# odd byte length primitive types and reinterpret()

**URL:** <https://discourse.julialang.org/t/odd-byte-length-primitive-types-and-reinterpret/9025>\
**Category:** General Usage\
**Created:** [February 12, 2018, 11:42pm UTC](https://discourse.julialang.org/t/odd-byte-length-primitive-types-and-reinterpret/9025 "2018-02-12T23:42:06Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![wherrera](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wherrera/32/5288_2.png) [@wherrera](https://discourse.julialang.org/u/wherrera)\
**Post date:** [February 12, 2018, 11:42pm UTC](https://discourse.julialang.org/t/odd-byte-length-primitive-types-and-reinterpret/9025/1 "2018-02-12T23:42:07Z")

</div>

julia\> primitive type Int24 24 end  
julia\> Int24(x::Int) = Core.Intrinsics.trunc\_int(Int24, x)  
Int24  
julia\> Int(x::Int24) = Core.Intrinsics.zext\_int(Int, x)  
Int64  
julia\> x = b"\x10\x45\x12\x20\x30\x40"  
6-element Array{UInt8,1}:  
0x10  
0x45  
0x12  
0x20  
0x30  
0x40

julia\> y = reinterpret(Int24,x)  
2-element Array{Int24,1}:  
Int24(0x124510)  
Int24(0x004030)

* * *

The 4th byte is ignored here. Julia seems to take every 4 bytes and truncate to Int24, so it uses the first 4 bytes for the first 3-byte number, but that is not what I want.

The above seems to show that julia forces reinterpret() to align on standard integer boundaries even when the primitive type is an odd byte length. Is this a design constraint on reinterpret() or is there a way to specify Int24 that does not cause the loss of the 4th byte of the UInt8 array with reinterpret() of 6 bytes into two integers?

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<div class="post-metadata">

**Author:** ![simonbyrne](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/simonbyrne/32/19_2.png) [@simonbyrne](https://discourse.julialang.org/u/simonbyrne)\
**Post date:** [February 12, 2018, 11:48pm UTC](https://discourse.julialang.org/t/odd-byte-length-primitive-types-and-reinterpret/9025/2 "2018-02-12T23:48:28Z")

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You can use `NTuple{3,UInt8}` (or create a `struct` with a single `NTuple{3,UInt8}` field):

```julia
julia> x = b"\x10\x45\x12\x20\x30\x40"
6-element Array{UInt8,1}:
 0x10
 0x45
 0x12
 0x20
 0x30
 0x40

julia> reinterpret(NTuple{3,UInt8}, x)
2-element Array{Tuple{UInt8,UInt8,UInt8},1}:
 (0x10, 0x45, 0x12)
 (0x20, 0x30, 0x40)

```

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<div class="post-metadata">

**Author:** ![foobar\_lv2](https://avatars.discourse-cdn.com/v4/letter/f/ee59a6/32.png) [@foobar\_lv2](https://discourse.julialang.org/u/foobar_lv2)\
**Post date:** [February 13, 2018, 12:25am UTC](https://discourse.julialang.org/t/odd-byte-length-primitive-types-and-reinterpret/9025/3 "2018-02-13T00:25:59Z")

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I think this is a bug; something somehow gets confused in the computations of the offsets/alignments and you end up reading/writing out-of-bounds memory. Could you open an issue or should someone else on this thread do so?

```julia
versioninfo()
#Julia Version 0.6.2

primitive type Int24 24 end
A=UInt8[i for i =1:12];

Ab=reinterpret(Int24, A)
#4-element Array{Int24,1}:
# Int24(0x030201)
# Int24(0x070605)
# Int24(0x0b0a09)
# Int24(0x000000) This is oob memory, potentially belonging to a different object.

unsafe_load(pointer(Ab,2))
#Int24(0x060504) correct

unsafe_load(pointer(Ab)+3)
#Int24(0x060504) correct
unsafe_load(pointer(Ab),2)
#Int24(0x070605) WRONG
Ab[2]
#Int24(0x070605) WRONG

```

Fixed on current master 0.7 (but a testcase would still be good, especially since @Keno plans to do something to reinterpret).

Edit: To be more precise, this issue has nothing to do with reinterpret and also happens on 0.7:

```julia
primitive type Int24 24 end
Int24(x::Int) = Core.Intrinsics.trunc_int(Int24, x)
Base.zero(::Type{Int24})=Int24(0)

A=zeros(Int24, 1000)
1000-element Array{Int24,1}:
#0.6:
# corrupted double-linked list 
#signal (6): Aborted

#0.7:
#corrupted double-linked list
#signal (6): Aborted

```

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<div class="post-metadata">

**Author:** ![wherrera](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wherrera/32/5288_2.png) [@wherrera](https://discourse.julialang.org/u/wherrera)\
**Post date:** [February 13, 2018, 1:43am UTC](https://discourse.julialang.org/t/odd-byte-length-primitive-types-and-reinterpret/9025/4 "2018-02-13T01:43:14Z")

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Ah, thanks for the clarification of the bug. I have a work around that involves only handling 3 bytes at a time. Would you be willing to open the issue?