrighted software. See Section 10 for a full copyright notice.
To generate a translation of a header file you run the ffigen command to generate an intermediate form of the C header files you want to translate, and then run the back-end on the resulting files to generate the foreign function interface for the library.
Your task, should you choose to accept it, is to implement the target-specific parts of the back-end for your particular target (which is to say, combination of host language implementation, operating system, architecture, foreign language implementation, and translation policy). You should be able to use the FFIGEN front-end and the target-independent parts of the back-end pretty much as they are.
How to implement the target-specific parts of the back-end is discussed in Section 6. Use of the front end is described in Section 2. The intermediate format is described in Section 4, and the target-independent parts of the back-end and their interface to the target-dependent part are described in Section 5. Finally, Section 7 covers some issues which need to be tackled in the future.
The command ffigen is run on a set of header files with preprocessor option and include file options. Arguments are processed in order. For each header file (type .h) and all the files it includes, a single preprocessor file (type .ffi) is produced.
The options are:
The intermediate format consists of s-expressions following this grammar:
<file> -> <record> ... <record> -> (function <filename> <name> <type> <attrs>) | (var <filename> <name> <type> <attrs>) | (type <filename> <name> <type>) | (struct <filename> <name> ((<name> <type>) ...)) | (union <filename> <name> ((<name> <type>) ...)) | (enum <filename> <name> ((<name> <value>) ...)) | (enum-ident <filename> <name> <value>) | (macro <filename> <name+args> <body>) <type> -> (<primitive> <attrs>) | (struct-ref <tag>) | (union-ref <tag>) | (enum-ref <tag>) | (function (<type> ...) <type>) | (pointer <type>) | (array <value> <type>) <attrs> -> (<attr> ...) <attr> -> static | extern | const | volatile <primitive> -> char | signed-char | unsigned-char | short | unsigned-short | int | unsigned | long | unsigned-long | float | double | void <value> -> <integer> <filename> -> <string> <name> -> <string> <body> -> <string> <name+args> -> <string> <tag> -> <string>Notes relating to the grammar:
The target-independent back-end is a Scheme program called process which reads the intermediate form into memory and performs some initial processing. It exports some global variables and a number of procedures which are used to access the structures in the database of intermediate records, and imports two target-dependent functions from the target-dependent back-end. This section describes the interfaces.
The global variables which hold the database are:
(define functions '()) ; list of function records (define vars '()) ; list of var records (define types '()) ; list of type records (define structs '()) ; list of struct records (define unions '()) ; list of union records (define macros '()) ; list of macro records (define enums '()) ; list of enum records (define enum-idents '()) ; list of enum-ident recordsEach of these contains a list of all the records of the type indicated by their names. Note that records may look different internally than in the defined intermediate form, so accessor functions (see below) should always be used.
In addition, there are two globals which are set but not used by the target-independent back-end:
(define source-file #f) ; name of the input file itself (define filenames '()) ; names of all files in the input
The main entry point to the back end is the procedure process, which takes a single file name as an argument. Process initializes globals, reads the file, and processes the records.
(define (process filename) ...)
Record processing consists of some general analysis and target-specific code generation. First, the target-specific procedure select-functions is called; it must set or reset the "referenced" bit in each record depending on whether the function is interesting to the back-end or not. After computing reachability of structured types and setting the referenced bits of those types which are reachable, a translation is generated by a call to the back-end function generate-translation, which takes no arguments.
(define (select-functions) ...) (define (generate-translation) ...)
A number of data structure accessors and mutators are also available. These are generic procedures which work on all of the record types.
(define (file r) ...) ; file name of record (define (name r) ...) ; name in records which have one (define (type r) ...) ; type in records which have one (define (attrs r) ...) ; attrs in records which have one (define (fields r) ...) ; fields in struct/union record (define (value r) ...) ; value of enum-ident record (define (tag r) ...) ; tag in struct/union/union/-ref record (define (referenced? r) ...) ; is record referenced? (define (referenced! r) ...) ; set referenced bit (define (unreferenced! r) ...) ; reset referenced bitArguably the tag accessor should go away and name should simply be used in its place. As it is, name is not defined on struct-ref, union-ref, and enum-ref records.
The procedure record-tag returns the tag of the record currently being held. It can also be applied to types.
(define (record-tag r) ...) ; get record tag
All records can have back-end specific values attached to them; usually these are cached names for operations on structured values, so for now the procedures which manipulate the back-end specific data are called cache-name to remember a value and cached-names to return the list of remembered values:
(define (cache-name r v) ...) ; remember value in record (define (cached-names r) ...) ; retrieve remembered valuesWe should probably replace this with a more general property-list-like mechanism.
In addition, two procedures extract parts of function types:
(define (arglist r) ...) ; function argument types (define (rett r) ...) ; function return type
Some utilities to deal with file names are also provided:
(define (strip-extension fn) ...) (define (strip-path fn) ...) (define (get-path fn) ...)
A string macro expander makes it easier to generate C code, for the back ends that need it. The macro expander is called instantiate and is called with a string template and a vector of arguments (which are also strings). The template contains patterns of the form @n where n is a single digit; when such a pattern is seen it is replaced with the corresponding value from the argument vector.
(define (instantiate template arguments) ...)
Two procedures, struct-names and union-names, take a structure (or union) and returns a list of all the typedef names which reference the structure directly.
(define (struct-names struct) ...) (define (union-names union) ...)
An association function which searches one of the record lists for a given record by the name field is also available:
(define (lookup key items) ...)
The procedure user-defined-tag? determines whether a tag was defined by the user or generated by the system:
(define (user-defined-tag? x) ...)
The procedure warn takes some arbitrary arguments and generates a warning message on standard output:
(define (warn msg . rest) ...)
Some standard predicates take a type and test its kind: primitive-type? is true if the argument is of a primitive type as outlined in the grammar above; basic-type? is true if the argument is a primitive type or a pointer type; array-type? is true if the argument is an array type, and finally, structured-type? is true if the argument is a struct-ref or union-ref type:
(define (primitive-type? t) ...) (define (basic-type? t) ...) (define (array-type? t) ...) (define (structured-type? t) ...)
To write the target-dependent back-end, you must decide on the policy for the translation and then implement the translation. The policy covers such issues as: which constructs in C are or are not handled; the translation for each handled construct; how non-handled constructs are dealt with (ignored, detected with warnings, detected with errors); how to deal with exceptional cases (consider the fgets example from the Manifesto).
For a concrete example, see the companion document FFIGEN Backend for Chez Scheme Version 5, which addresses many of the choices to be made and their possible solutions.
A number of features will be supported in the future:
A number of features will most likely be supported, but need to be investigated:
In addition, there are some issues to investigate in a larger perspective:
My goal is to support as many target languages as is reasonable, but I can't write all the translators myself (I lack the time and, in many cases, the knowledge). Targets that I will take care of include STk, and, if no-one beats me to it, Scsh, both Scheme systems. Someone has already volunteered to write the ILU back-end. Others are interested in back-ends for Modula-3 and Mercury.
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