# How to read big-endian data

**URL:** <https://discourse.julialang.org/t/how-to-read-big-endian-data/29132>\
**Category:** General Usage\
**Tags:** binaryio, io\
**Created:** [September 24, 2019, 8:27pm UTC](https://discourse.julialang.org/t/how-to-read-big-endian-data/29132 "2019-09-24T20:27:56Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![danielc](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/danielc/32/220_2.png) [@danielc](https://discourse.julialang.org/u/danielc)\
**Post date:** [September 24, 2019, 8:27pm UTC](https://discourse.julialang.org/t/how-to-read-big-endian-data/29132/1 "2019-09-24T20:27:56Z")

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Hello,

I have an IOStream and I want to read a series of floats that are stored in big-endian order, while my computer is usually little-endian. So there are a few parts to my question:

1. If I know that the computer is little-endian, can I read the float like this?:

```julia
value = bswap(read(io,Float32))

```

My thinking is that `read(io,Float32)` will at least give me the correct set of bytes, and `bswap()` seems to preserve the type.

1. What I wrote above is fine for a small file, but I eventually want to read several GiB of data. Is there a way to read many floats and then convert them all to little-endian? I know you can read all the bytes with

```julia
bytes = read(io,sizeof(Float32)*ncells)

```

But I don’t know what to do with those bytes once I have them? How do I put them into an array of floats?

1. To make my code more portable, it would be nice if it could test endianness. How do I determine whether the computer I’m currently running on is little-endian or big-endian?

Thanks!

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<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [September 24, 2019, 8:31pm UTC](https://discourse.julialang.org/t/how-to-read-big-endian-data/29132/2 "2019-09-24T20:31:45Z")

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I think `bswap` has no cost so you shouldn’t worry about performance when scaling it.

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**Author:** ![danielc](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/danielc/32/220_2.png) [@danielc](https://discourse.julialang.org/u/danielc)\
**Post date:** [September 24, 2019, 8:41pm UTC](https://discourse.julialang.org/t/how-to-read-big-endian-data/29132/3 "2019-09-24T20:41:17Z")

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The cost wouldn’t be with bswap but with disk IO. I don’t want to read a large file 4 bytes at a time. But in any case, I think I found a solution to that part of my problem: It looks like you can pre-allocate an array and use `read!()` to populate that array:

```julia
data = zeros(Float32,ncells)
read!(io,data)
for j in 1:ncells
    data[j] = bswap(data[j])
end	

```

That seems to work. So the `read!()` function must be inferring a few things from the type and length of the `data` array.

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<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [September 24, 2019, 8:42pm UTC](https://discourse.julialang.org/t/how-to-read-big-endian-data/29132/4 "2019-09-24T20:42:49Z")

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in any case you can always read all the bytes you need out once and then `bswap` when interpolating them as needed.

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<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [September 24, 2019, 10:53pm UTC](https://discourse.julialang.org/t/how-to-read-big-endian-data/29132/5 "2019-09-24T22:53:57Z")

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> [@danielc](#):
>
> Is there a way to read many floats and then convert them all to little-endian?

```julia
x = reinterpret(Float32, read(io, sizeof(Float32)*ncells))
x .= ntoh.(x)

```

(Note that you can use `ntoh` rather than `bswap`, which will do the right thing for big-endian data regardless of whether you are running on a big-endian or little-endian machine.)

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<div class="post-metadata">

**Author:** ![danielc](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/danielc/32/220_2.png) [@danielc](https://discourse.julialang.org/u/danielc)\
**Post date:** [September 24, 2019, 11:13pm UTC](https://discourse.julialang.org/t/how-to-read-big-endian-data/29132/6 "2019-09-24T23:13:30Z")

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Aha! Thanks!