Stream: helpdesk (published)

Topic: number of bytes remaining in IO

Colin Caine (Feb 14 2021 at 20:05):

Is there a way of finding out how many bytes remain in an IO stream? I want to be able to work out if e.g.peek(io, Int) will error without calling it.

I think this can probably only be done by trying to read the file and seeing what happens, then resetting the seek point (which is how peek works), but I thought I'd ask anyway.

Fredrik Ekre (Feb 14 2021 at 20:07):

You can use bytesavailable maybe?

Jakob Nybo Nissen (Feb 14 2021 at 20:07):

Returns zero for me for a nonzero file

Expanding Man (Feb 14 2021 at 20:17):

ohhhh, so interesting that you should mention this... I think this may be a bug

Expanding Man (Feb 14 2021 at 20:18):

are you on 1.6?

Expanding Man (Feb 14 2021 at 20:18):

I saw this also, maybe we should open an issue

Fredrik Ekre (Feb 14 2021 at 20:18):

Ah, right! I ran into this the other day.

Expanding Man (Feb 14 2021 at 20:18):

ok, we definitely need an issue

Expanding Man (Feb 14 2021 at 20:18):

let me ask on slack first

Fredrik Ekre (Feb 14 2021 at 20:20):

It might be correct though, since the io stream needs to block before you can read?

Expanding Man (Feb 14 2021 at 20:20):

no, I've seen it give 0 and then immediately readavailable returns something of length $>0$

Fredrik Ekre (Feb 14 2021 at 20:21):

Yea but that is because readavailable does block and fill the buffer I think: https://github.com/JuliaLang/julia/blob/2eeef2e231a55cac770543b6dd673e349adfd797/base/iostream.jl#L381

Expanding Man (Feb 14 2021 at 20:21):

I'm confused, shouldn't readavailable always return the number of bytes indicated before calling it by bytesavailable?

Expanding Man (Feb 14 2021 at 20:22):

that was my understanding of what they're supposed to do

Expanding Man (Feb 14 2021 at 20:23):

bytesavailable seems to work on IOBuffer btw, though it may be worth noting that IOBuffer never blocks

Fredrik Ekre (Feb 14 2021 at 20:23):

Yea I don't know actually.

Expanding Man (Feb 14 2021 at 20:26):

I believe this is something of a MWE

julia> open(io -> write(io, "what up"), "testfile.txt", "w+")
7

julia> f = open("testfile.txt")
IOStream(<file testfile.txt>)

julia> bytesavailable(f)
0

julia> readavailable(f)
7-element Vector{UInt8}:
 0x77
 0x68
 0x61
 0x74
 0x20
 0x75
 0x70

Expanding Man (Feb 14 2021 at 20:26):

I believe bytesavailable should return 7

Kwaku Oskin (Feb 14 2021 at 20:28):

# num bytes available without blocking

Kwaku Oskin (Feb 14 2021 at 20:28):

It is written in the source code.

Expanding Man (Feb 14 2021 at 20:28):

Yup, that's what the docs say, and the above does not block, so this looks like a bug

Kwaku Oskin (Feb 14 2021 at 20:29):

So, it's not the number of bytes from here till the end of the stream.

Kwaku Oskin (Feb 14 2021 at 20:29):

Mmmm....
You need to block to readavailable

Fredrik Ekre (Feb 14 2021 at 20:30):

I mean, you can not assume that readavailable will give you bytesavailable bytes I think

Expanding Man (Feb 14 2021 at 20:30):

why not?

Fredrik Ekre (Feb 14 2021 at 20:30):

Because that could have changed between calling bytesavailable and readavailable.

Expanding Man (Feb 14 2021 at 20:30):

well yeah, ok fair enough, but we are seeing it in examples where that is not the case

Kwaku Oskin (Feb 14 2021 at 20:31):

Hmmm.... I do not quite understand what is intended behaviour, but code itself is rather straightforward.

bytesavailable(s::IOStream) = @_lock_ios s ccall(:jl_nb_available, Int32, (Ptr{Cvoid},), s.ios)

function readavailable(s::IOStream)
    lock(s.lock)
    nb = ccall(:jl_nb_available, Int32, (Ptr{Cvoid},), s.ios)
    if nb == 0
        ccall(:ios_fillbuf, Cssize_t, (Ptr{Cvoid},), s.ios)
        nb = ccall(:jl_nb_available, Int32, (Ptr{Cvoid},), s.ios)
    end
    a = Vector{UInt8}(undef, nb)
    nr = ccall(:ios_readall, Csize_t, (Ptr{Cvoid}, Ptr{Cvoid}, Csize_t), s, a, nb)
    if nr != nb
        unlock(s.lock)
        throw(EOFError())
    end
    unlock(s.lock)
    return a
end

Fredrik Ekre (Feb 14 2021 at 20:31):

What example? In your example you get mroe bytes than bytesavailable says so I think that is fine.

Kwaku Oskin (Feb 14 2021 at 20:33):

So to readavailableit verify number of available bytes and if it is zero it fills the buffer and update this number.

Kwaku Oskin (Feb 14 2021 at 20:33):

So, I think, correct meaning of bytesavailable is not the number of bytes available in iostream, but number of bytes available in the intermediate buffer.

Kwaku Oskin (Feb 14 2021 at 20:34):

Purely technical thing.

Expanding Man (Feb 14 2021 at 20:34):

Yeah, I noticed that it's a direct ccall, kind of makes me suspect it's an intended thing, but I don't know why

Expanding Man (Feb 14 2021 at 20:35):

well, regardless, I find it all very confusing

Expanding Man (Feb 14 2021 at 20:35):

"number of bytes available before blocking" sounds very straightforward, and it's clearly not giving me that

Kwaku Oskin (Feb 14 2021 at 20:36):

It depends on how you define blocking

Kwaku Oskin (Feb 14 2021 at 20:36):

For me, blocking means that you temporarily "own" file handler to read next chunk of data from the file.

Expanding Man (Feb 14 2021 at 20:37):

well I did not do an @async or @spawn and it's not stopping the execution, so that seems to me like it's not blocking

Fredrik Ekre (Feb 14 2021 at 20:37):

Yea I am pretty sure that is what is meant.

Fredrik Ekre (Feb 14 2021 at 20:45):

What are you using it for? I thought I would have use for it the other day but in the end I only use read and write with async tasks.

Fredrik Ekre (Feb 14 2021 at 20:45):

https://github.com/JuliaLang/julia/issues/24526 has some review of this btw

Expanding Man (Feb 14 2021 at 20:46):

Ok, I just had a conversation on slack, I think our problem is just that we are conflating different types of blocking

Expanding Man (Feb 14 2021 at 20:46):

readavailable seems to normally only block on relatively fast stuff

Colin Caine (Feb 14 2021 at 21:47):

I feel like bytesavailable isn't what I want. I want to know if there are at least N bytes remaining in the file or stream, but I don't care how many are currently buffered. When N = 1, this is equivalent to eof(io), but I want to be able to pick other Ns.

Last updated: Nov 27 2025 at 04:44 UTC