Ptr{Void} instance finalizer

[Maurizio De Santis]

Hi!

congratulations for the Julia language, it's a great idea :-)

I'm learning Julia and I'm trying to write a libpq (PostgreSQL) wrapper. Executing this (julia version 0.3.0-744~ubuntu13.10.1):

module LibPQ

  typealias PGconn Ptr{Void}

  function pgconn_finalizer(x::PGconn)
    ccall( (:PQfinish, "libpq"), Void, (PGconn,), x )
    println("finalized")
  end

  c = ccall( (:PQconnectdb, "libpq"), PGconn, (Ptr{Uint8},), "connection params" )

  finalizer(c, pgconn_finalizer)

  println(c)

  gc()

end

I get an error:

$ julia libpq.jl
ERROR: objects of type Ptr{None} cannot be finalized
 in finalizer at base.jl:103
 in include at boot.jl:240
while loading ./libpq.jl, in expression starting on line 12

I guess I have to change the PGconn type alias from Ptr{Void} to something, but I am undecided about what to use.

[Jake Bolewski]

Ptr{Void} is an immutable type that cannot be finalized.

what you want to do is wrap the underlying pointer handle in a (mutable) Connection type.

type PGconn
    handle::Ptr{Void}
end


function pgconn_finalizer(c::PGConn)
    if c.handle != C_NULL
        ccall((:PQfinish, "libpq"), Void, (Ptr{Void},), x.handle)
        x.handle = C_NULL
    end

end

c = ccall( (:PQconnectdb, "libpq"), PGconn, (Ptr{Uint8},), "connection params" )
finalizer(c, pgconn_finalizer)
println(c)
gc()

[Jake Bolewski]

Oops! a little too hasty.

type PGconn
    handle::Ptr{Void}
end


function pgconn_finalizer(c::PGConn)
    if c.handle != C_NULL
        ccall((:PQfinish, "libpq"), Void, (Ptr{Void},), x.handle)
        x.handle = C_NULL
    end
end

ptr = ccall( (:PQconnectdb, "libpq"), Ptr{Void}, (Ptr{Uint8},), "connection params" )  
c = PGconn(ptr)


finalizer(c, pgconn_finalizer)
println(c)
gc()

Best,

Jake 

[Eric Davies]

Hey Maurizio,

You may be interested in my package PostgreSQL.jl. It's a libpq wrapper under active development in conjunction with John Myles White's forthcoming DBI.jl standardized database interface. Contributions are most welcome, though I understand if you want to take your own package from start to finish in order to learn Julia. It's limited but functional at the moment, and you can expect major updates in the coming week.

- Eric

[Maurizio De Santis]

Jake Bolewski: thank you, it solved the issue; I was near the solution but couldn't figure out the handle part.

Eric Davies: I wasn't aware of it; I'll be happy to contribute to it when/if I'll become fluent in Julia.

Sorry for the double post on julia-users (which is the right place where this discussion should have been happened, I guess), I have some issues with the google groups search and with the email notifications!