FFI examples

- read-eval-print loop (and emacs)
- define a function with arguments and a returned value, and define a variable
- call a Scheme function from C, and get/set Scheme variable values in C
- C++ and Juce
- load sndlib using the Xen functions and macros
- add a new Scheme type and a procedure with a setter
- redirect display output to a C procedure
- extend a built-in operator ("+" in this case)
- C-side define* (s7_define_function_star)
- C-side define-macro (s7_define_macro)
- define a generic function in C
- signal handling (C-C to break out of an infinite loop)
- notification in C that a Scheme variable has been set!
- Load C defined stuff into a separate namespace
- Error handling in C
- Hooks in C and Scheme
- Load a C module dynamically
- gmp and friends
- gdb
- WASM
- FFI notes

s7 exists only to serve as an extension of some other application, so it is primarily a foreign function interface. s7.h has lots of comments about the individual functions. Here I'll collect some complete examples. s7.c depends on the following compile-time flags:

    SIZEOF_VOID_P                  8 (default) or 4.
    WITH_GMP                       1 if you want multiprecision arithmetic (requires gmp, mpfr, and mpc, default is 0)
    HAVE_COMPLEX_NUMBERS           1 if your compiler supports complex numbers
    HAVE_COMPLEX_TRIG              1 if your math library has complex versions of the trig functions
    DISABLE_DEPRECATED             1 if you want to make sure you're not using any deprecated s7 stuff (default is 0)

    WITH_IMMUTATBLE_UNQUOTE        1 if you want "unquote" omitted (default is 0)
    WITH_EXTRA_EXPONENT_MARKERS    1 if you want "d", "f", "l", and "s" in addition to "e" as exponent markers (default is 0)
                                       if someone defends these exponent markers, ask him to read 1l11+11l1i
                                       (in 2 million lines of open-source Scheme, there is not one use of these silly things)
    WITH_SYSTEM_EXTRAS             1 if you want some additional OS-related functions built-in (default is 0)
    WITH_MAIN                      1 if you want s7.c to include a main program section that runs a REPL.
    WITH_C_LOADER		       1 if you want to be able to load shared object files with load.
    WITH_R7RS                      1 if you want r7rs support built-in; otherwise you need to build/load libc_s7.so and load r7rs.scm.
    WITH_NOTCURSES                 1 is you want a decent repl (based on notcurses) built-in.

See the comment at the start of s7.c for more information about these switches. s7.h defines the two main number types: s7_int and s7_double. The examples that follow show:

- read-eval-print loop (and emacs)
- define a function with arguments and a returned value, and define a variable
- call a Scheme function from C, and get/set Scheme variable values in C
- C++ and Juce
- load sndlib using the Xen functions and macros
- add a new Scheme type and a procedure with a setter
- redirect display output to a C procedure
- extend a built-in operator ("+" in this case)
- C-side define* (s7_define_function_star)
- C-side define-macro (s7_define_macro)
- define a generic function in C
- signal handling (C-C to break out of an infinite loop)
- notification in C that a Scheme variable has been set!
- Load C defined stuff into a separate namespace
- Error handling in C
- Hooks in C and Scheme
- Load a C module dynamically
- gmp and friends
- gdb
- WASM

#### A simple listener

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>
    #include "s7.h"

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();      /* initialize the interpreter */
      while (1)                       /* fire up a read-eval-print loop */
        {
          char buffer[512];
          fprintf(stdout, "\n> ");    /* prompt for input */
          fgets(buffer, 512, stdin);
          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{                         /* evaluate the input and print the result */
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response);
    	}}
    }

    /* if not using gcc or clang, make mus-config.h (it can be empty)
     *
     * gcc -c s7.c -I.
     * gcc -o repl repl.c s7.o -lm -I. -ldl
     *
     * run it:
     *
     *    repl
     *    > (+ 1 2)
     *    3
     *    > (define (add1 x) (+ 1 x))
     *    add1
     *    > (add1 2)
     *    3
     *    > (exit)
     *
     * for long-term happiness in linux use:
     *   gcc -o repl repl.c s7.o -Wl,-export-dynamic -lm -I. -ldl
     *   clang also needs -fPIC I think
     * freebsd:
     *   gcc -o repl repl.c s7.o -Wl,-export-dynamic -lm -I.
     * osx:
     *   gcc -o repl repl.c s7.o -lm -I.
     * openbsd:
     *   clang -o repl repl.c s7.o -I. -fPIC -Wl,-export-dynamic -lm
     */

Since this reads stdin and writes stdout, it can be run as a Scheme subjob of emacs. One (inconvenient) way to do this is to set the emacs variable scheme-program-name to the name of the exectuable created above ("repl"), then call the emacs function run-scheme: M-x eval-expression in emacs, followed by (setq scheme-program-name "repl"), then M-x run-scheme, and you're talking to s7 in emacs. Of course, this connection can be customized indefinitely. See, for example, inf-snd.el in the Snd package.

Here are the not-always-built-in indentations I use in emacs:

    (put 'with-let 'scheme-indent-function 1)
    (put 'with-baffle 'scheme-indent-function 0)
    (put 'with-sound 'scheme-indent-function 1)
    (put 'catch 'scheme-indent-function 1)
    (put 'lambda* 'scheme-indent-function 1)
    (put 'when 'scheme-indent-function 1)
    (put 'let-temporarily 'scheme-indent-function 1)
    (put 'let*-temporarily 'scheme-indent-function 1)
    (put 'call-with-input-string 'scheme-indent-function 1)
    (put 'unless 'scheme-indent-function 1)
    (put 'letrec* 'scheme-indent-function 1)
    (put 'sublet 'scheme-indent-function 1)
    (put 'varlet 'scheme-indent-function 1)
    (put 'case* 'scheme-indent-function 1)
    (put 'macro 'scheme-indent-function 1)
    (put 'macro* 'scheme-indent-function 1)
    (put 'bacro 'scheme-indent-function 1)
    (put 'bacro* 'scheme-indent-function 1)

To read stdin while working in a GUI-based program is trickier. In glib, you can use something like this:

>     static gboolean read_stdin(GIOChannel *source, GIOCondition condition, gpointer data)
>     {
>       /* here read from g_io_channel_unix_get_fd(source) and call s7_eval_string */
>       return(true);
>     }

>     /* ... during initialization ... */

>     GIOChannel *channel;
>     channel = g_io_channel_unix_new(STDIN_FILENO);  /* watch stdin */
>     stdin_id = g_io_add_watch_full(channel,         /* and call read_stdin above if input is noticed */
>     			       G_PRIORITY_DEFAULT,
>     			       (GIOCondition)(G_IO_IN | G_IO_HUP | G_IO_ERR),
>     			       read_stdin, NULL, NULL);
>     g_io_channel_unref(channel);

Here's a version that uses libtecla for the line editor:

>     #include <stdlib.h>
>     #include <stdio.h>
>     #include <string.h>
>     #include <libtecla.h>
>     #include "s7.h"

>     int main(int argc, char **argv)
>     {
>       GetLine *gl = new_GetLine(500, 5000); /* The tecla line editor */
>       s7_scheme *s7 = s7_init();
>       while (1)
>         {
>           char *buffer;
>           buffer = gl_get_line(gl, "> ", NULL, 0);
>           if ((buffer[0] != '\n') ||
>     	  (strlen(buffer) > 1))
>     	{
>               char response[1024];
>     	  snprintf(response, 1024, "(write %s)", buffer);
>     	  s7_eval_c_string(s7, response);
>     	  fprintf(stdout, "\n");
>     	}}
>       gl = del_GetLine(gl);
>     }

>     /*
>      *   gcc -c s7.c -I. -O2 -g3
>      *   gcc -o ex1 ex1.c s7.o -lm -I. -ltecla -ldl
>      */

A repl (based on repl.scm or nrepl.scm) is built into s7. Include the compiler flag -DWITH_MAIN:

    gcc -o nrepl s7.c -O2 -I. -Wl,-export-dynamic -lm -ldl -DWITH_MAIN -DWITH_NOTCURSES -lnotcurses-core

Common Lisp has something called "evalhook" that makes it possible to insert your own function into the eval loop. In s7, we have a "begin_hook" which sits at the opening of many begin blocks (implicit or explicit). begin_hook is a (C) function; if it sets its bool argument to true, s7 interrupts the current evaluation. Here is a version of the REPL in which begin_hook watches for C-g to interrupt some long computation:

>     /* terminal-based REPL,
>      *    an expansion of the read-eval-print loop program above.
>      * type C-g to interrupt an evaluation.
>      */
>     #include <stdlib.h>
>     #include <stdio.h>
>     #include <string.h>
>     #include <termios.h>
>     #include <signal.h>

>     #include "s7.h"

>     static struct termios save_buf, buf;

>     static void sigcatch(int n)
>     {
>       /* put things back the way they were */
>       tcsetattr(fileno(stdin), TCSAFLUSH, &save_buf);
>       exit(0);
>     }

>     static char buffer[512];
>     static int type_ahead_point = 0;

>     static void watch_for_c_g(s7_scheme *sc, bool *all_done)
>     {
>       char c;
>       /* watch for C-g without blocking, save other chars as type-ahead */
>       tcsetattr(fileno(stdin), TCSAFLUSH, &buf);
>       if (read(fileno(stdin), &c, 1) == 1)
>         {
>           if (c == 7) /* C-g */
>     	{
>     	  *all_done = true;
>     	  type_ahead_point = 0;
>     	}
>           else buffer[type_ahead_point++] = c;
>         }
>       tcsetattr(fileno(stdin), TCSAFLUSH, &save_buf);
>     }

>     int main(int argc, char **argv)
>     {
>       s7_scheme *s7;
>       bool use_begin_hook = (tcgetattr(fileno(stdin), &save_buf) >= 0);
>       if (use_begin_hook)
>         {
>           buf = save_buf;
>           buf.c_lflag &= ~ICANON;
>           buf.c_cc[VMIN] = 0;
>           buf.c_cc[VTIME] = 0;

>           signal(SIGINT, sigcatch);
>           signal(SIGQUIT, sigcatch);
>           signal(SIGTERM, sigcatch);
>         }
>       s7 = s7_init();

>       if (argc == 2)
>         {
>           fprintf(stderr, "load %s\n", argv[1]);
>           if (!s7_load(s7, argv[1]))
>             fprintf(stderr, "can't find %s\n", argv[1]);
>         }
>       else
>         {
>           while (1)
>     	{
>     	  fprintf(stdout, "\n> ");
>     	  fgets((char *)(buffer + type_ahead_point), 512 - type_ahead_point, stdin);
>     	  type_ahead_point = 0;

>     	  if ((buffer[0] != '\n') ||
>     	      (strlen(buffer) > 1))
>     	    {
>                   char response[1024];
>     	      snprintf(response, 1024, "(write %s)", buffer);

>     	      if (use_begin_hook)
>     		s7_set_begin_hook(s7, watch_for_c_g);
>     	      s7_eval_c_string(s7, response);
>     	      if (use_begin_hook)
>     		s7_set_begin_hook(s7, NULL);
>     	    }}}
>       if (use_begin_hook)
>         tcsetattr(fileno(stdin), TCSAFLUSH, &save_buf);
>     }

#### Define a function with arguments and a returned value, and a variable

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    static s7_pointer add1(s7_scheme *sc, s7_pointer args)
    {
      /* all added functions have this form, args is a list,
       *    s7_car(args) is the first arg, etc
       */
      if (s7_is_integer(s7_car(args)))
        return(s7_make_integer(sc, 1 + s7_integer(s7_car(args))));
      return(s7_wrong_type_arg_error(sc, "add1", 1, s7_car(args), "an integer"));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();
      s7_define_function(s7, "add1", add1, 1, 0, false, "(add1 int) adds 1 to int");
                                          /* add the function "add1" to the interpreter.
                                           *   1, 0, false -> one required arg,
    				       *                  no optional args,
    				       *                  no "rest" arg
    				       */
     s7_define_variable(s7, "my-pi", s7_make_real(s7, 3.14159265));

      while (1)                           /* fire up a "repl" */
        {
          char buffer[512];
          fprintf(stdout, "\n> ");        /* prompt for input */
          fgets(buffer, 512, stdin);
          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response); /* evaluate input and write the result */
    	}}
    }

    /*    doc7
     *    > my-pi
     *    3.14159265
     *    > (+ 1 (add1 1))
     *    3
     *    > (exit)
     */

#### Call a Scheme-defined function from C, and get/set Scheme variable values in C

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();

      s7_define_variable(s7, "an-integer", s7_make_integer(s7, 1));
      s7_eval_c_string(s7, "(define (add1 a) (+ a 1))");

      fprintf(stderr, "an-integer: %lld\n",
    	  s7_integer(s7_name_to_value(s7, "an-integer")));

      s7_symbol_set_value(s7, s7_make_symbol(s7, "an-integer"), s7_make_integer(s7, 32));

      fprintf(stderr, "now an-integer: %lld\n",
    	  s7_integer(s7_name_to_value(s7, "an-integer")));

      fprintf(stderr, "(add1 2): %lld\n",
    	  s7_integer(s7_call(s7,
    			     s7_name_to_value(s7, "add1"),
    			     s7_cons(s7, s7_make_integer(s7, 2), s7_nil(s7)))));
    }

    /*
     *    doc7
     *    an-integer: 1
     *    now an-integer: 32
     *    (add1 2): 3
     */

In more complicated cases, it is probably easier use s7_eval_c_string_with_environment. As an example, say we want to have a C procedure that calls the pretty printer function pp in write.scm, returning a string to C. We need to make sure pp is loaded, and catch any errors that come up. And we need to pass the C-level s7 object to pp. So...

    static const char *pp(s7_scheme *sc, s7_pointer obj) /* (pp obj) */
    {
      return(s7_string(
              s7_eval_c_string_with_environment(sc,
                "(catch #t                         \
                   (lambda ()                      \
                     (unless (defined? 'pp)        \
                       (load \"write.scm\"))       \
                     (pp obj))                     \
                   (lambda (type info)             \
                     (apply format #f info)))",
    	   s7_inlet(sc, s7_list(sc, 1, s7_cons(sc, s7_make_symbol(sc, "obj"), obj))))));
    }

and now when we want a pretty-printed representation of something: pp(sc, obj); The s7_inlet call is creating a local environment with the object "obj" bound in scheme to the name "obj" so that (pp obj) will find the "obj" that actually lives in C. You may need to give the full filename for write.scm, or add its path to the load-path list. In the latter case, `(require write.scm)` could replace `(unless (defined?...))`.

#### C++ and Juce, from Rick Taube

    int main(int argc, const char* argv[])
    {
      initialiseJuce_NonGUI();

      s7_scheme *s7 = s7_init();
      if (!s7)
        {
          std::cout <<  "Can't start S7!\n";
          return -1;
        }

      while (true)
        {
          s7_pointer val;
          std::string str;
          std::cout << "\ns7> ";
          std::getline(std::cin, str);
          val = s7_eval_c_string(s7, str.c_str());
          std::cout << s7_object_to_c_string(s7, val);
        }

      free(s7);
      std::cout << "Bye!\n";
      return 0;
    }

#### Load sndlib into an s7 repl

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>
    #include <unistd.h>

    /* assume we've configured and built sndlib, so it has created a mus-config.h file.
     * also assume we've built s7 with WITH_SYSTEM_EXTRAS set, so we have file-exists? and delete-file
     */

    #include "mus-config.h"
    #include "s7.h"
    #include "xen.h"
    #include "clm.h"
    #include "clm2xen.h"

    /* we need to redirect clm's mus_error calls to s7_error */

    static void mus_error_to_s7(int type, char *msg)
    {
      s7_error(s7,                               /* s7 is declared in xen.h, defined in xen.c */
    	   s7_make_symbol(s7, "mus-error"),
    	   s7_cons(s7, s7_make_string(s7, msg), s7_nil(s7)));
    }

    int main(int argc, char **argv)
    {
      s7 = s7_init();                     /* initialize the interpreter */
      s7_xen_initialize(s7);              /* initialize the xen stuff (hooks and the xen s7 FFI used by sndlib) */
      Init_sndlib();                      /* initialize sndlib with all the functions linked into s7 */

      mus_error_set_handler(mus_error_to_s7); /* catch low-level errors and pass them to s7-error */

      while (1)                           /* fire up a "repl" */
        {
          char buffer[512];
          fprintf(stdout, "\n> ");        /* prompt for input */
          fgets(buffer, 512, stdin);
          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response); /* evaluate input and write the result */
    	}}
    }

    /* gcc -o doc7 doc7.c -lm -I. /usr/local/lib/libsndlib.a -lasound -ldl
     *
     *   (load "sndlib-ws.scm")
     *   (with-sound () (outa 10 .1))
     *   (load "v.scm")
     *   (with-sound () (fm-violin 0 .1 440 .1))
     *
     * you might also need -lgsl -lgslcblas -lfftw3
     */

If you built libsndlib.so, it is possible to use it directly in the s7 repl:

    repl          ; this is a bare s7 running repl.scm via -DWITH_MAIN=1
    loading libc_s7.so
    > (load "/home/bil/test/sndlib/libsndlib.so" (inlet 'init_func 's7_init_sndlib))
    #t            ; s7_init_sndlib ties all the sndlib functions and variables into s7
    > (load "sndlib-ws.scm")
    tmpnam
    > (set! *clm-player* (lambda (file) (system (format #f "sndplay ~A" file))))
    > (load "v.scm")
    fm-violin
    > (with-sound (:play #t) (fm-violin 0 1 440 .1))
    "test.snd"

You can use autoload to load libsndlib when needed:

    (define (find-library name)
      (if (or (file-exists? name)
    	  (char=? (name 0) #\/))
          name
          (call-with-exit
           (lambda (return)
    	 (for-each
    	  (lambda (path)
    	    (let ((new-name (string-append path "/" name)))
    	      (if (file-exists? new-name)
    		  (return new-name))))
    	  *load-path*)
    	 (let ((libs (getenv "LD_LIBRARY_PATH")) ; colon separated directory names
    	       (start 0))
    	   (do ((colon (char-position #\: libs) (char-position #\: libs start)))
    	       ((or (not colon)
    		    (let ((new-name (string-append (substring libs start colon) "/" name)))
    		      (and (file-exists? new-name)
    			   (return new-name)))))
    	     (set! start (+ colon 1))))
    	 name))))

    (autoload 'clm
      (lambda (e)
        (load (find-library "libsndlib.so") (inlet '(init_func . s7_init_sndlib)))
        (set! *features* (cons 'clm *features*))
        (with-let (rootlet) (define clm #t))
        (load "sndlib-ws.scm")
        (set! *clm-player* (lambda (file) (system (format #f "sndplay ~A" file))))))

and use the repl's vt100 stuff to (for example) post the current begin time as a note list computes:

    (define (clm-notehook . args)
      ;; assume second arg is begin time (first is instrument name)
      (when (and (pair? args)
    	     (pair? (cdr args))
    	     (number? (cadr args)))
        (with-let (sublet (*repl* 'repl-let) :begin-time (cadr args))
          (let ((coords (cursor-coords))
    	    (col (floor (/ last-col 2))))
    	(let ((str (number->string begin-time)))
    	  (format *stderr* "~C[~D;~DH" #\escape prompt-row col)
    	  (format *stderr* "~C[K~A"  #\escape (if (> (length str) col) (substring str 0 (- col 1)) str)))
    	(format *stderr* "~C[~D;~DH"   #\escape (cdr coords) (car coords))))))

    (set! *clm-notehook* clm-notehook)

#### Add a new Scheme type and a procedure with a setter

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    /* define *listener-prompt* in scheme, add two accessors for C get/set */

    static const char *listener_prompt(s7_scheme *sc)
    {
      return(s7_string(s7_name_to_value(sc, "*listener-prompt*")));
    }

    static void set_listener_prompt(s7_scheme *sc, const char *new_prompt)
    {
      s7_symbol_set_value(sc, s7_make_symbol(sc, "*listener-prompt*"), s7_make_string(sc, new_prompt));
    }

    /* now add a new type, a struct named "dax" with two fields, a real "x" and a list "data" */
    /*   since the data field is an s7 object, we'll need to mark it to protect it from the GC */

    typedef struct {
      s7_double x;
      s7_pointer data;
    } dax;

    static int dax_type_tag = 0;

    static s7_pointer dax_to_string(s7_scheme *sc, s7_pointer args)
    {
      s7_pointer result;
      dax *o = (dax *)s7_c_object_value(s7_car(args));
      char *data_str = s7_object_to_c_string(sc, o->data);
      int data_str_len = strlen(data_str);
      char *str = (char *)calloc(data_str_len + 32, sizeof(char));
      snprintf(str, data_str_len + 32, "<dax %.3f %s>", o->x, data_str);
      free(data_str);
      result = s7_make_string(sc, str);
      free(str);
      return(result);
    }

    static s7_pointer free_dax(s7_scheme *sc, s7_pointer obj)
    {
      free(s7_c_object_value(obj));
      return(NULL);
    }

    static s7_pointer mark_dax(s7_scheme *sc, s7_pointer obj)
    {
      dax *o = (dax *)s7_c_object_value(obj);
      s7_mark(o->data);
      return(NULL);
    }

    static s7_pointer make_dax(s7_scheme *sc, s7_pointer args)
    {
      dax *o = (dax *)malloc(sizeof(dax));
      o->x = s7_real(s7_car(args));
      if (s7_cdr(args) != s7_nil(sc))
        o->data = s7_cadr(args);
      else o->data = s7_nil(sc);
      return(s7_make_c_object(sc, dax_type_tag, (void *)o));
    }

    static s7_pointer is_dax(s7_scheme *sc, s7_pointer args)
    {
      return(s7_make_boolean(sc,
    			 s7_is_c_object(s7_car(args)) &&
    			 s7_c_object_type(s7_car(args)) == dax_type_tag));
    }

    static s7_pointer dax_x(s7_scheme *sc, s7_pointer args)
    {
      dax *o = (dax *)s7_c_object_value(s7_car(args));
      return(s7_make_real(sc, o->x));
    }

    static s7_pointer set_dax_x(s7_scheme *sc, s7_pointer args)
    {
      dax *o = (dax *)s7_c_object_value(s7_car(args));
      o->x = s7_real(s7_cadr(args));
      return(s7_cadr(args));
    }

    static s7_pointer dax_data(s7_scheme *sc, s7_pointer args)
    {
      dax *o = (dax *)s7_c_object_value(s7_car(args));
      return(o->data);
    }

    static s7_pointer set_dax_data(s7_scheme *sc, s7_pointer args)
    {
      dax *o = (dax *)s7_c_object_value(s7_car(args));
      o->data = s7_cadr(args);
      return(o->data);
    }

    static s7_pointer dax_is_equal(s7_scheme *sc, s7_pointer args)
    {
      dax *d1, *d2;
      s7_pointer p1 = s7_car(args);
      s7_pointer p2 = s7_cadr(args);
      if (p1 == p2)
        return(s7_t(sc));
      if ((!s7_is_c_object(p2)) ||
          (s7_c_object_type(p2) != dax_type_tag))
        return(s7_f(sc));
      d1 = (dax *)s7_c_object_value(p1);
      d2 = (dax *)s7_c_object_value(p2);
      return(s7_make_boolean(sc,
    			 (d1->x == d2->x) &&
    			 (s7_is_equal(sc, d1->data, d2->data))));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();
      s7_define_variable(s7, "*listener-prompt*", s7_make_string(s7, ">"));

      dax_type_tag = s7_make_c_type(s7, "dax");
      s7_c_type_set_gc_free(s7, dax_type_tag, free_dax);
      s7_c_type_set_gc_mark(s7, dax_type_tag, mark_dax);
      s7_c_type_set_is_equal(s7, dax_type_tag, dax_is_equal);
      s7_c_type_set_to_string(s7, dax_type_tag, dax_to_string);

      s7_define_function(s7, "make-dax", make_dax, 2, 0, false, "(make-dax x data) makes a new dax");
      s7_define_function(s7, "dax?", is_dax, 1, 0, false, "(dax? anything) returns #t if its argument is a dax object");

      s7_define_variable(s7, "dax-x",
                         s7_dilambda(s7, "dax-x", dax_x, 1, 0, set_dax_x, 2, 0, "dax x field"));

      s7_define_variable(s7, "dax-data",
                         s7_dilambda(s7, "dax-data", dax_data, 1, 0, set_dax_data, 2, 0, "dax data field"));

      while (1)
        {
          char buffer[512];
          fprintf(stdout, "\n%s ", listener_prompt(s7));
          fgets(buffer, 512, stdin);
          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response); /* evaluate input and write the result */
    	}}
    }

    /* (in Linux);
     *    gcc dax.c -o dax -I. -O2 -g s7.o -ldl -lm -Wl,-export-dynamic -Wno-stringop-overflow
     *    dax
     *    > *listener-prompt*
     *    ">"
     *    > (set! *listener-prompt* ":")
     *    ":"
     *    : (define obj (make-dax 1.0 (list 1 2 3)))
     *    obj
     *    : obj
     *    #<dax 1.000 (1 2 3)>
     *    : (dax-x obj)
     *    1.0
     *    : (dax-data obj)
     *    (1 2 3)
     *    : (set! (dax-x obj) 123.0)
     *    123.0
     *    : obj
     *    #<dax 123.000 (1 2 3)>
     *    : (dax? obj)
     *    #t
     *    : (exit)
     */

#### Redirect output (and input) to a C procedure

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    static void my_print(s7_scheme *sc, uint8_t c, s7_pointer port)
    {
      fprintf(stderr, "[%c] ", c);
    }

    static s7_pointer my_read(s7_scheme *sc, s7_read_t peek, s7_pointer port)
    {
      return(s7_make_character(sc, fgetc(stdin)));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();

      s7_set_current_output_port(s7, s7_open_output_function(s7, my_print));
      s7_define_variable(s7, "io-port", s7_open_input_function(s7, my_read));

      while (1)
        {
          char buffer[512];
          fprintf(stdout, "\n> ");
          fgets(buffer, 512, stdin);
          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response);
    	}}
    }

    /*
     *    > (+ 1 2)
     *    [3]
     *    > (display "hiho")
     *    [h] [i] [h] [o] [#] [<] [u] [n] [s] [p] [e] [c] [i] [f] [i] [e] [d] [>]
     *    > (define (add1 x) (+ 1 x))
     *    [a] [d] [d] [1]
     *    > (add1 123)
     *    [1] [2] [4]
     *    > (read-char io-port)
     *    a                             ; here I typed "a" in the shell
     *    [#] [\] [a]
     */

In Snd, we want debug.scm (*debug-port*) output to go to the Snd listener text widget. The Snd function listener_append adds a string to that widget's text, so we define:

    static void (listener_write)(s7_scheme *sc, uint8_t c, s7_pointer port)
    {
      char buf[2];
      buf[0] = c;
      buf[1] = '\0';
      listener_append(buf);
    }

Then we define a Scheme-side variable, *listener-port*, to be a function port:

    s7_define_variable_with_documentation(s7, "*listener-port*",
      s7_open_output_function(s7, listener_write), "port to write to Snd's listener");

And tie it into *debug-port* via `(set! ((funclet trace-in) '*debug-port*) *listener-port*)`.

#### Extend a built-in operator ("+" in this case)

There are several ways to do this. In the first example, we save the original function, and replace it with ours, calling the original whenever possible:

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    static s7_pointer old_add;           /* the original "+" function for non-string cases */
    static s7_pointer old_string_append; /* same, for "string-append" */

    static s7_pointer our_add(s7_scheme *sc, s7_pointer args)
    {
      /* this will replace the built-in "+" operator, extending it to include strings:
       *   (+ "hi" "ho") -> "hiho" and  (+ 3 4) -> 7
       */
      if ((s7_is_pair(args)) &&
          (s7_is_string(s7_car(args))))
        return(s7_apply_function(sc, old_string_append, args));
      return(s7_apply_function(sc, old_add, args));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();

      /* get built-in + and string-append */
      old_add = s7_name_to_value(s7, "+");
      old_string_append = s7_name_to_value(s7, "string-append");

      /* redefine "+" */
      s7_define_function(s7, "+", our_add, 0, 0, true, "(+ ...) adds or appends its arguments");

      while (1)
        {
          char buffer[512];
          fprintf(stdout, "\n> ");
          fgets(buffer, 512, stdin);
          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response);
    	}}
    }

    /*    > (+ 1 2)
     *    3
     *    > (+ "hi" "ho")
     *    "hiho"
     */

In the next example, we use the method (inlet) machinery:

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>
    #include <math.h>

    #include "s7.h"

    static s7_pointer our_abs(s7_scheme *sc, s7_pointer args)
    {
      s7_pointer x = s7_car(args);
      if (!s7_is_number(x))
        {
          s7_pointer method = s7_method(sc, x, s7_make_symbol(sc, "abs"));
          if (method == s7_undefined(sc))                       /* no method found, so raise an error */
    	s7_wrong_type_arg_error(sc, "abs", 1, x, "a real");
          return(s7_apply_function(sc, method, args));          /*   else apply the method to the args */
        }
      return(s7_make_real(sc, 2.0 * (s7_double)fabs(s7_number_to_real(sc, x))));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();
      s7_define_function(s7, "our-abs", our_abs, 1, 0, false, "abs replacement");

      while (1)
        {
          char buffer[512];
          fprintf(stdout, "\n> ");
          fgets(buffer, 512, stdin);
          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response);
    	}}
    }

    /*    > (our-abs -1)
     *    2.0
     *    > (abs (openlet (inlet 'value -3.0 'abs (lambda (x) (our-abs (x 'value)))))))
     *    6.0
     */

#### C-side define* (s7_define_function_star)

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    static s7_pointer plus(s7_scheme *sc, s7_pointer args)
    {
      /* (define* (plus (red 32) blue) (+ (* 2 red) blue)) */
      return(s7_make_integer(sc, 2 * s7_integer(s7_car(args)) + s7_integer(s7_cadr(args))));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();
      s7_define_function_star(s7, "plus", plus, "(red 32) blue", "an example of define* from C");

      while (1)
        {
          char buffer[512];
          fprintf(stdout, "\n> ");
          fgets(buffer, 512, stdin);
          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response);
    	}}
    }

    /*
     *    > (plus 2 3)
     *    7
     *    > (plus :blue 3)
     *    67
     *    > (plus :blue 1 :red 4)
     *    9
     *    > (plus 2 :blue 3)
     *    7
     *    > (plus :blue 3 :red 1)
     *    5
     */

#### C-side define-macro (s7_define_macro)

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    static s7_pointer plus(s7_scheme *sc, s7_pointer args)
    {
      /* (define-macro (plus a b) `(+ ,a ,b)) */
      s7_pointer a = s7_car(args);
      s7_pointer b = s7_cadr(args);
      return(s7_list(sc, 3, s7_make_symbol(sc, "+"),  a, b));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();
      s7_define_macro(s7, "plus", plus, 2, 0, false, "plus adds its two arguments");

      while (1)
        {
          char buffer[512];
          fprintf(stdout, "\n> ");
          fgets(buffer, 512, stdin);
          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response);
    	}}
    }

    /*
     *    > (plus 2 3)
     *    5
     */

#### define a generic function in C

In scheme, a function becomes generic simply by `(apply ((car args) 'func) args)`. To accomplish the same thing in C, we use s7_method and s7_apply_function:

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    static s7_pointer plus(s7_scheme *sc, s7_pointer args)
    {
      #define plus_help "(plus obj ...) applies obj's plus method to obj and any trailing arguments."
      s7_pointer obj = s7_car(args);
      s7_pointer method = s7_method(sc, obj, s7_make_symbol(sc, "plus"));
      if (s7_is_procedure(method))
        return(s7_apply_function(sc, method, args));
      return(s7_f(sc));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();
      s7_define_function(s7, "plus", plus, 1, 0, true, plus_help);
      while (1)
        {
          char buffer[512];
          fprintf(stdout, "\n> ");
          fgets(buffer, 512, stdin);
          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response);
    	}}
    }

    /* gcc -c s7.c -I.
     * gcc -o ex15 ex15.c s7.o -I. -lm -ldl
     *
     *     > (plus 1 2)
     *     #f
     *     > (define obj (openlet (inlet 'plus (lambda args (apply + 1 (cdr args))))))
     *     obj
     *     > (plus obj 2 3)
     *     6
     */

#### Signal handling and continuations

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>
    #include <signal.h>

    #include "s7.h"

    static s7_scheme *s7;
    struct sigaction new_act, old_act;

    static void handle_sigint(int ignored)
    {
      fprintf(stderr, "interrupted!\n");
      s7_symbol_set_value(s7, s7_make_symbol(s7, "*interrupt*"), s7_make_continuation(s7)); /* save where we were interrupted */
      sigaction(SIGINT, &new_act, NULL);
      s7_quit(s7);                             /* get out of the eval loop if possible */
    }

    static s7_pointer our_sleep(s7_scheme *sc, s7_pointer args)
    {
      /* slow down our infinite loop for demo purposes */
      sleep(1);
      return(s7_f(sc));
    }

    int main(int argc, char **argv)
    {
      s7 = s7_init();
      s7_define_function(s7, "sleep", our_sleep, 0, 0, false, "(sleep) sleeps");
      s7_define_variable(s7, "*interrupt*", s7_f(s7));
      /* Scheme variable *interrupt* holds the continuation at the point of the interrupt */

      sigaction(SIGINT, NULL, &old_act);
      if (old_act.sa_handler != SIG_IGN)
        {
          memset(&new_act, 0, sizeof(new_act));
          new_act.sa_handler = &handle_sigint;
          sigaction(SIGINT, &new_act, NULL);
        }

      while (1)
        {
          char buffer[512];
          fprintf(stderr, "\n> ");
          fgets(buffer, 512, stdin);
          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response);
    	}}
    }

    /*
     *    > (do ((i 0 (+ i 1))) ((= i -1)) (format () "~D " i) (sleep))
     *      ;;; now type C-C to break out of this loop
     *    0 1 2 ^Cinterrupted!
     *      ;;; call the continuation to continue from where we were interrupted
     *    > (*interrupt*)
     *    3 4 5 ^Cinterrupted!
     *    > *interrupt*
     *    #<continuation>
     *    > (+ 1 2)
     *    3
     */

#### Notification from Scheme that a given Scheme variable has been set

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    static s7_pointer scheme_set_notification(s7_scheme *sc, s7_pointer args)
    {
      /* this function is called when the Scheme variable is set! */
      fprintf(stderr, "%s set to %s\n",
    	  s7_object_to_c_string(sc, s7_car(args)),
    	  s7_object_to_c_string(sc, s7_cadr(args)));
      return(s7_cadr(args));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();

      s7_define_function(s7, "notify-C", scheme_set_notification, 2, 0, false, "called if notified-var is set!");
      s7_define_variable(s7, "notified-var", s7_make_integer(s7, 0));
      s7_set_setter(s7, s7_make_symbol(s7, "notified-var"), s7_name_to_value(s7, "notify-C"));

      if (argc == 2)
        {
          fprintf(stderr, "load %s\n", argv[1]);
          if (!s7_load(s7, argv[1]))
            fprintf(stderr, "can't find %s\n", argv[1]);
        }
      else
        {
          while (1)
    	{
              char buffer[512];
    	  fprintf(stdout, "\n> ");
    	  fgets(buffer, 512, stdin);

    	  if ((buffer[0] != '\n') ||
    	      (strlen(buffer) > 1))
    	    {
                  char response[1024];
    	      snprintf(response, 1024, "(write %s)", buffer);
    	      s7_eval_c_string(s7, response);
    	    }}}
    }

    /*    > notified-var
     *    0
     *    > (set! notified-var 32)
     *    notified-var set to 32
     *    32
     */

#### Load C defined stuff into a separate namespace

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    static s7_pointer func1(s7_scheme *sc, s7_pointer args)
    {
      return(s7_make_integer(sc, s7_integer(s7_car(args)) + 1));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();
      /* "func1" and "var1" will be placed in an anonymous environment,
       *   accessible from Scheme via the global variable "lib-exports"
       */
      s7_pointer new_env = s7_inlet(s7, s7_curlet(s7), s7_nil(s7));
      /* make a private environment for func1 and var1 below (this is our "namespace") */
      s7_gc_protect(s7, new_env);

      s7_define(s7, new_env,
    	    s7_make_symbol(s7, "func1"),
    	    s7_make_function(s7, "func1", func1, 1, 0, false, "func1 adds 1 to its argument"));

      s7_define(s7, new_env, s7_make_symbol(s7, "var1"), s7_make_integer(s7, 32));
      /* those two symbols are now defined in the new environment */

      /* add "lib-exports" to the global environment */
      s7_define_variable(s7, "lib-exports", s7_let_to_list(s7, new_env));

      if (argc == 2)
        {
          fprintf(stderr, "load %s\n", argv[1]);
          if (!s7_load(s7, argv[1]))
            fprintf(stderr, "can't find %s\n", argv[1]);
        }
      else
        {
          while (1)
    	{
              char buffer[512];
    	  fprintf(stdout, "\n> ");
    	  fgets(buffer, 512, stdin);

    	  if ((buffer[0] != '\n') ||
    	      (strlen(buffer) > 1))
    	    {
                  char response[1024];
    	      snprintf(response, 1024, "(write %s)", buffer);
    	      s7_eval_c_string(s7, response);
    	    }}}
    }

    /*     > func1
     *     ;func1: unbound variable, line 1
     *     > lib-exports
     *     ((var1 . 32) (func1 . func1))
     *     ;; so lib-exports has the C-defined names and values
     *     ;; we can use these directly:
     *
     *     > (define lib-env (apply sublet (curlet) lib-exports))
     *     lib-env
     *     > (with-let lib-env (func1 var1))
     *     33
     *
     *     ;; or rename them to prepend "lib:"
     *     > (define lib-env (apply sublet
                                    (curlet)
                                    (map (lambda (binding)
                                           (cons (string->symbol
                                                   (string-append "lib:" (symbol->string (car binding))))
                                                 (cdr binding)))
                                         lib-exports)))
     *     lib-env
     *     > (with-let lib-env (lib:func1 lib:var1))
     *     33
     *
     *     ;;; now for convenience, place "func1" in the global environment under the name "func2"
     *     > (define func2 (cdadr lib-exports))
     *     func2
     *     > (func2 1)
     *     2
     */

#### Handle scheme errors in C

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    static s7_pointer error_handler(s7_scheme *sc, s7_pointer args)
    {
      fprintf(stdout, "error: %s\n", s7_string(s7_car(args)));
      return(s7_f(sc));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();
      bool with_error_hook = false;
      s7_define_function(s7, "error-handler", error_handler, 1, 0, false, "our error handler");

      if (with_error_hook)
        s7_eval_c_string(s7, "(set! (hook-functions *error-hook*)                    \n\
                                (list (lambda (hook)                                 \n\
                                        (error-handler                               \n\
                                          (apply format #f (hook 'data)))            \n\
                                        (set! (hook 'result) 'our-error))))");
      while (1)
        {
          char buffer[512];
          fprintf(stdout, "\n> ");
          fgets(buffer, 512, stdin);

          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
    	  s7_pointer result;
    	  int gc_loc = -1;
    	  const char *errmsg = NULL;

    	  /* trap error messages */
    	  s7_pointer old_port = s7_set_current_error_port(s7, s7_open_output_string(s7));
    	  if (old_port != s7_nil(s7))
    	    gc_loc = s7_gc_protect(s7, old_port);

    	  /* evaluate the input string */
    	  result = s7_eval_c_string(s7, buffer);

    	  /* print out the value wrapped in "{}" so we can tell it from other IO paths */
    	  fprintf(stdout, "{%s}", s7_object_to_c_string(s7, result));

    	  /* look for error messages */
    	  errmsg = s7_get_output_string(s7, s7_current_error_port(s7));

    	  /* if we got something, wrap it in "[]" */
    	  if ((errmsg) && (*errmsg))
    	    fprintf(stdout, "[%s]", errmsg);

    	  s7_close_output_port(s7, s7_current_error_port(s7));
    	  s7_set_current_error_port(s7, old_port);
    	  if (gc_loc != -1)
    	    s7_gc_unprotect_at(s7, gc_loc);
    	}}
    }

    /*
     *   gcc -c s7.c -I. -g3
     *   gcc -o ex3 ex3.c s7.o -lm -I. -ldl
     *
     * if with_error_hook is false,
     *
     *   > (+ 1 2)
     *   {3}
     *   > (+ 1 #\c)
     *   {wrong-type-arg}[
     *   ;+ argument 2, #\c, is character but should be a number, line 1
     *   ]
     *
     * so s7 by default prepends ";" to the error message, and appends "\n",
     *   sending that to current-error-port, and the error type ('wrong-type-arg here)
     *   is returned.
     *
     * if with_error_hook is true,
     *
     *   > (+ 1 2)
     *   {3}
     *   > (+ 1 #\c)
     *   error: + argument 2, #\c, is character but should be a number
     *   {our-error}
     *
     * so now the *error-hook* code handles both the error reporting and
     *   the value returned ('our-error in this case).
     */

#### C and Scheme hooks

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    static s7_pointer my_hook_function(s7_scheme *sc, s7_pointer args)
    {
      fprintf(stderr, "a is %s\n", s7_object_to_c_string(sc, s7_symbol_local_value(sc, s7_make_symbol(sc, "a"), s7_car(args))));
      return(s7_car(args));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();

      /* define test_hook in C, test-hook in Scheme, arguments are named a and b */
      s7_pointer test_hook = s7_eval_c_string(s7, "(make-hook 'a 'b)");
      s7_define_constant(s7, "test-hook", test_hook);

      /* add my_hook_function to the test_hook function list */
      s7_hook_set_functions(s7, test_hook,
    			s7_cons(s7,
    				s7_make_function(s7, "my-hook-function", my_hook_function, 1, 0, false, "my hook-function"),
    				s7_hook_functions(s7, test_hook)));
      while (1)
        {
          char buffer[512];
          fprintf(stdout, "\n> ");
          fgets(buffer, 512, stdin);

          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response);
    	}}
    }

    /*
     *    > test-hook
     *    #<lambda (hook)>
     *    > (hook-functions test-hook)
     *    (my-hook-function)
     *    > (test-hook 1 2)
     *    a is 1
     *    #<unspecified>
     */

#### Load a shared library

We can use dlopen to load a shared library, and dlsym to initialize that library in our main program. The tricky part is to conjure up the right compiler and loader flags. First we define a module that defines a new s7 function, add-1 that we'll tie into s7 explicitly, and another function that we'll try to call by waving a wand.

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"

    double a_function(double an_arg);
    double a_function(double an_arg)
    {
      return(an_arg + 1.0);
    }

    static s7_pointer add_1(s7_scheme *sc, s7_pointer args)
    {
      return(s7_make_integer(sc, s7_integer(s7_car(args)) + 1));
    }

    void init_ex(s7_scheme *sc);
    void init_ex(s7_scheme *sc)  /* this needs to be globally accessible (not "static") */
    {
      /* tell s7 about add-1, but leave a_function hidden */
      s7_define_function(sc, "add-1", add_1, 1, 0, false, "(add-1 x) adds 1 to x");
    }

And here is our main program:

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include "s7.h"
    #include <dlfcn.h>

    static void *library = NULL;

    static s7_pointer try(s7_scheme *sc, s7_pointer args)
    {
      /* try tries to call an arbitrary function in the shared library */
      void *func = dlsym(library, s7_string(s7_car(args)));
      if (func)
        {
          /* we'll assume double f(double) */
          typedef double (*dl_func)(double arg);
          return(s7_make_real(sc, ((dl_func)func)(s7_real(s7_cadr(args)))));
        }
      return(s7_error(sc, s7_make_symbol(sc, "can't find function"),
    		  s7_list(sc, 2, s7_make_string(sc, "loader error: ~S"),
    			         s7_make_string(sc, dlerror()))));
    }

    static s7_pointer cload(s7_scheme *sc, s7_pointer args)
    {
      /* cload loads a shared library */
      #define CLOAD_HELP "(cload so-file-name) loads the module"
      library = dlopen(s7_string(s7_car(args)), RTLD_LAZY);
      if (library)
        {
          /* call our init func to define add-1 in s7 */
          void *init_func = dlsym(library, s7_string(s7_cadr(args)));
          if (init_func)
    	{
    	  typedef void *(*dl_func)(s7_scheme *sc);
    	  ((dl_func)init_func)(sc);  /* call the initialization function (init_ex above) */
    	  return(s7_t(sc));
    	}}
      return(s7_error(sc, s7_make_symbol(sc, "load-error"),
    		      s7_list(sc, 2, s7_make_string(sc, "loader error: ~S"),
    			             s7_make_string(sc, dlerror()))));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();
      s7_define_function(s7, "cload", cload, 2, 0, false, CLOAD_HELP);
      s7_define_function(s7, "try", try, 2, 0, false,
                             "(try name num) tries to call name in the shared library with the argument num.");

      while (1)
        {
          char buffer[512];
          fprintf(stdout, "\n> ");
          fgets(buffer, 512, stdin);

          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response);
    	}}
    }

    /* Put the module in the file ex3a.c and the main program in ex3.c, then
     *
     * in Linux:
     *   gcc -c -fPIC ex3a.c
     *   gcc ex3a.o -shared -o ex3a.so
     *   gcc -c s7.c -I. -fPIC -shared
     *   gcc -o ex3 ex3.c s7.o -lm -ldl -I. -Wl,-export-dynamic
     *   # omit -ldl in freeBSD
     *
     * in Mac OSX:
     *   gcc -c ex3a.c
     *   gcc ex3a.o -o ex3a.so -dynamic -bundle -undefined suppress -flat_namespace
     *   gcc -c s7.c -I. -dynamic -bundle -undefined suppress -flat_namespace
     *   gcc -o ex3 ex3.c s7.o -lm -ldl -I.
     *
     * and run it:
     *   ex3
     *   > (cload "/home/bil/snd-18/ex3a.so" "init_ex")
     *   #t
     *   > (add-1 2)
     *   3
     *   > (try "a_function" 2.5)
     *   3.5
     */

All of this is just boring boilerplate, so with a little support from s7, we can write a script to do the entire linkage. The s7 side is an extension to "load" that loads a shared object file if its extension is "so", and runs an initialization function whose name is defined in the load environment (the optional second argument to load). An example of the scheme side is cload.scm, included in the s7 tarball. It defines a function that can be called:

    (c-define '(double j0 (double)) "m" "math.h")

This links the s7 function m:j0 to the math library function j0. See cload.scm for more details.

Here's a shorter example:

    add1.c:

    #include <stdlib.h>
    #include "s7.h"

    static s7_pointer add1(s7_scheme *sc, s7_pointer args)
    {
      if (s7_is_integer(s7_car(args)))
        return(s7_make_integer(sc, 1 + s7_integer(s7_car(args))));
      return(s7_wrong_type_arg_error(sc, "add1", 1, s7_car(args), "an integer"));
    }

    void add1_init(s7_scheme *sc);
    void add1_init(s7_scheme *sc)
    {
      s7_define_function(sc, "add1", add1, 1, 0, false, "(add1 int) adds 1 to int");
    }

    /* gcc -fpic -c add1.c
     * gcc -shared -Wl,-soname,libadd1.so -o libadd1.so add1.o -lm -lc
     * gcc s7.c -o repl -fpic -DWITH_MAIN -I. -ldl -lm -Wl,-export-dynamic -DUSE_SND=0
     * repl
     *   (load "libadd1.so" (inlet 'init_func 'add1_init))
     *   (add1 2)
     */

#### Bignums in C

Bignum support depends on gmp, mpfr, and mpc. In this example, we define "add-1" which adds 1 to any kind of number. The s7_big_* functions return the underlying gmp/mpfr/mpc pointer.

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>

    #include <gmp.h>
    #include <mpfr.h>
    #include <mpc.h>

    #include "s7.h"

    static s7_pointer big_add_1(s7_scheme *sc, s7_pointer args)
    {
      /* add 1 to either a normal number or a bignum */
      s7_pointer n;
      s7_pointer x = s7_car(args);
      if (s7_is_big_integer(x))
        {
          mpz_t big_n;
          mpz_init_set(big_n, *s7_big_integer(x));
          mpz_add_ui(big_n, big_n, 1);
          n = s7_make_big_integer(sc, &big_n);
          mpz_clear(big_n);
          return(n);
        }
      if (s7_is_big_ratio(x))
        {
          mpq_t big_q;
          mpq_init(big_q);
          mpq_set_si(big_q, 1, 1);
          mpq_add(big_q, *s7_big_ratio(x), big_q);
          mpq_canonicalize(big_q);
          n = s7_make_big_ratio(sc, &big_q);
          mpq_clear(big_q);
          return(n);
        }
      if (s7_is_big_real(x))
        {
          mpfr_t big_x;
          mpfr_init_set(big_x, *s7_big_real(x), MPFR_RNDN);
          mpfr_add_ui(big_x, big_x, 1, MPFR_RNDN);
          n = s7_make_big_real(sc, &big_x);
          mpfr_clear(big_x);
          return(n);
        }
      if (s7_is_big_complex(x))
        {
          mpc_t big_z;
          mpc_init2(big_z, mpc_get_prec(*s7_big_complex(x)));
          mpc_add_ui(big_z, *s7_big_complex(x), 1, MPC_RNDNN);
          n = s7_make_big_complex(sc, &big_z);
          mpc_clear(big_z);
          return(n);
        }
      if (s7_is_integer(x))
        return(s7_make_integer(sc, 1 + s7_integer(x)));
      if (s7_is_rational(x))
        return(s7_make_ratio(sc, s7_numerator(x) + s7_denominator(x), s7_denominator(x)));
      if (s7_is_real(x))
        return(s7_make_real(sc, 1.0 + s7_real(x)));
      if (s7_is_complex(x))
        return(s7_make_complex(sc, 1.0 + s7_real_part(x), s7_imag_part(x)));
      return(s7_wrong_type_arg_error(sc, "add-1", 0, x, "a number"));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s7 = s7_init();
      s7_define_function(s7, "add-1", big_add_1, 1, 0, false, "(add-1 num) adds 1 to num");

      while (1)
        {
          char buffer[512];
          fprintf(stdout, "\n> ");
          fgets(buffer, 512, stdin);
          if ((buffer[0] != '\n') ||
    	  (strlen(buffer) > 1))
    	{
              char response[1024];
    	  snprintf(response, 1024, "(write %s)", buffer);
    	  s7_eval_c_string(s7, response);
    	}}
    }

    /*
     *   gcc -DWITH_GMP=1 -c s7.c -I. -O2 -g3
     *   gcc -DWITH_GMP=1 -o gmpex gmpex.c s7.o -I. -O2 -lm -ldl -lgmp -lmpfr -lmpc
     *
     *   gmpex
     *   > (add-1 1)
     *   2
     *   > (add-1 2/3)
     *   5/3
     *   > (add-1 1.4)
     *   2.4
     *   > (add-1 1.5+i)
     *   2.5+1i
     *   > (add-1 (bignum 3))
     *   4
     *   > (add-1 (bignum 3/4))
     *   7/4
     *   > (add-1 (bignum 2.5))
     *   3.500E0
     *   > (add-1 (bignum 1.5+i))
     *   2.500E0+1.000E0i
     */

To tie mpfr's bessel-j0 into s7 at run-time:

    /* libgmp_s7.c */

    #include <gmp.h>
    #include <mpfr.h>
    #include <mpc.h>

    #define WITH_GMP 1
    #include "s7.h"

    static s7_pointer gmp_bessel_j0(s7_scheme *sc, s7_pointer args)
    {
      s7_pointer x, result;
      mpfr_t mp;

      mpfr_init2(mp, s7_integer(s7_starlet_ref(sc, s7_make_symbol(sc, "bignum-precision"))));
      /* initialize the mpfr variable mp to the current s7 bignum-precision */

      x = s7_car(args);
      if (s7_is_big_real(x))
        mpfr_j0(mp, *s7_big_real(x), MPFR_RNDN);
      else
        {
          if (s7_is_real(x))
    	{
    	  mpfr_set_d(mp, s7_real(x), MPFR_RNDN);
    	  mpfr_j0(mp, mp, MPFR_RNDN);
    	}
          else return(s7_wrong_type_arg_error(sc, "gmp_bessel_j0", 1, x, "real"));
        }

      result = s7_make_big_real(sc, &mp);
      mpfr_clear(mp);
      return(result);
    }

    void libgmp_s7_init(s7_scheme *sc);
    void libgmp_s7_init(s7_scheme *sc)
    {
      s7_define_function(sc, "bessel-j0", gmp_bessel_j0, 1, 0, false, "(bessel-j0 x) returns j0(x)");
    }

libarb_s7.c provides some extensions of the multiprecision math: Bessel functions and the like. It is based on the Flint and Arb libraries, flintlib.org and arblib.org. In Linux:

    gcc -fPIC -c libarb_s7.c
    gcc libarb_s7.o -shared -o libarb_s7.so -lflint
    repl
      > (load "libarb_s7.so" (inlet 'init_func 'libarb_s7_init))
      #f
      > (acb_bessel_j 0 1.0)
      7.651976865579665514497175261026632209096E-1

As of January 2024, libarb has been absorbed into libflint 3.0.

#### gdb

gdbinit has some debugging commands, intended for your ~/.gdbinit file.

    s7print interprets its argument as an s7 value and displays it
    s7eval evals its argument (a string)
    s7stack displays the current s7 stack (nested lets)
    s7value prints the value of the variable passed by its print name: s7v "*features*"
    s7let shows all non-global variables that are currently accessible
    s7history shows the history entries (if enabled)

gdbinit also has s7bt, a bt replacement that prints the gdb backtrace info, replacing bare pointer numbers with their s7 value, wherever possible:

    (gdb) s7bt
    #0  0x000055555567f7ca in check_cell (p=#<lambda (lst ind)>,
        func=0x5555559106e0 <__FUNCTION__.10273> "mark_slot", line=3976) at s7.c:28494
    #1  0x000055555567f84d in check_nref (p=#<lambda (lst ind)>,
        func=0x5555559106e0 <__FUNCTION__.10273> "mark_slot", line=3976) at s7.c:28507
    #2  0x0000555555563201 in mark_slot (p='list-ref #<lambda (lst ind)>) at s7.c:3976
    #3  0x0000555555564ce0 in mark_let (env=#<mock-number-class>) at s7.c:4506
    #4  0x0000555555563239 in mark_slot (p='mock-number-class #<mock-number-class>) at s7.c:3976
    #5  0x0000555555564ce0 in mark_let (env=(inlet 'mock-number-class #<mock-number-class> 'mock-number mock-number)) at s7.c:4506
    #6  0x0000555555563239 in mark_slot (p='*mock-number* (inlet 'mock-number-class #<mock-number-class> 'mock-number...)) at s7.c:3976
    #7  0x0000555555564ce0 in mark_let (env=(inlet '*features* (mockery.scm stuff.scm linux autoload dlopen...))) at s7.c:4506
    #8  0x0000555555565697 in mark_closure (p=reactive-vector) at s7.c:4590
    #9  0x0000555555566872 in mark_rootlet (sc=0x555555b41eb0) at s7.c:4813
    #10 0x0000555555566a2f in gc (sc=0x555555b41eb0) at s7.c:4897
    #11 0x000055555558e903 in copy_stack (sc=0x555555b41eb0, old_v=[sc->stack] #<stack>) at s7.c:9024

#### WASM

s7 can be compiled to web assembly. There are instructions at s7-playground (Christos Vagias) and s7-wasm (Iain Duncan).

#### FFI notes

- Errors
- GC protection
- Load
- Eval and Apply
- Define
- Function info
- C-objects
- IO
- Lets
- Symbols
- Numbers
- Lists
- Vectors
- C-pointers
- Strings
- Characters
- Hash-tables
- Iterators
- Hooks
- Constants
- Optimization
- And so on...

Errors

Most of the s7.h functions do little, if any, error checking. s7_car, for example, does not check that its argument is a pair. Partly this is a matter of speed; partly of simplicity. If we had elaborate error checks, we'd need some convention for passing error information back to the caller, and of course separate versions of each function for cases where all those checks are redundant. You can easily make your own C version of s7_car that includes error checks:

    static s7_pointer my_car(s7_scheme *sc, s7_pointer lst)
    {
      if (s7_is_pair(lst))
        return(s7_car(lst));
      return(s7_wrong_type_arg_error(sc, "my_car", 0, "a pair"));
    }

The s7.h error functions are:

    s7_pointer s7_error(s7_scheme *sc, s7_pointer type, s7_pointer info);

    s7_pointer s7_wrong_type_arg_error(s7_scheme *sc, const char *caller, s7_int arg_n, s7_pointer arg, const char *descr);
    s7_pointer s7_wrong_type_error(s7_scheme *sc, s7_pointer caller, s7_int arg_n, s7_pointer arg, s7_pointer descr);
    s7_pointer s7_out_of_range_error(s7_scheme *sc, const char *caller, s7_int arg_n, s7_pointer arg, const char *descr);
    s7_pointer s7_wrong_number_of_args_error(s7_scheme *sc, const char *caller, s7_pointer args);

    s7_pointer s7_current_error_port(s7_scheme *sc);
    s7_pointer s7_set_current_error_port(s7_scheme *sc, s7_pointer port);

s7_error is equivalent to the scheme error function, and like the latter, it takes two arguments: a symbol giving the error type, and a list giving the error data. In s7, all of the data lists are organized so that you can `(apply format #f data)` to get an error string. If you're using catch to handle errors, the error type is what catch looks for. So, the s7_wrong_type_arg_error call above could be:

    s7_error(sc, s7_make_symbol(sc, "wrong-type-arg"),
                 s7_list(sc, 3, s7_make_string(sc, "~S is a ~S, but should be a pair"),
                                s7_car(lst),
                                s7_type_of(sc, s7_car(lst))));

s7_wrong_type_arg_error takes the name of the caller, the argument number, the argument itself, and a description of the type expected. If the argument number is 0, that info is left out of the error message (that is, the caller takes only one argument). s7_out_of_range_error is similar. s7_wrong_number_of_args_error takes the caller's name and the offending arg list. The corresponding error types are 'wrong-type-arg, 'wrong-number-of-args, and 'out-of-range. A faster version is s7_wrong_type_error; here the caller is an s7 symbol or string, and the description is an s7 string.

Normally, s7_error sends its error message to the current error-port which defaults to stderr. In GUI-based apps, you may need to redirect the output to your interface. One method, used in Snd's snd-motif.c, captures the error output in an output string:

    old_port = s7_set_current_error_port(s7, s7_open_output_string(s7));
    ...
    result = s7_eval_c_string(s7, text);
    errmsg = s7_get_output_string(s7, s7_current_error_port(s7));
    s7_close_output_port(s7, s7_current_error_port(s7));
    s7_set_current_error_port(s7, old_port);
    ...

and if errmsg is not NULL, it posts it somewhere. (You'll also want to GC-protect the old port while it is idle).

s7_error does not return; its s7_pointer return type is just a convenience. It unwinds the scheme stack, closing files, handling dynamic-winds, looking for a catch that matches its type argument and so on, then longjmps to unwind the C stack. If a catch is found, its error handler becomes the new point of execution.

s7 has an internal debugger that checks everything for consistency. If you're writing C code that calls the s7.h functions a lot, it is sometimes helpful to turn on this debugger by adding the -DS7_DEBUGGING=1 compiler flag. s7 will run 10 to 20 times slower, but it will complain loudly about anything it doesn't expect. The internal debugger normally calls abort whenever it detects some error, so it's most convenient to run s7 in gdb if this flag is on. S7_DEBUGGING also provides various debugging functions that can be called from gdb. By far the most useful is display. It takes one argument, the object you want to display. For much more detail, there is object_to_let_p_p. If you're trying to look at a scheme object and have only its name in scheme code, use s7_name_to_value. heap_scan, heap_analyze, and heap holders provide a view of the heap. The scheme function (*s7* 'memory-usage) might also be useful in this context. s7_show_stack returns a brief list of the current Scheme stack. Due to tail calls and whatnot, the s7 stack is not as informative as the equivalent C stack would be. The s7 file gdbinit has a few gdb functions that might be helpful. See also (*s7* 'history).

If you find a scheme bug in s7, and want to take a stab at tracking it down, build s7 with S7_DEBUGGING=1, and run the buggy code. The internal debugger checks nearly everything the interpreter does, so s7 runs 10 to 20 times slower, but in most cases, it will pinpoint the line where things unravel. It also provides GC info about each cell (where it was last allocated or freed, etc) To get more context, there's also the switch in s7.c, SHOW_EVAL_OPS, which sends out an enormous log of the interpreter's adventures. Even more detailed info can be generated via the OPT_PRINT and DO_PRINT switches, though those are more oriented toward questions like "why wan't this code fully optimized". I always try to save a breaking test case in s7test.scm, and then run tools/auto-tester.scm overnight. The auto-tester is a sort of "fuzz tester" generating billions of random scheme code fragments, then evaluating them in various contexts.

GC protection

If you save an s7_pointer value in C, you may need to protect it from the garbage collector. In the example above, the first "..." is:

    gc_loc = s7_gc_protect(s7, old_port);

where gc_loc is (or should be) an s7_int. Since we're subsequently calling s7_eval_c_string, we need to GC protect old_port beforehand. After the evaluation,

    s7_close_output_port(s7, s7_current_error_port(s7));
    s7_set_current_error_port(s7, old_port);
    s7_gc_unprotect_at(s7, gc_loc);

The full set of GC protection functions is:

    s7_int s7_gc_protect(s7_scheme *sc, s7_pointer x);
    void s7_gc_unprotect_at(s7_scheme *sc, s7_int loc);
    s7_pointer s7_gc_protected_at(s7_scheme *sc, s7_int loc);

    s7_pointer s7_gc_protect_via_stack(s7_scheme *sc, s7_pointer x);
    s7_pointer s7_gc_protect_2_via_stack(s7_scheme *sc, s7_pointer x, s7_pointer y);
    s7_pointer s7_gc_unprotect_via_stack(s7_scheme *sc, s7_pointer x);

    s7_pointer s7_gc_protect_via_location(s7_scheme *sc, s7_pointer x, s7_int loc);
    s7_pointer s7_gc_unprotect_via_location(s7_scheme *sc, s7_int loc);

    s7_pointer s7_gc_on(s7_scheme *sc, bool on);

If you create an s7 object in C, that object needs to be GC protected if there is any chance the GC might run without an existing Scheme-level reference to it. s7_gc_protect places the object in a vector that the GC always checks, returning the object's location in that table. s7_gc_unprotect_at unprotects the object (removes it from the vector) using the location passed to it. s7_gc_protected_at returns the object at the given location. There is a built-in lag between the creation of a new object and its first possible GC (the lag time is set indirectly by GC_TEMPS_SIZE in s7.c), so you don't need to worry about very short term temps such as the arguments to s7_cons in:

    s7_cons(s7, s7_make_real(s7, 3.14),
                s7_cons(s7, s7_make_integer(s7, 123), s7_nil(s7)));

The protect_via_stack functions place the object on the s7 stack where it is protected until the stack unwinds past that point. Besides speed, this provides a way to be sure an object is unprotected even in some complicated situation where error handling may bypass an explicit s7_gc_unprotect_at call. s7_gc_protect_2_via_stack protects two objects in one stack location, saving stack space. The protect_via_location are intended for cases where you have a location already (from s7_gc_protect), and want to reuse it for a different object. s7_gc_on turns the GC on or off. Objects can be created at a blistering pace, so don't leave the GC off for a long time!

Load

    s7_pointer s7_load(s7_scheme *sc, const char *file);
    s7_pointer s7_load_with_environment(s7_scheme *sc, const char *filename, s7_pointer e);
    s7_pointer s7_load_c_string(s7_scheme *sc, const char *content, s7_int bytes);
    s7_pointer s7_load_c_string_with_environment(s7_scheme *sc, const char *content, s7_int bytes, s7_pointer e);
    s7_pointer s7_load_path(s7_scheme *sc);
    s7_pointer s7_add_to_load_path(s7_scheme *sc, const char *dir);
    s7_pointer s7_autoload(s7_scheme *sc, s7_pointer symbol, s7_pointer file_or_function);
    void s7_autoload_set_names(s7_scheme *sc, const char **names, s7_int size); snd-xref.c

s7_load is similar to the scheme-side load function. Its argument is a file name, and optionally (via s7_load_with_environment) an environment in which to place top-level objects. Normally the file contains scheme code, but if WITH_C_LOADER is set when s7 is built, you can also load shared-object files. If you load a shared-object file (a dynamically loadable library), the environment argument provides a way to pass in the initialization function (named 'init_func). For example, the repl in s7.c needs access to libc's tcsetattr, so it looks for libc_s7.so (created by libc.scm). If found,

      s7_load_with_environment(sc, "libc_s7.so",
        s7_inlet(sc, s7_list(sc, 2, s7_make_symbol(sc, "init_func"),
                                    s7_make_symbol(sc, "libc_s7_init")));

You can also include an 'init_args field to pass arguments to init_func. Here's an example that includes init_args:

    /* tlib.c */
    #include <stdio.h>
    #include <stdlib.h>
    #include "s7.h"

    static s7_pointer a_function(s7_scheme *sc, s7_pointer args)
    {
      return(s7_car(args));
    }

    s7_pointer tlib_init(s7_scheme *sc, s7_pointer args); /* void tlib_init(s7_scheme *sc) if no init_args */
    s7_pointer tlib_init(s7_scheme *sc, s7_pointer args)
    {
      fprintf(stderr, "tlib_init: %s\n", s7_object_to_c_string(sc, args));
      s7_define_function(sc, "a-function", a_function, 1, 0, true, "");
      return(s7_car(args));
    }

    /* in Linux:
       gcc -fPIC -c tlib.c
       gcc tlib.o -shared -o tlib.so -ldl -lm -Wl,-export-dynamic

       /home/bil/cl/ repl
       <1> (load "tlib.so" (inlet 'init_func 'tlib_init 'init_args (list 1 2 3)))
       tlib_init: (1 2 3)
       1
       <2> (a-function 1 2 3)
       1
    */

s7_load returns the last value produced during the load; so given "test.scm" with the contents:

    define (f x) (+ x 1))
    32

when we call s7_load:

    s7_pointer val;
    val = s7_load_with_environment(sc, "test.scm", s7_curlet(sc));

val is set to 32 (as a scheme object), and f is placed in the current environment. If "test.scm" is not in the current directory, s7 looks at the entries in its *load-path* variable, trying each in turn until it finds the file. If it fails, it returns NULL. s7_load_path returns this list, and s7_add_to_load_path adds a directory name to the list.

s7_load_c_string takes an array of bytes representing some scheme code (xxd -i file.scm can generate these arrays), and treats it as if it were the contents of a file of scheme code. So, unlike s7_eval_c_string, it can handle multiple statements, and things like double-quote don't need to be quoted. nrepl.c for example embeds the contents of nrepl.scm at compile time, then calls s7_load_c_string at program startup. It also includes notcurses_s7.c. The end result is a stand-alone program that doesn't need to load either nrepl.scm or notcurses_s7.so. The "content" argument should be a null-terminated C string. The "bytes" argument is the contents length, not including the trailing null, as in strlen. There are simple examples in ffitest.c.

> xxd is not ideal in this context because diffs become enormous. I use this code to turn nrepl.scm into nrepl-bits.h, following the original code's layout to minimize diffs:

>     (call-with-output-file "nrepl-bits.h"
>       (lambda (op)
>         (call-with-input-file "nrepl.scm"
>           (lambda (ip)
>     	(format op "unsigned char nrepl_scm[] = {~%  ")
>     	(do ((c (read-char ip) (read-char ip))
>     	     (i 0 (+ i 1)))
>     	    ((eof-object? c)
>     	     (format op "0};~%unsigned int nrepl_scm_len = ~D;~%" (+ i 1)))
>     	  (format op "0x~X, " (char->integer c))
>     	  (if (char=? c #\newline)
>     	      (format op "~%  ")))))))

> Then in nrepl.c:

>           #include "nrepl-bits.h"
>           s7_load_c_string(sc, (const char *)nrepl_scm, nrepl_scm_len);

> which replaces `s7_load(sc, "nrepl.scm")`.

s7_autoload adds a symbol to the autoload table. As a convenience, s7_autoload_set_names adds an array of names+files. The array should be sorted alphabetically by string<? acting on the symbol names (not the file names), and the size argument is the number of symbol names (half the actual array size). snd-xref.c in Snd has more than 5000 such names:

    static const char *snd_names[11848] = {
        "*clm-array-print-length*", "ws.scm", /* each pair of entries is entity name + file name */
        "*clm-channels*", "ws.scm",           /*   so clm-channels is defined in ws.scm */
        ...
        "zone-tailed-hawk", "animals.scm",
        "zoom-spectrum", "examp.scm",
    };

    s7_autoload_set_names(sc, snd_names, 5924);

Eval and Apply

    s7_pointer s7_eval(s7_scheme *sc, s7_pointer code, s7_pointer let);
    s7_pointer s7_eval_with_location(s7_scheme *sc, s7_pointer code, s7_pointer let, const char *caller, const char *file, s7_int line);
    s7_pointer s7_eval_c_string(s7_scheme *sc, const char *str);
    s7_pointer s7_eval_c_string_with_environment(s7_scheme *sc, const char *str, s7_pointer let);

    s7_pointer s7_apply_function(s7_scheme *sc, s7_pointer fnc, s7_pointer args);
    s7_pointer s7_apply_function_star(s7_scheme *sc, s7_pointer fnc, s7_pointer args);

    s7_pointer s7_call(s7_scheme *sc, s7_pointer func, s7_pointer args);
    s7_pointer s7_call_with_location(s7_scheme *sc, s7_pointer func, s7_pointer args, const char *caller, const char *file, s7_int line);
    s7_pointer s7_call_with_catch(s7_scheme *sc, s7_pointer tag, s7_pointer body, s7_pointer error_handler);

    s7_pointer s7_apply_1(s7_scheme *sc, s7_pointer args, s7_pointer (*f1)(s7_pointer a1));
    s7_pointer s7_apply_n_1(s7_scheme *sc, s7_pointer args, s7_pointer (*f1)(s7_pointer a1));
    /* and many more passing 2 to 9 arguments */

These functions evaluate Scheme expressions, and call Scheme functions (which might be defined in C originally). s7_eval evaluates a list that represents Scheme code. That is,

    s7_eval(sc, s7_cons(sc, s7_make_symbol(sc, "+"),
                   s7_cons(sc, s7_make_integer(sc, 1),
                      s7_cons(sc, s7_make_integer(sc, 2), s7_nil(sc)))),
                s7_rootlet(sc));

returns 3 (as a Scheme integer). This may look ridiculous, but see snd-sig.c for an actual use. s7_eval_c_string evaluates a Scheme expression presented to it as a C string; it combines read and eval, whereas s7_eval is just the eval portion.

    s7_eval_c_string(sc, "(+ 1 2)");

also returns 3. The expression is evaluated in rootlet (the global environment). To specify the environment, use s7_eval_c_string_with_environment.

s7_apply_function and s7_apply_function_star take an s7_function and apply it to a list of arguments. These two functions are the low-level versions of the s7_call functions. The latter set up various catches so that error handling is safe, whereas s7_apply_function assumes you have a catch already somewhere.

s7_call_with_location passes some information to the error handler, and s7_call_with_catch wraps an explicit catch around a function call: s7_call_with_catch(sc, tag, body, err) is equivalent to (catch tag body err). There are many examples of these functions in clm2xen.c, ffitest.c, etc.

The s7_apply_1 functions and its many friends are left over from long ago. I hope to deprecate them someday, but currently Snd uses them to excess. Each applies its function to the arguments.

Define

    void s7_define(s7_scheme *sc, s7_pointer env, s7_pointer symbol, s7_pointer value);
    bool s7_is_defined(s7_scheme *sc, const char *name);

    /* these s7_define* functions return a symbol */
    s7_pointer s7_define_variable(s7_scheme *sc, const char *name, s7_pointer value);
    s7_pointer s7_define_variable_with_documentation(s7_scheme *sc, const char *name, s7_pointer value, const char *help);

    s7_pointer s7_define_constant(s7_scheme *sc, const char *name, s7_pointer value);
    s7_pointer s7_define_constant_with_documentation(s7_scheme *sc, const char *name, s7_pointer value, const char *help);
    s7_pointer s7_define_constant_with_environment(s7_scheme *sc, s7_pointer envir, const char *name, s7_pointer value);

    s7_pointer s7_defun(s7_scheme *sc, const char *name, s7_function fnc,
                        s7_int required_args, s7_int optional_args, bool rest_arg,
    		    const char *doc, s7_pointer signature, s7_pointer let, s7_safety_t type);

    /* the rest of these s7_define|make_*_function are simply calling s7_defun */
    s7_pointer s7_define_function(s7_scheme *sc, const char *name, s7_function fnc,
                                  s7_int required_args, s7_int optional_args, bool rest_arg, const char *doc);

    s7_pointer s7_define_safe_function(s7_scheme *sc, const char *name, s7_function fnc,
                                       s7_int required_args, s7_int optional_args, bool rest_arg, const char *doc);

    s7_pointer s7_define_typed_function(s7_scheme *sc, const char *name, s7_function fnc,
    				    s7_int required_args, s7_int optional_args, bool rest_arg,
    				    const char *doc, s7_pointer signature);

    s7_pointer s7_define_unsafe_typed_function(s7_scheme *sc, const char *name, s7_function fnc,
    					   s7_int required_args, s7_int optional_args, bool rest_arg,
    					   const char *doc, s7_pointer signature);

    s7_pointer s7_define_semisafe_typed_function(s7_scheme *sc, const char *name, s7_function fnc,
    					     s7_int required_args, s7_int optional_args, bool rest_arg,
    					     const char *doc, s7_pointer signature);

    void s7_define_function_star(s7_scheme *sc, const char *name, s7_function fnc,
                                 const char *arglist, const char *doc);

    void s7_define_safe_function_star(s7_scheme *sc, const char *name, s7_function fnc,
                                      const char *arglist, const char *doc);

    void s7_define_typed_function_star(s7_scheme *sc, const char *name, s7_function fnc,
                                       const char *arglist, const char *doc, s7_pointer signature);

    s7_pointer s7_define_macro(s7_scheme *sc, const char *name, s7_function fnc,
                               s7_int required_args, s7_int optional_args, bool rest_arg, const char *doc);

    s7_pointer s7_define_expansion(s7_scheme *sc, const char *name, s7_function fnc,
    	                       s7_int required_args, s7_int optional_args, bool rest_arg, const char *doc);

    /* s7_make* functions return a function */
    s7_pointer s7_make_function(s7_scheme *sc, const char *name, s7_function fnc,
                                s7_int required_args, s7_int optional_args, bool rest_arg, const char *doc);

    s7_pointer s7_make_safe_function(s7_scheme *sc, const char *name, s7_function fnc,
                                     s7_int required_args, s7_int optional_args, bool rest_arg, const char *doc);

    s7_pointer s7_make_typed_function(s7_scheme *sc, const char *name, s7_function f,
    				  s7_int required_args, s7_int optional_args, bool rest_arg,
                                      const char *doc, s7_pointer signature);

    s7_pointer s7_make_typed_function_with_environment(s7_scheme *sc, const char *name, s7_function f,
    				  s7_int required_args, s7_int optional_args, bool rest_arg,
                                      const char *doc, s7_pointer signature, s7_pointer envir);

    s7_pointer s7_make_function_star(s7_scheme *sc, const char *name, s7_function fnc,
                                     const char *arglist, const char *doc);

    s7_pointer s7_make_safe_function_star(s7_scheme *sc, const char *name, s7_function fnc,
                                     const char *arglist, const char *doc);

    bool s7_is_dilambda(s7_pointer obj);

    s7_pointer s7_dilambda(s7_scheme *sc,
    		       const char *name,
    		       s7_pointer (*getter)(s7_scheme *sc, s7_pointer args),
    		       s7_int get_req_args, s7_int get_opt_args,
    		       s7_pointer (*setter)(s7_scheme *sc, s7_pointer args),
    		       s7_int set_req_args, s7_int set_opt_args,
    		       const char *documentation);

    s7_pointer s7_typed_dilambda(s7_scheme *sc,
    		       const char *name,
    		       s7_pointer (*getter)(s7_scheme *sc, s7_pointer args),
    		       s7_int get_req_args, s7_int get_opt_args,
    		       s7_pointer (*setter)(s7_scheme *sc, s7_pointer args),
    		       s7_int set_req_args, s7_int set_opt_args,
    		       const char *documentation,
     		       s7_pointer get_sig, s7_pointer set_sig);

    s7_pointer s7_dilambda_with_environment(s7_scheme *sc, s7_pointer envir,
    		       const char *name,
    		       s7_pointer (*getter)(s7_scheme *sc, s7_pointer args),
    		       s7_int get_req_args, s7_int get_opt_args,
    		       s7_pointer (*setter)(s7_scheme *sc, s7_pointer args),
    	               s7_int set_req_args, s7_int set_opt_args,
    		       const char *documentation);

The s7_define* functions add a symbol and its binding to either the top-level (global) environment or, in s7_define, the 'env' passed as the second argument. Use s7_set_shadow_rootlet to import the current let into rootlet.

    s7_define(s7, s7_curlet(s7), s7_make_symbol(s7, "var"), s7_make_integer(s7, 123));

adds the variable named var to the current environment with the value 123. Scheme code can then refer to var just as if we had said `(define var 123)` in Scheme.

s7_define_variable is a wrapper for s7_define; the code above could be:

    s7_define_variable(s7, "var", s7_make_integer(s7, 123)); /* (define var 123) */

except that s7_define_variable assumes you want var in rootlet.

s7_define_constant is another wrapper for s7_define; it makes the variable immutable:

    s7_define_constant(sc, "var", s7_f(sc));  /* (define-constant var 123) */

Most of the s7_define* functions return the name as a symbol; this reflects the way define worked in s7 until 2014. Backwards compatibility...

The rest of the functions in this section deal with tieing C functions into Scheme. s7_make_function creates (and returns) a Scheme function object from the s7_function 'fnc'. An s7_function is a C function of the form `s7_pointer func(s7_scheme *sc, s7_pointer args)`. The new function's name is 'name', it requires 'required_args' arguments, it can accept 'optional_args' other arguments, and if 'rest_arg' is true, it accepts any number of trailing optional arguments (that is, it isn't a true :rest argument for grubby historical reasons). The function's documentation is 'doc'.

Leaving aside the value returned, s7_define_function is the same as s7_make_function, but it also adds 'name' (as a symbol) to the global environment, with the function as its value. For example, the Scheme function 'car' is essentially:

    s7_pointer g_car(s7_scheme *sc, s7_pointer args) {return(s7_car(s7_car(args)));} /* args is a list of args */

It is bound to the name "car":

    s7_define_function(sc, "car", g_car, 1, 0, false, "(car obj)");

which says that car has one required argument, no optional arguments, and no "rest" argument.

The "safe" and "unsafe" versions of these functions refer to the s7 optimizer. If it knows a function is safe, it can more thoroughly optimize the expression it is in. "Safe" here means the function does not call the evaluator itself (via s7_apply_function for example) and does not mess with s7's stack.

The "typed" versions refer to the function's signature. Since "car" is safe, and has a signature, it is defined in s7.c:

    s7_define_typed_function(sc, "car", g_car, 1, 0, false, H_car, Q_car);

Here unless you use s7_define_unsafe_typed_function, the function is assumed to be safe. We've given it the Scheme name "car", which invokes the C function g_car. It takes one required argument, and no optional or rest arguments. Its documentation is H_car, and its signature is Q_car. The latter is `s7_make_signature(sc, 2, sc->T, sc->is_pair_symbol)` which says that car takes a pair argument, and returns any type object.

s7_defun underlies the clutter of functions above. All the arguments are as above except s7_safety_t type. It can be s7_unsafe, s7_semisafe, or s7_safe. If both the name and let arguments are NULL, you get the equivalent of s7_make_*_function; that is name is not defined in the let. In every case, you get the new function back (not a symbol).

s7_define_macro defines a Scheme macro; its arguments are not evaluated (unlike a function), but its returned value (assumed to be some sort of Scheme expression) is evaluated. It returns a symbol. s7_define_expansion is the same, but returns a read-time macro (an "expansion" in s7 jargon).

The function_star functions are similar, but in this case we pass the argument list as a string, as it would appear in Scheme. s7 makes sure the arguments are ordered correctly and have the specified defaults before calling the C function.

    s7_define_function_star(sc, "a-func", a_func, "arg1 (arg2 32)", "an example of C define*");

Now in Scheme, (a-func :arg1 2) calls the C function a_func with the arguments 2 and 32.

Finally, the dilambda function define Scheme dilambda, just as the Scheme dilambda function does. The dax example above gives read/write access to its x field via:

    s7_define_variable(s7, "dax-x", s7_dilambda(s7, "dax-x", dax_x, 1, 0, set_dax_x, 2, 0, "dax x field"));

Function info

    const char *s7_documentation(s7_scheme *sc, s7_pointer p);
    const char *s7_set_documentation(s7_scheme *sc, s7_pointer symbol, const char *new_doc);
    const char *s7_help(s7_scheme *sc, s7_pointer obj);

    s7_pointer s7_arity(s7_scheme *sc, s7_pointer obj);
    bool s7_is_aritable(s7_scheme *sc, s7_pointer obj, s7_int args);

    s7_pointer s7_setter(s7_scheme *sc, s7_pointer obj);
    s7_pointer s7_set_setter(s7_scheme *sc, s7_pointer obj, s7_pointer setter);

    s7_pointer s7_signature(s7_scheme *sc, s7_pointer func);
    s7_pointer s7_make_signature(s7_scheme *sc, s7_int len, ...);
    s7_pointer s7_make_circular_signature(s7_scheme *sc, s7_int cycle_point, s7_int len, ...);

    s7_pointer s7_lambda_body(s7_scheme *sc, s7_pointer p);
    s7_pointer s7_lambda_let(s7_scheme *sc, s7_pointer p);
    s7_pointer s7_lambda_parameters(s7_scheme *sc, s7_pointer p);
    s7_pointer s7_funclet(s7_scheme *sc, s7_pointer p);

    bool s7_is_function(s7_pointer p);
    bool s7_is_procedure(s7_pointer x);
    bool s7_is_macro(s7_scheme *sc, s7_pointer x);

These functions pertain mostly to functions, both those defined in Scheme and those in C. s7_help and s7_documentation return the documentation string associated with their argument. I find "documentation" tedious to type, and Snd uses "help", but other than the name, there isn't much difference between them. s7_set_documentation sets the documentation string, if it can.

s7_arity returns an object's arity, a cons of the number of required arguments, and the total acceptable arguments. s7_is_aritable returns true if the object can accept that number of args.

s7_setter is the object's setter, and s7_set_setter sets it, if possible.

s7_signature is the object's signature, a list of types (symbols like 'integer?) giving return and argument types. For a function defined in C, s7_make_signature and s7_make_circular_signature create the signature that is then associated with the function via s7_define_typed_function and its friends. In s7.c g_is_zero (the function that implements zero?) uses:

    s7_make_signature(sc, 2, sc->is_boolean_symbol, sc->is_number_symbol); /* return a boolean, argument is a number */

Similarly, g_add is:

    s7_make_circular_signature(sc, 0, 1, sc->is_number_symbol); /* returns a number, takes any number of numbers */

The two numeric arguments set the cycle start point (0-based) and the number of type symbols passed as arguments to it. So, char=? is:

    s7_make_circular_signature(sc, 1, 2, sc->is_boolean_symbol, sc->is_char_symbol);

which says there are two type entries (the "2"), and the cycle starts at the second (the "1" -- it's 0-based).

The s7_lambda functions only apply to functions defined in Scheme. They return the closure body (s7_lambda_body, a list), its definition environment (s7_lambda_let), and its argument list (s7_lambda_parameters). If the function is of the form `(define (f . args) ...)`, s7_lambda_parameters returns the symbol ('args in this case). s7_funclet returns the top let within the function (the let containing the argument names).

C-objects

    bool s7_is_c_object(s7_pointer p);
    s7_pointer s7_make_c_object(s7_scheme *sc, s7_int type, void *value);
    s7_pointer s7_make_c_object_without_gc(s7_scheme *sc, s7_int type, void *value);
    s7_pointer s7_make_c_object_with_let(s7_scheme *sc, s7_int type, void *value, s7_pointer let);

    s7_int s7_c_object_type(s7_pointer obj);
    void *s7_c_object_value(s7_pointer obj);
    void *s7_c_object_set_value(s7_pointer obj, void *value);
    void *s7_c_object_value_checked(s7_pointer obj, s7_int type);
    s7_pointer s7_c_object_let(s7_pointer obj);
    s7_pointer s7_c_object_set_let(s7_scheme *sc, s7_pointer obj, s7_pointer e);

    s7_int s7_make_c_type(s7_scheme *sc, const char *name);
    void s7_c_type_set_gc_free      (s7_scheme *sc, s7_int type, s7_pointer (*gc_free)   (s7_scheme *sc, s7_pointer obj));
    void s7_c_type_set_gc_mark      (s7_scheme *sc, s7_int type, s7_pointer (*mark)      (s7_scheme *sc, s7_pointer obj));
    void s7_c_type_set_is_equal     (s7_scheme *sc, s7_int type, s7_pointer (*is_equal)  (s7_scheme *sc, s7_pointer args));
    void s7_c_type_set_is_equivalent(s7_scheme *sc, s7_int type, s7_pointer (*is_equivalent)(s7_scheme *sc, s7_pointer args));
    void s7_c_type_set_ref          (s7_scheme *sc, s7_int type, s7_pointer (*ref)       (s7_scheme *sc, s7_pointer args));
    void s7_c_type_set_set          (s7_scheme *sc, s7_int type, s7_pointer (*set)       (s7_scheme *sc, s7_pointer args));
    void s7_c_type_set_length       (s7_scheme *sc, s7_int type, s7_pointer (*length)    (s7_scheme *sc, s7_pointer args));
    void s7_c_type_set_copy         (s7_scheme *sc, s7_int type, s7_pointer (*copy)      (s7_scheme *sc, s7_pointer args));
    void s7_c_type_set_fill         (s7_scheme *sc, s7_int type, s7_pointer (*fill)      (s7_scheme *sc, s7_pointer args));
    void s7_c_type_set_reverse      (s7_scheme *sc, s7_int type, s7_pointer (*reverse)   (s7_scheme *sc, s7_pointer args));
    void s7_c_type_set_to_list      (s7_scheme *sc, s7_int type, s7_pointer (*to_list)   (s7_scheme *sc, s7_pointer args));
    void s7_c_type_set_to_string    (s7_scheme *sc, s7_int type, s7_pointer (*to_string) (s7_scheme *sc, s7_pointer args));
    void s7_c_type_set_getter       (s7_scheme *sc, s7_int type, s7_pointer getter);
    void s7_c_type_set_setter       (s7_scheme *sc, s7_int type, s7_pointer setter);

    void s7_mark(s7_pointer p);

These functions create a new Scheme object type. See dax above for a simple example, s7test.scm and ffitest.c for several progressively more complicated examples. C-objects in Scheme usually correspond to an instance of a struct in C which you want to access from Scheme. The normal sequence is: define a new c-type via s7_make_c_type, call s7_c_type_set* to specialize its behavior, then to wrap a C object, call s7_make_c_object. s7_make_c_type takes an arbitrary name, used in object->string to identify the object, and returns an s7_int, the "type" mentioned in many of the other functions.

s7_c_type_set_gc_free sets the function that is called by the GC when a Scheme c-object is garbage-collected. You normally use this to free the associated C value (the instance of the struct). To get that value, call s7_c_object_value. It returns the void* pointer that you originally passed to s7_make_c_object. See free_dax in the dax example.

s7_c_type_set_gc_mark sets the function that is called by the GC during its marking phase. Any s7_pointer value local to your C struct should be marked explicitly at this time, or the GC will free it. Use s7_mark for this (see mark_dax). Don't allocate any s7 objects in the mark function (otherwise you'll get a recursive call of the GC).

s7_c_type_set_is_equal and s7_c_type_set_is_equivalent set the function called when s7 sees a c-object of the current type as an argument to equal? or equivalent?. When called, these functions can assume that the first argument is a c-object of the current type, but the second argument can be anything (see dax_is_equal).

s7_c_type_set_ref and s7_c_type_set_set are called when the c-object is treated as an applicable object in Scheme. That is, `(object ...)` in Scheme calls the function set as the "ref" function, and `(set! (object ...) new-value)` calls the "set" function. The arguments in the set! form are passed as a flattened list.

The rest of the s7_c_type_set* functions set the functions called when the c-object is an argument to length (s7_c_type_set_length), copy (s7_c_type_set_copy), fill! (s7_c_type_set_fill), reverse (s7_c_type_set_reverse), object->string (s7_c_type_set_to_string), and internally by map and a few other cases, s7_c_type_set_to_list. For the copy function, either the first or second argument can be a c-object of the given type. The getter and setter functions are optimizer helpers.

s7_c_object_value_checked is like s7_c_object, but it first checks that the object type matches the given type.

s7_c_object_let and s7_c_object_set_let manage the c-object's local environment. These two functions need to check that they are passed the correct number of arguments. See the block object in s7test.scm. The c_object_let provides methods normally. In Snd, marks can be passed into Scheme; the setup code is:

      static s7_pointer g_mark_methods;
      ...
      g_mark_methods = s7_openlet(s7,
                         s7_inlet(s7, s7_list(s7, 2, s7_make_symbol(s7, "object->let"),
                                                     mark_to_let_func)));
      s7_gc_protect(s7, g_mark_methods);
      xen_mark_tag = s7_make_c_type(s7, "<mark>");
      s7_c_type_set_gc_free(s7, xen_mark_tag, s7_xen_mark_free);
      s7_c_type_set_is_equal(s7, xen_mark_tag, s7_xen_mark_is_equal);
      s7_c_type_set_copy(s7, xen_mark_tag, s7_xen_mark_copy);
      s7_c_type_set_to_string(s7, xen_mark_tag, g_xen_mark_to_string);

The mark object's let (g_mark_methods) has a method for object->let. It is tied into each mark object:

    s7_pointer m;
    m = s7_make_c_object(s7, xen_mark_tag, mx);  /* mx is the C-side value */
    s7_c_object_set_let(s7, m, g_mark_methods);

and now if you type (object->let mark) in Snd's listener (where "mark" is an appropriate mark of course), object->let calls the object's object->let method. Don't forget to GC-protect the let!

s7_make_c_object_without_gc makes a c-object of the given type, but the gc_free function won't be called when the s7_cell that holds the C data is freed for reuse.

IO

    bool s7_is_input_port(s7_scheme *sc, s7_pointer p);
    bool s7_is_output_port(s7_scheme *sc, s7_pointer p);
    void s7_close_input_port(s7_scheme *sc, s7_pointer p);
    void s7_close_output_port(s7_scheme *sc, s7_pointer p);
    bool s7_flush_output_port(s7_scheme *sc, s7_pointer p); /* false=flush lost data */
    const char *s7_port_filename(s7_scheme *sc, s7_pointer x);
    s7_int s7_port_line_number(s7_scheme *sc, s7_pointer p);

    s7_pointer s7_open_input_file(s7_scheme *sc, const char *name, const char *mode);
    s7_pointer s7_open_output_file(s7_scheme *sc, const char *name, const char *mode);

    s7_pointer s7_open_input_string(s7_scheme *sc, const char *input_string);
    s7_pointer s7_open_output_string(s7_scheme *sc);
    const char *s7_get_output_string(s7_scheme *sc, s7_pointer out_port);
    s7_pointer s7_output_string(s7_scheme *sc, s7_pointer out_port);

    typedef enum {S7_READ, S7_READ_CHAR, S7_READ_LINE, S7_PEEK_CHAR, S7_IS_CHAR_READY, S7_NUM_READ_CHOICES} s7_read_t;
    s7_pointer s7_open_output_function(s7_scheme *sc, void (*function)(s7_scheme *sc, uint8_t c, s7_pointer port));
    s7_pointer s7_open_input_function(s7_scheme *sc, s7_pointer (*function)(s7_scheme *sc, s7_read_t read_choice, s7_pointer port));

    s7_pointer s7_read_char(s7_scheme *sc, s7_pointer port);
    s7_pointer s7_peek_char(s7_scheme *sc, s7_pointer port);
    s7_pointer s7_write_char(s7_scheme *sc, s7_pointer c, s7_pointer port);
    s7_pointer s7_write(s7_scheme *sc, s7_pointer obj, s7_pointer port);
    s7_pointer s7_display(s7_scheme *sc, s7_pointer obj, s7_pointer port);
    void s7_newline(s7_scheme *sc, s7_pointer port);
    const char *s7_format(s7_scheme *sc, s7_pointer args);
    s7_pointer s7_object_to_string(s7_scheme *sc, s7_pointer arg, bool use_write);
    char *s7_object_to_c_string(s7_scheme *sc, s7_pointer obj);

    s7_pointer s7_current_input_port(s7_scheme *sc);
    s7_pointer s7_set_current_input_port(s7_scheme *sc, s7_pointer p);
    s7_pointer s7_current_output_port(s7_scheme *sc);
    s7_pointer s7_set_current_output_port(s7_scheme *sc, s7_pointer p);
    s7_pointer s7_current_error_port(s7_scheme *sc);
    s7_pointer s7_set_current_error_port(s7_scheme *sc, s7_pointer port);

    s7_pointer s7_read(s7_scheme *sc, s7_pointer port);

Most of these correspond closely to the similarly named scheme function. s7_port_filename returns the file associated with a file port. s7_port_line_number returns position of the reader in an input file port. The "use_write" parameter to s7_object_to_string refers to the write/display choice in scheme. It can be #t == write, #f == display, or :readable. The string returned by s7_object_to_c_string should be freed by the caller. s7_output_string is the same as s7_get_output_string except that it returns an s7 string, not a C string.

s7_open_input_function and s7_open_output_function call their "function" argument when input or output is requested. The "read_choice" argument specifies to that function which of the input scheme functions called it. The intent of these two input functions is to give you complete control over IO. In the case of an input_function:

    static s7_pointer my_read(s7_scheme *sc, s7_read_t peek, s7_pointer port)
    {
      /* this function should handle input according to the peek choice */
      return(s7_make_character(sc, '0'));
    }

    s7_pointer port;
    s7_int gc_loc;
    uint8_t c;
    port = s7_open_input_function(sc, my_read);
    gc_loc = s7_gc_protect(sc, port);
    c = s7_character(s7_read_char(sc, p1)); /* my_read "peek" == S7_READ_CHAR */
    if (last_c != '0')
       fprintf(stderr, "c: %c\n", c);
    s7_gc_unprotect_at(sc, gc_loc);

Lets

    s7_pointer s7_rootlet(s7_scheme *sc);
    s7_pointer s7_shadow_rootlet(s7_scheme *sc);
    s7_pointer s7_set_shadow_rootlet(s7_scheme *sc, s7_pointer let);

    s7_pointer s7_curlet(s7_scheme *sc);
    s7_pointer s7_set_curlet(s7_scheme *sc, s7_pointer e);

    s7_pointer s7_outlet(s7_scheme *sc, s7_pointer e);
    s7_pointer s7_sublet(s7_scheme *sc, s7_pointer let, s7_pointer bindings);
    s7_pointer s7_inlet(s7_scheme *sc, s7_pointer bindings);
    s7_pointer s7_varlet(s7_scheme *sc, s7_pointer let, s7_pointer symbol, s7_pointer value);

    s7_pointer s7_let_to_list(s7_scheme *sc, s7_pointer let);
    bool s7_is_let(s7_pointer e);
    s7_pointer s7_let_ref(s7_scheme *sc, s7_pointer let, s7_pointer symbol);
    s7_pointer s7_let_set(s7_scheme *sc, s7_pointer let, s7_pointer symbol, s7_pointer val);

    s7_pointer s7_starlet_ref(s7_scheme *sc, s7_pointer sym);
    s7_pointer s7_starlet_set(s7_scheme *sc, s7_pointer sym, s7_pointer new_value);
    /* the old confusing names for the same functions: */
      s7_pointer s7_let_field_ref(s7_scheme *sc, s7_pointer symbol);
      s7_pointer s7_let_field_set(s7_scheme *sc, s7_pointer symbol, s7_pointer new_value);

    s7_pointer s7_openlet(s7_scheme *sc, s7_pointer e);
    bool s7_is_openlet(s7_pointer e);
    s7_pointer s7_method(s7_scheme *sc, s7_pointer object, s7_pointer method);

    /* these might go away someday */
    s7_pointer s7_slot(s7_scheme *sc, s7_pointer symbol);
    s7_pointer s7_slot_value(s7_pointer slot);
    s7_pointer s7_slot_set_value(s7_scheme *sc, s7_pointer slot, s7_pointer value);
    s7_pointer s7_make_slot(s7_scheme *sc, s7_pointer env, s7_pointer symbol, s7_pointer value);
    void s7_slot_set_real_value(s7_scheme *sc, s7_pointer slot, s7_double value);

Many of these are the same as the corresponding scheme function: s7_rootlet, s7_curlet, s7_outlet, s7_sublet, s7_inlet, s7_varlet, s7_let_to_list, s7_is_let, s7_let_ref, s7_let_set, s7_openlet, and s7_is_openlet.

s7_starlet_ref and s7_starlet_set refer to *s7*, the let that holds various s7 settings. To get the current default print-length,

    s7_integer(s7_starlet_ref(s7, s7_make_symbol(s7, "print-length")))

s7_method looks for a field in "object" with the name "method", a symbol. For example, in clm2xen.c, if mus-copy is called on an object that Snd does not immediately recognize (i.e. a generator), it looks for a mus-copy method, and if found, Snd calls it:

    s7_pointer func;
    func = s7_method(s7, gen, s7_make_symbol(s7, "mus-copy"));
    if (func != s7_undefined(s7))
      return(s7_apply_function(s7, func, s7_list(s7, 1, gen)));

The object searched can be anything that has an associated let: a c-object, a function or macro, a c-pointer, or of course a let.

s7_set_curlet and the slot functions might go away someday. They are currently used in Snd. For the adventurous however, here's a sketchy description. A slot in s7 is a location in a let (a variable binding in an environment to use more standard terminology). s7_make_slot creates a slot in "env" with the given symbol and value. s7_slot_value returns the value; s7_slot_set_value sets the value; s7_slot_set_real_value sets the mutable real value's numerical value. s7_slot takes a symbol and tries to find its currently active slot. s7_set_curlet sets curlet, returning the previous curlet.

s7_shadow_rootlet and s7_set_shadow_rootlet make it easier to import a let into rootlet. This is also aimed at code that is defining lots of functions and variables, using the default functions like s7_define_variable that place things in the rootlet, but the code actually wants all those objects stored in a let other than rootlet.

    s7_pointer cur_env, old_shadow;
    cur_env = s7_curlet(sc);
    old_shadow = s7_set_shadow_rootlet(sc, cur_env);
    /* define everything here */
    s7_set_shadow_rootlet(sc, old_shadow);

s7_set_shadow_rootlet returns the previous shadow rootlet, so this turns the current environment into a shadow rootlet while defining functions, then restores the old rootlet. Similarly notcurses_s7.c places everything in the *notcurses* let, but uses s7_set_shadow_rootlet to make these available in scheme as if they were in the rootlet:

      s7_pointer notcurses_let, old_shadow;
      s7_define_constant(sc, "*notcurses*", notcurses_let = s7_inlet(sc, s7_nil(sc)));
      old_shadow = s7_set_shadow_rootlet(sc, notcurses_let);
      /* ... here we have all the s7_defines ... */
      s7_set_shadow_rootlet(sc, old_shadow);

Symbols

    bool s7_is_symbol(s7_pointer p);
    const char *s7_symbol_name(s7_pointer p);
    s7_pointer s7_make_symbol(s7_scheme *sc, const char *name);
    s7_pointer s7_gensym(s7_scheme *sc, const char *prefix);

    bool s7_is_keyword(s7_pointer obj);
    s7_pointer s7_make_keyword(s7_scheme *sc, const char *key);
    s7_pointer s7_keyword_to_symbol(s7_scheme *sc, s7_pointer key);

    s7_pointer s7_name_to_value(s7_scheme *sc, const char *name);
    s7_pointer s7_symbol_value(s7_scheme *sc, s7_pointer sym);
    s7_pointer s7_symbol_set_value(s7_scheme *sc, s7_pointer sym, s7_pointer val);
    s7_pointer s7_symbol_local_value(s7_scheme *sc, s7_pointer sym, s7_pointer local_env);
    s7_pointer s7_symbol_initial_value(s7_pointer symbol); /* #_symbol's value */
    s7_pointer s7_symbol_set_initial_value(s7_scheme *sc, s7_pointer symbol, s7_pointer value);

    s7_pointer s7_symbol_table_find_name(s7_scheme *sc, const char *name);
    bool s7_for_each_symbol_name(s7_scheme *sc, bool (*symbol_func)(const char *symbol_name, void *data), void *data);
    bool s7_for_each_symbol(s7_scheme *sc,      bool (*symbol_func)(const char *symbol_name, void *data), void *data);

s7_is_symbol corresponds to scheme's symbol?, s7_symbol_name to symbol->string, s7_make_symbol is string->symbol, s7_gensym to gensym. The gensym prefix is the optional argument to gensym in scheme. By default the prefix is "gensym", so the gensym-created symbols are of the form {gensym}-nnn where nnn is some number. s7_is_keyword is keyword?, s7_make_keyword is string->keyword, and s7_keyword_to_symbol is keyword->symbol.

Normal symbols, and keywords do not need to be garbage-protected, but gensyms do.

s7_symbol_to_value finds the current binding of the symbol (using its string name), and returns its value, similar to symbol->value. To specify the environment in which to lookup the symbol, use s7_symbol_local_value. s7_symbol_set_value sets the value of the symbol in its current binding.

s7_symbol_table_find_name finds the symbol given its name. s7_make_symbol is the same if the symbol already exists, but s7_symbol_find_by_name returns NULL if there isn't any symbol by that name. s7_for_each_symbol_name and s7_for_each_symbol traverse the symbol table, calling "symbol_func" on each symbol. symbol_func is a boolean function that takes as arguments the symbol name and the void* data pointer. The latter can carry along whatever state your function needs. s7_for_each_symbol_name also includes some s7 constants like #f.

The C declaration above says s7_for_each_symbol is a C function that returns a boolean, and takes three arguments, an s7_scheme* pointer, a function (symbol_func), and a void* pointer (data). The function passed (symbol_func) also returns a boolean, and takes two arguments, a char* (name), and the same void* pointer that was passed to s7_symbol_for_each. If symbol_func returns true, the outer function immediately returns true, ending the symbol table traversal. Sketched in scheme, it might be:

    (define (s7_for_each_symbol s7 symbol_func data)
      (call-with-exit
        (lambda (return)
          (for-each
            (lambda (symbol-name)
              (if (symbol_func symbol-name data)
                  (return #t)))
            (symbol-table))
          #f)))

An example is snd-completion.c.

Numbers

    bool s7_is_number(s7_pointer p);
    char *s7_number_to_string(s7_scheme *sc, s7_pointer obj, s7_int radix);

    bool s7_is_integer(s7_pointer p);
    s7_int s7_integer(s7_pointer p);
    s7_pointer s7_make_integer(s7_scheme *sc, s7_int num);
    s7_int s7_number_to_integer(s7_scheme *sc, s7_pointer x);
    s7_int s7_number_to_integer_with_caller(s7_scheme *sc, s7_pointer x, const char *caller);

    bool s7_is_real(s7_pointer p);
    s7_double s7_real(s7_pointer p);
    s7_pointer s7_make_real(s7_scheme *sc, s7_double num);
    s7_pointer s7_make_mutable_real(s7_scheme *sc, s7_double n);
    s7_double s7_number_to_real(s7_scheme *sc, s7_pointer x);
    s7_double s7_number_to_real_with_caller(s7_scheme *sc, s7_pointer x, const char *caller);
    s7_double s7_number_to_real_with_location(s7_scheme *sc, s7_pointer x, s7_pointer caller);

    bool s7_is_rational(s7_pointer arg);
    bool s7_is_ratio(s7_pointer arg);
    s7_pointer s7_make_ratio(s7_scheme *sc, s7_int a, s7_int b);
    s7_pointer s7_rationalize(s7_scheme *sc, s7_double x, s7_double error);
    s7_int s7_numerator(s7_pointer x);
    s7_int s7_denominator(s7_pointer x);

    bool s7_is_complex(s7_pointer arg);
    s7_pointer s7_make_complex(s7_scheme *sc, s7_double a, s7_double b);
    s7_double s7_real_part(s7_pointer z);
    s7_double s7_imag_part(s7_pointer z);

    s7_double s7_random(s7_scheme *sc, s7_pointer state);
    s7_pointer s7_random_state(s7_scheme *sc, s7_pointer seed);
    bool s7_is_random_state(s7_pointer p);
    s7_pointer s7_random_state_to_list(s7_scheme *sc, s7_pointer args);
    void s7_set_default_random_state(s7_scheme *sc, s7_int seed, s7_int carry);

    bool s7_is_bignum(s7_pointer obj);
    mpfr_t *s7_big_real(s7_pointer x);
    mpz_t *s7_big_integer(s7_pointer x);
    mpq_t *s7_big_ratio(s7_pointer x);
    mpc_t *s7_big_complex(s7_pointer x);
    s7_pointer s7_make_big_integer(s7_scheme *sc, mpz_t *val);
    s7_pointer s7_make_big_ratio(s7_scheme *sc, mpq_t *val);
    s7_pointer s7_make_big_real(s7_scheme *sc, mpfr_t *val);
    s7_pointer s7_make_big_complex(s7_scheme *sc, mpc_t *val);

Most of these correspond to the obvious scheme functions, so I'll only touch on the less-obvious cases. s7_make_mutable_real returns a real number object whose value can be changed directly. In snd-sig.c, for example, we have a C procedure that applies a scheme function to every sound sample in an audio file. We do not want to create a new object for the scheme function's argument list on every call! So, we start by creating the mutable real:

    yp = s7_make_slot(s7, let, arg, s7_make_mutable_real(s7, 1.5));

"let" is the let for the evaluation, "arg" is the real's name as a symbol in that let, and we make its initial value 1.5 (for no particular reason). Then on every sample, we call the function:

    s7_slot_set_real_value(s7, yp, data[kp]); /* set yp's value to data[kp] */
    data[kp] = opt_func(s7, res);             /* call opt_func */

s7_number_to_real returns any real number as an s7_double. If it can't convert its argument, it signals an error, which is annoying because it doesn't know where that error occured in scheme. So s7_number_to_real_with_caller gives you a way to tell it at least the caller's name. s7_number_to_real_with_location is the same function, but the caller argument is a s7 symbol or string (this is much faster than using a char*).

For the bignum functions, see Bignums in C.

Lists

    bool s7_is_pair(s7_pointer p);
    s7_pointer s7_cons(s7_scheme *sc, s7_pointer a, s7_pointer b);

    s7_pointer s7_car(s7_pointer p);
    s7_pointer s7_cdr(s7_pointer p);
    s7_pointer s7_set_car(s7_pointer p, s7_pointer q);
    s7_pointer s7_set_cdr(s7_pointer p, s7_pointer q);
    s7_pointer s7_cadr(s7_pointer p);
    etc...

    bool s7_is_list(s7_scheme *sc, s7_pointer p);
    bool s7_is_proper_list(s7_scheme *sc, s7_pointer p);
    s7_pointer s7_make_list(s7_scheme *sc, s7_int length, s7_pointer initial_value);
    s7_int s7_list_length(s7_scheme *sc, s7_pointer a);
    s7_pointer s7_list(s7_scheme *sc, s7_int num_values, ...);
    s7_pointer s7_list_nl(s7_scheme *sc, s7_int num_values, ...);
    s7_pointer s7_array_to_list(s7_scheme *sc, s7_int num_values, s7_pointer *array);
    void s7_list_to_array(s7_scheme *sc, s7_pointer list, s7_pointer *array, int32_t len);
    s7_pointer s7_list_ref(s7_scheme *sc, s7_pointer lst, s7_int num);
    s7_pointer s7_list_set(s7_scheme *sc, s7_pointer lst, s7_int num, s7_pointer val);

    s7_pointer s7_reverse(s7_scheme *sc, s7_pointer a);
    s7_pointer s7_append(s7_scheme *sc, s7_pointer a, s7_pointer b);
    s7_pointer s7_assoc(s7_scheme *sc, s7_pointer obj, s7_pointer lst);
    s7_pointer s7_assq(s7_scheme *sc, s7_pointer obj, s7_pointer x);
    s7_pointer s7_member(s7_scheme *sc, s7_pointer obj, s7_pointer lst);
    s7_pointer s7_memq(s7_scheme *sc, s7_pointer obj, s7_pointer x);
    bool s7_tree_memq(s7_scheme *sc, s7_pointer sym, s7_pointer tree);

These functions are mostly obvious: s7_car corresponds to scheme car, etc. s7_list_nl is a version of s7_list. s7_list will accept a list of arguments that does not fill the list (that is, "num_values" does not match the actual number of values), leaving null pointers that will later wreak havoc. The list of arguments to s7_list_nl on the other hand must be null-terminated, and must match "num_values", or you'll get an error. s7_tree_memq is like s7_memq, but searches an entire tree structure. not just the top-level list. s7_array_to_list takes an array of s7_pointers and returns a list of them (similar to s7_vector_to_list).

Vectors

    s7_pointer s7_make_vector(s7_scheme *sc, s7_int len);
    s7_pointer s7_make_and_fill_vector(s7_scheme *sc, s7_int len, s7_pointer fill);
    s7_pointer s7_make_normal_vector(s7_scheme *sc, s7_int len, s7_int dims, s7_int *dim_info);
    bool s7_is_vector(s7_pointer p);

    s7_int s7_vector_length(s7_pointer vec);
    s7_int s7_vector_rank(s7_pointer vect);
    s7_int s7_vector_dimension(s7_pointer vec, s7_int dim);
    s7_pointer *s7_vector_elements(s7_pointer vec);
    s7_int s7_vector_dimensions(s7_pointer vec, s7_int *dims, s7_int dims_size);
    s7_int s7_vector_offsets(s7_pointer vec, s7_int *offs, s7_int offs_size);

    void s7_vector_fill(s7_scheme *sc, s7_pointer vec, s7_pointer obj);
    s7_pointer s7_vector_copy(s7_scheme *sc, s7_pointer old_vect);
    s7_pointer s7_vector_to_list(s7_scheme *sc, s7_pointer vect);

    s7_pointer s7_make_byte_vector(s7_scheme *sc, s7_int len, s7_int dims, s7_int *dim_info);
    uint8_t *s7_byte_vector_elements(s7_pointer vec);
    bool s7_is_byte_vector(s7_pointer p);
    uint8_t s7_byte_vector_ref(s7_pointer vec, s7_int index);
    uint8_t s7_byte_vector_set(s7_pointer vec, s7_int index, uint8_t value);

    s7_pointer s7_make_int_vector(s7_scheme *sc, s7_int len, s7_int dims, s7_int *dim_info);
    s7_pointer s7_make_int_vector_wrapper(s7_scheme *sc, s7_int len, s7_int *data, s7_int dims, s7_int *dim_info, bool free_data);
    s7_int *s7_int_vector_elements(s7_pointer vec);
    bool s7_is_int_vector(s7_pointer p);
    s7_int s7_int_vector_ref(s7_pointer vec, s7_int index);
    s7_int s7_int_vector_set(s7_pointer vec, s7_int index, s7_int value);

    s7_pointer s7_make_float_vector(s7_scheme *sc, s7_int len, s7_int dims, s7_int *dim_info);
    s7_pointer s7_make_float_vector_wrapper(s7_scheme *sc, s7_int len, s7_double *data, s7_int dims, s7_int *dim_info, bool free_data);
    s7_double *s7_float_vector_elements(s7_pointer vec);
    bool s7_is_float_vector(s7_pointer p);
    s7_double s7_float_vector_ref(s7_pointer vec, s7_int index);
    s7_double s7_float_vector_set(s7_pointer vec, s7_int index, s7_double value);

    s7_pointer s7_make_complex_vector(s7_scheme *sc, s7_int len, s7_int dims, s7_int *dim_info);
    s7_pointer s7_make_complex_vector_wrapper(s7_scheme *sc, s7_int len, s7_complex *data, s7_int dims, s7_int *dim_info, bool free_data);
    bool s7_is_complex_vector(s7_pointer p);
    s7_complex *s7_complex_vector_elements(s7_pointer vec);
    s7_complex s7_complex_vector_ref(s7_pointer vec, s7_int index);
    s7_complex s7_complex_vector_set(s7_pointer vec, s7_int index, s7_complex value);

    s7_pointer s7_vector_ref(s7_scheme *sc, s7_pointer vec, s7_int index);
    s7_pointer s7_vector_set(s7_scheme *sc, s7_pointer vec, s7_int index, s7_pointer a);
    s7_pointer s7_vector_ref_n(s7_scheme *sc, s7_pointer vector, s7_int indices, ...);
    s7_pointer s7_vector_set_n(s7_scheme *sc, s7_pointer vector, s7_pointer value, s7_int indices, ...);

s7_make_vector returns a one-dimensional vector of the given length; its elements are initialized to the empty list, (). s7_make_and_fill_vector is similar, but the initial element is set by the "fill" parameter. This value is simply placed in every vector location, not copied, so if you pass a cons, then change its car, that change is reflected in every element of the vector. s7_make_normal_vector returns a possibly multidimensional inhomogenous vector (a "normal" vector, as opposed to an int-vector or a float-vector).

s7_is_vector is the same as vector?, s7_vector_length is length. s7_vector_rank returns the number of dimensions in a vector, and s7_vector_dimension returns the size of the given dimension. s7_vector_elements returns the s7_pointer array that holds that vector's elements. s7_vector_dimensions fills "dims" with the lengths of the corresponding dimensions. s7_vector_offsets does the same for the successive dimensional offsets. In a multidimensional vector, you can get the s7_vector_elements index by summing each index * offset[dimension]. s7_vector_to_list is vector->list. s7_vector_fill is fill! (as applied to a vector of course), and s7_vector_copy is copy.

s7_make_int_vector returns an int-vector. Its elements are s7_ints (int64_t), and the array of s7_ints can be accessed via s7_int_vector_elements. Similarly for float-vectors (the elements are s7_doubles which are C doubles), and complex-vectors (s7_complex which uses C doubles). s7_make_int_vector, s7_make_float_vector, and s7_make_complex_vector can return multidimensional vectors. The "dims" parameter specifies the number of dimensions, and the "dim_info" parameter the individual dimensions. If dims is 1, dim_info can be NULL. s7_make_float_vector_wrapper provides a way to pass a C array of doubles through scheme; it wraps up the array as a scheme float-vector. Similar are s7_make_complex_vector_wrapper and s7_make_int_vector_wrapper. If the s7_make_*_vector_wrapper "free_data" parameter is true, s7 will free the "data" array when the float-vector is garbage-collected. In ffitest.c, the g_block example calls:

    v1 = s7_make_float_vector_wrapper(sc, len, g1->data, 1, NULL, false);

when checking if two blocks are equivalent. Since this data is actually being shared with a block object, we don't want s7 to free it when the g_blocks_are_equivalent function is done. g1->data is freed by g_block_free when the c-object is garbage collected.

s7_vector_ref and s7_vector_set apply to one-dimensional vectors; the "_n" cases apply to multidimensional cases. All four functions can be used on any type of vector.

s7_make_complex_vector_wrapper creates a complex-vector that accesses data allocated elsewhere, for example a gsl_vector_complex (see s7test.scm).

C-pointers

    bool s7_is_c_pointer(s7_pointer arg);
    bool s7_is_c_pointer_of_type(s7_pointer arg, s7_pointer type);
    void *s7_c_pointer(s7_pointer p);
    void *s7_c_pointer_with_type(s7_scheme *sc, s7_pointer p, s7_pointer expected_type, const char *caller, s7_int argnum);
    s7_pointer s7_make_c_pointer(s7_scheme *sc, void *ptr);
    s7_pointer s7_make_c_pointer_with_type(s7_scheme *sc, void *ptr, s7_pointer type, s7_pointer info);
    s7_pointer s7_make_c_pointer_wrapper_with_type(s7_scheme *sc, void *ptr, s7_pointer type, s7_pointer info);
    s7_pointer s7_c_pointer_type(s7_pointer p);

These functions are equivalent to s7's c-pointer?, c-pointer, and c-pointer-type. C-pointers in s7 are aimed primarily at passing uninterpreted C pointers through s7 from one C function to another. The "type" field can hold a type indication, useful in debugging. s7_c_pointer_of_type checks that the c-pointer's type field matches the type passed as the second argument. As a convenience, s7_c_pointer_with_type combines s7_c_pointer with s7_is_c_pointer_of_type, calling s7_error if the types don't match. Nothing else in s7 assumes the type field is actually a type symbol, so you can use the type and info fields for any purpose. s7_make_c_pointer_wrapper_with_type is akin to s7_make_string_wrapper (see below). It creates a temporary c_pointer object outside the heap and GC.

Strings

    bool s7_is_string(s7_pointer p);
    const char *s7_string(s7_pointer p);
    s7_pointer s7_make_string(s7_scheme *sc, const char *str);
    s7_pointer s7_make_string_with_length(s7_scheme *sc, const char *str, s7_int len);
    s7_pointer s7_make_string_wrapper(s7_scheme *sc, const char *str);
    s7_pointer s7_make_string_wrapper_with_length(s7_scheme *sc, const char *str, s7_int len);
    s7_pointer s7_make_permanent_string(s7_scheme *sc, const char *str);
    s7_pointer s7_make_semipermanent_string(s7_scheme *sc, const char *str);
    s7_int s7_string_length(s7_pointer str);

These handle s7 strings. s7_is_string corresponds to scheme's string?, and s7_string_length to scheme's string-length. s7_string returns the scheme string's value as a C string. Don't free the returned string! s7_make_string takes a C string, and returns its scheme equivalent. s7_make_string_with_length is the same, but it is faster because you pass the new string's length (s7_make_string has to use strlen). s7_make_permanent_string returns a scheme string that is not in the heap; it will never be GC'd or freed by s7. s7_make_semipermanent_string is similar, but the string will be freed if you call s7_free. s7_make_string_wrapper creates a temporary string. This saves the overhead of getting a free cell from the heap and later GC-ing it, but the string may be reused at any time. It is useful as an argument to s7_call and similar functions where you know no other strings will be needed during that call. s7_make_string_wrapper_with_length is the same but passes in the string length.

Characters

    bool s7_is_character(s7_pointer p);
    uint8_t s7_character(s7_pointer p);
    s7_pointer s7_make_character(s7_scheme *sc, uint8_t c);

s7_is_character is equivalent to character?. s7_character returns the unsigned char held by the s7 object p, and s7_make_character returns an s7 object holding the unsigned char c.

Hash-tables

    bool s7_is_hash_table(s7_pointer p);
    s7_pointer s7_make_hash_table(s7_scheme *sc, s7_int size);
    s7_pointer s7_hash_table_ref(s7_scheme *sc, s7_pointer table, s7_pointer key);
    s7_pointer s7_hash_table_set(s7_scheme *sc, s7_pointer table, s7_pointer key, s7_pointer value);
    s7_int s7_hash_code(s7_scheme *sc, s7_pointer obj, s7_pointer eqfunc);

These functions are the C-side equivalent of hash-table?, make-hash-table, hash-table-ref, and hash-table-set!.

Iterators

    s7_pointer s7_make_iterator(s7_scheme *sc, s7_pointer e);
    bool s7_is_iterator(s7_pointer obj);
    bool s7_iterator_is_at_end(s7_scheme *sc, s7_pointer iter);
    s7_pointer s7_iterate(s7_scheme *sc, s7_pointer iter);

These are the C equivalents of make-iterator, iterator?, iterator-at-end?, and iterate.

Hooks

    s7_pointer s7_hook_functions(s7_scheme *sc, s7_pointer hook);
    s7_pointer s7_hook_set_functions(s7_scheme *sc, s7_pointer hook, s7_pointer functions);

These access the list of functions associated with a hook. See hooks for a discussion of hooks, and C and Scheme Hooks for a short example. The scheme equivalent is hook-functions (a dilambda).

Constants

    s7_pointer s7_f(s7_scheme *sc);
    s7_pointer s7_t(s7_scheme *sc);
    s7_pointer s7_nil(s7_scheme *sc);
    s7_pointer s7_undefined(s7_scheme *sc);
    s7_pointer s7_unspecified(s7_scheme *sc);
    s7_pointer s7_eof_object(s7_scheme *sc);

    bool s7_is_unspecified(s7_scheme *sc, s7_pointer val);
    bool s7_is_null(s7_scheme *sc, s7_pointer p);
    bool s7_is_boolean(s7_pointer x);

    bool s7_boolean(s7_scheme *sc, s7_pointer x);
    s7_pointer s7_make_boolean(s7_scheme *sc, bool x);

    bool s7_is_immutable(s7_pointer p);
    s7_pointer s7_set_immutable(s7_scheme *sc, s7_pointer p);

These return the standard scheme or s7 constants: #f, #t, (), #<undefined>, #<unspecified>, and #<eof>. Also the s7 function unspecified?, and the scheme functions null?, and boolean?. s7_make_boolean returns #t or #f depending on its argument.

s7_set_immutable makes its argument immutable, and s7_is_immutable returns true if its argument is immutable. They parallel s7's immutable! and immutable?.

Optimization

    typedef s7_double (*s7_d_t)(void);
    void s7_set_d_function(s7_scheme *sc, s7_pointer f, s7_d_t df);
    s7_d_t s7_d_function(s7_pointer f);
    etc...

    s7_pfunc s7_optimize(s7_scheme *sc, s7_pointer expr);

These functions tell s7 to call a foreign function directly, without any scheme-related overhead. The function to be called in this manner needs to take the form of one of the s7_*_t functions in s7.h. For example, one way to call + is to pass it two s7_double arguments and get an s7_double back. This is the s7_d_dd_t function (the first letter gives the return type, the rest give successive argument types, d=double, i=integer, v=c_object, p=s7_pointer). We tell s7 about it via s7_set_d_dd_function. Whenever s7's optimizer encounters + with two arguments that it (the optimizer) knows are s7_doubles, in a context where an s7_double result is expected, s7 calls the associated s7_d_dd_t function directly without preparing a list of arguments, and without wrapping up the result as an s7 object.

Here is an example of using these functions; more extensive examples are in clm2xen.c in sndlib, and in s7.c.

    static s7_pointer g_plus_one(s7_scheme *sc, s7_pointer args)
    {
      return(s7_make_integer(sc, s7_integer(s7_car(args)) + 1));
    }

    static s7_int plus_one(s7_int x) {return(x + 1);}

    s7_define_safe_function(sc, "plus1", g_plus_one, 1, 0, false, "");
    s7_set_i_i_function(sc, s7_name_to_value(sc, "plus1"), plus_one);

s7_define_safe_function defines a Scheme function "plus1", telling the optimizer that this function is safe. A safe function treats the arglist passed to it as immutable and temporary (that is, it just grabs the arguments from the list). A few s7_* functions are unsafe, and that makes anything that calls them also unsafe. The main one is s7_apply_function, but it is possible that s7_eval, s7_eval_c_string, s7_call, s7_load_c_string, and s7_values are unsafe in some cases. If your code calls a Scheme function from C via s7_apply_function, it is not safe from the optimizer's point of view. If the optimizer knows a function is safe, it can use prebuilt lists to pass the arguments (saving in the GC), and can combine it in various ways with other stuff. If an unsafe function handles its argument list safely, declare it with s7_define_semisafe_typed_function.

If the safe function knows its return and argument types, there is another level of optimization that can call it without setting up an arglist or "unboxing" values, basically a direct call in C. In this example, the s7_set_i_i_function call tells the optimizer that if plus1 is seen in a context where the optimizer knows it is receiving an s7_int argument, and is expected to return an s7_int result, it can call plus_one directly, rather than g_plus_one.

There are more of these functions in s7.c that could be exported via s7.h if you need them.

By the way, to optimize scheme code (for speed), first use functions: the optimizer ignores anything else at the top level. Then perhaps check lint.scm and the profiler. Don't use something dumb like call/cc. Avoid append. Use iteration, not recursion. Perhaps take the hot spot and do it in C. callgrind might also be helpful, but it can be hard to map from callgrind output to the original scheme code.

s7_optimize is the third-level optimizer. It is a bit hard to explain, but basically you pass it some scheme code, and it returns either NULL or a function that can be called to evaluate that code. There are several examples in snd-sig.c. Here's an example where we want to call `(list 1 2 3)` a bazillion times:

    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>
    #include "s7.h"
    int main(int argc, char **argv)
    {
      s7_scheme *s = s7_init();
      s7_pfunc f;
      s7_pointer body;
      s7_int gc_loc;
      int i;
      body = s7_list_nl(s, 4, s7_make_symbol(s, "list"),
    	                  s7_make_integer(s, 1),
    	                  s7_make_integer(s, 2),
    	                  s7_make_integer(s, 3),
    	                  NULL);
      gc_loc = s7_gc_protect(s, body);
      f = s7_optimize(s, s7_list(s, 1, body));
      if (!f) fprintf(stderr, "oops\n");
      else
        for (i = 0; i < 2000000; i++)
          f(s);
      s7_gc_unprotect_at(s, gc_loc);
    }

s7_optimize can also be used to take advantage of the direct C function calls mentioned above:

    static s7_pointer g_d_func(s7_scheme *sc, s7_pointer args)
    {
      /* a normal C-defined s7 function that simply returns (scheme) 1.0 */
      return(s7_make_real(sc, 1.0));
    }
    static s7_double opt_d_func(void)
    {
      /* a version of g_d_func that returns (C) 1.0 */
      return(1.0);
    }

    /* now make it possible to call opt_d_func in place of g_d_func */
    s7_float_function func;
    s7_pointer symbol;

    symbol = s7_define_safe_function(sc, "d-func", g_d_func, 0, 0, false, "opt func");
    s7_set_d_function(sc, s7_name_to_value(sc, "d-func"), opt_d_func);

    /* and try it (this saves creating an s7 real, accessing its value, and GC-ing it eventually) */
    func = s7_float_optimize(sc, s7_list(sc, 1, s7_list(sc, 1, symbol)));
    fprintf(stderr, "%f\n", func(sc));

And so on...

    s7_scheme *s7_init(void);
    void s7_quit(s7_scheme *sc);
    void s7_free(s7_scheme *sc);
    void s7_repl(s7_scheme *sc);

    bool s7_is_eq(s7_pointer a, s7_pointer b);
    bool s7_is_eqv(s7_scheme *sc, s7_pointer a, s7_pointer b);
    bool s7_is_equal(s7_scheme *sc, s7_pointer a, s7_pointer b);
    bool s7_is_equivalent(s7_scheme *sc, s7_pointer x, s7_pointer y);

    void s7_provide(s7_scheme *sc, const char *feature);
    bool s7_is_provided(s7_scheme *sc, const char *feature);

    s7_pointer s7_stacktrace(s7_scheme *sc);

    s7_pointer s7_history(s7_scheme *sc);
    s7_pointer s7_add_to_history(s7_scheme *sc, s7_pointer entry);
    bool s7_history_enabled(s7_scheme *sc);
    bool s7_set_history_enabled(s7_scheme *sc, bool enabled);

    s7_pointer s7_dynamic_wind(s7_scheme *sc, s7_pointer init, s7_pointer body, s7_pointer finish);
    s7_pointer s7_make_continuation(s7_scheme *sc);
    s7_pointer s7_values(s7_scheme *sc, s7_pointer args);
    bool s7_is_multiple_value(s7_pointer obj);
    s7_pointer s7_copy(s7_scheme *sc, s7_pointer args);
    s7_pointer s7_fill(s7_scheme *sc, s7_pointer args);
    s7_pointer s7_type_of(s7_scheme *sc, s7_pointer arg);
    bool s7_is_syntax(s7_pointer p);
    bool s7_is_valid(s7_scheme *sc, s7_pointer arg);
    s7_pointer s7_function_let(s7_scheme *sc, s7_pointer obj);

    void (*s7_begin_hook(s7_scheme *sc))(s7_scheme *sc, bool *val);
    void s7_set_begin_hook(s7_scheme *sc, void (*hook)(s7_scheme *sc, bool *val));

s7_init creates a scheme interpreter. The returned value is the s7_scheme* used by many of the FFI functions. s7_quit exits the interpreter. The memory allocated for it by s7_init is not freed unless you call s7_free. (s7_free also frees its s7_scheme* argument, you may need to call s7_quit before s7_free to clean up the C stack, and as in multithreaded cases, global variables may need to be reinitialized after calling s7_free). s7_repl fires up a REPL. s7_is_eq and friends correspond to scheme's eq?, eqv?, equal?, and equivalent?. s7_provide and s7_is_provided add a symbol to the *features* list, or check for its presence there.

s7_stacktrace is like stacktrace; it currently ignores (*s7* 'stacktrace). The s7_history functions deal with the (*s7* 'history) buffer. s7_dynamic_wind is dynamic-wind in C. The parameters "init", "body", and "finish" are the same as in scheme (i.e. #f or a thunk). s7_make_continuation is call/cc; there is an example above. s7_values is values, s7_copy is copy, s7_fill is fill!, s7_type_of is type-of, s7_is_syntax is syntax?.

s7_is_multiple_value returns true if its argument (a list) is the result of calling values or s7_values. (values) returns a special object that is eq? to #<unspecified> but not C == to it (that is, there are two things in s7 that represent the unspecified object, one being the result of calling values with no arguments). (values obj) returns obj, which is not considered a multiple-value. Don't use s7_values in the new code portion of a C-side scheme macro definition (one from s7_define_macro). Use the symbol 'values instead.

s7_is_valid is a debugging aid; it tries to tell if an arbitrary value is pointing to an s7 object. Set the compile-time switch TRAP_SEGFAULT to 1 before using this function!

s7_function_let is an experiment. Its argument should be a c_function from s7_make_c_function and friends. It returns the let associated with that function, normally rootlet, but s7_make_typed_function_with_environment has an "envir" argument that sets this value. See g_f2 and make_f2 in ffitest.c for an example.

Finally, the begin_hook functions are explained above.

An example of s7_values:

    static s7_pointer vals(s7_scheme *s, s7_pointer args)
    {
      return(s7_values(s, args));
    }

    int main(int argc, char **argv)
    {
      s7_scheme *s = s7_init();
      s7_define_function(s, "vals", vals, 0, 0, true, NULL);
      s7_pointer let = s7_eval_c_string(s, "(+ (vals 1 2 3))");
      s7_display(s, let, s7_current_output_port(s));
      s7_newline(s, s7_current_output_port(s));
    }

related documentation: s7 s7-scm