> Yes you warned me that it may sometimes segv, but what else can I do to >> get the pointer to a variable (a scalar or a composite type)? >> I asked it once in >> https://groups.google.com/forum/?fromgroups=#!topic/julia-users/i8DO3pBAHPU >> and my safe was to box it in pointer([M]). What else should I have done? >> > You can’t get a pointer to it, because an immutable doesn’t have a memory > location associated with it. What you can do is create a new immutable with > the values from the old immutable, plus any modifications you wanted to > make. > OK, I think I had understood that but I'm not finding a way to do it in my case. The point is what I need to replace is not the content of something pointed by a pointer. Instead it's (for example) the contents of the member 'rgb_low' which in itself is another immutable
https://github.com/joa-quim/GMT.jl/blob/master/src/libgmt_h.jl#L1486 no problems is creating a new 'Array_4_Cdouble' but how do I put in rgb_low, that is an immutable and for which I have no pointer? (that's why I was chasing the pointers of the member's type) Thanks > >> To send in a pointer to an integer to C that wants for example >> >> function blabla(int *n) {} >> >> what I do is >> >> n = [0] >> (wrapper to blabla) blabla(pointer(n)) >> n = unsafe_ref(n) >> >> How should I proceed here instead of using pointer([n])? >> > This usage is fine. You are calling pointer(n) here (not pointer([n])), > and you keep the variable n around until all uses of it are finished > (inside of blabla). > > >> The link you send me >> https://github.com/JuliaLang/julia/blob/jn/ccall3/doc/manual/calling-c-and-fortran-code.rst#passing-pointers-for-modifying-inputs >> >> gives as example "width = Ref{Cint}(0)" but that errors >> >> julia> width = Ref{Cint}(0) >> ERROR: UndefVarError: Ref not defined >> >> This will only work on Julia 0.4 after that pull-request is merged. On > 0.3, you can use an Array{Cint,0} to achieve a similar effect. > > I'm more than happy to follow your advice but I need an alternative. >> >> Thanks >> >> Joaquim >> >> >>> >>> >>> On Tue, Feb 24, 2015 at 11:36 AM J Luis <[email protected]> wrote: >>> >>>> >>>> >>>> use unsafe_load() to convert the value to a julia object, then >>>>> unsafe_store() to write the new struct back. >>>>> >>>>> at some point, I want to modify unsafe_load/unsafe_store to take a >>>>> fieldname symbol as the second argument to make this functionality more >>>>> direct available. however, there hasn't seemed to be a request / issue >>>>> open >>>>> for it yet. >>>>> >>>> >>>> PLEASE, do. >>>> >>>> I have the same type of problem but I'm not able to solve it with the >>>> solutions proposed in this thread. >>>> See, I have this and want to change the *rgb_low* >>>> >>>> julia> gmt_lut = unsafe_load(C.range, 1); >>>> >>>> julia> gmt_lut.rgb_low >>>> GMT.Array_4_Cdouble(0.583333333333333,0.0,1.0,0.0) >>>> >>>> which is a >>>> >>>> immutable Array_4_Cdouble >>>> d1::Cdouble >>>> d2::Cdouble >>>> d3::Cdouble >>>> d4::Cdouble >>>> end >>>> >>>> so I do >>>> >>>> julia> z = GMT.Array_4_Cdouble(0.0, 0.0, 0.0, 0.0) >>>> GMT.Array_4_Cdouble(0.0,0.0,0.0,0.0) >>>> >>>> julia> unsafe_store!(pointer([gmt_lut.rgb_low]), z) >>>> Ptr{GMT.Array_4_Cdouble} @0x00000000819faf60 >>>> >>>> no errors but I don't know where new *z* immutable landed on because I >>>> keep seeing the old value >>>> >>>> julia> gmt_lut.rgb_low >>>> GMT.Array_4_Cdouble(0.583333333333333,0.0,1.0,0.0) >>>> >>>> I suspect part of it is dues to the pointer([gmt_lut.rgb_low]) command >>>> because when I do several of those on line I keep getting different >>>> addresses >>>> >>>> julia> pointer([gmt_lut.rgb_low]) >>>> Ptr{GMT.Array_4_Cdouble} @0x0000000082b7baf0 >>>> >>>> julia> pointer([gmt_lut.rgb_low]) >>>> Ptr{GMT.Array_4_Cdouble} @0x0000000082ba00d0 >>>> >>>> julia> pointer([gmt_lut.rgb_low]) >>>> Ptr{GMT.Array_4_Cdouble} @0x0000000082ba0670 >>>> >>>> but than how can modify this field(s)? >>>> >>>> >>>> >>>> >>>> >>> >
