I'm having some difficulty with the following prototypes:
memcached_mget (memcached_st *ptr,
char **keys,
size_t *key_length,
unsigned int number_of_keys);
char *memcached_fetch (memcached_st *ptr,
char *key,
size_t *key_length,
size_t *value_length,
uint32_t *flags,
memcached_return *error);
I have MEMCACHED, in which I'm holding the pointer to memcached_st. I have MEMCACHED_RETURN, which I use to store the return pointer from various functions.
memcached_mget asynchronously gets the passed keys' values, and memcached_fetch retrieves them one by one. However, when watching the server debug output, when memcached_fetch is called, it's reporting an error (either SERVER_END or PROTOCOL_ERROR, and I can find documentation for neither). However, I think that the problem is newlisp dies with a bus error and disconnects in the middle of a transaction.
Here is the function I'm using (it's not yet cleaned up):
(when MEMCACHED
(letn ((num-keys (length list-keys))
(keys (pack (trim (dup "lu " num-keys)) (map 'string list-keys)))
(lengths (pack (trim (dup "d " num-keys)) (map 'length (map 'string list-keys)))))
(setq MEMCACHED_RETURN (memcached_mget MEMCACHED keys lengths num-keys))
(when (= (result) "SUCCESS")
(setq res '())
(dolist (key list-keys)
(let ((key-length (length key))
(val-length 0)
(flags 0)
(val nil))
(setq val (memcached_fetch MEMCACHED
key (address key-length)
(address val-length) (address flags)
(address MEMCACHED_RETURN)))
(push (list key (get-string val)) res -1))))))
res)
At the end, that get-string is where the bus error happens. memcached_fetch is returning 0 (NULL), which, according to libmemcached's docs, means I've retrieved all of the values... but it does this on the first call. Not quite sure where to go from here; the docs are not clear and I'm no expert in C.
Note: that 'result' function reads the error/status of MEMCACHED_RETURN. Success returns the string "SUCCESS", which suggests that the memcached_mget call succeeded.
Any ideas?
PS docs for libmemcached: http://docs.tangent.org/libmemcached/memcached.html
Here are some comments:
(1) char * * keys, array of string pointers
The string (array) addresses have to be anchored to a symbol, so the strings don't get moved around (memory managed), but stay in a fixed place in memory.
Instead of:
do:
(set 'keys (pack (dup "lu" num-keys) s-keys)
Here is a simple example to illustrate the same:
(set 'ptr (pack "lululu" sa)) ; ptr -> char * *, ptr to array of ptrs
> (get-string (get-int (+ (address ptr) 0)))
"ABC"
> (get-string (get-int (+ (address ptr) 4)))
"DEF"
> (get-string (get-int (+ (address ptr) 8)))
"GHI"
(2) size_t *key_length
size_t should be 32-bit not 16-bit. And as in (1) you have to anchor addresses in memory to build an array of size_t unsigned integers.
Instead of:
do:
; or using the previous s-keys
(set 'len-s-keys (map length s-keys))
(set 'lengths (pack (dup "lu" num-keys) len-s-keys))
Simple explicit example:
(set 'ptr (pack "lululu" sa))
(set 'l-sa (map length sa))
(set 'lengths (pack (dup "lu" 3) l-sa))
> (get-int (+ (address lengths) 0))
1
> (get-int (+ (address lengths) 4))
2
> (get-int (+ (address lengths) 8))
3
So now you can do:
Hope this helps ;-)
Is let-binding not sufficient? When I write the function with symbols locally bound in the variable and your tips:
(setq klen (address (length key))
vlen (address 0)
flags (address 0))
(setq res (memcached_fetch MEMCACHED key klen vlen flags (address MEMCACHED_RETURN)))
(if (= 0 res) nil (get-string res)))
(define (get-keys list-keys , res num-keys s-keys keys len-s-keys lengths)
(setq MEMCACHED_RETURN nil)
(when MEMCACHED
(setq num-keys (length list-keys))
(setq s-keys (map string list-keys))
(setq keys (pack (dup "lu" num-keys) s-keys))
(setq len-s-keys (map length s-keys))
(setq lengths (pack (dup "lu" num-keys) len-s-keys))
(setq MEMCACHED_RETURN (memcached_mget MEMCACHED keys lengths num-keys))
(when (= (result) "SUCCESS")
(setq res '())
(dolist (key list-keys)
(push (list key (fetch key)) res -1))))
res)
...it works perfectly (and thanks!). Don't let-bound variables have their own addresses, or did I simply not bind enough of the intermediate variables to symbols?
Quote
Yes, absolutely sufficient, I was just to lazy to write out the whole 'let' -form and used 'set' for that reason, 'let' should work just the same.
I've posted the new code to artful code (static.artfulcode.net/modules). Thanks for your help, Lutz.