# Need a function similar to python struct.unpack
**URL:** https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991
**Category:** General Usage
**Tags:** question
**Created:** [June 29, 2023, 7:18pm UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991 "2023-06-29T19:18:23Z")
**Posts on this page:** 13
**Page:** 1
### Author:  [@jjgomezcadenas](https://discourse.julialang.org/u/jjgomezcadenas)
#### Post date: [June 29, 2023, 7:18pm UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/1 "2023-06-29T19:18:23Z")
Hello everybody.
I need to read binary file from a specific module (HydraHarp) which comes with python and matlab codes for decoding the file.
The python code uses extensively the struct.unapck functions. For example:
tyEmpty8 = struct.unpack(“\>i”, bytes.fromhex(“FFFF0008”))[0]
For reference, this will yield the value: -65528
This is equivalent to the MatLab code:
tyEmpty8 = hex2dec(‘FFFF0008’);
I need a similar conversion in Julia (I am using 1.9.1 in a MacOs with ARM processor) and I have been unable to reproduce the value using several approaches (including PyCall). Any help would be very welcoed!
---
### Author:  [@mkitti](https://discourse.julialang.org/u/mkitti)
#### Post date: [June 29, 2023, 7:38pm UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/2 "2023-06-29T19:38:05Z")
I’m confused. Is the hex in a string or is it binary in a file. Here’s the literal function starting from a string.
```julia
julia> fromhex(hex) = first(reinterpret(Int32, [parse(UInt32, "0x"*hex)]))
fromhex (generic function with 1 method)
julia> fromhex("FFFF0008")
-65528
```
I got a feeling that this is not what you want.
You probably want something that is more like this.
```julia
julia> write("test.bin", 0xFFFF0008)
4
julia> read("test.bin", Int32)
-65528
```
In the above case, the file “test.bin” contains 4 bytes in little endian order.
```julia
julia> read("test.bin")
4-element Vector{UInt8}:
0x08
0x00
0xff
0xff
```
The above bytes are in little endian order. `08` comes first, and `FF` comes last. It seems like you need this to be read in big endian order.
```julia
julia> write("test.bin", hton(0xFFFF0008))
4
julia> read("test.bin")
4-element Vector{UInt8}:
0xff
0xff
0x00
0x08
julia> ntoh(read("test.bin", Int32))
-65528
```
Now `0xFF` comes first, and `0x08` comes last.
---
### Author:  [@cchderrick](https://discourse.julialang.org/u/cchderrick)
#### Post date: [June 29, 2023, 7:38pm UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/3 "2023-06-29T19:38:42Z")
```julia
reinterpret(Int32, hex2bytes("FFFF0008")) .|> hton
```
Output this:
```julia
1-element Vector{Int32}:
-65528
```
This only works for unpacking single datatype.
---
### Author:  [@jjgomezcadenas](https://discourse.julialang.org/u/jjgomezcadenas)
#### Post date: [June 29, 2023, 7:51pm UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/4 "2023-06-29T19:51:23Z")
> [@mkitti](#):
>
> `julia> `
Thanks a lot for your explanations! Actually I had both type of problems, some data are “hard wire” in the code (e.g, tyEmpty8 = struct.unpack(“\>i”, bytes.fromhex(“FFFF0008”))[0]) and some data are read from the input file (e.g, tagInt = struct.unpack(“\
### Author:  [@frylock](https://discourse.julialang.org/u/frylock)
#### Post date: [June 29, 2023, 8:33pm UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/5 "2023-06-29T20:33:01Z")
Another relevant post on [SO](https://stackoverflow.com/questions/64579524/unpack-binary-data-with-specific-format-in-julia). Looks very similar to @mkitti’s answer.
Also [this post](https://www.juliabloggers.com/learn-with-me-julia-structs-and-binary-i-o-3/) on Julia Bloggers.
---
### Author:  [@xgdgsc](https://discourse.julialang.org/u/xgdgsc)
#### Post date: [June 29, 2023, 11:54pm UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/6 "2023-06-29T23:54:15Z")
Using [GitHub - analytech-solutions/CBinding.jl: Automatic C interfacing for Julia](https://github.com/analytech-solutions/CBinding.jl) should be very easy for such things as you can define the structure in C code directly.
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [June 30, 2023, 5:40am UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/7 "2023-06-30T05:40:12Z")
> [@mkitti](#):
>
> `julia> fromhex(hex) = first(reinterpret(Int32, [parse(UInt32, "0x"*hex)]))`
This seems a bit redundant, why not
```julia
fromhex(hex) = reinterpret(Int32, parse(UInt32, "0x"*hex))
```
?
---
### Author:  [@jondea](https://discourse.julialang.org/u/jondea)
#### Post date: [June 30, 2023, 5:48am UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/8 "2023-06-30T05:48:34Z")
You can avoid the string creation by manually specifying the base
```julia
fromhex(hex) = reinterpret(Int32, parse(UInt32, hex; base=16))
```
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [June 30, 2023, 5:59am UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/9 "2023-06-30T05:59:39Z")
> [@jondea](#):
>
> `fromhex(hex) = reinterpret(Int32, parse(UInt32, hex; base=16))`
There’s also
```julia
fromhex(hex) = parse(UInt32, hex; base=16) % Int32
```
though I’m not 100% certain it’s exactly the same (on my phone here).
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [June 30, 2023, 8:39am UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/10 "2023-06-30T08:39:43Z")
> [@cchderrick](#):
>
> `reinterpret(Int32, hex2bytes("FFFF0008")) .|> hton`
```julia
julia> hex2bytes("FFFF0008")
4-element Vector{UInt8}:
0xff
0xff
0x00
0x08
julia> hex2bytes("FFFF008") # <- valid number
ERROR: ArgumentError: length of iterable must be even
```
> [@xgdgsc](#):
>
> Using [GitHub - analytech-solutions/CBinding.jl: Automatic C interfacing for Julia](https://github.com/analytech-solutions/CBinding.jl) should be very easy for such things as you can define the structure in C code directly.
What would be the advantage of doing this in C?
---
### Author:  [@mkitti](https://discourse.julialang.org/u/mkitti)
#### Post date: [July 2, 2023, 8:09am UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/11 "2023-07-02T08:09:24Z")
I got curious enough about this that I started to implement the Python `struct` library in Julia.
```julia
(@v1.9) pkg> activate --temp
Activating new project at `/tmp/jl_DFEY1E`
(jl_DFEY1E) pkg> add https://github.com/mkitti/PythonStructs.jl
Updating git-repo `https://github.com/mkitti/PythonStructs.jl`
Resolving package versions...
Updating `/tmp/jl_DFEY1E/Project.toml`
[ac2a11db] + PythonStructs v1.0.0-DEV `https://github.com/mkitti/PythonStructs.jl#main`
Updating `/tmp/jl_DFEY1E/Manifest.toml`
[ac2a11db] + PythonStructs v1.0.0-DEV `https://github.com/mkitti/PythonStructs.jl#main`
julia> using PythonStructs
julia> unpack(">i", hex2bytes("FFFF0008"))[begin]
-65528
julia> unpack(" unpack(">I", hex2bytes("FFFF0008"))[begin]
0xffff0008
julia> hex2bytes("FFFF0008") |> unpack(">l") |> first
-65528
```
> **[GitHub - mkitti/PythonStructs.jl: An implementation of the Python struct...](https://github.com/mkitti/PythonStructs.jl)**
>
> An implementation of the Python struct standard library in Julia. - GitHub - mkitti/PythonStructs.jl: An implementation of the Python struct standard library in Julia.
Also see the [initial documentation](https://mkitti.github.io/PythonStructs.jl/dev/).
---
### Author:  [@jjgomezcadenas](https://discourse.julialang.org/u/jjgomezcadenas)
#### Post date: [July 2, 2023, 8:53am UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/12 "2023-07-02T08:53:09Z")
Thanks a lot, this is really useful. The python library provides useful tools for binary data manipulation that are not obvious in Julia. I will go ahead and use your functions in my application
---
### Author:  [@jjgomezcadenas](https://discourse.julialang.org/u/jjgomezcadenas)
#### Post date: [July 3, 2023, 3:15pm UTC](https://discourse.julialang.org/t/need-a-function-similar-to-python-struct-unpack/100991/13 "2023-07-03T15:15:23Z")
I have implemented the functions of your package in my code and they work just right. Thanks a lot.
juan jose gomez cadenas
[jjgomezcadenas@gmail.com](mailto:jjgomezcadenas@gmail.com)