# When using channels, errors do not get raised, and computation stalls
**URL:** https://discourse.julialang.org/t/when-using-channels-errors-do-not-get-raised-and-computation-stalls/100291
**Category:** General Usage
**Created:** [June 13, 2023, 5:14pm UTC](https://discourse.julialang.org/t/when-using-channels-errors-do-not-get-raised-and-computation-stalls/100291 "2023-06-13T17:14:15Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@lmiq](https://discourse.julialang.org/u/lmiq)
#### Post date: [June 13, 2023, 5:14pm UTC](https://discourse.julialang.org/t/when-using-channels-errors-do-not-get-raised-and-computation-stalls/100291/1 "2023-06-13T17:14:15Z")
This is intended behavior, a known issue, or an unknown bug?
```julia
julia> function test()
buff = Channel{Int}(3)
for _ in 1:3
put!(buff, 0)
end
@sync for _ in 1:30
Threads.@spawn begin
take!(buff)
s = 0.0
for j in 1:10^6
s += log(j)*exp(j)*s
end
error()
put!(buff, 1)
end
end
end
test (generic function with 2 methods)
julia> test() # stalls, but does not raise any error.
^CERROR: InterruptException:
```
(edited post, localizing better the problem)
---
### Author:  [@ffevotte](https://discourse.julialang.org/u/ffevotte)
#### Post date: [June 13, 2023, 7:34pm UTC](https://discourse.julialang.org/t/when-using-channels-errors-do-not-get-raised-and-computation-stalls/100291/2 "2023-06-13T19:34:29Z")
I think this is an unfortunate behavior, which is more or less intended due to the way [`@sync`](https://docs.julialang.org/en/v1/base/parallel/#Base.@sync) works:
> `@sync`
>
> Wait until all lexically-enclosed uses of `@async`, `@spawn`, `@spawnat` and `@distributed` are complete. All exceptions thrown by enclosed async operations are collected and thrown as a `CompositeException`.
In your case, the first 3 tasks error out and are the only ones to complete because all other tasks wait for input. This means `@sync` can’t return and show you the errors.
With [`errormonitor`](https://docs.julialang.org/en/v1/base/parallel/#Base.errormonitor), you can at least see the errors thrown by the failing tasks in real time:
```julia
julia> function test()
buff = Channel{Int}(3)
for _ in 1:3
put!(buff, 0)
end
@sync for _ in 1:30
t = Threads.@spawn begin
take!(buff)
s = 0.0
for j in 1:10^6
s += log(j)*exp(j)*s
end
error()
put!(buff, 1)
end
errormonitor(t)
end
end
test (generic function with 1 method)
```
```julia
julia> test()
Unhandled Task ERROR:
Stacktrace:
[1] error()
@ Base ./error.jl:44
[2] macro expansion
@ ./REPL[11]:13 [inlined]
[3] (::var"#13#14"{Channel{Int64}})()
@ Main ./threadingconstructs.jl:373
Unhandled Task ERROR:
Stacktrace:
[1] error()
@ Base ./error.jl:44
[2] macro expansion
@ ./REPL[11]:13 [inlined]
[3] (::var"#13#14"{Channel{Int64}})()
@ Main ./threadingconstructs.jl:373
Unhandled Task ERROR:
Stacktrace:
[1] error()
@ Base ./error.jl:44
[2] macro expansion
@ ./REPL[11]:13 [inlined]
[3] (::var"#13#14"{Channel{Int64}})()
@ Main ./threadingconstructs.jl:373
^CERROR: InterruptException:
```
---
### Author:  [@lmiq](https://discourse.julialang.org/u/lmiq)
#### Post date: [June 13, 2023, 7:37pm UTC](https://discourse.julialang.org/t/when-using-channels-errors-do-not-get-raised-and-computation-stalls/100291/3 "2023-06-13T19:37:58Z")
Good to know, thanks, hard to debug something like that, though, it would be better if `errormirror()` was the default behavior, and possibly allowing its suppression by some `@sync` option.
---
### Author:  [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)
#### Post date: [June 13, 2023, 8:18pm UTC](https://discourse.julialang.org/t/when-using-channels-errors-do-not-get-raised-and-computation-stalls/100291/4 "2023-06-13T20:18:54Z")
There is also [`Experimental.@sync`](https://github.com/JuliaLang/julia/blob/84fc130f7a51fcd67feb9b4ece4c725772e92c6d/base/experimental.jl#L87), which errors directly. This is pretty convenient when developing at least.