[Haskell-beginners] Re: ffi woes

At 11:15 PM +0100 2/19/10, Jose A. Ortega Ruiz wrote:

Stephen Tetley writes:

...

Hi

I think I've found a version 30 from Debian's archive - the essid string is fixed length, rather than sized by essid_len (even though essid_len is now in the struct). Is this the same as the version you have?

Yes, it is. I've tried to read the essid field both using peekCString, as in

s <- (#peek struct wireless_info, essid) peekCString s

and providing explicitly the maximum length of essid (34) to peekCStringLen. Do you mean that for a struct field that is a of type char[LEN] those peek functions are innappropriate?

(I've checked using C that essid_len actually gets the correct value, and that the contents of essid is null-terminated, i.e. wi.essid[wi.essid_len] == '\0' is always true).

Thanks, jao

...

/* Structure for storing all wireless information for each device * This is a cut down version of the one above, containing only * the things *truly* needed to configure a card. * Don't add other junk, I'll remove it... */ typedef struct wireless_config { char name[IFNAMSIZ + 1]; /* Wireless/protocol name */ int has_nwid; iwparam nwid; /* Network ID */ int has_freq; double freq; /* Frequency/channel */ int freq_flags; int has_key; unsigned char key[IW_ENCODING_TOKEN_MAX]; /* Encoding key used */ int key_size; /* Number of bytes */ int key_flags; /* Various flags */ int has_essid; int essid_on; char essid[IW_ESSID_MAX_SIZE + 2]; /* ESSID (extended network) */ int essid_len; int has_mode; int mode; /* Operation mode */ } wireless_config;

Best wishes

Stephen

Though I'm not an FFI expert, I'll take a stab in case it might help you. It seems to me that

s <- (#peek struct wireless_info, essid) peekCString s

would read a pointer from the essid field, which is clearly not what you want. I think you'd want to use #ptr. Maybe something like the following (untested!): l <- (#peek struct wireless_config, essid_len) wc p <- (#ptr struct wireless_info, essid) wc -- wireless_config.essid is a char[MAX_LEN] peekCStringLen (p, fromIntegral (l :: CInt)) Dean