Thanks Jameson,
I want to later use `x` in auto-generated expressions, and if it's a Ref I
will have to use `x.x` to get the actual data which makes generating the
expressions more tricky, I thought I could perhaps avoid it.
Here's a more complete snippet:
~~~
type Foo
x::Int64
y::Vector{Int64}
end
Foo() = Foo(0, zeros(Int64, 10))
bar = :(f.x + f.y[2])
expr = :(
f = Foo()
for i=1:10
ccall((:myfunc_x, lib), Void, (Ptr{Void}, ), pointer(h.x)) #fill the
scalar f.x
ccall((:myfunc_y, lib), Void, (Ptr{Void}, ), pointer(h.y)) #fill the
array f.y
r = $(eval(bar)) #do something with f.x and f.y
end
#done with f, OK to clean up
)
~~~
Does this seem like an OK approach with Refs and is there a way to avoid
needing to do `h.x.x` in the expressions?
On Tuesday, 1 September 2015 23:51:34 UTC+2, Jameson wrote:
>
> use `Ref{Clong}` for the calling convention to pass an auto-allocated
> boxed c-long.
>
> it's usually best to avoid `pointer` and `pointer_from_objref`. memory is
> not "guaranteed by julia": it will be cleaned up as soon as the gc detects
> that you are no longer referencing the julia object. using `pointer` and
> `pointer_from_objref` hides memory from the gc, making it easier for you to
> create memory bugs in your program.
>
>
> On Tue, Sep 1, 2015 at 5:46 PM Joosep Pata <joose...@gmail.com
> <javascript:>> wrote:
>
>> I want to modify the data stored in an elementary julia object (e.g.
>> Int64) using external C code. I can do this if the type is wrapped in an
>> array, but is it possible to do something like `pointer(x::Float64) ->
>> address of data in x` with the memory being guaranteed by julia?
>>
>> Here’s a short snippet to describe:
>> ~~~
>>
>> #C side
>> void myfunc(long* x) {
>> x[0] += 1;
>> }
>>
>> #julia side, works
>> x = Int64[0]
>> ccall((:myfunc, lib), Void, (Ptr{Void}, ), convert(Ptr{Void}, pointer(x)))
>> println(x)
>> >> [1]
>>
>> #julia side, desired
>> x = 0
>> ccall((:myfunc, lib), Void, (Ptr{Void}, ), pointer(x))
>> println(x)
>> >> 1
>> ~~~
>>
>> I tried pointer_from_objref but as I understand this gives me the pointer
>> of the whole julia type with meta-data etc. If I write to this address in
>> C, I fear I’ll have problems.
>
>
