[cffi-devel] Calculating size of C struct .?!

[Luís Oliveira]

On 2006-maj-10, at 17:19, Frank Goenninger wrote:
> typedef unsigned long	tcflag_t;
> typedef unsigned char	cc_t;
> typedef long		speed_t;	/* XXX should be unsigned long */
>
> struct termios {
> 	tcflag_t	c_iflag;	/* input flags */
> 	tcflag_t	c_oflag;	/* output flags */
> 	tcflag_t	c_cflag;	/* control flags */
> 	tcflag_t	c_lflag;	/* local flags */
> 	cc_t		c_cc[NCCS];	/* control chars */
> 	speed_t		c_ispeed;	/* input speed */
> 	speed_t		c_ospeed;	/* output speed */
> };
 >
> with NCCS being #define'd to 20.

CFFI> (defctype tcflag :unsigned-long)
TCFLAG
CFFI> (defctype cc :unsigned-char)
CC
CFFI> (defctype speed :long)
SPEED
CFFI> (defcstruct termios
         (iflag tcflag)
         (oflag tcflag)
         (cflag tcflag)
         (lflag tcflag)
         (cc cc :count 20)
         (ispeed speed)
         (ospeed speed))
TERMIOS
CFFI> (foreign-type-size 'termios)
44
CFFI> (foreign-alloc 'termios)
#.(SB-SYS:INT-SAP #X00600450) ; pointer to 44 newly allocated bytes
CFFI>


> I need to allocate space for this struct and therefore want to know  
> its size ... As I don't need the struct itself I thought about  
> calculating the size but I figure there's the problem of getting  
> all the padding to count to the size...

The size depends on the target platform. Different combinations of  
CPUs/OSs have different rules for struct alignment and different  
sizes for types, so your best bet is to define the struct anyway.

-- 
Luís Oliveira
http://student.dei.uc.pt/~lmoliv/