Re: Common FFI declarations
- From: Jay Carlson <nop@xxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Tue, 8 May 2012 12:10:35 -0400
On May 8, 2012, at 10:09 AM, Mike Pall wrote:
> Sadly, it's not that easy to automatically generate the target
> definitions. And the host compiler may not grok the target includes.
> So one probably has to parse the output of $(CROSS)$(CC) -E and
> grab only the needed definitions, but without all of the internal
> cruft.
Here's a snippet from preprocessor-fodder and a Makefile to go with it:
====
/* We would like to avoid including half the C library. Therefore
lines generated between BARRIER1 and BARRIER2 are snipped
from output. */
#include <stdint.h>
#include <stddef.h>
BARRIER1
#include <stdio.h>
#include <errno.h>
#include <math.h>
[...]
BARRIER2
/* We don't care about the internals of stdio, but some
debug functions mention it. */
typedef struct { void*dummy;} FILE;
extern FILE *stdin, *stdout, *stderr;
#include "config.h"
#include "libfoo/foostring.h"
#include "libfoo/fooformat.h"
[...]
#include "lua-fooformat-const.h"
====
lua-foo.i: lua-foo.E
$(CC) -x c -E $(CPPFLAGS) $(CFLAGS) $< | \
awk '/^BARRIER1/,/^BARRIER2/ {next} /^#/ || /^[ \t]*$$/ {next}
{print}' >$@
====
The contents of lua-foo.i are then suitable for feeding into the ffi parser.
Wait, so what's that lua-fooformat-const.h file at the end? It exists because
libfoo uses "#define FOO_MYCONST expr" a lot. It can *mostly* be mechanically
generated:
perl -l -n -e '/^#define\s+(\w+)\s/ && print "static const int l$1 = $1;";'
$s/libfoo/fooformat.h >lua-fooformat-const.h
This generates a lua-format-const.h containing:
====
static const int lFOO_FLAG_OPT_X = FOO_FLAG_OPT_X;
static const int lFOO_FLAG_OPT_Y = FOO_FLAG_OPT_Y;
====
which the previous preprocessor stage will deal with. It is stupid; if you have
#ifdef NEW_API
#define FOO_FLAG_OPT_X 8
#else
#define FOO_FLAG_OPT_X 10
#endif
it will spit out two static const int lFOO_FLAG_OPT_X declarations. I
hand-edited that file anyway to deal with other stuff. It seemed like less work
than writing a smarter #define-reader.
> With a cross-compiler you can't run the target
> program.
(This is so cute I can't resist posting about it over and over again.)
This is not necessary in many cases, surprisingly. Autoconf gained this
functionality when I wasn't looking. AC_LANG_BOOL_COMPILE_TRY will evaluate
boolean expressions at compile time.
http://git.savannah.gnu.org/cgit/autoconf.git/tree/lib/autoconf/c.m4?h=branch-2.59#n229
. The basic trick is to declare
static int test_array[1 - 2 * !(your_boolean_expression)];
which is a compile-time error when it evaluates to declaring a negative length
array.
AC_COMPUTE_INT will just compile and run a printf() of an expression when
compiling natively, but falls back to _AC_COMPUTE_INT_COMPILE when it's not
able to run programs. In essence, it can use AC_LANG_BOOL_COMPILE_TRY to
binary-search the integers for the value of the expression, one trial
compilation at a time.
http://git.savannah.gnu.org/cgit/autoconf.git/tree/lib/autoconf/general.m4?h=branch-2.59#n2462
is the actual implementation.
Jay
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