setjmp で遊ぶ

README.md

$ clang -v

clang version 3.0 (tags/RELEASE_30/final)
Target: i386-pc-linux-gnu
Thread model: posix

$ clang lambda_macro.c
$ ./a.out

lambda defined.
call lambda!
lambda called.
1 times called. str = hello
return lambda!
call lambda!
lambda called.
2 times called. str = world
return lambda!
lambda defined.
call lambda!
lambda called.
hoge
return lambda!
call lambda!
lambda called.
fuga
return lambda!

lambda_macro.c

#include <stdio.h>
#include <string.h>
#include <stdarg.h>
#include <setjmp.h>

typedef struct lambda {
  jmp_buf lambda_location;
  jmp_buf return_location;
  void **args;
} lambda;

#define LAMBDA_ARGC_MAX 256

#define LAMBDA_ARGS_NEW(ptr, ...)				 \
  {								 \
    void *__lambda_args_tmp[LAMBDA_ARGC_MAX] = { __VA_ARGS__ };	 \
    ptr->args = calloc(LAMBDA_ARGC_MAX, sizeof(void *));	 \
    memcpy(ptr->args, __lambda_args_tmp, LAMBDA_ARGC_MAX);	 \
  }

#define LAMBDA_ARGS_FREE(ptr)			\
  {						\
    if (ptr && ptr->args) free(ptr->args);	\
  }

/*
 * LAMBDA_CALL(lambda_ptr, arg1, arg2 .. argN);
 */
#define LAMBDA_CALL(ptr, ...)				\
  {							\
    if (setjmp(ptr->return_location) == 0) {		\
      puts("call lambda!");				\
      LAMBDA_ARGS_NEW(ptr, __VA_ARGS__);		\
      longjmp(ptr->lambda_location, 1);			\
    } else {						\
      puts("return lambda!");				\
      LAMBDA_ARGS_FREE(ptr);				\
    }							\
  }

/*
 * LAMBDA(lambda_ptr,
 * {
 *   // Some code.
 * });
 */
#define LAMBDA(ptr, body)					\
  {							\
    if (setjmp(ptr->lambda_location) == 0) {		\
      /* ラムダ定義時はなにもしない */			\
      puts("lambda defined.");				\
    } else {						\
      /* LAMBDA_CALL() によって呼ばれたとき */		\
      puts("lambda called.");				\
      body;						\
      longjmp(ptr->return_location, 1);			\
    }							\
  }

void example1()
{
  lambda l;
  lambda *lp = &l;
  int count = 0;
  LAMBDA( lp,
  {
    count++;
    char *str = lp->args[0];
    printf("%d times called. str = %s\n", count, str);
  });
  LAMBDA_CALL(lp, "hello");
  LAMBDA_CALL(lp, "world");
}

lambda *example2_defines_lambda_in_a_function()
{
  lambda *lp = malloc(sizeof(lambda));
  /* 変数の宣言とか参照はかなり危うい。
   * int count = 42;
   */
  LAMBDA( lp, {
      puts(lp->args[0]); // 変数を受け取ったりはできるが…
      /* たぶんセグフォが落ちであろう。
       * char *str = "Segumentation fault";
       * printf("%d times called. str = %s\n", count, str);
       */
  });
  return lp;
}

void example2()
{
  lambda *l = example2_defines_lambda_in_a_function();
  LAMBDA_CALL(l, "hoge");
  LAMBDA_CALL(l, "fuga");
}

int main(int argc, char *argv[])
{
  example1();
  example2();
  return 0;
}