update ccall documentation

[vtjnash]

Struct support has been merged -- documentation needs to be updated accordingly

[ihnorton]

Also need to document callbacks. I'm happy to do that and structs once struct stuff stabilizes. (least I can do)

[ihnorton]

note re: callbacks, cfunction is understandably finicky about return types so it is necessary to convert() the return in order to make the return sig match:
function f1(cu::Ptr{Void}) return convert(Uint32, 1) end
otherwise you get:
function_ptr: type signature of f1 does not match
in cfunction at base.jl:143

[stevengj]

Please also document the possibility of using dlopen and dlsym for the function argument of ccall in the manual. (See #2316).

[pao]

Please make that a new issue, rather than reopening one that is specifically for a different topic.

[ihnorton]

i'll send a doc pr tonight for those and the other C-related things in #2242.

[nolta]

Should this have been closed? According to the docs, "Currently, it is not possible to pass structs and other non-primitive types from Julia to C libraries."

[ihnorton]

i think this was closed accidentally, but struct passing is not fully functional yet, see #1831 #2236 #2149 .

[pao]

Oh, yeah, I misunderstood the original reopening. Wasn't reading carefully enough. Sorry @stevengj.