# What is the IO interface?
**URL:** https://discourse.julialang.org/t/what-is-the-io-interface/107350
**Category:** Internals & Design
**Tags:** question
**Created:** [December 8, 2023, 10:28pm UTC](https://discourse.julialang.org/t/what-is-the-io-interface/107350 "2023-12-08T22:28:30Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@mkitti](https://discourse.julialang.org/u/mkitti)
#### Post date: [December 8, 2023, 10:28pm UTC](https://discourse.julialang.org/t/what-is-the-io-interface/107350/1 "2023-12-08T22:28:30Z")
@jameson recently mentioned an `IO` interface in a [review](https://github.com/JuliaLang/julia/pull/52249/files#r1420986656). The lack of documentation on this matter was [mentioned several years ago](https://discourse.julialang.org/t/abstract-interfaces-dicts-ios-etc-going-to-be-documented/20377).
My best guess at that moment is that the core interface is inferrable from this part of boot.jl focusing on output:
[https://github.com/JuliaLang/julia/blob/551b37adf55acc7e9ffbe8c42623735186a2ec12/base/boot.jl#L660-L687](https://github.com/JuliaLang/julia/blob/551b37adf55acc7e9ffbe8c42623735186a2ec12/base/boot.jl#L660-L687)
# High Level Required
- `write(io::IO, x::UInt8)::Int` - Default implementation via `Base.io_pointer(io::IO)`
- `write(io::IO, x::String)::Int` - Default implementation via `Base.unsafe_write(io::IO, `Ptr{UInt8}`, nb::Int)`.
- `show(io::IO, @nospecialize x)::Nothing`
- `print(io::IO, x::AbstractChar)::Nothing`
# Low-level interface
- `Core.io_pointer(io::IO)::Ptr{Cvoid}`
- `unsafe_write(io::IO, x::Ptr{UInt8}, nb::UInt)::Int` - Default implementation via `Base.io_pointer(io::IO)`
- `unsafe_write(io::IO, x::Ptr{UInt8}, nb::Int):;Int` - Default implementation via `Base.io_pointer(io::IO)`
# Optional
- `print(io::IO, x::String)` - Default implementation via `write(io::IO, x::String)`
- `print(io::IO, x)` - Default implementation via `show(io::IO, x)`
- `print(io::IO, x, a...)` - Default implementation via `print(io::IO, x)`
- `println(io::IO)` - Default implementation via `write(io::IO, x::UInt8)`
- `println(io::IO, x...)` - Default implementation via `print(io::IO, x, a...)` and `println(io::IO)`
# Other functions
- `lock(io::IO)`
- `unmark(io::IO)`
- `unlock(io::IO)`
- `reseteof(io::IO)`
- `buffer_writes(x::IO, bufsize=Base.SZ_UNBUFFERED_IO)`
- `isopen(io::IO)`
- `close(io::IO)`
- `closewrite(io::IO)`
- `flush(io::IO)`
- `bytesavailable(io::IO)`
- `readavailable(io::IO)`
- `eof(io::IO)`
- `copy(io::IO)`
- `wait_readnb`
- `wait_close`
- `read(io::IO, String)`
- `unsafe_read(s::IO, p::Ptr{UInt8}, n::UInt64)`
- `unsafe_read(s::IO, p::Ptr, n::Integer)`
- `unsafe_read(s::IO, p::Ref{T}, n::Integer)`
I’m getting lost in this.
My sense is that we are missing several abstract subtypes of IO or IO traits.
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [December 9, 2023, 1:50am UTC](https://discourse.julialang.org/t/what-is-the-io-interface/107350/3 "2023-12-09T01:50:56Z")
> [@mkitti](#):
>
> - `show(io::IO, @nospecialize x)::Nothing`
> - `print(io::IO, x::AbstractChar)::Nothing`
IIRC, when you’re not in bootstrap mode, then `show` is implemented via `print` and [`print` of `Char` is implemented in terms of `write`](https://github.com/JuliaLang/julia/blob/master/base/char.jl#L252-L253).
`io_pointer` is an internal thing that’s used for `IO` types that correspond to [libuv](https://libuv.org/) I/O streams. I don’t think other `IO` types should implement it as long as they implement their own `write(io, byte::UInt8)` and `unsafe_write(io, bytes::Ptr{UInt8}, num_bytes::UInt)` methods.
(I suspect that `boot.jl` is not always the best guide to this sort of thing, because bootstrapping is kind of a weird environment.)
---
### Author:  [@ianshmean](https://discourse.julialang.org/u/ianshmean)
#### Post date: [December 9, 2023, 4:29am UTC](https://discourse.julialang.org/t/what-is-the-io-interface/107350/4 "2023-12-09T04:29:45Z")
Thanks for looking into this. The docs definitely could improve.
In a similar vein I started collecting gotchas/tips on IO with a view to expanding the docs, but it never matured enough to start a PR. Just in case it’s helpful: [https://hackmd.io/nlqvf7cjT6qcfW76cHLHqw](https://hackmd.io/nlqvf7cjT6qcfW76cHLHqw)