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To: geda-help AT delorie DOT com
Subject: Re: [geda-help] Help, how can I help?
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Comments: In-reply-to Andrew Poelstra <asp11 AT sfu DOT ca>
   message dated "Mon, 23 Jan 2012 12:57:25 -0800."
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Message-Id: <20120123233338.98CDD820B35D@turkos.aspodata.se>
Date: Tue, 24 Jan 2012 00:33:38 +0100 (CET)
From: karl AT aspodata DOT se (Karl Hammar)
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Andrew Poelstra:
> On Mon, Jan 23, 2012 at 12:13:55PM -0800, Colin D Bennett wrote:
> > On Sun, 22 Jan 2012 15:43:06 -0800
> > Andrew Poelstra <asp11 AT sfu DOT ca> wrote:
> > 
> > > Function pointers may not be casted to function pointers of
> > > different types. (This is very irritating, but it's what the
> > > standard says.)
> > 
> > I don't see how it's irritating.  It's just the only sane thing to
> > do.  How is a function pointer usefull unless you know the argument
> > types?
> > 
> 
> Because a lot of times you have unnecessary arguments -- i.e., suppose
> you had an object that looked like
> 
> typedef struct stream_t {
>   /* some public data */
>   int  (*open)  (struct stream_t *, int mode);
>   /* some other functions */
> } stream_t;
...

If you want variable length argument list, why don't you use a variable
length argument list?

$ cat a.c
#include <stdarg.h>
#include <stdio.h>

typedef struct stream_t {
  /* some public data */
  int  (*open)  (struct stream_t *, ...);
  /* some other functions */
} stream_t;

int f(struct stream_t *this, ...);
int g(struct stream_t *that, ...);

int main(void) {

  stream_t a;

  a.open = f;
  a.open = g;

  f(&a);
  g(&a, 10);

  return 0;
}

int f(struct stream_t *this, ...) {
  printf("hello f\n");
  return 0;
}

int g(struct stream_t *that, ...) {
  va_list ap;
  int mode;

  va_start(ap, that);
  mode = va_arg(ap, int);
  va_start(ap, that);  

  printf("mode %d\n", mode);
  return 0;
}
$ gcc  a.c 
$ ./a.out 
hello f
mode 10
$

///

Since you pass a struct to the open(), why don't you just put the
"int mode" into the struct:

typedef struct stream_t {
  /* some public data */
  int  (*open)  (struct stream_t *);
  int mode;
  /* some other functions */
} stream_t;

then the open who needs it can get it from the struct.


Regards,
/Karl Hammar

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