[cffi-devel] Calculating size of C struct .?!
Hi again ;-) I do have the following C struct: 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. 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... How would I do that ? Thx!! Frank
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/
Am 10.05.2006 um 21:53 schrieb 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
Ah - now that you write it ;-) The bit I was missing is the :count directive ...
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.
Yep. And it works. Thanks!!! Frank
participants (2)
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Frank Goenninger -
Luís Oliveira