Bryan Matsuo wrote:
I am working on some Go (golang) bindings for the LMDB library and I have
some interest in exposing the functionality of mdb_set_compare (and
mdb_set_dupsort). But it is proving difficult and I have a question about the
Calling mdb_set_compare from the Go runtime is challenging. Using C APIs with
callbacks comes with restrictions. I believe it impossible to bind these
functions way that is flexible, as one would expect. A potential change to
LMDB that would make binding drastically easier is having MDB_cmp_func to take
a third "context" argument with type void*. Then a binding could safely use an
arbitrary Go function for comparisons.
Is it possible for future versions of LMDB to add a third argument to the
MDB_cmp_func signature? Otherwise would it be acceptable for a variant API to
be added using a different function type, one accepting three arguments?
Thanks for the consideration.
 Go bindings -- https://github.com/bmatsuo/lmdb-go
 Cgo pointer restrictions --
 Cgo documentation -- https://golang.org/cmd/cgo/
I see nothing in these restrictions that requires extra information to be
passed from Go to C or from C to Go.
There is a vague mention in 
"A particular unsafe area is C code that wants to hold on to Go func and
pointer values for future callbacks from C to Go. This works today but is not
permitted by the invariant. It is hard to detect. One safe approach is: Go
code that wants to preserve funcs/pointers stores them into a map indexed by
an int. Go code calls the C code, passing the int, which the C code may store
freely. When the C code wants to call into Go, it passes the int to a Go
function that looks in the map and makes the call."
But it's nonsense in this case - you want to pass a Go function pointer to C,
but the only way for C to use it is to call some *other* Go function? Sorry
but there is no other Go function for the mdb_cmp() function to call, the only
one it knows about is the function pointer that you pass.
If this is what you're referring to, adding a context pointer doesn't achieve
anything. If this isn't what you're referring to, then please explain exactly
what you hope to achieve with this context pointer.
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/