I guess what I find most confusing is that there would be a difference, since adding 1 to a pointer only adds one byte, not one element size.
> p1 = pointer(zeros(UInt64)); Ptr{UInt64} @0x000000010b28c360 > p1 + 1 Ptr{UInt64} @0x000000010b28c361 I would have expected the latter to end in 68. the two argument pointer function gets this “right”. > a=zeros(UInt64); > pointer(a,1) Ptr{Int64} @0x000000010b9c72e0 > pointer(a,2) Ptr{Int64} @0x000000010b9c72e8 I can see arguments multiple ways, but when I’m given a strongly typed pointer (Ptr{T}), I would expect it to participate in arithmetic in increments of sizeof(T). On March 25, 2015 at 6:36:37 AM, Stefan Karpinski (ste...@karpinski.org) wrote: That does seem to be the issue. It's tricky to fix since you can't evaluate sizeof(Ptr) unless the condition is true. On Tue, Mar 24, 2015 at 7:13 PM, Stefan Karpinski <ste...@karpinski.org> wrote: There's a branch in eltype, which is probably causing this difference. On Tue, Mar 24, 2015 at 7:00 PM, Sebastian Good <sebast...@palladiumconsulting.com> wrote: Yep, that’s done it. The only difference I can see in the code I wrote before and this code is that previously I had convert(Ptr{T}, pointer(raw, byte_number)) whereas here we have convert(Ptr{T}, pointer(raw) + byte_number - 1) The former construction seems to emit a call to a Julia-intrinsic function, while the latter executes the more expected simple machine loads. Is there a subtle difference between the two calls to pointer? Thanks all for your help! On March 24, 2015 at 12:19:00 PM, Matt Bauman (mbau...@gmail.com) wrote: (The key is to ensure that the method gets specialized for different types with the parametric `::Type{T}` in the signature instead of `T::DataType`). On Tuesday, March 24, 2015 at 12:10:59 PM UTC-4, Stefan Karpinski wrote: This seems like it works fine to me (on both 0.3 and 0.4): immutable Test x::Float32 y::Int64 z::Int8 end julia> a = [Test(1,2,3)] 1-element Array{Test,1}: Test(1.0f0,2,3) julia> b = copy(reinterpret(UInt8, a)) 24-element Array{UInt8,1}: 0x00 0x00 0x80 0x3f 0x03 0x00 0x00 0x00 0x02 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x03 0xe0 0x82 0x10 0x01 0x00 0x00 0x00 julia> prim_read{T}(::Type{T}, data::Array{Uint8,1}, offset::Int) = unsafe_load(convert(Ptr{T}, pointer(data) + offset)) prim_read (generic function with 1 method) julia> prim_read(Test, b, 0) Test(1.0f0,2,3) julia> @code_native prim_read(Test, b, 0) .section __TEXT,__text,regular,pure_instructions Filename: none Source line: 1 push RBP mov RBP, RSP Source line: 1 mov RCX, QWORD PTR [RSI + 8] vmovss XMM0, DWORD PTR [RCX + RDX] mov RAX, QWORD PTR [RCX + RDX + 8] mov DL, BYTE PTR [RCX + RDX + 16] pop RBP ret On Tue, Mar 24, 2015 at 5:04 PM, Simon Danisch <sdan...@gmail.com> wrote: There is a high chance that I simply don't understand llvmcall well enough, though ;) Am Montag, 23. März 2015 20:20:09 UTC+1 schrieb Sebastian Good: I'm trying to read some binary formatted data. In C, I would define an appropriately padded struct and cast away. Is is possible to do something similar in Julia, though for only one value at a time? Philosophically, I'd like to approximate the following, for some simple bittypes T (Int32, Float32, etc.) T read<T>(char* data, size_t offset) { return *(T*)(data + offset); } The transliteration of this brain-dead approach results in the following, which seems to allocate a boxed Pointer object on every invocation. The pointer function comes with ample warnings about how it shouldn't be used, and I imagine that it's not polite to the garbage collector. prim_read{T}(::Type{T}, data::AbstractArray{Uint8, 1}, byte_number) = unsafe_load(convert(Ptr{T}, pointer(data, byte_number))) I can reinterpret the whole array, but this will involve a division of the offset to calculate the new offset relative to the reinterpreted array, and it allocates an array object. Is there a better way to simply read the machine word at a particular offset in a byte array? I would think it should inline to a single assembly instruction if done right.